Selection of proper parameter for slope analysis 2

[geolover]

Hi all

I have a problem for selection proper parmeter for calculating safe slope of excavation.I have a soil report including c, fi for CU and UU triaxial test for clay which is slope dominant soil.

c-uu= 85 kpa, fi-uu=0
c-cu-5kpa, fi-cu=30

this slope will execute only for construction of retaining wall and is primary.which set of parameter for this case is correct, note that results is very different.please advise me and show relevant referece you may know.

 

[paganel]

UU is for short term analysis.

 

[hwg89]

Beware of tension cracks wich can open up when unloading. For the eventual toplayer with vertical cracks it is not safe to use c-UU. Reference: Theroretical soil mechanics. Karl Terzaghi.

 

[oldestguy]

If the soil is one that undergone drying and wetting and has developed "structure", cohesion values may be meaningless. When dry there is no "gluing" of the "peds", thus no cohesion. Some might call these shrinkage cracks.

For some reason most soils courses tell nothing about pedology conditions that the agricultural soil scientist works with. I know one department head of the geotech section of a large university was floored when soil Structure was discussed at a job meeting.

In short, you may have to significantly reduce the values of parameters dealing with cohesion.

 

[soil1999]

For excavation, soil is under unloading condition/ extenstion . So the shear strength obtained from CU or UU (triaxial compression) should be somehow lower than (maybe by 10 to 20 %) those obtained from Triaxial extenion that should be performed otherwise to simulate the strength under excavation case.

 

[RookieGeo]

Determine FoS for TSA and ESA and do a quick sensitivity analysis by lowering cohesion. ESA is usually more critical. Which parameters you use really depends on the length of excavation and how quickly pore water drains. The presence of sand layers may affect this among other factors.

 

[molerat2210]

Why don't you follow the OSHA Standard Excavation Handbook. It's the law in the USA. May be Type B soil for 1H to 1V unless groundwater, >20 feet, surcharge, vibration, in which case it could be Type C.

 

[geolover]

hi

thanks a lot for your recommendations and hints.

regards.

 

[BigH]

To use ESA, you will need to accurately be able to deine the changes in pore air and pore water pressures. This is what makes the ESA difficult to use except for the long term conditions.