It is not uncommon to encounter copious amounts of water when digging trenches for pipelines or excavating for foundations at construction sites. Since water table depth varies at different places, it is also possible that some places may not encounter any water while digging. However; in most cases, water is encountered, which needs to be drained effectively to stabilize the soil so that construction work can proceed. Usually construction crews utilize water pumps to dewater the area. This water is often let out into wetlands, lakes or other water bodies, which have lower ground water levels; although, there are times when this water is openly released, causing erosion of surrounding soil. If dewatering is not managed well, it can cause seepages, inflows and instability of the excavation, erosion of the side slopes of the excavation or excess groundwater pressure on retaining structures from hydrostatic loads, leading to delays or even failure of the project.
Trenchless technology has brought with it the option of using horizontal wells, perforated pipes or well point systems, situated around the construction site to remove the water prior to or during excavation. This is cost effective, quick and safer than other dewatering systems. The best part about dewatering using trenchless methods is the fact that surface disruption, by means of huge excavation pits, and diversion of vehicular traffic is eliminated. Depending on the type of project undertaken, and the level of the groundwater table, dewatering method needs may vary. Safe practices have to be followed irrespective of the method of dewatering. (Learn more in "Trenchless Operations Safety Do’s and Don’ts".)
Groundwater or surface water from rain or streams needs to be drained before construction can begin. This process of draining is called dewatering. Since many constructions require going below the groundwater table, different techniques are utilized depending on the soil condition, dimension of the excavation, and the amount of groundwater encountered.
Sump pumps are commonly used; this method requires allowing groundwater to enter into the excavated area and then pumping it away with heavy duty pumps that can handle mud and solids. This method, though effective, is, however, prone to cause instability to the walls of the excavation and can also cause other problems during the construction phase. Moreover this method cannot be used for more complicated dewatering needs. (Read on in "When Ground Improvement is Needed During Trenchless Rehabilitation.")
The basic necessity of any construction is its foundation. Digging is inevitable and so is the presence of ground water. Work for foundation and other sub surface structures cannot be carried out in wet conditions. The ground should be relatively dry and stable to prevent sinkholes during excavation and collapse of pit walls, which can be a serious problem. To ensure stability of the slopes of the excavated pit and prevent groundwater from seeping into the excavation, lowering the groundwater level before excavating is a good technique. Below are a few methods of dewatering using trenchless technology.
For shallow depths of excavation, especially for laying pipelines, wellpoint dewatering is a quick and effective method. It temporarily removes groundwater from soil in a specific area so that pipelines, foundations, sub surface structure or soil remediation can be carried out. It consists of small diameter wells that are connected to a header pipe and a wellpoint pump. The pump draws up water through notches in the well points by creating a vacuum in the header pipe.
These pumps are designed for high air handling capacity to handle gasses, air and water. Air control is important as excess air can cause cavitation, reducing the efficiency of the pump.
For long excavations for pipelines, a flexible, perforated pipe is installed horizontally using a trenching machine. The un-perforated end of the pipe is brought to the ground surface and attached to a wellpoint suction pump.
Horizontal Directional Drilled (HDD) Wells
Horizontal directional drilled wells are used where permanent dewatering solutions are required, like those needed beneath existing structures. This method is also used for remedying contaminated groundwater and tunnel construction. The areas associated with these structures are usually inaccessible, and HDD wells provide a great dewatering solution.
Deep wells are placed just outside the area of a proposed excavation site. It consists of a series of bored wells fitted with submersible pumps at the bottom. Each well is capable of creating a cone of depression around itself while drawing water to the surface. Depending on the depth of the wells, several such wells, when pumping together, can effectively lower the groundwater level over a large area.
Similar to deep wells, these wells also use a vacuum pump at the surface in addition to the submersible pumps at the bottom. This makes it twice as effective by reducing pore water pressure in fine-grained soils that do not drain well.
In soils such as silt, clay and silty sands with low permeability, the eductor system works to control the pore water pressure. Where wellpoints and deep wells may not be effective in dewatering, eductors can be used to stabilize the slopes and the base of the excavation. High pressure water is circulated from a tank at the surface down to the eductor in the well fitted with a small diameter nozzle and venturi. A vacuum is created up to the level of the eductor, which then draws the groundwater into the well from where it is removed using a riser pipe. (Read on in "Ground Improvement Requirements When Working With Sandy Soil".)
Good planning and the appropriate method for dewatering are necessary to make the construction project cost effective and successful. Trenchless methods of dewatering are quickly gaining popularity because they are versatile and very effective. A proposed site can be drained before excavation begins, reducing the time required to drain out collected water from an excavated pit. The dewatering operation can be carried out even during the construction phase to keep the area dry and stable. With several options available for dewatering depending on the soil conditions encountered, it becomes more economical for a project when the right dewatering technique is used based on tests conducted to determine the water content of the soil in which the construction is to be carried out.
Trenchless dewatering also significantly reduces the chances of contamination and settlement. However; all dewatering techniques may impact the flow of water to supply wells, wetlands and springs in the vicinity. It is necessary to carry out Impact Assessment to mitigate any adverse effect on the environment.