OPTIMISATION OF DEWATERING SYSTEMSMartin Preene, Preene Groundwater Consulting, UKErik Loots, Loots Groundwater International, Netherlands

SYNOPSIS

• Dewatering and groundwater control

• Why optimise?

• Methods of optimisation

• Potential problems and possible priorities for optimisation

PRACTICE PROFILE

Preene Groundwater Consulting is the Professional Practice of Dr Martin Preene and provides specialist advice and design services in the fields of dewatering, groundwater engineering and hydrogeology to clients worldwide

Dr Martin Preene has more than 25 years’ experience on projects worldwide in the investigation, design, installation and operation of groundwater control and dewatering systems. He is widely published on dewatering and groundwater control and is the author of the UK industry guidance on dewatering (CIRIA Report C515 Groundwater Control Design and Practice) as well as a dewatering text book (Groundwater Lowering in Construction: A Practical Guide to Dewatering)

DEWATERING AND GROUNDWATER CONTROL

Groundwater control, commonly known as dewatering, can involve:

• Pumping methods: sump pumping, wellpoints, deep wells, ejector wells

• Exclusion methods: low permeability cut-off walls, grout curtains, artificial ground freezing

Pumping methods

Exclusion methods

WHY OPTIMISE?

• Dewatering is often one of the first geotechnical processes on a project

• It is not ‘codified’ in the same way as some other geotechnical processes

• Dewatering guidance documents tend to take a ‘toolkit’ approach to the selection of methods

• The designer can be faced with a bewildering range of dewatering design and implementation options

METHODS OF OPTIMISATION

Empirical optimisation• Design based on experience, local knowledge and rules of thumb – can be

very effective on simple projects

Analytical optimisation• Based on ‘textbook’ type analytical equations. Suited to relatively simple

hydrogeological conditions. It is important to use equations that are appropriate to the hydrogeological conceptual model

METHODS OF OPTIMISATION

Numerical optimisation• Numerical groundwater modelling is used far more than 10 years ago, because costs for

software, hardware and training have dramatically reduced• Can allow rapid comparison of different scenarios, but must be based on a valid

hydrogeological conceptual model

Observational optimisation• Use of construction observations (e.g. drawdown, pumped flow rates) • Can be planned from the start to deal with complex or poorly defined conditions• Can be used in the field as ‘troubleshooting’ measure if problems occur once a project has

started

POTENTIAL PROBLEMSLack of clarity in objectives of optimisation• Traditionally optimisation focuses on pumping rates, but there are other aspects (environmental

impacts, timescales, etc). Optimising one aspect may conflict with othersData quality and quantity• Poor data quality and quantity can be a problem. No amount of modelling and analysis can

correct false or poorly determined parametersErrors in conceptual model• Many significant dewatering problems can be traced back to an inappropriate conceptual model.

This can cause the designer to ignore a design condition that is, in fact, importantInappropriate dewatering method• Most dewatering techniques are only appropriate in a limited range of ground conditions. A

change in method partway through site works will be expensive and cause delays

POSSIBLE PRIORITIES FOR OPTIMISATION

Several different strategies can be adopted for optimisation

CONCLUSION• The wide range of dewatering techniques means that optimisation can be

useful

• There is no perfect optimisation method, and optimising in some aspects may create conflicts in others

• Required conditions for effective optimisation include:– Clarity of objectives– Adequate site investigation data– Valid hydrogeological conceptual model– Selection of appropriate dewatering method at the earliest stage possible

OPTIMISATION OF DEWATERING SYSTEMSMartin Preene, Preene Groundwater Consulting, UKErik Loots, Loots Groundwater International, Netherlands

Contact:Martin Preene: mp@preene.comErik Loots: erik@lootsgwt.com

© M Preene and E Loots 2015