Ejectors

Ejectors are at times the only realistic method to achieve pore water pressure relief in low permeability soils, such as Silts and fine Sands. Project Dewatering Ltd can offer the solution where the flow rates are too small to run an effective Deep Well system or the depth is beyond the economic and spatial use of Wellpoints.

Ejectors have the advantage of having no realistic limit of suction lift, as do Wellpoints (practically they operate upto 30-50m), and can even run at times of no water entry into the well. Further to this, there ability to pump both air and water can be advantageous, as if the ejector wells are sealed a vacuum will be automatically developed in the well, and improve the gravity drainage towards the well by creating a vacuum assisted drainage path.

Ejectors are installed as per Deep Wells, with a similar well construction. Project Dewatering Ltd Project has probably the best drilling capability of any groundwater company in the UK. We can use a variety of drilling methods to deal with all ground conditions.

The principle of an ejector system, is that groundwater is drawn into a strainer at the base of the ejector body, by the creation of a vacuum. The vacuum or low-pressure zone is caused by supply water being pumped at high pressure (typically 7-8 Bar) through a nozzle. Upon its exit into the venturi, supply water is mixed with the induced groundwater and is piped to the surface through a larger diameter return pipe.

Submerged items

Ejector unit

uPVC well liner
(100mm dia Screen & Casing)

Supply & Return riser Pipe

Filter Pack & Bentonite/Grout Seal

Surface items

Sealing Cap

Supply & return control lines 

Supply & Return
Collection Main

Pumping Station
(Pumps & Tank)

They are simplistic in their surface layout, basically re-circulating supply and groundwater with any excess water, i.e. the total extracted groundwater being piped off to discharge. Returned water will enter a static tank via a common collection pipeline. This tank acts as settlement and supply tank. It contains four chambers separated by baffle plates. Chamber 1 is the entry point; water then passes in a settlement chamber before flowing over a V-notch into the supply chamber. Any excess water flows over a second V-notch to a final discharge tank, allowing the ‘total’ extracted groundwater to be discharged by gravity or pumped to a designated point.

The supply chamber is coupled to the high-pressure pump, which pumps water along the common supply pipeline, to individual supply pipes, at each ejector well. The pump is a multi-stage horizontal rotary pump, powered by a silenced diesel or electric engine. This is coupled to a standby pump that automatically starts if the duty pump fails. Plus this is an alarm facility that is
triggered if the supply pressure falls below a set level.

 

 

Pressure Relief Wells

Pressure relief wells are simply solution to controlling excess pore water pressures in confined aquifers. The artesian or sub-artesian head within a confined aquifer can be dangerous if the overburden pressure is equal or less than that of the confined pore water pressure, i.e. if not dealt with it will result in piping or heaving of the base of the excavation.

There are two basic means of relieving this pressure; one is the use of passive pressure relief wells the other is the utilising, active pumping pressure relief wells, which can consist of either Deep wells or Wellpoints.

It is always recommended to adopt an active pumping pressure relief wells, as the control ability is higher. A passive relief system although economically attractive can generate difficult working conditions, with groundwater seeping onto the excavation floor, needing a progressive sumping arrangement. They can also prove difficult to seal effectively upon completion.

A variation of a passive well is Sand drains, which is provides a drainage path between two aquifers. Typically, they are used to drain a perched aquifer, which cannot dewater by gravity drainage into a dewatered lower aquifer. They are effective, in particular where the perched water would cause stability problems to an open-cut excavation, or to speed up the consolidation of Silts and soft Clays.