This edition of the blog discusses the different approaches that can be taken when designing construction dewatering and mine dewatering systems.
WHAT IS DEWATERING DESIGN?
Dewatering is more correctly described as groundwater control, and is the process of temporarily controlling groundwater for construction excavations or mining projects. As described in a previous blog there are two principal groups of groundwater control technologies: exclusion methods and pumping methods:
Pumping and exclusion methods may be used in combination.
Dewatering design is the process of developing a workable and economic solution to a groundwater problem that will affect an excavation. In some cases design starts with a clearly defined and understood groundwater problem, and the main design objective is to develop an appropriate solution. In other situations the nature of the groundwater problem may not be well understood, and the initial stages of design are effectively an investigation and definition of the problem, before specific solutions can be developed (this is often the case when groundwater problems are encountered part way through a project).
In essence, there are three elements to dewatering design: modelling, analysis and judgement.
Modelling: This does not automatically mean numerical modelling. In fact, the most important element is a conceptual groundwater model, which is the foundation of good dewatering design. For some complex or important dewatering designs, numerical models may be developed as part of later analysis.
Analysis: This can include numerical calculations, analyses or numerical modelling carried out, for example to estimate the required pump flow rate, or to assess potential environmental impacts. It is essential that the analysis methods used are appropriate to the conceptual model developed earlier.
Judgment: Modelling and analysis alone are not sufficient to ensure the design of effective groundwater control systems. An element of judgment is required to ensure that the proposed design is realistic and practicable. Judgment cannot be learned theoretically, but is developed through accumulated experience. But as a guide, judgement might involve comparing the proposed dewatering design with established empirical guidance, or with examples of similar dewatering systems in comparable ground conditions.
Successful design will typically require all three elements to be applied in some combination. Where dewatering design produces results that are perceived to be unsuccessful, it is often the case that one of the elements of design has been neglected.
POSSIBLE APPROACHES TO DEWATERING DESIGN
The are three principal approaches to the design of groundwater lowering or dewatering systems:
Empirical: A design based largely on experience, local knowledge and ‘rules of thumb’.
Analytical and Numerical: Use of hydrogeological design equations, either manually or by spreadsheet, and/or use of two- or three-dimensional numerical groundwater flow models.
Observational: Use of construction observations to design and refine the dewatering system.
EMPIRICAL DESIGN
Empirical design uses experience of previous projects nearby or in comparable conditions. For example, there are a number of simple dewatering cases that are sufficiently common that, once site investigation has confirmed there are no unusual complications, they can be designed purely empirically – almost by rule of thumb, based on the established capabilities of standard dewatering equipment. For example, shallow trench excavations in homogenous sand deposits of moderate permeability can almost always be dewatered by wellpoint systems with a spacing of 1 to 2 m between wellpoints. This has been practically proven over many decades. The empirical method is not applicable in more complex (or less clearly identified) geometries and ground conditions. If applied in such circumstances it can lead to considerable difficulties.
When geotechnical engineers become involved in dewatering design, the use of empirical design is sometimes viewed as being less rigorous compared to numerical or analytical methods. However, there is a huge track record of empirical methods providing successful dewatering designs. One of the reasons why this is the case is that, provided the correct groundwater control method is selected, a given dewatering technology can often successfully deal with modest variations in ground conditions. However, this means it is essential that the designer selects the correct dewatering technology for a project.
ANALYTICAL AND NUMERICAL DESIGN
The analytical and numerical approach can be used in cases whether or not there is empirical experience of the case in hand. The approach may involve fairly simple calculations or analyses, or may require more complex numerical modelling. The results of the calculations or modelling are used to specify the number and type of wells, pumps, etc., that will be required. Problems often arise when the theoretical approach does not take into account the limitations and advantages of the various dewatering techniques. If these issues are not considered, impractical or uneconomic designs may result.
The analytical approach uses hydrogeological equations (as might be found in a textbook) to estimate pumped flow rates and drawdowns. It is typically suited to relatively simple hydrogeological conditions with few complex boundaries (rivers, faults, other abstractions). Each type of analytical equations is only applicable to a relatively narrow range of hydrogeological boundary conditions, and gross errors can result if used in the wrong conditions
Numerical modeling is used far more in dewatering design and optimisation than it was a decade ago. This popularity is because the software is cheaper and easier to use than previously, and also because modern software can easily demonstrate results visually for non-technical project clients. Numerical modeling offers the flexibility to take into account known or inferred variations in the aquifer within the range of influence. This might include assessing the effects of a nearby river, another dewatering project, or a natural barrier in the aquifer. But again, applied inappropriately, gross errors can result.
OBSERVATIONAL DESIGN
Observational design uses construction observations (for example pumped flow rates and groundwater drawdown levels) to allow design and construction methods to be altered incrementally to match the behaviour of the ground and groundwater as part of a deliberate process of design, construction control, monitoring and review. The observational method can be often applied to pumped groundwater control systems. This is because they can be easily modified (by the addition of extra wells or by using pumps of different capacity) and because easily observable parameters (such as drawdown and discharge flow rate) can be used to interpret how the system is performing.
The observational method can be useful to deal with local variations in ground conditions. On larger projects it may be the best solution to address these variations locally (using the flexibility of the observational method) instead of engineering the overall system based on the worst-case conditions, as might be necessary if the dewatering system was conservatively designed at the start with little flexibility.
CONCLUSION
In practice, the best dewatering designs incorporate elements of all the above types of design. The analytical and numerical method requires a ‘conceptual model’ of the ground and groundwater regime to be developed, following which calculations and/or numerical modelling are carried out. The empirical method should be used as a ‘sanity check’ to ensure that the proposed groundwater lowering system is realistic and practicable. For example, if the output of a theoretical design recommends a single stage of wellpoints to achieve a drawdown of 8 m this is clearly not going to work (wellpoint systems rarely achieve a drawdown of more than 5 to 6 m). More subtly, if a wellpoint spacing of, say, 15 m was recommended, this should be looked into more closely, since this is outside the normal range of wellpoint spacings – there may be problems with the conceptual model, methods of analysis, or selection of dewatering method.
One defining feature of the design of any geotechnical process (including dewatering) is that there will be some uncertainties in the ground. These uncertainties may result from the site investigation being of limited or inappropriate scope. Alternatively, even following a comprehensive site investigation, the sheer variability and complexity of the revealed ground conditions may give rise to uncertainty in design.
Complexity of calculations or modelling is not automatically a good thing, and sometimes basic principles can be lost and fundamental problems obscured if users and clients are dazzled by sophisticated models, fancy outputs and slick presentations. There are many cases where simple and fairly basic calculations are perfectly acceptable, and may be preferred in many cases, provided they are compared with an empirical approach.
The eminent geotechnical engineer Professor Ralph Peck (the originator of the observational method) once said:
‘If you can’t reduce a difficult engineering problem to just one 8-1/2 x 11-inch sheet of paper, you will probably never understand it.’ *
This is certainly true of dewatering designs, as well as many other geotechnical problems.
* as quoted in Ralph B. Peck, Educator and Engineer — The Essence of the Man (2007) by John Dunnicliff and Nancy Peck Young, p. 114.
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