Retaining wall / Slope stability design 2

[kieran1]

Can anyone give me any pointers as in the best way to go about checking the stability of an existing slope on which i am construction a 7.0m high reinforced concrete retaining wall which is retaining a highway. I have designed the wall but i am concerned about the concentration on load at the top of the slope which has a gradient of 1:2

 

[Ginger]

You could dig out the design methods by either Bishop or by Fellenius which are to be found in most standard undergraduate soil mechanics text books. These methods check the equilibrium of slices of soil and are simple to follow from the texts (but can be time consuming if you need to be absolutely certain about the minimum factor of safety.) You will need to make an educated guess as to where the critical slip circle lies and then apply your foundation load at the ground level (for simplicity). Then undertake the analysis.

hope this helps
Andy Machon

 

[ishvaaag]

You have sheets in the Mathcad collaboratory that may help you on a preliminar assessment through circular and logarithmic spiral surfaces of failure.

 

[geomo]

For highway projects it is common for the local DOT to take responsibility for global stability. Some commercial projects require wall construction in the DOT right-of-way or walls that support DOT facilities. In those cases the local DOT will probably have the same jurisdiction as they would for their own contracts. Check with geotechnical engineers for the DOT or commercial developers, whichever applies, to see if they have already assessed the problem.


Some jurisdictions will require a global stability analyses before issuing a permit for construction, and may even require that a geotechnical report include those analyses for general site development. Assigning shear strengh parameters, friction angle, cohesion, and unit weight of soil, should be carefully considered before performing the analyses. Too conservative and things won't work on paper that might be just fine in the real world. On the other hand, aggresive shear strength values need to be substantiated with laboratory testing.

Whoever is ultimately responsible for global stability should be a geotechnical engineer or have experience in that field, and should be fully knowledgeable of the local geologic conditions. It is also a good idea to get some peer review from other engineers before making a final jugdement regarding global stability. Check around. Someone who knows your area will know what works and what doesn't, technically, economically and politically.

 

[kieran1]

Thanks Gremo, my local DOT have asked me to prove the global stability before they will adopt the project upon completion. This is common in my country.
I'm thinking of using a geotextile sheeting to build up the slope behind the wall in stages and improve the angle of repose and hence reduce the pressure on the wall. Do you think this will be of any benefit Kieran
BEng(hons),CEng, M.I.Struct.E, M.I.E.I.

 

[geomo]

The reinforced slope will help you to make that portion steeper, but a steeper slope on top of a retaining wall will actually increase the surcharge load, whether reinforced or not. Where steepened slopes are used over a retaining wall, there aren't always clear methods of determining how that surcharge load is estimated. The load on the wall is based on the weight of soil above it, and not so much the strength of that soil overburden. The surcharge issue is separate from global stability.

Consider that global stability generally refers to what is going on behind or beneath the wall, so reinforcing the top might not provide the solution that is wanted. If the wall is also reinforced with geogrid, geotextile, steel strips, etc. then the whole system will work together and the soil reinforcement at the lower part of the wall can enhance the global stability of the system by providing strength where it is most beneficial. Putting reinforcement at the top might not keep the soil from under the wall from sliding out.

Have you considered a mecanically stabilized earth (MSE) wall? That could help with global stability. If that is not feasible, i.e. reinforcement lengths require excavation too close to existing road or adjoining property, then some sort of foundation improvement below the proposed wall might be in order.