ABSTRACT:
Design and construction of roads and foundations on steeply dipping slopes and weak bedrock creates challenging rockslope engineering problems because of the kinematic potential for failure into the rock excavations. Nevada Power Company (NPC) is constructing the 83km (50mile) Harry Allen Mead (HAMD) 500 kV power transmission line, their largest. Because ideal terrain was occupied by towers owned by another utility, tower pad HAMD 24/1 and access road had to be constructed on the dip slopes of the adjoining ridge. The slopes created design problems because they dip unfavorably into the excavation creating potentially large planar failures and rockfall. During construction NPC encountered engineering and geological challenges, which necessitated design modifications: adversely dipping rock structure, very weak and fractured unstable rock, rockfall, backbreak from preshear blasting, and poor bonding of resin grout. To stabilize the backwall of the tower pad and road, over 1311m (4300 ft) of rockbolts and dowels were installed.
1 INTRODUCTION
1.1 Project
The Nevada Power Company (NPC) is constructing the Harry Allen Mead (HAMD) 500 kV power transmission line, NPC’s largest. Los Angeles Department of Water and Power (LADWP) owns a high power transmission line that traverses prime terrain in a pass formed from the shoulder of Lava Butte and the steeply dipping limestone slope of the Horse Spring Formation (Figs. 1-2). The new power line parallels the line operated by LADWP. However, because the present power line occupies prime terrain in the pass tower pad HAMD 24/1 and access road had to be constructed on the dip slope of the adjoining ridge. To achieve the proper tower pad and access road size, vertical cut slopes were proposed. These dip slopes created rockslope design problems in that the slopes dip unfavorably into the excavation of the proposed structures creating potentially large planar failures and rockfall.
