Question about wick drain

[npthao121]

Dear all!
I have questions that necessary to be consulted with all of you. We are developing a residential area project with low story building (maximum is 3 stories with impact load about 30-40 ton/column) such as house, school, store …The soil condition is very weak containing organic silty clay & mud. The underground water level is about 1 meters from the ground. The depth of weak soil layer is 18 meters. The whole area is leveled with sand with 2.5 meters high. Therefore, surcharge load is about 5ton/m2. As we know well, the whole area will be settlement

For the building foundation we have applied the pile foundation with RC pile 300x300 mm driven until 21 meters. The superstructure is cast-in-situ concrete frame. The building is quite stable. But the ground floor is moving down as well as the other areas, except the building foundation position.

For the ground floor of building we can resist the settlement by applying reinforcement concrete floor connected to the building structure. The other area we are considering to use PVD (prefabricated vertical drain) solution for the whole area.

So I have two questions.

If we apply the PVD for the whole area including building, parking, park …and wait until the soil is stable. Can we use the shallow foundation instead of pile foundation for buildings?
If the shallow foundation is acceptable, which way is cheaper between using PVD plus shallow foundation and concrete ground floor plus pile foundation in general?

I know that I am asking you to give me a favor, but please give me any advices if you can!
Thank in advance! Hope to hear the reply soon!
Vahoang!

 

[Focht3]

The answer to your question about piling now or shallow foundations later is "It depends." What does your geotechnical consultant say?

The addition of 2.5 meters of sand will cause the soft clay to consolidate and settle. It will also induce negative skin friction on the precast concrete piles, so I doubt that 21 meter piles will be long enough. Again, ask your geotechnical consultant.

Wick drains can work - with enough surcharge and time. But it may not be as cheap as deeper piles and a suspended ground level floor.

 

[BigHarvey]

I doubt that you can escape piled foundations given the loads and fill thickness indicated. Nevertheless vertical drains will allow you to get rid of the negative skin friction ( if full consolidation is reached before construction of the piles of course )which will mean big savings. Another advantage of treating with vertical drains is that you will solve the problems linked to the settlement of services lines ( sewers, water and so on ).
You will certainly have to invest in site investigation and the services of a geotech engineer.