QA Issues with MSES Backfill

1
QA Issues with MSES Backfill

• NESGE Meeting, Boston MA
• October 2007
• Don Dwyer, NYSDOT

2
Well Discuss

• Our QA process for MSES backfill
• Issues with backfill quality on a specific
  project that were detected by our QA procedures
• Steps that were taken to assure that quality
  material was used on that project

3
QA Procedure for MSES Backfill

• Contractor/supplier builds stockpile
• Sampling 4 samples
• Testing Gradation, plasticity, durability
  (soundness), chemical

4
Chemical Criteria

• Resistivity gt 3,000 ohm-cm, OR
• Resistivity gt 1,000 ohm-cm, AND
• Sulfates lt 200 ppm, AND
• Chlorides lt 100 ppm

Resistivity between 1,000 and 3,000 ohm-cm is in
the gray zone
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Acceptance/Rejection Criteria

• All 4 samples must meet all spec requirements
• Tests on samples obtained from behind the wall
  (routinely taken QA samples) can result in
  rejection

This leads us to our issue..
6
Background

• Project Drury Lane Interchange
• Back to back MSES walls
• Max height 8 m
• Length 320 m

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(No Transcript)
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MSES Backfill Original Construction

• 20 stockpiles, constructed from Feb 06 to May
  07
• Walls constructed over the winter (!?!?)
• 7 stockpiles rejected for resistivity
• 4 in a row 12-15
• Resistivity btw 523 and 805 ohm-cm

9
Quantities Tested

• Approved 33,613 m3
• Rejected 22,244 m3
• Contract quantity needed 14,460 m3

Comment 1 Boy, we do a lot of testing. Comment
2 With the abundance of approved material,
surely there wont be any problems!
10
Problems

• 3 QA samples taken in spring, fail resistivity.
• Resistivities are 605, 612 and 612 ohm-cm
• Piles 12-15 are
• 12 805, 681, 681, 667
• 13 571, 585, 619, 550
• 14 1060, 970, 709, 633
• 15 942, 523, 942, 784

This closely matches rejected stockpiles 12-15.
Hmmm.
11
So then

• An additional QA sample is taken (just as the
  wall is being completed)
• This sample is in the gray zone (1,390 ohm-cm),
  and fails the sulfate requirement with a 244 ppm
• Piles 5 and 6 are
• 5 R1,383 sulfate259
• 6 R1,390 sulfate336

This closely matches rejected stockpiles 5 and 6.
Hmmm some more.
12
Stop!

• Entire backfill is suspect
• Samples are taken, at 6 various locations and
  elevations
• 5 of the 6 fail!
• Contractor ordered to remove and replace the
  backfill at his cost

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Contractors Argument
The backfill was material Approved by the State.
If its later rejected, the State is responsible.
If rejection is based on QA testing, then
approval means nothing and the contractor is
working at risk.
14
States Argument
QA testing on material from behind the wall is to
catch mistakes, such as drawing from the wrong
stockpile(s). Any QA sample that fails to meet
spec must have come from a source other than an
Approved stockpile.
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MSES Backfill QA Procedure for Reconstruction

• Removed material could not be used again
• Piles 17-20 were reconstructed, sampled and
  tested. Renamed 21-24.
• One more stockpile (25) built

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MSES Backfill QA Procedure for Reconstruction - 2

• Geotech Bureau personnel provided on-site
  inspection of source
• Signed truck delivery tickets
• Took QA samples from stockpiles
• Took QA samples from behind walls

A stockpile sign fell off the pile and was
replaced, incorrectly, on a different pile by
Supplier personnel. Our inspector caught the
mistake before more than a few loads were
delivered.
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MSES Backfill QA Procedure for Reconstruction - 3

• The entire wall reconstruction was completed in
  under 2 weeks
• All samples passed, although Stockpile 25 showed
  resistivity in the gray zone, it passed the
  other testing. The QA sample taken reflected
  these test values.

18
Statistical Summary and Conclusions

• Initial Construction
• Backfill material did not match the approved
  stockpile material
• Backfill material did match stockpiled material
  when all stockpiles were taken into account

Conclusion The contractor used both approved and
rejected material in construction of the wall
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Statistical Summary and Conclusions - 2

• Reconstruction
• All stockpiles used consisted of approved
  material ( 21-25 used in wall)
• Backfill material matched the stockpiled material

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Last Thoughts

• When it comes to requiring a supply of approved
  material, make sure you get what you intend to
  get
• Pay attention to delivery inspection, tickets
• Take QA samples from behind the wall to verify
  that the material is the same as that in the
  stockpiles

21
Afterward

• The test procedure (AASHTO T-288) and lab methods
  are being challenged
• Currently conducting repeatability studies on our
  lab test procedures (initial results show we are
  within /- 1 SD)