Criticality and Risk Assessment for Pipe Rehabilitation in the City of Santa Barbara Sewer System
Available at: https://digitalcommons.calpoly.edu/theses/1424
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Dr. Shikha Rahman
Aging sewer infrastructure is posing greater and greater risk to the health and well-being of City residents. Issues can range from pipe blockages in sewer laterals to Sanitary Sewer Overflows. This thesis develops a risk analysis method that can be used by municipalities to maintain and rehabilitate sewer assets. Risk combines the effect of Likelihood of Failure (LOF) and Consequence of Failure (COF) to perform a complete two-dimensional analysis that allows for relative comparison between different pipes in the system. The LOF rating has been equated to pipe integrity while the COF rating was related to the environmental, economic, and social consequences to pipe failure.
In order to estimate pipe integrity Closed Circuit Television (CCTV) scores from the City of Santa Barbara were used in combination with spatial and physical properties associated with each pipe. The CCTV scores were simply integer values between 0 and 5 based on the National Association of Sewer Services Company’s (NASSCO) Pipeline Assessment Certification Program (PACP) results. The quantitative parameters included pipe material and age, distance from restaurants, distance from any above ground water source, pipe depth below the ground surface, pipe length, and vehicular traffic volumes. The sensitivity analysis compared the given structural integrity scores with the predicted scores based on the weighted scoring method. It isolated four out of six of the parameters tested that affected the structural integrity of sewer pipes: material and age (45%), pipe depth (20%), Vehicular Traffic (10%), and distance from an above-ground water source (25%). A program was created in the C programming language that iteratively determined the percentage for each factor. These percentage factors are used to obtain the predicted structural integrity score for all the pipes.
Like the LOF rating, the COF rating consisted of scores between 0 and 5. The COF rating used pipe diameter, distance from commercial zones, distance from critical infrastructure, and vehicular traffic volume as parameters for quantifying the environmental, economic, and social consequences. These factors were determined from review of past literature and given approximately equal weighting when determining the COF rating values. The environmental factor, pipe diameter, was given a percentage factor of 30%; the economic factor, distance to commercial zones, was given a percentage factor of 30%; and the social concerns, distance to critical infrastructure and vehicular traffic volume, were given percentage factors of 20% each.
Finally, the risk for each pipe was determined in Geographic Information Systems (GIS) by combining the predicted structural integrity score or LOF rating and COF rating value for each pipe. This generated color-coded maps that showed distinct pipes that had the most critical predicted structural integrity scores, highest consequence, and the pipes with the most risk. This process could be used by any City to create a maintenance and rehabilitation schedule and plan for future CCTV inspections.
Civil Engineering Commons, Construction Engineering and Management Commons, Environmental Engineering Commons