What could be the reason?

[fargofarmer]

I am desigining an approximately 18-foot tall and segmental block retaining wall that has a 3 to 1 slope on the toe side. A wet pond is planned for construction at the toe of the above mentioned slope. I ran a slope stability analysis of the wall using SLIDE program for long term (when pond is filled with water), rapid drawdown and no water in the pond (end of wall construction) conditions. I used a friction angle of 28 degrees and zero cohesion for both retained and foundation soils in the analysis. For no water in the pond case, I got a factor of safety of 1.35. For the long term condition, I got a FS of 1.55. Can somebody tell the reason why I am getting low factor of safety under no water in the pond condition? Is it reasonable to consider the no water in the pond case as short term condition?

best regeards

 

[BigH]

For the short term - does your critical circle actually lie within the retained fill? Do the critical circles match closely? Without seeing your computations and assumptions, I cannot see the dry condition having a lower FS - as the water in the pond and also equilizing within the retained fill and backfill to the retained fill will minimize the "weight" of the fill in the slice in resisting (sigma'*tan(phi) vs sigma*tan(phi).

 

[VAD]

You need to consider that with the water in the pond case that there is a buoyancy effect on some slices which reduces the driving forces depending on the location of critical circle and hence could lead to a higher factor of safety than the no water case.

 

[fargofarmer]

Dear BigH and VAD,

Thank you very much for your quick replies.
For short term- yes, my critical circle lies within the retained fill and the circles match closely. Basically, the on-site soils are typically sands and silts. Therefore, I assumed friction angle of 28 degrees and zero cohesion. I totally agree with your reasoning. I attached my typical critical circles here. The minimum required factor of safety for short condition is 1.3, right? does my no water case come under short term? Or do you guys think my assumption is wrong?

Thanks a lot

 

[fargofarmer]

Here is the model for pond with water case

 

[fargofarmer]

I'm sorry. I didn't attach the file properly earlier.

 

[TDAA]

Your water is providing a vertical load on the lower slices, thus pushing the to of the failure up the slope.

 

[moe333]

As TDAA states, the ponded water provides a large reisting force that forces the critical surface upslope where the reinforcing layers are also providing a reisting force.

 

[oldestguy]

Have you good data to show your 105 p/cf density is correct? 28 degrees friction angle to me shows very loose stuff.

That may be OK for loose bone dry clean sand, but most likely above the water table it is higher, due to usual soil moist conditions.

Also, if the program just subtracts 62.4 from the non-submerged condition, that does not the result in the actual correct submerged unit weight in most cases.

Also, when the pond is full of water is a flat WT under the hillside correct?

Before you get excited about your current question, I'd examine other factors here that put a question in correctness of either result.