Our latest case study discusses our design, development and construction of two 4” long coupled heavy duty Self-Priming Pumps we are despatching for dewatering on a current construction project. The pumps will be used for the removal of water during diaphragm wall and cofferdam construction. 

What is a diaphragm wall? 

It’s a structural concrete wall built in a deep trench excavation, either cast in situ or using precast concrete components. Diaphragm walls are often used on congested sites, close to existing structures where there is limited headroom, or where the excavation is so deep it would require large amounts of soil to be removed. 

Diaphragm walls are suitable for most sub-soils and only a small amount of vibration and noise are generated during installation, which makes them suitable for works close to existing buildings.  

What is a cofferdam? 

This is a watertight enclosure built within a body of water to allow builders to dewater construction sites. It’s important to maintain a dry work site in waterlogged areas, so that the building work can be carried out safely. 

There are different types of cofferdams based on varying sizes, configurations and materials. These include earthen cofferdams, braced cofferdams, double-wall sheet piles, and cellular cofferdams which come in two types – the diaphragm type with circular arcs on the side, and the circular option with connected circular cells.  

The dewatering process  

Once the diaphragm walls and cofferdam are constructed, the dewatering process can begin. This involves temporary lowering of the groundwater level by pumping from purpose-built bores fitted in the work area. Groundwater is removed from inside the walls of an underground structure and pumped back into the ground outside the structure, through a series of pipes and reinjection bores.  

The main dewatering techniques 

Construction dewatering can be carried out using a variety of methods including wellpoints, deep wells, bypass and flood control. In wellpoint and deep wells, submersible pumps are installed in a drilled well shaft, while in bypass and flood control, pumps are placed in the area that needs to be dewatered.  

• Wellpoints 

A wellpoint is a small-diameter, closely-spaced shallow well, which offers highly economic and flexible groundwater control, where drawdown requirements are less than 5m to 6m in depth. 

Wells are drilled around the excavation area and submersible pumps are fitted in the well shaft. These pumps are connected to a header pipe, allowing the groundwater to be drawn up by the pumps into the wellpoints and then discharged. 

• Deepwells 

With deepwells, one or a number of individual wells are drilled, and submersible pumps are placed in each shaft. The deepwell technique is best suited to deep excavations where large amounts of water need to be moved. A range of deepwells is installed either internally or externally and can control groundwater along with a physical wall, such as a sheet-pile, secant-pile or diaphragm wall.  

• Bypass dewatering 

When sewer lines need maintenance, the sewage flow is pumped around the damaged pipe section using dewatering pumps, which are fitted upstream of the maintained pipe section. This bypass dewatering technique is also widely used in irrigation and construction projects. 

• Flood control 

In construction, tunnelling and mine work, site managers need to be constantly prepared for potential site flooding. Flood control is a range of methods which can be used to reduce or completely prevent flooding caused by storm water and heavy rainfall. These often involve having dewatering pumps nearby that can move large volumes of water against low head pressure.  

• Tunnelling dewatering 

Tunnels are complex worksites where massive amounts of water need to be removed to stabilise the ground and prevent flooding of the work area.  

Tunnel boring machines and drilling equipment need a reliable supply of cooling water, which must be recovered, extracted and treated after use. Therefore, tunnelling projects need to have many dewatering pumps on site, from small submersible pumps to very large units to carry out significant dewatering on site. 

How did our pumps meet the brief? 

Here at North Ridge Pumps, we’re not tied to a single pump technology, so we can provide the best pump for a particular application and designed for the lowest lifetime cost. In fact, we can use our expertise to offer a bespoke solution, not something straight off the shelf. 

For this particular project brief, we calculated that a robust design of a heavy duty self-priming pump was needed. This was because the project required pumps to be operating for long periods, prime quickly and handle fluids which might contain a variety of solids. 

Our pumps are unlike other designs of self-priming pump as they are built with an armoured volute, which protects the main casing from abrasive wear, as well as high speed impacts from solids.  

What’s the difference between pump casing and volute? 

A pump casing is often referred to as a volute. A volute is a spiral-like geometry with an increasing through-flow area, reducing the speed of the fluid and increasing the static pressure. The fluid exiting the impeller is then diffused towards the casing discharge nozzle. 

We built our cofferdam pumps using ductile iron which ensures that each unit is durable, handles solids up to 50mm x 24mm in diameter without loss of flow, and will perform well for many years to come.  

The pumps needed to meet a range of duty points from 50M³H to 150M³H at up to 60m head, so we fitted the pumps with motors suitable for operation via Variable Frequency Drive (VFD).  

What’s more, as our cofferdam pumps needed to meet three duty points, we constructed them with two control panels in a metal enclosure with integrated Danfoss inverters. This not only achieves the three duty points, but also activates the pumps via float switches. 

Made for harsh environments 

The enclosures were built in full metal enclosures with IP55 protection, for installation in rugged, hostile environments.  

In addition, the panels were built with a variety of features for pump protection and automated dewatering. These include on/off pump control via 4ma to 20ma input, pressure switch and float control, motor overload and phase failure protection. There’s also a back-up battery with GSM connection, so that operators can monitor the operating status of the pumps 24/7. 

In total, both dewatering pumps have three separate float switches for fully automatic control at low level for stopping the pumps, high level for starting the pumps, and for alarm and increased flow rate. Probes were also supplied to enable the pumps to operate in areas where float switches may not be suitable, such as in some tanks or narrow pits.  

If you have a site you’re looking to dewater or are simply want a high volume transfer pump in an easy to maintain, low maintenance, reliable surface-mounted unit, speak to North Ridge Pumps to see how we can help.