RMR Rock Properties for Shallow Foundation Design

Journal of Engineering Technology,  Fall 2004  by Rose, Andrew T

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Methods for estimating the settlement of foundations on rock are available from ASCE and AASHTO. Both methods use the same general elastic deformation equation with slightly different coefficients for consideration of different footing geometries. For layered rock systems where the elastic deformation moduli E vary with depth, ASCE gives a useful equation for determining a weighted deformation modulus.5 The calculations in Appendices A and B include estimates of the elastic settlements using both the ASCE and AASHTO procedures for an applied load P^sub applied^ of 350 kips (1558 kN).

Table 6 summarizes the calculated ultimate bearing capacity, allowable bearing pressure, allowable load, and elastic settlement for this example using the ASCE and AASHTO procedures.3,4,5,6

The AASHTO procedure gave an ultimate bearing capacity that is 63% higher than the valued determined by the ASCE procedure. For foundations on any type of broken rock, a presumptive allowable bearing pressure of 20 kips/ft.2 (958 kPa) is recommended.3 This presumptive value essentially is used when no other data is available or for very preliminary designs and project feasibility studies. In this example, more detailed geologic data and test results are available to support using higher allowable bearing pressures. The allowable bearing pressure of 21.9 kips/ft.2 (1049 kPa) from the ASCE procedure represents a conservative value in foundation design and analysis. The ASCE bearing capacity would be slightly higher if soil overburden were included in the calculations (see Appendix A). The AASHTO procedure does not account for soil overburden in determining the bearing capacity. As a result, soil overburden was neglected from both procedures for consistency. The elastic settlement estimated by the AASHTO procedure was 42% larger than the ASCE estimate. Both estimates of elastic settlement are small and are negligible from a foundation design and performance perspective.

Course Implementation and Assessment

Introduction of the RMR^sub 76^ rock classification system and rock strength and deformation parameter correlations have been introduced into a senior-level civil engineering technology course in foundation design. Students estimate the RMR76 value for the rock, use correlations for estimating rock strength and deformation parameters, and analyze shallow foundations with the ASCE and AASHTO approaches.2,5,6,8,14 The RMR^sub 76^ rock classification system provides students an introduction to rock mechanics concepts and terminology and allows for discussion of the factors influencing the strength and deformation behavior of rock. Although the exercise currently uses boring log data and previously obtained intact rock core strengths to estimate the RMR^sub 76^ value for the rock mass, a natural extension of this application would be to include a laboratory component to actually observe and log rock cores and measure the uniaxial compressive strength or point load index of the intact rock core to determine the actual RMR^sub 76^ value for the rock. Designing shallow foundations on rock with the ASCE approach utilizes the same theory and similar equations as used for designing foundations on soil, so students can make the transition easily. Use of the AASHTO equations not only provides a comparison of rock foundation design procedures but also exposes students to another design specification they may encounter in professional practice. Students understand that the geologic conditions in which many of them will seek jobs include shallow rock conditions, so they recognize the advantage of including this topic in the course and appreciate the learning opportunity. Student performance on related homework assignments and exams indicates they are adequately learning and applying the concepts.