Rehabilitation of a Failing Anchored Retaining Wall - Nicholson ...

Richards/Thome, Page 3
3. Lack of corrosion protection on the initial thread bars used as tieback supports.
Conclusions of the study suggested that while each defect in the wall is believed to be a
contributing factor in the failure, the combination of all three factors acting simultaneously
probably initiated the global failure. Improper drainage from behind the wall increased the
earth pressures acting on the wall leading to overload the tiebacks, failing in part due to the
lack of sufficient corrosion protection around the ground anchors. Finally, subsurface
investigations concluded that the toe of the slurry wall was actually embedded in a stratum of
slickensided clay, which created a less resistant path for the failure plane.
Condition of Old Anchors
Bar anchors were installed in 1978. As noted above, the bearing plates and nuts reportedly
exhibited problems during testing. By 1999, several of the anchors had failed causing the
wall to lean significantly. Detailed condition assessment of the anchors was not a component
of our work, since the rehabilitation did not rely on any of the existing anchors.
As part of demolition of leaning wall panels, several anchors were exhumed. The breaks were
typically within the wall or just behind the wall as shown in Photos 3 and 4. The pipe was a
typical anchor sleeve in the diaphragm wall.
Photo 3 Breaks in Anchors behind Wall
Photo 4 Breaks in Anchors behind Wall
Grouting of the sleeves seemed to have some deficiencies as shown in Photos 3 and 4 and also
several of the other anchors which had seepage and grass growing around the bearing plates.
It is also unclear how access for secondary grouting was provided.
The broken bar had pitting corrosion and had been protected by plastic sheath as shown in
Photo 5. It did not appear that grease had been used or remained inside the plastic sheath.
Water could then get to the bars through the incomplete secondary grouting and then travel
down the bar. However, the bars would then have been expected to fail near the top. This
suggests that shear loads may have also been present at the back face of wall to ground
interface due to possible filling behind the wall or lateral wall movement allowing downward
soil movement behind the wall.