soaking the subsoil by water before making the shallow foundation

[geokarabu]

Hello, I have a question. I am designing the shallow foundations for a prodaction hall in arid zone in Egypt. In the report of the geotechnical investigation there is recommended to soak the excavation by the continuous flood of water for 2days before the foundation works start. It should help to compact the cemented silty sand layers with large void ratio. Is it common process?

Thank you for your answer.

 

[Ron]

While I'm not familiar with that particular soil, I am familiar with partially cemented silty sands. Soaking without compaction will not do a great deal to reduce the void ratio (compact). I would scarify the soil, try to increase the moisture content to slightly above optimum, then mechanically compact it.

 

[cvg]

if these are collapsible soils which it sounds like they are, then soaking might cause them to collapse. that might be a good thing, but you cannot guarantee it will happen or that the entire intended foundation area will settle. If you cannot tolerate a potential future foundation collapse due to wetting or earthquake, then you need to design a deeper foundation or scarify and recompact.

 

[Mercycity]

Dear geokarabu,

As my colleague said, this may be collapsible soil and in order to collapse the soil,you have to flood it and use heavy energy compactor which can compact the soil to a depth of least 2B where B is a width of foundation. If you don't have such equipment you have to dug a foundation to a depth of 2B and replace in a layer of 150mm compacted at 95% Mod AASHTO Density up to a foundation level. Control tests should be observed. Addition of 4% cement in the insitu soil can also used to improve the soil. Flooding the foundation and after that you continue with construction it can lead to early cracking of the building because of immediate settlement which is common to sandy soil of this nature. Building without carrying out above treatment the cracking of the structure will be observed after the heavy rain and this can be very serious. I some times dealing with this kind of problems in our areas so i know what i am talking about.

 

[Patgeotech]

Certainly sounds like a collapsible soil is the issue. How deep did the voids (pin-holes) extend into the subsoils? Wetting the soils may weaken the cemented bonds, but one needs to include some form of suitable vibrational compaction to ensure the breaking of the cemented bonds (assuming they are weakly cemented?). Depending on the percentage of silt in the sand (and assuming the sand is fine-grained), one has to watch out for "matressing" affects, which may cause time delays or even weaken the compacted soils.