What are the Criteria for Selection of Dewatering Methods?

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Dewatering is the process of removing excess water from saturated soil to facilitate the excavation process and create safe working conditions.

Various dewatering methods can be employed during excavation works to facilitate construction operations. The selection of suitable dewatering techniques could help avoid the extra costs and complete the project within the designated time frame.

Engineers need to understand the different types of dewatering methods and their features to choose the right one when needed. In addition, different criteria, such as the nature of the work and construction site condition guide the engineers toward selecting proper and cost-effective dewatering methods. 

Criteria for Selection of Dewatering Methods

1. Factors controlling selection of dewatering methods
2. Location and features of the project site
3. Types, size, and depth of planned excavation
4. Type and thickness of soil layers to be excavated
5. The water table at the project site
6. Permeability of soil that would be located underneath the foundation structure
7. Timeline of the project
8. Importance of the project
9. Cost of installation and operation of the dewatering system
10. Factors governing the cost of the dewatering system, including pumping requirement, power availability and its type, labor resources, time period of pumping, time available for the construction work, required speed of dewatering, and presence of structures around the project site.

How to Choose Dewatering Methods?

The following table can be used by the site engineers to select dewatering methods based on the soil type and required dewatering depth.

Table-1: Dewatering Methods and Selection Criteria

Dewatering Method Types	Conditions of Applicability
Open sump pump	Suitable for dewatering in cohesive soils, where the percolation rate is slow and where the water table is not much higher than the final elevation of the excavation base. It is suitable for a depth of up to 3 m. The open sump pump can be used in well-graded coarse soils or fissured rocks.
Wellpoint system	Economical and cost-effective when dewatering has to be carried out in a ground area where the groundwater level is at a lower depth. It is suitable for the cases where the water table needs to be lowered up to a depth of 6 or 8 m. The soil below the water table should have good draining such as sand; otherwise, this method should not be used. Therefore, this type of dewatering system is effective in soils constituted primarily of sand fractions or other soil containing seams of such materials.
Shallow Well	Shallow wells are used in permeable soils when the well-point systems would be expensive and often at inconveniently close centers. It is a suitable dewatering option in gravels to silty fine sands and water-bearing rocks. A shallow well is ideal up to a depth of about 8 m.
Deep well system	It is a good choice for dewatering that requires penetration into a permeable layer of shale, soil, or rock. The deep well system works well in soils or rocks where permeability is between moderate (sands) to high (gravels). This technique is mainly used to construct deep shafts, tunnels, or caissons. A deep well is used to extract water from depths greater than 8 m.
Electro-osmosis	Suitable for fine-grained cohesive soils such as clays which can be drained or stabilized using electric current.
Ground Freezing	Ground freezing is used for groundwater cutoff, earth support, temporary underpinning, stabilizing earth for tunnel excavation, arresting landslides, and stabilizing abandoned mineshafts. Freezing acts as water cutoff and earth support resulting in the elimination of sheeting and bracing.

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