Press Pit Design

[pbw]

I have designed a pit (size 19’x10’x9.5’ deep) that will be located in an existing warehouse. Excavation will be down to 11’ below finish floor. A contractor has proposed drilling in concrete piers 18” dia. practically side by side as shoring. Then installing the 15” retaining wall. This seems like tremendous overkill and is very expensive.

Soil is cohesive so the contractor could bench or cut back at a 45 degree slope. However, this would require demolition and repouring of existing concrete floor. There is approximately 18’ of clearance AFF, which probably limits the installation of sheet piling.

I am looking for ideas on how to support the pit's sides during excavation. All feedback is welcome.

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[Qshake]

Despite your initial reaction, the use of drilled shafts might be a good solution for a number of reasons. Bear with me as we don't know all of the issues at hand. Since you're working within what might be a constrained area the use of drilling equipment is far less intrusive than that required to seat sheet piling. Obviously it is intrusive to over excavate the pit as well. As for cost and time, the drilled shaft construction can be expedited through drilling and placing concrete in every other shaft with the contractor coming back to finish the remaining. What would be nice is: if the contractor redesigned the vertical retaining system such to include the drilled shafts as much as they are permanent. This would eliminate the need for the 15" wall. This assumes that the wall and flooring are or could be seperate.

Alternatively, with such a little excavation using an internal bracing system might do the trick. This could be constructed of sheet piling (to hold the walls of the excavation) with a metal pipe connected to the sheet to prevent the walls collapsing inside the excavation. Naturally, a stage construction would be necessary to work around the struts. And you probably wouldn't want to do the whole excavation at one time. [sig][/sig]

 

[tps]

You asked for ideas and these are just that. Consider trenching and building the walls first. Conventional trench shoring could be used. Then excavate the pit after the walls are cast placing shores as required during the excavation of the main pit, if necessary for the final design configuration. Then cast the bottom slab.

Alternative idea. Use caisson technology where the walls are built integrally above ground and the structure lowered by subexcavation. This will eliminate the trench shoring.
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