Trench In front of Cantilevered Wall 1

[CE475]

I am designing a cantilevered soldier pile wall. There is a 7.5' deep utility trench 5' from the face of the wall. The elevation of rock is at the top of the trench. The rock is weathered limestone with an RQD of 75. The retained soil is clay with a Qu of 1 tsf. The maximum height of the wall measured from the bottom of the trench is 25 ft. How would you go about modeling this?

Here is what I have done. I am using Cwalsht for the design. I assume the passive wedge acts over 3 shaft diameters. I have the soldier piles spaced at 6'-0" and am using 2' diameter shafts. To model the rock I assumed a clay with a cohesion of 4000. My problem is when I model a near vertical face in cwalsht for the rock it is giving me unconservative results (7' of penetration, an elevation near the bottom of the trench). I am having a hard time trying to come up with the contribution of the rock between the drilled shaft and the trench. Does anyone have experience with this?

 

[PEinc]

(25' - 7.5') = 17.5' high cantilevered sheeting wall. This is a pretty high cantilever, especially with a rock cut only 5' off the sheeting. I would not approach this with a cantilevered, soldier beam wall. I would first look at using a soil nail wall, if possible, from OG to the top of rock. Then I would consider whether or not the rock needed to be line drilled and rock bolted. If you must use soldier beams, I would use tiebacks and then check whether it is cheaper to drill in the soldier beams with rock sockets or drive the soldier beams to rock and then secure the bottoms of the beams with toe ties.

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[CE475]

Tie backs are not feasible. The right of way line is too close.

 

[PEinc]

And soil nails?

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[CE475]

Soil nails are out too. The right of way line is basically on top of the wall. Cantilvering the wall is the only viable solution.

 

[PEinc]

Can't you get a temporary easement for temporary (but left-in-place) tiebacks or nails?

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[PEinc]

Can you brace across to another, parallel sheeting wall?

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[CE475]

Just getting an inch of property from the owner was a headache. The contractor we are working for does not want to use tie backs.

 

[PEinc]

If this is a utility trench, maybe it would be better to sheet both sides down to rock and cross brace the sheeting walls to each other.

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[BigH]

Instead of digging a long trench in front of the wall full length, why not open up the trench in short sections with a significant (to be determined) spacing in between? We used to do this type of "leap frogging" for putting in stabilizing berms for tailings dams where we had to key the berm deep below the dam. By doing this judiciously, the wall may not "know" that there is any loss of material in front. Of course, the construction process will be slower - but - this is a choice of economics vs construction system.

 

[CE475]

A little more information. This is for a roadway. There will be a permanent MSE wall installed after this is in place. We cannot brace the soldier pile wall to another wall. The "leap frogging" idea seems valid, but for soldier piles spaced at 6' i don't think it will be feasible since the pipe getting installed will be longer than that.

 

[PEinc]

This job seems ripe for an alternate, or value engineered, permanent, soil nail wall. Eliminating the MSE wall and its geogrids or strips could very well provide sufficient easement for the permanent soil nails. The face of the finished soil nail wall would be in the same location as the face of the MSE wall. If you need to build a temporary sheeting wall to build another, permanent retaining wall, you usually have the wrong type of permanent wall.

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[CE475]

You still need to install the utilities.

 

[PEinc]

I assume that the utilities are supposed to be located in front of the originally planned MSE wall, not beneath it. If so, the utilities will be out in front of an alternate wall also. I never saw anyone place new utilities beneath an MSE wall where they would be inaccessible for repairs if needed.

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[PEinc]

AND..... how can you be certain that your 17.5' high, cantilevered soldier beams won't pry apart the thin, probably fractured, 5' max. width by 7.5' high rock bench in front of the sheeting wall? If the rock pries apart, your 17.5' cantilever could act more like a 25' cantilever. Also. how can you be sure that the rock utility trench does not get excavated wider and closer to the soldier beams than planned? Do what you want; but I think this is being approached incorrectly (based on the little information provided).

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[Geostructsparks]

It seems like you should design the wall for level ground, excavate to that ground level, then put in a stiff waler on the wall and diagonal braces from the waler as you dig the utility trench to temporarily take up the active load on the cantilever wall. The braces would be to a temporary footing (timber mat, etc) on the opposite side of the trench. If you design it right, you could remove one or more braces temporarily to install pipe and have the load redistributed on the waler to adjacent braces. It seems you could toenail a strong horizontal load into this limestone. Keep in mind that if you brace the wall too high up, then the toe may still kick out.

It would be useful to know what your utility is, I presume its long, segmental pipe (what stick length compared to your 6' soldier pile spacing?). You may want to specify using shorter pipe sections. I presume this is not something that can be directional drilled.

I agree with the previous concern that your contractor may not be able to dig a vertical trench in fractured rock, and may effective excavate all the way back to the soldier piles. An option might be to require the utility contractor to pre-excavate and cast concrete beams on the top either side of the trench (say 3' deep, in short segments), then use these for bracing against with heavy jacks at spacing as needed while digging to full depth. The jacks also could be rearranged to allow insertion of longer sticks.

 

[csd72]

I would put concrete struts at each soldier to the opposite side of the trench to transfer the lateral load.

Find out from the utility company what pipe length and diameter they are using and check that this can be inserted between the concrete struts. If you need higher spacings than us a concrete transfer beam between the struts.

 

[mudman54]

CWALSHT is a good program for modeling odd shapes with it's sweep-search method; you should be able to do the geometry in front of the wall including the trench.

Have you considered a double sheetpile wall?

 

[wmta]

This whole discussion amazes me. Does anyone ever use drawings to show what they are talking about. The type of design are critical to new structures and how you build the project. The simple solution is to install rakers and a waler to brace the wall. A 17'6 cantilever is a major structure with very high deflections. Just be careful with this type of problem without drawings and a detailed layout.