Continued Use of RMR and 2012 LRFD Procedures

The newly revised of Section 10.4.6.4 of the 2014 LRFD Bridge Design Specifications has significantly changed the methodology for estimating rock mass strength.  In particular, the 2014 code has changed from using the Rock Mass Rating (RMR) system to using the Geotechnical Strength Index (GSI) system for characterizing the properties of a rock mass.  Each system has its advantages and disadvantages; however, the GSI is less applicable to West Virginia rock types since the GSI system requires the rock to behave isotropically.  Generally our rock is very anisotropic with distinct bedding and fracturing orientations.

In addition, the RMR system has been in practice by the WVDOH and their consultants for many years now.  Guidance for the use of the RMR system was provided in previous WVDOH research (RP 204) and was verified by comparison to drilled shaft load tests.  The RMR system is compatible with our PSSLOPE software that was developed through another WVDOH research project (RP 210).  Also, strength parameters are simpler to estimate using the RMR system and the Bieniawski, 1983, correlations than with the GSI system and the Hoek-Brown, 2002, correlations.  We estimate that the GSI system results in less conservative skin friction, end bearing, and modulus values.

Consequently the WVDOH, with the concurrence of the ACEC Geotechnical Committee, are requiring the continued use of the RMR system as presented in the 2012 code.  The methodology for estimating bearing resistance of both spread and deep foundations, as recommended in RP 204, are to continue. The methodologies for estimating rock mass modulus and skin friction are to continue as presented in the 2012 code until such time as the GSI system is demonstrated as applicable to the rocks of West Virginia.

 

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