Slope Stability Assessment in the field 10

[Geo66]

I am a new graduate with a site assignent and looking for some tips on slope stability assessments in the field. Other than geological build-up, stratification, slope materials and surface water conditions, what else would you check for stability assessment in the field? Any instability signs that I can look for?

Any help is much appreciated.
Regards,
Geo66

 

[Stego1]

Hi

You can look for tension cracks and slip scarps along the top of the slope.
Tree trunks, fence posts etc can also indicate past or ongoing movement.
Try looking at the slope from a distance as well as close up.

If you are expecting movement during your site assignment maybe you could knock some stakes in at various points on the slope and measure their angle two or three times a day with your clinometer. Also you could install a number of marker pins over the slope and get them surveyed in. then survey them again two or three times a day to measure any vartical or lateral movement.

Hope this helps

 

[epongra2]

As an engineering geologist, I would strongly suggest getting ahold of both an engineering geology and a geomorphology text to familiarize yourself on these matters.

Stego1 provides some good advice for a recent slope failure or one that is currently moving. But, how do you begin to figure out if a slope has moved in the past, say hundreds or even thousands of years ago when much of the evidence has eroded or disappeared with time?

With all due respect to Stego1, be careful with using tree trunks and/or fence posts. All slopes, i.e. both natural and man-made, are affected by a natural process known as creep. Creep is a near-surface phenomenon, and it isn't technically viewed as a slope failure.

 

[Delsks]

I think Stego1 has given you some good advice especially when it comes to tension cracks. While bent trees and fence posts may not indicate a text book slope failure if the topsoil is creeping in such a way to move trees and fence posts building foundations on (or through) that material may require extra thought!

It is a tough question you ask, and to answer it confidently you really need to know the history of the area. You need to familiarise yourself with the way the ground behaves in that area when it does fail. Sand, Clay, Limestone, Granite will all fail in different ways you need to look for early indications.

I don't mean any disrespect (as I found myself in a similar position when I started) but sometimes you need a bit more experience to make those calls. Try spending time with other engineers or geologists who know that area and know what to look for.


Good luck with it.

There is no such thing as gravity!!!!
The Earth Just Sucks

 

[msucog]

look for signs of activity by man (not just recent activity)...(we/man do tend to screw up everything in some form or fashion over time).

 

[Geo66]

Thank you all for your valuable suggestions. My findings will be reviewed with a senior engineer in the field before making any conclusions. You are absolutely right Delsks, I have a long way to go..

By the way, how can you tell if tension cracks are serious in nature or not?

Thanks again.

 

[Stego1]

You can measure their rate of growth both across the crack and in depth. Also if there is evidence of water ingress or the potential for water ingress. If they look dodgy remember to stand upslope of them ;o)
Regarding trees etc, I agree that creep can affect these in the long term however they are still worth thinking about in the context of the slope you are looking at. The best thing to do is have a look and then think about what everything is (or might be) telling you.
As mentioned above, there is no substitute for a more experienced colleague who can pass on their knowledge. Buy them a beer or two seems to work quite well!

 

[Mickney]

Stego1 offers great advice. However, dont confuse tension cracks with desiccation cracks. Make sure you walk a good distance away from the crest, particularly for very tall slopes. Tension cracks associated with global movement will form some distance from the crest. Note any load applications that are new or were not part of the slope stability analysis. Note the cover on the face of the slope. Also, if you suspect movement, you could install slope inclinometers through newly advanced boreholes. RocTest is a group that could help with the materials and reading instruments.

 

[Riggly]

Slope stability problems are not always obvious. Slow slope movement could be as little as a fraction of an inch a year, to sevaral feet per second(the latter is an obvious failure). Abramson, Lee, Sharma, and Boyce in "Slope Stability, and Stabilization Methods", list a number of factors that can trigger slope failure (think a lot of it was take from Varnes - landslides ). Maybe you can look at those, and see whether any of them can impact the site. You mentioned surface water condition, but you can specfically look at the drainage condition- how is water draining from that slope-is it concentrated? I observed a signicant circular failure once in the field, and I noticed that the scarp was located almost at the end of a broken curb at the top of the slope. Sometimes these failures are not always easy to identify because there can be combination - for example the failure had caused a crib wall to collapse. The question was, was it the crib wall that failed or the slope? The other thing that might might not be very easy is determing whether the failure is shallow or deep. Is the area fill or natural soil? Look at the condition of the toe of the slope- is it water logged, is there cracking or toppling of retaining wall (if any) at the bottom of the slope? Is there undermining? Is the slope vegetated? What kind of vegetation? Are they fibrous roots, shallow, deep? Is there any heavy structure closeby? What is the slope angle? What type of slope: natural, reinforced? Take as much measurements as you can, getting a model on paper does help. Can you get soil borings at the top and toe (is it accessible?) Slope stability issues is not easy to show on photos- but they may help.

 

[Patgeotech]

Just one thing to add to the brilliant advice above, I would suggest (if the site allows) that you speak with the people that live in the area - they know what has been going on in the area. On a very few occasions this has helped me to solve the problem.

As a graduate, you need to be exposed to as many senior guys as possible as we all have our different ways and manners in the field. Do not be scared to ask questions, even if they seem stupid, sometimes that question may have been overlooked by your geotechnical team and most importantly - use your common sense, something that is lacking nowadays.

 

[Bluefoot]

There are a series of open publication online which may be helpful for your question. It is the Layman's Guide to Slope Maintenance by CEDD of Hong Kong.

 

[liutenantz]

hello,
do you know JinPing-I hydropower station in Sichuan,China?

 

[fattdad]

According to the original post you would first have an understanding of the geologic setting (i.e., bedrock, strike and dip of rock layers, schistocity, soil layers, bedding angles, soil types, whether marine, lacustrine, alluvial, glacial), you would also have some understanding on the nature of surface water flow (i.e., whether there is some overland flow running down the slope, at the toe of the slope) and you would have some understanding on the percent grades down slope. You would also have some understanding on the location of the top and toe of slope.

Important to this whole process is also the overall objective of the slope assessment. Are you building a reservior and want to know whether innundation will trigger slides, are you doing an excavation at the toe to build a road, are you constructing a building at the top, etc.?

If you have the information in (my) paragraph 1, I'd go to the field and look for hummocky topograpy, I'd look for tension cracks, I'd look for bent trees (notwithstanding the earlier comments) and I'd bring a hand auger to confirm desktop soil review. I'd also look at other slopes in the area and document slope grades in similar geologic settings using a clinometer (angle measuring device). If you have a 24 degree slope but other (seemingly) stable slopes are present at 31 degrees, that's good information to document. What this means depends on the geologic setting, however, as a steeper slope may be stable when it crosses bedding as opposed to when it's parellel to bedding.

Slope stability can be a challenge. But a fun one!

f-d

¡papá gordo ain’t no madre flaca!

 

[epongra2]

The assessment of air photos and topographic maps is invaluable in the process, IMHO. Not enough people look at these, and I don't know why.

Also, do not get into the all too familiar habit of simply looking at the subject property. If the site is on a large slide, the ground at the site may show no evidence of movement. However, if you step back and start looking at a much larger area, the signs begin to reveal themselves.

 

[dirtsqueezer]

I agree with epongra about air photos. That should go back to your college experience. I had two classes on that myself, geologic hazards and, well, arial photography. Also geomorph, I guess. One thing I did when I started out with slope stabilities here in the Puget Sound is go to your local engineering department. They will have on public record the reports of local engineering firms on the very same topic. It gives you a little more experience on the topic in general as well as your local geology. They were free for me to look at. My region actually puts out slope stability maps.

http://www.ecy.wa.gov/programs/sea/landslides/maps/maps.html

Worst case scenario is that you have some ancient slippage plane to worry about. The maps would be a good guide, but if it's a bad area, they could recommend boreholes. Anyway, there's lots of information on the web about the subject. I would go with your local engineering office, though. Free and gobs of info. Good luck.

 

[Bluefoot]

Hong Kong has a well illustrated GEOGUIDE 5 Layman's Guide for Slope Maintenance. Some of the material may be helpful.

 

[roxndrt]

I would enlist the services of a good engineering geologist, one with extensive field experience if possible. Indications of past instability / ancient landslides can be subtle and easily missed by the inexperienced eye. We have a saying in my company that geologists are very useful -- every engineer should own one (apologies to any geologists out there).

Do a thorough search of resources that might be available for your area -- geologic maps, agency archives, City or County hazard mapping... Some cities, for example Seattle, have GIS databases of historic landslides. There's nothing quite so embarassing as missing a landslide feature that has been mapped in the published literature.

Topo maps and aerial photo stereo pairs are good resources, but again interpretation requires a trained eye. Some areas have started to do LIDAR mapping, which is much more detailed and very useful for evaluating potential landslide areas. You might want to check and see if any LIDAR imaging is available for your area.

Best of luck in your study.

 

[HouseBoy]

Here is a link to a landslide group in Cincinnati, Ohio. Not sure if it is convenient for you but they have a workshop on field identification scheduled for October.
I know the presenters and they are very knowledgeable.

http://www.ohiovalleylandslides.com/