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program was recommended to evaluate the quality of the backfill in the other
areas of the wall. Due to time constraints, the failed section was rebuilt following
the original design but using imported dense graded aggregated and a slightly
higher-strength grid to account for installation damage due to the size of the rock
fragments in the fill.
4 SECOND WALL FAILURE
About 2 months after the repair of the first failed section was complete, several
tension cracks were observed in the asphalt pavement located behind other
sections of the wall. The contractor sealed these cracks with tar, but they
reopened within a few days. The cracks became wider and longer, and movement
of the concrete curb was also noticed. However, no bulging of the wall was
readily apparent at this time. Again, a monitoring and subsurface investigation
program was recommended to evaluate the cause of this movement.
No action was taken initially; however, as the cracks became wider, a
vertical displacement became obvious, and the wall face started to bulge outward
near the middle and lean in at the top. A subsurface exploration program was
conducted by another independent geotechnical consultant, which included
performing standard penetration testing (SPT) and obtaining bulk samples and
undisturbed Shelby tube samples from several locations behind the wall.
The SPT results indicated that the upper 3m to 4.5 m of the soil were in a
very loose to loose state. In-place density results from the Shelby tubes indicated
that the field compaction generally varied from 80 to 90% of the maximum dry
density based on the proctor results from the bulk samples obtained in the second
study. Direct shear testing on the samples from the Shelby tubes indicated
effective friction angles ranging from about 30.5 to 33.7
.
The cracking and deformation of the ground surface behind the wall
continued, and a vertical displacement of about 0.2 m developed at the back of the
reinforced zone. The bulging near the middle of the wall and leaning inward at
the top continued to progress. The wall finally failed about 5 months after the
other section was repaired and about one year after original construction.
The failure was similar to the first one, with blocks and drainage gravel piled up
at the base of the wall and the reinforced mass still standing. However, the
reinforced zone underwent much more movement and exhibited a clear failure
surface as was evidenced by the resulting scarp at
8
with an average of 32
8
the ground surface.
This failed section was dismantled and rebuilt under the observation of the
consultant who performed the second field exploration program.
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