You can use a series of anchors below the water table (that is, as long as the anchor contractor can cut or drill holes through the wall without losing too much soil from the water running in through the holes).
As an alternate, if your sheeted excavation is not too large, you may want to internally brace the support walls.
Maybe you should talk to a local tieback anchor contractor, explain your situation, and get advice.
BigH & PEinc thanks for your replies.
I am hired to plan a retaining structure (a secant-piled or steel-pile wall) to retain a foundation pit of 90m to 60m having a depth of 20m. The GWL is 6m below ground.
I am not sure to propose a dewatering process, though I am quite aware that I have to deal with greater moments. Because the pit is quite big the dewatering process is going to cost a lot. In addition the average water level to be lowered is around 14m and it will not be easy to engage.
The pit is too big to be braced. May be the corners can be braced. I have some doubts about the ground tie back anchors' strength in sand medium.
I do appreciate your valuable contributions.
This all depends on the shear strength of the sand. If you provide for the appropriate free length and fixed length, include provisions for secondary grout and require proof testing of all completed tie-backs, why not? The bigger question (which you have acknowledged) is how to deal with the lateral loading from the soil and hydrostatic pressure acting on the wall (and in-turn conveyed to the tie-backs). I'd expect quite a section modulus will be required in conjuction with numerous ties to solve this problem. . . .
I concur with fattdad on proof testing "ALL ANCHORS" and although sand could be a tricky animal, it is quite feasible to anchor in it. As PEinc pointed out make sure the contractor knows what he is doing when it comes to drilling and grouting anchors below the water table. As to the dewatering, I do not see how you can get away from it. On a site this big, increasing the embedment depth of the pile will only insure the pile toe stability and protect against piping in its vicinity, but the problem remains further away. Unfortunatly, it will cost regardless how you look at it.
I don't think that dewatering will be out of line - some deep wells should draw the water level down so that you can excavate in the dry. With sheets, you will have to worry not only about your sand anchors but 14m of head on the bottom perhaps making the bottom "live" (e.g., boiling). I still think that dewatering is appropriate and should not be dismissed out of hand.
I appreciate all valuable contributions. Experience is beyond the scope of all books.
Being a young geotechnical engineer and working as a researcher I feel like I am a bird lacking one wing.
That wing is the experience acquired in the field.
The present situation is that I am hired to estimate a retaining structure that is going to be built in the field that I mentioned before. I was wondering that if I would be too conservative if I recommend a dewatering system and my second concern was the effectiveness of the ground anchors.
Now I feel confident to have any kind of arguments with the client's engineers.
Thank you all!
Should I bother you if I ask one last question about anyone experienced in installing a bored pile in a sand medium. I think that we should bore with the casing and after concreting the casing can be removed. But will it be possible to remove a 30 meter casing? Because it is very common in Turkey to use secant piles as retaining structures it is cheaper than steel sheet piles the client would insist on installing secant-piled-wall.
You can most likely drill the bored piles openhole with bentonite or other supporting fluid mechanism to prevent cave. Why not consider a diaphragm wall if you are needing to go secant rather than sheets. You basically dig under slurry a trench then tremie in your concrete wall after installing cages, etc. Used on many projects.
Is it possible to instal a 30m casing and retreive it. ?
Yes, if you have the right machine.
I just came back from a trip to Dubai and witnessed 30m piles in sand using a double rotating head machine. The machine rotates and vibrates the casing down in one direction and the C.F.Auger was removing the soil while rotating in the opposite direction. I believe the company was Korean.
