The process of dewatering involves the removal of excess water from soil that is saturated, which is done to facilitate the excavation process and ensure safe working conditions. To achieve this, various dewatering methods can be employed during excavation works, and selecting the appropriate one can help avoid additional costs and ensure completion of the project within the designated timeframe.
For engineers to choose the right dewatering method when required, it is essential that they have an understanding of the different types of dewatering methods and their characteristics. Additionally, various criteria such as the nature of the work and the condition of the construction site play a crucial role in guiding engineers towards selecting a suitable and cost-effective dewatering method.
Criteria for Selection of Dewatering Methods
The selection of dewatering methods is dependent on several factors. One such factor is the location and features of the project site. Another factor is the types, size, and depth of the planned excavation. It is also important to consider the type and thickness of soil layers that will be excavated. The water table at the project site is another key factor to be considered, along with the permeability of the soil that would be located underneath the foundation structure. The timeline of the project and its importance are also important factors to be taken into account.
Cost is another important factor that governs the selection of dewatering methods. This includes the cost of installation and operation of the dewatering system, as well as factors governing the cost of the system such as pumping requirements, power availability and its type, labor resources, time period of pumping, time available for the construction work, required speed of dewatering, and the presence of structures around the project site. All of these factors need to be carefully considered in order to select the most appropriate dewatering method for a given project.
How to Choose Dewatering Methods?
The table provided serves as a useful reference for site engineers when determining the appropriate dewatering method based on the specific soil type and desired dewatering depth. It acts as a guide to help engineers make informed decisions when faced with dewatering challenges on construction sites. By considering the soil type and desired dewatering depth, engineers can refer to the table to select the most suitable dewatering method that will effectively remove excess water from the site, ensuring safe and efficient construction activities. This table serves as a valuable tool for site engineers in choosing the optimal dewatering approach tailored to the unique conditions of the construction site.
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. |
FAQs
What is dewatering?
Dewatering refers to the practice of eliminating surplus water from soil that has become saturated. The aim of this process is to make excavation easier and create a secure work environment. Typically, dewatering is carried out to reduce the water level in the soil so that excavation can take place without any issues. It is an essential process that is employed in construction and mining activities to prevent groundwater from impeding work progress. By extracting excess water from the soil, the stability of the excavation site is improved, and the safety of workers is enhanced.
What factors should be considered before dewatering?
Dewatering is a process used to remove water from the ground, and it is important to understand the type of ground for which this process is necessary. Additionally, before beginning any dewatering activities, it is crucial to check for signs of erosion and instability in the area to ensure safety.
Choosing the correct dewatering method is also important, as different methods may be more effective for different types of ground and water conditions. Weather conditions should also be taken into account when planning dewatering activities, as heavy rainfall or high winds may affect the process.
The right pump must be selected for the job as well, as the pump’s capacity and efficiency can impact the effectiveness of the dewatering process. Finally, it is essential to understand the water table conditions of the ground, as this can affect the duration and intensity of the dewatering process. By considering these factors, the dewatering process can be carried out safely and effectively.
What are the different dewatering methods?
- An open sump pump is a device used to remove water from a pit or sump. It consists of a pump that sits in the pit and pumps water out through a discharge pipe. Open sump pumps are commonly used in residential basements and are designed to handle relatively small volumes of water.
- A wellpoint system is a method of dewatering an area with a high water table. It involves installing a series of small wells, known as wellpoints, around the perimeter of the area to be dewatered. A vacuum pump is used to lower the water level in the surrounding soil, allowing construction or excavation work to take place.
- A shallow well is a type of well that is typically less than 30 feet deep. These wells are commonly used in rural areas to provide water for household use, such as drinking, cooking, and washing. Shallow wells can be dug by hand or using a drilling rig.
- A deep well system is a type of well that is typically more than 100 feet deep. These wells are used to access groundwater for drinking water or irrigation purposes. Deep wells require specialized drilling equipment and are typically more expensive to install than shallow wells.
- Ground freezing is a technique used in construction to create a temporary barrier of frozen soil or rock around a construction site. This technique can be used to prevent water from entering the construction area or to stabilize soil or rock formations. Ground freezing involves circulating a refrigerant through a series of pipes that are installed in the ground.
- Electro-osmosis is a technique used to reduce the water content of soil. This technique involves applying an electrical current to the soil, which causes the water to migrate towards the electrodes. Electro-osmosis is commonly used in construction to stabilize soil and prevent landslides.
- Which method is suitable for dewatering fine-grained soil?
- Fine-grained cohesive soils, such as clays, can be effectively drained or stabilized through the use of electro-osmosis. This method involves the application of electric current to the soil, which causes the movement of pore water and ions towards the cathode or anode, depending on the polarity of the electric field. The resulting flow of water can help to reduce the soil’s pore pressure, improve its drainage characteristics, and enhance its overall stability.
- Electro-osmosis is a particularly useful technique for cohesive soils, as these types of soils tend to have very small pore sizes and low permeability, which can make it difficult to achieve effective drainage through traditional methods such as gravity or vacuum pumping. By applying an electric field to the soil, however, electro-osmosis can induce a flow of water through the soil’s pores, even in the absence of any hydraulic gradient.
- Overall, the use of electro-osmosis represents a promising approach for the drainage and stabilization of fine-grained cohesive soils, and has been successfully applied in a variety of engineering applications, including slope stabilization, embankment construction, and foundation engineering.
Which method is suitable for dewatering fine-grained soil?
Electro-osmosis is a viable method for draining or stabilizing fine-grained cohesive soils, specifically clays, through the use of electric current. This technique is well-suited for soils with fine particles that tend to stick together, as it harnesses the power of electricity to facilitate drainage or stabilization processes. Fine-grained cohesive soils, such as clays, can be effectively treated using electro-osmosis, which exploits the unique electrical properties of these soils to improve their drainage or stability.