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Testing of stiff clay samples on slopes

Testing of stiff clay samples on slopes

Testing of stiff clay samples on slopes

(OP)
Hi all,
I am carryng out routine soil investigation on slopes during slope cutting; the objective is to counter check the design parameter during design stage. Boreholes and test pits are conducted. We were unable to collect undisturbed samples in the borehole. The samples are fractured stiff clay. The spt N valus start with 40 up to 5 feet below the ground & hit 50 after 3 core runs. The recovery is pretty poor. We then dug trial pits & collected block samples with a 1 cubic feet box as the container. The sample is like a stack of thin stiff clay plates about half an inch thick. The clay plates are relatively hard but can be broken by hand. Visible weak planes can be seen between each thin clay layers. I believed if the  shear test is conducted on a single thin clay sample, the C & Phi values would be high. However, it is impossible to get an undisturbed sample since the sample would crack during cutting. Furthermore, the failure would most likely to occure in the weak planes even if I'm able to
test a thin half inch sample. I need the shear strength parameters to detemnine the slope FOS on site. I do not think remoulding a disturbed sample would yield a result that would reflect site condition. Any suggestion on this tricky problem would be very much appreciated.

Thanx in advance

P/S: The slope is about 3000 ft above sea level & the formation is mainly stiff sandy clay. I think the terrain is
few million years away from forming sandstones.

RE: Testing of stiff clay samples on slopes

You need to run two types of strength tests.  The first would be direct shear with the fractures oriented along the shear plane of the machine.  The second would be a standard triaxial drained shear test (or undrained with pore pressure monitoring).  

For the analysis you need to use a software package that allows anisotropic strenghts.  Then you will be able to see what the impact of the proposed construction is on the existing slope.

RE: Testing of stiff clay samples on slopes

(OP)
Hi,
Thanks or the reply. I've done both shear tests you mentioned with some other samples collected. As I said, the difficult part for this particular sample is to prepare the test samples without causing cracks. The reason I'm concern is the adjacent slope has collapsed. The failure may be considered localised failure judging from the size. 40 ft wide by 30 ft height.
 
Any suggestions on how to prepare a intact sample? My lab rats gave up after 4 tries.

   

RE: Testing of stiff clay samples on slopes

Try and try again is the only advice I can give on preparing the samples.

You could also backin to a shear strength using the existing failure.

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