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Pipe excavation in silt clay soil and pipe safety

  1. May 9, 2014 #1
    Problem: Excessive settlement

    I was considering adding new material under the pipe to avoid settlement. But I ran into another problem which has to do which the pipe's edges. How to avoid shears there in silt clay soil? pipe depth is 4m.

    Last edited: May 9, 2014
  2. jcsd
  3. May 10, 2014 #2


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    Science Advisor

    Normal practice here in a dynamic clay soil is to surround the pipe with free flowing bedding sand. Water drainage provided through the bedding sand also tends to immobilise the surrounding soil.

    We are a little short of information.
    What is the pipe diameter, internal pressure and material? What fluid does it carry?
    Is the trench depth constant or does it have steps in it due to poor excavation technique?
    Why 4 m deep, seasonal soil heave or deep frost? Flooding? Heavy vehicle traffic?
    How dynamic is the soil at 4m deep once settlement has completed?
  4. May 14, 2014 #3
    It carries sewage under pressure. Material hpvc. 60 cm pipe diameter, 4 m deep and 1.5 water table in slit clay soil. Considering pipe safety.

    Should we replace the material under the pipe. If so, how deep? Can we use the excavated material for fill?

    Thanks for your reply!
  5. May 16, 2014 #4


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    Science Advisor

    How is settlement “excessive”. So long as the pressure is applied evenly to the pipe there should be no problem.

    I do not know your local design rules and I have not seen an analysis of the soil, so I can only make suggestions.

    If the excavated soil does not contain rocks and if it flows well then placing the pipe on an even bed before back filling with the excavated soil should present no problems.

    If the excavated soil is not free flowing then the pipe should be laid on a bed of, and surrounded by, free flowing sand. The pipe will be slightly buoyant below the water table and should be surrounded evenly by hydrostatic pressure. Free flowing sand is usually very low cost compared with the cost of one sewage pipe repair.
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