builder over-dug, backfill with bags of rocks?? pics

[sporf]

Just started excavation with a local builder in central Austin and already hitting problems: it’s all clay to 10’ down. Dig is now on hold pending a geotechnical engineering firm’s core samples and recommendations. Builder advised that other pools in the area only required 4-8’ of excavation so lucky me.

It looks like it may end up on piers. I’ll update once I have soil data and would be very curious to hear others’ experience in deep clay.

 

[Texas Splash]

Welcome to TFP! That clay belt that runs through our state is a major pain. We're surrounded by it as well and had to have some foundation work done a few years back. Of course anything above ground like fencing is always wobbly. Very frustrating. The guys who firmed-up our foundation on one side punched steel piers about 40' down before they hit bedrock.

 

[sporf]

Finally got the report back. Seems like mostly good news, excerpts with the most interest bits highlighted:

"brown fat clay is found on the surface that overlies goldish tan and gray lean clay (Eagle Ford Formation). The goldish tan and gray lean clay extends to a depth of 18.1 feet and overlies dark gray clayshale. The dark gray clayshale extends to a depth of 32.5 feet and overlies gray limestone rock (Buda Formation). The gray limestone rock extends to the termination of the boring at a depth of 37.5 feet below existing grade. The brown fat clay is high in plasticity with a plasticity index of 35. The goldish tan and gray lean clay is moderate in plasticity with P.I.'s ranging from 19 to 24 and contains medium hard to very hard limestone (flagstone) layers and concretion layers. The dark gray clayshale is stiff to very stiff, with a peppershale (bentonite bed) layer at the base. The gray limestone is hard and solid."

"It is our understanding a swimming pool is planned for the back yard (west side) of the residential lot. It is further our understanding the swimming pool will be approximately 4 feet to 6 feet below existing grade. We made a brief site visit and observed the excavation of the pool has been excavated to a depth of approximately 7 feet. The underlying soils are low to moderate in plasticity and are densely deposited and will provide excellent support for the pool. The pool bottom is gradually sloping from the deep end to the shallow end and fill will be needed to shape the pool bottom. We recommend the fill consist of compacted crushed limestone rock base material meeting the attached Select Fill Specifications. In lieu of compacted Select Fill we recommend crushed gravel may be placed on top of the lean clay under the pool bottom. The gravel should consist of one-inch diameter crushed limestone rock (no fines). The pool be designed as a mat foundation or sprayed gunite shell with a bearing capacity of 1,500 PSF for a uniform allowable loading on Select Fill or gravel fill over the goldish tan and gray lean clay. In order to mitigate the effect of movement due to the underlying moderately expansive tan and gray lean clays, it should be considered to design the pool bottom with a highly stiffened reinforced concrete slab. Fill sand should not be used under the pool bottom. A thickened edge beam or perimeter beam extending 6 inches into undisturbed soil is recommended, but optional. The swimming pool walls should be designed for an equivalent fluid pressure of 65 PCF for the at-rest condition. We also recommend designing the pool walls assuming no pressure is being exerted from water in the pool (the pool is empty). We recommend that backfill soils for the swimming pool walls, if needed, have a plasticity index (P.I.) not exceeding 18 and be compacted to at least 95% of maximum dry density as determined by TxDOT test method TEX-113-E. Soil moisture should be within 3% of optimum."

 

[sporf]

The builder over-excavated the whole pool to about 7', has to backfill, and I'm not sure they're following the engineer's guidance. Has anyone seen backfill done with bags of crushed rock like this? This thread on overdig backfill compaction seems relevant and so does this thread on sand bags used as a perimeter.

I don't see how the bags can be compacted the way the engineer specified: "fill will be needed to shape the pool bottom. We recommend the fill consist of compacted crushed limestone rock base material meeting the attached Select Fill Specifications", "The gravel should be tamped with a jumping jack compactor to prevent settlement", and "Select fill shall be placed in lifts not to exceed 8 inches loose measure and compacted to 95% or greater". The bags have big voids between them and the bag-bag surfaces slide instead of interlocking like the sharp rock. see attached pic of exact select fill spec and guidance from the engineer who did the soil report.

 

[sporf]

Has anyone seen bags of rocks used to backfill? The builder isn't very responsive and is arguing that "this is normal" even though it contradicts the instructions from the soil survey licensed PE.

 

[Pv2]

I saw something like that in a high end pool build thread here a year or two ago. It was in Austin Texas, I think. other than that I don't know anything other than stuff like that is common for shaping stairs and benches.

 

[sporf]

@Pv2 do you mean this thread on sand bags used as a perimeter ? @bdavis466 didn't like it.

 

[.ben]

This is my opinion only, as I have no direct experience with pool construction practices, other than my own.

I cannot say if this is normal or not. And yes, this absolutely does not conform with the language from the report. And no, I would not be happy if this were my pool.

However, the geotechnical (soils) engineer is not responsible for the structural design of the pool. The pool structure (should) be designed by a structural engineer (SE) who uses the soils information from the geotechnical engineer. Right or wrong, many pools in Texas are built without an SE. Are your plans stramped by a structural engineer? If so, my next step would be to ask for a letter from the SE that the sandbags are OK.

If your pool is built without an SE and you PB insists on the sandbags I am unsure what recourse you have. I suppose you can press the PB to explain why they are not following the recommendation from the report (the obvious reason is money). Maybe try the building department/inspector?

Maybe there are code requirements for proper compaction, but I have no idea.

Maybe I don't understand the video, but this seems like a huge amount of backfill. Why was the pool over-excavated? Is this from a change you made to the design or did the PB screw up?

 

[Pv2]

@Pv2 do you mean this thread on sand bags used as a perimeter ? @bdavis466 didn't like it.

that was the one

 

[sporf]

Maybe I don't understand the video, but this seems like a huge amount of backfill. Why was the pool over-excavated? Is this from a change you made to the design or did the PB screw up?

It is a substantial amount of backfill. I don't understand why: the drawings in the contract and permit all show the deep end at 5'6" and shallow end at 3'6" with steps and a spa. I would have expected them to excavate about a foot or so below the depth of the steps and spa and shallow end, but they dug the whole pool to about 7' (maybe 8', hard to measure)

Edit: found the actual drawings in the info box I assume the excavator used, they match my expectation.