shoring a deep excavation

[jonathanpye]

hi
I'm looking to shore a deep archaeological dig... 75ft x 75ft on plan, and 75ft deep. Casting concrete into the ground ie. bored pile wall or diaphragm wall is not an option as this might damage archaeology, nor is steel sheet piling. It will need to be some form of sectional steel drop box I think. The ground is dry and well graded granular material, pretty compact. Anyone had any similar experience or who can make any suggestions ?
Appreciate any assistance you guys can give !

 

[hokie66]

Sounds like you will need a version of an old fashioned caisson. You dig a bit to drop the box, then add another section on top. Repeat until you reach the required depth.

 

[aeoliantexan]

I agree that a cylindrical shape would be advantageous, as it makes use of the structurally-efficient compression ring. However, excessive friction may keep the caisson from settling in this soil.

Corrugated tunnel liner plate is often used for shoring vertical cylindrical shafts. You dig a couple of feet unsupported, assemble another ring of bolted sections, and repeat. Periodically, fill any space left outside the liner with grout pumped through plugged holes in the liner sections. If necessary, the strength can be increased with intermittent ring beams. This method should be well suited to the soil conditions you describe.

Check the websites of Contech and others.

I am curious as to how 75 feet of dry granular soil got placed over the artifacts. Is this a sand dune?

 

[PEinc]

Can you use soldier beams and timber lagging, braced or tiedback? Another expensive option is ground freezing.

http://www.PeirceEngineering.com

 

[Ginger]

Wouldn't suggest trying ground freezing if there is no groundwater. Would be extremely expensive.

However, you don't have to discount sheet piles as an option. I would suggest you employ an experienced sheet piling contractor to install the sheets. You can use the "Dig and Push" method for installation with limited risk of damaging the artifacts.

The sheets would be pre-driven to a depth well above the artefacts' horizon using a vibrating hammer (ideal for dry granular material. Excavation could then commence with steel frames being installed at certain levels as the excavation proceeds. This requires experience, but with judicious placing of the frames, you get the bottom frame located with approx 1m of the sheet pile toe. Once you have constructed this frame, you sequentially drive the sheet piles 1m, then excavate, then drive1m then excavat. Further frames will be added to support the sheets until you reach the required horizon. The sheet piles will be cantilevering below the bottom frame in this method and care will be needed to ensure frames are added to prevent overstressing of the sheet piles and the frames. the toe of the sheets must ALWAYS remain buried to prevent inrushing of materials.

Regards

Andy

 

[PEinc]

Yes, I remembered the "dry" comment after I posted. I hoped nobody noticed. Oh well.

I believe that it would be very expensive to incrementally dig and push steel sheet piling as the "dig" progresses. I would not want to pay for a sheeting crew and crane to wait around while archaeologists played in the dirt. It would be cheaper to intall soldier beams and then let the installer demobilize and leave. It would also be difficult to excavate with the steel sheeting projecting significanly above original grade. Drilled-in, braced soldier beams and lagging would be my preference (if the site is dry).

http://www.PeirceEngineering.com

 

[USGeotech]

Consider using a slide rail shoring system. There are several manufactures of slide rails or similar systems. I prefer the slide rail system available in the US through United Rentals Trench Saftey.

A 75'x75' excavation will require many internal braces but since, I presume, most of the excavation will be done by hand for this archaeological dig, braces at about 20' cc should not impede the progress of the excavation.

 

[chicopee]

We have used guard rails placed horizontally and then anchored with tendons inserted in predrilled holes and then grouted for very heigh temporary retaining walls.
You may want to check that method.