RECOGNIZING AND INVESTIGATING COLLUVIAL SLOPES IN WESTERN PENNSYLVANIA AND DEVELOPING ENGINEERING SOLUTIONS IN ADVANCE OF CONSTRUCTION PROJECTS

Duquesne Light Company of Pittsburgh, Pennsylvania is in the process of implementing a reliability upgrade to its electrical transmission system in western Pennsylvania. Many of these upgrades are being performed at existing substations. The Logans Ferry substation in Plum Borough, Allegheny County was upgraded to have the capacity to transmit 345 kilovolts of electricity and the existing substation was expanded westward onto approximately 3.2 acres of a southwest-facing slope. An inspection of a topographic map for the site indicated very irregular contour lines highly indicative of a colluvial slope. Indeed, the outline of at least one slump was clearly recognizable on the topographic map. Field reconnaissance of the site confirmed that the slope was terraced from several episodes of sliding and the presence of red soil and hydrophilic vegetation confirmed that the site was underlain by colluvium. Nine test borings were drilled to delineate subsurface conditions during which continuous split barrel and Shelby tube samples were collected in an effort to visually identify the presence of shear zones and for shear strength testing. Soil analyses included routine grain size distribution and reversible direct shear tests. The reversible direct shear tests were performed to obtain the residual friction angle of the colluvium, which was between 8 and 17 degrees. The investigation determined that the site was underlain by up to 80 feet of claystone that formed a substantial colluvial slope likely containing numerous shear planes, although no recognizable shear plane was identified in the soil samples. Two soldier beam and concrete lagging retaining walls, having a total length of approximately 1,000 were designed and constructed to support the proposed substation expansion area at a cost of approximately $2,500,000. Four slope indicators were installed to monitor post construction movement. As of this abstract date, there has been no significant post construction movement of the wall.