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Chapter 1
Groundwater Lowering
A Personal View and Introduction by Pat M. Cashman
Over a thousand years ago King Canute learned by experience that control of water cannot be achieved by words alone.
Pat M. Cashman
Many engineering projects, especially major ones, entail excavations into water-bearing soils. For all such excavations, appropriate system(s) for the management and control of the groundwater and surface water run-off should be planned before the start of each project. In practice, this can only be done with knowledge of the ground and groundwater conditions likely to be encountered by reference to site investigation data. The control of groundwater (and also surface water run-off) is invariably categorized as âtemporary worksâ and so is often regarded by the client and their engineer or architect as the sole responsibility of the contractor and of little or no concern to them. In many instances, this philosophy has been demonstrated to be short-sighted and ultimately costly to the client.
Sometimes, as work proceeds, the actual soil and groundwater conditions encountered may differ from what was expected. Should this happen, all concerned should be willing to consider whether to modify operations and construction methods as the work progresses and as more detailed information is revealed. Based upon this philosophy, I advocate, particularly for large projects, frequent âengineering-orientedâ reappraisal meetings between client or owner, or both, and contractor (as distinct from âcost-orientedâ meetings). This will afford the best assurance that the project will be completed safely, economically and within a realistic programme time and cost.
On a few occasions, I have been privileged to be involved in the resolution of some difficult excavation and construction projects when the engineer succeeded in persuading the client to share the below-ground risks with the contractor. During the progress of the contract, there were frequent engineering-oriented meetings with the contractor to discuss and mutually agree how to proceed. I believe that the engineers concerned with these complex projects realized that it would not be in the best interests of their client to adhere rigidly to the traditional view that the contractor must take all of the risks. They were enlightened and had a wealth of practical experience, and so had a realistic awareness that the soil and groundwater conditions likely to be encountered were complex. Also, they realized that the measures for effecting stable soil conditions during construction might not be straightforward. The few occasions when I have experienced this joint risk-sharing approach have, without exception in my view, resulted in sound engineering solutions to problems that needed to be addressed: they were resolved sensibly, and the projects were completed within realistic cost to the client. Furthermore, claims for additional payments for dealing with unforeseen conditions were not pressed by the contractor.
I found these experiences most interesting and enlightening, and I learned much by having direct access to different points of view of the overall project as distinct from my own view as a specialist contractor. I find it encouraging that in recent years, the target form of contract â the client and the contractor sharing the risks of unforeseen conditions â is being implemented more frequently. Thereby, the contractor is confident of a modest but reasonable profit, and the client is not eventually confronted with a multitude of claims for additional payments, some of which may be spurious, but all requiring costly and time-consuming analysis and investigation.
There are three groups of methods available for temporary works control of groundwater:
- a) Lowering of groundwater levels in the area of construction by means of water abstraction, in other words groundwater lowering or dewatering
- b) Exclusion of groundwater inflow to the area of construction by some form of very low-permeability cut-off wall or barrier (e.g. sheet-piling, diaphragm walls, artificial ground freezing)
- c) Application of a fluid pressure in confined chambers such as tunnels, shafts and caissons to counterbalance groundwater pressures (e.g. compressed air, earth pressure balance tunnel boring machines)
Rudolf Glossop (1950) stated:
The term drainage embraces all methods whereby water is removed from soil. It has two functions in engineering practice: permanent drainage is used to stabilise slopes and shallow excavations; whilst temporary drainage is necessary while excavating in water-bearing ground.
This book principally addresses the subject of temporary drainage, though many of the principles are common to both temporary and permanent requirements.
The book is intended for use by the practical engineer (either contractor or consultant or client); but it is intended particularly for the guidance of the specialist âdewatering practitionerâ or advisor. In addition, it is commended to the final-year graduate or masterâs student reading civil engineering or engineering geology as well as to the civil engineering-oriented hydrogeologist. It is deliberately addressed to the practitioner involved in the many day-to-day small- to medium-scale dewatering projects for which a simplistic empirical approach is usually adequate. It is anticipated that the typical reader of this work will be one quite comfortable with this philosophy but one who is aware of the existence of â though perhaps wishing to avoid â the purist hydrogeologist philosophy and the seemingly unavoidable high-level mathematics that come with it.
We, the writers and the readers, are pragmatic temporary works engineers â or, in the case of some readers, aspiring to be so â seeking the successful and economical completion of construction projects. For the small- and medium-size projects (which are our âbread and butterâ), there seems to be little practical justification for the use of sophisticated and time-consuming techniques, when simpler methods can give serviceable results. The analytical methods described in this book are based on much field experience by many practitioners from diverse countries and have thereby been proven to be practicable and adequate for most temporary works assessment requirements. I consider that J P Powers stated a great dewatering truism: âThe successful practitioner in dewatering will be the person who understands the theory and respects it, but who refuses to let theory overrule judgementâ (Powers et al., 2007).
Extensive use is made of the DupuitâForchheimer analytical approaches. I am conscious that purists will question this simplistic approach. My riposte â based on some 30 or more years of dealing with groundwater lowering problems â is that in my experience and that of many others, this empirical philosophy has resulted in acceptably adequate pumping installations; always provided, of course, that due allowance is made for the often limited reliability of available ground and groundwater information. Acquired practical field experience is required to assess the quality of the site investigation data. Whenever possible, reference should be made to other excavations in adjacent areas or in similar soil conditions to verify oneâs proposals. Towards the end of the book, in Chapter 27, there are some brief descriptions of a number of relevant case histories that the authors have dealt with in the past.
I readily acknowledge that for a groundwater lowering system design pertinent to large-scale and/or long-term projects â for example construction of a dry dock, a nuclear power station or an open cast mining project â more sophisticated methods of analysis will be appropriate. These can provide reassurance that the pragmatic solution is about right, but do we ever know the âpermeability valueâ to a similar degree of accuracy?
The underlying philosophy of this publication is to address the pragmatic approach. It follows that three questions arise:
- How does water get into the ground, and how does it behave whilst getting there and subsequently behave whilst there?
- What is the inter-relationship between the soil particles and the groundwater in the voids between them?
- How can groundwater and surface water run-off be controlled and so prevented from causing problems during excavation and construction?
A thorough site investigation should go a long way towards providing the answers to these questions. Unfortunately, experience indicates that many engineers responsible for specifying the requirements for project site investigations consider only the designerâs requirements and do not address the other important considerations, namely â how can this be built? Often, the site investigation is not tailored to obtaining data pertinent to temporary works design requirements or to problems that may occur during construction.
The contractor should not expect always to encounter conditions exactly as revealed by the site investigation. Soils, due to the very nature of their deposition and formation, are variable and rarely, if ever, isotropic and homogeneous, as is assumed in many of the analytical methods. The contractor should carry out the works using their professional skills and abilities and should be prepared to adjust if changed circumstances are revealed as the work proceeds.
Throughout the planning, excavation and construction phases of each project, safety considerations must be of paramount importance. Regrettably, the construction industry historically has a poor safety performance record.
Let us consider another professional discipline! Hopefully, no surgeon would contemplate commencing an operation on a patient without carrying out a thorough physical examination and having the information from x-ray; ECG; urine, blood and other test results; and any pertinent scans available beforehand. The surgeon will realize that these may not indicate everything and that during the operation complications may occur, but the possibilities of such âsurpriseâ occurrences will have been minimized by having reliable site investigation data concerning the patient.
Likewise, if the clientâs engineer/designer provides comprehensive ground and groundwater information at tender stage, the âsurpriseâ occurrences during construction should be minimized. An experienced contractor, with the co-operation of an experienced client/engineer, should be able to agree how to adjust working techniques to deal adequately with the changed circumstances as, ...
