Determining safe slope for construction in cohesive soil

[hairman]

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.

 

[Focht3]

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!

 

[BigH]

Focht3 - good points. Add-on: If clay is that stiff, it is likely overconsolidated and this is not good for long term near vertical cuts due, as you say, to softening, etc. associated with depressurization of soils near slope face. With his encumbrances noted, probably would be best to soil nail and shotcrete - but correctly pointed out; get a local geotech who is 'into' excavations and use him like your best friend!! On any excavations - this or others - always observe for cracks, other signs of distress. Set up some markers and shoot "down the line" to see if any are moving, etc.

 

[dirtsqueezer]

How large a work area do you need? Is it a pipe one could use a very wide trench box for, or do you need to wait for 28-day compressive strenght results?

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!

 

[Myoho]

You definelty need to use a positive support system to shore this excavation.

 

[hairman]

Everyone seems to be pointing to an active shoring system, however, it seems as if a sloped system may be appropriate. The excavation is a building excavation approximately 200 feet across. The slope is a homogeneous simple slope in clay with an unconfined compressive strength of 4 TSF. Pavement approximately 15 feet back from the top of the cut will keep the soil from becoming saturated.

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?

 

[VAD]

Hello Hairman:

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.

 

[dirtsqueezer]

The FHWA puts it in terms of hazard and risk. If you've got a steep, granular slope above a wooded gulley in the middle of nowhere, then your only concern is, perhaps, the deer passing by, and is low risk. In your case, it sounds like you've decided it is a low hazard situation. But what about the extreme catastraphic event if that slope does slide into the building? Is there a possibility of erosion of the toe over time that might compromise human life in the building after 10 years? Are there any seismic considerations? Will traffic and vibrations from the road above affect your new excavation? I think there are definitely a few things to consider even without seeing the site, if you're looking at the long term. I would at least get it checked out by a consultant. My company does slope stabities that involve site visit/records search that are inexpensive and relatively quick. You don't need to be a hero, but you might just want to do something to cover yourself.

Let us know what you think. Good luck!

 

[Focht3]

Where is the site located? What is the geologic name of the formation that the clay is a part of? How long is "short term"?

 

[Grouser]

Side wall inspections should be no less frequent than every 15 minutes if the trench is occupied. A properly secured, sturdy ladder/escape route must be readily available for trench evacuation. Surface cracking means impending failure. Get them out!

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?

 

[Focht3]

Grouser is right on all points - except that OSHA requires two escape routes.

I'm a firm believer in Safety First!

 

[PEinc]

In trenches 4 feet or deeper, OSHA 1926.651, (c), (2) requires means of egress (ladder, stairs, ramp, etc.) spaced so that no person in the trench is required to travel more than 25 feet laterally to escape. This could be significantly more than 2 escape routes.