Type of Foundation

[LIJIN]

Hello everyone, A two storied house having ground floor area 2900 square feet is being built on a weak soil strata. From the soil testing data, it was observed that up to a depth of 2m, the soil is fine sand with standard penetration test, N value=4. 2m-2.7m fine sand with N = 2. 2.7m-4.5m: Sandy clay; N=10. 4.5m-6m: Clayey sand; N = 15. 6m-7.5m stiff lateritic clay with pebbles; N = 20. 7.5m-9m stiff lateritic clay with pebbles; N = 11. 9m-10m stiff lateritic clay with pebbles; N = 12.
Water table is just below the soil surface at a depth 20cm.
What type of foundation shall be adopted?
The soil testing data has been attached

 

[oldestguy]

This forum is not here for designing foundation types. The report is considered far too brief for yur job. However, that report calls for piles, but gives no scoop on what kind of pile the recommend, depths, cost, etc.. It would appear that another engineer (hopefully experienced) there would be more suitable than the "engineer" you have. There also may be other options for foundations, which local experienced engineers can elaborate on.

 

[BUGGAR]

"For pile foundation adopt single under reamed piles taken to rest on stiff
lateritic clay at 6m."

There you are, what else do you need?

 

[concretemasonry]

Location is very important in terms of possible wind and seismic factors. Could it be India?

Could the house actually be a "ground plus two" structure with parking underneath or protection from flood surges? In that case, they would be involved.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[LIJIN]

Sir, location is India..no seismic activity was reported. Adopting pile is uneconomical. And also there was a 1 storied building in the same site which was demolished some 5 years ago. All surrounding plots are covered houses built in random rubble/ column-tie beam footing. Can anybody suggest me a better option.

 

[JedClampett]

Make sure your insurance is paid!

 

[LIJIN]

Irangi poda nayeente mone

 

[oldestguy]

Look at some type of "floating" slab system. A common one for motels in USA on soft ground is a grid of trenches, filled with concrete, reinforced (making them beams in effect) , topped with a concrete slab. For light structures, these grids could be much smaller and more widely spaced. The object is to keep the building in ne piece, with minimal differential settlement.

 

[BigH]

Really, the only problem you seem to have is in the upper 2.7 m (say 3 m for variability across the site). This zone has N values in the range of 2 to 4 - meaning very loose. The underlying clayey sand with N about 10 and underlying layers are competent. This seems sort of like the Calcutta flood plain where the upper 6 m is very soft.

My suggestion is to do the following - place about 2.5 m to 3 m of fill across the building footprint and say 3 m beyond. Put in a crude settlement plate for monitoring the settlement. As the material is fine sand, it should be almost instantaneous. Let the fill sit for a minimum of a week. Remove the fill. Raise grade, if needed, with river sand - again, place it up to 3 m beyond the building footprint. Construct your building. The preload will have taken out the settlement that would be induced by the structure on the upper loose zones. You can then follow strip footings with a slab on top as per Oldestguy suggestion.

For sites such as these, the use of preloading is a very useful tool in your foundation arsenal.

 

[oldestguy]

OK as to Big H's thoughts, but it depends on how much settlement you can take. But, assuming the slab evens it out within the structure, you may or may not need pre-loading. Once you build it and get some rather immediate settlement, maybe just patch the cracks.

However, to save money on pre-loading, consider this. Instead of covering the whole site with one fill layer, do this.

Place a "windrow" of fill, perhaps 4 meters high one meter wide on top, at one end of the site.
With a settlement platform there see how long for settlement to stabilize. With that guide move the windrow gradually across the site, called a rolling surcharge.

My experience with this method is that it works quite well on all sorts of compressible stuff, including loosely dumped fill as thick as 8 meters.

Always keep the top width as it is moved. One dozer working usually is all that is needed, but I have had back-hoes move it with piles as high as 7 meters.

Usually 3 meters high rolling surcharge is used with success.

When reading those settlement platforms, place your instrument on the pile and read the bench mark as a long survey rod left permanently attached to a nearby power pole.