SOE to top of Bedrock & Dewatering

[BNGeo]

I'm looking at a project that requires excavation to about 25 feet depth over an area of about 20,000 sf, with bedrock at higher than the target depth (let's say rock at 20 feet on average). Groundwater is at about 5 feet. For this scenario, I'm anticipating temporary SOE would be installed to the top of rock (tied back, braced or pinned for stability).

Do you expect that the contractor would find sheet piling to be a good option here? They would not seal at the interface of the sheets/rock (it is hard rock and rock is sloping). So there would be seepage through the sheets/rock interface, as well as whatever seepage through the interlocks.

Also, would it be appropriate to install wellpoints or deep wells for this condition? Would the wells typically go into the bedrock or would they terminate right at the bedrock surface?

The bedrock is mostly RQD~80%, but one of five cores had an anomalous 20% RQD. Add to this that if it is determined to used controlled blasting to remove the rock, there will likely be additional rock fracturing that will increase seepage through the rock.

Thanks!

 

[SlideRuleEra]

In my area, well points are typically installed to the top of rock with deep wells used in the rock (sedimentary limestone - very porous). Depth of 25' is pretty much the limit for single-stage vacuum well points. Because the water table profile is lowest at the location of each well point, deep wells will almost certainly be required to keep the water table below 25' over the entire excavation.

IMHO, sheet piling is not practical for an excavation that large. Expect significant leakage where the sheet pile tip meets the rock.

For our electric generating station projects we had to dewater for a 20'+ excavation (well points and deep wells) several acres for over 1 1/2 years. No sheet piling used.

Dewatering can affect neighboring properties, we had problems with that. Consider your consequences if this happens.

 

[BNGeo]

Thank you SlideRuleEra. So it sounds like your suggestion on a better approach than sheets would be open cut with a robust dewatering system, while considering/managing the effects of drawdown on neighboring properties.

 

[SlideRuleEra]

BNGeo - Yes, an open cut has advantages.

Not much you can do to mitigate nearby potential problems. Just common sense, dewater as small an area, to as shallow a depth, for the shortest time practical. It may not be necessary to dewater to the initial full depth for the duration. As work progresses, consider decreasing depth of ground water removed... if appropriate. Also make sure proper insurance is in place.

For a large area dewatering (like your project), usually best to require dewatering to take place 24/7... except for short downtime for necessary maintenance.

Consider reviewing a contractor's dewatering plan to see if necessary wells, piping, pumps, etc. do not interfere with his or others work areas. Pay special attention to how the extracted ground water discharge is handled.