Overhead Transmission Line
Concrete drilled shaft foundations, for two electrical utility projects, each support a single steel monopole tower structure for new overhead electrical transmission and generation lines. Surface defects in the concrete were discovered near the top of three drilled shafts. Working for the contractor, SGH investigated the extent of the defects, evaluated the structural adequacy of the drilled shafts, designed temporary excavation support systems and concrete repairs, observed the repair work, and performed a root-cause analysis of the defects.
SGH worked closely with the contractor and owner. We prepared and executed a comprehensive non-destructive and destructive testing program that included the following:
- Visual survey of the drilled shafts
- Hammer sounding and rebound hammer testing of the in-place concrete
- Extraction of cores to assess concrete quality and compressive strength
Our testing revealed defects, including soft concrete, voids, honeycombing, and sand inclusions. Based on these findings, we conducted a detailed structural analysis of the foundations and developed a repair plan that allowed the contractor to conduct the repairs while keeping the drilled shafts, towers, and power lines in service. Our root-cause analysis showed that the defects at the three drilled shafts were isolated to these shafts due to the installation techniques that were used.
Two of the three drilled shafts are located on steep slopes. For these, we designed the temporary support of excavation systems, site regrading, and site restoration.
SGH provided a quick response and solution to the problem that prevented any additional schedule delays.