Determining safe slope for construction in cohesive soil
Determining safe slope for construction in cohesive soil
(OP)
I need to determine a safe slope for a very stiff clay (Unconfined Compressive Strength - 4 tons per square foot) for a 14 foot cut. The exposure will be short term. I know that OSHA allows a 3/4:1 slope (53 degrees) for an "A" soil, but I need to slope at a steeper angle due to utilities and other encumberances. The soil will not become saturated due to pavement. Any information regarding methods for calculation of slope and other parameters that would be required to make the calculation would be appreciated.





RE: Determining safe slope for construction in cohesive soil
Hire a local geotechnical consultant - who is familiar with trenching, shoring design and microtunneling - to assist you. To proceed without their assistance is both foolish and dangerous.
You have very limited options - shore and brace the excavation, use a trench box or microtunnel. If the utilities or other structures are within 30 feet of the trench edge, you will probably have to shore and brace the excavation or microtunnel. If you have a long reach with this condition, or you are in an urban setting with a traffic control problem, you should consider microtunneling as your first option.
While the clay is quite strong, keep in mind that minor defects control slope and excavation behavior. And time. Remember that excavation collapse is a sudden, catastrophic event. Once it occurs, it's too late - if you're in the trench, you're probably dead.
Safety First!
RE: Determining safe slope for construction in cohesive soil
RE: Determining safe slope for construction in cohesive soil
I worked on a project with similiar constraints only more granular soil, and we used sheet piling design that I thought worked well. The contractor had some extra piling that he anchored to bedrock with rock bolts and propped up with h beams. I didn't think the FHWA engineers would go for it, but in the end we proceeded with their stamp of approval, along with that of the engineer's. The work was on St. Helens, so slightly greater possibility for catastrauphic events, but it was a relatively cheap fix and very safe, I thought. You might not be close enough bedrock for an anchor, though it may be possible to find some other way.
Best of luck!
RE: Determining safe slope for construction in cohesive soil
RE: Determining safe slope for construction in cohesive soil
Using Stability Number charts in a Geotechnical text, I am finding that I have a Safety Factor for cohesion of about 10 even with a proposed slope of 60 degrees.
Am I missing something?
RE: Determining safe slope for construction in cohesive soil
Your numbers are no doubt correct. However, what I think the other respondents were trying point out was the issues thatare associated with excavations inwhich people have to work and which have utilities adjacent to them. Safety is a priority in this respect. These days the OH&S are tough on this type of work and in fact some issues that may be overlooked are readily picked up by their review.
Let us examine the problem and ask a few questions which you can no doubt answer. If you cannnot then you need to address those.
Are you confident that your soil stratum is homogeneous and that the strength you quoted exists with depth to the level proposed for your excavation and beyond.
How far is the utilities from the proposed top of cut.
What type of utilities are present. If they are ruputured what would the result be.
Will any workers be in close proximity of the slope. What protection will be provided to them against any debris -rocks etc rolling off pavement and falling on their heads. This may sound stupid but OH&S looks at these aspects.
What is the pavement composed of. Is there a chance that the base course would ravel out and materials fall into the excavation re above.
What is the likely behaviour of the slope whan wetted by rainfall.
Where will the rainfall surface runoff from the pavement go.
Is there someone who will be monitoring the excavation slopes on a daily basis.
Have you advised the road authority and utilities companies of the intended excavation and have they provided approval.
The above is a partial hazard asesment list which one should create to determine whether actions will be safe or not. Others can be added if you take a look from out of the tunnel.
Having said the above, if you are convinced that all will be okay and your FOS in relation to time of exposure and effects of weather will not reduce significantly then the ball is in your court.
If nothing happens then mark this against your experience. If something does then it is part of your experience as well and one that you will not likely forget depending on outcome.
The above are meant to ensure that you examine the issues and determine the risks before taking the leap.
Whatever you decide ensure that all working in and around the excavation are aware of what precautions may be needed for them to work safely and ensure that those in the hole are recognized. There should be some protective barrier that keeps traffic from travelling too close to the excavation edge and to ensure that spoil is not placed too close to the top as well. You no doubt know of these various fundamentals.
RE: Determining safe slope for construction in cohesive soil
Let us know what you think. Good luck!
RE: Determining safe slope for construction in cohesive soil
RE: Determining safe slope for construction in cohesive soil
What is being constructed in the trench? How soon can partial backfilling occur? How soon and/or close to the full depth excavation will compaction occur? How will backfilling occur, by excavator bucket or by loader from hole edge?
How important is it for excavation to remain open/what will be the results if sidewall collapse occurs?
RE: Determining safe slope for construction in cohesive soil
I'm a firm believer in Safety First!
RE: Determining safe slope for construction in cohesive soil