Installation Issues of Braced Excavation with Sheet Piles

[MrEngineerUS]

I've been informed that the contractor is hitting 6-7" diameter rocks during the installation of the sheet piles for a braced excavation I've designed. They would like to essentially not drive the piles to the required depth.

The braced excavation is roughly 7 feet deep and the piles should be driven to at least 9.5 feet per my design. They want to stop driving piles that encounter these rocks at generally 1.5'-2' short of the designed depth. There are two levels of walers and struts that are to be installed as the excavation progresses.

My question is what is typically done in a scenario such as this? Do I just tell them that it needs to be installed to the required depth and that it's a means-and-methods item? Can they just locally excavate and remove the rocks as they are encountered? I've been told that it isn't just a pile here and there but could be 3-4 piles in a row with this condition.

Thanks for any comments! I don't have much experience with the installation and resulting field issues of these sorts of walls.

 

[PEinc]

If ground water is not a problem, you could drive the SSP to or very close to subgrade and be done because you have two tiers of bracing. However, you may need to redesign the SSP and bracing because there would be no toe embedment below subgrade. You may need to drive the SSP, install both tiers of bracing at the upper brace elevation, and then lower the lower tier of bracing as the hole is excavated.

http://www.PeirceEngineering.com

 

[jayrod12]

I vote for driving the entire line of piles as far as possible, and then looking at locally excavating to remove the rocks where you couldn't get down to the depth needed. provided it's only one or two sheets in a row locally excavating it shouldn't be an issue.

 

[MrEngineerUS]

Thanks for the replies. Looks like most of the piles driven to within 12-18" of the design depth. I suspect additional analysis will be required to make sure the system is still viable with the reduced embedment.

I'm not sure if moving the bottom level of support with the progressing excavation will be an option with some other complications within the pit (utilities).

Local excavation may be an option. Do you know if it is possible to install the top waler system, continue excavation, then locally excavate to remove obstructions, then continue to drive those piles that were obstructed? I'm concerned with 1) Damage to the waler system while continuing to drive the pile and 2) Buckling of the pile as it is no longer fully supported on both sides by soil while being driven.

 

[SlideRuleEra]

Do you know if it is possible to install the top waler system, continue excavation, then locally excavate to remove obstructions...

IMHO, not a wise move. After excavation, the sheeting will be unstable since it has no embedment to keep it from moving inward. The bottom of the sheeting could "kick in" suddenly, without warning - very dangerous. That's why the lower ring is dropped as the excavation proceeds, as PEinc recommended.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[PEinc]

If the sheeting can be driven to within 12 to 18 inches of the design tip elevation, then it can be designed for two tiers of bracing without any embedment below Subgrade - unless ground water is a problem. Sheeting with multiple tiers of bracing does not necessarily need embedment below subgrade. The brace levels need to be chosen properly so that the bottom cantilevered portion of the sheeting is not too great and unstable.

http://www.PeirceEngineering.com