Driven Pile Retaining Wall 1

[farqy]

I am looking into the preliminary design for a driven pile retaining wall, which will run between the bottom of a 35degree slope and a proposed housing development.

The slope is generally poorly cemented sands, with some clay lenses. The SI is currently pending but we hope it will pick up the clay horizon expected to be sitting at around 15m BGL.

The major problem with the site conditions is the presence of the water table which sits at the bottom of the slope. Therefore any excavations for foundation work for the houses will become inundated with groundwater.

My thought is to drive the retaining wall down to the clay layer and thus intercepting the groundwater from across the site. Therefore some management of the water is required to avoid the buildup of porewater pressures behind the retaining wall.

Has anyone got any experience of managing groundwater flow behind a driven retaining wall?? and know of any good systems?

Any help, guidence, pointers will be greatly appreciated.

 

[DRC1]

You can dewater above the clay (assuming the material is somewhat permiable and granular) with electric pumps in a drilled in casing. You are probably too deep for well points. This is done in some instances for temporary construction, but is expensive. Typically the sheeting system is designed for a head differental and no provisions are made for draining the water behind the wall. The wall is not impervious and will leak water through the joints. Provisions for draining this water should be made.

 

[farqy]

Cheers DRC1,

I am also looking at the use of a counterfort drains around my retaining wall. This is something i have very limited experience of.

Does anyone have any pointers, in terms of useful texts/papers which could get me started?

Cheers

F

 

[SlideRuleEra]

Where I work, trying to predict what the (high) ground water will do is like "chasing after moonbeams" - no matter what you do, you can't really solve the problem.

One option may be to forget about trying to cut off the ground water and address house foundation dewatering on a case-by-case basis. Although the "static" water level may be high, once an excavation is dug, the "flow rate" into the hole may be quite low - easily controlled during construction with simple diaphragm pumps, etc. Perhaps some test pits dug now will give a clue if this approach is practical for your site.

For documents on sheet piling, check this page of my website

http://www.slideruleera.net/Steel-Piling.html

http://www.SlideRuleEra.net

 

[BigH]

A lot depends on the permeability of the sand layer - how much water can be expected to flow into an excavation?Foundations taken to reasonable depths (say 4 to 5 ft depending on frost requirements) can likely be done, even in medium sands using the pumping from filter equiped sumps and perimeter ditching while digging the foundatons.

My question is why is the retaining wall piled?

 

[oldestguy]

Another thought.

I've worked on a few jobs where we wanted to keep ground water from moving laterally. One was a waste disposl site with nasty stuff in the water. This and others had an impervious layer to go to. The barrier was a trench filled with either ordinary local clay, or a bentonite slurry.

These worked very well, as planned, but ground surface was near level.

For this site, I'd be wary of bentonite, due to it being not significantly heavier than water, with little resistance to lateral pressures.

However, I question at this site, digging the trench and not having a slope movement toward it while trench is open. Perhaps with braced sheeting or trenching box it might be done. This may need a stability check first, with open trench in the calcs.

Will such a barrier have water then rise up and flow over the top causing "springs" and other mischief? So, before any barrier is done you had better know the natural ground water flow conditions.

Of course one could consider a sub-drain up-stream from the barrier, but that too has questions on how to do it at slope base.

 

[farqy]

We need to provide support to the base of the slope as the toe has to be excavated to make room for the housing foundations (this is expected to leave an unsupported face of roughly 8 - 9metres in places) . Original thoughts were based around a soil nailing solution, but the requirements for managing groundwater across the site made us review that. Therefore we looked at the possibility of using the piled retaining wall to provide the lateral support and to act as a barrier for the groundwater.

Further SI and pezo testing are pending... but should be carried out ASAP. Generally working on limited data at the moment.

All assistance is greatly appreciated.

 

[oldestguy]

More thoughts, this one on stabilizing.

I've done a few jobs where drainage of moving slopes was done by methods similar to those done by California DOT.

Drains are installed by drilling into the slope at the base, installing near-horizontal perforated pipe and, if possible, sand filter surrounding. It is chancy, since the drains are perpendicular to the flow situation.

We also have augmented these by coming down vertically at the top of the slope, meeting the horizontal pipe with a sand filled hole, tapping seepage zones.

It has a lot of trial and error associated with it, but it has stopped slippage. These were clay slopes with sand seams.

 

[BigH]

Is your piled wall a secant wall (piles "touching" each other"??) 8 to 9 metres will present a pretty good overturning moment at the base which is one reason that soil nails or other tie-back solutions have some advantages. Oldestguy has a point - horizontal drains can work well to draw the groundwater level well back into the slope. Actually there was a system of horizontal vacuum drains developed at the University of British Columbia and used in Malibu I believe among other places. Stability is greatly enhanced by mitigating the effects of the water. The vertical drain component is good if you have layered soils as your horizontal drains wouldn't be expected to intersect all such layerings in the slope.

 

[farqy]

We are looking at a secant wall, as to limit water flow through joints. Also the piled wall solution we are looking at will incorporate tie-backs.

I am intrigued by the Horizontal drainage system. Can you recommend any sources of further information?

Regards

F

 

[TDAA]

You may still be able to do soil nails, and add a drain prior to shotcrete.

Also, with a bored pile wall, you could leave a space between the piles to allow the pressures to dissipate. Drainage can be added between the piles to carry water to the toe, and into a perforated pipe to convey the water away.

 

[BigH]

Chuck Brawner (professor emeirtus at University of British Columbia) developed the system along with a studen of his (I want to say Pankas but not sure of the spelling). You can find Chuck's coordinates and give him a shout or email. Genuinely a terrific guy - he'll be at the 59th Canadian Geotechnical Engineering Conference in Vancouver the first week of October I am sure. I'll try to find some more references for you. Just don't forget that if you cut off the flow of water, the water table in the slope will rise and . . .

 

[BigH]

It was Pakalnis. I did a google and got some good stuff. The first reference highlights a book that Chuck just wrote - top of page 3, left, of the review mentions the vacuum drains. The second is a paper that Chuck wrote that was reprinted in a trade magazine. Some other good papers, too, not all about vacuum drains, but horizontal drains.

Use of Vacuum horizontal drains for slope stability mitigation

http://www.bitech.ca/pdf/brawner_book_review.pdf

http://www.imwa.info/bibliographie/01_1_001-106.pdf

http://www.highwaygeologysymposium.org/hgsindex.pdf

Pakalnis

http://www.tkk.fi/Yksikot/Rakennus/Pohja/Preloading_and_vertical_drains.pdf

http://www.epa.vic.gov.au/bus/comments/docs/supplementary_international_peer_review2.pdf

http://www.isope.org/publications/proceedings/ISOPE/Proc-2003-CD-vol2_toc_0422-FINAL.pdf

http://www.eng.jcu.edu.au/Current Students/Postgraduate/School of Engineering - MEngSc Thesis.pdf

http://www.slopeindicator.com/pdf/application guide.pdf

 

[eric1037]

We used horizontal drains to stabilize a bluff along the shore of Lake Michigan. We used a directional drilling machine to install perforated pipe from the top of the slope to the bottom and installed a manifold at the bottom to gravity drain the slope.

The main advantage is that the sytem doesn't require power. For your case, you may need a sump near the bottom to collect GW and pump it out. Another method is to do a sand or gravel-filled trench upslope from the wall that intercepts and directs it to a place where it can be pumped. A thorough stability check would be needed for this case, along with careful planning on construction sequencing.