Design of a Large Diameter Water Line at Active Fault Crossing

A proposed 84-in. steel water line intersects the active normal dip-slip Woodgate Fault, of which the estimated vertical movement rate is approximately half inch per year with a down-to-the-coast dip angle of 77 degrees. This surface water transmission main is owned by the North Harris County Regional Water Authority (NHCRWA) which comprises 335 square miles and includes approximately 710,000 residents. The anticipated vertical and horizontal movements within the design lifespan pose a significant risk to the water line since local buckling may occur due to excessive ground movement. Other challenges, such as (1) constraint of right of way (ROW), (2) balancing cost and reliability, and (3) accessibility for operation and maintenance (O&M), further complicate the design. To meet all design requirements, multiple design alternatives were evaluated, and an optimized "design and maintain" approach was selected. The design concept is verified and refined by finite element analysis (FEA) using a nonlinear Winkler-based soil spring model. It is found that the proposed O&M guidelines can reduce the risk of the water line by limiting the compressive strain induced in the steel carrier pipe, hence preventing local buckling. Also, the proposed approach reduces the overall construction cost while maintaining an acceptable level of reliability, and the O&M is facilitated by providing maximum accessibility to the couplings.