May 17, 2018 Town Hall Drive Stormwater Management

May 17, 2018 Withers & Ravenel 115 McKenan Drive Cary, NC 27511 Attn: Ms. Jennifer Diaz Re: Report of Subsurface Investigation Town Hall Drive Stormwater Management Facility Morrisville, North Carolina GeoTechnologies Project No. 1-18-0354-EA Ms. Diaz: GeoTechnologies, Inc. has completed the authorized subsurface investigation to evaluate subsurface conditions and provide geotechnical engineering recommendations for the above referenced project in Morrisville, North Carolina. Subsurface conditions on the site were investigated with 4 hand auger test borings at locations depicted on Figure 1. Samples from each location were collected for laboratory testing. Furthermore, a seasonal high water table evaluation was performed at each location by a soil scientist. The report is attached to this one. This report presents the findings of our investigation and our recommendations pertaining to construction of the new stormwater pond. It is our understanding that a new stormwater management facility will be constructed on the south side of Town Hall Drive next to the Indian Creek Trailhead at 101 Town hall Drive. The area of the proposed pond is currently wooded. We understand that grading will consist of cuts and fills on the order of about 6 feet. Subsurface conditions: The subsurface conditions at the site consist of about 3 inches topsoil in the borings. Underlying the topsoil, soils consisting of silty/clayey sands, sandy clays, and sandy silts were encountered. The silty/clayey sands and sandy clays were typically encountered to a depth of about 4 to 5 feet below grade. Underlying these soils, very stiff Triassic sandy silts were encountered to the boring termination depth of about 5 feet. Groundwater was not encountered upon boring completion. However, the on-site soils are conducive to perched water conditions and groundwater will fluctuate during the seasons. A series of laboratory tests were perfomed on select samples including Atterberg limits (ASTM D-4318), sieve analysis (ASTM D-422), natural moisture content (ASTM D-2216), and standard Proctor (ASTM D-698). Based upon the test results, the soils tested classify at SM, SC and CL according to the Unified Soil Classification System. The results of our tests are attached. Recommendations: The following recommendations are made based upon a review of the attached test boring and laboratory data, our understanding of the proposed construction, and past experience with similar projects and subsurface conditions. Additionally, should subsurface conditions adverse to those indicated by this report be encountered during construction, those differences should be reported to us for review and comment.

Withers & Ravenel Re: Town Hall Drive Stormwater Facility May 17, 2018 Page: 2

Site Grading Considerations: Grading should begin with stripping of topsoil and removal of vegetation from the proposed pond area including embankments. Based on the results of the test borings we anticipate that most areas in the beneath embankment will areas should be stable, however, the subgrade should be evaluated by a geotechnical engineer prior to placement of fill for the embankments. Any areas found unsuitable by the geotechnical engineer should be repaired as recommended. The magnitude and type of repair will need to be determined during the construction phase. Typical repair options include discing and drying with re-compaction of wet soils or undercutting to stable soils. In-place discing, drying and re-compaction may be attempted as a method of repair for wet near-surface soils during the warmer months of the year when drying of soils is feasible. If this is not successful, the soils should be undercut to firm bearing and the area then backfilled to design subgrade with properly compacted structural fill or the soils should be stabilized with lime or cement. Repair costs should be budgeted in the project. We anticipate that the on-site soils (sans topsoil) will be suitable for reuse as structural fill. The table below outlines our structural fill compaction recommendations.

SOIL TYPE COMPACTION RECOMMENDATIONS

On-site soils excluding topsoil (SM, SC, CL, ML)

• 95% standard Proctor maximum dry density, increased to 98% for final foot of subgrade beneath slabs & pavements. Moisture content within 2% optimum moisture.

• 10 inch loose lift thickness for structural fill, decreased to 6 inches in trenches.

Off-site structural fill (ML, CL, SM, SC)

We expect that the majority of excavations can be performed with conventional construction equipment. The Triassic silts encountered at a depth of about 5 feet below grade were very stiff and could not be effectively penetrated with hand augers, however, this is not indicative of partially weathered rock (PWR). If it is desired to further evaluate the materials below a depth of 5 feet to determine if it is PWR, we recommend performing borings with a drill rig or using a large excavator such as a CAT 330 with a narrow bucket and rock teeth.

Permanent Slope Considerations: In this area, dry and well compacted unreinforced fill slopes built at

2.5H:1V should be stable. Steeper fill slopes can be used with properly designed and installed geosynthetic reinforcement. Cut slopes should also be on the order of 2.5H:1V or flatter. To protect from surface erosion, slopes should be quickly vegetated. Storm Water Pond Considerations: The provided drawing indicates that an 8 inch thick clay liner be installed if the SHWT is not within 6 inches of the normal pool elevation. In recent years we have seen several wet ponds in the area which appear to meet plan infiltration criteria (typically 0.01 in/hr), but which do not perform as expected. This is due to the fact that the boundary conditions of the field infiltration test typically do not match those of the pond under normal pool conditions. Essentially, the hydraulic head (and therefore

Withers & Ravenel Re: Town Hall Drive Stormwater Facility May 17, 2018 Page: 3

gradient) of the pond under normal operating conditions is higher (often significantly higher) than the head from the standard double ring infiltrometer test. Common problems we have seen associated with this issue include water levels which cannot support the required vegetation and unacceptably low water levels for a project amenity such as a fountain.

Our experience has been that, when required, a soil liner constructed from a properly tested (lab and

field) and placed low permeability soil can be used to maintain water levels reasonably close to the intent of the infiltration specification, even when accounting for the hydraulic head/liner thickness at normal pool levels. However, the higher the head and/or thinner the liner, the more impervious the liner material should be. As such, a soil liner material may be acceptable for one combination of hydraulic head/liner thickness, but not for another. If the pond will be incorporated as an architectural amenity, it is recommended that a manufactured synthetic liner be used in conjunction with a soil liner or as a replacement. In this case, if a synthetic liner is not used, consideration should be given to installing a make-up well to maintain pond water levels.

Closing: GeoTechnologies, Inc. appreciates the opportunity to have provided you with our services on this project. Please contact us if you should have questions regarding this report or if we may be of any further assistance. Sincerely, GeoTechnologies, Inc. Mark R. Potratz, P.E. NC License No. 25955 MRP/pr-els Attachments

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2.5'

2.5'

2.5'

Specimen Identification LL PL PI Fines Classification

PLASTICITY

INDEX

CL

ML

CH

MH

LIQUID LIMIT (LL)

ATTERBERG LIMITS' RESULTS

36

46

42

34

21

22

28

20

CL-ML

B-1

B-2

B-3

B-4

ASTM D-4318

15

24

14

14

83.0

48.5

36.8

72.3

Brown Fine Sandy Silty Clay

Brown Silty Clayey Coarse to Fine Sand

Brown Clayey Silty Coarse to Fine Sand with Fine Aggregate

Brown Fine Sandy Silty Clay

PROJECT JOB NO.DATE

Town Hall Dr. Stormwater - Morrisville, NC 1-18-0354-EA5/15/18

3200 Wellington Court, Ste 108Raleigh, NC 27615

Date Recieved:

Dates Tested: 5/9-5/15/2018

5/9/2018

0

10

20

30

40

50

60

70

80

90

100

680.00124680.0124680.1246812468102468100

Town Hall Dr. Stormwater

1"

Boring No. Elev./Depth Nat. W.C. L.L. P.L. P.I.

Project: Job No.:

Date:

Soil Description or Classification

Morrisville, NC

1-18-0354-EA

2.5' 15.0 Brown Fine Sandy Silty Clay

5/15/18

3/8"

Per

cen

t F

iner

By

Wei

gh

t

U.S. Standard Sieve Sizes

Grain Size In Millimeters

GRAIN SIZE DISTRIBUTION

3/4"

1/2"

B-121.6

#4 #10 #20 #40 #60 #100 #200

GRAVEL SAND

21.0

FINES

COARSE FINE COARSE MEDIUM FINE SILT SIZES CLAY SIZES

36.0


Date Recieved:


5/9/2018

0

10

20

30

40

50

60

70

80

90

100

680.00124680.0124680.1246812468102468100


1"


Project: Job No.:

Date:


Morrisville, NC

1-18-0354-EA

2.5' 24.0 Brown Silty Clayey Coarse to Fine Sand

5/15/18

3/8"

Per

cen

t F

iner

By

Wei

gh

t




3/4"

1/2"

B-217.9

#4 #10 #20 #40 #60 #100 #200

GRAVEL SAND

22.0

FINES


46.0


Date Recieved:


5/9/2018

0

10

20

30

40

50

60

70

80

90

100

680.00124680.0124680.1246812468102468100


1"


Project: Job No.:

Date:


Morrisville, NC

1-18-0354-EA

2.5' 14.0 Brown Clayey Silty Coarse to Fine Sand with Fine

Aggregate

5/15/18

3/8"

Per

cen

t F

iner

By

Wei

gh

t




3/4"

1/2"

B-318.3

#4 #10 #20 #40 #60 #100 #200

GRAVEL SAND

28.0

FINES


42.0


Date Recieved:


5/9/2018

0

10

20

30

40

50

60

70

80

90

100

680.00124680.0124680.1246812468102468100


1"


Project: Job No.:

Date:


Morrisville, NC

1-18-0354-EA

2.5' 14.0 Brown Fine Sandy Silty Clay

5/15/18

3/8"

Per

cen

t F

iner

By

Wei

gh

t




3/4"

1/2"

B-420.2

#4 #10 #20 #40 #60 #100 #200

GRAVEL SAND

20.0

FINES


34.0


Date Recieved:


5/9/2018

75

80

85

90

95

100

105

110

115

120

125

130

135

0 5 10 15 20 25 30 35 40 45

2.70

2.60

MOISTURE-DENSITY RELATIONSHIP

5/15/18

ASTM D 698

16.5%

S-1

Brown Silty Clayey Sand

%

LL PI

DR

Y D

EN

SIT

Y (

Po

un

ds

Per

Cu

bic

Fo

ot)

WATER CONTENT (Percent Dry Weight)

Job No:

110.2 PCF

1-18-0354-EA

Morrisville, NC

Method of Test:

Maximum Dry Density:

Optimum Moisture Content:

Natural Moisture Content:

Atterberg Limits:

Soil Description:

Town Hall Dr. StormwaterJob Name:

Job Location:

Boring No:

Sample No:

Depth:

TEST RESULTS

FOR SPECIFIC GRAVITY EQUAL TO:

CURVES OF 100% SATURATION

Date:

2.80

Date Recieved:


5/9/2018


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May 17, 2018 Town Hall Drive Stormwater Management