Are you gambling with your dewatering project?

Vahid Sohrabi, Ph.D., P.Geo.


Introduction

Dewatering is a key component of projects where mining and construction activities are to be carried out where groundwater is present. Archeological findings have found that dewatering predates written history. Such historical dewatering projects were mostly conducted based on the gravity transportation of water where terrain conditions were favorable. Lifting water with buckets was the other option until mechanical devices were gradually developed.

One of the earliest recorded dewatering projects in modern times dates back to the 1830s in England for the construction of the Kilsby railroad tunnel. In this project, a series of sumps and wells was installed and pumped to remove water from a large area of quicksand encountered during construction.

Many more extraordinary construction projects with critical dewatering components were conducted. The following are examples: Erie Canal (1825), Croton Aqueduct (1842), Transcontinental Railroad (1869), Eads Bridge (1874), Brooklyn Bridge (1883), New York City Subway (1904), and Holland Tunnel (1927).

Up to the 1940s, controlling groundwater was primarily based on field experience, more of an art than any sort of science. In the 1940s, with the integration of geotechnics and hydrogeology, a more scientific approach to dewatering developed gradually.

Today, the application of advanced dewatering techniques has enabled engineers to better accommodate more complex projects and to run activities successfully in complicated situations. However, although modern practice tends to embrace more engineering and science-based design, field experience still plays a major role in the early identification of latent challenges in real-time at any particular project site.

Here is a tricky situation. The use of engineering- and science-informed planning is now a recognized approach in the design and operation of dewatering activities. Also, we know that site-specific conditions can show substantial variation. Nevertheless, for a variety of reasons, there is still a lingering tendency to engage only a minimum of site-specific technical input. So, intuitively, we would know that for a dewatering scheme to be truly successful in terms of both cost-efficiency and technical effectiveness, it would need to be tailored to site-specific conditions, particular mining or construction scenarios at hand, and their sequential timing. In turn, this would mean that a “cookie-cutter” approach to dewatering from site to site would potentially have a strong likelihood of incurring unnecessary extra costs to offset in real-time what turns out to be, at best, a moving target of sub-optimal performances.

In the following sections, we firstly list the origins of unwanted outcomes of dewatering. Then, we briefly touch upon the potential consequences of such unwanted dewatering outcomes. Finally, our technical consulting services at AquaGroundTech are introduced. Our objective is to ensure that our clients will have access to cost-effective consultation and designs that minimize or eliminate the potential for costly, challenging outcomes to develop.

Origin of Undesirable Outcomes in Dewatering

As mentioned above, depending on specific conditions at any project site, if and as necessary, groundwater control systems must be designed, optimized, and applied in a manner that is compatible with those conditions. This means dewatering should not be treated as repetitive or a set of more-or-less identical tasks for different project sites. The story is, however, different in reality. In many projects, the dewatering component is underestimated or accorded low priority for a variety of reasons. Examples of common reasons would be funding constraints and tight project timelines. An insufficiently funded site characterization program, where it leads to a consequent dewatering scheme design shortcoming, will possibly result in catastrophic outcomes.

So, one may pose a question as to what are the main causal factors that have the potential to contribute to the triggering of some type of unfavorable outcome in respect to a new dewatering scheme. The following is a list of candidate determinants that have the potential to cause unwanted outcomes:

1- Bidding competition between contractors for a single project

The competitive bidding process is a double-edged sword! Yes, it obliges each bidder to sharpen the pencil cost-wise. At the same time, this can or will compromise the quality of service. Does a client really want a summer student or the most junior/barely experienced personnel gathering key engineering data at their project site? It happens! That’s one way for a contractor to drive field costs down. Nine times out of ten, with luck, this might just even work. Besides, an actual engineer in a climate-controlled office is signing off on the client’s project regardless of who collected the field data, right!? The tenth time, the project site has a particular twist, which is missed in the field, and then financial over-runs and time consumed are both off to the races!

Commonly, RFPs (request for proposals) only ask for the resumes of the proposed principal players in each bidder’s team. The caliber of intermediate and junior personnel may be unknown at the time of contractor selection. Alas, typically senior engineers are too expensive in terms of their hourly rates to be heavily involved at the data gathering/site characterization stage. In any event, the contractor’s project manager can “nail ‘em on the extras” when “unexpected” situations arise during project implementation. Classically, additional charges will be enabled by “weasel clauses” embedded in the lower-bid proposals; that may well be one way the lowest-bidder’s cost projection was driven down in comparison to a more robust technical proposal.

2- Insufficient field geological/hydrogeological understanding

A thorough understanding of the subsurface conditions at the site is necessary for the development and implementation of a successful dewatering operation.

Identifying the geology and soil parameters such as density, permeability, stratigraphy, and consolidation is the first step. Secondly, the hydrogeological characteristics of the site must be defined to determine the groundwater circulation regime and expected initial water level and drawdown during dewatering. In construction projects, geotechnical data may well be gathered precisely owing to their direct influence on construction activities. However, attention to hydrogeological aspects of the site is typically minimal in projects that encounter failure.

3- Level of dewatering experience where dewatering practice is not common

In areas where dewatering is uncommon, issues during construction are more prevalent as consulting companies and contractors are less experienced with dewatering and groundwater control activities. As a result, missing crucial aspects that have an important bearing on dewatering is more frequent. Field knowledge and design considerations are integral parts of a successful dewatering program, which may be scarce in such areas.

4- Attention to surrounding infrastructures and natural terrain features

Some other parameters to consider are surrounding structures, local climate, and seasonal groundwater/surface water level fluctuations, which could affect the perspective on subsurface dewatering at any site.

The presence of natural wetlands, historical structures, highways, residential and office buildings in the vicinity of a dewatering project are examples of features or artifacts that likely ought to be taken into consideration.

5- Application of dewatering parameters/design of other sites in the project

To reduce the cost of a project, a path that some companies follow is to cut seemingly unnecessary expenditures. Usually, dewatering design requirements and practice are one of the casualties in such a cost-cutting process. For example, the number of groundwater sensors (i.e. slimline tubings inserted into groundwater, called piezometers), to enable essential groundwater measurements to be made, maybe at a bare minimum. Or there may be none at all! Instead, where available, data from other sites in the general vicinity may be rationalized as constituting a sufficient surrogate representation.

The usual desire to conserve and expend funds in a prudent manner may promote susceptibility for straying into a false economy whereby the significance of an important complementary dewatering scheme is downgraded. So, as a result of one or more of these factors, surprises may be encountered in respect to groundwater control during real-time activities, which in turn would likely result both in unanticipated extra costs and in fast-breaking and/or rather random delays.

6- Improper planning of discharged water management

In its natural condition, the groundwater may be too mineralized to meet regulatory benchmarks in respect to discharge criteria for water quality. Ideally, this possibility is proactively addressed by collecting groundwater samples from the on-site piezometers and having these samples analyzed by a laboratory that is recognized by the regulatory agencies. Mitigation measures for any natural chemical constituents for which there is an exceedance can then be devised.

Depending on the flow rate, the discharging water itself may have some potential to cause erosion to the local terrain. Mitigation measures may thus need to be devised to minimize erosion, while also ensuring that attendant conveyance of turbid water to a receiving stream is minimized or eliminated.

Therefore, from two different key perspectives, particular consideration is needed in regards to the management of the discharging groundwater.

7- Local and provincial regulations

There may be a variety of regulations regarding dewatering activities in different countries, states, provinces, and municipalities.

As an example, Alberta’s Water Act in Canada requires approval and/or license to be obtained before undertaking an activity like dewatering and before diverting the water. In addition, you may need to have authorization under a Municipal Government Act.

Owing to the bureaucratic rigor under which governmental regulations are commonly exercised, even an unintentional departure from published benchmarks or criteria corresponding to where dewatering is going to be practiced may result in legal intervention, financial loss, and delay.

Potential Outcomes of Improper Dewatering 

A dewatering scheme, along with its attendant timeline schedules, will be most technically efficient and cost-effective if it is purpose-designed, organized, and implemented. Such a dewatering scheme is optimally compatible with the particular demands that are associated with the particular site setting and its specific construction requirements. At the same time, regulatory requirements and off-site risks should be given more than passing consideration in case their implications are perhaps not immediately evident.

Improper dewatering/groundwater control plan, design, and operation may result in adverse effects in a range from sub-optimal to deleterious outcomes for the whole project. Inappropriate dewatering consideration and design can delay the completion of a planned project. Based upon industry surveys, dewatering-related issues led to 8.2% of construction delay claims in Malaysia. By 2003, claim percentage as high as 50% in construction projects was noted due to dewatering issues in Canada.

The adverse effects of dewatering for projects ranging from construction to mining can be categorized into two groups:

1- Direct influence

  • Geotechnical damage
  • Impact on surface water resources features
  • Contamination
  • Depletion of water resources

 2- Indirect influence

  • Delay awaiting permitting/approval
  • Delay owing to the adverse impact of applied dewatering scenario
  • Delay corresponding to required changes to dewatering plan
  • Changes to construction plan with additional engineering and design costs
  • Loss of significant budget and time
  • Legal complaints
  • Possibility of project-related complaints and liability generations

AGT Hydrogeology Team Dewatering Services

Our hydrogeology team is able to provide you with some immediate insights as to the dewatering needs of a site or sites that you chose to enter. We can review your dewatering plan and generate a report with a gap analysis and critique of aspects that may have initially not given prominence in your plan and worth having more precise attention for a successful dewatering scheme.

If you find that your dewatering operation has the potential to be burdened by several broader inter-related challenges or implications, we can provide you with consulting services in terms of developing a suitable corresponding design. Please contact us at ino@aquagroundtech.com or via +1-587-887-4124.

 

Photo Reference

*** https://www.youtube.com/watch?v=5H6AFQjR-oY

 

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