Assessing scour of rock underlying abutments and piers at West … · 2016-03-25 · Assessing scour of rock underlying abutments and piers at West Virginia bridges . A Presentation

Assessing scour of rock underlying abutments and piers at West

Virginia bridges

A Presentation to:

14th Annual Geohazards Technical Forum,

August 2014


Virginia bridges

Background

Project Objectives and Tasks

Research Methods

Results

Application & Implementation


Virginia bridges

Background


Research Methods

Results


The rock scour to be addressed here refers to

removal of rock by hydraulic action from below

bridge abutments and piers. (Excludes

concrete spillways, etc.)

Rock scour is a rock-water interaction

phenomenon

What is rock scour?

Scour = f (rock mass properties,

hydraulic energy exerted

onto rock over time)

NCHRP Report 717: Scour at Bridge Foundations on Rock, by Keaton et al. (2012) represents the state of the art for rock scour research.

Methodology for depth and time rate of scour. *

* Abrasion of degradable rock

West Virginia DOH issued RFP-273 to learn applicability of NCHRP-717 to bridges in our state. Project known as RP-273.

Study motivation (cont.)

RP-273 Research Team

RAHALL TRANSPORTATION

INSTITUTE

ADMINISTRATIVE P.I.

Wael Zatar, Ph.D.

Co-Principal Investigator, Geology &

Geotechnical

William Niemann, Ph.D., P.G., E.I.T.

Special Technical Advisor

Jeffrey Keaton, Ph.D., P.E.

P.G. AMEC

GANNETT FLEMING, INC.

Geotechnical Manager – Terry Downs, P.G.

Senior Engineering Geologist – Brian Greene,

Ph.D., P.G.

Co-Principal Investigator, Hydrology &

Hydraulics

Isaac Wait, Ph.D., P.E.

From NCHRP Project 24-29 / Report 717: Scour at Bridge Foundations on Rock

Rock scour can occur in the following four modes:

Abrasion (grain-scale plucking) of degradable rock. *

Quarrying and plucking of durable, jointed rock.

Dissolution of soluble rocks.

Cavitation.

* provides predictive method

Abrasion (grain-scale plucking) of

degradable (non-durable) rock.

Abrasion Mode of Scour

(Coon Creek bridge,

Summers Co.)

Dislodgement/Quarrying Mode of Scour

(Roaring Creek bridge, Pendleton Co.)

Project Objectives

How erodible are rocks in

West Virginia?

What are the stream energy

conditions at bridges in West

Virginia?


Virginia bridges

Background


Research Methods

Results


Site Assessment / Mapping • Select drilling locations

• Map bedrock / bedload

• Assess mode of scour

• Measure scour magnitude

in three dimenions

= Desired boring

locations

Characterizing Scour Depths

Scour Depth is one of two factors that define “Scour Number”

Coring / Sampling Target Zone

10

’ T.

Z.

Rock Coring Hetager Drilling, Inc.

Coring Summary

• 15 bridges

• 10 districts

• 15 counties

• 17 days

• 694 feet of drilling

• 30 rock cores

• 391 feet of rock coring

• 45 samples for Modified Slake Durability (MSD)

Core Logging Sample management

Modified (Continuous)

Slake Durability Test

(modified from ASTM 4644)

Simulates rock scour by abrasion

from bedload.

Eliminates wetting/drying cycles

of conventional slake durability.

60-min. increments (vs. 10 min.)

Total test duration 4-8 hours.

(vs. 20 min.)

45 core and surface samples



Non-durable

Durable

R² = 0.9595

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

0 100 200 300 400 500

We

igh

t Lo

ss (

%)

Time (min)

Example test data



(modified from ASTM 4644)

Equivalent Scour Depth

D(X*60 min) = Vi (X*60 min) / unit area

-----------------------------------------------------------------

D(X*60 min) = depth of scour corresponding to test interval

x (= 1,2,3 …9) minutes (L),

Vi (X*60 min) = incremental volume lost during test interval

x (= 1,2,3 …9) minutes, (L3),

unit area = area in appropriate units (L2).

Vi = Mi / γsat

Vi = incremental volume lost during 60-minute interval (L3),

Mi = incremental mass lost during 60-minute test interval (L3),

γsat = natural (field) saturated specific gravity of the rock

being tested (M/L3).

Equivalent Stream Power

ω = L / t [ W(X) + W(X+1) ] / [2 A(1/8) ]

ω = equivalent stream power. [force* L/T/L2],

L = equivalent length. [ L ],

t = incremental time of test interval. [ T ].

[ W(X) + W(X+1) ] / 2 = average weight of sample during test

interval (force). [ force ]

A(1/8) = water-filled area of drum (1/8 of total area). [ L2 ]

-----------------------------------------------------------------

Plotting Modified Slake

Durability Test Data

Geotechnical Scour Number: a measure of

abrasion scour depth

per unit stream power

y = 2.42E-04x - 1.75E-03

R² = 7.95E-01

y = 1.38E-04x - 1.15E-03

R² = 5.43E-01

y = 1.00E-04x - 7.83E-04

R² = 8.87E-01

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

7.0E-04

6.0 6.5 7.0 7.5 8.0 8.5 9.0

Eq

. Sc

ou

r D

ep

th (

ft)

Eq. Stream Power (ft-lb/s//ft2)

ROA-SEXP-01-01

ROA-01-11.2'-11.9'

ROA-02-11.2'-11.6'

Linear (ROA-SEXP-

01-01)Linear (ROA-01-11.2'-

11.9')Linear (ROA-02-11.2'-

11.6')

presumed mode of scour:

dislodgement

Sandstone: massive, hard, jointed

Plotting Modified Slake

Durability Test Data


Virginia bridges

Background


Research Methods

Results


Scour Mode by Rock Type

Geographic Distribution of Scour Mode

Applying the NCHRP 717 model

for abrasion scour

Field

measurement

Hydrologic / Hydraulic Modeling based on historical data

Lab

Scour Number vs. GSN

Site Dominant

Rock Type

Scour

Mode

Scour Number

(ft/daily

ft∙lb/s/ft2)

GSN (ft /

daily

ft∙lb/s2/ft2)

Scour

Number /

GSN

GSN* (ft /

daily

ft∙lb/s2/ft2)

Scour

Number

/ GSN*

Audra Sandstone PQP 1.80E-02 5.55E-06 3,243 2.41E-04 75

Beverly Siltstone PQP 2.70E-02 1.40E-05 1,929 1.32E-04 205

Bridge Fork Sandstone AB/QP 5.00E-02 5.60E-06 8,929 1.39E-04 360

Caldwell Run Shale DAB 1.20E-01 5.60E-06 21,429 1.86E-04 645

Clear Fork Sandstone AB/QP 7.70E-04 4.05E-06 190 2.25E-04 3.4

Coon Creek Sandstone PAB 4.70E-03 8.99E-06 523 5.32E-04 8.8

Grassy Run Sandstone DQP 2.00E+00 1.79E-05 111,732 1.08E-04 18,467

Laurel Fork Sandstone AB/QP 5.50E-01 6.47E-06 85,008 1.17E-04 4,701

Leatherwood Sandstone AB/QP 1.90E-01 1.18E-05 16,102 2.06E-04 922

Leatherwood Shale AB/QP 3.35E-05 4.69E-05 0.7 8.97E-04 0.04

Mish Limestone PQP 3.60E-02 2.89E-06 12,457 2.07E-04 174

Paden Fork Sandstone AB/QP 2.70E-01 4.42E-05 6,109 7.84E-05 3,444

Ritter Park / 5th

St Sandstone AB/QP 7.80E-03 7.46E-05 105 8.68E-05 90

Roaring Creek Sandstone PQP 2.90E-03 1.04E-05 279 1.38E-04 21

Median

Values 3.15E-02 9.70E-06 4,676 1.63E-04 189

Conclusion: The lab-based GSN value is not a reliable predictor

of Scour Number at the project sites. Scour mode is at issue.

Comparison of Surface and Core GSN*


Virginia bridges

Background


Research Methods

Results


Application and Implementation

Risk-based bridge categories

TIER ↓ / GROUP → A B C

1 1A 1B 1C

2 2A 2B 2C

3 3A 3B 3C

Tier 1 – No further Action

Tier 2 – Abrasion Confirmed → predict scour per NCHRP 717

Tier 3 – Significant Quarrying Probable → further evaluate

Group A – low stream power

Group B – medium stream power

Group C – high stream power

Application and Implementation 1 – Rock Scour Decision Tree and Field Inspection Guidance


Database of sites where scour depth, mode, and tier classification is defined.

Recommended action: expand database to include

additional sites in WV. Once sufficiently large,

database can be used to set risk-based energy group

categories.

Site Average Annual Stream

Power, Ωaverage annual [ft∙lb/ft2] Scour Mode

Decision Tree

Score

Scour Tier

Vertical Scour Depth (in)

Min Max

Clear Fork 296,961 AB/QP 4.5 III 3 3

Roaring Creek 196,913 PQP 7 III 6 7

Audra 193,437 PQP 6.5 III 30 40

Coon Creek 144,645 PAB 2.5 II 1 5

Beverly 82,293 PQP 6.5 III 4 6

Bridge Fork 54,444 AB/QP 4.5 III 2.5 4.5

Ritter Park / 5th St 31,871 AB/QP 5.5 III 8 10

Leatherwood 13,263 AB/QP 4 III 5 12

Thank you

The following slides were

not used in the presentation

Equations for scour in sand typically are used to

predict scour depths in erodible rock based on

hydraulic loading associated with peak discharge.

“[State] DOTs are encouraged to initiate studies for the

purpose of obtaining field measurements of scour and

related hydraulic conditions at bridges for evaluating, verifying, and improving existing scour prediction

methods.”

– Hydrologic Engineering Circular No. 18, Section 1.7.1, Federal Highways Administration, U.S. Department of Transportation, 2001.

Study motivation

From NCHRP Project 24-29 / Report 717: Scour at Bridge Foundations on Rock:

Current methodology for scour prediction around bridge foundations considers rock as either “erodible” or “non-erodible.” (WVDOH: “erodible” means RQD<50%)

Equations for scour in sand typically are used to predict scour depths in erodible rock based on hydraulic loading associated with peak discharge.

As a consequence, predictions of scour in erodible rock frequently seem to overestimate the extent and depth of scour. Resulting elevations for piers and abutments on erodible rock may be established at levels that require unnecessary, very expensive, and difficult excavation.

Non-erodible rock is assumed not to scour.

0

2

4

6

8

10

12

0 2000 4000

Stre

am V

elo

city

(ft

/s)

Flow Rate (cfs)

0

2

4

6

8

10

12

0 1000 2000 3000 4000

De

pth

(ft

)

Flow Rate (cfs)

PadenForkBridge

RoaringCreekBridge

CoonCreekBridge

Stream Power depth

velocity

Scour Number vs. GSN


Virginia bridges

Background


Research Methods

Results


Hybrid Mode of Scour

(Paden Fork bridge)

Coring

• Overburden unsampled

• NX Coring in Target Zone

Boring Logs

Depth to Rock

Lithology

Recovery

RQD (ASTM 6032)

Project Tasks

RP-273

1. Literature Review

2. Site Selection

3. Field and Lab Testing

4. Data Analysis

5. Reporting

Additional:

1. State Survey

2. Contract for Core Drilling

3. Best Management Practices

Geographic Distribution of GSN*

Hydrologic Assessment

Stream Gage Data

Available at 3 of 15 sites

USGS Regression Equations

What is the relationship between

time and flow rate?

Watershed Modeling

Basin Model Rainfall Characterization

Runoff

Hydrograph


time and flow rate?

Hydraulic Modeling


flow rate (Q), Velocity, and Depth?

Decision Tree – Example


2. Methodology to characterize Stream Power where stream gage

data is unavailable.

Recommended action: assess Average Annual Cumulative Stream Power at bridge sites to characterize

the stream energy available for scour.

Increment

(return period

range)

λ size

Interval Cumulative

Stream Power, Ω

(ft∙lb/ft2)

2-5 0.300 18,609

5-10 0.100 62,344

10-25 0.060 131,684

25-50 0.020 216,851

50-100 0.010 303,734

100-200 0.005 400,484

200-500 0.003 525,547

500- 0.002 599,790

Avg. Annual Cum. Stream Power,

Ω (ft∙lb/ft2): 31,871

5th Street, Ritter Park

RP-273: Criteria for Predicting Scour of Erodible Rock in

West Virginia

Leatherwood Bridge, Kanawha Co., WVDOH District 1

Deck

Left Abutment

Right Abutment Flow

Flow

Boring 1

Boring 2

Coring / Sampling – Plan View

Documents

Assessing scour of rock underlying abutments and piers at West … · 2016-03-25 · Assessing scour of rock underlying abutments and piers at West Virginia bridges . A Presentation