Settlement Issues Bridge Approach Slabs (Final Report Phase I) Report NM04MNT-02 Prepared by: University of New Mexico Department of Civil Engineering MSC01 1070 1 University of New Mexico Albuquerque, NM 87131-0001 December 2006 Prepared for: New Mexico Department of Transportation Research Bureau 7500B Pan American Freeway NE Albuquerque, NM 87109 In Cooperation with: The U.S. Department of Transportation Federal Highway Administration

i

1. Report No. NM04MNT-02

2. Government Accession No. 3. Recipient's Catalog No.

5. Report Date December 2006

4. Title and Subtitle Settlement Issues Bridge Approach Slabs (Final Report Phase 1) 6. Performing Organization Code

7. Author(s) Lary R. Lenke

8. Performing Organization Report No.

10. Work Unit No. (TRAIS)

9. Performing Organization Name and Address Transportation Engineering Research Program (TERP) Department of Civil Engineering MSC01 1070 1 University Of New Mexico Albuquerque, New Mexico 87131-0001

11. Contract or Grant No. CO 4654

13. Type of Report and Period Covered Interim Report July 2004 - December 2005

12. Sponsoring Agency Name and Address Research Bureau New Mexico Department of Transportation 7500 East Frontage Road P.O. Box 94690 Albuquerque, NM 87199-4690

14. Sponsoring Agency Code

15. Supplementary Notes 16. Abstract The New Mexico Department of Transportation (NMDOT) Bridge Section in conjunction with various NMDOT district engineers have documented settlement issues with bridge approach slabs, i.e., the ubiquitous bump at the end of the bridge, as a pressing maintenance, safety, construction and design issue. Bridge approach slab problems affect approximately 25% of U.S. bridges (about 150,000 structures nationwide) and at least $100 million (1997 dollars) is spent every year on repairs dealing with this issue. Based on population proportions, the author estimates the cost in New Mexico is approximately $750,000 (2005 dollars). The bump is of interest to the New Mexico Department of Transportation (NMDOT) because of maintenance costs and liability issues. A literature review identifies geotechnical issues with the natural soil foundation and the embankments as the leading contributor to the bump. Inadequate subsurface investigation, analysis, and subsequent stabilization of deep-seated foundation problems are factors in long term settlements. Poor material selection, compaction criteria, and compaction control can cause long term settlement problems in the approach embankment. Erosion and drainage concerns can be contributing factors to approach settlements, but tend to be of secondary concerns compared to geotechnical issues. Too short approach slabs can be problematic as they tend to exacerbate the bump. A field evaluation of 19 New Mexico bridges suggest that most, if not all, bump problems in the state are associated with geotechnical issues. The lengths of approach slabs in New Mexico are deemed too short. Recommendations for dealing with the bump are thorough geotechnical analysis and engineering implementation of this analysis in the design and construction of the natural soil foundation and embankment materials. High quality QA/QC (quality assurance-quality control) is absolutely required. Lengthening of approach slabs is also recommended. 17. Key Words: bridge, approach slab, settlement, foundation, embankment, bump-at-the-end-of-the-bridge

18. Distribution Statement Available from NMDOT Research Bureau

19. Security Classif. (of this report) None

20. Security Classif. (of this page) None

21. No. of Pages 100

22. Price None

ii

Settlement Issues Bridge Approach Slabs

(Final Report Phase 1)

by

Lary R. Lenke

University of New Mexico Department of Civil Engineering MSC01 1070

1 University of New Mexico Albuquerque, New Mexico 87131-0001

Under Contract Number: CO4654

A Report on Research Sponsored by

New Mexico Department of Transportation Research Bureau

In Cooperation with

The U.S. Department of Transportation Federal Highway Administration

December 2006

NMDOT Research Bureau

7500B Pan American Freeway PO Box 94690

Albuquerque, NM 87199-4690

2006 New Mexico Department of Transportation

iii

PREFACE The research reported herein explores the fundamental causes of bridge approach settlement, i.e., the bump at the end of the bridge. A literature review explores the basic underlying causes such as approach geometry, geotechnical exploration and analysis, material selection, structural issues, drainage, and QA/QC. A cursory field investigation was performed of 19 bridges in New Mexico that were identified with concerns about bridge approach settlement problems. Based on the literature and field evaluations it is concluded that most problems in New Mexico are of geotechnical in nature. Deep-seated foundation problems of the natural soil foundation that are not adequately addressed during field exploration, design, or construction are one issue. The second geotechnical issue is the proper control of the embankment and backfill in terms of material selection, compaction specifications, abutment geometry, and QA/QC. Of secondary importance is the approach slab geometry. Current New Mexico design standards limit the approach slab length to 14 feet. Current state of the art recommends a length of 20 feet to help in mitigating any bump caused by approach slab settlement.

NOTICE

DISCLAIMER

The United State Government and the State of New Mexico do not endorse products or manufacturers. Trade or manufacturers names appear herein solely because they are considered essential to the object of this report. This information is available in alternative accessible formats. To obtain an alternative format, contact the NMDOT Research Bureau, 7500B Pan American Freeway, Albuquerque, NM 87109 (P.O. Box 94690, Albuquerque, NM 87199-4690) or by telephone (505) 841-9145.

This report presents the results of research conducted by the author(s) and does not necessarily reflect the views of the New Mexico Department of Transportation. This report does not constitute a standard or specification.

iv

ABSTRACT

The New Mexico Department of Transportation (NMDOT) Bridge Section in conjunction with

various NMDOT district engineers have documented settlement issues with bridge approach

slabs, i.e., the ubiquitous bump at the end of the bridge, as a pressing maintenance, safety,

construction and design issue. Bridge approach slab problems affect approximately 25% of U.S.

bridges (about 150,000 structures nationwide) and at least $100 million (1997 dollars) is spent

every year on repairs dealing with this issue. Based on population proportions, the author

estimates the cost in New Mexico is approximately $750,000 (2005 dollars). The bump is of

interest to the New Mexico Department of Transportation (NMDOT) because of maintenance

costs and liability issues. A literature review identifies geotechnical issues with the natural soil

foundation and the embankments as the leading contributor to the bump. Inadequate

subsurface investigation, analysis, and subsequent stabilization of deep-seated foundation

problems are factors in long term settlements. Poor material selection, compaction criteria, and

compaction control can cause long term settlement problems in the approach embankment.

Erosion and drainage concerns can be contributing factors to approach settlements, but tend to be

of secondary concerns compared to geotechnical issues. Too short approach slabs can be

problematic as they tend to exacerbate the bump. A field evaluation of 19 New Mexico

bridges suggest that most, if not all, bump problems in the state are associated with geotechnical

issues. The lengths of approach slabs in New Mexico are deemed too short. Recommendations

for dealing with the bump are thorough geotechnical analysis and engineering implementation

of this analysis in the design and construction of the natural soil foundation and embankment

materials. High quality QA/QC (quality assurance-quality control) is absolutely required.

Lengthening of approach slabs is also recommended.

v

ACKNOWLEDGMENTS

The author expresses his appreciation to Mr. Virgil Valdez and Mr. Rais Rizvi, research staff at

the New Mexico Department of Transportation Research Bureau. Mr. Valdez supported the

author greatly during the field investigation efforts described. Appreciation is also extended to

the following members of the NMDOT Research Advisory Committee (RAC): Bob Meyers,

Jimmy Camp, Ted Barber, Bobby Gonzales, Rae Van Hoven, Susan Gallaher, Sherman Peterson,

David Trujillo, Jr., and Eric Lowe. Mr. Steve Von Stein with the New Mexico Division of

FHWA is thanked for his support. Thanks, also, to Lee Frieberg, and Peter Brakenhoff of HDR

Engineering for providing the experimental abutment details for the Washington Street Bridge

over I-40.

vi

TABLE OF CONTENTS

Page INTRODUCTION. 1LITERATURE REVIEW WITH DISCUSSION.. 3LITERATURE REVIEW CONCLUSIONS AND RECOMMENDATIONS.. 22FIELD EVALUATION OF NEW MEXICO BRIDGE APPROACHES. 27

Big-I, North-to-West Departure.. 28Bridge No. 9135, US 550, M.P. 29. 28Bridge No. 8375, I-25, M.P. 252, South Bound. 34Bridge No. 8376, I-25, M.P. 252, North Bound. 34Bridge No. 6554, I-40 West Bound Over BNSF Railway Mainline (W. Gallup).. 36Bridge No. 6553, I-40 East Bound Over BNSF Railway Mainline (W. Gallup)... 38Bridge No. 8335, West Bound I-40 at Exit 16 (W. Gallup)... 41Bridge No. 8336, East Bound I-40 at Exit 16 (W. Gallup). 45Mountain Valley Road, NM 217 Over I-40 47Paseo del Norte at Coors Blvd 53Paseo del Norte at Interstate 25 (I-25) 57Pennsylvania Street Over I-40 (Albuquerque) 58Bridge 9311, US 84, South Bound. 63Bridge 9312, US 84, South Bound (Camel Rock Exit).. 64Bridge 9309, US 84, South Bound (Flea Market Exit)... 64US 84 South Bound (Opera Drive, Exit 168). 64US 84 South Bound Over Rio Tesuque.. 71Bridge 9310, US 84, South Bound (S. Tesuque Exit) 71Bridge 8942, US 84 Overpass at NM 502.. 74

CONCLUSIONS AND OBSERVATIONS FROM FIELD EVALUATION.. 77RECOMMENDATIONS.............. 82

Recommended Field Trials. 85REFERENCES.................. 90

vii

LIST OF TABLES

Table 1. Contributing Factors to The Bump (Briaud, et al (2)) 7Table 2. Methods to Minimize the Bump (Briaud, et al (2)) 15Table 3. Approach Slab Dimensions by State (Hoppe, (5)). 18Table 4. State Backfill Material Specifications for Approach Slabs (Hoppe, 5). 19Table 5. State Compaction Specifications for Approach Slabs (Hoppe (5))... 20Table 6. Compaction Requirements at Abutment (Briaud, et al (2))... 21Table 7. New Mexico Bridges with Concerns About the Bump.. 27Table 8. Experimental Design for Bridge Abutments on Washington Street

Bridge. 86

viii

LIST OF FIGURES

Figure 1. Causes of "the Bump" (Briaud et al. (2))... 5Figure 2. Bridge Approach Settlement Problems by State (Hoppe (5)) 6Figure 3. Range of Most Erodible Soils (after Briaud, et al (2), reproduced

from White, et al (3)). 9

Figure 4. Range of Most Erodible Soils with Typical Quality Soil Types and New Mexico Base Courses ...

10

Figure 5. Approach Slab Settlement Geometry..................... 11Figure 6. Laboratory Model for Simulating the Bump (Seo and Briaud (6)

and Seo (7))........................ 12

Figure 7. Approach Slab Joint Details at Pavement Edge (after Briaud, et al (2), reproduced from White, et al (3))......................................................

17

Figure 8. Drainage Structure, Left Edge, Big-I North Bound-to West Bound Departure................

29

Figure 9. Drainage Structure, Right Edge, Big-I North Bound-to West Bound Departure....

29

Figure 10. Detail Between Barrier Wall and MSE Wall. 30Figure 11. North End Approach, Bridge No. 9135, US 550... 31Figure 12. North End Approach (Note Cracking and Evidence of Pressure

Grouting).... 31

Figure 13. Estimate of South Bound Approach Slab Settlement, Bridge No. 9135 32Figure 14. Use of Rip-Rap for Erosion Control on Bridge No. 9135.. 33Figure 15. Example Drainage Structure at Bridge No. 9135................... 33Figure 16. South Bound Departure, I-25 at Arroyo Tonque... 35Figure 17. Bump on South Bound Departure, I-25 at Arroyo Tonque 35Figure 18. Cracking in Wheel Paths of South Bound Departure, I-25 at Arroyo

Tonque.... 36

Figure 19. Poorly Maintained Joint Between Deck and Approach Slab (Bridge No. 8375)

37

Figure 20. Drainage Adjacent to Abutments and Approach Slabs at Arroyo Tonque....

38

Figure 21. Settlement at West Bound Departure Slab, Bridge No. 6554 39Figure 22. Evidence of Settlement Between Abutment and Embankment at

Bridge No. 6554. 39

Figure 23. Additional Evidence of Settlement Between Abutment and Embankment at Bridge No. 6554...............

40

Figure 24. East Bound Approach Slab on Bridge No. 6553 40Figure 25. Movement at Right Approach Abutment on Bridge No. 6553.. 41Figure 26. Cavity Under Concrete Facing on Abutment on Bridge No. 6553 42Figure 27. Concrete Embankment Facing Movement Relative to Right

Departure Abutment on Bridge No. 6553.. 42

Figure 28. Approach Slab, Bridge No. 8335, Exit 16, West Gallup... 43Figure 29. Lateral Drop Off at Edge of Approach Slab on Bridge No. 8335.. 44Figure 30. Joint Concrete Embankment Facing Against Vertical Bridge Fascia

(Bridge No. 6553).............................. 44

ix

LIST OF FIGURES (CONT.)

Figure 31. Watertight Median Between Bridges at Exit 16 45Figure 32. Departure Slab on Bridge No. 8336 (East Bound, West Gallup)........... 46Figure 33. Approach Slab on Bridge No. 8336 (East Bound, West Gallup)... 46Figure 34. North Bound Approach Slab on NM 217 at I-40................... 48Figure 35. Excessive Settlement on North Bound Approach on NM 217.. 48Figure 36. Settlement at North Bound Approach Slab, NM 217 49Figure 37. Embankment Subsidence and Approach Settlement, South End

Departure.... 50

Figure 38. South Bound Approach, NM 217.. 50Figure 39. Approach Slab Joint, NM 217 51Figure 40. North Bound Departure Slab, NM 217.................. 51Figure 41. North End Bump, NM 217 at I-40.. 52Figure 42. Settlement at Northeast End of NM 217 Bridge at I-40. 53Figure 43. West Bound Approach, Paseo del Norte at Coors Blvd. 55Figure 44. East Bound Departure, Paseo del Norte at Coors Blvd.. 55Figure 45. West Bound Approach Slab with Evidence of Alkali-Silica Reactivity

(ASR). 56

Figure 46. Concrete Embankment Protection on Paseo del Norte at Coors Blvd... 56Figure 47. Drainage Barrier Between Bridge Barrier Wall and MSE Wall 57Figure 48. Damaged Approach Slab Joint Caused by ASR 59Figure 49. Area Between Bridge MSE Wall and Bridge Barrier Wall... 59Figure 50. MSE Wall, Paseo del Norte at I-25, West Abutment. 60Figure 51. North Bound Approach on Pennsylvania Street Over I-40 60Figure 52. Approach Slab Joint on Pennsylvania Street Bridge Over I-40. 61Figure 53. Concrete Slope Protection with Drainage Gutter................... 62Figure 54. Positive Drainage Control at Base of Concrete Slope Protection.. 62Figure 55. South Bound Departure Slab, Bridge No. 9311. 65Figure 56. South Bound Approach Slab (note cracking and slab jacking).. 65Figure 57. South Bound Departure Slab (note slab jacking)............... 66Figure 58. Concrete Slope Protection on Bridge No. 9311. 66Figure 59. South Bound Approach Slab, Bridge No. 9312. 67Figure 60. South Bound Departure Slab, Bridge No. 9312. 67Figure 61. South Bound Approach Slab, Bridge No. 9309. 68Figure 62. Mulch Embankment Erosion Control, Bridge No. 9309 68Figure 63. South Bound Departure at Opera Drive (US 84)... 69Figure 64. South Bound View of Bridge at Exit 168 (US 84) 69Figure 65. Drainage Channel on Embankment at Exit 168 (US 84)... 70Figure 66. South Bound Lanes Over Rio Tesuque (US 84) 72Figure 67. Gabion Erosion Control at Rio Tesuque (US 84).......... 72Figure 68. Bridge 9310, US 84, South Bound (S. Tesuque Exit) 73Figure 69. Concrete Slope Protection on Bridge No. 9310. 73Figure 70. North Bound Approach System, US 84 at NM 502........... 75Figure 71. Watertight Backfill Between Barrier Wall and Bridge Fascia... 75Figure 72. Drop Inlet, Bridge No. 8942 Over NM 502... 76

x

LIST OF FIGURES (CONT.)

Figure 73. Example of Abutment with Corbel 85Figure 74. Control Abutment #2, North End, Washington St. Bridge 88Figure 75. Experimental Abutment #1, South End, Washington St. Bridge... 88Figure 76. Submerged Approach Slab System (Hoppe (5)) 89

INTRODUCTION

The New Mexico Department of Transportation (NMDOT) Bridge Section in conjunction with

various NMDOT district engineers have documented settlement issues with bridge approach

slabs, i.e., the ubiquitous bump at the end of the bridge, as a pressing maintenance, safety,

construction and design issue. Briaud, Maher, and James (1) state:

The bump at the end of the bridge is a common but complex problem that

involves a dizzying range of design factors, including soil settlement in

embankments, approach fill material, abutment foundation type, abutment type,

structure, type, joints, approach slab, paving and construction methods.

They further claim that bridge approach slab problems affect approximately 25% of U.S.

bridges (about 150,000 structures nationwide) and at least $100 million is spent every year on

repairs dealing with this issue (1997 dollars). Briaud, et al (2) claim the cost to Texas caused by

approach slab distress is $7 million (2002 dollars). Based on population proportions, the author

estimates the cost in New Mexico to be approximately $750,000 (2005 dollars).

White, et al (3) state that bridge approach settlement and the formation of the bump is a

common problem drawing considerable resources for maintenance, creating a negative

perception in the minds of transportation users. They define the term bridge approach, not just

in terms of the approach slab alone, but in terms of a more holistic definition as the area from the

abutment to a significant distance (about 100 feet) away from the bridge structure (abutment).

This definition recognizes that the backfill and embankment regions beyond the approach slab

can be significant contributors to settlements in the bridge approach region. Furthermore,

attention to design, construction, and quality control-quality assurance (QA/QC) is just as

important in this region as the region in close proximity to the approach slab itself.

2

The bridge approach settlement issue, i.e., the bump, is of interest to the New Mexico

Department of Transportation (NMDOT) because of maintenance costs and liability issues. An

improperly designed or constructed approach can require expensive modifications after the fact

such as mud jacking, soil modification, mill and inlay of hot mix asphalt (HMA), surface

grinding and treatments, or even complete replacement of the approach system. Liability issues

are concerned with damage to the vehicle chassis caused by the bump or even the potential for

vehicular crashes.

3

LITERATURE REVIEW WITH DISCUSSION

New Mexico is not the only state that has expressed concern about the bump at the end of the

bridge. An early study sponsored by the National Cooperative Highway Research Program

(NCHRP) was conducted by Wahls (4). Wahls lists the following fundamental causes for the

bump,

1) time-dependent consolidation of the embankment foundation (natural soil),

2) time-dependent consolidation of the approach embankment,

3) poor compaction of abutment backfill (caused by restricted access of standard

compaction equipment),

4) erosion of soil at the abutment face, and

5) poor drainage of the embankment and abutment backfill.

Figure 1 is a schematic (after Briaud, et al (2)) of the cross-section of a bridge abutment

(e.g., on pile foundations) including the embankment foundation soil (natural soil), the approach

embankment (and backfill), the abutment face embankment, the approach slab, and the approach

sleeper slab. Consistent with Wahls (4), note that compressibility of the foundations natural soil

is of concern (deep-seated foundation problem; difficult to mitigate after the fact), the

embankment and backfill soils above the natural soil that support the approach system, i.e., the

approach slab, the sleeper slab and the surfacing material before the approach slab, whether

HMA or portland cement concrete (PCC) surfacing. Compressible natural foundation soils (such

as clay, or collapsing soils) can result in long term deformation of the pavement system, before

the bridge abutment, if not properly treated in advance. Compressible embankment and backfill

materials under and in front of the approach system can result in long term settlements as well.

This can be caused by poor material selection of the embankment and backfill materials, and also

4

because of poor compaction control (i.e., low density, resulting in a highly deformable

embankment mass). Poor compaction can also be caused, as Wahls (4) notes, by limited access

or difficult access within the confined working space behind the bridge abutment. Moisture

sensitive materials may be problematic as well (expansive soils and soils sensitive to

freeze-thaw).

Note in Figure 1 that erosion of embankment fill and erosion under the approach slab and

approach system can lead to settlement problems at the approach slab. Furthermore, slope

stability of the embankment system can lead to settlement of the approach system, too. Other

mechanisms causing the potential bump are caused by thermal movements of the bridge

system (integral bridges in particular) and the approach system before the bridge abutment.

Lastly and perhaps most importantly, is the design of the approach slab and sleeper slab.

Improper design can lead to structural failure of the approach slab, while improper sleeper slab

geometry can lead to settlement problems as well. Erosion problems can be caused by improper

design of these elements.

Hoppe (5) surveyed the 48 contiguous state DOTs regarding their experience with bridge

approach settlement issues. The map of Figure 2 shows the states results regarding the degree

or severity that the states have experienced. Twenty state DOTs claim to have bridge approach

settlement issues, predominantly in the west, north central, and central regions of the country.

Six states claim to have no problems. Interestingly, three of which are New England states

(where one might expect to find freeze susceptible materials), and one is Texas, a state that has

done quite a bit of work on this problem in recent times. Thirteen states state moderate bump

problems, all in the midwest, east, or northeast region; one being Iowa, a state that more recently

has claimed significant problems (see White, et al (3)). It is interesting that most of these states

5

might tend to have more problems with the bump because of the likelihood of moisture

sensitive soils to exist in these areas. Nine states did not respond to Hoppes survey (5). With

33 of 48 (or 69%) of the state DOTs responding with concerns about bridge approach settlement

issues, it is clear that a vast majority view this as a problem of some degree.

Figure 1 Causes of "the Bump" (Briaud et al. (2)).

Briaud, et al (2) surveyed most of the states Departments of Transportation (DOTs) and

found, based on 48 states responding, that the most significant factors contributing to the bump

at the end of the bridge are those as listed in Table 1. Table 1 displays the top eight factors of a

total of the twenty factors cited by state DOTs. However the eight cited in Table 1 account for

6

68% of the survey citations. This suggests that most of the significant factors are those in

Table 1.

Figure 2 Bridge Approach Settlement Problems by State (Hoppe (5)).

Based on the survey data shown in Table 1 suggests that many states have found long

term settlement of the natural foundation soils to be the number one cause of the bump. One

can only surmise that careful attention in the design, foundation soil evaluation, and treatment of

these soils prior to the construction stage has lead to these long term settlement concerns.

Rankings three, and four in Table 1 are concerned with the quality of the embankment. Poor

backfill and embankment materials can lead to long term settlement problems as well as erosion

caused by water drainage under the approach slab system. For example, Briaud, et al (2) give

soil gradation guidelines for types of soils that are erosion resistant and those that are prone to

erosion. This is clearly shown in Figure 3 where it is clear that a gradation band of material in

the sand size down to silt size material is a bad choice for embankments and backfill unless

suitably protected (e.g., by appropriate drainage design or erosion control systems).

7

Figure 4 shows the upper erodible band, from Figure 3, along with typical quality

American Association of State Highway and Transportation Officials (AASHTO) soil types as

well as New Mexico Class I and Class II base courses. The base course materials along with the

AASHTO A-1-a material are clearly acceptable and superior to the other materials presented.

Interestingly, the AASHTO A-1-b material appears unacceptable based on the erosion criteria of

Briaud, et al (2). The A-2 material is comparable to the quality base courses shown. While the

A-3 material meets the erosion criteria, it is not a well graded material, hence perhaps useful for

embankment material, but not backfill. All other AASHTO soil types would be unacceptable as

well.

Table 1 Contributing Factors to The Bump (Briaud, et al (2))

Ranking Contributing Factor1 Fill on Compressible Foundation2 Approach Slab Too Short3 Poor Fill Material4 Compressible Fill5 High Deep Embankment (> 33 ft (10 m))6 Bad Drainage7 Severe Erosion8 Poor Joint Design and Joint Maintenance

Returning to Table 1, the number two ranking by state agencies was an issue with respect

to the design of the approach slab length. Many state DOTs felt that their slabs were too short.

An approach slab that is too short will aggravate the bump because it will accentuate the

traveling publics perception of the bump because of the vehicular response. For simplicity here,

assume a level roadway pavement approach to a bridge, and a level bridge deck. These two

sections of level pavement are connected by the approach slab. The slope of the approach slab is

simply the change in elevation between the beginning of the approach slab (at the sleeper slab)

8

and the bridge abutment divided by the length of the approach slab. Represented in equation

form this slope is:

L

ssS af

= (1)

In equation 1, S is the slope of the approach slab, L is the length of the approach slab, and

sf and sa are the settlements of the foundation (embankment and natural soil foundation) and

abutment, respectively. Figure 5 depicts a typical bridge with abutment and approach slab

system. Typically the abutment settlement is approximately zero for a well designed bridge

abutment system. Therefore the slope is driven by the foundation settlement and length of the

approach slab alone. Now the mechanism that gives a vehicle a jolt and the occupant, too, as

they traverse an approach is not just the slope, but the rate of change of slope, or the curvature.

For the simple case here where it was assumed that the pavement and deck were level, the

curvature is simply equal to the slope as defined in Equation 1. Therefore, as the length of the

slab decreases, the slope increases (for a given foundation, embankment, and embankment

depth) resulting in a more severe bump for the vehicle and vehicle occupant. Hence, one way

of minimizing the bump is to lengthen the approach slab. The question then becomes what

curvature, or slope, is acceptable so as to make the bump acceptable. Note that if the

pavement approach and bridge deck are not level the resultant curvature maybe increased or

decreased.

Embankment depth (or height) is number five in Table 1. For a given compressibility of

the embankment or backfill, the foundation settlement (sf) is based on integrating this

compressibility over the entire depth of the embankment. Clearly then, the thicker the

embankment the more the resultant settlement. Hence, a desirable goal is to minimize the

embankment depth when ever possible.

9

Figure 3 Range of Most Erodible Soils (after Briaud, et al (2), reproduced from White, et al (3)).

Items six, seven, and eight in Table 1 all relate to drainage and control of water behind

the abutment, under the approach slab, and into or off of the embankment. Proper drainage

design cannot be overemphasized in any civil engineering work, and the bridge approach system

is no exception. Moving water off and away from the bridge deck, bridge approach slab and

pavement approach is necessary for the integrity of this bridge system and its longevity, but also

necessary for the traveling public. Erosion from under the approach slab or off of the

embankment sides or face can lead to approach slab failure, slope instability, and slope

subsidence. Intrusion of water into the embankment or under the approach slab through joints

because of poor design or poor maintenance can lead to erosion and settlement of the bridge

approach system.

Seo and Briaud (6) and Seo (7) approached the bump problem from an experimental

laboratory point of view. They designed a circular test track that would very quickly allow for

10

the repeated loading of a bridge approach model. Figure 6 shows a cross-sectional radial view of

their test device. The depth of a soil model (D2) in their experiments can be varied up to a depth

of 12 inches. The width of the test track soil bed is also 12 inches (looking radially into the page

of Figure 6). The outside diameter of the track is 60 inch while the inside diameter is 36 inch.

Gradation SpecificationsSelect AASHTO Soil Types

NMDOT Base Courses

0.07

5 m

m, #

200

0.15

0 m

m, #

100

0.30

0 m

m, #

50

0.60

0 m

m, #

30

1.18

mm

, #16

2.36

mm

, #8

4.75

mm

, #4

9.5

mm

, 3/8

in.

12.5

mm

, 1/2

in.

19.0

mm

, 3/4

in.

37.5

mm

, 1-1

/2 in

.

50.0

mm

, 2 in

.

0

10

20

30

40

50

60

70

80

90

100

-1.124938737 -0.624938737 -0.124938737 0.375061263 0.875061263 1.375061263

Sieve Size, mm

Perc

ent P

assi

ng

A-1-a

A-1-b

A-3

A-2

Class I B.C.

Class II B.C.

Erodible

Figure 4 Range of Most Erodible Soils with Typical Quality Soil Types and New Mexico Base Courses.

Using dimensional similitude they were able to derive numerous pi terms using

Buckingham Pi theory based on the following assumed functionality for predicting the sleeper

slab displacement along the length of the sleeper slab (note that sf is the specific case of the

displacement at the sleeper slab location ),

),,,,,,( 221,11 NVDELDIEW= (2)

11

Figure 5 Approach Slab Settlement Geometry.

In Equation 2, the independent variables are as follows:

1) W is the weight of the vehicle traversing the test bed of the circular test track (W=mg;

mass of vehicle times the acceleration due to gravity),

2) E1 and E2 are the modulus of elasticity of the pavement approach slab material and

embankment material, respectively,

3) the product E1I1 is the stiffness of the approach slab, where,

4) I1 is the moment of inertia of the approach slab cross-section (per unit width), (i.e., the

slab stiffness),

5) L is the approach slab length,

6) D1 and D2 are the slab thickness and the depth of the embankment material, respectively,

7) V is the vehicle speed traversing the model track, and

8) N is the number of repetitive loads caused by the model vehicle traversing the track

12

The results of this laboratory model showed the most important contributing factors to

the bump were as follows:

1) number of cycles of loading (N) over the approach slab is proportional to the increase in

the bump,

2) shorter approach slabs result in higher displacements of the approach slab,

3) inherently stiffer soils that are more highly compacted result in reduced deflections of the

approach slab,

4) the vehicle velocity has an effect on the increase on the magnitude of the bump,

5) weight of the vehicle traversing the bump relates to the eventual magnitude of the

bump settlement.

Figure 6 Laboratory Model for Simulating the Bump (Seo and Briaud (6) and Seo (7)).

Summarizing Seos efforts (6, 7), there are several factors affecting the bump at the end of

the bridge. The number of cycles is paramount in the degree of settlement in the bridge approach

system. Soils are inherently viscoelastic and will creep over time with the number of load

cycles. The effect of repeated cycles can be minimized by using competent embankment and fill

13

materials that exhibit minimal viscous effects (for example non-plastic soils, or soils of low

plasticity index (PI). The Youngs modulus of the soil is of extreme importance. Select

materials for embankment and backfill are necessary and must be compacted to proper density

and moisture content to meet specifications. The proper approach slab and design are required to

minimize the bump. This includes an adequate length and structural thickness to mitigate the

vertical acceleration of a traversing vehicle caused by high curvature from pavement-to-approach

slab-to-bridge deck. The weight of the vehicle is directly related to the settlement; this a

common understanding of any deformable body or mass (e.g., Hookes law). Along with the

viscous nature and higher vehicle weight, one can expect increased deformations under the

approach slab. Lastly, vehicle velocity causes additional loading (dynamic) on the approach slab

system because of viscous effects, but also because of increased curvature of the bridge deck

approach with time. In conclusion it seems that the work of Seo (6, 7) has confirmed many of

the effects noted by DOTs in the survey of Briaud, et al (2) and summarized in Table 1.

Briaud, et al (2) further pose the question to the various DOTs about their methods for

minimizing the bump all of which appear to be methods before the fact, including proper

design, material specifications, and good conduction practices in conjunction with quality

assurance-quality control (QA/QC). It is interesting to note that many of the causes of the

bump mentioned in Table 1 can be easily mitigated in a converse fashion (i.e., the antidotes in

Table 1 are quite well listed in Table 2). Of the 19 methods for minimizing the bump from

Briauds survey, 79 % of the responses are those listed in Table 2.

Again as one looks through the literature cited so far, one sees some very common

threads. Quality embankment and materials are necessary to minimize settlements caused by

repeated loadings. Such materials tend to be non-viscous and are much more free draining

14

materials. These materials must be properly compacted in accordance with the specifications.

During the design phase one can conclude that the depth of embankment and fill should be

minimized were practical to mitigate settlement. Further more, the length of slab should be

lengthened and stiffened to minimize the bump caused by excess vertical curvature on the

approach.

Hoppe (5) surveyed the various DOTs in 1999. Of thirty-eight DOTs that responded to

the question, Are approach slab settlements a significant problem? Thirty-two (84%)

responded yes or moderate (21 yes, 11 moderate). New Mexico responded in the affirmative.

Arizona, Maine, New Hampshire, Texas, and Vermont all responded in the negative. When

asked about the advantages of using approach slabs, 30 of 37 (81 %) states responding cited

improved smoothness while 15 of 37 (41 %) mentioned reduction of impact loads on the

approach system. Nine DOTs mentioned a desire for uniform settlement of the approach slab

(9/37 = 24 %). The above three issues are all tied to the issue of high curvature on the approach

caused by excess settlement, assumedly caused by poor embankment material specifications and

QA/QC as well as the length of approach slab. The use of a properly designed approach slab

was mentioned by six DOTs to aid in drainage control. Three mentioned lower maintenance

costs, while two stated that approach slabs have no advantage (Kentucky and Maryland).

Conversely, the disadvantages to using approach slabs were higher initial costs (17 of 23

respondents), while 12 of 23 mentioned higher maintenance costs in contrast to the three

agencies that mentioned lower maintenance costs above. New Mexico did not respond to this

question posed by Hoppe (5).

Thirty-one of 36 (86 %) responding DOTs use approach slabs on over 50 % of their

interstate bridge system, 30 of 36 (84 %) on over 50 % their primary system, while only 17 of 36

15

(47 %) use them on over 50 % their secondary route system. New Mexico claimed to use bridge

approach slabs on 80 % across their entire route system.

Table 2 Methods to Minimize the Bump (Briaud, et al (2))

Ranking Mitigating Factor1 Embankment on Strong Soil Foundation2 Strong and Long Enough Approach Slab3 Well Compacted or Stabilized Fill Material4 Good Fill (Well Graded)5 Good Drainage6 Shallow Embankment (< 10 ft (3 m))7 Good Construction Practice and Inspection8 Adequate Tine Between Placement of Fill and Pavement

For bridges with conventional abutments, 14 responding states use approach slabs, while

two do not. For integral abutment bridges, 18 DOTs claim to use approach slabs, 8 do not use

them. For the 38 and 37 states responding, respectively, to this question, it appears that many

states may feel it is unnecessary to use approach slabs for conventional bridges than for integral

bridges, on the other hand it appears that 8 are adamant in not using them for integral bridges

where as only two do not use them for conventional bridges (Kentucky and Maryland said no to

both conventional and integral types). High volume traffic was cited as a compelling reason for

incorporating approach slabs in both conventional and integral bridges. Other reasons cited were

pavement type, expected settlement, and height of embankment, all of which can be contributing

factors to the bump. New Mexico uses approach slabs for both conventional and integral

abutment type bridges.

Table 3 from Hoppe (5) presents typical approach slab dimensions for the various states

surveyed. Slab lengths vary from a low of 13 ft (Kansas) to a high of 40 ft (Louisiana). The

average length based on the tabulated data is 22 ft. Note that New Mexicos length is 15 ft., well

16

below the average (only Kansas has shorter approach slabs). Slab thickness varies between 8 in.

and 18 in.; clearly, this thickness is largely dependent on the length of the slab and other

structural and foundation considerations. The last column in Table 3 shows the width of the

approach slab, i.e., whether the slab is only as wide as the paving lanes, or runs from

curb-to-curb. Here curb-to-curb is the width of the bridge decking (width between bridge

guardrails, or barriers). A majority of the responses favored the curb-to curb design including

New Mexico. Another important aspect of encouraging curb-to-curb design is for erosion

control and effective drainage of water away from the bridge structure and approach slab system.

Figure 7(a) (after Briaud, et al (2), reproduced from White, et al (3)) depicts a poorly designed

approach slab that will allow water into the backfill and embankment materials promoting

erosion and weakening of these granular materials. Figure 7(b) shows a system that will prevent

infiltration into the soils below the approach slab (this approach is commonly one used in New

Mexico for mechanically stabilized earth (MSE) wall systems). Stewart (8) suggests that the

pavement should even be placed cantilever over the wingwalls to further mitigate infiltration

below the approach slab. In other words the pavement (or approach slab) should be sandwiched

between the barrier walls and wingwalls in Figure 7.

Table 4 from Hoppe (5) presents state backfill material specifications per Hoppes survey

in 1999. A number of states use the same material specifications for both of their backfill

material and their embankment material. However, most states have a different requirement for

the backfill material on top of the embankment or the material adjacent to the abutment wall.

Thirty-three percent (13/39) have a fineness requirement such that less than 15 % material is

finer than the # 200 sieve (75 m). Perusing the miscellaneous comments in the last column one will note that the typical backfill material and, assumedly, in some cases the embankment is a

17

porous granular material. Note that Iowa allows for the use of geogrid and Wyoming requires

the use of fabric reinforcement. Since the time of Hoppes survey (5), New Mexico now requires

A-1-a or granular base course for the backfill material.

Figure 7 Approach Slab Joint Details at Pavement Edge (after Briaud, et al (2), reproduced from White, et al (3)).

Table 5 displays the compaction specifications by state as surveyed by Hoppe (5). For

most states the nominal loose lift thickness prior to compaction is 8 in. (except Connecticut,

Kentucky, and New York which base the thickness on the final compacted lift thickness). The

typical dry density requirement is 95 % of maximum standard proctor density. Note that seven

out of thirty-nine (18 %) respondents indicate using compaction requirements greater than 95 %.

It is interesting to note that of these seven states, two responded to Hoppes initial question of

Are approach slab settlements a significant problem? (see Figure 2 (5)) with no answers

(Arizona and New Hampshire) and three with answers of moderate (Connecticut, Florida, and

Maryland), suggesting that increased compaction effort may be significant. New Mexico did not

respond to Hoppes survey for Table 5. However, New Mexico requires 8 inch loose lifts

compacted to 100 % of standard proctor density.

18

Table 3 Approach Slab Dimensions by State (Hoppe, (5))

State Length (ft)

AverageThickness (in)

AverageWidth Limited

ToAL 20 9 PavementAZ 15 N/A a N/A a

CA 20 12 Curb-to-CurbDE 24 N/A a N/A a

FL 20 12 Curb-to-CurbGA 25 10 Curb-to-CurbIA 20 11 PavementID 20 12 N/A a

IL 30 15 Curb-to-CurbIN 20.5 N/A a N/A a

KS 13 10 Curb-to-CurbKY 25 N/A a Curb-to-CurbLA 40 16 Curb-to-CurbME 15 8 Curb-to-CurbMA N/A a 10 N/A a

MN 20 12 PavementMS 20 N/A a Curb-to-CurbMO 25 12 N/A a

NV 24 12 Curb-to-CurbNH 20 15 N/A a

NJ 25 18 N/A a

NM 15 N/A a Curb-to-CurbNY 17.5 12 Curb-to-CurbND 20 14 Curb-to-CurbOH 22.5 14.5 N/A a

OK 30 13 Curb-to-CurbOR 25 13 Curb-to-CurbSD 20 9 N/A a

TX 20 10 N/A a

VT 20 N/A a N/A a

VA 24 15 PavementWA 25 13 PavementWI 20.5 12 N/A aWY 25 13 Curb-to-Curb

Average 22.0 12.3 N/A a

Standard Deviation 5.1 2.3 N/A a a N/A: Information not available or not applicable

19

Table 4 State Backfill Material Specifications for Approach Slabs (Hoppe, 5)

State Same/Different from Regular Embankment

% PasingNo. 200 Sieve

75 mMiscellaneous

AL Same --- a A-1 to A-7AZ Different --- a No CommentsCA Not Stated < 4 Compacted Pervious MaterialCT Different < 5 Pervious MaterialDE Different --- a Borrow Type CFL Same --- a A-1,A-2-4 through A-7 (lLiquid Limit < 50)GA Same --- a Georgia Class I, II, or IIIID Not Stated --- a A Yielding MaterialIL Different --- a Porous, GranularIN Different

20

Table 5 State Compaction Specifications for Approach Slabs (Hoppe (5))

State Loose LiftThickness

(in.)

%Compaction Miscellaneous

AL 8 95 No CommentsAZ 8 100 No CommentsCA 8 95 * *For Top 30 inchCT 6 * 100 * Compacted Lift IndicatedDE 8 95 No CommentsFL 8 100 No CommentsGA 100 No CommentsID 8 95 No CommentsIL 8 95 * * For Top; Remainder Varies with Embankment DepthIN 8 95 No CommentsIA 8 None * * One Roller Pass per inch ThicknessKS 8 90 No CommentsKY 6 * 95 * Compacted Lift Indicated; Moisture +2 % / -4 % of OptimumLA 12 95 No CommentsME 8 --- a At or Near Optimum MositureMD 6 97 * * For Top 12 inch; Remainder is 92 %MA 6 95 No CommentsMI 9 95 No CommentsMN 8 95 No CommentsMS 8 --- a No CommentsMO 8 95 No CommentsMT 6 95 At or Near Optimum MositureNE 95 No CommentsNV 95 No CommentsNH 12 98 No CommentsNJ 12 95 No CommentsNY 6 * 95 * Compacted Lift IndicatedND 6 --- a No CommentsOH 6 --- a No CommentsOK 6 95 No CommentsOR 8 95 * * For Top 36 inch; Remainder is 90 %SC 8 95 No CommentsSD 8 - 12 * 97 * 8 inch for embankment; 12 inch for bridge end backfillTX 12 --- a No CommentsVT 8 90 No CommentsVA 8 95 + or - 20% of Optimum Moisture (i.e., percent of percent moisture)WA 4 * 95 Top 24 inches Are 4 inch Lifts; Remainder are 8 inch LiftsWI 8 95 * * Top 6 ft, Within 200 ft of Abutment; Remainder is 90 %WY 12 * --- a * Use of Reinforced Geotextile Layers

a No specification or none cited; NM did not respond, see p. 17 (herein) for NM requirements

21

Table 6 from Briaud, et al (2) is abutment compaction. Here, as for Hoppe (5), Briauds

survey shows that most states use 8 in. loose lifts for their backfill. Most states use 95 % criteria

for their compaction control, most of which is standard proctor or a standard state method. A

few states specify modified proctor specifications; 95 % for Colorado, New Hampshire, and

Rhode Island. Recall that these three states responded to Hoppes survey (5) with no response,

or only moderate approach slab problems. Of the 13 states listed in Table 6, a total of 9

responded with no, moderate, or no response (Arkansas and Colorado), while four responded in

the affirmative to having approach slab issues (California, Missouri, Ohio, and South Carolina).

Note there are some discrepancies between Table 5 and Table 6. Arizona claims only 95 %

compaction in Table 5 versus 100 % in Table 6. In Table 5 the stated values for New Hampshire

are 12 in. lifts and 98 % compaction, but in Table 6 they are 8 in. and 95 %, respectively. Ohios

compaction requirement in Table 6 is 98 to 102 %, while South Carolinas lift thickness is

reduced to 6 in. State specifications may have changed slightly in the interim of Briauds (2)

work and Hoppes (5), thus explaining these modest differences.

Table 6 Compaction Requirements at Abutment (Briaud, et al (2))

State Maximum Loose Lift Thickness, in. Relative CompactionAZ 8 95% Standard ProctorAR 4 95% Standard ProctorCA 8 95% State Test Method ProctorCO 6 95% Modified ProctorCT 6 100% Modified ProctorDE 8 95% State Test MethodME 8 98% State Test MethodMI 8.8 95% State Test MethodMO Not Specified 95% Standard ProctorNH 8 95% Modified ProctorOH Not Specified 98% - 102% Standard ProctorRI 10 95% Modified ProctorSC 6 95% State Test Method

22

LITERATURE REVIEW CONCLUSIONS AND RECOMMENDATIONS

Wahls (4) emphasizes the importance of an adequate subsurface investigation for any bridge

approach system to be successful. This investigation should be integrated with the subsurface

investigation for the bridge structure. Such an investigation must incorporate adequate

information on depth, thickness, and classification of all soil strata. The strength,

compressibility, and permeability of critical strata are paramount information. This subsurface

investigation should be accomplished by a geotechnical engineer in complete cooperation with

the bridge engineer for the project. The AASHTO subsurface investigation manual (9) provides

guidelines for drilling and sampling along the bridge structure and approach slab areas.

Analysis of the foundation behavior in terms of compression and stability must be

considered for any bridge structure and its associated bridge approach system. Settlement

analysis, based on the laboratory information gleaned from the soil samples taken during the

subsurface investigation, is used to compute anticipated settlements in the natural soil foundation

material and in the embankment material. The applied loads for analysis are those contributed

by the self weight of the soil structure and that of the bridge structure and pavement system.

Consideration must be given to both compressible clay soil foundations and cemented collapsible

soil types.

Stability analysis of the soil foundation and embankment caused by applied loads is

necessary, as these loads can induce lateral failure in either or both soil structural elements.

Wahls (4) suggests the following techniques for foundation improvement of the natural

foundation soils when the subsurface and geotechnical analysis call for such improvement (some

of these may also be advantageous for improvements of the embankment system approaching the

bridge abutment).

23

1) Removal and replacement,

2) In-situ densification by one of the following methods:

a. Precompression (e.g., using future embankment),

b. Temporary surcharges,

c. Vertical drains (drain wicks),

d. Dynamic compaction,

e. Compaction piles (vibroflotation),

3) Reinforcement methods using one of the following:

a. Stone columns,

b. Lime columns,

c. Embankment piles (spaced along bridge approach system

Briaud (2) notes in his concluding remarks that the bump tends to be more severe when

one or more of the following conditions exist: 1) high (deep) embankment, 2) abutment on piles,

3) high average daily traffic (ADT), 4) existence of compressible natural soil foundation, 5) high

intensity rain events, 6) extreme temperatures (particularly a concern with integral abutments),

and 7) steep approach gradients.

Based on the above conclusions and his survey of many state DOTs, Briaud (2) makes

the following recommendations for best practice with regard to the design of bridge approach

systems:

1) Treat the bump at the end of the bridge as a stand-alone design issue with

prevention as a design goal.

2) Assign the above design goal responsibility to an engineer.

24

3) Stress teamwork and open-mindedness toward achieving this design goal amongst

all engineering staff involved (bridge engineering, geotechnical engineering,

pavement engineering, construction and maintenance engineering).

4) Perform proper settlement analysis of all soil structural elements (natural soil

foundation and embankment materials) in order to estimate long term settlement of

bridge and approaches.

5) Design approach slab based on this settlement analysis.

6) Provide for adequate expansion and contraction between the bridge structure and the

approach system.

7) Incorporate properly designed drainage and erosion protection systems for the bridge,

bridge abutment, and bridge approach system.

8) Use and enforce proper specifications.

9) Use knowledgeable inspectors for the job, especially for the geotechnical aspects.

10) Inspections of joint details, grade specifications, geotechnical aspects, and drainage

are critical.

Briaud (2) further suggests a criterion for the maximum slope of Equation 1 as follows:

2001=

Lss

S af (3)

or rearranging,

( )af ssL 200 (4) For example, if a settlement analysis indicates a differential settlement between the abutment and

the beginning of the approach slab ( af ss ) equal to 1.5 in., then the length of the approach slab

must be greater than 300 in., or 25 ft.

25

Seo, et al (6) and Seo (7) conclude the following based on the literature, laboratory and

field work cited:

1) The main reason for the bump is due to settlement of the embankment caused by

weak natural foundation soils or to settlement of the embankment fill itself. Erosion

of voids under the pavement contributes to the bump, as well. Abutment

displacement caused by pavement growth, slope instability or temperature change can

contribute to the bump, too.

2) The bump is more severe for higher (deeper) embankments, abutments on piles,

high ADT, soft compressible natural soils, intense rain, extreme temperature

variations, and steep approach gradients.

3) The bump is less severe when an approach slab is used, appropriate fill materials

are required, good compaction and/or stabilization is employed, effective drainage is

incorporated into the design, and good construction practice along with quality

QA/QC are used. A waiting period between fill placement and pavement placement

should be provided, as well.

4) Based on experimental simulation, soil with higher compaction (higher Youngs

modulus) develops less bump at the approach than soils of lower modulus (lower

compaction). In addition, the bump was found to increase with vehicle velocity,

vehicle weight, and number of cycles of repetitive loading (i.e., ADT). The bump

was found to increase with a decrease in approach slab length.

5) A tolerable bump satisfies Equation 3, i.e., a tolerable bump has a slope of 1/200 or

less.

Recommendations from Seo, et al (6), and Seo (7) are as follows:

26

1) Within 100 feet of the abutment, use quality backfill material with a plasticity index

(PI) less than 15, with less than 20 % passing the # 200 sieve (75 m), and with a coefficient of uniformity greater than three.

2) Within 100 feet of the abutment, compact the embankment and backfill materials to

95 % of modified proctor density. Also, the use of thinner lifts to obtain density is

recommended.

3) Approach slabs, at a minimum, should be 20 ft in length, and designed to support full

traffic loading in free span (this accounts for any unexpected erosion beneath the slab

of softening of the soil under the slab). The width of the sleeper slab supporting the

approach end of the approach slab should be minimally 5 ft (in order to adequately

prevent bearing failure with the backfill material under the slab).

27

FIELD EVALUATION OF NEW MEXICO BRIDGE APPROACHES

As part of this project, field evaluation of numerous New Mexico bridges was performed. The

bridges investigated were bridges that NMDOT personnel (primarily RAC members) cited as

having bridge approach settlement issues (the bump). Table 7 lists 19 bridges that were

investigated and evaluated during late spring 2005. The author was accompanied by Mr. Virgil

Valdez of the NMDOT Research Bureau during these field evaluations. Each of the bridges will

be discussed sequentially along with photographic documentation in order to provide the reader

with the authors impression of the bump problem, its severity, and root cause.

Table 7 New Mexico Bridges with Concerns About the Bump

Bridge Location Route No.Bridge

No. Additional Comments1 Albuquerque Big-I (I-25/I-40) N/A North-to-West Departure2 M.P. 29 US 550 9135 North End3 M.P. 252 I-25 8375 South Bound at Arroyo Tonque4 M.P. 252 I-25 8376 North Bound at Arroyo Tonque5 West Gallup I-40 6553 East Bound (over BNSF Railway mainline)6 West Gallup I-40 6554 West Bound (over BNSF Railway mainline)7 West Gallup I-40 8335 Exit 16, West Bound8 West Gallup I-40 8336 Exit 16, East Bound9 Mountain Valley Rd. NM 217 N/A NM 217 over I-40 (east of Sedillo Exit)

10 Albuquerque Paseo del Norte N/A Paseo del Norte at Coors Blvd.11 Albuquerque Paseo del Norte N/A Paseo del Norte over I-2512 Albuquerque Pennsylvania St. N/A Over I-4013 US 84 Corridor US 84 9311 South Bound14 US 84 Corridor US 84 9312 South Bound, Camel Rock Exit15 US 84 Corridor US 84 9309 South Bound, Flea Market Exit16 US 84 Corridor US 84 N/A Opera Drive, Exit 16817 US 84 Corridor US 84 N/A Over Rio Tesuque18 US 84 Corridor US 84 9310 South Tesuque Exit19 US 84 Corridor US 84 8942 Interchange w/ NM 502

28

Big-I, North-to-West Departure

Figure 8 is a view of the reported problem on the Big-I (I-25/I-40 interchange). This drop-inlet

(DI) structure is intruding into the travel lane (see yellow line). It is also abutting up against the

departure slab (an approach slab if you will). Impact loading appears to be problematic because

of the location and the width of this slab. It would appear that this is just a poor design and or

construction detail. This DI should have been positioned farther away from the departure slab

and its width minimized to prevent encroaching on the driving lane. A similar drainage structure

was placed on the opposite side of the driving lane as shown in Figure 9. This DI does not

encroach on the driving lane and is removed from the departure slab (but probably not enough).

Clearly the location and geometry of such structural appurtenances can have a significant

influence on the distress of the pavement and approach slabs in their vicinity.

Figure 10 shows the area between the bridge barrier wall and the mechanically stabilized

earth (MSE) wall that supports the spread foundations for supporting the bridge structure. This

area shows no signs of erosion or drainage problems. It seems that by minimizing the area and

covering it with bituminous HMA is quite an effective solution for these MSE wall structures.

Bridge No. 9135, US 550, M.P. 29

Figure 11 is a view looking from the north bound lanes toward the south bound lanes of the north

end approach slab of Bridge No. 9135 on US 550. There is obvious extreme distress as evident

by the extreme diagonal cracking in this approach slab. Figure 12 is a closer view of the south

bound lanes where it is clear that pressure grouting has been used to jack the slab up because

of excessive settlement of the approach slab system. Figure 13 gives an indication of the amount

of settlement based on the movement between the approach barrier system and the bridge barrier

wall. The amount of this settlement based on this figure is approximately 4 inches.

29

Figure 8 Drainage Structure, Left Edge, Big-I North Bound-to West Bound Departure.

Figure 9 Drainage Structure, Right Edge, Big-I North Bound-to West Bound Departure.

30

Figure 10 Detail Between Barrier Wall and MSE Wall.

Based on conversations with the NMDOT geotechnical engineer (Bob Meyers), it would

appear that the cause for the original excessive settlement on this bridge approach slab system

had to do with inadequate subsurface investigation or improper design of the embankment

system based on the subsurface investigation. There is evidence that soft natural soils prone to

consolidation may have existed at this bridge abutment. Appropriate measures were not used to

adequately consolidate these soft soils before construction. Hence, there was long term

settlement after embankment placement and bridge construction. Efforts to mitigate this after the

fact were the use of slab-jacking techniques using high pressure grout.

31

Figure 11 North End Approach, Bridge No. 9135, US 550.

Figure 12 North End Approach (Note Cracking and Evidence of Pressure Grouting).

32

Figure 13 Estimate of South Bound Approach Slab Settlement, Bridge No. 9135.

The lesson from Bridge No. 9135 is to perform an adequate subsurface investigation

along the length of the bridge approaches and bridge itself to allow for the adequate estimation of

potential settlements not only of the bridge, but of the approach slab system. Good geotechnical

information and analysis will tell one what the soils response will be to applied loads, and the

physics behind this sound geotechnical analysis will not lie, regardless of schedule and cost

constraints. In this case, an ounce of prevention would easily have been worth a pound of cure.

Figure 14 provides an excellent example of good drainage and erosion control on the

embankment face underneath the bridge. Here rip-rap is effectively used to prevent scour on the

face which could cause erosion under the approach slab and bridge abutments. Figure 15 shows

a DI where one questions why the structure was not butted up against the barrier wall. However,

overall the drainage at Bridge No. 9135 appeared to be quite well designed and functional.

33

Figure 14 Use of Rip-Rap for Erosion Control on Bridge No. 9135.

Figure 15 Example Drainage Structure at Bridge No. 9135.

34

Bridge No. 8375, I-25, M.P. 252, South Bound

Figure 16 shows the south bound departure end of the bridge at Arroyo Tonque (Bridge

No. 8375. Figure 17 provides a visual of the magnitude of the bump traversing from the bridge

deck onto the departure approach slab system (~1 to 2 in.). This bridge was built in the early to

mid 1980s and it appears that there has been effort in the past to mitigate this bump by placement

of HMA. It appears that the problem with this approach system is one of compressible soils,

either the embankment material, that may have been placed to loosely, or deep-seated

compressibility with the natural soil foundation material. Figure 18 provides additional evidence

that soft soil exists in the embankment because of the cracking of the HMA in the wheel paths

(fatigue caused by excessive deformation). Figure 19 shows the quality of maintenance and

disrepair of the joint between the bridge deck and the approach slab. Maintenance by cleaning

and replacement when necessary is required to prevent stress buildup between the bridge

structure and the approach slab and pavement system. Such stresses can cause damage to the

decking, the abutment, and can cause distortions of the approach slab as well.

Bridge No. 8376, I-25, M.P. 252, North Bound

The north bound bridge (No. 8376) at Arroyo Tonque exhibits essentially the concerns as the

south bound bridge (No. 8375). There is a similar bump on the departure end of this bridge.

Poorly maintained joints are evident. The channel of the arroyo below the bridge is lined with

rip-rap which seems to have adequately scouring, erosion, and problems for the approach slab

system. In general the drainage appears quite adequate for these two bridges at Arroyo Tonque.

However, one might question the drainage details laterally adjacent to the abutments and

approach slabs (Figure 20). Here there is lack of an adequate drainage structure to direct runoff

away from the bridge and bridge approach system. More design attention is suggested here.

35

Figure 16 South Bound Departure, I-25 at Arroyo Tonque.

Figure 17 Bump on South Bound Departure, I-25 at Arroyo Tonque.

36

Figure 18 Cracking in Wheel Paths of South Bound Departure, I-25 at Arroyo Tonque.

Bridge No. 6554, I-40 West Bound Over BNSF Railway Mainline (W. Gallup)

This bridge shows severe departure slab settlement as shown in Figure 21. In terms of the

severity of the bump, this may have been the worst one investigated and evaluated during the

project. Heavy truck traffic undoubtedly contributes to the severity of this bump along with the

age of this bridge (late 70s or early 80s). Figure 22 provides graphical evidence of the

magnitude of settlement between the bridge abutment and the embankment (which includes the

material under the approach slab). Here the concrete covering the embankment face is seen to

have moved vertically approximately 4 to 6 inches relative to the abutment. This embankment is

37

quite deep to allow clearance for the railroads freight vehicles. This coupled with perhaps

inadequate compaction or material selection may be a contributing factor to this bump. Its

possible that there are also some deep-seated foundation problems with the natural soil.

Figure 23 shows additional pictorial evidence of the settlement and subsidence of the

embankment away from the bridge abutment. Here there is about a 2 to 3 inch gap that has

developed between the concrete facing and the abutment which clearly is an avenue for water to

enter and further exacerbate the problem. While not shown here, the joints on this bridge are

poorly maintained and in disrepair.

Figure 19 Poorly Maintained Joint Between Deck and Approach Slab (Bridge No. 8375).

38

Figure 20 Drainage Adjacent to Abutments and Approach Slabs at Arroyo Tonque.

Bridge No. 6553, I-40 East Bound Over BNSF Railway Mainline (W. Gallup)

The east bound bridge (No. 6553) over the BNSF railway mainline in West Gallup has many of

the same issues as the west bound bridge previously discussed. Figure 24 shows the east bound

approach slab which has more modest settlement problems compared to the west bound

departure slab of Bridge No. 6554. Figure 25 shows an extreme amount of vertical relative

movement at the right abutment on the approach end. This relative displacement is on the order

of 6 inches. Note the opening between the concrete embankment facing and the abutment.

39

Figure 21 Settlement at West Bound Departure Slab, Bridge No. 6554.

Figure 22 Evidence of Settlement Between Abutment and Embankment at Bridge No. 6554.

40

Figure 23 Additional Evidence of Settlement Between Abutment and Embankment at Bridge No. 6554.

Figure 24 East Bound Approach Slab on Bridge No. 6553.

41

Figure 25 Movement at Right Approach Abutment on Bridge No. 6553.

Figure 26 shows the lateral magnitude of this opening underneath the concrete facing on

the embankment. The inserted tape measure extends into this cavity almost 150 inches. This

suggests soil movement and perhaps erosion as well. Figure 27 (similar to Figure 23) shows the

lateral movement of the concrete facing away from the right departure abutment. This opening is

on the order of 4 inches, or so. Clearly this is evidence of settlement, or subsidence, and

provides a point of water entry and potential further settlement and erosion problems.

Bridge No. 8335, West Bound I-40 at Exit 16 (W. Gallup)

West bound Bridge No. 8335 at Exit 16 in West Gallup exhibits very modest bumps in the

approach and departure slabs. Figure 28 is a west bound view that showing the approach slab

with a very low magnitude bump. Also note in this figure the excellent placement of the

drainage structure, well away from the approach slab and abutting up to the barrier wall.

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Figure 26 Cavity Under Concrete Facing on Abutment on Bridge No. 6553.

Figure 27 Concrete Embankment Facing Movement Relative to Right Departure Abutment on Bridge No. 6553.

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Figure 28 Approach Slab, Bridge No. 8335, Exit 16, West Gallup.

Figure 29 is a close up of the approach slab. This photo shows the minimal magnitude of

the longitudinal bump, however the lateral vertical displacement from the edge of the open

graded friction course (OGFC) is of concern; perhaps this drop off (close to 3 or 4 inches) should

be minimized or tapered farther laterally to the edge of pavement. Figure 30 is a photograph of

the joint between the concrete embankment cover and the vertical fascia of the bridge (note this

is not an MSE wall bridge). The joint here appears to be fairly watertight. Figure 31 presents

the watertight bituminous material placed in the median between the west bound and east bound

bridges at Exit 16.

Bridge No. 8335 overall has minimal bump problems and has very good drainage

features. One drawback, as in previous bridges discussed, is poor maintenance at the joints

between the bridge deck, approach slab, and approach pavement.

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Figure 29 Lateral Drop Off at Edge of Approach Slab on Bridge No. 8335.

Figure 30 Joint Concrete Embankment Facing Against Vertical Bridge Fascia (Bridge No. 6553).

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Figure 31 Watertight Median Between Bridges at Exit 16. Bridge No. 8336, East Bound I-40 at Exit 16 (W. Gallup)

The approaches and departures for the east bound bridge at Exit 16 in West Gallup are more

severe than those for the west bound side. Figure 32 shows the severity of the departure slab end

of this bridge. The settlement at this joint between the approach slab and the bridge deck is

about 1 in. Note there is evidence of some spalling and subsequent maintenance along the joint.

Figure 33 shows the approach slab looking in a westerly direction. Here there is evidence of

cracking at the diagonal corners and some indications of stress and disrepair along the length of

the joint.

The two bridges at Exit 16 in Gallup are not as bad as other bridges surveyed. The

drainage features appear quite well designed and in general maintained. Maintenance appears

good except for the necessity of joint cleaning. Settlements are likely due to embankment issues

or perhaps vertical alignment between the bridge, approach slab, and pavement.

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Figure 32 Departure Slab on Bridge No. 8336 (East Bound, West Gallup).

Figure 33 Departure Slab on Bridge No. 8336 (East Bound, West Gallup).

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Mountain Valley Road, NM 217 Over I-40

The Mountain Valley Road Bridge over I-40 is a two lane bridge with a north/south alignment.

This bridge has no access from I-40, and is the sole bridge providing access from NM 333 on the

south to properties north of I-40 between the Sedillo and Edgewood exits. This bridge was

recently widened and is undoubtedly the most problematic of the bridges surveyed in this report.

From a geotechnical standpoint it is classical in what can transpire from design to construction

and QA/QC.

The original design called for widening the embankment along the existing right-of-way

by building a sliver-slope on the east side embankment. This sliver-slope, while not terribly

wide, was considered an adequate solution for widening the approaches and bridge itself.

Another interesting issue was that the contractor for this project was well versed in concrete flat

work but not in bridge construction and the details associated with bridge construction (note that

this bridge was a component of a larger reconstruction and rehabilitation effort on Interstate 40).

The real coups-de-grace for some of the failures to follow was undoubtedly poor quality control

of the embankment. It would appear that poor compaction methods were used with minimal

oversight by inspectors. This embankment control problem lead to instability in the eastern

embankment slope with resultant settlement of the approach slabs. Poor compaction control

underneath the approach slabs and pavement approaches appears to have aggravated the situation

as well.

Figure 34 shows the north bound approach. Note the severe longitudinal cracking in the

HMA pavement and the concrete slab prior to the approach slab itself. Also note the cracking in

the HMA adjacent to the guard rail. A view of the settlement along this guard rail alignment is

shown in Figure 35. Here it is evident that the slope has subsided on the order of 4 in.

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Figure 34 North Bound Approach Slab on NM 217 at I-40.

Figure 35 Excessive Settlement on North Bound Approach on NM 217.

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Figure 36 provides a visual of the amount of settlement at the south approach slab joint

(~1 in.). Figure 37 is similar to Figure 35 showing the south end of the bridge on the west

abutment. Here again is clear evidence of excessive settlement and subsidence at the

embankment and approach slab system. Figure 38 is similar to Figure 34 showing the south

bound approach with similar cracking. Here the crack openings are on the order of 1 inch wide

which is an obvious entry point for water which can cause further softening and compressibility

of the base course and subgrade layers. Figure 39 shows what appears to be an excessively wide

approach slab joint. Such a joint will allow a large volume of compressible material to enter

causing high distress over time as the joint fills tighter and tighter. Figure 40 is the north bound

departure slab system, again showing distresses that were described for the south end of the

bridge. Figure 41 depicts the excessive bump at the north approach slab (in excess of 2 in.).

Figure 36 Settlement at North Bound Approach Slab, NM 217.

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Figure 37 Embankment Subsidence and Approach Settlement, South End Departure.

Figure 38 South Bound Approach, NM 217.

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Figure 39 Approach Slab Joint, NM 217.

Figure 40 North Bound Departure Slab, NM 217.

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Figure 41 North End Bump, NM 217 at I-40. Figure 42 shows the relative settlement and subsidence between the approach slab system and

the embankment and approach system foundation. Here the lateral crack has opened

approximately 1-1/2 in., while the vertical relative displacement is about 3 in. Note that the curb

has also moved laterally away from the approach slab system. The crack opening and curb

movement have compromised good drainage features of this bridge system.

Concluding on the NM 217 Bridge, one can clearly point to poor QA/QC practices during

the construction. Probably excessive lift thickness and low relative density are the prime

culprits. The narrow sliver-slope design perhaps should have been avoided by complete removal

of the north and south approach system prior to replacement. This would have avoided any

instability in the embankment. While there is definitely an extreme amount of vertical

movement, additional causes may be attributed to poor vertical alignment between the bridge

approach system (pavement and approach slab) and the bridge deck. Drainage, while perhaps

53

not the best, does not appear to be a real cause of the high levels of distress noted on this bridge.

It is worthwhile noting that the author returned to the site approximately 6 months later and did

not note any significant worsening of the distresses. This suggests that most of the movement

occurred very quickly after reconstruction was complete.

Figure 42 Settlement at Northeast End of NM 217 Bridge at I-40.

Paseo del Norte at Coors Blvd.

Paseo del Norte is a limited access expressway connecting Interstate 25 to Coors Boulevard on

the west side of Albuquerque. Hence, the intersection of Paseo del Norte and Coors Blvd. is an

extremely busy center-point interchange. Figure 42 is west looking view of the west bound

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approach on the west bound bridge of Paseo del Norte. Here it is clear that there is a major

bump attributed to excessive settlement of the embankment (which is quite high for clearance

over Coors Blvd.) or poor vertical alignment of the approach system and the bridge deck, or a

combination of the two.

Figure 43 shows exactly why it is a bad idea not to extend the approach or departure slabs

full width to the bridge barrier wall (recall Figure 7). In this photograph the approach slab has

moved relatively upward to the surrounding HMA material. This requires additional

maintenance along the longitudinal joint as well as the lateral joint. Lack of a full width section

of the approach slab here exacerbates the relative movement; hence the desire to make such slabs

full width. Figure 45 shows the west bound approach slab in more detail showing evidence of

alkali-silica reactivity (ASR). Based on the age of this bridge it is possible that ASR has

developed over time. Expansion stresses from ASR can potentially lead to slab expansion and

distress in the approach slabs, approach joints, and vertical uplift of the slabs and pavement

preceding the approach slabs.

The Paseo del Norte Bridge over Coors Boulevard arguably has some of the best drainage

features of any of the bridges surveyed. Figure 46 shows the concrete slope protection used

along the entire embankment length of the bridge. Examination of this concrete slope protection

found few flaws in terms of its water tightness. Figure 47 provided another example of the

attention to drainage detail. Here the use of HMA between the outer side of the bridge barrier

wall and the inner side of the MSE wall is used effectively to prevent water infiltration into and

under the bridge abutment approach system.

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Figure 43 West Bound Approach, Paseo del Norte at Coors Blvd.

Figure 44 East Bound Departure, Paseo del Norte at Coors Blvd.

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Figure 45 West Bound Approach Slab with Evidence of Alkali-Silica Reactivity (ASR).

Figure 46 Concrete Embankment Protection on Paseo del Norte at Coors Blvd.

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Figure 47 Drainage Barrier Between Bridge Barrier Wall and MSE Wall.

Paseo del Norte at Interstate 25 (I-25)

The Paseo del Norte Bridge at I-25 is well known to have had extreme alkali-silica reactivity

(ASR) problems. Besides the structural concrete used for the bridge, the adjacent portland

cement concrete (PCC) paving materials have also exhibited ASR. ASR expansion stresses

caused by ASR expansion can lead to extreme damage at the joints connecting the bridge deck to

the approach slab and the approach slab to the preceding concrete pavement. Such stresses can

cause spalling and resultant crack widening. This is a maintenance problem requiring constant

joint filling with bituminous materials. In addition, if the joint is not periodically filled with joint

58

sealant, then the joint can become filled with compressible materials resulting in further damage

by stress buildup. Figure 48 is a graphic example of ASR and the extreme damage by this

expansion, spalling, and filling with debris. This process can lead to bumps at the end of the

bridge because of potential uplift of the slabs. The compromised joint is also an easy avenue for

the penetration of water into and underneath the base and subgrade materials supporting the

approach slab system. The presence of water will soften these unbound materials, making them

more compressible and prone to settlements.

In contrast to the Paseo bridge at Coors, the Paseo bridge at I-25 does not show the same

degree of drainage detail for the area between the bridge barrier and the MSE wall. Figure 49

shows clearly the vegetation, and assumedly the lack of HMA at this location. This is either a

design detail or a construction detail missed in the field. Figure 50 is the west MSE abutment for

this bridge. Visual perusal of the abutments at Paseo del Norte and I-25 did not show any

evidence of lateral movement or displaced embankment material from the joints between the

panels used to construct the MSE wall. Note that the face of the abutment appears to be quite

vertical and all of the joint openings are quite uniform. Note carefully the presence of ASR in

the concrete element supporting the bridge girders.

Pennsylvania Street Over I-40 (Albuquerque)

The complete reconstruction of the Pennsylvania Street Bridge over I-40 in Albuquerque is of

recent vintage (rebuilt within the last couple of years). For the most part there is very little

evidence of extreme settlement caused by compressible backfill, embankment, natural

foundation soils or displacements caused by instability. The north bound approach view on

Pennsylvania (Figure 51) shows little evidence of a bump. Figure 52 shows the 6 foot straight

edge clearly indicating little or no bump across the approach slab joint.

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Figure 48 Damaged Approach Slab Joint Caused by ASR.

Figure 49 Area Between Bridge MSE Wall and Bridge Barrier Wall.

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Figure 50 MSE Wall, Paseo del Norte at I-25, West Abutment.

Figure 51 North Bound Approach on Pennsylvania Street Over I-40.

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Figure 52 Approach Slab Joint on Pennsylvania Street Bridge Over I-40.

The drainage features on the Pennsylvania Bridge are quite extraordinary. Concrete

slope protection was used on all embankment sides of the abutment as well as the area between

the back of the MSE wall and the base of the spread footing supporting the bridge abutments. In

addition a drainage gutter was constructed to provide adequate drainage away from the base of

the MSE wall. Figure 53 of the southerly abutment shows this excellent concrete slope

protection with the gutter at the top of the MSE wall. The drainage gutter is sloped to allow

drainage around to the base and toe of the lateral embankment. Figure 54 shows this positive

drainage control around the southerly abutment to the base of the northwest toe of the west

embankment.

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Figure 53 Concrete Slope Protection with Drainage Gutter.

Figure 54 Positive Drainage Control at Base of Concrete Slope Protection.

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Bridge 9311, US 84, South Bound

A number of bridges along the US 84 corridor between Santa Fe and NM 502 are known to have

had approach slab distresses caused by excessive settlement. The settlement, based on the

authors understanding, is attributed in large part to compressible foundations soils that were not

adequately prepared in advance of the embankment and bridge construction. These natural soils

were compressible soils for which the geotechnical engineer had recommended consolidation

methods such as vibroflotation or precompression of the natural soil foundation. Unfortunately,

these methods were not performed because of schedule and budgetary constraints. The

following examples along this corridor clearly show how excessive the fix can be after the fact

and how difficult it is to fix a wrong that should have been made right at the beginning based on

solid geotechnical investigative work and analysis.

Bridge No. 9311 is a bridge that experienced approach slab settlement shortly after

construction. As mentioned, this is attributed to deep-seated foundation problems, but it is

possible that some of the settlement is caused by poor compaction control in the embankment as

well. Figure 55 shows the south bound departure slab where a bump is evident in the slab.

Figure 56 shows the south bound approach slab. Here there is evidence of extreme cracking

(caused by settlement) followed at a later time by slab jacking (the yellow areas) using an easily

pumpable grout suitable for this application. Slab jacking was the chosen solution to mitigate the

settlement of the slabs on this bridge and to obtain better vertical alignment between the

approach pavement, approach slab, and bridge abutment.

The drainage features of the bridges along the US 84 corridor are quite excellent.

Figure 58 shows the typical type of concrete slope protection used to remove water away from

the approach slabs, embankments, and abutments. One will note the existence of a vertical crack

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paralleling the vertical construction joint between the concrete under the bridge face and the

concrete that wraps around to the embankment toe. This cracking is difficult to explain, but was

also noted on other bridges with this detail on the US 84 corridor.

Bridge 9312, US 84, South Bound (Camel Rock Exit)

Figure 59 shows the level of cracking distress in the south bound approach slab of Bridge

No. 9312 again caused by settlement of the foundation soils. Note that slab jacking has already

occurred in this slab. Figure 60 is the south bound departure slab. Here is an indication, based

on the 6 foot level, of how much settlement has occurred based on the difference in slopes of the

bridge deck and the departure slab.

Bridge 9309, US 84, South Bound (Flea Market Exit)

Figure 61 shows a side view of the south bound approach slab on Bridge No. 9309. It is evident

that there is some modest settlement present in this slab. Note the presence of an epoxy se