The criteria of selecting waterproofing system are Hydrology concept (e.g.
- environmentally sensitive area,
- permanent water table,
- the degree of water-tightness required,
- drained or undrained concept,
- the water pressure,
- the impacts and conditions during construction,
- the kind of chemicals in ground water,
- their concentrations and their possible influences on chosen materials.
The basic design of waterproofing systems in Tunnel construction is of two types:
- Drained System
- Pressure System
The drained system allows the water to channelize and hence drain out of the system, also commonly known as Umbrella system.
The pressure system holds the water behind the membrane for perpetual period, commonly known as Submarine system.
Primary Drainage- Flexodrain system
A primary drainage allows spot- wise collection of water ingress, normally located where the anchors/ bolts penetrate the shotcrete lining, with drainage channels. It helps in two ways:
- No flowing water in the excavated profile, safe working process.
- Dry substrate for waterproofing membrane installation
A drainage mesh or dimpled membrane is generally used to allow water to drain out of umbrella system and installed on the shotcrete using PVC Disks with fasteners. Same PVC disks shall be used to hold the membrane in place. Important recommendation is full faced drainage layers such as dimpled sheets shall not be in direct contact with the PVC membrane due to the risk of plasticizer migration from the membrane into the drainage layer, which might result in loss of durability. A drainage layer can never act as the permanent waterproofing element. In that regard it does not matter whether the drainage layer has an open or a closed structure. The overlays of dimpled sheets etc even when sealed with tape, will never be watertight for the service life of the tunnel. That means sooner or later the PVC membrane will anyway act as the only waterproofing barrier. The drainage layer only provides the space for the water to flow into the pipes. To avoid multi- application of drainage layer and separation sheet, it is recommended to use factory attached geotextile on drainage layer, which eases the application process.