Dewatering problem - Basement Design

[Muhammad Hashmi]

Assalam O Alykum

In one of our planned project on which construction not yet started , the problem is that water table is very high that is 2.5m from the average NGL ground level ( +0.2m). The Building is double basement+ ground + seven stories. The client already refuses to go for Tension piles. Therefore, we are going to proceed with the raft foundation.

Ground level is +0.7m and the bottom of raft is = 9.60. The total water pressure = 9.6-2.5=7.1x10 = 71 Kn per meter square approximately.

I made exercise and it has been found that we cannot stop dewatering unless at least 4 roofs are casted ( upto 2nd floor )to get the factor of safety 1.1 against uplift. ( Concrete volume)

On the other hand, client wants to stop dewatering as soon as possible maximum up to ground floor is casted because it has cost 7000 SR per day. 

What we suggest is after casting the ground floor stop the dewatering but make the holes in the raft foundation at every 2m x 2m grid interval x+y. If in case water table rises 

The building will sink but not float , but how we can protect the water proofing under the raft. The magna dense aggregate that has the density 40 Kn per meter cube is also not easily available that can increase the dead weight of structure.

Can anybody advise

[MIK]

Walikom Salam.

What will be the problem if you stack excavated material (filled in bags) on the raft as a dead load after constructing the lower Basement floor to balance the load from the uplift during construction???

[Muhammad Hashmi]

Thanks Muhammad.  Can any one else advise?

[Ayesha]

You need to tell the client what is required. Since concrete is heavier than water, one way would be to increase the basement thickness till you achieve enter counter dead weight or install piles and design basement floor as raft so it is anchored to ground.