Ejectors
The ejector system is made up of an array of
wells. The wells may either be closed or
sparsely spaced. However, the commonality in the wells is that the jet pump,
called an ejector is used for pumping. Initially, the ejector dewatering
technique was developed in North America in the 1950s and 1960s. During this
period, the jet pumps used in domestic supply wells were first applied to
groundwater lowering problems. Ever since, the technique has been a common
occurrence in implementation in Europe, the former USSR and the Far East.
However, in the United Kingdom, ejectors were rarely applied prior to the late
1980s. The implementation was adopted during the A55 Conwy Crossing Project.
This project made use of ejectors on a large scale throughout the U.K (Powrie
and Roberts 1990)
An ejector is suitable for pore water pressure
reduction projects in low-permeability soils such as very silty sand, silt, or
clay with permeable fabric. The main reason for using the ejectors in this
soils is due to the fact that they aid in drainage. Their characteristics such
as the ability to reduce the flow rate and the creation of a vacuum makes them
ideal for these usage.
Advantages of
ejector systems
The ejector system works by circulating
high-pressure water. The water is usually from a tank and a supply pump
placed at ground level. The flow follows a down riser pipe and
through a small diameter nozzle and venturi located in the ejector in each
well. The water passes through the nozzle at high velocity, thereby creating a
zone of low pressure. A vacuum of approximately 9.5m from the level of the
ejector is created. The vacuum draws groundwater into the well through the well
screen, where it joins the water passing through the nozzle, piped back to
ground level via a return riser pipe, and then back to the supply pump for
recirculation. Two header mains are needed. A supply main feeds high-pressure
water to each ejector well, and a return main collects the water coming out of
the ejectors. This consists of the water supply and the groundwater as it is
drawn into the well. For the recirculation processes to continue this pipework
is needed.
Another advantage of an ejector system is that
ejectors have the ability to pump both air and water. Therefore, if the
ejectors are installed in low-permeability soil, a vacuum will be developed in
the well. This is one of the main reasons that ejectors are suitable for use in
low-permeability soils, where the vacuum is needed to enhance drainage of soils
into the wells. Another advantage is that the method is not constrained by the
same suction lift limit as a wellpoint system. Drawdowns of 20–30 m below the
pump level can be achieved with commonly available equipment, and drawdowns in
excess of 50 m have been achieved with systems capable of operating at higher
supply pressures. These characteristics mean that ejector systems are mainly
applicable in the following ways:
1.
As a vacuum-assisted pore water pressure control method in low- permeability
soils.
2.
As a form of “deep wellpoint” in soils of moderate permeability as an
alternative to a two-stage wellpoint system or a low-flow-rate deep well
system.
It is worth noting some of the practical
limitations and drawbacks of the ejector system. One of the
major drawback is the low mechanical efficiency of ejector
systems. This means that the energy levels are low. In low- to
moderate-permeability soils, where flow rates are small, this may not be a
major issue, but in higher-permeability soils, the power consumption and energy
costs may be huge in comparison to other methods. As a result,
the ejector system is rarely used in soils of high permeability. Another
concern is that an ejector systems are prone to gradual loss of
performance due to nozzle wear or clogging. This is due to the high
water velocities through the nozzle. Regular monitoring and maintenance can be
used to mitigate this problem. However, adopting this approach may lead
to long-term operations being less straightforward.
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