physico mechanical properties of rock materials and details of different laboratory as well as field tests for determining behaviour of different rock materials in the field of mining and civil engineering
1 Physico-Mechanical Properties of Rock Materials Siva Sankar Ulimella M.Tech Under Manager Project Planning, SCCL Email: [email protected]A Rock material is an aggregate of mineral particles The performance of the rock, under a particular condition depends upon physical and mechanical properties of rock materials The Physical properties may be known as Index properties, which describes the rock material and helps in classifying them The Mechanical properties may be known as Strength properties and they will give an information about the performance of rock materials, when subjected to a particular loading system. When we talk of Rock Strength we generally understand that: Rock material is generally strong in compression. PHYSICO-MECHANICAL PROPERTIES OF ROCKS
� A Rock material is an aggregate of mineral particle s
�The performance of the rock, under a particular con dition depends upon physical and mechanical properties of rock materials
� The Physical properties may be known as Index properties , which describes the rock material and helps in clas sifying them
� The Mechanical properties may be known as Strength propertiesand they will give an information about the perform ance of rock materials, when subjected to a particular loading s ystem.
When we talk of Rock Strength we generally understa nd that:
� Rock material is generally strong in compression.
PHYSICO-MECHANICAL PROPERTIES OF ROCKS
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� Rocks exhibit a brittle type behaviour when unconfi ned, but become more plastic as the level of confinement increases.
� Conditions in the field are primarily compressive and vary fromunconfined near the opening walls to confined at s ome distancefrom the opening.
� The strength of a rock is affected not only by fact ors that relate to its physical and chemical composition such as its m ineralogy, porosity cementation, degree of alteration or weath ering, and water content, but also by the methods of testing, includ ing such factors as sample size, geometry, test procedure, and loading rate.
PHYSICO-MECHANICAL PROPERTIES OF ROCKS
Physical Properties of Rock Material
The physical properties of rocks affecting design and construction in rocks are:
• Mineralogical composition , structure, and texture;• Specific gravity G• Unit weight • Density• Void ratio e• Porosity n• Moisture content w• Degree of saturation, S• Coefficient of Permeability k• Electrical and Thermal properties• Swelling • Anisotropy • Durability
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Mineralogical composition is the intrinsic property controlling the strength of the rock Although there exist more than 2000 kinds of known minerals, only about nine of them par take decisively in forming the composition of rocks. They are:
• Quartz
• Feldspar
• Mica
• Hornblende (Amphiboles)
• Pyroxenes
• Olivine
• Calcite
• Kaolin, and
• Dolomite
Physical Properties of Rock Material
Specific gravity is the ratio of the density of solids to the density of water.
WS
S
V
MG
ρ1⋅=
(where SM = mass of solids and SV -volume of solids)
Unit weight ( )γ
V
W=γ
( W is the total weight of the sample and V the total volume of the sample) Density is a measure of mass per unit of volume. Density of rock material various, andoften related to the porosity of the rock. It is sometimes defined by unit weight andspecific gravity. Most rocks have density between 2,500nd 2,800 kg/m3.
Physical Properties of Rock Material
Dry Density, Bulk Density, and Saturated Density
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Void ratio (e) is the ratio of the volume of voids (VV) to the volume of solids (VS)
S
V
V
Ve =
we
G
V
WW
dDry +
=⋅+
==11
γγγ
Porosity (n) describes how densely the material is packed. It is the ratio of the non-solidvolume (VV) to the total volume (V) of material. Porosity therefore is a fraction between 0 and 1.
V
GWV
e
e
V
Vn WSV )/(
1
γ−=
+==
V
V
eS=
+1
1
(The unit weight of water = 1 g/cm3 = 1 t/m3 = 9.81 kN/m3 = 62.4 lb/ft3)
Physical Properties of Rock Material
Physical Properties of Rock Material
� Porosity decreases with increasing age of the rock and depth of the rock
� Porosity is a measure of water – holding capacity of a rock material
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Physical Properties of Rock MaterialMoisture Content (M): it is the ratio of weight of water in the voids to the weight of dry solids in the rock sample
M = Ww / Ws, where M = Moisture Content, Ww = Weight of water, and
Ws = Weight of Solids
Degree of saturation (S): it is defined as the volume of water in the void to the total volume of voids in the rock sample
S = Vw / Vv, where Vw = volume of water, and Vv = volume of voids
The rockmass having higher porosity has higher degree of saturation
Permeability ( k): the ability of porous material to allow a liquid to pass through its pores, units: cm/sec, or m/secQ = k i AQ= discharge through area, i= hydraulic gradient
Electrical properties: Most of the rocks are dielectric in nature and measurement of Dielectric constants used for data interpretation Electric resistivity method used in geophysical prospecting
Physical Properties of Rock MaterialThermal Properties: Increase in temperature makes rock weaker due to the formation of cracks in the rockmass
Coefficient of thermal expansion of the rocks: increase in length due to a change in temperature
Swelling: it is an increase in volume of the mass due to suction of water or due to contact of water for a longtime
Swelling is more in weaker type rocks
Anisotropy: properties of the elements of the rock mass are not similar in every direction, due to sequence of rock formation, i.e., due to existence of bedding planes, etc.
Anisotropic material has some weakness in a particular direction
Sedimentary rocks have high degree of anisotropy
Durability : it is the resistance to destruction.
If rock is more durable means it will last for a longer period when put into use.
It depends upon the nature of environment against which the rock is going to be used. Swelling index or slake durability test is used to describe nature of weathering
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EXAMPLES 1 .A cylindrical specimen of moist clay has a diameter of 38 mm, height of 76 mm and mass of 174.2 grams. After drying in the oven at 105 0 C for about 24 hours, the mass is reduced to 148.4 grams. Find the dry density, bulk density and water content of the clay. Assuming the specific gravity of the sample grains as 2.71, find the degree of saturation.
Solution
Strength and Deformation Properties of Rocks
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Idealized diagram showing the transition from intact rock to a heavily jointed rock mass with increasing sample size.
• Figure above illustrates the difficulty in finding a realistic failure criterion for rock masses, it shows the transition from intact rock material to a heavily jointed rock mass.
• The underground excavation designer is concerned with all stages in this transition.
• The stability of the entire system of u/g openings which make up a mine depends upon the behaviour of the entire rock mass surrounding these openings.
• The rockmass may be heavily jointed that it will tend to behave like an assemblage of tightly interlocking angular particles with no significant strength under confined conditions.
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In considering the behaviour of rock as an engineering material in transition from intact rock to heavily jointed rock mass, the quantity and quality of experimental data decrease rapidly as one moves from the intact rock sample to the rock mass. Because small samples are easy to collect and to test under a variety of laboratory conditions.
Experimental difficulties increases significantly in tests on samples with a single set of a joint to multiple sets. Further the full scale testing on jointed rock mass is a real challenge both in terms of testing as well as expense .
Taking all these factors into consideration, it is seen that thefailure criteria which will be of significant use to the underground excavation designer should satisfy the following requirements:
•It should adequately describe the response of an intact rock sample to the full range of stress conditions likely to be encountered underground. These conditions range from uniaxial compression, tension, to triaxial compression
•It should be capable of predicting the influence of one or more sets of discontinuities upon the behaviour of a rock sample.
•It should provide some form of projection, even if appropriate, for the behaviour of a full scale rock mass containing several sets of discontinuities.
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Mechanical or Strength Properties of Rocks
Strength : Ability of a material to resist an externally applied load, but
In Rock mechanics, strength is the Force per unit Area required to bring about rupture in a rock mass at a given environmental conditions.
Classification of strength: depending upon type of loading and the stresses, the strength in general may be classified as
� Compressive Strength
� Tensile strength, and
� Shear Strength
For determining the above strength values the tests are conducted either on intact rock specimens in the laboratory tests or on rockmass in the field, i.e., insitu strength tests
In the laboratory there are direct Methods for the determination of above strength values in the laboratory and also indirect methods for the determination of above strength values roughly in the laboratory or at the field site
Compressive Strength
The compressive strength of a material is a measure of its ability to resist uniaxial compressive loads without yielding or fracture.
The most common measure of compressive strength is the Uniaxial compressive strength or unconfined compressive strength. It is one of the most important properties used in design, analysis and modeling.
Direct Methods: 1.Uni axial Compression Test2.Tri axial Compression Test Indirect Method :1.Point Load Test2.Schmidt hammer Test
Mechanical or Strength Properties of Rocks
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Direct Method: It requires a preparation of sample as accordance to ISRM (International Society of RockMechanics).
Uniaxial compressive strength (UCS) of rock material and deformation behavior under loading is verified by applying compressive load until failure occurs in the core by a fracture in the middle using high capacity Compressive testing machines
Mechanical or Strength Properties of Rocks
(a) Test specimens shall be right circular cylinders having a height to diameter ratio of 2.0-3.0 and a diameter preferably of not less than NX core size , approximately 54 mm. The diameter of the specimen should be related to the size of the largest grain in the rock by the ratio of at least 10:1.
(b) The ends of the specimen shall be flat to 0.02 mm and shall not depart from perpendicularity to the of the specimen by more tha n 0.001 radian (at 3.5 mm) or 0.05 mm in 50 mm.
(c) The sides of the specimen shall be smooth free of abrupt irregularities and straight to within 0.3 mm over the lull length of the specimen.
(d) The use of capping materials or end surface treatments other than machining is not permitted.
(e) The diameter of the test specimen shall be measured to the nearest 0.1 mm by averaging two diameters measured at right angles to each other at about the upper-height, the mid-height and the lower height of the specimen. The average diameter shall be used for calculating the cross-sectional area. The height the specimen shall be determined to the nearest 1.0 mm.
ISRM Standards for Testing of Rock Specimens in Lab oratory
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(f) Samples shall be stored, for no longer than 30 days . in such a way as o preserve the natural content, as far as possible, and tested in that condition. This moisture condition shall be reported in accordance with “Suggested method for determination of the water content of a rock sample”.
(g) Load on the specimen shall be applied continuously at a constant Stress rate such that failure occur within 5 -10 min . of loading, alternatively the stress rate shall be within the limits of 0.5—1.0 MPa/s.
(h) The maximum load on the specimen shall be recorded in newtons (or kilonewtons and mega-newtons where appropriate) to within 1%.
(i) The number of specimens tested should be determined from practical considerations but at least five are preferred.
ISRM Standards for Testing of Rock Specimens in Lab oratory
Where: D = diameter of specimen, in.L = length of specimen, in.C a= measured compressive strength, lb/in.C0 = corrected (computed) compressive strength of an equivalent L/D = 2 specimen.
If the length-to-diameter ratio of the rock specimen is less than 2 , the measured compressive strength, Ca should be corrected to give the standardized compressive strength, C0, by means of the following equation:
Uniaxial compressive strength (UCS)
UCS is given by the ratio of load at failure or rupture to cross-sectional area of the specimen
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Compressive Testing Machines
Universal Testing Machine
Manual
INSTRAN Testing Machine
With UTM, the axial displacement w.r.t. load is to be recorded manually with the help of proving ring, while lateral deformation recorded using dial gauges or strain gauges coupled to LVDT,If provided.
With Instran machine Displacement between Loading Platens will give axial displacement of the specimen under loading and directly get recorded in connected computer
Lateral displacement will be recorded in computer using the special attachment shown below or manually recorded using strain gauges coupled to LVDT
Lateral Displacement Measurement
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Uniaxial stress-strain curves for different rock types
Peak stress
Post peak characteristicsare different in differentrock types
Uniaxial compression
Class I: is a stable fracture propagationwhich means that when the max load bearing capacity is exceeded, still some external work has got to be done for further destruction of the specimen
Class II: unstable fracture propagation takes place such that the amount of energy stored in the specimen at the moment when its max load carrying capacity is just exceeded is sufficient to maintain the crack growth
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Post failure behaviour of rock in compressionCyclic loading
The behaviour of the rock under compression until t he rockhas lost its strength is as shown in the following figure.
Load deformation Curve if loading-unloading is not followed
Compressive StrengthThe load-deformation characteristics in UCS for loading andunloading cycles follow the following behaviour:
1. On loading , the curve eventually joins that for a specimen in which the axial displacement increases with time
2. As displacement continues in the post-peak region, the portion of the total displacement that is irrecoverable increases
3. The loading-unloading-loading loop shows some hysteresis
4. The apparent modulus of the rock which can be calculated from the slope of the reloading curve, decreases with post-peak deformationand progressive fragmentation of the specimen
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Rock Failure characteristics in UCS
Spalling phase
Shear Fracture
The fracture pattern of specimen is divided into 8 distinct regions.
I-III are marked with closure of pre-existing cracks as well ascoalescence of random crack formation, crack growth and sliding
on existing crack surface
IV extension of the small fractures parallel to the line of loading. The cracks appear at the center of the specimen height and dilation is prominently seen. The peak strength is also reached
V Spalling of the dilated specimen starts at the beginning of the region V of stress-strain curve, continues in the region VI followed by a steeply inclined shear fracture plane and it grows into theregion VII
VIII Loose mass of the broken material is held together due to friction
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Failure modes in compression
Rock Specimens before & After failure in Uni-Axial compression
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Triaxial compression of rock samples – Direct Method
When the rock specimen is subjected to confining pressure in addition to vertical pressure, the strength exhibited by rock specimen is known as Triaxial compressive strength
Axial loading by Compressive testing machine and Confining pressure usually oil pressure from external source
This test also helps in determining shear strength parameters of rock material from the Mohr’s envelope drawn from test results
Usually tests on atleast five specimens, each at a different confining pressure needed to define peak strength envelope Sigma 1 Vs Sigma 3
Stress Strain Curve in TriaxialCompression
Axi Symmetric Triaxial compression
Triaxial Cell
Mohr’s Envelope
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Figure Complete axial stress-axial strain curves obtained in tri-axial compression tests on Tennessee Marble at various confining pressures (Wawersik & Fairhurst
1970).
Tri-axial compression
Effects of Confining PressureA number of important features of the behaviour of rock in tri-axialcompression can be seen, such as with increasing co nfining
pressure ,
(a) the peak strength increases;(b) there is a transition from typically brittle to fully ductile
behaviour with the introduction of plastic mechanis m ofdeformation;
(c) the region incorporating the peak of the axial stress-axial strain curve flattens and widens;
(d) the post-peak drop in stress to the residual st rength reduces and disappears at high confining stress.
The confining pressure that causes the post-peak reduction in strength disappears and the behaviour becomes fully ductile (48.3 MPa in the figure), is known as the brittle-ductile transition pressure. This brittle-ductile transition pressure varies with rock type.
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CONFINED
UNCONFINED
LOA
D
DISPLACEMENT
Effects of Confining Pressure
A series of triaxial compression tests was carried out on a limestone with a constant confining pressure of 69 MPa, but with various level of pore pressure (0-69MPa). There is a transition from ductile to brittle behaviour as pore pressure is increased from 0 to 69 MPa. In this case, mechanical response is controlled by the effective confining stress (σ3' = σ3 – u).
Effects of Pore water Pressure
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Rock Specimens before & After failure in Triaxial co mpression
A.Point Load Strength Index Test
Point load test of rock cores can be conducted diametrically andaxially. In diametrical test, rock core specimen of diameter D is loaded between the point load apparatus across its diameter. The length/diameter ratio for the diametrical test should be greater than1.0.
Uncorrected point load strength, Is, is calculated as:
2e
s DPI =
Where:P = Load at failure in (kN)De= equivalent diameter for a circular core (m)
Compressive Strength - Indirect Test:
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UCS = 14 x Is for Indian Coal measure rocks
UCS = 21 x Is in other cases
UCS = Uniaxial Compressive Strength
Is = point Load strength
Compressive Strength - Indirect Test:
Compressive Strength - Indirect Test:
Schmidt or rebound Hammer Test:
It normally tests on surface hardness of rock sample as it is also easy to use and handle. The sample can be in core or in block shape and it is non-destructive typeof test. The best part of the test is that the sample used for the previous test can be used again.
Schmidt or rebound Hammer
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Tensile strength Tests
Tensile strength of a material is defines as the maximum tensile stress which a material is capable of developing
In nature rockmass is rarely subjected to direct tension, but it is subjected to tensile stresses
Rocks are weak in tension
Direct Tests:
In this Rock specimen is subjected to uni-axial tensile loading along its axis.
The principal difficulties associated with tensile tests on rock the prevention of failure within the grips and the elimination of bending in the specimen.
Indirect Method:
Brazilian Test : (Mellor & Hawkes,1971)
Where T is the tensile strength, P is the maximum compressive load recorded during the test, D is the diameter, and t is the thickness of the test specimen.
Rock Specimens before & After failure in Brazillian Test
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Brazilian test in which tensile failure is induced in a disc by compressing it across a diameter.
Point load Test :
Point load is approximately 0.8 times the uni-axial Tensile strength
Tensile strength Tests
UTS = 0.8 x Is
UTS = Uniaxial Tensile Strength
Is = point Load strength
Shear strength Tests
Shear strength of may be defined as the maximum resistance to deformation due to shear displacement caused by shear stress
Shear strength in a rockmass is derived from the surface frictional resistance along the sliding plane, interlocking between individual rock grains and cohesion in sliding surface of the rock.
Shear strength Tests
� It mostly deals with the shear strength and shear behavior of the shearing and weakness planes of the rock which hold together a rock specimen.
� This is the most expensive laboratory strength tests, as it requires special kind of methodology for acquiring the samples from the site as fracture planes to be tested and utmost relatively complex testing procedures
In general there are two methods for evaluation of Shear Strength of rocks;
1. Direct Shear Test
a. Shear Box Test
b. Shear Test on Rock Cubes
2. Indirect Shear Test – Punch shear Test
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Shear Box Test:
Constant Normal Load (CNL)
Portable shear Testing apparatusComplete setup of Shear Testing apparatus with online acquisition system
Arrangement for shear Testing
Constant normal load (CNL)
nσ
τ
= Constant
Rock slope stability
（non-reinforced）
Free to move
Constant Normal Load Condition
(After Barton)
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Direct shear test apparatus
(1) The Constant Normal Load (CNL) is applied on single rock joint through a loading yoke connected to a loading lever.
(2) The shear displacement is applied through the advancement of a lead screw which is pushing the shear box assembly. A high sensitivity proving ring (5 MPa) is usedfor measuring the shear load.
Proving Ring
Vertical dial gauge
Specimen
Loading Yoke
Shear Box
Lead Screw
Turret Gear BoxHorizontal dial gauge
Direct shear test apparatus
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Sample Preparation
Leveling the Sample Samples after mold is set
Two halves of the joint ready for molding One half p laced in concrete mold
Joint roughness Coefficient Measurement
Fig: Surface profiler Fig: Brush profiler
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Shear behaviours of rock joint (i = 150)
Fig. Shear stress-horizontal displacement and dilation curves at
0.502 mm/min shear rate (asperity angle i = 150)
0.25 Mpa
1.5 Mpa
1.Mpa0.5 Mpa
Peak shear stress region
Residual stress region
Larger shear stresses are obtained under higher normal stress levels
Positive dilation is shown in the residual region and negative dilation is generated when a shear starts.
1. Mobilization of friction with beginning of stress. This usually occurs with in the first 1mm of shear displacement.
2. Mobilization of roughness with the beginning of dilation.
3. Peak shear strength at which contribution from JRC is maximum.
4. Beyond peak stress roughness is gradually destroyed with the declining of dilation.
Fig; Ideal shear Stress Vs Displacement Curve
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Relation between strength Properties
Uni-axial Compressive Strength = 7.5 times of Shear strength
= 10.5 times of Tensile Strength
= 14 to 21 times of Point Load Strength Index
Elastic Properties of Rocks :
Elastic constants are evaluated by Uniaxial compression, Uniaxial Tension or Flexural Strength tests and choice depends up on the type of loading expected in field
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Elastic properties of rocks
Fig. Stress-strain curve with yield point, peak strength, post-peak ductile and brittle behaviour.
Elastic properties of rocks
Fig Stress strain relationship for determination of Young’s modulus (E) and Poisson’s ratio
Secant Modulus of elasticity
Tangent Modulus of elasticity
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Elastic properties deformation in rocks
Modulus of ElasticityRate of change of strain as a function of stress. The slope of the straight line portion of a stress-strain diagram. Tangent modulusof elasticity is the slope of the stress-strain diagram at any point. Secant modulusof elasticity is stress divided by strain at any given value of stress or strain. It also is called stress-strain ratio.
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Rock Material Classification Compressive Strength (MPa)
