Dewatering shafts and tunnels
a. The requirements and design of systems for dewatering shafts and tunnels in cohesionless, porous soil or rock are similar to those previously described for open excavations, As an excavation for a shaft or tunnel is generally deep, and access is limited, deep – wells or jet-eductor wellpoints are considered the best method for dewatering excavations for such structures where dewatering techniques can be used. Grout curtains, slurry cutoff walls, and freezing may also be used to control groundwater adjacent to shafts or tunnels.
b. Where the soil or rock formation is reasonably homogeneous and isotropic, a well or jet-eductor sys-
tern can be designed to lower the water table below the tunnel or bottom of the shaft using methods and formulas presented in paragraphs 4-1 through 4-4. If the soil or rock formation is stratified, the wells must be screened and filtered through each pervious stratum, as well as spaced sufficiently close so that the residual head in each s tra turn being drained is not more than 1 or 2 feet. Dewatering stratified soils penetrated by a shaft or tunnel by means of deep wells may be facilitated by sealing the wells and upper part of the riser pipe and applying a vacuum to the top of the well and correspondingly to the filter. Maintenance of a vacuum in the wells and surrounding earth tends to stabilize the earth and prevent the emergence of seepage into the tunnel or shaft.
c. In combined well-vacuum systems, it is necessary to use pumps with a capacity in excess of the maximum design flow so that the vacuum will be effective for the full length of the well screen. Submersible pumps installed in sealed wells must be designed for the static lift plus friction losses in the discharge pipe plus the vacuum to be maintained in the well. The pumps must also be designed so that they will pump water and a certain amount of air without cavitation. The required capacity of the vacuum pump can be estimated from formulas for the flow of air through porous media considering the maximum exposure of the tunnel facing or shaft wall at any one time to be the most pervious formation encountered, assuming the porous stratum to be fully drained. The flow of air through a porous medium, assuming an ideal gas flowing under isothermal conditions, is given in the following formula:
Qa = Ap(D – hw)k §
Qa = flow of air at mean pressure of air in flow system p, cubic feet per minute A„ = pressure differential (Pi—p2) in feet of water pi = absolute atmospheric pressure p2 = absolute air pressure at line of vacuum wells D = thickness of aquifer, feet hw = head at well, feet
k = coefficient of permeability for water, feet per minute
pw = absolute viscosity of water Pa = absolute viscosity of air § = geometric seepage shape factor (para 4-3)
The approximate required capacity of vacuum pump is expressed as
absolute atmospheric pressure
(feet of water) (4-13)
(cubic feet per minute)
where p represents mean absolute air pressure
in feet of water. Wells, with vacuum, on 15- to 20-foot centers have been used to dewater caissons and mine shafts 75 to 250 feet deep. An example of the design of a deep-well system supplemented with vacuum in the well filter and screen to dewater a stratified excavation for a shaft is shown in figure D-8, and an example to dewater a tunnel is shown in figure D-9.
d. In designing a well system to dewater a tunnel or shaft, it should be assumed that any one well or pumping unit may go out of operation. Thus, any combination of the other wells and pumping units must have sufficient capacity to provide the required water table lowering or pressure relief, Where electrical power is used to power the pumps being used to dewater a shaft or tunnel, a standby generator should be connected to the system with automatic starting and transfer equipment or switches.
4-11. Permanent pressure relief systems. Permanent drainage or pressure relief systems can be designed using equations and considerations previously described for various groundwater and flow conditions. The well screen, collector pipes, and filters should be designed for long service and with access provided for inspection and reconditioning during the life of the project. Design of permanent relief or drainage systems should also take into consideration potential encrustation and screen loss. The system should preferably be designed to function as a gravity system without mechanical or electrical pumping and control equipment. Any mechanical equipment for the system should be selected for its simplicity and dependability of operation. If pumping equipment and controls are required, auxiliary pump and power units should be provided. Piezometers and flow measuring devices should be included in the design to provide a means for controlling the operation and evaluating its efficiency,