significance of igneous, metamorphic and sedimentary rock. MRMR classification system.

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Rock mechanics
[engineering properties of rock & MRMR classification system]

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Engineering Properties of Rocks
Rock:
Rock is a combination of different minerals. When different types of
minerals are joined together, they form a rock.
Rocks are mainly of three types
1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
Engineering Properties of ROCKS:
Rocks have very much importance in engineering point of view. All civil
engineering structures are built on rock so, engineering properties of
rocks are very much important.
In the following paragraphs, engineering properties of different types of
rocks are given.
Engineering Properties of IGNEOUS Rocks
The rocks formed by the molten rocks (Magma or lava) are called igneous
rock.
It has two main types
• Intrusive Igneous Rocks
• Extrusive Igneous Rocks
Intrusive Rocks:
Important features of igneous rocks are following
Granular texture
Massive structure
Relatively homogeneous composition
Some times highly altered with weather
Massive igneous rocks such as batholiths may affect tunneling, mining
slope stability. These rocks are also used as construction material. Tabular
intrusive rocks such as dikes may create more construction problems
than massive rocks because of the lack of homogeneous composition.

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Extrusive Rocks:
Extrusive rocks are found in crystalline texture. The origin of these rocks
are greatly influence their engineering properties. Main characteristics
which influence the engineering works are followings.
Variable composition and texture.
Strength durability and permeability.
Strong unconfined compressive strength >200mpa.
Columnar Jointing.
Engineering Properties of SEDIMENTARY Rocks
The following are the important properties of sedimentary rocks in
engineering point of view.
Compressive strength and deformability of sandstone is influenced by its
porosity, the amount and type of cement, and matrix material, grain
contact and composition. Siliceous cement is stronger than calcareous
cemented sandstones. Pore water plays a significant role in the
compressive strength and deformation characteristics of sandstone. It
can reduce the unconfined compressive strength by 30 to 60%.Shale
mineral content influences geotechnical properties; most important is
the quartz-clay mineral ratio. The liquid limit of clay shale increases with
increasing clay-mineral content.
Swelling properties of certain shale have proven detrimental to the
integrity of engineering structures. Swelling occurs by the absorption of
free water by clay minerals (montmorillonite) in the clay fraction of the
shale. Highly fissured over consolidated shale have greater swelling
tendencies than poorly fissured clayey shale, the fissures providing access
for water.
Porosity of shale may range from slightly under 5% to just over
50%.Cemented shale are stronger and more durable than compacted
shale. The elastic moduli of compaction shale range between 140 and
1400 Mpa: well cemented shale has elastic moduli in excess of 14000
Mpa.
Clay shale usually has permeability of the order 10-8 m/s to 10-12 m/s.
However, sandy and silt shale and closely jointed cemented shale may
have permeabilities as high as 1 x 10-6 m/s.
Grain size and Cementation influence engineering properties of
carbonate sediments. Carbonate sediments can sustain high overburden
pressures, and retain high porosities at considerable depth. Generally,

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the density of these rocks increases with age, and porosity is reduced.
Important sedimentary rocks:
Following are some important sedimentary rocks
Sandstones vary from thinly laminated micaceous types to very thickly
bedded varieties. They may be cross-bedded and are invariably jointed.
With the exception of shale sandstone, sandstone is not subject to rapid
surface weathering. The dry density and porosity of sandstone are
influenced by the amount of cement and/or matrix material occupying
the pores. Usually the density of sandstone tends to increase with
increasing depth below the surface.
Limestone When dolomitize, undergoes an increase in porosity of a few
percent. Joints in limestone have generally been subjected to various
degrees of dissolution. Sinkholes may develop where joints intersect. And
these may lead to subterranean caverns. The dissolution leads to an
increase in mass permeability. Enlargement of the pores enhances water
circulation encouraging further solution. This s brings about an increase
in stress within the remaining rock framework, which reduces the
strength of the rock and leads to increasing stress corrosion.
Chalk The unconfined compressive strength of chalk ranges from
moderately weak to moderately strong. The strength of chalk is reduced
when saturated. The Upper Chalk from Kent is particularly deformable,
typical values of Young’s modulus being 5 X 103 Mpa. It exhibits
elastic-plastic deformation. Discontinuities govern the mass permeability
of chalk. Chalk is also subject to dissolution along discontinuities.
Anhydrite is a strong rock, gypsum and potash are moderately strong,
and rock salt is moderately weak. Evaporitic rocks exhibit various degrees
of plastic deformation before failing. Creep may account for 20 to 60% of
the strain at failure. Rock salt is most prone to creep.
Gypsum is more readily soluble than limestone. Sinkholes and caverns
can develop in thick beds of gypsum. Massive anhydrite can be dissolved
to produce seepage flow rates which increase in a rapidly accelerated
manner. Heave is another problem associated with anhydrite. When
anhydrite is hydrated to form gypsum, there is a volume increase of
between 30 and 58%, which exerts pressures between 2 and 69 MPa.
Salt is even more soluble than gypsum, slumping, brecciaing and collapse
structures occur in rocks overlying salt beds.
Engineering Properties of METAMORPHIC Rocks

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Metamorphic rocks are divided into two
categories Foliates and Non-foliates.
Foliates are composed of large amounts of micas and chlorites. These
minerals have very distinct cleavage. Foliated metamorphic rocks will split
along cleavage lines that are parallel to the minerals that make up the
rock. Slate, as an example, will split into thin sheets.

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(MRMR) this classsification systerm developed for mining
purpose.
In 1990 Laubscher, developed the Mining Rock Mass Rating (MRMR)
system by modifying the Rock Mass Rating (RMR) system of Bieniawski.
In the MRMR system the stability and support are determined with the
following equations:
RMR = IRS + RQD + spacing + condition
in which:
RMR = Laubschers Rock Mass Rating
IRS = Intact Rock Strength
RQD = Rock Quality Designation
spacing = expression for the spacing of discontinuities
condition = condition of discontinuities (parameter also dependent
on groundwater presence, pressure, or quantity of groundwater inflow
in the underground excavation)
MRMR = RMR * adjustment factors
in which:
adjustment factors = factors to compensate for: the method of
excavation, orientation of discontinuities and excavation, induced
stresses, and future weathering
The parameters to calculate the RMR value are similar to those used in
the RMR system of Bieniawski. This may be confusing, as some of the
parameters in the MRMR system are modified, such as the condition
parameter that includes groundwater presence and pressure in
the MRMR system whereas groundwater is a separate parameter in
the RMR system of Bieniawski. The number of classes for the parameters
and the detail of the description of the parameters are also more
extensive than in the RMR system of Bieniawski.
PITFALLS AND UNCERTAINTIES
The basic functions of the MR-MR classification system are to:

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 subdivide (classify) the rock man into zones, based on similar
behaviour
 provide a basis for communication between various mining
disciplines and
 formulate design parameters for the actual mine design.
The MRMR system is one of the methods to characterise the rock
mass competency. It is important to understand that rock mass
competency is not only influenced by its inherent geological
parameters (material strength and quantity and strength of the
defects). but also the change introduced by the mining activities
(induced stress, blasting damage, exposure to weathering. relative
orientation of the defects and excavations. water). These 'man made'
changes often have detrimental effects on the rock mass competency
and thus stability of the openings. and cannot be ignored.
The most common errors in classifying rock masses include:
 avenging values across geotechnical domains:
 mixing natural and mining induced defects (joints and fractures):
 mixing in situ rock mass ruing (IRMR) and modified rock mass
rating (MRMR) values:
 not considering the variability (distribution) of values of individual
parameters:
 ignoring rock strength anisotropy and its orientation:
 avenging the intact rock strength (IRS) of weak and strong zones:
 averaging joint conditions for individual discontinuity sets;
 not considering discontinuities other than joints:
 ignoring the orientation of the structural irregularities (small and/or
larg scale joint expressions):
 ignoring or misusing the sampling error adjustment:
 wrongly adjusting for alteration:
 wrongly adjusting for weathering:
 not recognising internal rock defects such as discontinuous natural
fractures. stuns. foliation. cemented joints. schistosity. bedding.
preferred mineral orientation and micro fractures:

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 applying mining adjustments without considering spatial
relationship and time leg weathering, blasting):
 mixing localised failures with casing: and
 altering the classification system to suite the local 'needs' and
then using stability graph, and ground
support tables based on original (unaltered) ratings.