Structural properties and durability of a sprayed waterproofing membrane for tunnels

A sprayed waterproofing interlayer between primary and secondary tunnel linings has the potential to create composite structural interaction between them, mainly in circumferential flexure. This depends on the bond between sprayed surfaces and on the structural and chemical properties of the material. Data on bond is reviewed. Recent tests in tension, compression and shear to determine the structural properties of a particular elastomeric waterproofing material are presented, analysed and discussed. Values for Young’s modulus and the shear modulus G are proposed, for serviceability verifications that include limitation of crack width caused by excessive local flexure. It is concluded that where sufficient bond is present, composite action arising from transfer of circumferential shear between the linings can be relied upon to improve performance in service for short-term and accidental situations. This allows for reduction of stiffness during the first few years. There is no evidence from tests lasting up to 2.2 years that significant further reduction would occur, but prediction of 100-year performance remains difficult