Slope stability evaluation

[nagatalluri]

Guys

Can you please suggest a geophysical method for evaluation of slope stability (doing a geophysical array to get the soil properties along the slope). In my understanding seismic refraction and MASW are the most famous techniques that are used for geophysical exploration related to geotechnical applications. Can any one comment on which one is better for evaluation of slope stability in Clay soils.

Thanks
NT

 

[oldestguy]

About the best I can recall as to the use of geophysical methods in site investigations would be the electrical resistivity and the seismic methods. However, here is a general link with respect to oil exploration listing these and others.

http://dnr.louisiana.gov/assets/TAD/education/BGBB/5/techniques.html

My experience has been mostly using these methods for finding depth to rock. I doubt that they would be of much use for slope stability evaluations other than finding general properties, With test borings as a reference next to a resistivity or seismic investigation, it may be possible to extrapolate that data beyond the boring locations.

 

[Mccoy]

Geophysical methods are NOT for evaluation of slope stability.
As OG suggests, they may be used to estimate depth of bedrock (refraction) or Vs,30 (MASW, REMI) in seismic areas (simplified local seismic response). Or maybe depth of GW level.
Depth of bedrock wopuld be useful in some instances to have a feeling for the geometry of the problem but not for quantitative analyses.

Vs derived from MASW-REMI may be used to estimate Go and then Eo but I would never dream of using such parameter In FEM methods, because the Elastic moduli from geophysics represent a spatial average in function of the wavelenght or Rayleigh waves involved, so they may totally ignore weak layers. Also, Poisson in geophysics is different from Poisson in geotechnical problems and last but not least an adequate degradation scheme is then needed, which would not be so clear for slopes.

NO WAY to estimate phi and c' or Su from geophysics, I read some reports of people doing that from MASW investigations but that's just insane, as an expert witness in court I would hammer them straight on the head with the same 18-pounds hammer they use to produce surface waves.

http://www.mccoy.it

 

[killswitchengage]

Well to my knowledge , you can use those tests just to try and approximatly find the failure surface
It may come as a change in density or resistivity profil

 

[nagatalluri]

Sir

Thanks a lot for your response. We are planning to do a bore hole on the top of slope and one at the toe of slope and MASW over the slope to get understanding of the slope stability. The main reason for doing an MASW is cost also as it is cheap compared to a bore hole (and lab testing) to get soil parameters.

Do you suggest any specific investigation procedure to better evaluate slope stability for slopes in clay.

Our project is located in Atchison county, Missouri near Tarkio.

Thanks
Naga

 

[Mccoy]

Guys, just to make things clear.
From the OP question:

Can you please suggest a geophysical method for evaluation of slope stability

I answered NO, in relation to the choice of the input geotechnical parameters.

Of course geophysical investigation can be and is advantageously used to investigate wide areas and define the landslide mass in relation to borehole soundings with an economic optimization. I agree on that.

The use of various methods can be tricky though. MASW is accurate only in the surface layers, then the reliability drops dramatically, depending upon the layering. if there is a contrast of impedance (rigidity contrast, rigid layer underneath a soft layer) The energy of surface waves is mostly reflected so no way we can accurately predict what's underneath the contrast.
REMI tends to be more accurate in deeper layers
Refraction is pretty good until there are no velocity inversions, which the method ignores. Also in refraction energy is dissipated soon so we need a dense arrangement of survey lines, 3D tomographic refraction being the best, at a co$t.
Nakamura microtremors are pretty economical with the right instrumentation but are not carried out everywhere I don't know even if in Japan, the birthplace of Nakamura, they are being routinely carried out.

Nagatalluri, I would suggest you read the linked article, which is a pretty good review. Also, you might want to consult a credible service company explaining clearly your specific needs, the conditions on the terrain (steepiness is also a factor) and what they can do for you.
It is much different if there is a shallow bedrock or competent layer or if there is not any. In the latter case, geophysical investigation may not always yield credible or useful results.

https://hal.archives-ouvertes.fr/insu-00522819/document

http://www.mccoy.it

 

[Mccoy]

If the URL I linked above does not open up, it doesn't from my PC, I'm attaching the article here

www.mccoy.it

 

[killswitchengage]

@nagatalluri : could you be more specific , are you studying the susceptibility of a given slope to failure or has the slope already failed ?. I believe stability of slopes in clays depends on their geometry( Height , inclination,Beta etc ) and the existence or not of discontinuities such as sand/silt beds that may liquefy or prove to be aquifers . the possible existence of a rock bed under +/- shallow depth can affect the overall stability . Anyway, all in all you must conduct lab tests with samples , relying on seismic refraction methods can be useful especially in wide areas where multiple boreholes can be costly. Resistivity methods are highly regarded as i was told , it can give you a 3D map of the soil profil . But dont ever rely on these last two.

 

[nagatalluri]

Sir

Thanks for your response. Regarding the slope we have the slightest of information now. Based on the local geology the soils are clays (Glacial). The main intent of studying slopes is to understand the effects of a wind turbine foundation on the top of the slope and associated risks to clay slope stability. As explained in my previous posts we are planning to do boring at the top of the slope and toe of the slope. For the slope we are trying to use Geophysical method to get an idea what is below along the slope because depending on slope extent doing multiple boreholes may be expensive.

Any suggestions??

Thanks
NT

 

[nagatalluri]

Mr. Mccoy, I really appreciate your time and effort in sharing the case study paper.

It was really helpful for a basic understanding of geophysical methods for slope evaluation.

Thanks again

NT

 

[killswitchengage]

Ok now i get it . First of all , try to study the stability of the slope like i stated using only static loading ( the soil with and without the turbine ). After that proceed by doing a dynamic study of the site . Here , seismic refraction can help provide you with dynamic modulus of the soil for ex dynamic poisson coefficient and dynamic shearing modulus etc . Try to study the impact of dynamic loading from winds energies transferred to the foundation . If by any chance there is just a tiny sand liquifable layer you are risking a lot by ignoring it . dont forget to do also an earthquake study too

 

[GeoPaveTraffic]

No offense intended, we all had to learn sometime; but learning the basics on an internet form is not a good idea. I hope someone involved with this project has some experience with slopes and slope stability. I have worked on several landslide in that part of Missouri and it takes experience. I really don't need the experience of fixing a slide with a wind turbine on top of it.

As for your original question about geophysics, I cannot think of any geophysical method that is likely to give any useful information for slope stability analysis that isn't gained from the required borings and laboratory testing. I know that it is difficult to drill on a slope, however, if you have a slide; you often have to drill where it is difficult.

Best of luck.

Mike Lambert

 

[Mccoy]

I'm going to attach two examples of refraction seismic profiles along unstable slopes. I was not directly involved in these projects, I've just been recently discussing them with the interested colleague because they are located not far from my place. Usually the profiles are checked by borings. Site 2 has a very good correspondence between borings and seismic layering.
The geotechnical parameter represented is Vp, velocity of compression waves, which is not useful for landslide stability analysis by numerical method.

Nonetheless, we have a soil profile along a landslide section which would be very costly to acquire by borings alone. The profile must be interpreted by geophysical engineer, geotechnical or geological engineer and must be of course coherent with the borings and all other known evidence. As oldestguy already said, that's basically an extrapolation process. Not all profiles are good, depending on the operator and local conditions.

These are 2D profiles, 3D models yield a tridimensional representation of the landslide body, the more the detail the higher the co$t.


www.mccoy.it

 

[nagatalluri]

Mr. MaCoy

I could not download the attached file. Can you please email it to talluri.tinnu@rediffmail.com.

Regarding comment from GeoPave, I am only trying to get an insight based on members experience. In fact, we have a geotechnical consultant with Geophysical capabilities to do the investigation. Before they recommend a method, I am trying to understand which geophysical methods are apt with respect to price and past experiences so that we have an engineered judgement (which is acceptable) rather than a correct judgement but very expensive.

Thanks
NT

 

[Mccoy]

NT, sent, pls advise if it doesn't hit your mailbox.
I myself cannot download the file I attached in the previous post, trying again here.


www.mccoy.it

 

[Mccoy]

Again, the link to the uploaded document is a blank one. Don't know what's the prob, going to work it out another way Tomorrow, by linking the images from an image sharing site.

http://www.mccoy.it

 

[Mccoy]

From my preview I can now see directly the images. In the first one the irregular layering of the landslide in process is evident, as the blue more competent strata are also evident.
In the second one the flowing body (in red colours) is also evident (confirmed by borings), whereas the stable mass is not much more competent than the unstable one (about 50 m/s Vp in difference).

http://www.mccoy.it

 

[oldestguy]

NT: Once you have soil strength data, ground water information, etc.with an analysis, perhaps you might spend some time with topography and potentials for increased moisture content and resulting future weakness. From my experience while today a site may be stable, future changes in ownership, vegetation, topography and drainage can change that stable situation drastically.

 

[nagatalluri]

Oldestguy

Thanks for your response. I am wondering if you have encountered any situation where the water table depth is shallower on the slope than at the Toe ? The water table elevation at toe is around 30 feet. The reason is if this is the case then the type of geophysical technique to be used must be chosen carefully to prevent any misinterpretation due to WT.

McCoy:

If i understand correctly, the first image is P-wave velocity whereas the second image is Shear wave.Can you comment on the technique used in the both situations, I mean which technique was used ?

Thanks
NT

 

[oldestguy]

NT in answer to the question: Yes. The reason I give for finding the water elevation at toe of slope as higher in a few cases I have seen is due to the effect of variable infiltration of surface water. In these cases the soil was generally a layer of weathered loess type clay over silty glacial till and the slope diverting rain water rapidly to flatter areas where it soaked in. Since depth obviously is more under the slope, I used elevation as the reference. In my case I was evaluating potential contamination due to a future landfill at these areas.