Terry R. West

Development of a probabilistic approach for rock wedge failure

Abstract For rock slope engineering, uncertainty and variability are inherent in data collected on orientation and strength of discontinuities, yielding a range of results. Unfortunately, conventional deterministic analysis based on the factor of safety concept, requires a fixed representative value for each parameter without regard to the degree of uncertainty involved. Therefore, the deterministic analysis fails to properly represent uncertainty and variability, so common in engineering geology studies. To overcome this shortcoming, the probabilistic analysis method was proposed and used for more than a decade in rock slope stability analysis. However, most probabilistic analyses included a deterministic model as part of the analysis procedure causing subsequent problems, which went uncorrected. The objectives of this paper are to develop a solution for these difficulties in probabilistic analyses and to propose an appropriate simulation procedure for the probabilistic analysis of rock wedge failures. As part of the solution, probability of kinematic instability and probability of kinetic instability are evaluated separately to provide a proper, combined evaluation for failure probability. To evaluate the feasibility of this new probabilistic approach, the procedure is applied to a practical example, a major, highway rock cut in North Carolina, USA. Results of the probabilistic approach are compared to those of the deterministic analysis; findings are significantly different, indicating that the deterministic analysis does not depict rock slope variations, particularly where significant scatter in parameter data occurs.

Sampling bias of discontinuity orientation caused by linear sampling technique

Abstract Orientation data of discontinuities are of paramount importance for rock slope stability studies because they control the possibility of unstable conditions or excessive deformation. Most orientation data are collected using linear sampling techniques, such as borehole fracture mapping and the detailed scanline method (outcrop mapping). However, data acquired by such linear sampling techniques are subject to bias, owing to the orientation of the sampling line. Even if a weighting factor is applied to orientation data to reduce this problem, the bias will not be significantly reduced when certain sampling orientations are involved. If the linear sampling orientation nearly parallels the discontinuity orientation, most of these parallel discontinuities will be excluded from the survey results. This phenomenon can cause serious misinterpretation of discontinuity orientation data because critical information is omitted. In the case study, orientation data collected using the borehole fracture mapping method (vertical) were compared to those based on orientation data from the detailed scanline method (horizontal scanline). Differences in results for the two procedures revealed a concern that a representative orientation of discontinuities was not accomplished. Equal area, polar stereo nets were used to determine the distribution of dip angles and to compare the data distribution for the borehole method with that for the outcrop scanline.

Influence of Petrography of Argillaceous Carbonates on Their Frost Resistance in Concrete

Argillaceous carbonate rocks, when used as coarse aggregate, have been found to cause severe pitting and popouts in several Indiana highways within one winter after construction. These rocks were studied in detail petrographically to determine texture, structure, and mineral composition, and in particular the amount, type, and mode of distribution of argillaceous material. Results of petrographic examination were compared with other test data including specific gravity, absorption-adsorption values, pore characteristics, and loss in freeze-thaw. This comparison suggests a close relationship between petrographic characteristics and freeze-thaw resistance of argillaceous carbonates. A comparison of petrographic information with field performance shows that those rocks that contain more than 20% silt and clay distributed uniformly throughout the rock mass result in pitting and popouts while those in which silt and clay are concentrated as streaks and laminations are not necessarily unsound. This is contrary to several previous investigations that concluded that laminated carbonates should be considered nondurable without specific concern for their total argillaceous content.

DEVELOPMENT OF A PROCEDURE TO IDENTIFY AGGREGATES FOR BITUMINOUS SURFACES IN INDIANA

Adequate friction resistance is needed to prevent pavement slipperiness allowing vehicles to stop in a reasonable distance. For stone mastic asphalt surfaces, friction resistance is mainly a function of the interaction between the aggregates exposed at the road surface and vehicle tires. Aggregate performance is reduced with time by wear and polishing as a consequence of vehicular traffic. In this research a method to investigate performance based on physical, chemical and petrographic factors has been evaluated. The objective was to develop a laboratory method to test Indiana dolomite, limestone, sandstone, and gravel aggregates to predict friction resistance in the field and determine causes for the range of values among these aggregates. Assessment of gravel sources was based on individual rock types and their proportions comprising the gravel. Initial Friction Values (IFV) and Polished Values (PV) were determined in the laboratory with the British Wheel and Pendulum test and field values obtained from the towed friction trailer. For two laboratories involved a significant difference in IFV and PV was obtained so that further verification is required. Correlations between parameters were established which provide predictions of friction resistance based on laboratory specimens. A database of physical and chemical properties should be collected on aggregates used or considered for bituminous wearing courses. This includes the testing required for Class A aggregates plus elemental Mg and elemental Ca content.