Sticking the waterproof layer of the coil to the water-facing surface of the underground engineering structure is usually called external waterproofing, and is attached to the back surface to be waterproof inside.
The outer waterproofing of the coil can protect the main structure of the underground engineering from the harmful effects of groundwater; the waterproof layer can be pressed by the earth pressure and can resist the penetration of the pressurized groundwater together with the load-bearing structure.
     The internal waterproofing method cannot protect the main structure, and a set of lining structure is required to press the waterproof layer to resist the penetration of the pressurized groundwater. Sometimes, even an anchor bolt is needed to connect the waterproof layer and the supporting structure into a whole.
     Therefore, the underground works generally excavated and constructed do not use the internal waterproofing method. Only the underground works of the underground excavation construction must adopt the internal waterproofing method when the coil material is waterproof and the external waterproofing method cannot be adopted.