There is a sand layer in the foundation pit; what is the best way to deal with it?
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Seeking advice: there is a sand layer in the foundation pit – what is the best way to deal with it? This project is a sewage treatment project that involves pouring concrete to create an underground tank. There is a sand layer at a depth of 7 meters below the surface; the thickness of this sand layer is unknown. I would appreciate advice on the best way to deal with this sand layer.
We have a pool here that faces the same issue; it’s still made of collapsible loess. The initial design called for a foundation of sand and gravel piles, but the final solution was to replace everything with fill material – digging down to the Gobi layer and then filling it with 70% soil and 30% lime mixture, before constructing the main structure. This is just my personal opinion, so it shouldn’t be taken as definitive evidence
It’s just a wastewater treatment tank; the load-bearing capacity is fine, but leak prevention is essential.
First, how big is the pool? Digging 7m down – that should be quite deep, right? I’m not aware of your location; it is recommended to conduct a bearing capacity test. If that doesn’t work, dig down to the bearing stratum in your area; the expanded section can be backfilled with sand-lime mix or reinforced with concrete to reach the desired elevation of the pool’s base layer.
The geology varies in different regions; I think filling treatment could be considered.
The bearing capacity of sand layers is quite high, but it must be uniform. As a general requirement, geological surveys (drilling, coring) and geological analysis are carried out prior to design, with a geological survey report provided to enable the design team to proceed with the design. During construction, it is also necessary to place probes at regular intervals; in areas with complex geology, it is essential to carry out replacement and compaction work to ensure even stress distribution across the foundation. For ordinary ponds, graded sand is the best choice. (For reference)
Replacing everything is the safer option.
Additionally, it should be determined whether the “sand and soil” can be compacted; if it is quicksand, it must be completely replaced.
First, conduct a detailed engineering geological survey; the survey report should include the parameters necessary for design, so that a design plan for foundation pit support can be developed!
We are also constructing a septic tank here, and we encountered the same situation mentioned by the original poster. However, the sand layer here isn’t very thick; therefore, we involved both a design firm and an investigation agency in the project. In the end, the design firm advised that the entire sand layer be removed, and the areas that were above the designed elevation were filled with graded sand and gravel up to that elevation before a base layer was laid
I think: 1. For sewage treatment tanks, the load-bearing capacity requirements are not very strict; it’s sufficient to dig 7 meters deep (or even deeper), compact the soil using a rammer, and backfill it with white rock. Continue to compact during backfilling. The foundation of our company’s 22-meter reaction tower is on riverbed soil, with only 8 meters dug. 2. When backfilling reaches 3 meters above the ground level, it should be poured with reinforced concrete. 3. Add PVC material during foundation pouring to prevent leakage. Hehe, the above are my own opinions. Please take a look and see if they’re useful; feel free to give some points as encouragement.