Applications of rock classifications

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ROCK CLASSIFICATIONS AND IT’S USE IN DESIGN

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INTRODUCTION

• Rock mass classification systems are used for various engineering design and stability analysis.

• These are based on empirical relations between rock mass parameters and engineering applications, such as tunnels, slopes, foundations, and excavability.

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ROCK MASS CLASSIFICATION SYSTEMS

Systems for tunneling: Quantitative• Rock Mass Rating (RMR)• Q-system• Mining rock mass rating (MRMR)Other systems: Qualitative• New Austrian Tunnelling Method (NATM)• Size Strength classificationSystems for slope engineering• Slope Mass Rating (SMR)• Rock mass classification system for rock slopes• Slope Stability Probability Classification (SSPC)

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PURPOSE• 1. Identify the most significant parameters influencing the

behaviour of a rock mass.• 2. Divide a particular rock mass formulation into groups of

similar behaviour – rock mass classes of varying quality.• 3. Provide a basis of understanding the characteristics of each

rock mass class• 4. Relate the experience of rock conditions at one site to the

conditions and experience encountered at others• 5. Derive quantitative data and guidelines for engineering design• 6. Provide common basis for communication between engineers

and geologists

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ADVANTAGES AND DISADVANTAGES OF DIFFERENT ROCK MASS CLASSIFICATION

SYSTEMS• RMR classification system ADVANTAGES:1. Rock mass strength is evaluated by RMR system.2. It works well to classify rock mass quality.3. RMR system is used in many projects as one of the indicators

to define the support or excavation design. DISADVANTAGES:4. A great deal of judgment is needed in the application of rock

mass classification to support design.5. RMR value doesn’t give us rock mass properties.6. These give only empirical relation & have nothing to do with

rock engineering classification in its true sense.

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4. The relatively small database makes the system less applicable to be used as an empirical design method for rock support.

5. RMR cannot be used as the only indicator, especially when rock stresses or time dependent rock properties are of importance for the rock engineering.

• NATM classification system: ADVANTAGES: 1. NATM can be applied successfully in a large no. of tunnels in

poor and difficult ground conditions.2. As compared to traditional tunneling, considerable cost

saving is gained, as well as reduced construction time.

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• Q- system of rock mass classification: ADVANTAGES:1. Together with the ratio between the span or height of the

opening and an excavation support ratio (ESR), the Q value defines the rock support.

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DISADVANTAGES:1. The accuracy of estimation of rock support is very difficult to

evaluate.2. In the poorer rock (Q<1) system may give erroneous design.3. The true nature of rock mass (e.g. swelling, squeezing or

popping ground ) is not explicitly considered in the Q- system.4. The value is used as the only indicator to define the classes in

question.

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• RMi classification system: ADVANTAGES:1. Rmi value can be applied as input to other rock engineering methods

to estimate the deformation modulus for rock masses.2. The system applies best to massive & jointed rock masses where the

joints in the various sets have similar properties.3. It may also be used as a first check for support in faults & weakness

zones. DISADVANTAGES:4. Requires more calculation than RMR & Q- system.5. For special ground conditions like swelling, squeezing & fault zones,

etc. the rock support should be evaluated seperatlly for each & every cases.

6. Like other empirical method, it is not possible to evaluate the accuracy of the system.

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ROCK MASS CLASSIFICATION USED IN DESIGN

Q Classification

Q = RQD/Jn x Jr/Ja x Jw/SRF

• I. Relative block size (RQD/Jn) • II. Inter-block shear strength (Jr/Ja) • III. Active stresses (Jw/SRF)

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Temporary mine openings. ESR = 3 - 5

Permanent mine openings, water tunnels for hydro power (excluding high pressure penstocks), pilot tunnels, drifts and headings for large excavations.

1.6

Storage rooms, water treatment plants, minor road and railway tunnels, surge chambers, access tunnels.

1.3

Power stations, major road and railway tunnels, civil defence chambers, portal intersections.

1.0

Underground nuclear power stations, railway stations, sports and public facilities, factories.

0.8

Barton et al (1974) defined an additional parameter called the Equivalent Dimension, De, of the excavation.De= excavation span diameter or height (m) /excavation support ratio, ESR ESR is related to the intended use of the excavation and to the degree of security, which is influence on the support system to be installed to maintain the stability of the excavation

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RMR ClassificationRMR= A.1+A.2+A.3+A.4(E)+A.5+BSix parameters are used to classify a rock mass

using the RMR system:1. Uniaxial compressive strength of rock material.2. Rock Quality Designation (RQD).3. Spacing of discontinuities.4. Condition of discontinuities.5. Groundwater conditions.6. Orientation of discontinuities.

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MINING ROCK MASS RATING (MRMR)• MRMR = RMR * 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 main differentiators of the MRMR 2000 system compared to previous versions of the Q-system, and Bieniawski RMR systems are:-

• Scale concept in material strength (intact rock > rock block > rock mass)

• Inclusion of cemented joints and veinlets• Abandonment of the Rock Quality Designation (RQD) as

an input parameter• Mining adjustments (in comparison to Q)

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• The lack of accountability for the basic rock

mass parameters such as intact rock strength and strength of defects, the tradeoff against its simplicity is its poor reliability in highly fractured, massive, or highly anisotropic conditions.

• The RMR method simply does not have the resolution that may be required for a more accurate assessment of fragmentation, cavability, and other mine design aspects.

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Example of the problems with RQD assessment of highly fractured or massive rock masses

Example of difference between RQD and fracture frequency-based IRMR. The IRMR based on fracture frequency (solid line) is considered more representative of actual rock mass conditions

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As gravity is the most significant force to be considered, the instability of the block depends on the number of joints that dip away from the vertical axis.

Adjustments are made where joints define an unstable wedge with its base on the sidewall.The instability is determined by the plunge of the intersection of the lower joints,

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Factors in the Assessment of Mining-induced StressThe following factors should be considered in the assessment of mining-induced

stresses:• drift-induced stresses;• interaction of closely spaced drifts;• location of drifts or tunnels close to large stopes;• abutment stresses, particularly with respect to the direction of advance and

orientation of the field stresses (an undercut advancing towards maximum stress ensures good caving but creates high abutment stresses, and vice versa)

• uplift;• point loads from caved ground caused by poor fragmentation• removal of restraint to sidewalls and apexes.• increases in size of mining area causing changes in the geometry.• massive wedge failures; • influence of major structures not exposed in the excavation but creating the

probability of high toe stresses or failures in the back of the stope.• presence of intrusives that may retain high stress or shed stress into

surrounding, more competent rock.

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Blasting creates new fractures and loosens the rock mass, causing movement on joints, so that the following adjustments should be applied

Technique Adjustment, %

Boring 100

Smooth-wall blasting 97

Good conventional blasting 94

Poor blasting 80

Adjustments must recognize the life of the excavation and the time-dependent behaviour of the rock mass

Parameters Possible adjustment, %

Weathering 30-100

Orientation 63-100

Induced stresses 60-120

Blasting 80-100

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table below shows how the support techniques in alphabetical symbols, increases in support pressure with the decrease in MRMR value

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• The “Slope Mass Rating” (SMR) is obtained from RMR by adding a

factorial adjustment factor depending on the relative orientation of joints and slope and another adjustment factor depending on the method of excavation.

• SMR = RMRB + (F1 x F2 x F3) + F4

(i) F1 depends on parallelism between joints and slope face strike. Its range is from 1.00 to 0.15. These values match the relationship: F1 = (1 – sin A)2

where A denotes the angle between the strikes of slope face and joints. (ii) F2 refers to joint dip angle in the planar mode of failure. Its value varies

from 1.00 to 0.15, and match the relationship: F2 = tg2Bj denotes the joint dip angle. For the toppling mode of failure F2 remains 1.00.

(iii) F3 reflects the relationship between slope and joints dips.(iv) F4 (adjustment factor for the method of excavation has been fixed

empirically.

Slope Mass Rating

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NEW AUSTRIAN TUNNELING METHOD• NATM: This method has been developed basically in Austria• Its name make use of providing flexible primary lining in shape

of shotcrete , wire mesh, rock bolts ,lattice girder. • In case of weaker rock mass the use of pipe forepole/pipe

roofing is also resorted for crown support which in turn lead to less overbreak as well as ensure safety during the execution.

• The main aspect of the approach is dynamic design based on rock mass classification as well as the insitu deformation observed.

• There various approaches classification of the rock mass most predominantly used here- RQD, RMR and Q factor of the rock mass.

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Components of Execution in NATM

i) Sealing Shotcrete – Shotcrete 25-50mm generally( fig 4)ii) Fixing of Lattice Girder – lattice girder is 3 Bars of steel

reinforcement placed at three corners of triangle with 8mm steel bar for connection.Easy to handle comparison of steel ribs. (fig 5)

iii) Fixing of wire mesh –generally used 6mm thick wires (fig 6)iv) Primary Lining with Shotcrete – In layers each not thicker

than 150mm (fig 7)v) Rock Bolting (fig 8)vi) Pipe Forepoling – Used for crown support for next

Excavation cycle ( for Rock Class after III only) ( fig 6)Note: Wire mesh is not used for Fibre Reinforced Shotcrete

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Face recently opened sealed with Shotcrete (Figure 4) Lattice Girder

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Fixing of Wire Mesh and Pipe Roofing/Forepoling ( Figure 6)

Shotcreting with CIFA Robotic Arm (Figure 7)

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Rock Bolting In Progress with Rocket BoomerFigure 8

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Major engineering rock mass classification systems currently in use

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APPLICATION OF ROCK MASS CLASSIFICATION SYSTEMS IN COAL

MINING

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CMRI-RMR•Central Mining Research Institute (Jharkhand, India) Indian School Of Mines introduced CMRI-Rock Mass Rating.•Five parameters are used in this classification.

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• Rock Mass Rating (RMR) is the sum of five parameter ratings.• If there are more than one rock type in the roof, RMR is

evaluated separately for each rock type and the combined RMR is obtained as:

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CMRR USBM Classification Concept (1995)

• The Coal Mine Roof Rating (CMRR) was developed to fill the gap between geologic characterization and engineering design.(USBM-United States Bureau Of Mines)

• Considers the parameters:Cohesion/roughness of weakness planes (0–35),Joint spacing and persistence (0–35) andCompressive strength(0–30)

• Equations for intersection stability, bolt length and bolt density have also been given.

• The safe intersection span was obtained from failed and stable cases

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For Development works-Bord and Pillaring

Where, B (width of galleries/splits), • D (average rock density) • Unit weight of rock, t/m3

• F is safety factor • A safety factor of 1.5 is generally considered enough.

Using CMRI - RMR Classification: in gallery junctions

Support load = 5 * y * F * B *[1-(RMR/1002)] t/m2

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Rock loads are calculated similar to CMRI-RMR:

• Bieniaweski RMR:Pr = y * B * (100-RMR)/100 t/m2

• CMRR USBM:Pr = 5.7 log10 H-0.35CMRR + 6.5 t/m2

Where,γ is theunitweightofrock,t/m3,• B is the width of gallery , m, and • F is the factor of safety• RMR is the average rockmass rating of the immediate roof after

adjustment.• H is depth of Cover in feet and Pr in Kilo pounds/sq.ft in case of

CMRR USBM

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APPLICATION OF Q-SYSTEM IN DEPILLARING WORKS

• Roof pressure could be estimated by the relations based on the Q value as RMR has its own limitations for depillaring works.

• For joint set number (Jn)> 9, the roof pressure (Pr) Proof = 2/Jr x (5Q)-1/3

• For Jn < 9, Proof = 2/3 Jn1/2 /Jr x (5Q)-1/3

• Jn = no joints were observed in the roof = 4 for galleries = 12 for junctions = 20 for goaf edges

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Visualization of rock mass classification systems

• In most widely used rock mass classification systems, such as RMR and Q systems, up to six parameters are employed to classify the rock mass

• Visualization of rock mass classification systems in multi-dimensional spaces is explored to assist engineers in identifying major controlling parameters in these rock mass classification systems

• The study reveals that all major rock mass classification systems tackle essentially two dominant factors in their scheme, i.e., block size and joint surface condition.

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• It is based on the fact that human beings are overpoweringly visual creatures

• Visualization is the task of generating images that allow important features in the data to be recognized much more readily than from processing raw data by other means, for example like statistics.

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Visualization in two-dimensional space

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• The plots from Figures it reveal one common feature of these widely used rock mass classification systems, that is, the most important controlling factors are block volume and joint surface condition

• When parameters are condensed to only these two parameters, the classification functions are best represented by planar surfaces in linear (RMR) or log scales (Q), or by surfaces that are very close to planar surfaces in log scales (GSI and RMi)

• Thus all the rock mass classification systems are essentially the same

• It is concluded that any new development of rock mass classification system should therefore start with careful consideration of the block size and joint surface condition characterization.

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Visualization in three-dimensional space

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Geotechnical investigation for support design in depillaring panels in Indian coalmines

• In Indian coalmines, CMRI- RMR and NGI-Q Systems are mostly used for formulating design of support in rock engineering.

• In India, Bord and Pillar Method of mining is very much in practice in underground coalmines

• CMRI- RMR and NGI Q- Systems are used for estimation of rock load for design of support in a coalmine

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• RMR system is used for design of support system in roadways during development stage of the mine

• Q system is used for design of support during final extraction (depillaring)

• STUDY AREA - GDK 8 Incline mine, located in Godavari Valley near Godavari Khani in Karimnagar District (AP), is under Ramagundam (RG) II Area of Singareni Collieries Company Ltd (SCCL)

• Geotechnical studies have been conducted to design a suitable support system for existing galleries, splits, slices and goaf edges for depillaring panels

• RMR System has been used to determine the RMR of the roof rock in existing galleries and splits in depillaring area, using five parameters

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Various projects in India where rock mass classification is used

Nathpa Jhakri H. E. Project, H.P• Nathpa Jhakri H.E. Project envisages the utilization of about

488 m drop in river Sutlej between Nathpa and Jhakri in Himachal Pradesh on the Indo-Tibet National Highway about 150 km from Shimla

• The fully underground project consists of concrete gravity dam, four underground desilting chambers, 10.15 m diameter and 27.3 km long head race tunnel, 301 m deep underground surge tank, three pressure shafts, underground Powerhouse etc

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Sardar Sarovar Project, Gujarat• The multipurpose Sardar Sarovar Project, was constructed on

river Narmada in the state of Gujarat with 1450 MW installed capacity (including river bed and canal bed powerhouses).

Sanjay Vidyut Pariyojna, H.P• Sanjay Vidyut Pariyojna is located underground in a hill which

runs nearly east-west in Sungra in Kinnaur district of Himachal Pradesh. Hill is flanked by the river Sutlej in the South and Bhaba in the North. The size of the powerhouse is 71 m (L) x 20.5 m (W) x 12.25 m (H)

Baspa H.E. Project, H.P• Underground power house complex of Baspa Hydro-electric

project is located on the left bank of river Sutlej about 800 m u/s

• The power house cavity of Baspa is 92 m (L) x 18 m (W) x 39.75 m (H).

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Few other major projects where rock mass classification is used –• Border Roads Organisation (BRO) has also taken up the task of

connecting the Lahaul and Spiti valley with Kullu-Manali through 9 km long all weather road tunnel at Rohtang Pass to connect Leh and Ladakh area of Jammu and Kashmir

• Yamuna Hydroelectric Scheme Stage II (Chhibro Power House), Uttrakhand

• Lakhwar H.E. Project, Uttrakhand• Chamera H.E. Project, H.P• Kadamparai Pumped Storage H.E. Project, Tamilnadu• Chukha H.E Project, Bhutan

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Some of the major softwares used are –

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