Shallow Foundation & Underground Water Table???

[TekEngr]

I am designing a shallow foundation for a G+3 hospital structures, according to soil investigation report the bearing capacity of soil at depth of 1m is 0.75 kg/cm2 (medium stiff clay type) and at depth of 2m the bearing capacity is 1.55 kg/cm2 (medium dense soil)but the problem is that the ground water table is encountered at 1m depth (due to the sea site), so therefore i have some following doubts in my mind any one can help and give some suggestions in this regards?

Does the shallow foundation is suitable for this type of ground situation loads are not very high because of light gauge steel structure????

My foundation should start from 1m or 2m??? If I start from 1m the bearing capacity is very less and if I start from 2m then water table will be above foundation???

Any suggestion for special site treatment to prevent my foundation from underground water table , increase in bearing capacity and future settlement??

Regard's

 

[hokie66]

I think those kg/cm2 numbers mean about 75kPa and 155 kPa. Based only on the information you have provided, and the high water table, I think driven piles would be a better solution for your hospital.

 

[TekEngr]

Yes your understandin is correct bearing capacity is 75 Kpa and 155 Kpa.

Before this hospital project porposal was in RCC structure and foundaion was piling which was expensive option.  That is why we change RCC structure to light gauge steel structurr to reduce self weight and economize the foundation.

Do we have other option instead of piling,  some thing like ground tratment,  jet grouting etc??

 

[SlideRuleEra]

Spread footing, mat foundations, etc. work well below the water table. Construction can be a problem if the Contractor does not correctly dewater the excavation. Since the project site is near the sea, there may be a lot of ground water, but this can be handled by an experienced Contractor.

Ask the geotechnical engineer if the ground water elevation is affected by any tidal action in the sea. If the project site is very close to the sea, that could be the case.

The design issue to avoid is having the bottom of the foundation located at or near the ground water elevation. Either the footing should bear on soil above the water table or below the water table, not on soil that is sometimes "wet" and sometimes "dry". If groundwater elevation is more or less constant at 1 meter depth, foundations bearing on soil at 2 meters depth should work well.

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[hokie66]

Not a foundation issue, but using light gauge steel framing adjacent to the sea could present all sorts of corrosion/durability problems.

 

[oldestguy]

If you have no cold weather with frost effects, a shallow footing grade is usually OK.

 

[TekEngr]

Yes!! I was thinking the same as above comment that  mat foundation should be ok @ 2m levell.  But still i have one doubt in my mind at the time of foundation construction we will do dewatring but in future due to earthquake is there any chance of soil settelment and any chance in increasing of water table does these things will effect my foundation in future?? any suggestions for prevention in this regards???? 

 

[SlideRuleEra]

shahg123 - Ask the geographical engineer the questions about soil settlement due to an earthquake. The answer depends on the soil properties.

If the foundation is a built below the water table (say, at 2 meters), then the water table gets higher in the future, this should not be a problem. The soil the foundation is bearing on remains fully submerged and unchanged.
A problem would occur if the foundation was built above the water table. In the future the water table gets higher, submerging the bearing soil.

Building near the sea has many risks - high winds, with or without flooding, tsunami, salt air corrosion of steel (per hokie66's comment), etc.
IMHO, settlement due to earthquake is far down the list of concerns (Would an earthquake that powerful cause a tsunami that would wash away the hospital?). To address all these hazards will cost a lot.

You asked about ground treatment, jet grouting, etc.... Hokie66's suggestion to drive piling is expensive... but it is probably the best and most cost effective way build near the sea.

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[TekEngr]

By the way sea is 10 KM away from the construction site.
so finally i decided to go with mat foundation.
thanks you all for your valuable suggestions.