Assuming inclined or horizontal layers in stability models

[geotechguy1]

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?

 

[fattdad]

Making stuff up is not part of generally-accepted geotechnical practice.

f-d

ípapß gordo ainÆt no madre flaca!

 

[GeoPaveTraffic]

I agree with fattdad, don't make things up. However, is there a basis for using and incline? Is there any geologic information that would indicate how the material slopes? If you are dealing with a coluvium slope, then assuming an incline is very reasonable.

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

 

[geotechguy1]

Assuming that the layers are horizontal is also 'making stuff up', is it not? Both are just assumptions. From my own experience at the beginning of my career, it appears that geotechnical engineering practice is built on a bedrock of making stuff up, or at least that the line between 'making stuff up' and 'engineering judgement' is so blurred that I doubt you could convince a judge of one or the other. I'm certain that if you posed a geotechnical question to 20 consultancies in my province, you would recieve 20 different answers, most of them contradictory.

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.

 

[GeoPaveTraffic]

I do not have a specific reference for performing a geologic recon. However, any experienced geotechnical engineer or geologist should be able to perform one.

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

 

[PEinc]

The boring locations should be staggered so that you can get some idea of the sloping of the strata.

http://www.PeirceEngineering.com

 

[emmgjld]

There is an underlying theme to the original & subsequent posting by geotechguy1 which concerns me.
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.

 

[geotechguy1]

In our area 'accepted practice' for about 80% of the firms in the industry is that these assessments are done either with no drilling and assumed soil based on surficial geology maps, with soil parameters based on engineering judgement/local experience (aka, making stuff up), or a single borehole. Actually, I believe for some of the major river valleys the setbacks are established based on research in the 70s and 80s that was done based on aerial photography. They worked out the 'ultimate angle' of slopes along the valleys and then determined the setback by projecting a slope with that angle out from the current toe. Later some developers pressured the cities to allow them to build houses much closer to the bank edge than originally recommended, ultimately leading to several houses losing most of their rear yards to landslides and some costly litigation.

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...

 

[EireChch]

We hand drill two 5m deep boreholes. One above and below the building platform for a house. Shear vane testing and penetromerer testing 2m from bottom of hole. Probably a days work for one technician.

 

[PEinc]

Five meters seems too shallow for a slope stability analysis.

http://www.PeirceEngineering.com

 

[geotechguy1]

For my case the slope height is about 10m and we drilled 15 which I think is reasonable. The consensus at various firms I've worked at is to drill to 1.3 to 1.5 times the slope height. I also drilled a shallower hole 15m back from the crest at the house location to about 6m.

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.

 

[EireChch]

The soils were Waitemata Residual soils in Auckland NZ, weathered from sandstone and mudstone rocks. Competent soils (which we classes as 10 blows per 50mm) were usually encountered within 10m and more often within 7m, hence why we drilled to 5m with shear vane testing every 0.5m and then 2m scala penetrometer off the base so we had geotechnical strength profile for at least 7m. If we didnt find compentent within 7m we can hand drill to 6-7m depth however this becomes a bit difficult as raising a 7m long auger out of the ground to ly it down and take a sample was a challenge. Soils were generally stiff to hard 50kPa to 200kPa+. Occasionally you may get a soft spot but the soils were generally good. Drilling to 5m with penetrometer off the bottom and logging would take on average 3-4hrs for a competent technician.

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

 

[GeoPaveTraffic]

geotechguy1,

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

 

[epongra2]

If indeed there is an old slide on the slope, then inherently there is at least one surface or layer that isn't horizontal, and it also happens to likely be the most critical with respect to future stability.

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.

 

[GeoPaveTraffic]

epongra2,

Just a note, you can have a failure plane on with flat soil layering.

Mike Lambert

 

[epongra2]

GeoPave,

Of course, but then there is that head scarp area. That flat failure surface at depth has to break towards the ground surface somewhere.

 

[BigH]

The bottom line is to do enough borings and sufficiently deep to properly characterize the subsurface conditions. See attached for a cross section from Krynine & Judd. As shown, stratigraphy can be quite complex.

 

[Okiryu]

Wow, that is a very nice section. And it was done in the late 30's! I guess the computers made us lazy and less detail...

 

[geotechguy1]

BigH,

Please tell me where I can find clients with the budget to pay for that many borings!

 

[BigH]

Well, that is the issue for all geotechnical investigations really, isn't it? As the plethora of geotechnical organizations have grown with cut-throat competition and prices, then, rather than results drive many project awards, the owner ends up getting what he pays for (or doesn't pay for). Remember that your reputation is on the line if you accept projects with risks far outweighing the potential benefits.

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.