Assuming inclined or horizontal layers in stability models
Assuming inclined or horizontal layers in stability models
(OP)
Recently I've been tasked with various slope stability assignments, usually pertaining to development of subdivisions or acreages in rural areas where it is necassary to establish a required setback distance from a slope.
Typically we drill boreholes near to the slope crest, to depths extending below the toe elevation of the slope. I've built up most of my models based on the assumption of horizontal layers based on the profile encountered in the boreholes. A colleague of mine uses inclined layers, although he 'makes up' the inclination.
Any thoughts?
Typically we drill boreholes near to the slope crest, to depths extending below the toe elevation of the slope. I've built up most of my models based on the assumption of horizontal layers based on the profile encountered in the boreholes. A colleague of mine uses inclined layers, although he 'makes up' the inclination.
Any thoughts?





RE: Assuming inclined or horizontal layers in stability models
f-d
ípapß gordo ainÆt no madre flaca!
RE: Assuming inclined or horizontal layers in stability models
Additionally, analyzing a slope based solely on information at the crest is a good way to get into trouble. At a minimum you should have geologic recon data for the slope which will aid in developing your stratigraphy.
Good luck.
Mike Lambert
RE: Assuming inclined or horizontal layers in stability models
Do you have any references for geologic recon? I did walk the slope and take some gps survey data, and from that I've ascertained that there appears to be a evidence of a historical slide (the upper slope is steeper, with a bench that is back-rotated towards the upper slope, followed by a lower, flatter slope, with a small stream running adjacent to the toe). Although based on the tree cover it must be a relatively old slide on a human timescale.
RE: Assuming inclined or horizontal layers in stability models
As for your observations during your site walk, sounds like you got a tiger by the tail. If you suspect an old landslide on the slope, you have a lot more field work, including additional borings to perform.
Mike Lambert
RE: Assuming inclined or horizontal layers in stability models
www.PeirceEngineering.com
RE: Assuming inclined or horizontal layers in stability models
You are producing models for slope stability analysis in which the attitude (strike & dip) of geologic (old & new) bedding is not known well enough to approximate. This is not proper, plain & simple. I have been in a lot of situations where accurate bedding attitude was difficult to define accurately, but to not be able to represent it on the model with proper orientation, boggles my mind. I am sorry to sound so harsh, but this is how I see the issue.
And now you are in a situation with a suspected existing slope failure in which modeling includes a feature upon a modeled feature, This is not a case for 'rough approximations'. You need to get with someone who understands THOSE PARTICULAR SLOPES & GEOLOGY. Please excuse the shouting.
RE: Assuming inclined or horizontal layers in stability models
Anyway, I'm getting off topic. It sounds like the best I can do is run some sensitivity analysis with different beddings. It sounds like others work in areas where drilling must be substantially cheaper or where there is some regulatory imperative to do more extensive investigation work. Alas that isn't the case in my area. Perhaps I should relocate...
RE: Assuming inclined or horizontal layers in stability models
RE: Assuming inclined or horizontal layers in stability models
www.PeirceEngineering.com
RE: Assuming inclined or horizontal layers in stability models
What do you use to hand drill and what are the soils like? We have a hard time hand drilling anything over 5 ft. Maybe we need beefier techs.
RE: Assuming inclined or horizontal layers in stability models
We drilled with a 50mm dia hand auger. Similar to a post hole auger (not a helical type blade), you would drill down approx 0.15m and then spin the auger without any pressure to "snap" off the sample.
this was the standard for a residential house
RE: Assuming inclined or horizontal layers in stability models
I think you have hit on the real problem. Too many geotechnical engineers, at least in many parts of the US, are doing work that is substandard from a technical point of view. From a legal point of view they are fine since they are meeting the incredibly low standard of care that has been established over the last couple of decades.
So you have to ask yourself if the fee you are getting for doing this kind of work is worth the hassle of eventually getting drug into court and having to explain why you did what you did and why you didn't do more. It will be even harder to explain if you really would like to have done more, but just didn't want to ask the client for more money to do the job right.
This is the reason I don't do residential development work and current do very little work for any developers. They are, by and large, only interested in getting the bare minimum done as required by regulators or the structural/civil engineer they have hired for the project.
Best of luck.
Mike Lambert
RE: Assuming inclined or horizontal layers in stability models
Depending on the degree of weathering at the surface (i.e. time), knowledge of local geology, the quality of topo data, etc., it is not all that difficult for an experienced professional to draw a section through the slide and reasonably infer / draw the basal slide surface or surface(s), at least in the 2D sense. In lieu of the appropriate type of investigation, it is better than nothing.
RE: Assuming inclined or horizontal layers in stability models
Just a note, you can have a failure plane on with flat soil layering.
Mike Lambert
RE: Assuming inclined or horizontal layers in stability models
Of course, but then there is that head scarp area. That flat failure surface at depth has to break towards the ground surface somewhere.
RE: Assuming inclined or horizontal layers in stability models
RE: Assuming inclined or horizontal layers in stability models
RE: Assuming inclined or horizontal layers in stability models
Please tell me where I can find clients with the budget to pay for that many borings!
RE: Assuming inclined or horizontal layers in stability models
I was involved in a job in a western Canadian province that I was not "permitted" to bid on for the GeoE work . . . 3 "respected" geotechnical firms were requested to provide proposals for the work . . . a high profile transportation project. One firm came in at 10k, one at 30k and one at 75k. I was, though, asked what I would have bid - without any thought I said over 50k. The highest bidder got the project and it was fortunate in that several issues were discovered under the more extensive investigation. This particular owner realized that the higher priced firm had sufficient funds to do a proper job and they got the job. An educated owner.