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Risks associated with groundwater recovery post-dewatering

Risks associated with groundwater recovery post-dewatering

(OP)
I am interested in any knowledge or references people may have regarding the potential for and extent of risks associated with groundwater recovery post dewatering.

APOLOGIES: re-posted from other forum as this one more appropriate

The site original GWL was approximately +1m elevation (original site ground level between +1.5 and +3) and has been lowered to -20m in some areas over 12 months. Ground conditions consist of approx 7-10m of slightly silty gravelly SAND overlying highly to completely weathered chalky calcareous limestone. There are localised areas of cohesive deposits, mostly slightly clayey SILT within the superficial deposits. All superficial deposits are being ground improved using dynamic compaction, vibrocompaction and rolled compaction in layers, finished reclamation level is +4.5m.

The areas of concern are those that are to be developed as port container terminals that will have large paved platforms for container stacking and movements. Most other structures are likely to be piled into the underlying rock and therefore risks are appreciable and can be managed.

The concern I have is the effect the recovery of the groundwater may have on both the granular and cohesive superficials once the dewaterign system is turned off in approximately 2 years. My initial thoughts are given the levels of compaction undertaken there will be little impact where the ground comprises only granular soils as the settlement has alaredy occured during compaction. However the areas where there are 'cohesive' deposits (slightly clayey SILT) would seem to pose more of a risk due to the impacts of longer term consolidation and changing gw levels.

Does anyone have any experience of this risk, are there ways to model the potential impacts or design out the risk? As always your help is much appreciated.

Cheers

RE: Risks associated with groundwater recovery post-dewatering

Raising groundwater should actually help you in terms of consolidation. Long term settlements of clay happen when you lower the groundwater table, which increases the effective stress in lower layers causing settlement.

If you want to model this, I would suggest finding a simple consolidation example online or in a text book. Using the given values, work the example with the groundwater table high then lower it. Then rework the example with the groundwater table low and raising it.

If you don't have access to something like this, let me know and I'll find one for you to work.

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