Trenchless technology is the answer to many of the woes encountered in open cut construction and rehabilitation of pipelines. Methods such as microtunneling and pipe jacking, cured-in-place pipe (CIPP) and sliplining have made life much easier for contractors as well as the public in general.
However, problems such as encountering groundwater during excavation have to be dealt with, whether employing open-cut construction methods or trenchless methods. While trenchless construction methods are, as the name implies, essentially trenchless, they do require shafts to be excavated in order to reach the required depth for tunneling.
When water is encountered, the shaft has to be dewatered.
What is Dewatering?
Water in an excavation needs to be drained effectively as part of ground improvementsto stabilize the soil, allowing construction work to proceed. Usually construction crews utilize water pumps to dewater the area. (Read: Dewatering: Selecting the Best Pump for Your Application.)
This water is often let out into wetlands, lakes or other water bodies, which have lower ground water levels. Sometimes this water is let out in the open, 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 Dewatering Methods
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 needs may vary. Safe practices have to be followed irrespective of the method of dewatering. (Read: Trenchless Operations Safety Dos and Don’ts)
In many places where there is a short time frame and high cost of traditional dewatering, horizontal directional drilling (HDD) is carried out to place perforated pipes below the water table. This method was undertaken in Melbourne, Australia, to install a dewatering system for the AU$6 billion Level Crossing Removal Project.
This helped to create a comfortable environment to carry out bulk excavation work below the train lines without disruption of services. This method of dewatering using HDD is cost effective and time saving.
Some trenchless dewatering methods are:
For shallow depth of excavation, especially for laying pipelines this method helps remove groundwater from soil in a specific area so that pipelines, foundations, subsurface 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 Wells
This method is used where permanent dewatering solutions are required, like those needed beneath existing structures. It 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.
These wells are placed just outside the area of a proposed excavation site. Deep wells consist 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 doubly 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.
Minimizing Dewatering for Shaft Construction
Often shafts for trenchless projects have to be sunk in very difficult ground conditions. (Read: Soil Types and How They Affect Trenchless Construction.) In most shaft sinking techniques, the groundwater table has to be lowered to enable the construction to be done in relatively dry conditions without groundwater ingress. However, lowering the groundwater using pumping wells can have adverse effects on surrounding infrastructure causing settlement and damage to property.
According to Ben Ablett, Principal Design Engineer at Golder, there is a way to combine watertightness with temporary and permanent ground support for shaft construction. Jacked precast segmental shaft construction can be carried out through strata containing groundwater without having to dewater. The method utilizes a precast gasketed lining system that allows the shaft lining to be constructed as the shaft is gradually sunk.
The use of hydraulic jacks to push precast shaft linings into the ground provides a quick, reliable and efficient means to construct shafts. This implies that it is possible to sink shafts through water-bearing strata, seal and then drain them without undertaking expensive dewatering techniques and preventing damage to surrounding structures. The precast rings are provided with gaskets that provide an immediate water tight seal during construction while bolting the rings together.
The compressed gaskets limit the amount of water ingress to an acceptable level for use in the transportation, water and utility industries.