Design of tieback soldier pile wall

[sango]

I had a site formation design on a sloping ground. The marjor construction activities including:
(1) installation of soldier pile 1.5m c/c
(2) carrying out excavation into stages with tieback and wailing installation.

The soldier piles, wailing beams and tiebacks form a rigid frame system to retain maximum 36.0m height of vertical cut faces. The sub-soil conditions generally compose of 4.0m thick colluvium overlaying 2.0m weathering fine ash TUFF. Grade III or above, fine ash TUFF forms the bottom bedrock layer. Therefore, joints pattern in the rock mass controls the design of the wall.

A number of open piezometers were installed in the rock portion, and the monitoring results showed that the groundwater level was almost at the top of rock portion. As no precise information on the joint patterns prior to the excavation, a hypothetical planar failure (30m height) with steep dip angle (82 degree), which based on the results of impression packer tests, was assumed at the rock portion. This rock wedge induced enormous lateral loading (over 2000 kN/m) to the wall if assuming a "triangular" hydrostatic water developing fully behind the wedge.

Can anyone suggest different water pressure distribution for the wedge analysis?

 

[curvbridger]

Your post is not clear to me.

At 36m below the water table, you will have MANY construction problems dealing with water. You may need to install well points around your excavation to lower the water table during construction.

I suggest using open lagging joints which will permit some water movement throught the wall and reduce your pressures.

What are you building?

curvbridger

 

[Focht3]

Yes, you need to provide more details. Tiebacks and walers may not be the best solution if you need to control the influx of water. And 36 meters is a very tall vertical cut; your loads are not unexpected. You really need to look at the rock mechanics aspects of this design - and look hard at dewatering your site. Soldier piles on 1.5 meter spacing may not be sufficient, either.

Then again, your design may be adequate. Give us more information. Do you have experience excavating in this material? How permeable is it? Is it brittle? (I suspect it is.) What is near the excavation?

 

[sango]

Sorry for the unclear information. Let me clarify the situation.

A large platform was proposed to form in a slope by excavtion. Therefore, a retaining wall in form of solider piles with tiebacks was constructed to facilitate the excavation.

The soldier pile wall was retaining about 6.0m soil composing of colluvium and completely decomposed Tuff (PW0/30) and 30.0m sound rock, slightly decomposed Tuff (PW90/100). The properties of sound rock layer are controlled by the nature of discontinuities of rock joints. Its intact strength estimated from uniaxial compression testsis very high, about 200Mpa.

Steel lagging with weepholes were provided between the solider piles for the soil portion. Therefore, the influx of water is purely controlled by the permability and presence of discontinuities in rock.

The active force in the soil portion can be estimated easily by classic theories such as Rankie or Columb method. However, the active force induced the rock portion is difficult to estimate, unless the whole excavation is nearly completed and detailed information on the nature of discontinuities can be revealed.

As there are certain limitations of ground investigation for determining the properties of rock joints (eg. persistent and extent of joints are not known), design assumptions have to make for the analysis. Reference to our local practice, a various size of wedges is considered to estimate the active force in rock portion. The dip and dip direction of wedges are assumed based on the result of impresssion packer test. The design assumptions are then verified during excavation and you may find that the design is over-conservative after construction!

You may surprise that why the soldier piles penetrated the 30.0m sound rock layer (open cut in rock is not considered in this case). The reasons are followins:

(1): vertical cut is required to allow more area for future development.

(2): ease for further excavation in the rock portion (in case of large joints spacing and small extent of joint persistant)

(3):a number of trees behind the cut line are required to retain due to environmental reasons.

So much for the background, my questions are:

(1) Standpipes are recommened in our local code to measure the ground water level in rock portion. The benefit of standpipes allows intersecting a number of joints by its long response zone. However, the water pressure is unknown. For piezometer, water pressure measured by piezometer is only relevant to the joints that intersect the response zone and it is critical to design the installation depth. Any suggestion on groundwater monitoring in rock material?

(2) The standpipes installed in rock portion (the interface of soil and rock sealed up by cement-bentonite) revealed that the groundwater level was almost at the top of rock portion. For planar wedge analysis, a hydrostatic water pressure acting on the joint is assumed to be maximum at the base of joint (height of wedge x unit weight of water), reducing nearly to zero where joint daylight in the interface of soil and rock. The calculated water forces is enormous, and the retaining system cannot support these water forces. Any thing wrong in the assumption on water pressure?

 

[Focht3]

You need a rock mechanics expert. That's not my area, but I'll give some limited suggestions (that a rock mechanics expert may disagree with.)

If your problem is the piezometric (water) level, then you can reduce or eliminate the force with a dewatering system. You can use horizontal drains - they don't have to be vertical. You do need to be able to control the drain discharge - in case you hit an underground seam that has both high flow and high pressure, and begins to erode the intact rock. Erosion of the rock could cause serious instability problems.

Do you know what the hydrologic setting is? What is the source of the water? Is it even possible to dewater the excavation? I can't answer these questions - you need local help. Do you know a good local engineering geologist and/or groundwater hydrologist - that doesn't work for one of your competitors - that could assist you?

If the wall is permanent, then you may have legal and environmental issues that would preclude this approach.

 

[gibbycu]

Focht3 is right, this is an issue for someone with a rock mechanics background. Richard Goodman of UC Berkeley has written many excellent papers and books on the subject of block sliding and failures. You should be able to find some of these books and papers in an engineering library of a local university