Lateral earth pressure at rest for a standing cliff

[T2ioTD]

A client is asking for the lateral earth pressure which his building will be subject to, and that facing a open cut excavation of 27 m. The soil is very stiff silty sand and/or sandstone. The excavation is standing undisturbed since more than 10 years, and the building was started at the time then kept unfinished. Now the client is intending to continue working on it.
Plugging the values into Excel, and using Lateral Earth Pressure at rest, I am having very high values. And the values given by software are even higher. (Plz see below). To alleviate the pressure the pressure on the walls the client is proposing to link the building slab directly to the cliff. Hence the forces on the slab in the picture below.
Since the values seem too high, I am questioning the use of the lateral earth pressure at rest in this case. In the formula of Ko, the cohesion is not used since the soil is not mobilised. But the soil has already moved ?!

 

[MTSOE]

I don't know where this project is but the soil properties you have assigned are very strange to me. They are not indicative of a deep open cut that has remained standing for 10 years. Were these values provided in a geotechnical report?

 

[T2ioTD]

The geotechnical report is very missing. It doesn't mention C and Phi values. I back calculated these from the open cliff.

 

[EireChch]

you dont describe SAND as stiff, granular soils are referred to as loose to very dense.

You most likely have a cemented SAND or SANDSTONE, a CLAY would not stand 27m high for 10 years. A cemented SAND would be better called an intermediate geotechnical material as its between a soil and rock.

You could pick a huge range of parameters and get to cut stand vertically in an analysis, however it doesn't mean your parameters are reflective of the actual ground conditions.

Lateral earth pressures shouldn't be your focuses. As you are involved in the project, you have an obligation to highlight that the stability of the cut face needs to be assessed. Just because it was standing for 10 years doesn't me it will stand for a day more. A few weeks of bad weather etc could easily cause a slip.

You need to do a site investigation and rotary core a borehole so you can test the materials strength. You also need to have the face logged by a competent geologist to work out the discontinuities set. This data should be fed into a kinematic analysis

 

[HTURKAK]

To alleviate the pressure the pressure on the walls the client is proposing to link the building slab directly to the cliff. Hence the forces on the slab in the picture below....

You may say just a guess but IMO, the building wall thk, the shear walls implies that the structure is not designed for such a lateral soil loading. Based on my past experience , such a structure shall have transversal walls ( similar to buttress retaining walls ) to transfer the soil loads to the foundation. In addition to the above comments , the structure and the slope shall be checked for slope stability and sliding stability.
Probably the simplest approach would be the option of , keeping the set back and provide permanent anchored retaining system .

 

[Greenalleycat]

Looking at that picture it is one hell of a site
I feel that designing to a soil bearing pressure is almost an academic exercise only: if that cliff decides to go, your house is going with it
Your retaining wall at the back is basically just doing its bit to protect the exposed face from weathering, retaining any backfill you put in, and then hoping nature plays ball
I don't see how you could design any reasonable house to restrain the actual failure plane there

 

[EireChch]

Very true GAC - its a global stability problem first, then a retaining wall design.