then equipped with an individual submer- sible turbine or right-angle drive pump. As discussed above, wellpoints are Grid of closely spaced wellpoints installed in low hydraulic conductivity coal ash Wellpoints Wellpoints are likely the most versatile and widely used dewatering tool. Wellpoints are small diameter vertical tubes, typically 1.5 to 2 in (38 to 51 mm), with slotted or perforated bottoms. The wellpoints may be installed by jetting using pressurized water, typically in loose granular formations like beach sand, or installed by conventional drilling methods. Wellpoints may be economically installed on close center-to-center spacing, making them ideal for addressing soils with low hydraulic conductivity or for sites with coarse over fine (e.g., sand over clay) geological interfaces. Both of these conditions require close spacing of devices to overcome the difficulty of removing groundwater from these environments. Wellpoints are connected by a common header pipe, and pumped by a single pumping station. The pumping station has three functions: 1. To apply vacuum to the header pipe and to the wellpoints by pumping air. 2. To separate the air/water mixture drawn toward the pump, usually by applying the vacuum to a “knockout tank” placed in front of the pumping station, where the air will tend to rise and be drawn to the source of vacuum. 3. To pump the water that passes through the knockout tank and to expel it to the discharge point. The major drawback of wellpoints is that, since they operate on vacuum, they are limited in how far they can draw the water down. Practical achievable drawdown by wellpoints in the field are often cited as 84 • DEEP FOUNDATIONS • MAY/JUNE 2017 being between 15 and 18 ft (4.6 and 5.5 m), but this will depend on soil type, distance from the wellpoint line to the point of observation, intensity of pumping, and a number of other factors. The author has personally observed wellpoints pumping under 1 gpm (3.8 l/min) to over 50 gpm (189 l/min). Deep Wells Deep wells can be simple and cost-effective dewatering tools, but their applicability is much more limited than wellpoints. In a typical application, deep wells will be installed around an excavation perimeter on a wide spacing, typically 30 ft (9.1 m) or greater between wells, and under the right geological conditions deep wells may be the lowest cost method of dewatering an excavation. Deep wells generally consist of large-diameter slotted screen and casing, typically 4 in (102 mm) or larger, installed in a jetted or drilled hole. Each deep well is often used to pump from low hydraulic conductivity aquifers or aquifers with complex geology. Deep wells are generally pr e f e r abl e in aqui f e r s tha t a r e homogeneous, have a relatively high hydraulic conductivity, and are deep compared to the proposed depth of excavation and the amount of drawdown required. These are the conditions that deep wells rely upon to be effective, as these conditions allow each well to affect a large area and to not be limited in capacity by native geology. Unlike wellpoints, deep wells are not inherently limited in drawdown. The amount of drawdown is limited only by the geology of the site, the depth of the deep wells and the size of the pumps in the wells. Deep wells may yield from under 1 gpm (3.8 l/min) to well over 1,000 gpm (3,785 l/min). Ejectors Ejectors combine some characteristics from both wellpoints and deep wells. In an ejector system, a pump at the surface forces pressurized water down a dedicated pipe in a well and through a small nozzle. When the water passes through the nozzle, it creates a drop in pressure (i.e., suction) that draws water into the well from the aquifer. The originally pressurized water and the newly pumped water combine above the nozzle and return to the surface. Excess water overflows to a discharge point, and the remaining water flows back Widely-spaced deep wells in a uniform sandy aquifer
