Use of cohesion in gravity retaining wall designs

[em35]

Hello All-
I am in the process of designing a modular block retaining wall that must be a gravity type wall - no geogrid. There is a road in front of the wall and the property line is immediately behind the wall. The wall is being placed to retain a cut in an existing bank so that a road can be built at the base of the wall. There is enough horizontal space between the rear of the wall and the property line for 1' to 2' of drainage stone. The existing slope above the wall is as steep as 2H:1V. The site soils are silty clays and the geotech gave me 1000 psf cohesion and 13 deg friction angle (phi). My question is - should all cohesion be ignored in the retained zone? This is what I encounter in my textbooks. I understand that over the long term clay soils (in theory) act like heavy liquids. Is this reasonable or overly conservative? If I only use 13 deg phi I can't get the design to "work" even though the wall is only 7' to 8' tall. Any help on a method to figure a long-term reduced cohesion or a method of analysis using c & phi would be appreciated.

 

[ishvaaag]

I once made a worksheet for slope stability, was then looking the books, inner friction and cohesion 1000 lbf/ft^2 a long term situation, a safety factor on the cohesion value of 2 (mid TO long term) is indicated. It would be 1.25 for short term. Then for the long term situation, horizontal slope (that differs of your case) with the reduced value of cohesion to 500 lbf/ft2 and 13 deg fi standing and a surcharge load of 400 kgf/m2 on the slope above I get Hc=3.58 m or 11.75 ft for a vertical cut, which would be stable long term with in this height, average.

A look in a table from D.W. Taylor (a procedure I coded in another worksheet) gives the for the same case (without the refinements of a surcharge load) critical height for a vertical cut Hc=3.75 m or 12.31 ft, being here the safety factor 1.5, i.e., the actual critical height (no term specified, buat at least midterm) would be 1.5 times higher.

A third worksheet establishes that for the standing 500 psf and fi=13 deg, earthquake acceleration 0.1g and vertical acceleration 0.05g the 1V:2H slope itself has a 1.76 safety factor against failure for up to a total height (gain og height in the slope) of 5 m or 15 ft. For a total gain in height in the slope of 10 m or around 34 ft the safety factor becomes 1.05. By another worksheet estimate, long term safety for the 10 m tall slope, no EQ forces involves becomes 0.93, whilst it is 1.87 for the 5 m tall slope.


We have NOT determined a critical height for a vertical cut in a slope as is your case, buy anyway it looks promising on that, as it is made sometimes in graphical estimates of the active push, THIS critical height, as estimated for an horizontal surface is deduced first from the active wedge. Hence for these approximate graphical evaluations you would only be getting active pressure if your wall was to be higher than is wanted to; it falls below such critical height.

Anyway you can search your books and formulations for a proper statement of the active push taking into account what above. If the NO active pressure would stand, then a padding situation of the wall in the backfill could be the action to take, let others to give their opinion.