Building under water table 4

[republic08]

We are designing a 2- level parking structure with 10 storey structure above, which is close to a lake. Water table above raft is causing a net uplift of about 60kPa.

Infact water table has to drop down below U/S of raft for construction of raft, but it can effect the lake.

What would be the strategy here. We are thinking to add soil anchors to raft, so that raft can be self supported without waiting for structure dead load to balance.
Net uplift is aroung 60kPa.

1. Is it possible that expansion joints can resist these and how they will be maintained under such a high pressure.
2. City department is allowing to reduce the water table to 1m below normal in a long run, but not more.

Appreciate for your help, how to approach.

 

[hokie66]

For a basement 6 metres below ground, JAE's suggestion of using a slurry diaphragm wall is probably the most common solution. It is nigh on impossible to make these walls commpletely watertight, so a trench drain (or spoon drain) is often employed to catch the water at the perimeter, with a piped and pumped system to take it away. The vertical joints between slurry panels and the anchor penetrations are where most of the leakage occurs.

A slab to resist 6 metres of uplift will be heavy, and should be reinforced for a strong degree of crack control, around .6% steel, eliminating the need for control joints. Water retaining structure, but still should have a membrane under the slab.

I don't like the idea of popouts. Nobody wants to flood a basement filled with expensive automobiles. The uplift is controlled by the structure. The drainage system is only for leakage.

 

[Zambo]

hokie66, I agree the base slab will be heavy, perhaps in the range 600-900mm thick. There are 2 ways to make the concrete watertight (obviously good joint detailing is required) but there are 2 concepts for the main slab area:

1. a waterproof membrane. These are expensive and have to be installed properly.

2. waterproofing the concrete. This can be by densifying such as with the use of silica fume. Or by using waterproofing admixtures.

Logic would suggest from a cost point of view that for thick slabs a membrane would be used, why densify or add an expensive admixture for a large mass of concrete. For thinner slabs treating the concrete may prove more economical than using a membrane.

For my last similar project we had a 1,100mm thick base slab (at 15m below ground level) We finally went for a waterproofing admixture as we doubted that a membrane would be fitted correctly around all the pile connections.

 

[dik]

Hokie... I've never experienced a 'catestrophic' failure of pop outs... the water sort of slowly 'oozes' in and that is after the sump system fails...

Dik

 

[hokie66]

dik,

My point is that the entire basement is subject to hydrostatic pressure, and there is no sump or drainage system to cope with water outside the envelope. The drainage and sump is for taking away leakage. The leakage is already inside, and I don't see how popouts would help in that.

 

[dik]

A properly functioning sump and underslab drainage system prevent the hydrostatic pressure; they have the effect of 'locally' drawing down the water table.

Dik

 

[hokie66]

I don't think there is any justification for using drainage to lower the water table 6 or 7 metres as in this instance. If you design the building so that gravity loads and/or tension piles resist the uplift, then hydrostatic pressure is solved. In the OP's case, the water table is allowed by the authorities to be drawn down only marginally.

 

[Zambo]

According to the thread starter the building area ia around 300,000 square feet. I do not believe that the methods either during construction or the permanent state recommended by dik will be appropriate for a structure of this size. The continuous pumping requirements alone make it unfeasible. For a smaller footprint and shallower basement maybe it would work depending on the soil conditions.

I don't think this basement (unless it is in a very remote area) can be carried out without excavation support. So either it is sheet piles as a temporary measure followed by a watertight retaining wall, or combine the temporary and permanent states using diaphragm wall or secant piles.

The choice of whether to use sheet piles or diaphragm wall can be worked out according to the most economical solution. Basically you need to find out the required level of the toe of the wall (to reduce water flow to a minimum). Once you know this you can decide whether the depth of diaphragm wall required works out to be cost effective compared to sheet piles which of course can be retrieved after use.

 

[hokie66]

In addition to those two options, another method sometimes employed is to leave the sheet piling in place as the outside wall form for either a cast or sprayed concrete wall, thus avoiding working between the sheet piling and the permanent wall.

 

[republic08]

Depth of exterior wall / sheet piling system below impervious layer is important. If the impervious layer is quite deep (I will attach borehole data for further idea), is it economical to go that deep or the option of continuous pumping is cheaper.

Continuous Pumping of water table below bottom of raft is required anyways for construction of raft and unless the walls are higher than normal water table, but my concern in this case is the level of the nearby lake.

If we are lowering the water table for this particular building, can it affect the nearby structures.

 

[BernieCA]

HI GUYS, want to share...all system go, am also right now managing the construction of 3 level basement designed to have 23 floors on top. Excavation is now at -10 meters w/out water and interistingly to note that my site is just around 50 meters from a very deep river being used by huge ships.

We have constructed 800mm thick perimeter diaphragm wall 38.50 meter deep struct into very dense soil. Set 15 water weels inside the basement and pumps out at -13m just 50cm below the bottom of Pile cap. Outside water level at -2.5m and got no problem with water anywhere. Anyway I have 2200sq.m basement floor area on a barrette pile foundations.

A watertight concrete is much to be desired in the bottom slab and 45~50 Mpa concrete is more logical to create a watertight concrete, no need membrane but to apply crytaline water proofing slurry which is far more cheaper. You have only the D/Wall slab connections to worry on water seapage. minimal water ingress into the D/wall is to be expected since no watertight concrete could be constructed under bentonite mixed water?...

 

[Zambo]

BernieCA,good to hear your recent construction experience.

as you have now excavated to -10m how are you supporting the diaphragm wall until the permanent floors are cast?

Will you be casting the basement slab at -13m before the 23 floor superstructure is partially constructed?

Have your barrettes been designed for tension under uplift forces, or is your construction method such that there will not be uplift?

I agree that the diaphragm wall to slab connections are critical for leakage. I also think that the worst location is where the slab meets the diaphragm wall at a joint between wall panels - do you agree?

 

[hokie66]

BernieCA and Zambo,

It would be interesting to hear your thoughts on leakage prevention methods which you have experience with at 1) diaphragm panel joints, 2) basement slab to wall joint, and 3) anchor penetrations.

 

[republic08]

Here are the 2 bore holes sheets.

 

[republic08]

borehole sheet 2