Phase(s): Preliminary Engineering
Date: March 2, 1998
Recent changes in the method of controlling ground water both during and after construction have significantly affected the results obtained in the construction of subway tunnel structures for the Washington Metropolitan Area Transit Authority (WMATA). WMATA has adopted a revised waterproofing method to be applied when tunneling through rock or soft ground material. This method of waterproofing is being applied to various types of tunneling used by WMATA: the New Austrian Tunneling Method (NATM), conventional tunnel boring machines (TBM), and the Earth Pressure Balancing Method (EPBM). The revised waterproofing method has also been implemented for the construction of underground stations.
The WMATA Standard Specifications allow for water leakage at rates of 0.08 to 0.14 gallons per 250 linear feet of tunnel depending on the type of tunnel structure. Earlier tunnel designs did not incorporate a waterproofing membrane within the final tunnel liner construction nor in the station area. Significant water infiltration and corrosion were common problems with the previous design.
WMATA adopted the new waterproofing method in 1983/1984 as a portion of a construction contract value engineering change proposal when it decided to utilize the NATM method of tunneling for the Outer B Route. The NATM method was first used in the United States to excavate rock on the Outer B Route. This new method is currently being utilized in the construction of WMATA's Mid E and Outer F Routes.
Installation of this system begins with a smooth substrate layer of geotextile material attached to the tunnel crown and side walls by a steel nail and a PVC washer disk assembly. The geotextile material serves two functions. The first one is to provide a drainage path for water infiltration that is directed to a collection system located approximately one foot above the tunnel invert. The second function of the geotextile is to provide a protection barrier between the initial liner surface and the waterproofing geomembrane. Once the geotextile material is securely fastened, the synthetic geomembrane (comprised of a PVC sheet material) is wrapped around the tunnel crown and sides and heat welded to the previously installed PVC washer disk assemblies. Membrane material is overlapped a minimum of six inches and the seams are heat welded. Double seams are normally utilized but, single seams are used occasionally. When a single seam is used, a second layer of geomembrane material is welded in place over the single seam to completely cover it. The integrity of double seams are determined by maintaining an air pressure of 30 psi for ten minutes. Single seams are evaluated using a test liquid coupled with a vacuum pump.
At final cast-in-place liner construction joints and certain other locations, PVC water stop is attached to the membrane by heat welding. The waterstop is used to define discrete liner segments (typically 50 feet in length). If water infiltration is later detected, it can be confined to a particular segment which can then be repaired. After the waterproofing geomembrane has been installed, grout pipes are placed at specified locations prior to pouring the final concrete lining. The final concrete liner is then poured directly against the installed waterproofing system. If water intrusion later becomes a problem, these pipes can be accessed to inject a grout material that will seal the leak and provide an additional waterproof barrier.
The permanent concrete liner is protected from water intrusion by the geomembrane and the geotextile. The geomembrane acts as an impervious barrier and the geotextile serves to capture the water. The intercepted water flows to the bottom of the tunnel sides where it is transported by a special drain collection system. This water control system has resulted in significantly drier tunnel sections on the Outer B Route.
As a result of the success achieved in NATM excavated rock, WMATA decided to adopt this new method of waterproofing for soft ground tunneling utilizing the NATM, TBM, or EPBM. The TBM and EPBM methods involve the installation of initial precast liner segments. Once these segments are installed and contact grouting has been completed, the waterproofing system is installed using a method similar to that utilized for the NATM rock tunnel. The waterproofing details are similar except, in the case of soft ground tunnels, the PVC membrane is fully wrapped around the final tunnel liner and the invert.
A similar method of waterproofing has also been applied to the stations. Vertical walls adjacent to the support of excavation utilize a waterproofing system consisting of a layer of geotextile, a geomembrane, a protective board, waterstop, and at selected locations a sheet metal barrier. The geotextile, membrane and waterstop are installed in a similar manner as described for the tunnel lining. The protection board is bonded to the membrane using an adhesive. The board protects the membrane from potential damage which may be caused during steel reinforcement installation, form work erection, and concrete placement. In situations that require welding of metals which can produce sparks and extreme temperatures, a protective metal sheet is installed to prevent damage to the protection board and membrane. Dome construction utilizes a similar waterproofing system that is installed at both the top and bottom of the slab. Waterstop in the station and service room areas is installed at locations indicated on the drawings to establish compartmental sections similar to the method used for tunnel waterproofing. Grout pipes are installed prior to the placement of structural concrete to facilitate the injection of grout. The grout is injected if leaks are later encountered.
The new waterproofing system, coupled with proper installation techniques and a rigorous quality control inspection process, has vastly improved the dryness of WMATA's underground tunnels and stations. WMATA anticipates the waterproofing system will remain intact for a long time. The incorporation of compartmental sectioning permits a logical approach to repairing future leaks. It is anticipated that this new system for water control will result in significant long term operations and maintenance cost reductions. However, rail car air filters will require more frequent replacement due to increased dust levels created by the drier tunnel conditions (this represents a relatively minor increased maintenance cost).
Any transit agency that is anticipating the construction of underground facilities may wish to consider using this waterproofing method. In Europe, its use in highway tunnels has substantially reduced water intrusion and, therefore, ice buildup in mountain tunnels.