dewatering system to yield water, meaning that installing a larger pump will be a wasted effort and result in greater energy costs for no improvement in results. Furthermore, the impermeable layer just below the excavation will also hinder dewatering. If the aquifer extended well below the excavation, the dewatering devices could be installed deeper to induce vertical drainage. However, in this case, the devices may only be installed to the top of the impermeable layer, forcing them to sip off the top of the layer. This will result in mounds of residual groundwater between the wells, unless the well spacing is close enough to prevent it. The classic dewatering approach to this problem would be a closely spaced system of wellpoints or ejectors. In the difficult conditions described above, the biggest challenge is not pumping the water out of the devices but providing enough opportunity for the water to enter the system. Therefore, we can see how an increased dewatering effort may be required even in a scenario where a low system flow is to be expected. The opposite case is well illustrated with a short case history. Moretrench was contracted to perform dewatering over a large area in a coastal plain aquifer, which consisted of homogeneous fine sand and was deep relative to the amount of groundwater lowering required. Moretrench was told to assume a hydraulic conductivity of about 200 gpd/ft2 (10-2 cm/s). Based on the information given, 19 deep wells were installed including appropriately-sized pumps, piping and electrical
systems. Subsequent pump testing revealed that the aquifer was actually 10 to 15 times more permeable than previously thought. Luckily, analysis of the test results showed that no additional drilling was required, and the project could be dewatered simply by upgrading the pumps and associated mechanical systems. Although the system flow rate was significantly greater than originally anticipated, the dewatering system was able to meet the project goals with only a modest increase in cost due to favorable geology. The high transmissivity of the aquifer allowed groundwater to flow easily to the deep wells, and all that was needed was an increase in pumping horsepower. It is apparent, then, that proper dewatering design relies on a detailed site investigation and more information than is commonly included in most geotechnical reports: • Borings that clearly identify coarse over fine soil interfaces. The previously described interface problem can occur even with very thin layers that are often missed with standard split spoon samples every 1.5 metres. This risk could be reduced by using continuous sampling or by combining standard borings with cone penetrometer testing, offering near-continuous sampling. • Borings that extend well beyond the proposed depth of excavation to confirm that the aquifer may be relied upon below the excavation or that an interface condition exists. • An estimate of hydraulic conductivity based on in-situ testing, such as slug tests, or pref-
Two similar dewatering scenarios. 36 PIC Magazine • June 2017
erably a pump test. Values of hydraulic conductivity in nature may vary by more than 14 orders of magnitude. Therefore, it is often worthwhile to use the geotechnical investigation stage of a project to narrow the range. This leads us finally to the secret to dewatering. People who subscribe to the two erroneous statements above believe that it is about figuring out the best way to pump water out of a well. However, those in the know understand that the true secret to dewatering is convincing the water to enter the well in the first place. l Gregory M. Landry, P.E., is chief dewatering engineer for specialty geotechnical contractor Moretrench. He is responsible for the supervision of the company’s engineering team and other professionals who perform design and analysis, cost estimating, installation, and management of dewatering and groundwater control systems. He also oversees Moretrench’s groundwater modeling team, combining the latest in computer-driven techniques and traditional analytical methods backed by hands-on experience. He can be contacted at glandry@moretrench.com. This article was originally published in DFI’s bi-monthly magazine, Deep Foundations, May/June 2017 issue. DFI is an international technical association of firms and individuals involved in the deep foundations and related industry. Deep Foundations is a member publication. To join DFI and receive the magazine, go to www.dfi.org for further information.