Pipe excavation in silt clay soil and pipe safety

[M. next]

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.

Thanks

 

[Baluncore]

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?

 

[M. next]

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!

 

[Baluncore]

Problem: Excessive settlement

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.

 

[alphy]

for bringing up this issue with pipe excavation in silt clay soil and pipe safety. Excessive settlement can be a major concern when working with this type of soil, as it is known for being highly compressible.

One possible solution to avoid settlement is to use a technique called preloading. This involves placing a temporary load on the soil before the pipe is installed, which allows the soil to settle and compact before the pipe is put in place. This can help reduce the amount of settlement that occurs after the pipe is installed.

In regards to the issue of shears at the pipe's edges, there are a few potential solutions. One approach is to use a protective layer of geotextile fabric around the pipe, which can help distribute the load and reduce the potential for shearing. Another option is to use a pipe with a larger diameter, which can help spread the load and reduce the stress on the edges.

It's also important to carefully assess the soil conditions and design the pipe installation accordingly. For example, if the soil is particularly soft or unstable, it may be necessary to use additional reinforcement techniques such as soil nails or geogrids.

Overall, it's important to carefully plan and engineer the pipe installation in silt clay soil to ensure safety and prevent excessive settlement. By considering factors such as preloading, protective layers, and reinforcement techniques, we can minimize the risks and ensure a successful and safe installation.