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Rock Mass Classification

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Rock Mass Classification and also a brief description of Rock Mass Rating (RMR), Rock Structure Rating (RSR), Q valves and New Austrian Tunneling method (NATM)

Rock Mass Classification and also a brief description of Rock Mass Rating (RMR), Rock Structure Rating (RSR), Q valves and New Austrian Tunneling method (NATM)

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Rock Mass Classification Rock Mass Classification Presentation Transcript

  • Presentation Topic: Rock Mass Classification Submitted To: Prof. Sohail Mustafa Submitted By: Ahmed Younhais Tariq 7th Semester Institute of Geology THE UNIVERSITY OF AZAD JAMMU & KASHMIR MUZAFFARABAD
  • Rock Mass Classification:
  • Introduction to Rock Mass Classification:  Rock mass classification schemes have been developed to assist in (primarily) the collection of rock into common or similar groups.  The first truly organized system was proposed by Dr. Karl Terzaghi (1946) and has been followed by a number of schemes proposed by others.  Terzaghi's system was mainly qualitative and others are more quantitative in nature.  The following subsections explain three systems and show how they can be used to begin to develop and apply numerical ratings to the selection of rock tunnel support and lining.  This section discusses various rock mass classification systems mainly used for rock tunnel design and construction projects.
  • Terzaghi's Classification:    Today rock tunnels are usually designed considering the interaction between rock and ground, i.e., the redistribution of stresses into the rock by forming the rock arch. However, the concept of loads still exists and may be applied early in a design to "get a handle" on the support requirement. The concept is to provide support for a height of rock (rock load) that tends to drop out of the roof of the tunnel.
  • Rock Mass Rating (RMR):
  • Rock Mass Rating (RMR): The Rock Mass Rating (RMR) system is a geomechanical classification system for rocks, developed by Z.T Bieniawski between 1972 and 1973.
  • Introduction:  During the feasibility and preliminary design stages of a project, when very little detailed information is available on the rock mass and its stress and hydrologic characteristics.  The use of a rock mass classification scheme can be of considerable benefit.  This may involve using the classification scheme as a check-list to ensure that all relevant information has been considered.
  • At the other end of the spectrum, one or more rock mass classification schemes can be used to build up a picture of the composition and characteristics of a rock mass to provide initial estimates of support requirements, and to provide estimates of the strength and deformation properties of the rock mass.
  • Parameters of RMR: Six parameters are used to classify a rock mass using the RMR system: Uniaxial compressive strength of rock material Rock Quality Designation (RQD) Spacing of discontinuities Condition of discontinuities Groundwater conditions Orientation of discontinuities
  • Each of six parameters is assigned a value corresponding to characteristic of rock. These values are derived from field surveys. The sum of the six parameters is the "RMR value", which lies between 0 and 100. RMR =Ja1 + Ja2 + Ja3 + Ja4 + Ja5 + Ja6
  • Classification table for the RMR: RMR Rock quality 0 - 20 Very poor 21 - 40 Poor 41 - 60 Fair 61 - 80 Good 81 - 100 Very good
  • Applications Of Rock Mass Rating:  Rock Mass Rating has found wide application in various types of engineering projects such as tunnels, slopes, foundations, and mines.  Rock mass classification systems have gained wide attention and are frequently used in rock engineering and design. However, all of these systems have limitations, but applied appropriately and with care they are valuable tools.
  •  Now RMR system is applied to coal and hard rock mining.  The RMR system is also applicable to slopes and to rock foundations. This is a useful feature which can assist with the design of slope near the tunnel portals as well as allow estimates of deformability of rock foundation for bridges and dams.  Other special uses includes applications to assess rock rippability, cuttability and cavability.
  • RMR may be applied for classification of the stability and support estimates of tunnels and rock caverns, preferably in jointed rocks. It may be used for planning purposes. It is less useful for prescription of rock support during construction. It is not likely that RMR is suitable to express the effects of pre-grouting. 
  • Using Rock Mass Classification Systems: The two most widely used rock mass classifications are Bieniawski's RMR (1976, 1989) and Barton et al's Q (1974). Both methods incorporate geological, geometric and design/engineering parameters in arriving at a quantitative value of their rock mass quality. 
  • The similarities between RMR and Q system from the use of identical, or very similar, parameters in calculating the final rock mass quality rating.
  • PARAMETERS:
  • Uniaxial compressive strength of rock material: The strength of rock can be evaluated using a laboratory compression test on prepare core. But for rock classification purposes it is satisfactory to determine compressive strength approximately using the point load test on intact pieces of drill core.
  • Uniaxial compressive strength of rock material: UCS of a material is verified by applying compressive load until failure occur due to fractures in core sample.
  • When stresses exceeds the bearing limit it cracks the core sample. These cracks are produce along the weaker zones. When crack produced then we can note the clock reading. That point shows the maximum compressive strength of rock.
  • Point load Index (MPa) Unconfined Rating Compressive Strength (MPa) >200 15 100-200 12 2-4 50-100 7 1-2 25-50 4 Don’t use 10-25 2 Don’t use 3-10 1 Don’t use <3 0 >8 4-8
  • Orientation of Discontinuities:  Orientation of the joints relative to the work can have an influence on the behavior of rock.  Bieniawski recommended adjusting the sum of first five rating numbers to account for favorable or unfavorable orientation.  No points are subtracted for very favorable orientation of joints up to 12 points are deducted for unfavorable orientation of joints in tunnels and up to 25 for unfavorable orientation in foundation.
  • The orientation of joint sets cannot be found from normal routine drilling of rock masses but can be determined from drill core with special tools or procedures. Logging of the borehole using a television or camera down hole will reveal orientation of joints and absolute orientation will also be obtained from logging shafts and adits.
  • Adjustment in RMR for joint orientations: Assessment of influence of orientation on the work Rating increment for tunnels rating increment for foundations Very favorable 0 0 favorable -2 -2 fair -5 -7 unfavorable -10 -15 Very unfavorable -12 -25
  • Modifications to RMR for mining:  Rock Mass Rating (RMR) system was originally based upon case histories drawn from civil engineering.  Laubscher developed the Mining Rock Mass Rating (MRMR) system by modifying the Rock Mass Rating (RMR) system of Bieniawski.
  •  In the MRMR system the stability and support are determined with the following equations: RMR = IRS + RQD + spacing + condition In which: RMR = Rock Mass Rating IRS = Intact Rock Strength RQD = Rock Quality Designation Spacing = expression for the spacing of discontinuities Condition = condition of discontinuities (parameter also dependent on groundwater presence, pressure, or quantity of
  • Comparison of MRMR and RMR: MRMR = RMR x adjustment factors In which: Adjustment factors = factors to compensate for: the method of excavation, orientation of discontinuities and excavation, induced stresses, and future weathering.  The adjustment factors depend on future (susceptibility to) weathering, stress environment and orientation.
  •  The combination of values of RMR and MRMR determines the socalled reinforcement potential.  A rock mass with a high RMR before the adjustment factors are applied has a high reinforcement potential, and can be reinforced by, for example, rock bolts, whatever the MRMR value might be after excavation.
  • Parameters of MRMR:  The parameters to calculate the RMR value are similar to those used in the RMR system. This may be confusing, as some of the parameters in the MRMR system are modified, such as the condition parameter that includes groundwater presence and pressure in the MRMR system whereas groundwater is a separate parameter in the RMR system.  The number of classes for the parameters and the detail of the description of the parameters are also more extensive than in the RMR system.
  • Rock Structure Rating (RSR):
  • Rock Structure Rating (RSR): Rock Structure Rating(RSR) is a quantitative method for describing quality of a rock mass and then appropriate ground support.
  • Categories of RSR: There are considered two general categories: Geotechnical parameters: Rock type; joint pattern; joint orientations; type of discontinuities; major faults; shear sand folds; rock material properties; weathering or alteration. and Construction parameters: Size of tunnel; direction of drive; method of excavation.
  • Parameter A : Geology General appraisal of geological structure on the basis of:  Rock type origin (igneous, metamorphic, sedimentary). Rock hardness (hard, medium, soft, decomposed). Geologic structure (massive, slightly faulted/folded, moderately faulted/folded, intensely faulted/folded).
  • Parameter A : Geology
  • Parameter B: Geometry Effect of discontinuity pattern with respect to the direction of the tunnel drive on the basis of:    Joint spacing. Joint orientation (strike and dip) Direction of tunnel drive.
  • Parameter B: Geometry
  • Parameter C:Effect of Groundwater Effect of groundwater inflow and joint condition on the basis of: Overall rock mass quality on the basis of A and B combined.  Joint condition (good, fair, poor).  Amount of water inflow (in gallons per minute per 1000 feet of tunnel). 
  • Parameter C:Effect of Groundwater
  • Q- Values:
  • Introduction: Barton et al. (1974) at the Norvegian Geotechnical Institute (NGI) proposed the Rock Mass Quality (Q) System of rock mass classification on the basis of about 200 case histories of tunnels and caverns. It is a quantitative classification system, and it is an engineering system enabling the design of tunnel supports.
  • Factor Affecting:  The concept upon which the Q system is based upon three fundamental requirements: a. Classification of the relevant rock mass quality, b. Choice of the optimum dimensions of the excavation with consideration given to its intended purpose and the required factor of safety, c. Estimation of the appropriate support requirements for that excavation.
  • Q value: The Q-system for rock mass classification is developed by Barton, Lien and lunde. It expresses the quality of rock mass, on which, the design and support recommendations are based for the underground excavations. The Q- value is determined by the following formula: Q = RQD/Jn x Jr/ Ja x Jw/SRF
  • Where, RQD = Rock Quality Designation Jn = Joint Number Jr = Joint Roughness Ja = Joint Alteration Jw = Joint Water Reduction Number SRF = Stress Reduction Fraction
  • RQD J J J w n a J =Degree of Jointing(or block size) r SRF =Joint Friction(inter-block shear strength) =Active Stress
  •  Q values can be determined in different ways, by geological mapping in underground excavation, on the surfaces , or alternatively by core logging. The most correct values are obtained from geological mapping underground. Each of Six Parameters is determined according to description found in tables.  The Q values varies between 0.001 and 1000. Please note that it is possible to get higher values and slightly lower values by extreme combinations of parameters. In such odd cases one can use 0.001 and 1000 respectively for determination of support.
  • RQD (Rock Quality Designation):  “RQD is the sum of length ( between natural joints ) of all core pieces more than 10 cm long as a percentage of the total core length.”  RQD will therefore be a percentage between 0 to 100. If 0 is used in the Q formula it will give a Q value of 0 and therefore all RQD values between 0 to 10 are increased to 10 when calculating the Q value.
  • RQD (Rock Quality Designation):  RQD is used as a simple classification system for the stability of rock masses. Using the RQD values, 5 rock classes are defined: S. No RQD RQD Value 1 Very Poor (>27 joints per m3) 0 - 25 2 Poor (20 - 27 joints per m3) 25 - 50 3 Fair (13 - 19 joints per m3) 50 - 75 4 Good (8 - 12 joints per m3) 75 - 90 5 Excellent (0 - 7 joints per m3) 90 - 100
  • In the underground opening it is usually possible to get a three dimensional view of rock mass. A three dimensional RQD may therefore be used. That’s means that the RQD m values are estimated from the no of joints per .  The following formula may be used : RQD = 115 - 3.3 Jv m J Where is the number of joints per  3 3 v
  • Precautions: RQD is intended to represent the rock mass quality in situ. When using diamond drill core, care must be taken to ensure that fractures, which have been caused by handling or the drilling process, are identified and ignored when determining the value of RQD. 
  • Stress Reduction Factor: It describes the relation between stress and rock strength around an underground opening. The effect of stresses can usually be observed in an underground opening as spalling, slabbing, deformation, squeezing, dilatancy and block release. However, sometime may pass before the stress phenomena are visible.
  • Joint Roughness Number:  It depend on joint wall surfaces. If they are undulating, planner, rough or smooth. Joint description is based on roughness in two scales: The terms rough, smooth and slickenside refer to small structures in a scale of cm and mm. This can be evaluated by running a finger along the joint wall; small scale roughness will then be left. 2. Lange scale roughness is measured on a dm to m scale and is measured by lying a one meter long ruler on the joint surface to determine the large scale roughness 1.
  • Jr Rock wall contact , and Rock wll contact before 10 cm of shear movement A Discontinuous Joints 4 B Rough or irregular, Undulating 3 C Smooth, Undulating 2 D Slickenside, Undulating 1.5 E Rough, Irregular, Planar 1.5 F Smooth, Planer 1 G Slickenside, Planar 0.5  No rock wall contact when sheared H Zone containing clay minerals thick enough to prevent rock wall contact when sheared 1
  • Joint Set Number:  Shape and size of the blocks in the rock mass depends on the joints geometry.  There will often be 2 to 4 joint sets at a certain locations.  Joints in it will be nearly parallel to one another and will display a characteristic joint spacing.  Joints that do not occur systematically or that have a spacing of several meters are called random joints.  However, the effect of spacing strongly depend upon the span or height of the underground opening .  If more than one joint belonging to a joint set appears in a underground opening, it has an effect on the stability and should be regarded
  • Table for determination of joint set number: Joint set Number: Jn A Massive, no or few joints 0.5-1.0 B One joint set 2 C One joint set plus random joints 3 D Two joint sets 4 E Two joint sets plus random joints 6 F Three joint sets 9 G Three joint sets plus random joints 12 H Four or more joint sets , randomly heavily jointed “Sugar Cube “ etc 15 I Crushed Rocks, earth like 20
  • RQD J   =Degree of Jointing(or block size) n This fraction represents the relative block size in the rock masses. In addition to RQD and Jn. It is also useful to make notes of the real size and shape of the blocks, and the joint frequency.
  • Joint Alternation Number:   In addition to the joint roughness the joint infill is significant for joint friction. When considering joint infill, two factors are important; thickness and strength. These factors depends on the mineral composition. In the determination of joint alternation number, the joint infill is divided into three categories ; (a, b and c) based on thickness and degree of rock wall contact when sheared along the joint planes.
  • Joint water reduction Factor:   Joint water may soften or washout the mineral in fills and there by reduce the friction on the joint planes. Water pressure may reduce the normal stress on the joint wall and cause the blocks to shear more easily. A determination of joint water reduction factor is based on inflow and water pressure observed in a underground opening. The lowest Jw values(Jw < 0.2) represent large stability problems.
  • J J =Joint Friction(inter-block shear strength) r a
  • New Austrian Tunneling method (NATM):
  • New Austrian Tunneling method (NATM):  The term New Austrian Tunneling Method Popularly Known as NATM got its name from Salzburg (Austria).  It was first used by Mr. Rabcewicz in 1962. It got world wise recognition in1964.  The first use of NATM in soft ground tunnel in Frankfurt (Europe) metro in 1969.  The basic aim of NATM is for getting
  • Definition of NATM: “The New Austrian Tunnelling Method is a support method to stabilize the tunnel perimeter by means of sprayed concrete ,anchors and other support and uses monitoring too control stability”.
  • Principles of NATM: Mobilization of the strength of rock mass Shotcrete protection Measurements Primary Lining Rock mass classification Dynamic Design
  • Summary of procedure in NATM: SHOTCRETING AT THE EXCAVATED AREA(PRIMARY LINING) PLACING OF THE WIREMESH ALONG THE FACEOF THE TUNNEL ERECTION OF THE LATTICE GIRDER ALONG THE FACE OF THE TUNNEL PERTICULAR TYPE OF ROCKBOLTING SHOTCRETING THE WHOLE ARRENGEMENT(SECONDARY LINING)
  • NATM Process on site:
  • POINTS TO CARRY OUT A SUCCESSFUL NATM PROCESS: Consideration of rock mechanics Selection of a proper profile Design of flexible support and slender lining (in rock) Careful excavation Maintenance of rock strength, avoidance of loosening and over-breaks Direct contact of rock/soil and support Continuous control by geotechnical measurements  Installation of support without delay and in correct sequence.
  • Thanks