Temporary cut slope on dense silty material

[AlexWong122]

Hi all,

I recently joined a new company and one of the mandate is to inspect temporary excavation slope stability for house basement construction. Due to the space limitation, I was told that for slope height less than 8', the common practice is to make the temporary cut slope about 1H:7V (on dense silty material)and cover the whole slope with poly liner and call it good for the next 30 days (if it rains heavily then the slope need to be re-inspect). I am a civil engineer and my past experience is usually a temporary cut slope of 1H:1V for soil and 1H:4V for rock. I don't know too much about geotechnical engineering but this is much different than all my previous experience. However as I go around the city (Vancouver, Canada) I do see a lot of sites (maybe 50%) are using this cut slope (i.e. 1H:7V) for soil.

Attached is one of the example.

Can anyone share some knowledge to me with this topic?

Thanks,

Alex

 

[oldestguy]

I'd check the Province and any other jurisdiction codes on this subject. I'd not think you will find any "temporary" slope rules. Consider all steep slopes as possible risks, even hours old, without actual test of the soil properties unless the code says otherwise. Contractors may "get away" with steep slopes until that fatal slip occurs. With actual tests, such as pocket penetrometer readings, you may go wrong unless you check for shrinkage cracks. I sp0eak from experience with a near fatal test pit cave-in.

 

[EireChch]

Those side slopes look too steep in my opinion. However I have been to many sites where i see them cut like that by contractors. I would always state that i recommend that the side slope be cut at 1;1 (45 degrees). The contractor will moan and most likely not do it but at least you have fulfilled your duty as an engineer (IMHO).

 

[bdbd]

Alex, this will always be a problem.

Because, even if you make some advanced direct shear tests, triaxials, and you are certain that your parameters are correct, and even you have performed most advanced calculations such as FEM, LEM, DEM or whatever, you may find that slope is unstable. But when they don't listen to you and cut it like that anyway, it may be stable for a while. And then, they will have a look on your face that makes you very mad, like "you know nothing, John Snow".

I am not sure why this occurs, and I haven't seen anybody who can predict the time of failure, so 30 days or 1:7 is very much based on the experience in the city. I would never allow that, but they will do it, and they will be successful. Some say that this is due to matric suction which holds together the soil as a cohesion like behavior. Maybe there is somebody here who can explain this.

 

[oldestguy]

The possible reasons for those cuts remaining standing likely are many. For instance, in many parts of the USA the loess soils have some degree of cementing and near vertical cuts remain for years, many apparently forever. A little cohesion goes along ways,even if only by capillary attraction.
This topic probably can be researched so that we actually can allow steep slopes based upon some measurable criteria that is not there yet.

 

[EireChch]

I have seen natural Loess faces that are vertical and stand 5m high. I wouldnt stand beneath one though.

I agree that it is capillary action / matric suction that 'bonds' the soil together however this is all lost when the soil becomes saturated.

Those guys have to have instances where some faces collapse or partially collapse. All it takes is a lump of soil, the size of a basket ball to fall on your leg and ruin your knee (im thinking an ACL rupture!) or break a bone!

 

[theCorkster]

The intersection between the theoretical and real world is the geo-professional's sand box, so to speak.

The true challenge is to evaluate what the actual soil parameters are that you should use to characterize the conditions and incorporate in your analysis of stability. Look to the influence of the geologic processes involved in the soil's development. I recently prepared an analysis for a temporary cut slope that by simple soil mechanic characterization of c and phi would not meet OSHA requirements. However, reviewing the geologic deposition process and aging of the material revealed that the undisturbed structure of the materials provided more cohesion than would be implied by a simple soil classification, and was more than sufficient to provide temporary stability during the construction phase. Visiting with some local engineering geologists will really advance your understanding of the unreasonable behavior of these slope based on incomplete understanding of the materials.