Settlement behind retaining wall 7

[eric1037]

I have a bit of a conundrum. We have a project that is a new school building. Part of the school has a lower level and part is slab-on-grade. At the interface between the two levels, there is a below-grade retaining wall. The wall retains about 13 to 14 feet of soil. The subgrade materials at the bottom of footing consist of loose to medium dense sands with a maximum allowable net bearing pressure of 2,000 psf. The design bearing pressure was verified prior to construction of retaining wall foundations.

During some of the utility installation, the plumbers discovered a significant void (4 to 5 inches) under the floor slab of the slab-on-grade portion adjacent to the retaining wall.

Our company observed the backfilling behind the retaining wall. The fill consisted of a relatively clean sand. The fill was placed in lifts of about 8 to 12 inches and compacted to 95% of the modified proctor. The compaction was done using a large vibratory roller and a walk-behind plate compactor close to the wall to avoid additional stress on the wall.

We observed the outlet of the retaining wall drainage system and did not see any evidence of sand at the outfall. We did not see any evidence of wall movement or settlement. No cracks - nothing.

Our approach right now is to perform several soil borings through the slab-on-grade behind the retaining wall. We will determine the relative density of the fill as it is right now. In addition, we are verifying the Maximum Dry Density by performing a new Proctor and performing a sieve analysis.

We have also recommended performing a ground penetrating radar scan of the slab to determine if the void is widespread or localized.

Lastly, we have recommended video survey of any utilities in the vicinity of the observed voids.

I guess I find it relatively hard to believe that the sand behind the wall could settle 4 to 5 inches under its own weight. Especially since we observed the fill placement and tested the fill as it was placed. We have somewhat ruled out settlement of the subgrade since we have not seen any movement or settlement of the retaining wall.

Have any of you experienced anything of this nature? Am I missing anything? Should I be performing any other tests or investigations?

Thanks in advance!!

 

[wallguy]

Sounds like you did all you could do and more and had reasonable outcome. I do have a question. Did the wall installer step the zone between the retained soil and the reinforced zone? As you know if they make a slope cut it allows for a slip zone and differiential settlement. I have seen this area cause some problems. I try to get installers to always step their back fill so they overlap the two zones.