Soil testing for slope stability analysis
Soil testing for slope stability analysis
(OP)
Hi,
I've assigned borehole on site to collect soil samples & data for slope design.
Unfortunately, I'm getting disturbed samples since the formation is very stiff sand to sandstone of SPT 50. What other tests should I assign?
In fact, is there a rule of thumb in soil test assignment for slope?
How to determine the overall stability of the slope?
P/S: Any pointers in regards to the geologist's report? What are the key elements I should be looking into?
thanx
I've assigned borehole on site to collect soil samples & data for slope design.
Unfortunately, I'm getting disturbed samples since the formation is very stiff sand to sandstone of SPT 50. What other tests should I assign?
In fact, is there a rule of thumb in soil test assignment for slope?
How to determine the overall stability of the slope?
P/S: Any pointers in regards to the geologist's report? What are the key elements I should be looking into?
thanx
RE: Soil testing for slope stability analysis
RE: Soil testing for slope stability analysis
Rockjoint
rockjoint@yahoo.com
RE: Soil testing for slope stability analysis
longisland -
I'd try running direct shear tests on uncemented sand samples - to get a lower-bound indication of the angle of internal friction. Be sure to run tests over the full range of expected overburden values. And try to test those layers, seams, etc. that are most likely to give the slope its future stability problems.
Intact samples are more problematic. If you must have them, then use a backhoe and hand carve some block samples. Keep them moist - and test as soon as possible. If you can trim triaxial samples, try CU with pore pressure measurements. Or try to core with geotech drill rig set up to core soft to medium hard rock to get rock samples.
But remember that slope stability is not based on the greatest strength, rather the issue is the weakest link. Look in the geologist's report for whether the sandstone is massive or fractured, the bed tilt (adverse or favorable), weak seams, etc. It's the small, usually overlooked details that can cause an otherwise stable slope to fail. Get very detailed geotech logs and use 3D software to give you a spatial perspective. Review it with the geologist - he may have insights that you don't.
And don't forget to look at any slope failures in the area of your site.
RE: Soil testing for slope stability analysis
RE: Soil testing for slope stability analysis
Unless things have changed - a sand is NEVER very stiff. Dense, compact (or medium dense) etc. but never given a consistency term.
Important: I would look particularly carefully at whether there is any layering in your sand deposits. If the slope is to be a cut with high initial water levels (do put in piezometers or standpipes), there can be disastrous effects effects when the cut starts to "lower" the groundwater level. Instead of dropping in a "homogenous" fashion, the water level will be shunted out at fine interlayers thus undercutting the sand.
Another consideration - unless the slope is very high - say more than 15m or so as a rough initial guess, elaborate testing of the sand is likely not warranted. Estimates of phi values can be made and your slope may not be overly critical to variations. With such a very dense sand, it may be somewhat overconsolidated and may stand at a greater angle initially than later. Suction effects may be appropriate to check out. But, the groundwater regime is of utmost concern, though.
As for the sandstone, earlier response is correct if the sandstone plays an imporant part role in the slope (as opposed to the fdn) - especially if there are dips into the pit (assuming cut) or clayey infills of fractures.
If this is for foundation design of an embankment, the above still holds to a degree, but N=50 is a pretty darn good material for a foundation base. Be careful if a water dam. You will likely need cutoffs in foundation.
Hope this helps in addition to the learned notes earlier.
Best regards to all.
RE: Soil testing for slope stability analysis
Someone assigned to perform a slope stability analysis should be a lot more familiar with the topic or should be working under the direct supervision of an engineer experienced in this type of analysis.
I highly recommend that longisland get help with this one.
RE: Soil testing for slope stability analysis
Best Regards,
Dirtdoc1
RE: Soil testing for slope stability analysis
Points well made about the direct shear box. I seldom use them - and I don't own one. But the results can give an indication of lower-bound behavior where no field performance can be used as guidance.
You can't run "undrained" direct shear tests on sand - period. Saturation is an issue, but apparent cohesion should not be a problem unless there is appreciable clay binder in the sample.
I do understand lab testing, and I have run a lot of direct shear tests myself. I also run CBRs, consols, triaxial CUs and CDs and Proctors. And I still run index tests periodically - moistures, Atterbergs, UDWs, gradations, hydrometers, pH tests, etc. Sometimes the lab test results don't tell the whole story; you have to get your hands dirty.
But let's not get too wrapped up in the testing. Remember that the point of doing the tests is to provide meaningful data to the designer. Slope stability design is different than many other design assignments: the weak zones are the key, not the "average" condition. Hence my interest in the lower-bound sand strength results.
PEinc:
You have a good point: the original question did not indicate that longisland has a great deal of experience with slope design. I chose to give longisland the benefit of the doubt based on some of his other postings, and assumed that he was having trouble because he wasn't getting good samples. That may have been a bad assumption on my part.
BigH:
As usual, a very helpful posting. It is well worth reading.
RE: Soil testing for slope stability analysis
thanks for the input.
I'm working under a geotech engineer with 20-year-experience. Testing is not my field to begin with. I was assigned to monitor the field tests on site. To my dismay, most borehole(mazier sampling) came out with very poor samples, low recovery & low RQD. It's frustrating to collect only disturbed samples when I need intact ones for triaxial tests.
I've consider collecting block samples but the layer of interest is around 6 to 10 m.
In short, I'm looking for ways to collect undisturbed samples. I've discussed this matter with the geotech engineer in charge & some of my geotech friends; in fact we have something in mind.
On the other hand, this forum has been one of the most helpful resource; I believe there's always some better ways to solve a problem.
again, thanx a bunch
RE: Soil testing for slope stability analysis
The triple tube should have worked, and probably still is your best bet for getting an undisturbed sample. I had to do some looking to find out what the mazier sampler is,it is very similar to the Christensen 94mm sampler we use here in the states. If I'm not mistaken Atlac Copco bought out Christensen. There are a couple of reasons why you had disturbed samples:
1 The shoe on the end of the inner barrel is too long (this is the most likely reason). Try a shorter one.
2) You have the wrong type of bit on the outer barrel for the formation you are drilling. In formations that have a tendancy to wash-out i.e.; dense sand and soft sandstone, you should use a face discharge bit.
Also there is an option on that barrel to use a thin wall "shelby" tube. This may work but you probably will have a hard time extruding the sandstone part if the sample.
Core recovery in soft and broken formations is as much technique as anything. Like I said in my earlier post try to find a driller that has experience coring in this particular formation, if you can.
RE: Soil testing for slope stability analysis
RE: Soil testing for slope stability analysis
I empathize with your problem. doc11 has given some good suggestions - sounds like he has good experience with similar equipment and techniques.
Since you were in the field during the sampling, do you think the poor quality samples were due to drilling techniques, "loose" soil conditions, or trying to sample within the shear zone? Or was it due to the lack of enough fines to provide a binder for the samples?
Keep in mind that your inability to obtain "good" samples may be telling you something (indirectly) about the site -
RE: Soil testing for slope stability analysis
I have the following to share with you guys and hope it will be of help:
1. I do echo the problems in sampling in terms of disturbed samples etc... and SPTs were mostly refusal with some weaker layers every now and then.
2. I very much favor Focht3's opinion in terms of getting to know the strength of the weaker layers as the system is as strong as its weakest link.
3. Must keep a close eye on the driller as he could botch things very easily if he is impatient or inexperienced.
4. Initially I ran several direct shear tests on the relatively weaker layers. No matter what the complaint is about such a test it is still a practical and useful test and in my opinion quite indicatif. Unfortunately Geotechnical Engineering is still very much an art and as such we should try to favor the practical side of things rather than the scientific side (although I have nothing against it, but sometimes too much theory pins us down).
5. For the cemented layers, I sent a double tube split barrel 101mm dia. with a diamond bit, slowed the RPM and the only pressure applied was basically the weight of the rods, shortened the run length and no water pressure. It was not a great success but did manage to get a couple of samples for a U.C test (perhaps a triaxial test would have been better but I had no access to one at the time, but shortly we will own one. That established sort of an upper bound (although it is expected to be higher than that).
6. Once the lower and upper bounds were established, I ran a sensitivity analysis (basically an if-so type analysis) and each case was considered separately. Whenever the weaker zones were of significant thickness (in the order of 2m and up) I utilized the lower bound parameters, otherwise I used average + standard deviation.
RE: Soil testing for slope stability analysis