nevada dot’s experience with mse wall corrosion nevada dot mse experience.pdf · nevada dot’s...

35th Southwest Geotechnical Engineers Conference

April 2010

Nevada DOT’s Experience

with MSE Wall CorrosionPresented By

J. Mark Salazar, P.E.

Chief Geotechnical EngineerNevada Department of Transportation

Contributing Engineers:

Abbas Bafghi, P.E., NDOT Senior Geotechnical Engineer

Todd Stefonowicz, P.E., NDOT Asst. Chief Bridge Engineer

Craig Smart, P.E., HDR Consulting Engineer

John Thornley, P.E., Consulting Engineer for Hattenburg Dilley & Linnell


April 2010


April 2010

Background

• 1974 ~ NDOT’s first MSE Wall

• Over 150 MSE Walls built since 1974

• Steel reinforcements are required

• 2004 ~ Extensive corrosion discovered

accidently at MSEW I-515 at Flamingo

Flamingo Interchange

Wall Location

Example of Discovered Corrosion


April 2010


April 2010

Background• NDOT/FHWA does preliminary MSEW

evaluation & condition assessment

– Barry Berkovitz, P.E., FHWA

• Results:

– MSE Walls are near end of service lives

– Rehabilitation is necessary

– NDOT lets emergency contract to repair tallest

wall

– McMahon & Mann Consulting Engineers

(MMCE) hired for expert corrosion evaluation

Drilling Operations –Track Mounted

Finished Tieback Wall and Soundwall


April 2010

MMCE’s In-depth

Corrosion Evaluation

• 11 test pits exposing

top 3 bar mats

• 43 backfill samples

• 37 bar mat samples

• 2800 wire diameter

measurements

• PR monitoring on

45 bar mats and

36 steel coupons

Exhumed Reinforcement Sample


April 2010


April 2010

Observations & Testing Results

• Maximum section loss observed at least

2 ft behind face of wall.

• Moisture content of MSE backfill 1%-13%

(6% median); 25%-40% degree of saturation.

• Drainage structures influenced the rate.

• On average, reinforcements have lost more

than 60% of their capacity.


April 2010


April 2010

Observations & Testing Results

• Reinforcement at condition expected at

50 years…not 20 years.

• Actual corrosion rate estimated to be

5-29 μm/yr (12 μm/yr ~ FHWA design manual).

• No material anomalies were identified in the

nongalvanized steel reinforcements.


April 2010


April 2010

Corrosion Study Conclusions

• Aggressive very corrosive backfill and

moisture intrusion were primary causes

of accelerated corrosion.

• All three walls were found to be in similar

condition and were close to the end of

their service life.

• The PR measurements compare well

with direct physical site observations.


April 2010


April 2010

Corrosion Study Recommendations

• Retrofit walls or remove from service.

• Conduct further studies at other Las Vegas

MSEW locations to identify problem

and extent.

• Continue PR corrosion monitoring of

I-515 Flamingo MSE Walls.

• Conduct laboratory studies to better

understand corrosion mechanisms and

contributing factors.


April 2010


April 2010

NDOT Research Project

• 2007~ NDOT/UNR initiate investigation of

corrosion of MSE Walls in Nevada.

• Phase 1 Tasks

– Develop NDOT MSEW inventory

– Synthesize available field inspection

database on the behavior of NDOT MSEWs

– Review McMahon Report to identify most

significant factors responsible for I-515

Flamingo MSEW corrosion


April 2010


April 2010

NDOT Research Project

Phase 1 Tasks (cont)

– Identify and synthesize data on important

factors that affect corrosion of NDOT’s

MSE Walls

– Assemble data on MSE Wall

performance, corrosion history, and

specifications from other states

– Select MSE Wall candidates for

Phase 2 Investigations


April 2010


April 2010

Phase 1 Project

Findings & Conclusions

NDOT’s source approval process has not effectively predicted the corrosive nature of backfill actually used in MSEW construction.

Historical Corrosion Limits

Specification

Pre-1986 1986 1992 1996 to Present

NDOTNDOT 1986

EditionAASHTO 15th

Editionb

NDOT 1996 & 2001 Editions

AASHTO 16th, 17th, & 2007

LRFD Editions

pH 6.4 to 9.5 5 to 10 5 to 10 5 to 10 5 to 10

Resistivity (ohm-cm)

1,000 min. 3,000 min. 3,000 min. 3,000 min. 3,000 min.

Chlorides (ppm)

500 max. 200 max. 50 max. 100 max. 100 max.

Sulfates (ppm)

2,000 max. 1,000 max. 500 max. 200 max. 200 max.

*

Statistical Analysis of MSE BackfillData from the 2005 investigation by MMCE was

compared to original data from backfill approval in

1985. The data was converted to test method

AASHTO T 288 equivalent soil resistivity.

Measured

Parameter

(avg.)

Approved

Backfill (1985)

Sampled

Backfill (2005)

Significantly

Different

Soil Resistivity

(ohm-cm)*3470 1301 Yes

Chlorides (ppm) 124 48 No

Sulfates (ppm) 62 2435 Yes


April 2010


April 2010

Phase 1 Project


Nevada DOT’s T235B “Method for the Determination of Minimum Resistivity of Soil” test method substantially under-predicts material corrosivity as compared to the AASHTO T 288 standard test method.


April 2010


April 2010

Soil Resistivity Test Methods

• Nevada T235B

– Conductivity of water from saturated

soil is measured

• AASHTO T 288

– Soil resistivity is measured in a

2-pin soil box

Nevada T235B

AASHTO T 288

y = 0.859x0.963

R² = 0.925

100

1000

10000

100 1000 10000

AA

SHTO

Re

sist

ivit

y Te

st R

esu

lts

(oh

m-c

m)

NDOT Resistivity Test Results (ohm-cm)

Correlation Between Two Resistivity Testing Procedures (NDOT Study Data)

114 tests included

from NDOT test data.

3000

3000

Current AASHTO Resistivity

Specification

Cu

rre

nt N

DO

T R

esis

tivity

Sp

ecific

atio

n

10

1

24

13

1

10 10

5

8

10

5

30

0%

5%

10%

15%

20%

25%

0

5

10

15

20

25

30

1750 2250 2750 3250 3750 4250 4750 5250 5750 6250 6750 7250 7500+

Pe

rce

nta

ge o

f To

tal A

pp

rove

d T

est

s

Nu

mb

er

of

Ap

pro

ved

Bac

kfill

Te

sts

Resistivity (Nevada T235 Test Method, ohm-cm)

Resistivity of Approved NDOT MSE Backfills (1982 to 2008)

118 Tests are included.

The current resistivity

requirements are a minimum

of 3,000 ohm-cm (NDOT test

method).

3,0

00

ohm

-cm

eq

uiv

ale

nt u

sin

g

AA

SH

TO

T-2

88

Te

st M

eth

od

*

(Number of Tests)


April 2010


April 2010

Phase 1 Project


There is significant potential that NDOT has more MSE Walls experiencing high rates of corrosion due to unintentional use of aggressive backfill!

300

3000

AA

SHTO

T-2

88

Re

sist

ivit

y (o

hm

-cm

)

Contract Number (Number of Tests)

AASHTO Correlated Resistivity’s for Constructed MSE Walls in Nevada District 1 (1982 to 2008)

Flamingo

Walls

Cheyenne

Walls

Maximum Resistivity

Average Resistivity

Minimum Resistivity

1980s 1990s 2000sDecade of

Construction

*Data included from post construction investigation


April 2010


April 2010

Phase 1 Project


Surprise!

A 10 year old MSE Wall at Cheyenne and I-15 with significant corrosion was discovered accidently in 2008 during Phase 1 Study.

I-15 at Cheyenne MSEW Corrosion


April 2010


April 2010

Phase 1 Recommendations

• Existing MSE Walls

– Conduct further studies at other MSEW

locations that have been identified as having

potential corrosion problems.

• Future MSE Walls

– Modify NDOT backfill approval practice.

• Use AASHTO T 288 instead of NevadaT235B

• Require source and production testing

• Begin corrosion monitoring program


April 2010


April 2010

NDOT Policy Changes

• Feb 2010 NDOT converts to

AASHTO T 288 test method

• Source and production testing of MSEW

backfill now required for all projects

• New specifications for geogrid reinforced

MSEW developed for Standard Specs


April 2010


April 2010

NDOT Policy Changes

• Geosynthetic reinforced MSE Walls are now

being considered to support roadways

• Approved Tensar ARES Wall System to

NDOT MSEW QPL (project specific basis)

• Developing standard NDOT MSEW QPL

submittal procedure guidelines


April 2010


April 2010

Phase 2 Study Objectives

– Develop understanding of mechanisms

and extent of elevated corrosion of

NDOT’s MSE Walls

– Develop guidelines/procedures to evaluate

consequences of corrosion problems and

remaining MSE Wall service live

– Develop suggestions to remediate problems

found during Phase 2


April 2010


April 2010

Phase 2 Study Tasks

– Site specific corrosivity studies of top

8 problematic MSE Walls identified in

Phase 1

– Install steel coupons for future

corrosion evaluation by inspection

and PR method

– Develop procedure to estimate

remaining service life of walls


April 2010


April 2010

Phase 2 Study Tasks

– Refine correlation of AASHTO and NDOT

resistivity test methods

– Revisit original Phase 1 MSEW prioritization

procedure and prepare new priority list

– Develop more suggestions, if needed,

to remediate NDOT MSEW corrosion

problems depending on results of

Phase 2 Study


April 2010


April 2010

References

1. McMahon & Mann’s May 2005 Consultant’s

Report “Corrosion Evaluation of MSE Walls

I -515/Flamingo Road Las Vegas Nevada” is

available at:

http://www.nevadadot.com/reports_pubs/Geotechni

cal_Reports/SearchGeotechnicalReport.asp.

2. A draft of UNR’s Nov 2009 “Investigation of

Corrosion of MSE Walls in Nevada” is available at

the above link. The draft will be replaced with the

finalized version soon.

http://www.nevadadot.com/reports_pubs/Geotechnical_Reports/SearchGeotechnicalReport.asp

http://www.nevadadot.com/reports_pubs/Geotechnical_Reports/SearchGeotechnicalReport.asp

Questions?

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