RMR Rock Properties for Shallow Foundation Design

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

• E-mail
• Print
• Link

Rock Mass Rating (RMR) System

The RMR system assigns points for several different characteristics of the rock mass. These points are summed and an adjustment for joint orientations is applied, resulting in the RMR value for the rock layer. Although revisions of the RMR rating have been proposed, the 1976 version (RMR76) is the version commonly used in practice.2,11,12,13,14,15

The RMR^sub 76^ system considers four rock characteristics or properties that can be evaluated quickly and inexpensively in the field.14 Figure 1 presents the parameters contributing to the RMR76 value and the range of points for each.12,14

The parameters used to determine the RMR^sub 76^ for a rock layer are evaluated as follows:14

1. Intact rock strength from unaxial compression tests on rock cores. The point-load index can be used for all but very low strength rocks when no suitable test specimens are available for compression tests.

2. The RQD of the rock core indicating rock core quality and discontinuity spacing.9

3. Joint spacing also accounts for discontinuity spacing within the rock mass including joints, faults, bedding planes, and any other planes of weakness. It is evaluated from rock outcrops, core runs, and excavalion walls.

4. Discontinuity condition accounts for joint width, continuity, surface roughness, surface hardness, and joinl material. This condition is evaluated through observation of rock outcrops, excavation walls, and rock cores.

5. Groundwaler condition assesses the influence of groundwater flow on the rock mass. For shallow rock typical of spread footing foundations, groundwater pressures are generally less significant than for deep rock conditions.

An adjustment for joint orientation reduces the RMR76 value by 0 to 25 points.14 Joint orientation varies from very favorable to very unfavorable, relative to the applied loading. Evaluation of rock mass joint orientation is based on rock outcrops, published geologic data, rock cores, and excavalion walls. The final RMR^sub 76^ value ranges from 0 to 100. The higher the RMR^sub 76^ value, the better the rock in terms of slrength and deformation behavior.

Table 1 presenls the RMR^sub 76^ system, the subdivisions for each classification parameter, and the point value associated with each subdivision.14 A well-planned subsurface exploration program combined with strength testing of rock core specimens provides the classification parameters necessary to calculate the RMR76 value for a rock layer.

Property Correlations with Rock Mass Rating

Correlations with the RMR76 value provide values of lhe Mohr-Coulomb strength parameters commonly used for foundation design. Based on the calculated RMR76 value, estimates of the friction angle φ' and the rock mass cohesion c' for the rock mass are obtained.14 Due to different failure mechanisms and lhe subsequenl consequences of failure, different reductions to the RMR^sub 76^ value to account for joint orientations are recommended for rock slope and tunneling applications.14 Application of the RMR^sub 76^ system and the strenglh and deformation correlations presented in this paper are limited to shallow foundations in rock.