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.

 

[msquared48]

"Normal" is how far above the top of the raft foundation?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

Since you'll have to dewater for construction, why not continue dewatering until balance point is reached? You could also stage the dewatering level to match the construction progress.

 

[republic08]

Normal water table is 7.5m above top of lower parking floor. City department is allowing for lowering of 1m maximum, i.e 6.5m. So final water table depth is avg. 6.5m above top of lower parking floor.

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.

 

[dik]

How permeable is the soil... first question...

Dik

 

[Zambo]

I don't think it is feasible to construct underwater for your project. So I expect you have to conbine excavation support (e.g sheet piles. diaphragm wall, secant pile wall) together with a way of keeping the ingress of water under control. Than you have to dewater - pump by one means or another.

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.

 

[hokie66]

To address just one of your questions which the others have not, I think that trying to incorporate expansion joints in the raft is a very bad idea. Construction joints are probably inevitable, but they should be made continuous with the other sections of slab. A mat like this does not expand.

As to staging the dewatering as Ron suggested, I would think you would need the whole 12 storeys to resist 60 kPa uplift.

 

[Zambo]

hokie66

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.

 

[hokie66]

It seems to me that the only way of allowing temporary relief is to allow water to enter the lower levels, and I don't think that is acceptable. I wasn't discounting Ron's suggestion of reducing the pumping gradually to allow the water to rise on the outside in accordance with the resisting mass, but that does add another complication to what is already a complex construction project.

 

[republic08]

Soil mainly comprsies of silty sand and silty clay.

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.

 

[ron9876]

I am working on a structure that will also be under water. We plan to install sheeting, dewater and build. After the concrete is set we will pour walls and flood the lower levels until enough structure is built to hold the mat down. Then will pump water out of lower levels and so on.

 

[dik]

If silty sand or silty clay, then the water may be somewhat impermeable and you don't have to worry about draining the lake.

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

 

[dik]

That should read soil and not water...

 

[cvg]

note that silty sand is not impermeable, maybe you meant sandy clay or sandy silt?

 

[Zambo]

I agree with hokie66, don't use expansion joints. Apart from the construction difficulties and the watertightness I'm now assuming that you'll have a grid of tension piles so the expansion joints wouldn't be effective.

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.

 

[Zambo]

One more point regarding point 2 "City department is allowing to reduce the water table to 1m below normal in a long run, but not more."

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.

 

[republic08]

Thank you guys for your experienced and valued suggestions.

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.

 

[Zambo]

republic08,

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.

 

[JAE]

You might also consider a slurry wall system - with the silty sand this might not be feasible. Trench down and keep the trench filled with a bentonitic slurry. Tremie in concrete from bottom up with a mat a reinforcing or two in the concrete wall.

As you excavate downward, add soil nails or tiebacks.

Dewater as per the comments above.

 

[dik]

* Avoiding any expansion joints in the foundation system.

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