Dewatering of Silts 1

[geomtl]

I have an excavation extending 25 feet below the ground surface in compact to dense silts with trace sand. The GWT is located at approximately 15 feet. Excavation below the GWT as can be imagined is difficult, with the saturated soils very susceptable to remoulding with a minimal amount of disturbance.

The contractor insisted on using soldier piles as opposed to sheet piles to retain the soils.

My question is what type of deatering system might be used in this situation to lower the water below the final level of the excavation and how long would this normally take. I was thinking of looking into well points, is this a possibility?

Thanks, clivek

 

[SlideRuleEra]

Well points may work for this application, but it will be a close call. As you know, the theoretical maximum vacuum dewatering depth (at sea level atmospheric pressure) is about 33 ft. Because of vacuum system losses, 25 ft is a typical operating depth. If you can put the vacuum pump / header pipes in a shallow pit, say 2 or 3 feet deep, this should allow a total draw down of 27 or 28 ft below the ground surface, which should be enough.

Depending on the soils, it could take several days for the dewatering system to reach steady state. It will probably be necessary to operate the system more or less continuously until work below the water table is complete.

It is not always necessary to surround a structure with well points. For example, one row of well points parallel to pipe line excavation may be all that is needed. In your case, however, sounds like numerous wells will be needed.

Best Wishes

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