Excavation Dewatering rates/settlement

[tkvail]

Is there a good reference for dewatering rates for large excavations that are adjacent to existing structures, and/or mitigation for possible settlement of those buildings due to dewatering.

Any assitance would be helpful.
Thanks

 

[dgillette]

Rate of dewatering? Not if you want anything quick and dirty. There are a number of publications on dewatering, but they won't get you very far without fairly detailed stratigraphy, a conceptual dewatering design, and some considerable analytical effort or seat-of-the-experienced-pants judgment. There is a joint US Army Corps of Engineers/Navy/Air Force manual that gets a lot of use. Army TM 5-818-5 / Navy NAVFAC P-418 / Air Force AFM 88-5, Chapter 6. You can probably get it from NTIS.

As for the mitigation, try the chapter on underpinning by Edward White in Winterkorn and Fang's "Foundation Engineering Handbook", or "Foundation Engineering for Difficult Subsoil Conditions" by Leonardo Zeevaert. The latter practiced (practices?) in Mexico City where all the soft clays make for settlement problems any time someone digs a hole or builds a heavy structure.

Bon chance!

DRG

 

[fattdad]

The rate of dewatering will be contingent on the stratigraphy (correctly stated above), the location of the water table before and after dewatering and the type of excavation bracing that is used. Equally important is the tendency for you to develop the "quick" condition owing to hydraulic exit gradients, which may steer the contractor toward well points or deep wells to provide a stable dewatered subgrade.

Consider the flow path for a sheet-pile bracing design - much longer than the flow path for soldier pile and lagging. This would factor into the rate of dewatering.

Once you have some idea on the stratigraphy and the likely dewatering methods, you would then evaluate the drawdown that would likely be realized beneath the adjacent structure(s). With drawdown, the effective stresses in the soils beneath the adjacent structure(s) will increase. You then need to consider how the change in effective stress will consolidate (or compress) the subgrade soils beneath the building(s). This can be done using one-dimensional consolidation characteristics or elastic modulus (again depending on the stratigraphy.

Good luck. These are fun problems and I'd love to be working on it!

fatt-dad, c.p.g., p.e.
Richmond, Virginia

 

[pgyr]

The dewatering manual is free and can be downloaded at

http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_818_5.pdf

 

[fattdad]

pgyr, Thanks for that link! I've had this manual for years and it's nice to see it preserved in cyberland. There's great stuff in this gov-publication!

f-d