Geomechanical Classification Systems

• The presentation discusses geomechanical classifications of rock masses.
• It was created by Ana María Olalla A., and covers the period from October 2024 to March 2025.

Unit 4 Objectives

• Learn about the various geomechanical classification systems.
• Understand the criteria applied by different geomechanical classification systems.

Unit 4 Content

• An overview of geomechanical classification systems.
• Geomechanical classification systems, including RQD, RMR, Q by Barton, and GSI.
• Analysis of the applications of geomechanical classification systems in Civil Engineering.

General Concepts of Geomechanical Classifications

• Describing and measuring the characteristics and properties of the rock matrix, discontinuities, and rock mass parameters provides necessary data for overall geomechanical evaluation.
• These data allow the estimation of the quality and approximate resistance parameters of the rock mass in terms of cohesion and friction.
• Fracture properties strongly determine classification systems.

Classification systems

• Classification systems use different scales to assess rock mass quality
• The most used systems are:
• RQD (Rock Quality Designation) by Deer (1964-1967).
• Q system by Barton, Lunde, and Lien (1974)
• Rock Mass Rating System (RMR) by Bieniawski (1989)
• Geological Strength Index (GSI) by Hoek & Brown (1997).

Rock Quality Designation (RQD)

• Developed as a rock mass quality index.
• RQD classification relies on quantifying the frequency and spacing of discontinuities in rock masses.
• RQD is defined as the percentage of core sample containing solid rock segments or fragments.
• Solid Rock segments must be ≥ 100 mm (4 inches).
• RQD is expressed as:
  • RQD = 100 * (sum of fragment lengths / total length of the core run)

RQD Length

• Established as a lower limit for rock masses with equitable quality
• Accounted for joints and moderate spacing
• Macizo rocoso classification criteria:
  • 0–25%: Very poor rock quality
  • 25–50%: Poor rock quality
  • 50–75%: Moderate rock quality
  • 75–90%: Good rock quality
  • 90–100%: Excellent rock quality
• Deere in 1967 created the classification of rock masses using RQD method

RQD Determination – Direct Method

• ISRM recommends a minimum NX (54.7 mm) core size, and rotary drilling, for RQD determination.
• Measuring core fragments along the center line is part of Deere's procedure, recommended by ISRM.
• Fractures caused by the drilling process should be reassembled and counted as one.
• Artificial fractures can be recognised by their tight fit, fresh rough surfaces, cleanliness, lack of discoloration, and should be ignored.
• Unsolid fragments longer than 10 cm should not be counted.
• Solidity is required to separate rock degraded by weathering or hydrothermal effects.
• RQD index is sensitive to the core extraction length.
• Core lengths between 1 and 1.5 m should be of high priority.

RQD Determination – Indirect Methods – Seismic Method

• Seismic method exploits variations in elastic properties, affecting seismic wave velocity.
• The method is cost-effective and quick, and good for large rock mass volumes.
• The information obtained is:
• Location and arrangement of bedrock and geological structures in the subsoil.
• The effect of discontinuities can be estimated by comparing compression wave velocity in-situ with sonic velocity obtained from intact rock core samples.
• Differences are due to discontinuities
• Relative Velocity Index Formula (RQD):
• RQD = (VF/VL)^2 * 100
• VF: Wave velocity for rock mass in situ
• VL: Wave velocity for extracted core
• The relative velocity index can be used equivalently to RQD:
  • A macizo of very good quality is near 1.0
  • The value decreases as fracture grows

RQD Determination by Discontinuity Count

• The estimation of RQD is measured on surface.
• Proposed by Planström (1975) and based on the parameter Jv total number of discontinuities per cubic meter or selected rock volume.
• RQD % = 115 – 3.3 Jv for Jv > 4.5
• Cannot exceed 100
  • RQD % = 100 for Jv ≤ 4.5

Q-System

• The Q classification was developed in 1974 by Barton, Lunde, and Lien based on data from tunnels.
• Has been revised occasionally since then and is expressed as:
• Q = (RQD/Jn) * (Jr/Ja) * (Jw/SRF)
  • RQD should be ≥ 10
• Q = 115 – 3.3 Jv ≤ 100
  • RQD = Rock quality index
  • Jn = Number of joints per m²
  • Jr = Roughness factor of critically oriented joints family
  • Ja = Alteration and filling of critically oriented joints.
  • Jw = Water presence in joints
  • SRF = Stress reduction factor
  • Jv = Total number of discontinuities per cubic meter

More About Q ratings

• Substitute ratings for a given rock mass, and use the Q equation to get Rock mass quality, Q.
• Note Q can be seen as a function of only three parameters:
  • Block Size (RQD/Jn): Represents the structure of the rock mass.
  • Shear Strength Between Blocks (Jr/Ja): High values mean greater mechanical quality
  • Stress Condition Influence (Jw/SRF): A "environmental factor" that adds pressures tension and water flow

Influence of Q ratings

• Discontinuity families can have different roughness, and alteration degrees so the Q System utilizes the worst-case scenario.
• The tension in (Jw/SRF): includes pressures and flows of water, regions of shear stress, compaction, rock swelling, and the state of stress in situ.
• A high ratio comes from a drop in water pressure/flow, relations of strength in situ, and active strength.
• One can gauge what term carries more or less weight on Q index evaluation, thus its influence on rock mass quality, from the score obtained in each block.

Q-System Parameters

• For different rock settings, assign classifications to these parameters.
• Characterization of Tunnel Support System is the main aim.

Values For Jn – Joint Set Number

• Folations, schistosities, and slate beds impact Jn, which indicates the number of joint families.
• Discontinuities count as a joint family if these planes are well formed.
• Joints count as random.

Jr Values

• This variable represents the roughness/unevenness of joint surfaces and is rated based on several criteria.
• Discontinuous joints have a rating of 4.0.
• Rough or irregular joints (either undulating or planar) are rated 3.0 or 1.5
• Smooth, planar joints have lower ratings of 1.0 or 0.5
• If there isn't contact between the joints, that shifts the formula
• It depends if the zone contains Clay, gravel, pebbles etc.
• This addition is applied if discontinuity spacing is greater than 3 meters.
• Jr could hit 0.5

Ja values

• In Joint Alteration Number, surface alteration is key to stability:
• Joint Characteristics impact the Ja number
• Discontinuity with no infilling or stains have lower ratings
  • Closed, hard, unweathered joints score 0.75
  • Planar joints score 1.0 (if only stained)
• Discontinuity with altered surfaces, have higher values depending on filling and material.

Barton’s Jw Parameter

• The parameter, Jw, quantifies water pressure effect on joint shear strength
• Jw must relate to future groundwater conditions with the alteration of rock-mass permeability
• Jw value aligns with erosion of chemical elements
• Water decreases strength because water pressure counters joint shear strength.

Factor for presence of H2O in joint

• MPa stands for megapascals
  • Excavation is dry is
  • Water is important in highly compressed rock joints
• These estimates are crude.
• Jw has to match drainage if drainage is installed.
• Do not consider freezing. Low RQD can help reduce hydrodynamic activity. The practice of assessing the static depth deformation and seismic movement to the usage of correlation dependent by the seismic speed used.

Stress Reduction Factor SRF

• Stress Reduction Factor SRF conditions stresses in the rocks
• The Reduction factor's parameter: Measures how much pressure goes during caves with compressed, argillaceous rocks. There is effort due matrix, the prime effort before digging

SRF Values

• Factor = Conditions
  • Weak intersect (intersect when digging, block fall)
  • Strong rock with hard tension
  • Deformable rock.
  • Expansion A GSI-RMR relationship, (GSI = RMR89 - 5) when when RMR89 ≥ 23

About Q

• A Q index runs from goes from 0,001to 1000 is
  • Low end: A rock that is really poor to highly unexceptionable.

Rock Mass Rating (RMR)

• Proposed by Bieniawski in 1989.
• The stability of a rock is directly dependent on presences and its composition.
• The properties for stability:
• Strength
• RQD
• Spacing
• Water
• Position with cutting

Using RMR

• RMR is divided into units
• The limits of the units match structure. RMR needs the total values divided by 6 to be determined

Getting 6 Paramters in full use.

• RMR and the values is set up to check for a primary quality by checking all primary factors up to the design set by the conditions.
• RMR's total is within 5 categories.

GSI

• GSI (Geological Strength Index) was developed by Hoek and Brown in 1997.
• Applies to bad resisting parts.
• Mainly Matrix based.
• 2 major parts considered
• Grade and separation
• Conditions where break happened

Summary

• GSI is in a isotopic rock for its behavior, in short its is not going to work with rock that has structural failure or to close.

Resistance Values in GSI

Using Applied System Factors

Aplications In civil Design

• It is a current engineering practice in civil based on geomecanic.
• Used for forcast behavior and mainly terrestrial
• Good practice in building sites
• Used for easy reading for good results.

Advantes in Application

• Improves by requiring quality and order.
• It's easy to divide in order to show excavation
• Basic data are provided
• Used on sites to start off mech props , and roters.

Other Factors in the system

• Provides base and simple language.
• Are mainly used at the start, like at before plans.
• Used in areas where there is no cost.
• Main intrested is on site, like where tunnels.
• App 80 percent where found is atend.

Geomechinacs Use

• Good at finding slope.
• The system works by using:
• the elastic module
• Rotation points.
• it can be use to find good place at the starte , but has limits and is not the only one.

Support selection

• Selection system
• It uses rock bolting and spraying the surface and with steel supports

Applying this is good.

• It provides great mechanical value and is used commonly to estimate the method of digging with in a support enviroment.
• The problem is over applying it and making plans that are not in the base which becomes bad.

Last things in factors

• The current best applied use for GSIM,B or RMR
• The GSI can be used after other altercations had place. This creates a other use for the system by applying all methods and using one to check if it will work.

Links of system

• GSI= RMR89 when number is that of 23 the result will work.
• GSI= RMR89-5 value
• Bienawski: RMR89 = 9 In and a Q'+- 18
• 15 log Q' + 50

Constraints of System

• RQD does not account for fractures or way of reading, but the di, length and use.
• It can be wrong at an equal level if not read right. There can be bad connections because fractures had bad read on a good level.
• RQD can mess with precentiles, especially when not read well.
• Q values are wrong base on the rock used or swelling . This is 1.5 or 2 and even 20.

Summary

• There is a big trust on the test for tunnels and it uses everything
• RMR is the limited it's use for expansion.
• These factors work well together.