Geotechnical Investigation for Relocation of Two Towers

摘要

This paper presents results of a geotechnical investigation for relocating two towers to accommodate widening the existing highway of Caltrans SR-138 in Cajon Pass, San Bernardino County, California, USA. These two towers, numbered as 203-1-3 and 203-II-4, are part of the Victorville-Century Transmission Lines 1 and 2 owned by Los Angeles Department of Water and Power (LADWP). Tower 203-1-3 was located on the south side of SR-138 and proposed to be relocated 30.63 m (100.5 feet) away from the existing location along the transmission line to the southeast. Tower 203-II-4 was located on the north side of SR-138 and proposed to be relocated 23.53 m (77.2 feet) away from the existing location along the transmission line to the northwest. The geotechnical investigation consisted of a field exploration and laboratory testing program, engineering analyses, development of recommendations, and preparation of a report. The field exploration included drilling two hollow-stem auger borings and conducting two seismic refraction surveys. Based on the information obtained from the field exploration and laboratory testing program, the proposed tower relocation sites are generally underlain by a layer of dense to very dense sands and silty sands with gravel. In accordance with the subsurface conditions and anticipated tower loads, cast-in-drilled-hole (CIDH) reinforced concrete piers are recommended to support the new towers. Axial capacity and lateral response analyses were performed on 1.22-m (4-foot) diameter CIDH piers. The analyses considered the situation that one of the piers is located near a slope with a height of 2.44 m (8 feet) and a gradient of 1.667:1 (horizontal:vertical). The results of the analyses are summarized in the paper.