Rock-Mechanics_2023 4-7.pdf

ROCK MECHANICS EM Review 2023 Engr. Reginald Ratilla Public Geotechnics is the application of scientific methods and engineering principles to the acquisition, interpretation, and use of knowledge of materials of the Earth's crust and earth materials for the solution of engineerin...

ROCK MECHANICS EM Review 2023 Engr. Reginald Ratilla Public Geotechnics is the application of scientific methods and engineering principles to the acquisition, interpretation, and use of knowledge of materials of the Earth's crust and earth materials for the solution of engineering problems and the design of engineering works. Geotechnics embraces the fields of soil mechanics and rock mechanics, and many of the aspects of geology, geophysics, hydrology, and other related sciences. Geotechnics is practiced by both engineering geologists and geotechnical engineers. Public Two (2) Subdivisions of Geotechnics 1. Soil Mechanics - It is defined as the study of the physical properties of soil that will have a direct impact on the design of structures to be built on it. 2. Rock Mechanics - is the theoretical and applied science of the mechanical behavior of rock and rock masses. Rock mechanics deals with the mechanical properties of rock and the related methodologies required for engineering design. Geotechnics Rock Soil Mechanics Mechanics Public Examples of the application of geotechnics in mining include: the prediction, prevention or mitigation of design and predicted performance of slope collapse in open pit earthen structures such as dams design and predicted performance of earthen Type of ground support to be used in underground structures such as waste/ore stock pile development headings Public Examples of the application of geotechnics in mining include: Type of underground mining method to be employed Public TAILINGS DAM Public TAILINGS DAM is typically an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from the gangue. Tailings retention dams are similar to water retention dams in regard to soil properties, surface water and ground water controls, and stability considerations. They are suitable for any type of tailings and deposition method. Three types of tailings dam Upstream Downstream centerline Public Upstream The upstream method is the lowest initial cost and most popular design for a raised tailings embankment in low risk seismic areas. One of the reasons for this is mainly due to the minimal amount of fill material required for initial construction and subsequent raising which normally consists entirely of the coarse fraction of the tailings. Construction relies on the integrity of the tailings for stability. Public Downstream The downstream design was developed to reduce the risks associated with the upstream design, particularly when subjected to dynamic loading as a result of earthquake shaking. The installation of impervious cores and drainage zones can also allow the impoundment to hold a substantial volume of water directly against the upstream face of the embankment without jeopardizing stability. This construction method does not rely on the stability of the tailings and therefore can be more versatile during operation (e.g. increased rates of rise, more accommodating of changes in tailings properties). Public CENTERLINE The centerline method is really a compromise between both the upstream and downstream designs. It is more stable than the upstream method but does not require as much construction material as the downstream design. Like the upstream method the tailings are generally discharged by spigots from the embankment crest to form a beach behind the dam wall. When subsequent raising is required, material is placed on both the tailings and the existing embankment. The embankment crest is being raised vertically and does not move in relation to the upstream and downstream directions of subsequent raises, hence the term, centerline design Public FREEBOARD The freeboard of a tailings impoundment is a key parameter in both design stage as well as in the operation. Its importance resides in that an adequate freeboard minimizes the possible uncontrolled spilling of water and/or tailings on the dam, avoiding socials and environmental impacts downstream of impoundment. Freeboard is defined as “the vertical distance between the operating or. predicted water level in a storage and the level where water would overflow the dam” Public THEORIES OF ROCK MECHANICS Public Theories in Rock Mechanics The study of rock mechanics deals w/ stresses and deformation in solid materials, there are two (2) theories adapted, namely: Theory of Elasticity: In this theory, rocks have elastic properties when all strains created by externally applied loads are instantaneously and totally recoverable upon removal of all external loads. A material is perfectly elastic if it recovers completely upon removal of external loads. Theory of Plasticity: In this theory, rocks have plastic properties when all strains created by externally applied loads are permanent upon removal of all external loads. Public ROCK MASS It consist of intact rock but is more commonly formed from an array of intact rock blocks with boundaries formed by discontinuities. Public ROCK MASS COMPONENTS Rock Mass Intact Rock Discontinuities Mass Intact Rock – are blocks of rock that do not contain mechanical discontinuities and do have tensile strength Discontinuities – is a plane or surface that marks a change in physical and chemical characteristics in rock material. They are considered plane of weakness and includes: bedding planes, joints, fractures, faults, etc. Public ROCK MASS COMPONENTS Public Discontinuities It is the existence of discontinuities in a rock mass that makes rock mechanics a unique subject. The word “discontinuity” denotes any separation in the rock continuum having effectively zero tensile strength. Public Geometrical Properties of Discontinuity The main feature of rock mass geometry include spacing and frequency, orientation(dip direction/dip angle), persistence(size and shape), roughness, aperture, clustering and block size. There is however, no standardized method of measuring and characterizing rock structure geometry, because the emphasis and accuracy with which the separate parameters are specified will depend on the engineering objectives Public Public Public Public Public Public ROCK PROPERTIES Public Public Public  ROCK PROPERTIES PHYSICAL MECHANICAL STRENGTH THERMAL DURABILITY ELASTIC PLASTIC Public 1. PHYSICAL PROPERTIES A. Mineralogical Composition – is the intrinsic property controlling the strength of rock. It comprise the element present mostly came from rock forming-minerals such as quartz, feldspar, micas, hornblende, augite, olivine, calcite, kaolinite, dolomite. Rocks containing quartz as binder are the strongest followed by calcite & ferrous minerals as the cementing agent “Glue”. Rock with clayey binder are the weakest. Public Structure - applies to well- pronounced macroscopic features of the rock (columnar structure for example). It signifies special feature of rock such as position, arrangement, attitude of a system of joints, fractures, folds, faults, etc. Texture - refers to arrangement of its grains or particles on a freshly exposed rock surface easily seen by the naked eye. Public B. Specific Gravity – the ratio of the density of a substance to the density of a standard, usually water 𝜌!"#$ 𝑆𝐺 = 𝜌%&'(! C. Unit Weight - The unit weight Y of soil and rock above the groundwater table is expressed as the ratio of the total weight of the rock (soil) in air W to the total volume V of the rock, all voids included: )(*+,' 𝑌 = -"./0( Rocks containing heavy mineral have higher unit weight than rocks with lighter minerals. Usually, igneous and metamorphic rocks have greater unit weight than sedimentary rocks. The more porous the rock is, the less is its unit weight. Public D. Porosity of Rock Voids, i.e. pores and fractures in rock are very important forms of non-uniformity of structures and texture in rocks. Pores in rock are little, interconnected voids having connection to the air also. The presence of voids or pores in rocks affects negatively its mechanical –viz., strength – properties E. Void Ratio Is the ratio of the volume of voids, Vv to the volume of solids, Vs of the rock expressed in decimal fractions. It is a dimensionless number which simply shows how many times there are more voids than solids in the rock: Public F. Moisture Content, MC Is the weight of water Ww present in a rock expressed in percentage by oven-dry weight of the rock: G. Degree of Saturation, S The degree of saturation is the ratio of the volume of water to the volume of voids S = Vw/Vv H. Permeability is the property of porous material that permits the passage or seepages of fluids, such as water and/or gas, through its interconnecting voids. Public 2. THERMAL PROPERTIES The study of thermal properties of rocks has its purpose: - an increase in temperature lowers rock strength & increases ductility; Geothermal gradient is 25C – 30C/km of depth Changes in temperature in rock may bring about “Rock Exfoliation” Public 3. Mechanical Properties 1. Hardness - is the resistance to abrasion. A. Macro Hardness Test(>1kg) i. Rockwell Hardness Test The Rockwell hardness test method consists of indenting the test material with a diamond cone indenter. Public ii. The Brinell hardness test is a method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg. Public iii. The Vickers hardness test Is a method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. “Hardness testing has always appeared attractive as a means of estimating other mechanical properties such as tensile strength of a material” Public B. Micro Hardness Test (