A method of reinforcing existing underground water conduits to resist earthquakes and landslides and a reinforcement structural device adapted for insertion into underground water conduits without having to excavate are disclosed. The reinforcement structural device consists of a seamless woven tubular sheath impregnated with a hardening resin which is adapted for insertion into a network of underground water conduits without the need to excavate. The seamless woven tubular sheath consists of continuous longitudinal fiber warp yarn providing tensile strength along the longitudinal axis of the woven tubular sheath and spiralling circumferential fiber filling yarn providing tensile strength around the wall of the woven tubular sheath.