What is the affect of dewatering on pile foundations and soil around the pile ?
does is cause reduction in soil bearing capacity?
negative skin friction ?
pile settlements?
responces are appreciated
Here are several responses to your questions. Hope that they are useful for your knowledge.
(i) What is the effect of dewatering on pile foundations and soil around the pile?
Sometimes it is necessary to temporarily lower the groundwater level for the construction of foundations. The process of lowering the groundwater is called dewatering and is accomplished by inserting well points around the excavation for the foundations.
Due to the dewatering, the pore fluid properties, permeability, void ratio, pore size, homogeneity may be changed, the stress state of the soil is now completely different from its original state. The resistance to vertical loads on a pile is hence absolutely affected.
(ii) Does it cause a reduction in soil bearing capacity?
Definitely, It is. The change of groundwater level leads to a reduction in pore-water pressure changes, and the effective stress of soils must shift. It is obvious that the shear strength of soils is altered (friction angle, moduli such as), and the soil bearing capacity are also affected.
(iii) Negative skin friction?
Piles located in soft clays are subjected to negative skin friction called
downdrag. The settlement of the soil layer causes the friction forces to act in the same direction as the loading on the pile. Rather than providing resistance, the negative friction imposes additional loads on the pile. The net effect is that the pile load capacity is reduced, and pile settlement increases.
For a soft, normally consolidated soil, the negative skin friction is usually calculated over one-half its thickness. Negative skin friction (NSF) should be computed for a long-term condition (effective stress). As explained earlier, dewatering changes the effective stress, and NSF must also be changed
(iV) Pile settlements?
The dewatering increases shaft resistance of the pile but also increase negative skin friction. So, the allowable load capacity can be increased and pile settlement decreases without effect of NSF,. Whereas, if negative skin friction is considered, it depends on the effect of dewatering on negative skin friction to say pile settlements increases or decreases.
Yes, the dewatering techniques can affect the shear strength and bearing capacity of soil if it is done after the pile was constructed. the direction of the groundwater flow leads to eroding out the fine particles from the soil and this leads to settlements of the pile. But some researchers conclude that dewatering techniques increase the strength of the shear force.
Chalachew T. Terefe
Your answer is also interesting and right!. In addition to your comments, the aim of the dewatering techniques is to increase the shear strength of soil by decreasing pore-water pressure and increasing effective stress. However, this technique is more effective for sands/clayey sand than soft clay because of the permeability coefficient of soft clay smaller much. Furthermore, the negative skin friction usually occur in the case of clay than sands
Dewatering will increase Effective Stress and rearrangement of Soil particles may occur
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