Geology

Ø  Thin disc are cut of the cylindrical cores by a diamond saw in order to have L/D ratio at 0.5 or D = 2L as shown in figure 8.

Ø  The periphery of the specimen should be smooth.

Ø  The specimen is placed between the loading platens of the compression testing machine.

Ø  Compression load is applied to the specimen slowly till the failure takes place.

Ø  The rate of loading is normally 200 N / Sec, so that the failure takes place in about 5 minutes.

Ø  Failure takes place by splitting along the vertical diameter of the specimen.

Ø  Tensile strength of the specimen can be calculated as follows

Where

              σ_t = Tensile strength at failure in Kg / cm ²

              F = Failure load in Kg

                 D = Diameter of the specimen in cm

                L = Length of the specimen in cm

Ø A beam of the rock sample is cut out as shown in figure 9.

Ø The specimen is supported at two ends.

Ø At the centre of the support, load is applied. The load is increased till the failure takes palace.

Ø Tensile strength is calculated by the following relation

Where

σ _t = Tensile strength in Kg / cm²

 P = Applied load in Kg

 L = length between the supports

 B = Width of the specimen

 D = Depth (thickness) of the specimen

 N = Distance from the neutral axis to the farthermost fibreCm²

Ø  Miller (1965) gave a relationship between Tensile Strength (σt) and the Uniaxial Compressive Strength ( σ_u ).

When the force applied to a specimen in opposite direction having different planes, these forces tend to distort the shape of the body. The shear failure is resulted.

Ø  It is defined as the maximum resistance to deformation due to shear displacement caused by shear stress.

Ø  Shear strength in a rock mass is derived from the surface frictional resistance along a sliding plane, interlocking between the individual rock grains and cohesion in the sliding surface of the rock.                  

Ø  The pattern of joints, shear zones and faults in a rock mass reduces the effective shear strength of a rock mass.  

Ø  Shear strength of in-situ rock is highly anisotropic.

Ø  Shear strength in a direction parallel to the discontinuities is much less than the intact rock mass.

Ø  Dams, especially the concrete dams experience the shear stress due to the                                           Excessive water pressure.

Ø  The shear strength of a rock along a discontinuity is affected by a number of factors, including the Gal Fault gouge, etc. and "asperities' (Roughness) of the Discontinuity.                               

Ø  The problems of slope stability of rocks and stability of the structure against sliding are also linked with the shear strength of the rocks concerned.

Ø  Shear strength is a combination of cohesion and internal friction; expressed by Coulomb failure envelope.

Ø  Take a block of rock sitting on a natural slope rough joint surged

Ø  Tilt it until it starts to slide under its own weight (Figure 10a)

Ø  The smallest angle at which sliding occurs is measured.

Ø  From this data peak shear strength of the base of the block can be calculated.

Ø  Two pieces of core in contact with each other are fixed to the surface of a tilting table.

Ø  A third core, free to slide, is placed on the top.

Ø  Table is slowly inclined until sliding starts along the lines of contact, at which point the angle of tilt a is measured (Figure. 10b)

Ø  Stimpson (1981) showed that the base friction angle

Frictional resistance of rock surfaces that are neither so smooth as to exist stickslip oscillations nor so rough as to interlock and dilate during sharing.

 It has been observed that.

Ø  Friction angle is 30˚ for dry limestone

Ø  Friction angle is 41- 48 for the same cores when they are wet

Ø  In this test, the value of normal stress is fixed and somewhat less than the weight of the block.

Ø  During land sliding, higher values of normal stress are encountered; the extrapolation of this values will give error

Ø  To overcome these limitations, a direct shear test was designed where; the normal load can be varied

Commonly used methods ate;

a)      Shear box test          

b)       Direct shear test on rock cubes

c)       Punch shear test

Ø  Sample of rectangular shape with standard dimensions is prepared (Figure 11)

Ø  The sample is put in shear box, a sort of sliding machine with two halves.

Ø  A normal load is applied on the upper half while the horizontal force is applied in the lower half, till the failure occurs

Ø  Sample is prepared in such a way that the weakest plane should be along the failure surface.

Ø  Several tests should be conducted at different normal loads and a graph is plotted between normal load and the horizontal force. Shear strength is evaluated with the help of the graph.

Ø  Rock sample in the form of cube with standard dimensions is prepared. Length of the cube varies from 5cm to 15cm (figure 12)

Ø  Sample is placed cubically at an angle of 45°

Ø  Normal load is applied with compression machine till failure (shear) along a predetermined surface.

Ø  As the name implies, triaxial test comprised of failure caused by axial load when the specimen is laterally confined i.e. the intermediate principal stress, and the minor principal stresses are not zero. (Figure 29)

Ø  Triaxial compression test consists of a heavy steel cylinder filled with a fluid (e.g. kerosene) to duplicate the actual confining pressure of natural rock.

Ø  In triaxial test   σ₁ > σ₂ = σ₃ > 0

Ø  Cylindrical specimen is subjected to an equal all-around pressure (confining pressure)

Ø  Axial load (Major Principal Stress) is applied till the failure of the sample.

Ø  Values of the confining pressure and axial pressures are plotted graphically as Mohr's circles.

Ø  Compared with unconfined compressive strength, in triaxial test we get different stress-strain curves result in higher ‘E’ values and higher ultimate strengths.

Ø  Most rocks are strengthened by confinement.

Ø  Highly fissured rock may have very low strength at the surface, but under confinement, the strength is increased

Ø   The strength of a fissured rock can achieve an increase in strength by 10 times.

Ø  Procedure was developed by Otto Mohr in 1882 (Figure 30)

Ø  Provides representation of the variation of normal and shear stresses with direction.

Ø  Triaxial test can be plotted graphically in the form of Mohr's circles or Mohr’s half-circles, called Mohr’s diagrams.

Ø  Normal (confining) stress is plotted as the abscissa

Ø  Shearing stress is ordinate

Ø  Stresses are expressed as unit stresses, N / m²

Ø  Cylindrical sample is subjected to a known confining (lateral) pressure equally stresses all surfaces of the sample.

Ø  Axial stress is increased, until the specimen fails.

Ø  Two tests are run, where the sample is subjected to two confining pressure (the minor principal stresses, distance OA and OA' in the figure)

Ø  Distances OB and OB' are the axial stresses (the major principal stresses, required to break the rock in theses two sets, respectively.

Ø  Circles (or more conventionally, semicircles) are drawn using AB and A’B’ as diameters.

Ø  A common tangent to both circles can be drawn that will intersect to ordinal (shear stress) at point 'D'.

Ø  Distance OD is the “unit cohesion" (c)

Ø  Ф is the angle of internal friction.

Ø  Using trigonometry, the perpendiculars from the points of tangency T and T’, of Mohr's envelop dropped to the abscissa

Ø  The ordinate lengths are the values of the shearing stress in the failure plane in the corresponding tests.

Ø  Values of shearing stresses can be read from the ordinate scab.

Ø  It diagrammatically allows for determination of the stability of material under varying loads and confining conditions

Ø  If the envelop curve is known, an axial and confining stress can be plotted, a circle is drawn.

Ø  If any part of the circle lies outside the envelop failure should occur.

Ø  It allows for solution of the stresses on all planes within a specimen.

Ø  Griffith proposed that the rock material contains a large number of randomly oriented zones of potential failure in the form of grain boundaries.

Ø  Grain boundaries contain a number of open flaws which are approximately elliptical in shape

Ø  Very high tensile stress occurs on the boundaries of the suitability oriented elliptical openings even under compressive stress condition.

Ø  Griffith assumed that fracture initiated from the boundaries of an open flaw boundaries exceeds the local tensile strength of the material.

Ø  Natural flaws called Griffith cracks are present in igneous and sedimentary rocks                                                                                            

Ø  Grain boundary cracks in igneous rocks, pores in sedimentary rocks; flaws           within the crystals

Ø  Griffith's cracks act as stress raisers

Ø  Under compressive conditions, oblique cracks have the greatest tensile stress concentrations at their tips. (Like stressed glass breaks gradually)

Ø  When the level of uniaxial stress on the specimen is increased, an obliquely oriented critical crack is the first to "initiate" (start extending).

Ø  Cracks that grow in a uniaxial compressive strength test specimen curve towards the direction of applied load, which leads to a rapid increase in the diameter of the specimen.

Ø  Cracks in a polyaxial compressed specimen or element of rock curve in to a plane parallel to the major and perpendicular to the minor principal stress

Ø  Individual cracks usually stabilized when their growth has reached a critical level, allowing stress to be redistributed to adjacent flaws, which they propagate until rupture occurs.

Ø  Crack growth distributed throughout the specimen results in a progressive, weakening and increase in volume.

Ø  Therefore, increase in confining stress greatly increases the strength of a rock                          by inhibiting the growth of cracks.

Ø  Even a small amount of confinement can have a significant effect on strength

Ø  It is the plastic deformation

Ø  At constant load, there would be deformation of the body with respect to time.                             

Ø  The phenomenon of increase in strain during the course of time under              constant stress is defined as creep.

Ø  High internal stresses occur in the microscopic regions of the crystals of the rock.           

Ø  When the load is applied, these regions move easily, would result in a measurable plastic deformation.

Ø  Rise in temperature facilitate the dislocation process due to the vibration of atoms and molecules.

Ø  These vibrations become so strong that a constant external load gives rise to a steady increasing deformation which is known as creep.

Ø  In rock salt, the effect of creep can be observed at low stress level

Ø  The role of creep for an opening decrease with age (due to dissipation of stresses)                                                                                                            

Ø  Rate of creep increases with the depth of the opening from the surface of the earth.

Ø  Plasticity of some of the rocks e.g. rock salt, increases with increasing temperature, and, therefore, creep rate is greater at higher temperature.

Ø  Fine grained materials are more creep resistant than coarse grained at a low temperature, however, it has been reported that elevation of temperature reversed the case.

Ø  If stress is maintained, the rupture can occur at stresses as low as one-half the short term uniaxial compressive strength of the rock material.

Ø  Rock bursts happen because massive and brittle rocks can not creep. Instead carry amore and more stress, until they rupture explosively

Ø  If stress applied rapidly, the rock appears stronger (cracks have little time to develop and grow, little time to shear)

Ø  If stress is applied gradually, the rock appears' weaker (cracks have the time to propagate).

Ø  Creep can result from various internal mechanisms

Ø  At high shear stress, crystal planes slip past one another and dislocation in          the crystals lattice also generate movement (Halite).

Ø  In ionic bond (halite), movements can occur without crack generation if the stress and the rates are appropriate.

Ø  Damage to the strongly covalent Oxygen Silicon bonds of silicate rocks is permanent and communicative.

Ø  In porous rock and jointed rock, particularly if coated with clay, creep can occur at a local or microscopic level rather than throughout the mass.

Ø  A cylindrical sample is subjected to a constant load (Uniaxial stress) and measure the strain at different time intervals, keeping temperature constant.

Ø  Load is increased in stages and for each stage of loading, strain is a recorded different time interval.

Ø  Strain measurement equipment should be accurate to measure very small value of strain during the process of creep.

Ø  Creep measurement is sensitive to temperature and humidity

Ø  For silicate rocks ±2C^○ is acceptable.

Ø  For ductile rocks (shale and halite) ±25C^○ is ok

Ø  Creep rates vary according to the level of confining stress, so creep data is obtained under triaxial confinement

Ø  An opening 10.5 m width and 7 high was made in the miner

Ø  Creep measurement was done in this opening

Ø  Measuring equipment consists of measuring rod with a dial gauge and the standard length reference rod 6.6 m long.

Ø  Two pins, fixed. at the roof  (a) and the floor (b) of the opening respectively.

Ø  Distance between two pins was made with the help of dial reading.

Ø  Observations of dial Gauge were made at different time intervals (years).

Ø  Log of creep is plotted versus time on a semi-log paper

It is the behavior of the rock under wetting and drying conditions alternatively. It may also express as the resistance of a rock to drying and wetting condition.

All rock is more or less affected by wetting and drying. Material like un-weathered granites and well-cemented quartzite sandstone are durable because they can survive many cycle of wetting and drying without disintegration. They are nevertheless weaker when wet because of the action of water in micro cracks. The result of the strength tests should therefore state the water content of the rock at the time of testing.

In contrast, many clay-bearing rocks (shale and some weathered igneous rocks) and other containing minerals such as anhydride will swell or disintegrate when exposed to atmospheric wetting and drying. Not only they weak when wet, but they are also permanently weakened.

Slake durability can be assessed by immersing sample in water and note their rate of disintegration. In the ISRM form of slake durability test, the slaking process is standardized and the result are measured. Ten lumps of rock, each weighting 40 and 60 g are placed in a sieve mesh drum and oven dried. The drum is then immersed in a water bath and slowly rotated; sample of moderate to low durability progressively disintegrate and fragments leave the drum through the sieve mess. After 10 min, the drum with the remaining fraction of the sample is removed, redried and subjected to an identical second cycle of slacking. We calculate the weight of the dry sample remaining in the drum after two cycle of slacking, expressed as a percentage of the initial dry sample weight as I_d2

For more rock like material that slake very little, slaking index approaches to 100 %

Ø  For this method a regular specimen (Core Sample) is required. Dry this sample at 105-110 ^oC in oven for 24 hours and weight it as Ms.

Ø  Immerse this sample in water for 24 hours to saturate it, make it  surface dry and calculate its saturated weight as Msat.

                 Pore volume = Vv = (Msat – Ms) / ρw

                 Porosity = n = Vv / V χ 100 %