And, as a add-on to SlideRuleEra's note, put in wick drains (sand drains, band drains or other vertically installed drains) to reduce the drainage path. Chose the spacing to match your desired time for settlement (usually taken as 90% or so).
Be VERY careful with embankments on soft clay. If you load it too quickly, you could cause a massive failure of the clay and it will be almost non-repairable (is that a word?).
If you are going to surcharge it and use wick drains, I would recommend instrumenting the heck out of it. If the pore pressures increase out of control or if you see unacceptable movement, then you will need to stop construction and wait until pore pressures decrease. With clays, this could take months.
Be prepared for a long construction period no matter what you do.
You should have an instrumentation plan in place prior to embankment construction with specified tolerances for pore pressures, and vertical and horizontal movements.
One question:
What is the purpose of the embankment? If it is for water detention/retention, wick drains may not be the best way to go since they introduce seepage pathways.
eric1037 is correct - you have to be careful with any embankment on soft clay. We've had a few small failures of highway embankment of 5m high when contractor overloaded by end-dumping over the edge. We've put 9 to 10m high RE walls, though, on clay having Su values (initial) of less than 25kPa - we used the wick drains and stage loading with some instrumentation - we've built up to some 80% of the height already with settlements in the order of 600mm in places. Walls still holding their verticality. Still . . . the wick drains were not installed because of settlement but because the foundations were not strong enough to support the RE Walls.
The mode of failure depends on the strength and geometry, so it could be a circular failure or a sliding failure. In either case, the clay is sheared and remolded usually resulting in a significant loss of strength of the clay. The actually strength loss depends on the sensitivity of the clay, but it will definitely be enough to throw off your whole design.
If you have very sensitive clays the loss in strength could be very significant.
If the clay is soft, it is probably near it's liquid limit. I would recommend doing atterberg limit tests on some of the clay samples to determine.
You could also determine the sensitivity of the clay by performing unconfined compressive strength tests, then remolding and performing again.
You have the biggest concerns during construction. Once the embankment is in place and consolidation occurs, the strength of clays should increase.
Maury - eric gives good points - the other thing I would do, as you are a civil engineer, is to look up the section on slope stability in general soils textbooks - actually, Hutchinson's state of the art paper in Mexico 1969 is an excellent paper and will show you about sliding zones, etc.
I come from a school where we don't use the "hush" word failure - we use the word "distressed." You can search, also, for "slope failures" on the net and can get some good photos of various kinds.
Here comes another Question for Eric & BigH. What methods will be available for determination or evaluation of 'state of loaded clay'? I mean, how we can evaluate whether the current state will not cause 'distress' or 'failure'?
You should collect undisturbed samples and perform consolidation tests. In addition, you should also determine the sensitivity of the clay. In very soft clays you may need to do vane shear tests in-situ. After you gather all of the info, a slope stability analysis should be performed by a qualified geotechnical engineer.
I don't think you have offered enough information for a satisfatctory answer.
1. Is the soft clay located in the foundation of the embankment or in the embankment itself? The embankment will settle as the soft clay is consolidated either in the foundation or the embankment. however, the method for a solution will be different.
2. What is the usage of the embankment? Is it a reservoir dam or something else? When the settlement is under control, other functions of the embankment should not be sacrificed.
3. In what construction stage, do you want to reduce or increase the rate of settlement? Usually, it is desirable to increase the rate of settlement before the commencement of construction, but it needs to be kept under control, or reduced during construction, usually by slowing down the speed of construction.
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