Dewatering is a critical step in excavation projects to ensure a safe, dry work environment below the groundwater table. Proper planning and execution of dewatering processes prevent water-related problems such as soil instability, flooding, and construction delays. This article outlines the key procedures involved in developing an effective dewatering plan for excavations, including the selection of appropriate methods, determination of hydraulic parameters, evaluation of well capacity, and estimation of the number of wells required.
1. Selection of Dewatering Method for Excavation
The first step in establishing a dewatering plan is selecting the most suitable method based on site conditions. The choice largely depends on the type of soil and the depth of excavation. Table 1 below provides a guideline to select the optimal dewatering system:
| Dewatering Method | Suitable Soil Type | Suitable Excavation Depth (m) |
|---|---|---|
| Open sump | Gravel (fine, medium, coarse), coarse sand | Less than 4 |
| Well point | Coarse, medium, fine sand; fine gravel | 4 – 8 |
| Vacuum well point | Fine and medium sand; coarse and medium silt | 4 – 8 |
| Electro-osmosis | Fine sand; coarse, medium, fine silt | 4 – 8 |
| Deep well | Coarse, medium gravel; coarse and medium sand | 20 – 24 |
| Deep well + auxiliary vacuum pumps | Fine gravel; sand; coarse and medium silt | Greater than 28 |
Each method has specific equipment and operational principles:
- Open Sump System: Ideal for shallow excavations in gravelly or sandy soils (Fig. 2). Water collects in a sump and is pumped out.
- Well Point System: Consists of shallow wells with small diameter points connected to a header pipe and vacuum pump (Fig. 3).
- Electro-Osmosis System: Uses electrical current to move water through fine-grained soils where traditional methods struggle (Fig. 4).
- Deep Well System: Employed for deep excavations, uses large-diameter wells and submersible pumps to lower the water table (Figs. 5 & 6).
Proper selection is critical to optimize cost and efficiency while maintaining excavation safety.
2. Determination of Hydraulic Parameters
Hydraulic parameters describe how groundwater moves and is influenced by pumping. These parameters vary based on the dewatering formulas used:
- Coefficient of Permeability (k): Measures soil’s ability to transmit water. It is essential when applying equilibrium methods like Thiem’s equation.
- Coefficient of Transmissivity: Product of permeability and aquifer thickness, used in non-equilibrium methods such as Theis and Jacob equations.
- Coefficient of Storage: Represents the volume of water an aquifer releases from storage per unit surface area per unit decline in hydraulic head; typically ranges from 0.0005 to 0.001.
The most reliable ways to determine permeability include pumping tests and empirical formulas. Tests like falling head and constant head are less favored as soil sample disturbance can affect accuracy.
3. Determination of Well Capacity
Accurate evaluation of well capacity is vital for designing the dewatering system and estimating project costs. The capacity refers to the discharge rate of each well, usually expressed in cubic meters per second (m³/s). An empirical formula for well capacity is: Qw=π×k×hw×rwQ_w = \pi \times k \times h_w \times r_wQw​=π×k×hw​×rw​
Where:
- QwQ_wQw​ = Well discharge capacity (m³/s)
- hwh_whw​ = Groundwater level at which water flows into the well (m)
- rwr_wrw​ = Radius of the well (m)
- kkk = Coefficient of permeability
Determining hwh_whw​ precisely is challenging, so for preliminary estimates, it is often assumed equal to the groundwater level in the deep well. Given the uncertainties, applying a safety factor to the capacity is recommended.
For more accuracy, a step drawdown pumping test is conducted to directly measure well capacity under operational conditions (Fig. 7).
4. Estimation of Number of Wells
To maintain a dry excavation bottom, the groundwater level should be lowered approximately 0.5 to 1 meter below the excavation base. Achieving this requires an adequate number of wells. The estimation process includes:
- Calculate total water volume to be pumped: Use equilibrium or non-equilibrium hydraulic equations with the parameters determined earlier.
- Calculate capacity of a single well: Using the formula or test results described above.
- Divide total pumping volume by single well capacity: This gives the number of wells needed.
This calculation ensures the dewatering system can handle the inflow, maintain excavation stability, and prevent flooding.
Conclusion
Developing an effective dewatering plan for excavations requires careful selection of the dewatering method tailored to soil and excavation depth, precise evaluation of hydraulic parameters, accurate well capacity assessment, and proper estimation of well numbers. Adhering to these procedures not only optimizes project cost and safety but also ensures successful and timely excavation work free from water-related issues.