Temporary slope stability - Excavation 1

[richthorpe]

I've been set the task of designing a temporary batter behind a subway wall. It's got to withstand the standard highway loads. there is limited working space due to where the carriageway is and so I've suggested a 2:1 slope for about 2m from the subway wall then tappering down to 1:1. The soil is very stiff clay (cannot be indented with thumb). How can I check the stability of the slope. Would I use the standard formular for active pressure (Pa = Ka.gamma.h - 2Cu) and just assume a value for Cu? or is there another method of annalysis?

 

[GeoPaveTraffic]

You need to get a geotechnical engineer with experience with this clay to run a slope stability analysis of the cut condition. They will also need to know how long you expect to have the excavation open.

How tall is the cut? If the cut is very small, appoximately less than 10 feet (3 meters) the geotechnical engineer might be able to say it is stable based on experience.

 

[richthorpe]

unfortunately we can't get a geotechnical engineer in to assess it. The cut is to be about 4m high. I'm only really interested in assessing the 2:1 slope as i'm fairly sure the 1:1 slope will stand up fine. The slope is to be in place for approx. 5 months. I'm going to assume for the worste case that the traffic load will be directly above the top of the cutting! Is there a standard way of analysing this?

 

[eric1037]

If you absolutely must evaluate the soil yourself, WATCH OUT FOR SILT AND/OR SAND SEAMS!!! Even if the soil looks like it will stay up, a very small seam could cause a slump. Do you want to be responsible for the death of someone? If not, I highly suggest that you call in somebody with specific geotechnical experience to do the analysis.

 

[richthorpe]

currently we are having hand shear vane and shear box tests done to clarify what the soil is. Also boreholes and trial pits will be done to determine what the layers are. From visual inspection the clay is very stiff with small deposits of 6N or 6P structural fill. There must be theoretical equations to use in slope analysis. I've looked in a number of geotechnical journals and books but can't find anything to use.

 

[eric1037]

You can use a variety of methods for slope stability. Most geotech books have them. Most often, these types of analyses are performed with computer software packages. However, estimating the properties of the soil is really in the realm of the geotechnical engineer with experience in that area. Equations and software are easy to use. Estimating the soil properties is the tough part.

 

[BigH]

Rich - you indicated that you can't get a geotechnical in to assess the situation - yet, you talk about hand shear vanes and shear box tests - boreholes too. What gives - with this kind of effort, you should have a geotechnical leading this.
Eric pointed out something of interest - seams of sand and silt - or pockets even. Definitely important as is any fissures with the stiff clay. I'm surprised you are thinking hand shear vanes in a stiff clay that you can't indent with a thumb. Inconsistant.
As has been mentioned, there are computer programmes and the like; but, if you want to get a feel, pick up a Terzaghi and Peck (1967 version is better I think) and look in the book for Taylor's charts. This, and formulations for getting Stability numbers - and relating it to other properties will give you a handle on what the FS is. Of course, you need to think about water filled tension cracks, etc.
Cheers - but get a geotech to help you.

 

[DRC1]

Clays are difficult animals. They are governed by Cohesion at first, then as time progresses, they loose cohesin, but gain friction. In soft clays, te intial cut generally controls. In hard clays, sometimes the longterm strength controls. As stated seems, even very small seems, can change everything. Laboratory testing sould be done to acess both long, short term and remolded properties. Analysis should be done on both local stability and global stability. My big concern would be the snsitivity to the clay. If you have a highway above and a subway or construction below, many otherwise comptent clays will flow suddenly when exposed to excessive vibration. A 4 meter wall supporting a highway is nothing to guess at. You definatly need a geotech to look at this.

 

[jdonville]

richthorpe,

From strictly a liability perspective, GET THEE TO A GEOTECH! Of course, if you want to take the risk of killing somebody, I'm sure that your boss and client will understand...NOT!

Jeff

 

[richthorpe]

I've got myself some slope analysis software and am seeking the advise of a senior geotechnical engineer!

 

[BigH]

Hope you have insurance. Have you ever used Slope Analysis software? Do you have a good handle on what garbage looks like? Be wary. If you want to do this as a personal exercise - by all means, but let the experienced do it for the project.

 

[geodan]

Gasp. This is a mind boggling thread. I agree with BigH 110%!! If you want to do it as a learning exercise, by all means do it... but for a real project where PUBLIC health and safety is at risk???!!? I think it is a breach of professional ethics for someone inexperienced in soils to design such a slope.

I have spent years and many classes studying soils and slopes in particular. To think that anyone can just purchase a program and suddenly become an expert able to analyize slope stability is incredible. There are many, many things that need to be taken into consideration (much have been already listed by other posters) ESPECIALLY in clay slopes!!

As has been said by everyone already... hire an experienced geotechnical engineer or don't do it.