In 2017, large winter storms resulted in a mudslide below Seneca Lane in San Mateo County which swept away much of the existing hillside severing a sanitary sewer line serving numerous private residences. In order to restore functionality of the sewer line, prevent additional damage to public facilities, and prevent future slides, the County elected to construct a tieback wall. Cornerstone design and provided construction observation services for the tieback wall which consisted of steel beam soldier piles with two rows of reinforced concrete walers, pressure treated timber lagging, and prestressing steel tendons embedded back into the retained hillside.   The construction of the tieback wall also included the addition of sub-drains in order to prevent the pooling of water behind the wall. Cornerstone worked closely with the County and the contractor to successfully overcome difficult site access, time, and environmental constraints, even changing the pile sections in order to deliver a tieback wall that would protect against future damage to adjacent private properties and public facilities prior to the winter of 2020.