Physico mechanical properties of rock materials

Slide 1: 

Physico-Mechanical Properties of Rock Materials Siva Sankar Ulimella M.Tech Under Manager Project Planning, SCCL Email: uss_7@yahoo.com

Slide 2: 

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

Slide 3: 

Rocks exhibit a brittle type behaviour when unconfined, but become more plastic as the level of confinement increases. Conditions in the field are primarily compressive and vary from unconfined near the opening walls to confined at some distance from the opening. The strength of a rock is affected not only by factors that relate to its physical and chemical composition such as its mineralogy, porosity cementation, degree of alteration or weathering, and water content, but also by the methods of testing, including such factors as sample size, geometry, test procedure, and loading rate. PHYSICO-MECHANICAL PROPERTIES OF ROCKS

Physical Properties of Rock Material : 

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

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: : 

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

Physical Properties of Rock Material : 

Physical Properties of Rock Material Dry Density, Bulk Density, and Saturated Density

Physical Properties of Rock Material : 

Physical Properties of Rock Material

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

Physical Properties of Rock Material : 

Physical Properties of Rock Material Moisture 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/sec Q = k i A Q= 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

Slide 10: 

Physical Properties of Rock Material Thermal 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

Slide 11: 

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 : 

Strength and Deformation Properties of Rocks

Slide 13: 

Idealized diagram showing the transition from intact rock to a heavily jointed rock mass with increasing sample size.

Slide 14: 

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.

Slide 15: 

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.

Slide 16: 

Taking all these factors into consideration, it is seen that the failure 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.

Slide 17: 

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

Slide 18: 

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: Uni axial Compression Test Tri axial Compression Test Indirect Method : Point Load Test Schmidt hammer Test Mechanical or Strength Properties of Rocks

Slide 19: 

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

Slide 20: 

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 than 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 Laboratory

Slide 21: 

(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 Laboratory

Slide 22: 

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

Slide 23: 

Compressive Testing Machines Universal Testing Machine Manual INSTRAN Testing Machine

Slide 24: 

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

Uniaxial stress-strain curves for different rock types : 

Uniaxial stress-strain curves for different rock types

Uniaxial compression : 

Uniaxial compression

Post failure behaviour of rock in compressionCyclic loading : 

Post failure behaviour of rock in compressionCyclic loading The behaviour of the rock under compression until the rock has lost its strength is as shown in the following figure.

Compressive Strength : 

Compressive Strength The load-deformation characteristics in UCS for loading and unloading cycles follow the following behaviour: On loading , the curve eventually joins that for a specimen in which the axial displacement increases with time As displacement continues in the post-peak region, the portion of the total displacement that is irrecoverable increases The loading-unloading-loading loop shows some hysteresis The apparent modulus of the rock which can be calculated from the slope of the reloading curve, decreases with post-peak deformation and progressive fragmentation of the specimen

Rock Failure characteristics in UCS : 

Rock Failure characteristics in UCS

Slide 30: 

The fracture pattern of specimen is divided into 8 distinct regions. I-III are marked with closure of pre-existing cracks as well as coalescence 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 the region VII VIII Loose mass of the broken material is held together due to friction

Slide 31: 

Failure modes in compression

Slide 32: 

Rock Specimens before & After failure in Uni-Axial compression

Triaxial compression of rock samples – Direct Method : 

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 Triaxial Compression

Axi Symmetric Triaxial compression : 

Axi Symmetric Triaxial compression Triaxial Cell Mohr’s Envelope

Slide 35: 

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 Pressure : 

Effects of Confining Pressure A number of important features of the behaviour of rock in tri-axial compression can be seen, such as with increasing confining pressure, (a) the peak strength increases; (b) there is a transition from typically brittle to fully ductile behaviour with the introduction of plastic mechanism of deformation; (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 strength 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.

Slide 37: 

CONFINED UNCONFINED LOAD DISPLACEMENT Effects of Confining Pressure

Slide 38: 

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

Slide 39: 

Rock Specimens before & After failure in Triaxial compression

Slide 40: 

Compressive Strength - Indirect Test:

Slide 41: 

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:

Slide 42: 

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 type of test. The best part of the test is that the sample used for the previous test can be used again. Schmidt or rebound Hammer

Slide 43: 

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.

Slide 44: 

Rock Specimens before & After failure in Brazillian Test

Slide 45: 

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.

Slide 46: 

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; Direct Shear Test a. Shear Box Test b. Shear Test on Rock Cubes 2. Indirect Shear Test – Punch shear Test

Slide 47: 

Shear Box Test: Constant Normal Load (CNL) Portable shear Testing apparatus Complete setup of Shear Testing apparatus with online acquisition system Arrangement for shear Testing

Slide 48: 

Constant Normal Load Condition (After Barton)

Slide 49: 

Direct shear test apparatus

Slide 50: 

Proving Ring Vertical dial gauge Specimen Loading Yoke Shear Box Lead Screw Turret Gear Box Horizontal dial gauge Direct shear test apparatus

Slide 51: 

Sample Preparation Leveling the Sample Samples after mold is set Two halves of the joint ready for molding One half placed in concrete mold

Slide 52: 

Joint roughness Coefficient Measurement Fig: Surface profiler Fig: Brush profiler

Slide 53: 

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) 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.

Slide 54: 

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

Relation between strength Properties : 

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

Slide 56: 

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

Elastic properties of rocks : 

Elastic properties of rocks Fig. Stress-strain curve with yield point, peak strength, post-peak ductile and brittle behaviour.

Elastic properties of rocks : 

Elastic properties of rocks Fig Stress strain relationship for determination of Young’s modulus (E) and Poisson’s ratio

Slide 59: 

Elastic properties deformation in rocks

Rock Material Classification Compressive Strength (MPa) : 

Rock Material Classification Compressive Strength (MPa)

Point Load Strength Index : 

Point Load Strength Index

Angle of Internal Friction (Degrees) : 

Angle of Internal Friction (Degrees)