Project Dewatering – A Cut Above the Rest

Let’s be honest – dewatering is a rather niche industry to say the least, with a myriad of intricacies and complexities that would likely not be apparent to anyone without a working background in the field. For example, it might come as a surprise to those not in the know just how important monitoring is during the commissioning stage in allowing refinements to be made to the system while plant, equipment and staff resources are still in attendance. Commissioning data can then be used to validate design assumptions, and if necessary, back-analysis of field data can be used to quickly recalculate the assumed permeability. Once installed, ongoing monitoring is then key moving forward to:

 

  • Prove performance.
  • Show trends indicating loss of system efficiency (perhaps as a result of biofouling or clogging).
  • Provide warnings and alarms in the event of partial or full pump system failure.

 

Without monitoring, there is simply no way of quantifying how effective the system is, and those dewatering and groundwater control contractors who do not incorporate monitoring in their project management structure run a greater risk of pump system failure, and are also more likely to incur excessive costs due to a lack of efficiency.

 

This, however, is just the tip of the iceberg when it comes to the less-apparent considerations which need to be taken into account in order to make sure that a dewatering project runs smoothly and without incident. Project Dewatering prides itself on its large team of seasoned dewatering and groundwater control experts, and we would like to take this opportunity to share a little of their knowledge in this area, and to show why we are a cut above the rest. Here are four more common dewatering complications caused by oversights made during a project, and the corresponding workarounds that should be applied in order to avoid or correct them.

 

1.

The Complication

Pump damage, pollution of water bodies, and ground loss (thereby undermining structures and foundations), due to pumping of fines.

 

The Workaround

The specification of slotted screens and filter materials for deepwells and ejector wells need to be considered carefully in any well design. The aim here is to allow the most efficient pumping well possible, while still retaining fines. Pumping of fines can, however, lead to the complications listed above, and therefore a screen slot width should be specified, which will match the soil grading as shown by Particle Size Distribution (PSD) analysis, with a minimum filter medium, sized either equal to, or slightly larger than, slot width.

 

2.

The Complication

Inefficiency due to unnecessary ‘over-engineering’.

 

The Workaround

An effective dewatering system should always include some degree of contingency in regard to flow variation. A relatively small change in ground permeability can have significant impact on flowrates, so it is always easier and more practical to downgrade or ‘trim’ an individual pump if flows are less than expected than to have to install larger pumps, pipework infrastructure, electrical switch-gear and cabling to deal with higher flowrates. When calculating flowrates and well yields, it is always best practice to carry out a sensitivity analysis to determining the impact of a range of permeabilities on system requirements. Understanding these effects leads to a robust well-understood solution.

 

3.

The Complication

Inadequate well-yield.

 

The Workaround

When considering well design, another important aspect is the wetted screen length. The designer should always consider whether the residual wetted screen (in pumped conditions) is sufficient for maintaining an adequate well-yield. In cases where there is a thin aquifer, or water bearing layer, additional wells are needed to ensure sufficient aggregate wetted screen length for the system. For reference, approximate maximum well yields can be found in the graph below:

 

Approximate Maximum Well Yields

(Preene, Roberts, Powrie and Dyer. 2000)

 

4.

The Complication

Lack of servicing time in the event of a single pump or power supply failure.

 

The Workaround

Carrying out a simulated switch-off or system failure test is always recommended as part of the commissioning system. This allows the rate of water-level recovery to be determined which in turn informs decisions made on the appropriate service response time.

 

To summarise, management of an effective dewatering system involves a lot more than simply drilling activities and pump installation. A good contractor should always provide pro-active and ongoing monitoring in order to instil confidence in the solution they have supplied. We believe that our seasoned dewatering experts have the knowhow needed to not only tackle any complication that may arise during a project, but to put the necessary measures in place to prevent them from occurring in the first place. And on top of this, our inter-European partnership with Hölscher Wasserbau grants us access to the latest innovations in both equipment and techniques, and gives us a uniquely advantageous perspective within the UK dewatering industry.

 

If you would like to find out more about how Project Dewatering can help you with your particular groundwater problem, get in touch today to book a consultation with our experienced experts via 01473 658 807 or enquiries@project-dewatering.co.uk – We look forward to hearing from you.

 

 

Bibliography

Preene, M, Roberts, T 0 L, Powrie, W and Dyer, M R (2000) Groundwater Control: Design and Practice. Construction Industry Research and Information Association. CIRIA Publication C515.