Definition of the problem 8
To understand the behavior of jointed rock masses, one must start with the components that together make up the system – the intact rock material and the individual discontinuity surfaces. Depending on the number, orientation, and nature of the discontinuities, intact pieces of rock will shift, twist, or crush in response to the stresses imposed by the rock mass. Since there are a large number of possible combinations of block shapes and sizes, it is obvious to look for any behavioral trends common to all these combinations.
Before embarking on a study of the individual components and of the system as a whole, it is necessary to establish some basic definitions.
Rock for engineers 8
This state of affairs existed until about the time of the first congress of the then newly formed International Society for Rock Mechanics (ISRM) held in Lisbon, Portugal in 1966. It is therefore essential that both the structure of a rock mass and the nature of its discontinuities are carefully described alongside the lithological description of the rock type. i) JOINT. It is the collective term for most types of joints, weak bedding levels, weak schistosity levels, weakness zones and faults.
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 10 (a) Orientation: Attitude of discontinuity in space, described by the direction of inclination (azimuth) and the slope of the line of steepest declination in the plane of the discontinuity. Wall strength: The equivalent compressive strength of the adjacent rock walls of a discontinuity may be lower than the strength of the boulders due to weathering or alteration of the walls. Fill: Material that separates the adjacent rock faces of a discontinuity and is usually weaker than the overlying ridge.
Seepage : Water flow and free moisture visible in individual discontinuities or in the rock mass as a whole. The rock mass can be further divided by individual discontinuities. j) Size of the block: The dimensions of the stone block, derived from the mutual orientation of the intersecting sets and derived from the spacing of the individual sets.
Gypsum Plaster 11
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 11 (i) Number of sets: The number of joint sets comprising the crossing joint.
X-Ray Diffraction Analysis 11
SEM/EDX Analyses 12
Uniaxial compressive strength 13
Elastic Modulus 14
Intact rock mass 14
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 15 Effect of confining pressure, temperature and loading rate. Besides the on-site situation, there are so many factors that influence the strength of intact rocks. 1. The confining pressure increases the strength of the rock and the degree of axial strain hardening after yielding, so these effects decrease with increasing pressure.
At low confining pressure there is increasing dilation, which decreases to a maximum of 400 MN/m2 at higher confining pressure. The strength of the rock decreases with increase in temperature, with the effect being different on different rocks. The effect of pore water pressure depends on the porosity of the rock, the viscosity of the pore fluid, the sample size and the degree of extrusion; usually an increase in pressure reduces force.
Usually the strength increases with the rate of loading, but here opposite cases were observed.
Jointed Rock Mass 15
The final summary of these factors are: .. 1. Confining pressure increases rock strength and the rate of axial strain hardening after yielding these effects decrease with increasing pressures. Node intensity is the number of nodes per unit of normal distance to the plane of nodes in a cluster. It significantly affects the stress behavior of the rock mass, the strength of the rock decreases with the increase in the number of joints, this is well established based on studies by (Goldstein 1966, Walker1971, Lama1971).
This is because joint roughness has a fundamental influence on the development of dilation and consequently on joint strength during relative shear displacement. The change in "i" results from the random and irregular surface geometry of natural rocks, the ultimate strength of the rock, and the interplay between surface sliding and the shearing mechanism of the asperity. The strength of the rock material decreases with an increase in the volume of the test piece.
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 19 (1981) conducted experimental studies on scale effects on the shear behavior of rock joints by conducting direct shear tests on different sized specimens with different natural joint surfaces. Where Δv is the vertical displacement perpendicular to the direction of the shear force, Δh is the horizontal displacement in the direction of the applied shear force.
Strength criterion for anisotropic rocks 20
Influence of Number and Location of joints 21
Parameters Characterizing type of Anisotropy 21
It was observed that the variation n and β is similar to the variation of uniaxial compressive strength ratio σcr with the value for the corresponding β values.
Deformation Behaviour of rock mass 22
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 22 Rock exhibits maximum strength at 0˚ or 90˚ and minimum strength between 20˚ and 40˚ (Arora and Ramamurthy 1987) introduced a tendency parameter (n) to predict the behavior of different joint orientations in rocky behavior.
Failure modes in Rock mass 23
MATERIALS USED 27
Experiments are performed on model materials to obtain a uniform, identical or homogeneous specimen to understand the failure mechanism, strength and deformation behavior. It is noted that plaster of Paris was used as a model material to simulate the weak rock mass in the field. Many researchers have used plaster of Paris because of its ease of casting, flexibility, instant hardening, low cost, and easy availability.
The reduced strength and deformability compared to actual rocks has made gypsum one of the ideal materials for modeling in geotechnical engineering and is therefore used to prepare models for this study.
PREPARATION OF SPECIMENS 27
CURING 28
MAKING JOINTS IN SPECIMENS 28
ROURKELA NATIONAL INSTITUTE OF TECHNOLOGY Page 29 to know the strength and deformation behavior of intact and jointed rocks and shear parameters respectively.
EXPERIMENTAL SETUP AND TEST PROCEDURE 29
- DIRECT SHEAR TEST 29
- UNIAXIAL COMPRESSIVE STRENGTH TEST 30
- TRIAXIAL COMPRESSION TEST 33
In the uniaxial compressive strength test, cylindrical specimens were subjected to a principal principal stress until the specimen fails due to shear along the critical failure plane. The squareness of the axis did not deviate by 0.001 radian and the sample was tested in 30 days. NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA Page 31 between two steel plates of the testing machine and load at a predetermined rate along the axis of the specimen until the specimen breaks.
The load is divided by the bearing area of the sample, which gives the uniaxial compressive strength of the sample. At the peak load, the stress conditions are σ1=P/A & σ3=p, where P is the highest load that can be carried parallel to the cylinder axis, and p is the pressure in the confining medium where grade 68 hydraulic oil is used as fluid lock and the jacket is made of oil-resistant rubber (polyurethane). The circular base of the triaxial testing machine has a central pedestal on which the sample is placed and is enclosed in an impermeable jacket to strengthen the rock by applying confining pressure, p.
First, a confining pressure (σ1=σ3=p) was applied throughout the cylinder, and then an axial load (σ1- p) was applied, keeping the lateral pressure constant. The omnidirectional pressure is considered the minor principal stress, and the sum of the omnidirectional pressure and the applied axial stress is considered the major principal stress, on the basis that there are no shear stresses on the surfaces of the specimen. The applied axial stress is thus called the principal stress difference (also known as deviatoric stress).
Therefore, the triaxial compression experiment can be interpreted as a superposition of the uniaxial compression test on the initial state of all-round compression. ROURKELA NATIONAL INSTITUTE OF TECHNOLOGY Page 34 Where σ1 and σ3 = principal and minor principal stresses; σci = uniaxial compressive strength of undamaged specimens; αi = slope of the plot between (σ1-σ3)/σ3 and σci/σ3, for most intact rocks its mean value is 0.8; and Bi = material constant equal to (σ1–σ3)/σ3 when σci/σ3 = 1. Uniaxial compressive strength in orientation; αj and Bj = values of α and B for the direction in question.
The values of αi and Bi can be estimated by performing at least two triaxial tests at a confining pressure of more than 5% of σc for the rock. This expression is applicable in the ductile region and in most of the brittle region. It underestimates the strength when σ3 is less than 5% of σc and also ignores the tensile strength of the rock.
PARAMETERS STUDIED 35
The jointed specimens were placed inside a rubber membrane before testing by U.C.S to avoid sliding along the joints immediately after application of the load. Triaxial tests are performed with intact specimens (with five different confining pressures 2,3,4,5 & 6 MPa) and single joint specimens (with orientation angle from 0˚ to 90˚). Therefore, with the results of the triaxial compression test, strength parameters and material constants for gypsum (α & B) have been found. Direct shear tests were performed on jointed specimens of gypsum to know Cj and ϕj values at 0.1 Mpa, 0.2 Mpa and 0.3 Mpa respectively.
RESULTS FROM XRD, SEM AND EDX 38
DIRECT SHEAR TEST RESULTS 41
UNIAXIAL COMPRESSION TEST RESULTS 42
The uniaxial compressive strength of intact samples obtained from the test results has already been discovered. Similarly, the uniaxial compressive strength (σcj) and elastic modulus (Etj) for the bonded samples were evaluated after testing the bonded samples. In this case, the pooled samples are placed in a rubber membrane before testing to prevent slippage along the critical joints.
After obtaining the values of (σcj) and Eti for different orientations (β) of the joints, it was observed that the jointed specimens exhibited minimum strength when the joint orientation angle was at 30º. ROURKELA NATIONAL INSTITUTE OF TECHNOLOGY Page 49 TABLE.5.6: VALUES OF Etc, Er FOR JOINT COMPUTER (Single joint).
TRIAXIAL COMPRESSION TEST RESULTS 51
ROURKELA NATIONAL INSTITUTE OF TECHNOLOGY Page 53 Fig.5.17: σci/σ3 Vs σdi/σ3 at different confining pressures (for intact specimens), where αi and Bi are material constants.
