Building under water table
Building under water table
(OP)
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.
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.






RE: Building under water table
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Building under water table
RE: Building under water table
Is it possible that during continue dewatering process, the nearby lake become drained. Lake is around 1 km from site. Does this draining process cause a gradient, so lake might not be effected.
Ron,
What is meant by stage the dewatering level. Do you meant by changing water level once we progress above.
RE: Building under water table
Dik
RE: Building under water table
How do you plan to excavate this basement?
What does the foundation consist of? You suggest soil anchors and a slab which may work while the uplift is there, but when you apply load I imagine piles are needed for the compression. In this case it seems more likely that tension piles instead of soil anchors is the solution.
RE: Building under water table
As to staging the dewatering as Ron suggested, I would think you would need the whole 12 storeys to resist 60 kPa uplift.
RE: Building under water table
You're a design guy I'm a construction guy so I appreciate it when someone can just look at the numbers and come up with a guideline. So assuming you are correct (which I believe) then he will have uplift from the time of completion of the basement slab until he completes the structure, then he will have some compression. Tension piles seem like the solution.
I think he has to construct some kind of wall to keep the water out, as mentioned earlier sheet pile diaphragm wall etc. But he cannot rely on this structure (temporary or other) to stop the build up of uplift force it will just reduce the amount of pumping required to aid constructability.
If piles are the solution for the foundation what about allowing some relief from the uplift force (i.e water pressure outlets) until after the load from the superstructure is applied.
RE: Building under water table
RE: Building under water table
Soil anchors or piles will act in tension for sure. The idea of water pressure outlet seems good, but is it permanent or just for temporary relief to raft during construction.
It is not possible to wait for 10 storey loads to balance uplift forces with gravity load of structure.
So, there appears to be no need of expansion joint in structue for a massive area of say 300,000 sft. It can be dealt with allowing portions of the concrete to shrink during construction.
P.S.
Some of the building area outside tower part don't have much gravity loads in any case to balance the uplift, so there is necessity of tension anchors/piles in some part of the structure.
RE: Building under water table
RE: Building under water table
Should have a geotekkie involved and perimeter drainage and sub slab drainage to a suitable sump (my one project had a 7'x15'x60' sump) don't know how large you need.
I'd also install 'pop-outs' in the lower floor to relieve pressure should the drainage fail. These are 30" square panels in the concrete slab that are intended to lift, in the event of hydrostatic pressure.
Dik
RE: Building under water table
RE: Building under water table
RE: Building under water table
There are some good posts above and most indicate that you will have to have some kind of "wall" to keep the water out, sheet piles are mentioned by several posters. Now you need deep trial boreholes to hopefully find a toe level for these sheet piles which penetrates into a preferably impermeable strata (clay for example).
The problem is that even if you find a suitable strata into which you can drive the sheet piles you cannot assume that all the water will be excluded and therefore prevent uplift. There will always be some water getting through either through the clutches or around the toe of the sheet pile. This means that the amount of water that has to be pumped out will be much reduced but you still need to assume uplift on the base slab.
You suggest that the tower above doesn't cover the whole basement parking area so that some sections have to be held down by tension piles in the temporary and permanent state. There is also an added complexity if you want to flood the basement after construction or build in the "pop outs" which in many ways seems an excellent suggestion by dik.
All things considered I would have a go at designing using all tension piles so that the superstructure isn't needed to hold down the basement against uplift. Then you can see if the costs of this method are prohibitive.
RE: Building under water table
I believe this is not to assist with construction but is the maximum effect your project can have on the water table. In fact although they allow one metre it would be better for you if the effect could be kept to zero as this will reduce the amount of pumping you have to do.
So first you need to ensure you have a way to monitor the effect your project has on the water table. You may choose to start this early as the water table may be seasonal. Once you start construction then you have to check whether the water table is being lowered. If it is lowering by an amount that gives concern you may have to find where the water is getting through your sheet piles (or diaphragm wall or secant piles etc) and plug the leak by, for example grouting.
Of course the concern is damage to adjacent property, roads etc and the project insurer may want some involvement in the monitoring procedure.
RE: Building under water table
We will consider the following points and wellcome further suggestion if any:
* Avoiding any expansion joints in the foundation system.
* There is definately a need of perimeter sheet piling system for the basement area.
* Requiremnts of pop outs in case the drainage system fails.
* Option of trench drain. (Can it work for 7m water
head). We can check further with geo-tech for this.
RE: Building under water table
your decisions on the way forward for your project seem logical. Except for "option of trench drain". Are you thinking this drain is inside or outside the sheetpile wall. The 7m water head is the level of the water table above your lower basement slab, why would you want to drain the water table?
Once you enclose the building area in nominally watertight sheetpiles (of course there will be limited leakage) then all you have to do is dewater to get rid of the water trapped within your sheetpile box. I don't see a 7m head of water to drain by a trench.
Or perhaps I've misunderstood the purpose.
RE: Building under water table
As you excavate downward, add soil nails or tiebacks.
Dewater as per the comments above.
RE: Building under water table
Properly detailed joints can minimise unexpected cracking
* There is definately a need of perimeter sheet piling system for the basement area.
You may need this for construction, but unless you have soil that readily permits water to flow, I would suggest you use your money for a better drain, sump and underslab drainage.
* Requiremnts of pop outs in case the drainage system fails.
I've used these often and they work well. You may consider added sumps as a back up system.
* Option of trench drain. (Can it work for 7m water
head). We can check further with geo-tech for this.
The most I've used trench drains is approx 10'... have no experience with 22' or so... Make sure the trench drain is lined with a non-woven geotextile to reduce the 'silting up that will/may occur.
Dik
RE: Building under water table
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.
RE: Building under water table
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.
RE: Building under water table
Dik
RE: Building under water table
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.
RE: Building under water table
Dik
RE: Building under water table
RE: Building under water table
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.
RE: Building under water table
RE: Building under water table
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.
RE: Building under water table
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?...
RE: Building under water table
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?
RE: Building under water table
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.
RE: Building under water table
RE: Building under water table