Application of rock engineering in geotechnial consulting practice considered herein is in areas of: (a) bedrock verification for foundation support, (b) bearing capacity and settlements, and (c) excavations. A routine geotechnical investigation usually provides data on depth to rock, rock classification, and rock quality. In cases of rock outcrop, such outcrops are mapped for geologic structures and rock mass description. In addition, the regional geology and the general aspects of the groundwater regime are usually known from either published literature or previous experience. The predominant technique for ground and site investigation are usually drilling and geologic reconnaissance. Other techniques involving seismic and geophysical soundings, borehole camera, and oriented cores are used by by no means routine. Performing laboratory strength and compressibility tests is often made to be considered as routine. The use of the point load test is gaining increasing use in the United States and would, perhaps, become a routine field or laboratory test due to its simplicity and, more importantly, its relatively low cost. The purpose of this paper is to describe cases where mostly routine investigation and testing were used to arrive at engineering decisions. Bearing Capacity and Settlements The allowable bearing values for foundation support on rock is usually dictated by local building codes which presumably should reflect local experience. The interpretation of the code, however, can present a problem, especially if the interpretation is made by a designer under the delusion of knowledge in rock engineering. Forexample, the provisions of the New York City Code allow 5750k N/m2 (60 tons per square foot TSF) bearing on Hard Sound Rock, 3830 kN/m2 (40 TSF) on Medium Hard Rock, 1916 kN/m2 (20 TSF) on Intermediate Rock, and 766 kN/m2 (8 TSF) on Soft Rock. The above bearing values are in many cases based on examination on exposed rock at footing bottom without consideration for possible presence of softer rock at some depth below footing, and the criterion for acceptance for 40 TSF bearing is contingent on whether the rock "rings." In some cases, this approach may be safe, especially when rock is excavated over an area much larger than the footing and rock walls can be examined. However, in many cases, this approach may be too simplistic as discussed below. Figure 1 shows a subsurface profile at the location of one of the footings for a proposed 40-story structure in New York City hereinafter called Site A. Rock is Manhattan Schist, and the foundations for the building had to be installed in the basement of an existing building. Thus, borings were drilled and NX rock cores were recovered at essentially all locations of footings. In addition to RQD, it was possible to assign a rock mass rating as proposed by Bieniawski (1974, 1976). For example, the hard Schist from 3.1 m - 4.33 m (14 - 18 feet) in Figure 1 was assigned a rating of 11 for hardess, 17 for RQD, 10 for jointing, 12 for conditions of the jointing, and 0.0 for water conditions.
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Some Applications Of Rock - Engineering To Geotechnical Practice
Paper presented at the The 23rd U.S Symposium on Rock Mechanics (USRMS), Berkeley, California, August 1982.
Paper Number: ARMA-82-1015
Published: August 25 1982
Oweis, Issa S., and Walter W. Lilly. "Some Applications Of Rock - Engineering To Geotechnical Practice." Paper presented at the The 23rd U.S Symposium on Rock Mechanics (USRMS), Berkeley, California, August 1982.
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