Design of Retaining Wall.

[dcredskins]

I have a retaining wall located in front of the sheet piles as shown in the attached PDF, how much soil/lateral pressure do I consider in designing the retaining wall? To the point, how do I design the retaining walls? The sheet piles are already existing.

 

[countervail]

Are the sheet piles failing? I am wondering why a new wall is being introduced, this will have a bearing on the manner of design of the wall.

 

[dcredskins]

Countervail,

If you refer to the sketch, the client would like to raise the ground level in front of the existing sheet pile. The retaining wall is there to retain the backfill between the front of the sheet pile and back of the retaining wall (distance x as indicated in the sketch). FYI, sheet piles were designed to retain the soil behind.

 

[DonPhillips]

As you go through your design, consider possible contractor means and methods. I see the toe of the sheeting being undermined during construction of the retaining wall depending on how deep the sheeting extends. You may have to plan for some temporary tie-backs to keep it stable during construction or otherwise make sure the contractor does something to keep the bottom from failing.

Don Phillips

http://worthingtonengineering.com

 

[JAE]

I'm not sure the lateral earth pressure on the new wall simply becomes constant once it "intersects" the sheet piling. Remember, the triangular earth pressure is a graph of the pressure and isn't really a physical position in space.

Thus, when your pressure "line" intersects the face of the existing sheet piling, there really isn't any physical intersection. Thought of another way - use a differnet horizontal scale for your pressure diagram and you could conceivably have a full triangular load to the bottom within the space between the walls.

Another way to think about this is that lateral earth pressure is similar to the lateral pressure of water in a vessle....only soil creates lateral pressure about 0.33 times the vertical soil weight while water is 1:1. Suppose you had the space filled with water. The lateral water pressure would be higher at the bottom than 1 foot off the bottom, despite the sheet pile wall behind.

 

[LobstaEata]

First, I agree with DonPhillips that removal of soil in front of the sheeting will likely lead to a shift in the location of passive pressure zones supporting the sheeting. A cantilevered sheet pile wall is a soil interaction structure, so changes in the supporting soil in front of the wall will alter the stability characteristics of the sheet piles.

Second, keep in mind that the stiffest element ends up attracting the load, therefore it would be wise to design the concrete wall without considering this comparatively flexible sheeting as a limiter for active soil pressure behind the concrete wall. Passive pressure between the sheeting and concrete wall will be required to stabilize the sheeting regardless.

Since the development of passive soil pressure is roughly 3 times that of active soil pressure, one could argue that the passive zones needed in front of the sheeting would simply serve to transfer the active pressure to the stiffer concrete wall over time.

 

[civeng80]

dcredskins

I think also to get a feel of the problem physical dimensions should be included in your layout so that we may have a feel of the size of the job and the sort of soil conditions e.g. sand or granualar filling and perhaps clay backfill against the sheet piles ?

 

[dcredskins]

JAE,

Most of what you mentioned, I agree. However, refer to my sketch, you see the failure wedge line defines how much x distance is required to get full lateral pressure. There is difference between water and soil. Water is frictionless, however, soil is not (except some frictionless soil). In my case phi is around 30 degree. I am very positive that if the x distance is very less, the retaining wall will not see full lateral pressure (based on triangular diagram - Ka*gamma*H). I am trying to find H1 (H2 > H1 in my case) based on failure wedge line in respect to x distance (however, there is not failure wedge in case of water). The weight of the soil above the failure wedge line detects the soil lateral pressure. If x very less than H (height of retaining wall), I am not sure if the retaining wall will see the full lateral pressure.

 

[KBVT]

I'm not sure what the distance is between your sheet piles and your retaining wall but you may have a silo-type retaining situation where the pressure distribution becomes non-linear with depth. I had a similar issue when backfilling between a new building built close to an adjacent existing building. You may want to confer with a geotech to get a more accurate pressure distribution.

 

[JAE]

I think what KBVT is suggesting is worth looking into.

I'm not suggesting that the earth pressure isn't less. What your diagram indicated, and what I was questioning, was that the intersection of your lateral stress line and the sheet piling indicated the point at which the lateral pressure becomes constant below that intersection.

I don't believe it does simply for the fact that your stress line was drawn based upon an arbitrary horizontal scale and thus the intersection is an arbitrary point in space and has no relevence.

I agree that with a smaller gap between the walls that intuitively the soil pressure would be less than with no sheet piling there. I just don't know how it is affected and KBVT's suggestion to get a geotech involved might be a good step.