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Sloped Site

Sloped Site

Sloped Site

(OP)
Hi

I've been asked to look at some retaining works for a sloped site and have some general queries. See attached sketch.

How do you calculate the sloped fill element on retaining wall B as a surcharge load?
The site soil is a firm clay, is it correct to position wall B just outside the 45 degree zone of influence (for load spread) from wall B or should I push it further away from wall A? I'm concerned that the formation dig for wall A will run into the zone of influence (for load spread) of wall B.
When designing wall A I assume wall B will have no effect if it is located outside the 45 degree zone of influence (for load spread)?

Any advice welcome.

RE: Sloped Site

you have over steepened your original slope to about 1.2H:1V. the original slope is about 2H:1V. your geotech needs to check global stability. I think this looks very questionable. by the way, your profile elevations are mis-labeled, making it more difficult to determine the slope.

RE: Sloped Site

I see several issues with plan:

1. Excavation shown is not deep enough to construct Wall A of the "Proposed Dwelling".

2. The footing for Wall B is on fill. IMHO, not good.

3. The soil (fill?) is described as clay, not granular.

If the excavation is made deep enough to construct Wall A, the footing for Wall B is placed deeper (on undisturbed soil), and only granular backfill is used, then construction and lateral pressure problems become more manageable.

The above recommendations are not "cheap"... but should result in a design that actually works.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Sloped Site

The vertical and horizontal scales are not the same; the horizontal scale itself is variable from left to right. It's probably a good idea to draw the cross section using the same scale throughout to get a better idea of what is going on.

I tend to agree with cvg that the slope on the right of Wall B is probably too steep. A slope stability study is definitely needed.

BA

RE: Sloped Site

Why do you need two walls in the first place? Seems like one will do...

Mike McCann, PE, SE (WA)


RE: Sloped Site

On the slight difference between the two scales, if one only uses those and does not scale the drawing,he can evaluate what is going on. Perhaps one might look at alternatives to typical gravity retaining walls, such as tied back walls or MSE type walls. Are there local regulations for railings, flat areas or other protections for pedestrians on the slope above the wall? In any case the recommended global stability analyses are needed. The changes that may result with time also need to be incorporated in the design, such as heavy rain causing saturation of backfill, plugging of drainage (are they properly filtered?), shrinkage of any clay backfill causing cracks followed by heavy rain.

RE: Sloped Site

The site soil is clay - but that doesn't mean that Wall B would be on clay - I would think that he would use a granular soil fill - not only for support but for drainage which would be carried out around the outside of the house. Wall A would be suitably waterproofed for redundancy. As far as needing Wall B, it seems that the homeowner wants to have a backyard off the kitchen "level" with the kitchen.

Wall A: why wouldn't the wall be designed to be integral with the house - in other words the house wall would also be Wall A.

For computing the pressures, which hasn't really been discussed, there are computations for sloping fill above the wall - or one could use the Coulomb graphical method.

In looking at this, I might suggest to the right of Wall B to have a berm of a couple of metres. For the angle of the slope in the shaded area to the right of the top of B, one could use a fabric MSE slope which has been used many places in the past - and use granular fill in the stabilized section. This would act as a buttress to the clay slope above.

As pointed out by others, a global stability check should be carried out.

RE: Sloped Site

MSE would be expensive since it is a bottom up construction. soil nails and shotcrete facing, top down might be easier. either one would be incredibly expensive

RE: Sloped Site

you may want to extend your shoring down to the basement level since you will need to excavate and temporary shore that anyway in order to construct the basement retaining wall. But first off, you need a geotechnical investigation done to determine the slope stability and suitability of a soil nailed or MSE or other type of wall. A geologic profile would be a first step and you need soil strength parameters, friction angles, cohesion, shear strength, unit weight, etc. in order to check stability and to design the wall.

RE: Sloped Site

John:

While your last sketch may be what you end up with, at this stage all such options should wait until an experienced geotech engineer has completed a detailed investigation and has doe the initial stages of a global stability analysis. As you move along, more such stability checks may be needed for the option you want to do. If you do have a drainage capable fill behind a wall, be sure the geotech OK's the gradations used. Too many commonly used drain systems plug up because proper filtering was not in the job. A lot of void space may look good, but it also fills with mud.

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