Tunnel Business Magazine

AUG 2017

TBM: Tunnel Business Magazine is the market leader for North America. TBM is written for leading professionals in all aspects of tunneling and covers project stories, design elements, contracting strategies, legal issues, new technology and more.

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Page 23 of 47

Dewatering Design and the GBR The theory behind and basic operation of the three main tools of construction dewatering (wellpoints, deep wells and ejectors) have been well described elsewhere. Each type of system has pros and cons and is suitable for addressing dif- ferent project and site conditions. Because of this, many de- signers become fixated on whether their project needs well- points, deep wells, ejectors or some combination thereof. It is important to bear in mind that they all simply represent different ways of getting water out of a well. The truth is that the aquifer doesn't know which type of system is installed or about the details of the plumbing that make it work. The aquifer feels only the following things: 1. The spacing of the dewatering devices. How frequent- ly is there a pick-up point where the groundwater may enter the system? 2. The yield of the dewatering devices 3. Whether or not the device exerts vacuum on the for- mation. Furthermore, the design process for dewatering does not necessarily depend on the type of system selected and the correct system may not become apparent until design is well underway. The relevant question then is not "How can I best get water out of the well or dewatering device?" but "How, given the geological conditions at the site, can I ensure that groundwater will flow to my dewatering system at a suffi- cient rate to achieve the drawdown required?" The answer to this question generally requires a good understanding of the aquifer's properties, particularly hydraulic conductivity, and the geometry of the aquifer relative to the project. Hydraulic Conductivity Hydraulic conductivity is an intrinsic property of the aqui- fer that measures how readily it will transmit water. Aquifers of lower hydraulic conductivity require pumping relatively small amounts of water to achieve a unit decline in head but individual wells will a have small area of influence, meaning they must be spaced close together. By contrast, higher hy- draulic conductivity aquifers require a greater flow but may be spaced further apart. This presents a problem in terms of defining a baseline condition. Traditionally, flow rate is taken as a proxy for de- watering difficulty and, all else being equal, higher hydrau- lic conductivity corresponds to higher flow rate. Therefore, if a certain value of hydraulic conductivity is given as the baseline then the more onerous condition is assumed to be the case where the actual conductivity is higher than the baseline. This is true to the extent that a higher hydraulic conductivity will require mechanical upgrades to the dewa- tering system, i.e. bigger pumps, piping and electrical sys- tems. However, given the information presented above, a scenario where the hydraulic conductivity is lower than the baseline value could require more dewatering devices owing to the steeper shape of the drawdown curves. This gives the counterintuitive result that a more onerous condi- tion (for which the contractor is presumably entitled to ad- ditional fees or schedule relief) could occur for hydraulic conductivity values both above and below the baseline. Figure 2 shows a tunnel shaft in a low hydraulic conductiv- ity aquifer dewatered with tightly spaced low flow devices. Figure 3 shows an excavation dewatered with a high flow system of widely spaced deep wells. F E A T U R E S T O R Y TUNNELINGONLINE.COM 24 TBM: TUNNEL BUSINESS MAGAZINE // AUGUST 2017 Dewatering and the Geotechnical By Gregory M. Landry, P.E. The tunneling industry has done a good job of embracing the concept of the geotechnical baseline report (GBR). However, this positive development is undermined to a certain extent by a general misunderstanding of what constitutes a challenging vs. easy dewatering condition. This article will describe how a deeper understanding of the interaction between the dewatering system and the site geology can lead to improved geotechnical baseline parameters relating to dewatering. Figure 1. Effect of pumping a low vs. high hydraulic conductivity aquifer.

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