Rock Mass Classification

INDEX PROPERTIES OF

ROCKS AND ROCK MASS

CLASSIFICATIONS SYSTEMS

Rock Mechanics

The study of the Engineering Properties of Rocks is

termed Rock Mechanics, which is defined as follows:

The theoretical and applied science of the

mechanical behaviour of rock and rock masses in

response to force fields of their physical

environment.

It is really a subdivision of Geomechanics which

is

concerned with the mechanical responses of all

geological materials, including soils.

Application of Rock Mechanics

Evaluation of geological hazards;

Selection and preparation of rock materials;

Evaluation of cut-ability and drill-ability of rock;

Analysis of rock deformations;

Analysis of rock stability;

Control of blasting procedures;

Design of support systems;

Hydraulic fracturing, and

Selection of types of structures.

Rock Mechanics

Committee on Rock Mechanics of Geological Society of America defined Rock mechanics in the following terms

Rock mechanics is the theoretical andapplied science of mechanical behaviourof rocks; it is that branch of mechanicsconcerned with the response of rocks tothe force fields of its physical environment(JUDD, 1964)

Rock Mechanics (contd)

Rock mechanics deals with the properties of rocks and the special methodology required for design of rock related components of engineering schemes.

Knowledge of rock engineering for the civil engineer is vital.

The potential areas where knowledge of rock mechanics is essential for civil engineer are Foundations for tall buildings

Dams

Transportation Engg- design of cut slopes for highways, railways, canals, pipelines etc.

Rock Mass

An assemblage of the rock blocks separated by different types of geological discountinuties.(e.g faults, folds)

Folds- the wavy undulations in the rock bed are called folds. They consists of arches and trough in alternate manner. The size of the fold vary greatly from kilometers to centimeters

Fault is a fracture along which there has been relative displacement of beds which were once continuous. The fracture surface is called fault plane. The displacement may be less than meter, several meters or kilometers

Application of Rock Mechanics for

Dams In case of arch or buttress dams that impose high

stresses on the rock foundations or abutments

Active faults in the foundations

Hazards of possible landslides in the foundations e.g Vajont dam

Choice of material for rip-rap for protection of slopes, filter material, protection against wave erosion

Analysis of rock deformation and rock stability

Blasting for rock cleanup has to be engineered to preserve the integrity of the remaining rock and to limit the vibration to the neighboring structures.

Rock Descriptions

Concise and consistent rock descriptions are

needed when logging core, rock outcrops or

hand specimen in the laboratory.

The following checklist has to be followed

Intact Rock

Rock Name

Colour, mineral composition, alteration

Texture, grain size and shape

Porosity, density and water content

Strength, isotropy and hardness

Durability, plasticity, swelling potential

Rock Description (contd)

Jointing

Block size and shape

Number and types of joint sets

Characteristics of each set

This is an extension of the traditional

system of geologist to which parameters of

engineering significance has been added.

Rock Description (contd)

The rock name is usually written in capital letters followed by a list of descriptors SANDSTONE,

medium greenish grey, 60% quartz, 30 % limestone rock fragments, 10 % crystalline calcite cement,

micro-cross bedded,

fine to medium sand sizes with occasional quartz pebbles,

porosity (10-20) 14%,

easily broken by light hammer blow and slightly friable;

block size (5-40) 20 cm, typical shape 1:3:3

Geological name and family

characterisitics

Classical rock name without any qualification can be misleading in any engineering context.

Example is granite when weathered or faulted can behave more like a crumbly sand

Similarly modifiers play an important role in qualifying the name of the rock with intermediate grain size or mineral content.

Example is silty sandy SHALE means a rock with at least 50% clay sized particles.

Generic classification of rocks

Igneous

Metamorphic

Sedimentary

Igneous Family

An igneous rock is one that has solidified

from molten material (magma).

It may be crystalline or glassy or both.

Igneous rocks are further divided

according to their grain size and their light

or dark colours.

Metamorphic Rocks

A metamorphic rock is one derived from preexisting igneous, sedimentary or metamorphic rock as a result of marked change in temperature or stresses.

Dynamic metamorphism generates intense stresses locally which tends to deform, fracture and pulverize the rock.

Regional metamorphism affects an extensive large area through an increase in pressure and temperature.

Sedimentary Rocks

Sedimentary rocks are composed of broken

fragments derived from existing rocks or from

the weathering products of such rocks that have

been transported by rivers, wind or glaciers.

Sedimentary rocks are further subdivided into

different families

Sandstone family

Limestone family

Shale family

Salt family

Physical Characteristics

Color

Mineral

Chemical composition

Texture, grain sizes and shapes

The texture or fabric of the rock is the size, shape or the arrangements of the constituents on the scale of hand specimen, one or few centimeters in size.

The structure is the arrangement of the rock mass components on a scale of several meters.

Structure include features such as macrobedding, folding and faulting.

Texture and structural differences are diagnostic in distinguishing between igneous, metamorphic and sedimentary rocks.

Texture, grain size and shape

(contd) Grain size and size distributions can be given either

numerically as a range and a typical value.

The commonly accepted size designation are clay (finer than 0.002 mm), silt (0.002-0.06 mm), sand (0.06 -2 mm), gravel (2-60 mm), cobbles (60-200 mm) and boulder (coarser than 200 mm).

The shape of the fragmental or crystalline can be in terms of relative length of the orthogonal grain axis. Equi-dimensional (1:1:1)

Platy or discoid ( two long axes and one short)

Fibrous or prolate ( two short axes and one long axes)

Fragmental grain may be further subdivided into angular, sub-angular, sub-rounded, rounded or well rounded.

Porosity, density and water content

Porosity is pore volume expressed as % of

total volume

Dry density is the unit weight of the solids

divided by total volume of the specimen

Degree of saturation is the ratio of water to

pore space by volume.

Water content is the ratio of water to solids

by weight.

Mechanical Characteristics

Strength

Strength Index Test

Uniaxial compressive strength test

Point load strength test

Brazillian strength test

Hardness

Depending upon the test method we can

define scratch, indentation, abrasion,

impact and rebound hardness.

Durability, plasticity and swelling

Slake durability is define as the resistance

of the rock to wetting and drying cycles.

Plasticity is measured by performing

Atterbergs limit test

Swelling potential

Character of Rock Material

Rock Material-

intact material

small piece of rock

Measured on scale of hand specimen

Rock Mass

Properties measured on larger scale

Influence of jointing taken into consideration

Rock Mass

For geotechnical mapping unit (GMU)

Block Size, RQD and block shape

Number and types of joint sets

For each joint set within GMU

Orientation

Spacing

Persistence, surface texture and roughness

Aperture, filling and wall strength

Seepage

Block Size

Block Size- is defined as the average

diameter of a typical rock block.

It is measured by observing exposed rock

face at the surface or underground, or the

rock core obtained by drilling or pile of

broken rock.

RQD

The parameter core recovery is defined as

the ratio of recovered core length to the

total length drilled.

RQD is defined as the sum of the length of

core pieces longer than 10 cm expressed

as a percentage of a given total length

drilled.

RQD

RQD Value (%) Status

100-90 Excellent

90-75 Good

75-50 Fair

50-25 Poor

< 25 Very poor

Volumetric joint count

It is expressed in terms of averge number

of joints per unit area or unit volume of

rock mass.

Block Shape

Most rock masses have a characteristic shape of block that depends on the number of joint sets and their relative orientation and spacing.

Cubic blocks

Slabby block- single closely spaced set and two at wider spacing

Prismatic- two closely spaced set and one at wider spacing

What is Rock mass

We need to know the properties of the

intact rocks and

properties of discountinuties

How to apply

View the rock mass as series of pattern

and relationships

Develop RMR or other rock mass into

zones

Rock Mass Parameters

Near surface may be the following

Intact strength

Discountinity

Spacing

Orientation

Ground water

Depth

Insitu stresses

Engineering Rock Mass

Classification

Terzaghis Rock Mass Classification

Rock Quality Designation Index

Palmstorm Criteria

Rock Structure Rating

Geomechanics Classification

Rock Tunnelling Index

Terzaghis Rock Mass

Classification

Based in extensive experiences in steel arch

supported rail tunnels in the Alps, Terzaghi

(1946) classified rock mass by mean of Rock

Load Factor. Terzaghis descriptors are

Intact Rock

Stratified Rock

Moderately jointed

Blocky and seamy

Squeezing

Swelling

Terzaghi Rock Load Concept

Figure1: Terzaghis rock load concept

The rock mass is classified into 9 classes from hard and

intact rock to blocky, and to squeezing rock. The

concept used in this classification system is to estimate

the rock load to be carried by the steel arches installed

to support a tunnel, as illustrated in Figure 1. The

classification is presented in Table 1.For obtaining the support pressure (p) from the rock load factor

(Hp), Terzaghi suggested the equation below,

p = Hp H

Where is the unit weight of the rock mass, H is the tunnel depth

or thickness of the overburden.

Singh and Goel (1999) gave the following

comments to the Rock Load Factor

classification:

It provides reasonable support pressure

estimates for small tunnels with diameter up to 6

meters.

It gives over-estimates for large tunnels with

diameter above 6 meters.

The estimated support pressure has a wide range

for squeezing and swelling rock conditions for a

meaningful application.

DEER RQD