Laboratory Rock Mechanics

MNG 551 Laboratory #2

Rock Mass Classification

ASTM D6032 & 5878

Philipe Innecco Rosa

Lab Performed: 9/17/14

Lab Reported: 9/24/14

Purpose

The purpose of the lab exercise is to determine a rock mass classification through different methods. The objective of rock classification is to quantify and combine qualitative features with mechanical properties of rock masses, which are important to an engineering project. In general those classification methods consider structural aspects of rock masses such as joints, fractures, faults, groundwater, weatherability and strength of the rock. There are several rock classification methods, each one consider this properties differently based on the application that method was created for. It is important to understand those differences and take those methods as an initial stage of you engineering project.

Equipment

The following list of equipment was used to complete the lab exercise:

Rock Quality Designation and FERM Classification (RQD)

Drill core Tape measure FERM Classification Booklet

Bieniawski’s Rock Mass Rating (RMR)

A rock mass to classify Ability to perform laboratory and field testing RMR classification charts and tables

Coal Mine Roof Rating (CMRR)

A coal mine roof to classify or drill core sample from roof Ability to perform laboratory and field testing CMRR Software provided by NIOSH Ball-peen hammer Chisel Water bucket

Procedure

Rock Quality Designation and FERM Classification (RQD)

1. With a tape measure, measure the length of each core peace greater than 4in.2. Sum the lengths of core pieces greater than 4 in.3. Measure the total length of the drill core.4. Use the equation provided in section 2.4 to calculate RQD.5. Record the approximate description using table provided in 2.4.6. Use the FERM classification booklet to classify the rock type of each core piece.

Bieniawski’s Rock Mass Rating (RMR)

1. From the descriptions provided in the problem statement, determine ratings for parameters 1 to 5 in table 6.9A.

2. Use table 6.10 to determine the discontinuity strike and dip classification.3. Use the strike and dip classification to determine rating adjustment for joint orientations

in table 6.9B.4. Use table 6.9C to determine rock description.5. Use table 6.9D to determine cohesion and friction angle of the rock mass.6. Use table 6.4 to determine average stand-up time of an unsupported excavation.7. Use the equations provided in section 2.4 to estimate the modulus of deformation of the

rock mass.

Coal Mine Roof Rating (CMRR)

Part 1.

1. Open the CMRR program in the NIOSHI folder located in the start menu.2. Select File > New, and fill out the project description. Click next.3. Fill out the general information tab.4. Fill out the unit information tab.5. Fill out the specific information for each unit.6. Fill out the CMRR tab7. The CMRR is shown at the top right of the dialogue box.8. Save the plot of roof layers.9. Save the CMRR report.

Part 2.

1. Perform the chisel splitting test on the three different rock types to determine cohesion of the joint.

2. Perform the moisture sensitivity test on each rock type to determine the moisture sensitivity.

3. Perform the ball-peen hammer test on each tock type to determine the strength index.4. Determine the FERM classification of each rock type.5. Fill out the forms on the CMRR program and determine the CMRR.

Results

Rock Quality Designation and FERM Classification (RQD)

After following the procedures listed above, we were able to fill table1.

RQD DATA SHEETProject: Rock Mass Classification Date: 09/17/14Core Box I.D. no.: Recorders: Rachel, Thales, Luis Felipe and Philipe Total Length of Core Run, mm (in): 235.52 Checkers:Core Diameter, mm (in): Date Checked:

RunSketch or Photographic

Image of core

Length of Each Sound Piece of Core > 100

mmFERM Index

1

An image of entire core sample is provided

bellow

75.755.755.577

335

2 333

3 4 323

4

56

5.55

8.257.75

332

58

8.75 328

Lengths of Sound Pieces of Core > 100 mm (4-in): 96.25

RQD (% )= Lengths of Sound Pieces of Core>100mm(4−¿)Total Length of Core Run ,mm (¿)

RQD (% )=¿ 40.87 Rock Classification: PoorTable. 1 – RQD Data Sheet

Figure 1 – Core Sample

Bieniawski’s Rock Mass Rating (RMR)

After following the procedures listed above, we were able to fill table2 and determine the sample’s RMR value.

Rock Mass Rating1) Strength Rating 122) RQD Rating 173) Joint Spacing Rating 154) Joint Condition Rating 205) Groundwater Rating 76) Joint Orientation Adjustment -5Total Rating 66Class No. IIDescription Good RockCohesion (kPa) 300 – 400 kPaFriction Angle 35 – 45 degreesStand-up time 6 months for 8m spanModulus of Deformation (GPa) 32

Table. 2 – RMR Data Sheet

Coal Mine Roof Rating (CMRR)

Part 1.

After following the procedures listed above, we’ve obtained the following ground water CMRR value. Also the CMRR report is attached at the end of this paper.

Image. 2 – CMRR Final Value

Part 2.

After following the procedures listed above, we’ve obtained the information on table 3 and then we’ve obtained ground water CMRR value for this problem. Also the CMRR report is attached at the end of this paper.

Unit A Unit B Unit CThickness (ft.) 3 0.5 6.5FERM class 994 994 994Strength Index 2 2 2No. of Discontinuities

2 1 1

(1) Cohesion Weak Weak Weak(1) Roughness Wavy – Planer Weak - Planer Jagged - Wavy(1) Spacing 0.5 0.25 0.75(1) Persistence 50 30 30(2) Cohesion Weak - -(2) Roughness Wavy - -(2) Spacing 10 - -(2) Persistence 20 - -

Contact Quality S S SWater Sensitivity Slight Slight SlightWater Movement Light DripBolt 5

Table. 2 – CMRR Underground Data Sheet

Image. 3 – CMRR Final Value

Discussion

Rock Quality Designation and FERM Classification (RQD)

After the measurements we have obtained a total length of sound pieces of core of 96.25 in and the total length of core run was 235.52 in. Using the equation provided on table 1, we came up with a RQD of 40.87% which classifies the core sample as a poor rock. General Tunneler’s description, describes this as being shattered, very blocky and seamy.

This experiment were the most propitious to have any kind of error, because it consist in evaluating which discontinuity we should or should not considerate.

Bieniawski’s Rock Mass Rating (RMR)

With all information provided we were able to quantify the final RMR value which were 66, and from there classify the rock into class number 2 which describes it as a Good Rock Mass.

Following this method we can infer some of the rock properties. It has a cohesion value that ranges from 300 to 400 kPa, a friction angle that ranges from 35 to 45 degrees and a stand up time of six months for an eight meters span. There are several empirical equations for determination of a variety parameters from RMR, one of these were proposed by Bieniawski in 1978 and we can obtain the deformation modulus. This equation is shown on Rock Mechanics Lab Manual page 35. We’ve obtained a rock mass deformation modulus of 32 GPa.

We have also obtained some geomechanics characteristics for excavation and support in that rock mass. Due to its classification we can have a full face advancement from 1.0 to 1.5 meters with support at twenty meters from face. Support will consist in locally bolts in crown, three meters long, spaced 2.5 meters with occasional wire mesh. Also a 50 millimeters shotcrete in crown is required but there is no need for steel sets.

Considering that we were provided with the data for this exercise the only source of error on this experiment could be a misunderstanding on those charts by the student.

Coal Mine Roof Rating (CMRR)

Part 1.

We filled the CMRR software with the underground database provided. As a result we have obtained a final groundwater adjusted CMRR value of 52.9, which classify this as a moderate roof, and a six feet long roof bolt is required for anchoring this rock mass.

Part 2.

For this we had to complete the underground information by performing the chisel splitting test, moisture sensitivity test and ball-peen hammer test to determine the cohesion of the joint, moisture sensitivity and strength index respectively. All of those tests are simple and fast to do and combining them with the charts provided we were able to have a better understanding of the rock properties. Also those are most important properties we should know to apply CMRR, considering that we need a strong layer to have our bolt fixed in.

After filling the CMRR software with this database we’ve obtained a final groundwater adjusted CMRR value of 50.8, which classify this as a moderate roof, and a five feet long roof bolt is required for anchoring this rock mass.

The source of error on this experiment could be a careless performing on the test, which lead to improper results.

Conclusion

The purpose of this laboratory report was to have a better understanding about rock mass classification systems by applying each one of them on real cases scenarios. We have performed three major experiments, they are Rock Quality Designation, Rock Mass Rating and Coal Mine Rating. In every one we were trying to quantify qualitative data like geologic features for further engineering design. For the first test, Rock Quality Designation, we have obtained a RQD of 40.87% which classifies the core sample as a poor rock. For the RMR test we had a final value of

66, classifying that sample into a Good Rock Mass. We had two databases to fill the CMRR software and we had a final Moderate Roof classification for both of them.

Suggested Exercises

1) Any natural joints or fractures that crosses transversely the core sample should be considered when evaluating rock mass quality under RQD. Artificial discontinuities should not be considered.

2) RQD system is needed for a classification system, RQD alone is not a classification system. It is used to get a general evaluation of rock masses, in other words, it is the starting point for our research.

3) Bieniawski’s RMR system combine some factors to classify rocks into some groups with similar characteristics. It provide a good basis for understanding characteristics of the rack mass, so it is recommended for general use in preliminary design. RMR should be simple and meaningful, that’s why it is based on measurable parameters which can be determined quickly and cheaply in the field. Those parameters are:

1. Strength of Intact Rock Material. UCS or point load test.2. Rock Quality Designation (RQD).3. Discontinuity Spacing. Including all discontinuities.4. Joint Condition. This includes joint separation, persistence, roughness, and

infilling material. 5. Groundwater Conditions.

4) CMRR was developed in order to have a rock mass classification system that was specially tailored for evaluation of coal mine roofs. It is designed for bedded coal measure rocks and concentrates on the bolted horizon and its ability to provide a stable mine structure. CMRR considers three major parameters: Discontinuities, Strength and Moisture sensitivity.

5) The primary factor in assessing the strength of the immediate roof is discontinuities and discontinuities conditions.

6) Water movement can reduce rock strength, generating swelling pressures, or compromising support effectiveness by causing sloughing around roof bolt plates.

7) Four common rock mass classification systems are: Q-System, RMR, CMRR and GSI

Required Reading Exercises

1) Rock Classification systems are important because they provide a methodology for characterizing rock mass strength using simple measurements; they allow geologic information to be converted into quantitative engineering data and they also make it possible to compare ground control experiences between sites, even when geologic conditions are very different.

2) Rock Classifications have their weaknesses as well. Some of them are: Some of them only provide a general idea of the rock mass, therefore they should be used on early stages of an engineering design; each one tend to focus on properties and characteristics which are more important for the application they were developed for; they rate just one rock unit at a time.

3) A rock material is a small piece of rock, which is natural aggregate of minerals. This small piece may not contain all the geologic features present in a larger mass. A rock mass is a large quantity of rock containing all the geologic features such as joint, fractures, faults, folds, etc.

4) Structural features are related to discontinuities such as joints, bedding planes, faults and fractures.

5) Spacing, persistence, cohesion and roughness.