Dewatering is hydraulic modification of foundation soil; like other modification (Mechanical, chemical etc) hydraulic modification is an effective technique to improve soil condition in many respects.
We know any dewatering technique have an influence zone within which radius groundwater table is lowered and when this radius exceeds property line of any construction site, adjacent property will also subjected to some modification.
The modification in adjacent ground is also positive, if there have no structure or any type of excavation (in respect of civil engineering project but have negative effect in agricultural project). When there have building, sometimes may suffer severe damage. We will learn about this throughout this post.
We know when ground water is lowered under dewatering of soil deposit, effective stresses in increased. Consolidation and subsequent settlement of structure is occurred; in case of pile foundation though point capacity (deep supported) remain same skin resistance becomes low (even may result negative skin friction).
Considering one dimensional settlement, a rough estimate may be made as follows:
Here we are considering vertical settlement not considering permeability in two horizontal directions.
H= Thickness of soil layer subjected to consolidation
e=Initial void ratio (initial term is used as void ratio will be altered after dewatering)
σ’vo =Average effective stresses in this layer at initial stage (in vertical direction)
∆σ=Stress increment due to dewatering (lowering of ground water table)
Cc =Compression index.
Again ∆σ=∆h ϒw
Here considered ∆h= Reduction in ground water level.
The unit weight of soil considered not changed significantly under dewatering. Compression index (Cc) is determined from consolidation test. Sometimes empirical formula is used, but less reliable, like
Cc = 0.009 (LL-10)
Where LL=Liquid limit
Again soil layer may be overconsolidated or normally consolidated. So depending on stress history using recompression index, CR real value of settlement can be found (sometimes both CR and Cc are required).
So we can determine settlement value, now question is whether the structure will suffer such amount of settlement. In settlement process time is very important factor. If soil has very low permeability time becomes very long. If soil is very erratic, different zones or different layers suffer variable settlements.
Variable settlement or differential settlement is the main concern of settlement analysis. Here three things are determined:
1. Settlement rate
2. Total settlement
3. Differential settlement
Above equation produces total settlement which doesn’t mean damage to adjacent structure. When soil layer is very thick having very low permeability, the settlement rate will be very slow. In this case temporary dewatering from overlaying aquifer will produce little effect. Differential settlement of structure will be comparatively small and very little or no effect on function.
Now another point is the nature of structure and its foundation type. Raft foundation or piled raft foundation will suffer less under differential settlement. Steel framed structure having corrugated iron cladding will suffers less differential movement relative to multistoried brick structure.