How do we design retaining walls to retain stiff soils 1

[IJR]

Stiff soil may not be the right term, but I visited a site where a wall had to be built as basement wall for a new building squeezed between other buildings. After excavation no backfill could be squeezed in as usual and I noticed that the soil is sort of flaky hard red stone, which I believe could just stay there stiff for time except for weathering effects if left uncovered(my intuition).

How do you design such walls?. Is the relatively expensive anchoring justified

Thanks and nice weekend.

 

[Ginger]

The "stiff" material which is currently standing unsupported will not be self supporting for ever. There are a number of approaches one could adopt but it does depend on the material being retained. If the material is a weak sanstone (flaky hard red stone) then as it weathers it will become a loose to medium dense granular material. Therefore, you can model the wall in the long term as retaining a loose granular fill. The phi value will be around 28 degrees, so use a k0 value of around 0.53 for your long term analysis.

Alternatively, you could assume that the stone is robust and will be self supporting in the long term. The void between the wall and the ground may fill with rain/groundwater so you should also check for hydrostatic loading.

This will give you the loads applied to the wall. The design of the wall itself will depend on the nature of the structure (ie a gravity retaining wall or a cantilever). Andy Machon

 

[IJR]

Thanks Ginger. That was really helpful

 

[14243]

Hi IJR
In addition to Ginger's comments, I would like to give some suggestion. Provide a perforated drain pipe behind the wall to day light the ground water/rain or use sump pump. This will minimize the pressure on the basement wall and minimize the dampness on the inner face of the wall.

JeyR

 

[geomo]

RE: Ginger's reply, more questions to ask are:

How long will it take for rock to weather into a soil-like state?

What is the design life of the proposed structure, and can the rock be expected to remain intact during that life span?

If earth pressure from stable rock is a lower bound, and earth pressure from residual soil is an upper bound, what will be the earth pressure from transitional materials? Is it an average value? Is it time dependent, related to weathering processes over time?

Answering these questions, which go woefully unanswered in practice, will help solve the problem. Without answers to these questions, it is conservative to assume that the rock will become soil during the life of the structure, and an upper bound earth pressure value should be used. This is conservative with respect to design, but very costly for construction. If we geotechnical engineers could convince owners and developers that our expertise has value beyond boilerplate reports, they might be willing to take a bit more risk on investigation expenditures for the potential reward of saving lots of dough.

If the rock will remain stable, refer to the following reference:

Frydman, S and I. Keissar, "Earth Pressure on Retaining Walls Near Rock Faces", Journal of Geotechnical Engineering, Vol. 113, No. 6, June, 1987, ASCE.

 

[ishvaaag]

For single level underground, it may turn out that minimal geometrical reinforcement ratios maybe enough to contain even the rest state pressures; so the question is not as much on economy but on ppl wanting to follow the advice or legal imposition on what is good for earth retaining walls; one wall can stand but not have the required safety required by the codes.

 

[tomsklucki]

This posting may be a bit late to be useful, however:

Design the excavation as a rock cutting, with measures such as sprayed concrete, meshing, bolting or anchoring as appropriate, in order to maintain the self-supporting nature of the rock.

If you don't build a retaining wall then you don't have to design it to relevant codes - you will thus avoid an outcome that is bad engineering.