INTRODUCTION

Landsliding is a significant geologic process and geotechnical consideration in the northern Appalachian Plateau of the eastern United States, especially in the vicinity of Pittsburgh, Pennsylvania (Fig. 1). Flat-lying interbedded strong and weak sedimentary rocks have been acted upon by erosion, stress relief, weathering, creep and sliding processes to produce masses of marginally stable colluvium on many of the steep hillsides common to the area. The precarious equilibrium of colluvial masses is frequently upset by heavy precipitation and by construction activities, e.g., removal of toe support, loading the slope, changing surface and subsurface drainage (Gray et al. 1979, Hamel 1980).

Some of the largest colluvial landslides reported in this region occurred in 1968 and 1969 during construction of Interstate Route 79(I-79) 16 km northwest of Pittsburgh (Fig. 1). The recent landslides were initiated by excavating toes of ancient landslides which had occurred in a thick sequence of weak claystones. These recent landslides were studied in detail; clay samples from failure surfaces were obtained and tested in the laboratory to determine index properties and peak and residual shear strengths (Hamel 1969, Hamel & Flint 1969, 1972). The purpose of this paper is to describe these landslides with emphasis on sampling and testing of failure surface clays.

GEOLOGIC SETTING

A section of I-79 extends from Sta. 900 to Sta. 970 along the east wall of the valley of Kilbuck Run, a tributary of the Ohio River (Fig. 1). I-79 was located here in sidehill cuts to avoid the pre-existing road and dwellings along the stream in the valley bottom.

(Figure in full paper)

Site geology has been described by Hamel & Flint (1969; 1972). Rock strata are flatlying cyclothemic, coal measure types of the Pennsylvanian (Carboniferous) age Conemaugh Group. Sandstones, shales and claystones are predominant in the valley walls. Strata dip west, i.e., out of the east valley wall, at approximately 2%.

For purposes of this paper, the geologic setting can be characterized by the generalized slope cross-section in Fig. 2. The lower portion of the slope consists primarily of silty to sandy shale. This is overlain by a 20 m thick weak rock interval consisting principally of claystone. The weak rock interval is overlain by 20 to 30 m of massive sandstone which extends to the ridge top.

Deep seated ancient landsliding occurred in the weak rock interval and extended up to the ridge top (Fig. 2). This ancient landsliding appears to have involved lateral translation (block sliding) with some rotation (slumping) at the rears of slide masses. Conspicuous landslide benches developed on both valley walls at the level of the weak rock interval (Figs. 2 & 3). These benches had maximum widths of 60 m perpendicular to the slope.

The ancient landsliding is inferred to have occurred in Pleistocene time. Continental glaciers stopped 70 km north of Pittsburgh but the area was subjected to a periglacial climate with associated heavy precipitation. Details of Pleistocene history are sparse but two facts are known.

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