SR-280 Sinkhole Repair DO (EA 04-0V800)

Caltrans proposes repairing the roadway and drainage at post miles (PM) 6.82 and 7.1 on State Route (SR-) 280 in San Mateo County. Due to winter storms, sinkholes occurred at PM 7.1 between San Carlos Vista Point and Edgewood Rd and at PM 6.82 on the off-ramp at Edgewood Rd. Both sinkholes are on the shoulder of the mainline and were covered by steel plates. The sinkholes post a risk to SR-280 and endanger the health and safety of the public. The failed drainages post a future risk of subsequent sinkholes if they are not repaired. The sinkhole at PM 7.1 is adjacent to a 48-inch diameter culvert and 42-feet south of a drainage inlet (DI). The Damage Assessment Team concluded that the slope became over-saturated during a rain event and resulted in a large washout approximately 15-feet wide by 3-feet deep. The embankment has a 1.25H:1V slope. The existing ditch at the toe of the slope connected to the drainage inlet that was covered by the washout material necessitated the closure of Lane No. 1 to traffic. A culvert inspection determined that the failure of a concrete collar created the sinkhole. The sinkhole at PM 6.82 was caused by a culvert failure near the Edgewood Rd off-ramp near Lane No. 1. Geotechnical preliminary investigation recommends excavation of both sinkholes, backfilling with structural material, and then compaction. At the PM 7.1 sinkhole, the concrete slab on the roadway will be replaced and the concrete collar at the culvert will be repaired. At the PM 6.82 sinkhole, the existing culvert will be lined. The project will also install a hot mix asphalt (HMA) dike and tack coat. The damaged roadway will be paved with asphalt concrete (AC) and any damaged or removed pavement striping and delineation will be replaced. Permanent erosion control will be placed on the slope. Traffic control and temporary k-railing will be required during construction.