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. I also posted this in the Structural Engineering forum.

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[MathMan]

Have you considered driven sheet piles?

They are deep corrugated sheets of thick steel (1/4" to 1/2&quot. They have high inertia and they interlok at the edges.

After they are no longer required, they are removed, and they can be reused, which reduces material costs considerably.

Let me know if you think these could be used, I may be able to help.


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[TAC]

I think they make a small pile driver that should fit your clearance. However, I have never tried, so I don't know what is available.

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[nitpik]

THE DEPTH AND PLANAR DIMENSIONS OF YOUR PIT ARE SO SMALL THAT THAT YOU MIGHT CONSIDER FIRST SOME MINIMAL SOILS EXPLORATION )(AUGER DOWN TO 10') TO ASCERTAIN SOIL TYPE AND MOISTURE CONTENT IN MORE DETAIL ...IF THE SOILS ARE COMPETENT AND THE WATER CONTENT LOW, CONSIDER EXCAVATING TO SAY 3 ' WITH SMALL BACKHOE...THEN CONSTRUCT AND INSTALL A LINER EXTENDABLE TO FULL DEPTH AS IT IS EXCAVATED.

 

[Ginger]

Dear pbw

The two simplest methods for undertaking this work with limited headroom are:-

a) Excavate by hand / pneumatic clay spades and support the excavation with timber lagging and either timber/steel/hydraulic frames. The frames can be removed sequentially as the concrete walls are poured in lifts.

b) If you don't have decent carpenters then use driven sheet piles in short lengths (perhaps 2 lengths) and butt weld them together. this will allow you to use a piiling hammer within the restricted head room you have. For the depths you are looking at I would use an hydraulic hammer fitted to an excavator.

Both solutions will require you to leave the temporary works (except for the frames) in place.

Regards

Andy Machon
Andy@machona.freeserve.co.uk

 

[snafuman]

I recently installed some vinyl sheet piling on a project. They are vibrated down with a jackhammer. They worked quite well in the sandy soil we had. Because of your headroom, you might consider vibrating these PVC or steel pile instead of driving. However, because of your height 11' and the strength required, you may need steel sheeting. Also, if your soils are too cohesive, vibrating these into place may not be practical.

 

[derubertis]

If the soil is not saturated, you might want to consider soil cement columns. Check with any ground improvement contractor such as Hayward-Baker or Soletanche for advice and cost.

 

[ayfarm]

I would rent/use a trench box, steel or aluminum. I you can use a backhoe to dig, the steel would be cheaper.

roque1@roquee.com

 

[jheidt2543]

Over the years I've worked on a number of pits very similar to what you describe in buildings of similar clearance. Most of them we excavated with backhoes in an open cut. The major problems we encountered had to do with protecting existing building foundations from being undermined and high water tables.

Your excavation will be 11' deep so, your open cut hole (depending on soil conditions) will be approximately 30'long x 21'wide. You must determine if there are any building foundations within that zone that could be undermined. If so, then you either drive sheet piling protect them or underpin them to an elevation below the influence line of the excavation. You can excavate 4'-5' down prior to driving the sheeting, which will give you some additional head room. Also, there are some small pile heads that can be attached to a breaker hammer to drive short sections of sheeting in low head room areas for small sheeting jobs.

If you have ground water, set your deep well system, or well points up ahead of time so they can be working prior to starting the major excavation. It generally takes a few days to draw the water table down to your working level (again this is site specific and a few soil borings ahead of time can give you a pretty good idea of what to expect.)

In general, it is cheaper to take out and replace the floor slab in an open cut situation than to drive sheeting. But, you do need a pretty good sized area in the plant to be able to do it. And don't forget that you need an area on site to store the excavated material so it can be used for backfill.

Good luck!