Managing groundwater is a critical part of excavation projects—it can either support your success or become an obstacle that derails your efforts. This article zeroes in on the practical aspects of dewatering and depressurization, two foundational elements of groundwater control. Whether your project is short-term, involving quick excavations like utility trenches, or long-term, such as infrastructure or mining projects, effective planning is the key to minimizing risks and achieving project goals.
This article will help you navigate the complexities of groundwater control by exploring your options, understanding timeframes, addressing stakeholder priorities, and modeling and communicating your plans. By applying these insights, you might just sleep a little better as you work your way through your dewatering project. Let’s dive in.
First Know Your Options
Excavations below the water table require measures to control groundwater in the surrounding soil. The first step in planning and design is to know what your options are. Groundwater control methods generally fall into three categories:
- Physical Barriers: These include impermeable walls or barriers installed to exclude groundwater from the excavation. Ground freezing can also fall under this category, where the ground around the excavation is frozen to form a barrier that prevents groundwater flow.
- Pumped Well Systems: These systems involve pumping groundwater from specially installed wells or wellpoints to artificially lower the groundwater levels. Pumped well systems can operate independently or in combination with physical barriers for optimal results.
- Pressure Barriers: These involve applying high air pressure into confined excavations, such as tunnels, as a counterbalance to the groundwater pressure.
The choice between these methods—or a combination of them—depends on site-specific factors such as ground conditions, excavation depth, and project duration. If excavating highly permeable materials like gravel or preventing groundwater displacement outside the excavation (e.g., to avoid ground settlement or contaminant plume migration) is critical, a physical barrier is likely the best solution. A pressure barrier is sometimes the right choice for tunneling. However, in most cases, a pumped well system remains the most practical and effective option, which is why it serves as the focus of this article.
Note: in this article I will use the word “dewatering” to refer to both dewatering and depressurization. It’ll be easier to write and read that way.
Consider Short-Term vs. Long-Term Dewatering
Dewatering can be generally divided into two categories: short-term and long-term. Classifying your project by time scale can help you predict the types of challenges that you need to plan for and/or design against.
Short-Term Dewatering: Short-term dewatering projects typically last days to months. Examples include utility trenches, building foundations, and cut-and-cover tunnels. These projects influence smaller areas. So, the dewatering effects are localized near the project site, where you usually have most of your site data and measurements. This higher-resolution measurements of local ground conditions provides better data for localized dewatering forecasts. While uncertainty is always present, short-term projects tend to have the least uncertainty. Further, if the model forecast is wrong, issues show up early. This allows for quick fixes, like changing pumping rates or boosting water treatment capacity (if you planned for it!). Transient simulations are key for short-term systems because they consider the higher initial pumping rates required to extract groundwater from storage. Additionally, short-term dewatering projects often face less regulatory oversight, simplifying their implementation.
Long-Term Dewatering: These projects can take months to years. They are often required for large infrastructure or mining projects. These projects create larger areas of impact, which increases the need for data collection and planning. Unfortunately, it is always difficult and expensive to collect data the further you go away from the site. This lack of data for the wider area increases uncertainty in planning, design and forecasting. Long-term dewatering forecasting often use steady-state simulations, to forecast impacts further away from the excavation area. Steady-state models are useful for looking at long -term impact potential but they are sensitive to your model’s boundaries. Therefore, it’s important to think carefully about your model boundaries and try to use only real-world boundaries such as ‘impermeable’ rock outcrops or significant surface water bodies. One of the biggest challenges with long-term dewatering planning and design is that negative impacts may not show up any time soon. This makes responding to these impacts somewhere between difficult and impossible. The usual mitigation strategy is continuous monitoring of surface and groundwater followed by regular updates to your plan and your model. It is no surprise then that Regulatory oversight is often stricter on long-term dewatering projects.
This section simplifies groundwater control projects into short-term and long-term categories, but the distinction is more of a spectrum. For instance, long-term projects often have an early phase where understanding initial transient flow rates is critical, requiring separate consideration during planning. The key takeaway is that groundwater control is inherently time-dependent, and this dynamic nature must be incorporated into your planning and design process.
Wear the ‘4 Hats’ of Groundwater
Groundwater can be simultaneously perceived as “good” and “bad” on every excavation project. You can think of these perspectives as the “4 hats” of groundwater, which you will need to switch between to fully appreciate how to achieve your project objectives. This concept of switching hats as a metaphor for changing your perspective on problem solving originated with Dr. Edward de Bono’s book Six Thinking Hats. However, the general idea of switching hats to change perspectives is widely recognized beyond the book.
The “4 hats” of groundwater are:
- Groundwater as a Hazard: Example – groundwater can cause instability on your excavation walls and base.
- Groundwater as a Nuisance: Example – water ingress can create wet, soft working conditions in the excavation that need to be managed.
- Groundwater as a Valuable Resource: Example – your excavation may impact the city water well.
- Groundwater as an Environmental Component: Example – your excavation will reduce the baseflow, or extract water from to the nearby stream.
Being able to “switch hats” at the planning stage allows you to perceive and plan for potential problems before they become an actual problem. For example, early project planning often focus on flow rates, while other aspects—like settlement, contamination, or environmental impact—become evident later when mitigation is more difficult. This highlights the differing perspectives and priorities among stakeholders.
Here is an example of how different perspectives and priorities might look on a given project:
- Construction Teams: Their priority is schedule, and they only want to know how much water to pumping and how long it will take.
- Geotechnical Teams: Their priority is ground conditions in and around the excavation and so they are only concerned with settlement potential.
- Clients: Their priority is to prevent liability and cost of clean up if the dewatering causes a neighboring contaminant plume to migrate.
- Regulators: Their priority is to prevent environmental impacts, such as reduced baseflow to streams.
Understanding these diverse priorities ensures that the dewatering design addresses the needs and values of all stakeholders. Learning to switch “hats” and view the project from multiple perspectives enhances planning and design effectiveness. This approach also reduces the likelihood of being blindsided by issues, such as cracking in the foundations of an adjacent building due to ground settlement.
Answer the 5 Key Questions
While every groundwater dewatering project is unique, they all start with these five questions:
- How much water must be pumped to achieve the goal? (For transient flow-dominant projects, understanding how flow rates change over time is crucial.)
- How long will it take to achieve dewatering or depressurization objectives?
- What is the potential for ground settlement damage caused by removing groundwater?
- Could the cone of depression lead to contaminant plume movement?
- What are the potential impacts on nearby surface water bodies?
Note that all five questions may not be asked immediately or even by your client at the outset. However, unless you are working on an extremely small excavation with minimal dewatering over a very short period, it is reasonable to expect these questions to arise from stakeholders eventually. It’s always a good idea to be ready for these questions. To answer these questions, you will need to turn to groundwater modeling.
The Role of Groundwater Modeling
Groundwater modeling is a cornerstone of dewatering planning and design. There are two conventional approaches to model dewatering systems: analytical methods and numerical methods. The best method for your project is the one that allows you to complete your design within the limits of your project resources (see next section). However, it’s also true that having the ability to quickly test and update your assumptions is invaluable to the design process.
- Analytical Methods: These are quick to use and easy to understand, but they often rely on simplifying assumptions. While analytical methods can often fit site conditions by allowing for conservative assumptions, they usually address only one of the “4 hats” at a time. For example, there are analytical equations to estimate steady-state flow to an excavation, equations to estimate impacts on streams, and equations to estimate migration of contaminant plumes. Although it is possible to string multiple analytical equations together, doing so is cumbersome and rarely provides a holistic understanding of how the aquifer system behaves under all “4 hats.” Additionally, analytical solutions lack flexibility—if the excavation design changes, the chosen equation may no longer fit the new conditions. Another challenge is the sheer volume of available equations—for instance, G.A. Bruggeman’s Analytical Solutions of Geohydrological Problems alone contains over 1,500 equations—making it difficult to identify the best solution for a given project.
- Numerical Methods: These are invaluable for simulating even modestly complex site conditions. Numerical modeling allows for the consideration of all “4 hats” within the same model, enabling consistent assumptions and a more holistic understanding of how the aquifer system will behave. Common numerical methods in groundwater practice include finite element, finite difference, and finite volume approaches. However, these numerical codes are often inflexible. Once a model is built, making changes is time-consuming and costly. This inflexibility can be a drawback in dewatering projects, where design changes are common.
A third and perhaps lesser-known choice is Anaqsim, which provides a significant advantage on planning and design of groundwater control projects. Often referred to as a hybrid modeling approach, Anaqsim combines the speed and intuitive application of analytical equations with the complexity of numerical methods. Its flexibility allows users to design and completely revise their models within minutes, making it particularly useful for design-oriented simulations, such as those required in dewatering projects.
Communicating Your Plan and Design
Effectively communicating your groundwater control plan is an essential step in any project, as your audience often includes a broad range of stakeholders with diverse perspectives (recall the “4 Hats”). To ensure clarity and alignment among all parties, consider the following points:
- Keep It Short and Simple: Most stakeholders have no formal training in groundwater science, so making your report simple and straightforward is very helpful to them. Just assume that it is impossible to make your report too simple or too short.
- Clearly Define the Purpose: Clearly outline the purpose of your report, keeping the “4 Hats” in mind. Not all perspectives will be relevant to your project (e.g., contaminant plumes may not be present) but documenting that all possibilities were considered will help gain stakeholder trust and alignment.
- Acknowledge the Limits of Your Analysis: Every analysis is limited by available resources, including time, money, and knowledge. While time and money are self-explanatory, knowledge encompasses the quality and quantity of observations, measurements, and expertise available. Documenting these limitations ensures that stakeholders understand the scope and potential constraints of your design.
- Create an Assumptions Document: A pro-tip for improving communication is to create an “Assumptions Document” at the beginning of the project. This document should detail your purpose, available resources, and key assumptions. It will help keep all stakeholders aligned on the objectives, limitations, and resulting uncertainties of the dewatering plan.
Conclusion
Planning and designing a groundwater dewatering project is a multifaceted process that requires careful consideration and preparation. Success lies in:
- Knowing Your Options: Understanding the methods available, including physical barriers, pumped well systems, and pressure barriers, ensures you can select the right approach for your project.
- Knowing Your Project Timeframe: Short-term and long-term dewatering projects present unique challenges, and tailoring your strategy to the time scale is essential.
- Addressing Stakeholder Perspectives: Considering the “4 Hats” of groundwater helps align your design with the diverse priorities of stakeholders, from construction teams to regulators.
- Answering the Five Key Questions: Ensuring clarity on critical aspects like water volumes, potential settlement, and environmental impacts strengthens your project plan.
- Modeling Your Project: Leveraging appropriate tools—such as analytical methods, numerical methods, or hybrid approaches like Anaqsim—is crucial for effective design and forecasting.
- Communicating Clearly: Simplifying technical information and clearly documenting assumptions, limitations, and objectives fosters alignment and trust among all stakeholders.
Every groundwater dewatering/depressurization project is unique and will therefore present you with unique challenges. However, by integrating the concepts covered in this article, you will be better prepared and ready to manage those challenges.