Stability of Reservoir Bank 1

[CED]

Hello. I've got an interesting slope issue dealing with a bank along a reservoir. Here's the background:

Our firm has a client who is proposing to develop a flat plateau that overlooks a reservoir to one side and a ravine to the other. The slope along the reservoir ranges from 20 to 30 meters in height. The reservoir is located within a wide mountain valley. The dam associated with the reservoir became operational in 1978.

The valley is very wide, with a very gentle slope from the toe of the bank to the center of the valley where the river is located. Prior to the construction of the dam and flooding of the reservoir, the valley floor was used as an airstrip.

Topographic maps of the region, a recent survey of the toe of the slope (at low water levels) and info from the operators of the reservoir show that the high water level within the reservoir only comes up to about the toe of the slope, where a small bench/beach is located (about 3 meters wide) before the very gentle grading toward the center of the valley/reservoir. The face of the bank in question is vegetated by mosses, grasses and an occasional tree. There was no visible indication of creep of the slope.

The bank is at a constant slope of about 24 degrees, except for a small region that juts out toward the center of the reservoir that has a slightly steeper slope angle of 30 degrees.

A review of air photos dating back to 1971 show that the bank in question has not changed since that time. A site inspection did show some minor surface slumping located just above the high water mark in the region with the steeper slope, likely due to wave action from the reservoir when full.

From the air photos and reviewing other slopes in and around the site, I feel that the bank below the proposed development site is likely already at its stable angle of repose in the locations with the shallower angle.

Now for my question:

Time constraints and client funding limit a full-scale, long-term slope stability analysis. As such, I'm wondering whether or not there are other methods for determining a safe top-of-bank setback guideline for a development in this location, without actually accessing the stability of the slope itself. I'm familiar with the setback guideline formula developed by Dave Cruden for valley slopes, but it may not be applicable in this case.

Thanks,
CED

 

[GeoPaveTraffic]

I would run the existing slope through a slope stability program and adjust the strengths to reach a factor of safety of 1.0 or slightly higher. Then the proposed developement can be added to the stability model and the setback adjusted until the calculated factor of safety does not change from the existing conditions.

Also, congradulations on a well stated problem. I expect that you will get several recommendations/comments.

 

[VAD]

YOU may have a situation for which many may tink it is a waste of time to undertake an investigation as the area has been stable for a long time. I presume that the development is building related. In that case you still have to be careful. The aspect I would address is what are the likely changes to result from the proposed development. No one want to place a building right to the edge of a slope irrespective of how stable the slope may be. I would use at least 10 m setback from the edge of slope. On the other hand one can play with slope stability analyses.A FOS of at least 1.5 is recommended for building development. Chart solutions for toe circles can be used as well. Use setback distance where circle with FOS of 1.5 cuts top of slope.

 

[Mccoy]

CED,
do you have an idea of the geological nature of the soil making up the plateau?
One aspect I would consider, if the devolepment is built to last, is the very long-term (at least tens of years)stability of the slope edge, due to morphological dynamics and rate of erosion which, albeit slow, inevitably will tend to cause a regression of the plateau's top. Also, the development may interfere not just in terms of load, but in terms of anthropic water discharge and related problems.
The initial setback may eventually become smaller and smaller...

 

[cdh61]

CED
?
You should evaluate the site based on slope stability. But you most likely will find the impact from the development on stability to be negligible. Toe erosion may not just be caused by wave action, soil (toe) softening may also occur as a result of high water levels, thereby causing shallow
sloughing along the toe. I assume the ravine is also near 20 to 30 m in height? In this case, a setback distance of a few meters will have little to no impact on stability since the slip planes of an overall large failure will extend 10s of meters from the top of bank. Shallow failures near the top of bank can also occur without a large deep seated failure. Therefore, an assessment of
stability (and soils) will be required, and should be completed for safety.

regards

 

[Focht3]

Make the time to perform the analyses. While the impact of the new development may be negligible, you have a professional responsibility to consider the possibility that the slope could fail.

My ex-partner and I fought over this very issue about 9 years ago. He took a job - over my strenuous objections - that involved the design of an apartment complex on the bank of an active river. The client wouldn't pay for a slope stability study; so he put that in the proposal, disclaimed all responsibility for the slope, and provided foundation design recommendations for the apartment buildings on the edge of the river.

As I understand the course of events, many of the foundations failed before the project was completed. And his firm was sued. (We had already parted ways.) His liability insurance apparently settled rather than going to trial.

Don't ignore the risk - it could prove a costly mistake...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

to Focht3. It is important. For a small house, I might use rule of thumbs - but for a large apartment building - never - and don't put it too close to the river unless you have "bedrock" or dense till!

 

[CED]

Hello all. Thanks for the responses.

The geology consists of about 12 feet of silt/gravel till overlying siltstone bedrock.

For my own peace of mind, and on the advice presented here, I went ahead and performed a basic slope stability analysis using GSlope. Considering that our test pits at the site were fairly shallow (down to the bedrock at about 13 feet), I used some conservative values (lower than normal friction angles, etc.) just to see how things would come out.

The analysis came up with a setback in the order of 17 meters from the top of bank in order to achieve a factory of safety of 1.5 (the factor of safety at the crest of the slope was also just over 1.0). This setback at a FOS of 1.5 was about half of what I had computed using the Cruden formula, so I think I'll go ahead and utilize the formula as it seems to be quite conservative for this application.

Thanks again to everyone for the replies. The helpful advice and knowledge/experience of the people who regularly use this site continues to impress me!

Cheers.