Foundation Design on Weak Soil

[srazahz]

Hello,

I am designing the footings for a 6-story building.
The issue I am facing is that the soil strength is very low, and the spring constant is 250 kN/m per square meter.
The footings for the stair and elevator shafts (which are taking the lateral shear as well) are mat footings but I am unable to optimize them and bring the applied bearing pressure under the allowable value.

My question is, what are the possible ways of dealing with this?

 

[RPMG]

Micropiles (cheapest), helical piles, deep foundations in general. Another option is to inject a soil stabilizing compound, which is the most expensive.

 

[JedClampett]

You should engage a geotechnical engineer for a recommendation. All of RPMG's suggestions are good, but there are cases where the deep foundations carry the load from a bad layer to a worse layer. A six story building is going to cost a few million, invest $10,000 into a geotechnical report.
Foundation improvements are not that expensive, especially in relation to the cost of remedying problems.

 

[Menai]

There are some good options listed above; here are a couple of alternatives to explore. If there is sufficient stiffness to provide lateral confinement you could use vibrated stone columns to increase the general stiffness of the ground. If the soil is suitable it can be cheaper than piles. If you have the time you can pre-load the ground till it consolidates. This works well if you have a cut and fill balance and a suitable programme, as it may permit shallow footings.

Don’t jump in to either of these without taking some specific advice, based on your ground report.

Hopefully worth a look.

 

[Retrograde]

A six story building on very low strength soil = piles in my opinion.

 

[BAretired]

Without a subsoil investigation, we are just guessing. Get a soil report!

BA

 

[srazahz]

Thank you all for your responses they are very helpful, we are currently communicating with the Geotechnical Engineer to find a solution.