Retaining Wall

[Snatch]

Hi
I have designed a retaining wall to retain soil pressure and over burden pressure from a foundation of a house about 2 meters away. The height of the wall is 4.57m high. My design produced a 330 mm thk wall with T16@125 c/c main reinforcement, vertically ( designed as a cantilever). My concern is that the soil that is to be retained is hard rock, when cut it stand up vertically by itself, actually it has been doing that for years, without any form of physical weathering.
I used 18 kn/m3 density for soil, which is over estimated but supervisor thinks its adequate.
I would like any suggestion from anyone regarding my design philosophy or to come up with a thinner wall size

Thanks

 

[RiBeneke]

Your design does not seem unreasonable if you designed it using simple formulae.

To do a more precise analysis of possible forces on the wall would require a careful geotechnical analysis, and this may be costly. One would need to check for possible slip planes etc.

You must also be careful that any excavation for the base of the proposed wall does not cause instability in the slope.

Have you considered some form of ground anchor bolts as an alternative ?

Regards
Richard Beneke

 

[Ron]

I agree with Richard. You have approached your design conservatively as if you had a granular condition which would exert lateral pressure on your wall. As Richard said, a more detailed geotechnical analysis would be indicated for a refined analysis.

If this is, in fact, an intact rock layer, then your load from the house would not likely impact the wall due to the tendency of the rock to attenuate lateral strains and the house load to manifest mostly as shear stress in the rock at the outside of the foundation.

Using a rock anchor approach is a good idea. This could then be capped with a shotcrete facing that would provide aesthetic appeal and serve as a lateral strain "tell-tale", as well as providing some structural enhancement.

Ron

 

[Snatch]

Ron, Richard.

Thanks for responding to my question promptly.

I am working on a small island in the caribbean and there are no equipment on island to carry-out rock anchor bolt solution, (that was one of the options i thought about initially)


Dion

 

[Ginger]

I think that a 330mm thick cantilevered wall with a height of nearly 5m sounds about right. You can obviously taper the thickness as you progress towards the top of the wall, but if you make the wall too slender you can start to get excessive cracking on the back of the retained face due to deflections. The approach of a granular backfill is correct (if as I assume you are cutting the rock to make backfill). However, if the rock is sufficiently strong and massive that it can stand unsupported then why use a retaining wall at all? Why not build the structure on the rock?

If youare using this design, you should be extra vigilant not to compact your crushed rock backfill too vigorously as this does impart large lateral stresse on the wall which will be locked in.

Regards

Andy Machon
Andy@machona.freeserve.co.uk