Shear Test For Highly fractured stiff clay on cut slope 1

[longisland]

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.

 

[oldestguy]

This may be a screwy comment for your situation.

However, I have done several on-site shear tests for various purposes, generally to assess the safety factor of a completed job or to design a safe slope where one can change the slope to suit, such as at a waste product disposal site.

These have been direct shear tests out on the job. To get a few points on the plot, the normal stress on a corrugated one foot square plate is done by a screw jack pushing against a good sized weight, such as a truck, a back-hoe bucket resting on a strut. The normal load was measured with a proving ring.

The shearing force has been a "porta power" portable hydraulic jack system, with calibrated presure gage. Strain has been measured also, but usually was not a factor. To get sufficient strength in the frame that moves the plate, a box of heavy steel angle has been used.

In one case for a fly ash dump at a distant place, with no way to get a big resistance load, where I was flown there, I varied the vertical load by loading up a a 5 gal. bucket and doing the horizontal shearing by pulling manually with a 100 # capacity spring scale. This was weak stuff however.

I suppose if you need field data at an angle to horizontal, you could work up an inclined normal and shearing force arrangement. It sounds like your material is pretty strong and my methods would not be able to do do any shearing, unless you reduce the area of the shearing plate, increasing the ignorance factor.

You also would need some fast setting "glue" to adhere the natural undisturbed stuff to the shearing plate.

To get several data points you also would need several set ups I suppose, but in some materials I'd re-tet the same place at different normal loads to get residual strength rather than peak strength.

 

[oldestguy]

Hello again:

I was thinking about this again.

Your material is darn near the strength of weathered rock.
In such instances my experience is that a steep cut slope is used and no form of analysis is done.

A common slope in these parts for weathered rock is 1 or 2 horizontal to 10 vertical, depending on erodability of the stuff.

Then no slope is made more that about 20 feet high, with a step back terrace and another above, etc. The terrace shelves are sloped inward so water is diverted along the slope, not over the next below.

 

[casimmons]

I think some kind of in-situ test performed down a borehole is your best bet. The Dilatometer is widely used to obtain undrained shear strength values. The soils are probably too hard to push the DMT very far, but you could pre-auger to selected depths and run the test. Any slope failures will probably happen along the fractures in the clay, so in some ways the slope may behave like a rock slope and could be analyzed as such.

 

[longisland]

Hi all,
Thanks for the replies. oldestguy, I think you're suggesting an on site shear test. My problem is I can't get a truck since the area in question is 30 feet above road level. casimmons, you're right. the soil is too stiff for dilatometer to penetrate.

anyway, thanks again

 

[dgillette]

Is this a case where you need to worry about sliding on the bedding planes or fractures, rather than through the pieces? If there has been any previous shearing of those surfaces, you may need to look at it differently, assuming planes of very weak material (residual or at least post-peak friction angle). See Bjerrum's Terzaghi lecture in the September 1967 ASCE Jnl of the Soil Mechanics and Foundations Division (whose name evolved into the present Jnl of Geotechnical and Geoenvironmental Engineering). Bjerrum covers Ft. Peck Dam foundation, Panama Canal cuts, (both in shales), and others in stiff clays.

Regards,
DRG

 

[oldestguy]

Longisland:

I've done on-site shear tests in test pits dug with back-hoe, using the nearby ground as a reaction to shearing forces, and the back-hoe bucket for the normal load. To keep that capability constant, I rest the back-hoe bucket on a strut and use that dead weight for a reaction. You can't depend on the operator to keep a constant loading otherwise.

My field standard shearing plane size of 1 ft. sq. has worked out in many "soil" types. Most tests have been in difficult access areas also, no truck access.

As pointed out above by dqillette, if the bedding planes are such that the slide may be dependent on that strength, orienting the test on those would be ideal. You don't need a horizontal shearing plane and can tilt it quite a lot and still do the job. It takes some effort and perhaps some jig making first.

Can you get a track mounted tractor-back-hoe there? If so, most of the digging work is easier.

 

[DRC1]

It sounds like varved clay common in parts of NY & NE. It was formed in glacial lakes during the retreat of the last ice pack. During colder periods, the clay would setlle to the bottom of the lake. During warmer periods ice dams would receede relaesing water and the clay content of the sediments reaching the bottom would be near zero. This formed layers or varves, of strong clay and weak silt. It is a very tricky soil. It has very good strength, but its strength in a cut can change rapidly as many sheeting contractors in the area have learned. It does not like vibration or changes in water table. It will generally test well and perform fine until it suddenly fails unexpectedly. Best bets are to be conservative with lateral strength parameters, limit construction vibration from sheeting or piling and talk to local engineers for insight on the material, because it will varry from area to area.