INDEX PROPERTIES OF

ROCKS AND ROCK MASS

CLASSIFICATIONS

Geomechanics Classification

• The rock mass rating (RMR) system is a

rock mass quality classification developed

by

– South African Council for Scientific and

Industrial Research (CSIR), close associated

with excavation for the mining industry

(Bieniawski 1973).

Rock Mass Rating

• Geomechanics classification system incorporated eight parameters. The RMR system in use now incorporates five basic parameters below.

• Strength of intact rock material: Uniaxial compressive strength is preferred. For rock of moderate to high strength, point load index is acceptable.

• RQD: RQD is used as described before.

• Spacing of joints: Average spacing of all rock discontinuities is used.

• Condition of joints: Condition includes joint aperture, persistence, roughness, joint surface weathering and alteration, and presence of infilling.

• Groundwater conditions: It is to account for groundwater inflow in excavation stability.

Geomechanics Classification

• Bieniawski (1976) published the details of rock

mass classification system called Geomechanics

Classification or Rock Mass Rating

• Six basic paramter are used to identify the rocks

– Uniaxial Compressive strength of rock material

– Rock Quality designation

– Spacing of discontinuities

– Condition of Discontinuities

– Groundwater conditions

– Orientation of discontinuities

• Table is the RMR classification updated in 1989.

• Part A of the table shows the RMR classification

with the above 5 parameters. Individual rate for

each parameter is obtained from the property of

each parameter. The weight of each parameter

has already considered in the rating, for

example, maximum rating for joint condition is

30 while for rock strength is 15. The overall

basic RMR rate is the sum of individual rates.

• Influence of joint orientation on the stability of excavation is considered in Part B of the same table.

• Explanation of the descriptive terms used is given table Part C.

• With adjustment made to account for joint orientation, a final RMR rating is obtained;

• The table also gives the meaning of rock mass classes in terms of stand-up time, equivalent rock mass cohesion and friction angle.

• RMR was applied to correlate with excavated active span and stand-up time.

• This correlation allow engineer to estimate the stand-up time for a given span and a given rock mass.

Problem

Problem 1

• A granite rock mass containing 3 joint sets, average

RQD is 88%, average joint spacing is 0.24 m, joint

surfaces are generally stepped and rough, tightly closed

and unweathered with occasional stains observed, the

excavation surface is wet but not dripping, average rock

material uniaxial compressive strength is 160 MPa, the

tunnel is excavated to 150 m below the ground where no

abnormal high in situ stress is expected.

• Selection of RMR parameters and calculation of RMR

• A sandstone rock mass, fractured by 2 joint sets plus random

fractures, average RQD is 70%, average joint spacing is 0.11 m,

joint surfaces are slightly rough, highly weathered with stains and

weathered surface but no clay found on surface, joints are generally

in contact with apertures generally less than 1 mm, average rock

material uniaxial compressive strength is 85 MPa, the tunnel is to be

excavated at 80 m below ground level and the groundwater table is

10 m below the ground surface. Here, groundwater parameter is not

directly given, but given in terms of groundwater pressure of 70 m

water head and overburden pressure of 80 m ground. Since there is

no indication of in situ stress ratio, overburden stress is taken as the

major in situ stress as an approximation.

•

• Joint water pressure = groundwater pressure = 70 m x γw

• In situ stress = Overburden pressure = 80 m x γ

• Joint water pressure / In situ stress = (70 x 1)/(80 x 2.7) = 0.32

•

• Selection of RMR parameters and calculation of RMR

• A highly fractured siltstone rock mass, found to have 2 joint sets and

many random fractures, average RQD is 41%, joints appears

continuous observed in tunnel, joint surfaces are slickensided and

undulating, and are highly weathered, joint are separated by about

3-5 mm, filled with clay, average rock material uniaxial compressive

strength is 65 MPa, inflow per 10 m tunnel length is observed at

approximately 50 litre/minute, with considerable outwash of joint

fillings. The tunnel is at 220 m below ground.

• In the above information, joint spacing is not provided. However,

RQD is given and from the relationship between RQD and joint

frequency, it is possible to calculate average joint spacing, with the

equation below,

•

• RQD = 100 e–0.1λ (0.1λ +1)

•

• Joint frequency is estimated to be 20, which gives average joint

spacing 0.05 m