Procedures used for dewatering plans for an excavation includes selection of dewatering method, determination of hydraulic parameter, determination of the well capacity and Estimation the number of wells.
Fig.1: Dewatering Excavations
Set Up Dewatering Plan for Excavations
1. Selection of Dewatering Method for Excavation
The selection of dewatering method is the first step to establish dewatering plan. Table 1 can be used to select suitable dewatering method based on the type of soil and excavation depth.
Different types of excavation dewatering system are shown from Figure 2 through Figure 6.
Table-1: Selection of dewatering method based on the type of soil and excavation depth
|Dewatering methods||Suitable type of soil||Suitable depth of excavation, m|
|Open sump||Gravel (including fine, medium, and coarse), and coarse sand||Less than 4|
|Well point||Coarse; medium; and fine sand, and fine gravel||4-8|
|Vacuum well point||Fine and medium sand, coarse and medium silt||4-8|
|Electro-osmosis||Fine sand, coarse; medium; and fine silt||4-8|
|Deep well||Coarse; medium; and gravel, coarse and medium sand||20-24|
|Deep well+ auxiliary vacuum pumps||Fine grave, coarse; medium; and fine sand, coarse and medium silt||Greater than 28|
Fig.2: Open Sump Dewatering System
Fig.3: Well Point Dewatering System
Fig.4: Electro-Osmosis Dewatering System
Fig.5: Deep Well Excavation System
Fig.6: Deep Well Excavation System
2. Determination of Hydraulic Parameter
There are number of hydraulic parameters that need to be evaluated based on the formula used. For example, it is required to evaluate coefficient of permeability solely if equilibrium formula such as Thiem’s equilibrium equation is used.
Coefficient of permeability can be computed using pumping tests, falling head test, constant head test, or empirical formula. The most acceptable technique for coefficient of permeability is empirical formula and pumping tests.
Filling head and constant head tests are not desirable because the disturbance of the samples would prevent fully represent the in-situ soil.
If un-equilibrium equation is used for instance Theeis’s and Jacob equation, then coefficient of transmissivity and coefficient of storage need to be computed.
The coefficient of transmissivity is equal to the coefficient of permeability times the thickness of aquifer. The coefficient of storage ranges from 0.0005 to 0.001.
3. Determination of Well Capacity
It is considerably significant to evaluate the capacity of the well since it is the base for the design and cost of dewatering process execution.
Empirical formula has been developed to estimate the capacity of the well. The equation can be expressed as follow:
Qw: Well discharge capacity, m3/s
rw: Radius of the well, m
k: Coefficient of permeability
It should be known that, the evaluation of hw is very difficult, that is why it is acceptable to assume that hw is equal to the groundwater level in the deep well at the preliminary estimation.
Because the computations of well capacity have substantial significant, that is why the provision of a safety factor is highly recommended. It is possible to calculate well capacity using step drawdown pumping test which more reliable than the value obtained from empirical formula.
Fig.7: Capacity of Well
4. Estimation of Number of Wells
Generally, groundwater level shall be lowered around 0.5 to 1m below the bottom of excavation so as to keep the bottom of excavation dry. This cannot be achieved unless required numbers of wells are placed.
Steps required to be taken to estimate number of wells include:
- The calculation of total quantity of water required to be pumped using either equilibrium or non-equilibrium empirical equation as discussed above (determination of hydraulic parameters).
- Compute the capacity of each wells using equation 1 explained in the above section.
- Compute the number of wells by dividing total amount of water needed to be pumped by the capacity of single well.
Fig.8: Dewatering Wells Installed in an Excavation