Managing Subsurface Utility Risk in Large-Scale Water Infrastructure Projects: A Comparative Assessment of GPR, As-Built Records, and Trial Pits

Underground utilities represent a critical risk factor in large-scale water infrastructure projects, particularly in dense urban environments such as Riyadh, KSA. This paper presents a comparative technical assessment of three primary methods used for subsurface utility detection and verification in a TSE (Treated Sewage Effluent) network construction project: Ground Penetrating Radar (GPR), existing as-built drawings, and site trial pit investigations. The study evaluates the accuracy, reliability, and limitations of each method, with a specific focus on deep utilities exceeding 6 meters in depth.A real project case is analyzed where GPR failed to detect an existing utility pipe at approximately 13 meters depth, resulting in a microtunneling collision. The incident is used to explore root causes, including high soil cover levels, surface reinstatement conditions, and signal attenuation mechanisms. The paper further discusses the project-level implications of relying solely on GPR and historical records without adequate trial pit verification, including time delays, cost overruns, and risk exposure in large-scale KSA infrastructure programs.The findings highlight that while GPR and as-built records are valuable planning tools, they are insufficient as standalone methods for deep utility risk management. The paper concludes with a set of practical lessons learned and recommendations for site engineers and project managers, emphasizing the need for a hybrid verification strategy that systematically integrates GPR, document review, and targeted trial pits for high-risk sections.