Foundation design for Shallow Water table area?

[Abdur_Rasheed]

Dear Structural Engineers!
I have to design a three stories medicines storage structure (reinforced concrete). The water table is at 5-10 feet below the surface level and the soil is poor too. The bearing capacity is not yet investigated but the B.C of the surrounding area is lower. My question is how to design the foundation for the structure, whether mat foundation would be preferable or any other alternatives to be adopted?
Thanks!

 

[SlideRuleEra]

The water table is at 5-10 feet below the surface level and the soil is poor too.
...mat foundation would be preferable or any other alternatives to be adopted?

Consider driven piling.

A mat foundation may work, but the cost to build it can be high.
Because of poor soil, deep excavation and backfill with suitable soil may be required for a mat.
Deep excavation/backfill near or below the water table and in poor soil may require dewatering of the work area. Also, slopes of an open excavation in poor soil cannot be steep. Shoring of the soil may be required.

Cast-in-place piling in poor soil/high water table conditions can also be difficult.

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[oldestguy]

This question is best answered by involving a geotech engineer along with a series of test borings. Then, a cost estimate of the different alternative foundations should be made, along with investigating the consequences in the future in case of a water table at times near the surface.

 

[fattdad]

it's all about the strength. It's not about the water table.

All geotechnical strength tests are done under saturation. We are used to defining the strength under saturated conditions. If the strength is there it'll support the loads.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Ron]

The shallow water table might present constructability issues for your foundation; however, it does not dictate your foundation design (except with its influence on soil parameters). The soil conditions will dictate your foundation design and bearing capacity is only one of the parameters you'll have to consider.

I agree with SRE that driven piles would likely be your better option if you have poor soil conditions in the upper 10 feet or so. Overexcavation and backfilling can get expensive. Depending on the soil conditions, you might consider surcharging if your client has the time to wait for settlement to occur; however, you don't know if the soil conditions are amenable to a surcharge approach. That leads into OG's comment....you need to get a geotechnical engineer involved to give you appropriate soil conditions from which your foundation can be designed.