By Thomas J. Minihan
Tunnels and tunnel shafts present unique problems when dealing with groundwater. Surface access is always the critical issue and forms the basis for selecting tunneling as the excavation method used to install the piping system or access corridor required by the project. Limited access to the jobsite also restricts how dewatering equipment can be installed and used. To compound the dewatering problems, the combination of vertical shafts and horizontal tunnels stresses the abilities of conventional dewatering systems to manage the groundwater problems, especially in stratified soils with low permeability.
The usual groundwater control systems available to tunnel dewatering are deep wells, eductors, wellpoints, horizontal wells and sumps. All of these methods must be installed properly to prevent the removal of soil particles, which can lead to subsidence and settling of adjacent structures.
When the soils around the tunnel shafts and the tunnel are permeable, and the permeable soil extends 15 ft or more below the shafts and tunnel, deep wells do a fine job. The tunnel sections are hard to handle because surface access along the tunnel route may restrict the intervals necessary to set the line of wells required to create a continuous water table drawdown. If the permeable soil below the tunnel invert is shallower than 15 ft, the wells must be set very close together, which results in greater access problems. Running discharge piping to connect the line of wells can also result in surface access problems, particularly when the tunnel is under a major multi-lane highway or under buildings.
Eductor wells can function to a depth of 100 ft but are somewhat limited in pumping capacity. This makes them ideal for very low permeability soils especially when the soils are stratified. Because the spacing between eductors is narrow, usually 10 to 20 ft, surface access is again a problem. One major advantage of the eductor well is its ability to create a vacuum at the well tip. This helps when the tunnel and tunnel shaft inverts are close to or at an impervious layer. Eductors function by pumping water into the well casing and through a venturi at the bottom of the well. The high-pressure injection water creates a vacuum that pulls water into the casing pipe through the well screen. A continuous source of priming water is supplied by one manifold pipe line and the discharge water is carried off by a second manifold pipeline. Both manifolds follow the line of wells that follow the tunnel route. Sealed tunnels may not need wells along the route and may only require wells at the shafts, which reduces the surface access problem.
For shallow tunnels up to an average of 20 ft deep in low permeability soils, vacuum wellpoints are an option but because of the close spacing required it can be difficult to provide enough surface access along the tunnel route. A wellpoint system involves a combination pump package having a vacuum pump and a centrifugal pump powered by a common driver, either a diesel engine or an electric motor. The pump package is connected to a vacuum manifold piping system. The individual wellpoints are connected to the manifold piping usually on 6-ft centers.
A variation on the wellpoint system design can work in tunnel shafts to any depth and is particularly effective in highly stratified soil profiles. This variation involves a new type of wellpoint called a SilterVac geotextile point that can be jetted into permeable sands between impervious soil layers from inside the tunnel shaft liner plates or lagging. The SilterVac points are battered into the permeable zones then connected to a manifold pipe ring set inside the tunnel shaft. The manifold pipe is connected to a vertical pipe that extends from the bottom of the shaft to the top of the shaft. Numerous manifold pipe rings can be connected to one vertical pipe to dewater many individual permeable layers. At the top of the vertical pipe is a vacuum generator with an air/water separator. At the bottom of the vertical pipe is a submersible pump that pushes the collected groundwater to the surface for discharge away from the shaft.
Depending on the tunnel excavation advancement method used, the manifold piping can be advanced from the shaft into the tunnel for continuous dewatering along the tunnel route. This method of dewatering eliminates the problem of surface access and is an economical means of solving groundwater problems in difficult soil profiles. The system is entirely installed by hand, eliminating the need for drill rigs and large equipment.
Again in low permeability soils particularly when surface access is limited, horizontal wells are effective but are difficult to design and install properly. Horizontal wells can be installed close to the tunnel line and along interface layers between rock or clay and sands. The advantage of horizontal wells is that less groundwater must be pumped to dewater the alignment than with any other method. The pumping activity is concentrated close to the tunnel and shafts, limiting the impact on the area surrounding the tunnel route.
Horizontal wells are hard to install properly because of the difficulty of controlling the well development elements laterally instead of vertically. Without proper placement and development of the wells, the horizontal system design will not be effective. New geotextile composite materials make the effectiveness of the horizontal well system exceptional and installation more expeditious and economical; however, conventional horizontal directional drilling methods must be modified and meticulously managed to ensure success.
Horizontal wells are abandoned in place after the tunnel is completed but can be converted to permanent dewatering systems if desired. The new geotextile materials are inert and will provide more than 60 years of useful service.
When the soil profile where the tunnel is to be installed is stable or either impermeable or above the water table, simple sumps can be used to control perched water, seepage water and rain water. Usually, electric submersible pumps are chosen because of their simplicity and compactness.
As the shafts and the tunnel are installed, grouting outside the liner plate or lagging will reduce the amount of groundwater that must be controlled; however, in most cases, the dewatering system used to facilitate the shaft and tunnel installation must continue to be operated until the utility piping or corridor lining is completely installed to eliminate the external water pressure on the tunnel liner.
Tunneling is a unique excavation method and dewatering tunnels can be tedious and difficult to accomplish successfully. It is always best to seek the help of an experienced and innovative dewatering contractor who can mix the various dewatering methods to adjust to the soil changes encountered in a linear project site.
Thomas J. Minihan is Vice President of Griffin Dewatering Midwest LLC.