What are the Purpose of Dewatering Foundation Excavations?
There are various reasons that enforce dewatering of foundation excavation such as to keep excavation bottom dry, prevent ground water or soil leakage, prevent sand boiling, to prevent upheaval failure, prevent basement floatation. In this article, foundation dewatering motivation are discussed.
Purpose of Dewatering Foundation Excavations
Keep Excavation Bottom Dry
When velocity of groundwater in gravel or sand is high, then it is possible that groundwater moves into the excavation area. This will create significant issue and hinder the flow of construction. So, it is necessary to keep foundation excavation dry to perform an acceptable job.
Commonly, dewatering will lower groundwater level. It is specified to lower groundwater level to 0.5 to 1m below the excavation. As far as clay soil is concerned, groundwater would not create issue since water cannot travel through clay and consequently dewatering will not be necessary.
Fig.1: Lowering Groundwater Level
Prevent Ground Water or Soil Leakage
If leakage occurs in an excavation, it would create great danger since it could lead to failure of elements that support the foundation wall provided that the leakage is great. So, it would be necessary to dewater the excavation area to prevent such undesired result.
Leakage may occur in sandy or gravelly soil with high ground water level and diaphragm wall with bad joints or improper watertight sheet pile is used to support foundation wall. This is because water would travel through poorly detailed joints or though sheet pile which is not properly watertight.
Prevent Sand Boiling
When excavation is carried in sandy or gravelly soil, the groundwater level is lowered to at least 0.5m. As a result, there would be differences between groundwater level outside excavation zone and inside excavation area.
If the difference reaches the hydraulic gradient of soil, the sand boiling would occur. To prevent sand boiling, it is recommended to lower groundwater level outside excavation area. However, care should be practiced to avoid settlement.
Fig.2: Sand Boiling
To Prevent Upheaval Failure
When excavation is carried out in a clay soil and there is a permeable layer like sandy soil or gravelly soil beneath the soil layer, then groundwater pressure from the permeable layer would exert an upward pressure on soil layer.
If the groundwater pressure overcomes the weight of soil layer, then upheaval failure would occur. In order to prevent the upheaval failure, it is required to lower down the piezometric pressure of the permeable layer by pumping as illustrated in Figure 3.
Fig.3: Dewatering used to lower down piezometric pressure of underlying permeable layer to avoid upheaval failure in the excavation
Prevent Basement Floatation
After excavation is completed, the construction of basement would start. If the structure is constructed in sandy soil, then it possible that basement would experience floatation at the initial stage of construction. At this construction stage, the weight of the basement is small.
So, if the upheaval pressure is greater than the weight of the structure, then floatation will occur with differential heaves of the foundation.
When the worst scenario occurs, the floated basement or foundation would return to its original location and it should either be demolished or reconstructed. That is why dewatering should be considered to prevent the floatation of basements at early stage of building construction.