Co Relation of Pont Load With Uni Axial Compressive Strength

CHAPTER 01

INTRODUCTION

1.1 OVERVIEW

In geological exploration, mechanical rock properties are one of the most important parameters

that will be later used in the analysis and design of any engineering structures in rock mass. To

obtain these properties, the rock from the site is extracted normally by means of core drilling,

and then transported the cores to the laboratory where the mechanical testing can be conducted.

Laboratory test machine is normally huge and cannot be transported to the site. Onsite testing of

the rocks may be carried out by other technique, but only on a very limited scale.

This method is called point load strength testing. This test however provides unreliable results,

and lacks theoretical supports. Its results may imply to other important properties (e.g.

Compressive and tensile strengths), but only based on an empirical formula, which usually poses

high degree of uncertainty.

To save cost and energy that are consumed by drilling processes, rock core transportation, and

laboratory testing, a new method for on-site testing is needed. The researcher proposes to modify

the currently used point load testing machine to be able to yield the compressive and tensile

strengths of the rock specimens with irregular shapes. The new technique which is thereafter

called “modified point load (MPL) testing,” will be backed by solid theoretical ground.

The testing machine will also remain small and will be easy to operate on-site. If the new

Technique can be invented successfully; it may significantly reduce the cost, time and energy

that have been consumed by the convention methods.

1.2 DESCRIPTION

The PLT is an attractive alternative to the UCS because it can provide similar data at a lower

cost. The PLT has been used in geotechnical analysis for over thirty years. The PLT involves the

compressing of a rock sample between conical steel platens until failure occurs. The apparatus

for this test consists of a rigid frame, two point load platens, a hydraulically activated ram with

pressure gauge and a device for measuring the distance between the loading points. The pressure

gauge should be of the type in which the failure pressure can be recorded. A state of the art point

load testing device with sophisticated pressure reading instrumentation is shown in Figure 1.

Figure 1.1: Point Load Index Test

Indirect tensile strength is more useful than direct tensile strength in rock mechanics application,

partly because tensile stress field in rock mass is usually induced indirectly by compressive

deviatoric stresses and partly because direct tension is difficult to apply to rock specimens

without inducing any eccentric moments. The point load strength test (PLST) is one of the most

popular indirect tensile strength tests used in rock engineering.

The strength index measured in this test is called the point load strength index (PLSI), which is a

measure of the indirect tensile strength and has been correlated empirically to both the tensile

strength and compressive strength of rock. The PLST has been applied most commonly to

cylindrical specimens, either axially or diametric.

Figure 1.2: A Cylindrical Rock Specimen Subjected To The Diametrical Plst. The Origin Is

At The Centre Of Cylinder And The Indentors Act Along A Diameter Passing Through

The Origin.

1.3 PROPOSED CONCEPT

In the rock mechanics and engineering geology, the point load test is regarded as valuable field

test to give an estimate of the unconfined compressive strength. Well known in the is the scale

effect concerning the point load strength since the first compressive paper by Broch and Franklin

(1972) after carried out a large high number of point load tests in different rock types with

different devices.

Several other researchers have correlated the point load index with the uniaxial compressive

Strength of the rocks (e.g. Miller, 1965; Reichmuth, 1968; Bieniawski, 1975; Pells, 1975; Jaeger

and Cook, 1979; Turk and Dearman, 1986; Kaczynski, 1986 and Chau and Wong, 1996). Brook

(1977, 1985, and 1993) has also established a relation between the point load strength with the

uniaxial compressive strength of intact rocks. It is however recognized here that the tensile

failure is the dominant mode of failure for the point load specimen. As a result, the point load

strength should be related to the tensile strength of the rock rather than the compressive strength.

Strictly speaking, the dominant mode of failure for the point load specimens is governed by the

size or the distance the loading points. For small specimens, the failure should be in biaxial or

poly axial compression modes. For a large specimen, the biaxial tension will be predominant

mode of failure. Recognizing this phenomenon, an attempt is made here to distinguish between

the compressive and tensile failures under a wide range of specimen sizes. Theoretical derivation

and numerical simulation may be used to assist in describing the stress and strain distribution

between the loading points for various specimen sizes. Relationships between the point load

index and the compressive and tensile strength may therefore be established. The final goal is

that one can conduct point load testing on various specimen sizes, and use the results as an

indicator of compressive and tensile strengths of the intact rocks.

1.4 REASONS FOR DEVELOPING INDEX TESTS

The index tests are devised to overcome some of the difficulties encountered in the laboratory

test as follows.

1. Laboratory testing of rock material is elaborate, time consuming and therefore expensive.

2. Delay in assessment.

3. Index test are essentially field tests devised to obtain test results

Without much specimen preparation

With portable equipment

Correlated to strength and deformation properties for design calculations

Some test give representative of properties for design calculation

Sometimes open boreholes test for litho logical classification and structural

mapping can also be correlated to the index properties.

1.5 CLASSIFICATION AND TYPES OF INDEX TESTS

1.5.1 BRAZILIAN TEST

The tensile strength of a material is a measure of its ability to resist uniaxial tensile loads without

yielding or fracture. A direct-pull uniaxial test is difficult to apply to rock and in many cases

some type of indirect test is employed to determine tensile strength.

The Brazilian test, where a disc of the test material is loaded across a diameter, is often

employed.

Brazilian test can be correlated to tensile strength used for classification of rocks by strength for

drill ability, rock breakage and crushing classification.

t = 2P/λDt

Where;

t = uniaxial tensile strength

P = Load at failure on a portable machine

D = diameter of the core (m)

T = thickness of core (m)

1.5.2 POINT LOAD INDEX TEST

Point load index tester, a rock testing instrument for determining the Diametrical Point Load

Strength Index of rock cores and Irregular lumps which may be tested without any treatment.

The Point Load Test is primarily as index test for strength classified of rock materials.

This instrument is mainly intended for field measurements on rocks specimen, but it can be used

in the laboratory. The results of the test may also be used to predict the uniaxial compressive

strength of rock from correlations

Point load index for strength classification of rocks which can be used by geotechnical engineers

for predicting the strength and deformation properties of rock to the design of mining

excavations.

1.5.3 DYNAMIC IMPACT STRENGTH TEST

Dynamic impact strength test (Pomeroy, 1955; Ghose et al, 1964) to estimate resistance of coal

to degradation and applicable to rock workability studies in order to assess the degradability of

coal that control subsequent breakage

This may occur in the loading coal in face conveyor, during transfer from one conveyor to

another or into storage bunkers.

1.5.4 CONE INDENTER HARDNESS TEST

Cone indenter hardness test developed the British coal; Mining Research and Development

Establishment for assess strength of coal in underground coal mines.

The NCB Cone Indenter which was developed by the British National Coal Board to evaluate the

compressive strength of rock by measuring the force required to cause a specific indentation in a

small chip of rock.

It measures the penetration of tungsten carbide cone on a prepared core specimen in 38mm in

diameter and 10mm thick under the normal force.

1.5.5 SLAKE DURABILITY TEST

Sake durability is a simulated weathering test to determine abrasion resistance during wetting

and drying cycles of shale and similar soft rocks as used in embankments and other construction-

related applications. Samples are alternately tumbled in mesh drums through water medium and

oven-dried for two cycles. The percent loss of mass is referred to as the slake durability index.

Slake durability test, therefore applicable to assess the durability of rock for near surface

excavations or swelling of roof and floor strata in underground excavations.

1.5.6 SCHMIDT HAMMER TEST

A Schmidt hammer, also known as a Swiss hammer or a rebound hammer, is a device to measure

the elastic properties or strength of concrete or rock, mainly surface hardness and penetration

resistance.

The hammer measures the rebound of a spring-loaded mass impacting against the surface of the

sample. The test hammer will hit the concrete at a defined energy. Its rebound is dependent on

the hardness of the concrete and is measured by the test equipment. Schmidt hammer test, a

rebound test devised for field condition and can be used for estimating strength and deformation

characteristics of rock.

1.5.7 SHORE SCLEROSCOPE TEST

The Scleroscope test consists of dropping a diamond tipped hammer, which falls inside a glass

tube under the force of its own weight from a fixed height, onto the test specimen. The height of

the rebound travel of the hammer is measured on a graduated scale. The scale of the rebound is

arbitrarily chosen and consists on Shore units, divided into 100 parts, which represent the

average rebound from pure hardened high-carbon steel. The scale is continued higher than 100 to

include metals having greater hardness. The Shore Scleroscope measures hardness in terms of

the elasticity of the material and the hardness number depends on the height to which the

hammer rebounds, the harder the material, the higher the rebound.

Shore Scleroscope test, a non-destructive test for classifying hardness and, therefore, selecting

excavation machinery or blasting.

1.5.8 NEUTRON-NEUTRON LOGS TEST

The neutron log is sensitive mainly to the amount of hydrogen atoms in a formation. Its main use

is in the determination of the porosity of a formation.

The tool operates by bombarding the formation with high energy neutrons. These neutrons

undergo scattering in the formation, losing energy and producing high energy gamma rays. The

scattering reactions occur most efficiently with hydrogen atoms. The resulting low energy

neutrons or gamma rays can be detected, and their count rate is related to the amount of

hydrogen.

Neutron-neutron logs are carried out in open bore hole which can be correlated to rock density

and point load index test table 5.1 summarizes the index properties of rocks.

1.6 POINT LOAD INDEX TEST:

A cylindrical core obtained from the bore is approximately cut to the length to diameter ratio 1.5

to 1 and diametrically loaded against conical platens on a portable loading machine the load of

failure can be related to the point load index rock shown in equation 1.1.

Is=P/d2 (1.1)

Uniaxial compressive strength is correlated with point load index as shown in equation 1.2.

c =24 Is (1.2)

Where;

P = Load at failure

D = Diametrical distance between conical platens at failure (m)

Is = Point load index of rock

c = Uniaxial compressive strength of rock (MPa)

Is50 = Point load index for 50 mm diameter core (fig 1-size correction)

Idealized condition for this test is as follows.

Portable loading machine

Use of calibration chart for size correction (figure 1)

Minimum core diameter 50mm

Length-to-diameter ratio 1.5 to 1

Number of samples- 10 to 15

No slandered rate of loading

Loading platens 600 conical platen with 5 mm curvature tip.

CHAPTER 02

POINT LOAD INDEX TESTING

2.1 SCOPE

This test is intended as a method for measuring the strength of rock specimens in the field, and

uses portable equipment. Specimens in the form of either rock core (the ‘diametric’ and ‘axial’

tests) or of irregular lumps (the ‘irregular lump’ test) are broken by application of a concentrated

loud using a pair of conical platens.

A Point-Load Strength index I (50) is obtained and may be used for rock strength classification.

2.2 SUMMARY OF TEST METHOD

This index test is performed by subjecting a rock specimen to an increasingly concentrated load

until failure occurs by splitting the specimen. The concentrated load is applied through coaxial,

truncated conical platens. The failure load is used to calculate the point load strength index and

to estimate the uniaxial compressive strength.

2.3 SIGNIFICANCE AND USE

The uniaxial compression test is used to determine compressive strength of rock

specimens, but it is a time-consuming and expensive test that requires specimen

preparation. When extensive testing is required for preliminary and reconnaissance

information, alternative tests such as the point load test can be used in the field to reduce

the time and cost of compressive strength tests.

The point load strength test is used as an index test for strength classification of rock

materials. The test results should not be used for design or analytical purposes.

This test method is performed to determine the point load strength index (Is50) of rock

specimens, and the point load strength anisotropy index (Ia50) that is the ratio of point

load strengths on different axes that result in the greatest and least values.

Rock specimens in the form of either core (the diametrical and axial tests), cut blocks (the

block test), or irregular lumps (the irregular lump test) are tested by application of

concentrated load through a pair of truncated, conical platens. Little or no specimen

preparation is required.

However, the results can be highly influenced by how the specimen is treated from the

time it is obtained until the time it is tested. Therefore, it may be necessary to handle

specimens in accordance with Practice.

2.4 TEST SPECIMENS

SAMPLING: Rock samples are grouped on the basis of both rock type and estimated

strength. When testing core or block specimens at least ten specimens are selected. When

testing irregular-shaped specimens obtained by other means at least 20 specimens are

selected. Specimens in the form of core are preferred for a more precise classification.

DIMENSIONS: The specimen’s external dimensions shall not be less than 30 mm and

not more than 85 mm with the preferred dimensioning about 50 mm..

SIZE AND SHAPE: The size and shape requirements for diametric, axial, block or

irregular lump testing shall conform to the recommendations. The sides of the specimens

shall be free from abrupt irregularities that can generate stress concentrations. No

specimen preparation is required.

WATER CONTENTS: Determines the water content of each specimen after testing

since it can affect the value of the point load strength..

MARKING AND MEASURING SPECIMENS: The specimens shall be properly

marked and measured..

MARKING: The desired test orientation of the specimen shall be indicated by marking

lines on the specimen. These lines are used for centering the specimen in the testing

machine, and to ensure proper orientation during testing. These lines may also be used as

reference lines for measuring thickness and diameter.

MEASURING: Measure each dimension of a specimen at three different places, and

calculate the averages.

Figure 2.1: Different Size And Shape Of Cylindrical\Irregular Specimens With Contact Of

Axial And Diametric Force Of Point Load Test.

2.5 DIGITAL POINT LOAD APPARATUS:

The testing machine incorporates a loading system (comprising for example, a loading Frame:

pump: ram and platens).

A system for measuring the load P required breaking the specimen and a system for measuring

the distance D between the two platen contact points. It’s essential features are the following:

1. The loading system should be adjustable to accept and test available rock specimens for

example in the size range 25-100 mm for which a loading capacity up to 50 KN is

commonly required.

2. A quick-retracting ram helps to minimize delay between tests. Ram friction should be

low so as not to impair the accuracy of load measurement.

3. Spherically truncated conical platens (Fig. 4) are used to transmit loud to the specimen.

The 60” curie and 5 mm radius spherical truncation should meet tangentially, and the

platens should be hardened so that they remain undamaged during testing.

4. The platens should be accurately aligned so that each is coaxial with the other, and the

machine should be rigid to ensure that the platens remain aligned during testing. No

spherical scat or other non-rigid component is permitted in the loading system.

5. The load-measuring system should indicate the failure load P to an accuracy of 2 %

irrespective of the strength of specimen tested. It should incorporate a maximum

indicating device so that the reading is retained and can be recorded after specimen

failure.

6. It should be resistant to hydraulic shock and vibration so that the accuracy of readings is

maintained during testing.

7. The distance-measuring system should indicate the distance D between platen-contact

points to accuracy of 0.5 mm. It should be designed to allow zero check and adjustment

and should be robust so that its accuracy is maintained during testing.

The function of this apparatus is given below

This is a portable instrument which can be used in either the laboratory or in-situ to ascertain the

rock strength index of samples of rock or core with diameters up to 102 mm.

The values required for the calculation of the rock strength index are failure load and distance

between the conical points.

Hydraulic loading ram with manual pump

Capacity: 60 KN

Scale: 0-6000 daN (resolution 1 daN -6000 divisions)

Measurement: microprocessor based digital gauge (linearity-hysteresis ≤ ± 0.20 F.S)

battery run using lithium battery 3.6V -2/3 AA type.

Peak value memorized

Pair of conical points

Poly carbonate safety guard

Instrument dimensions: 240 × 280 × 660 (h) mm

Pump dimension: 500 × 150 × 230 (h) mm

Overall weight: 37 kg

Accessories and spares parts:

Ts 70 6/7 two testing plates, 40 mm diameter for compression test on cylindrical specimen

AD 010 microprocessor based digital gauge (battery run)

TS 706/6 set of two conical points.

Figure no 2.2 Point Load Portable Testing Device

Figure 2.3: Point Load Testing Machine

Figure 2.4: Loading Platens Of Point Load Tester

2.6 GENERAL INSTRUCTION FOR USING POINT LOAD EQUIPMENT

The instrument is basically a hydraulic compression machine with a dynamometer. The jack "A"

is moved upwards by the hand pump "B"; its descent is made by pushing directly on the piston

"C" after having opened the discharge valve "D". Naturally the discharge valve "D" must be

closed when the hydraulic jack "A" is to be activated to perform a test. The precision digital

gauge E measures load in daN.

The dial gauge has 3 keys indicating SET, ZERO and PEAK..

SET: when pressed this key turns on dial gauge -when pressed for 5 seconds this key

turns off dial gauge. If pressed, held for 3 seconds and then released this key enables the

user to enter the configuration (setting) menu of the instrument.

ZERO: when pressed for 3 seconds, this key enables display to be zeroed (tare). When

pressed for 6 seconds, this key disenables ZERO function of digital gauge and displays

digital gauge offset.

PEAK: when pressed for 2 seconds, this key activates PEAK function which enables

maximum pressure measured to be displayed after activation of the function. When

pressed for 5 seconds, the temperature in °C is shown. To revert to pressure readout press

key again.

The user now positions a rock fragment in the test chamber, closes valve "D" and activates the

hand pump. The piston stem is thus raised so as to block the rock sample between the two

loading points.

While piston is rising, before contact is made, the gauge should indicate zero.

By continual pumping, the lower display will show an increasing value which corresponds to the

force applied to the sample, expressed in daN. In the test the data of interest is the force

necessary to break the sample; it is therefore useful to preset the device so it only shows

increasing values and maintains the peak value on the display.

Peak value is obtained by pressing F key (PEAK).

To show that peak value is entered the display flashes.

To deactivate PEAK function, simply press F key (PEAK) again.

CHAPTER NO 3

POINT LOAD TESTING OF ROCK SAMPLES

3.1 PREPARATION OF ROCK SAMPLES

The Preparation procedure comprises of following steps.

1. A core specimen of various samples is cut to produce appropriate rock specimen for each test by a cutting machine.

Figure no 3.1 Core Sample Cutting Machine

2. After cutting of different rock specimens prepared for testing with various sizes and shapes.

Figure No 3.1.2 Various Cutting Sample For Testing

3. Select sample: Rock lumps, small size (less than 30 mm), standard size (30 to 85mm) and

large size (more than 85mm) are suitable for the irregular lump tests. D is the distance

between the loading points. Lump samples are selected with dimensions such that

l>0.5D, 0.3 W<D<W; i.e. the ratio, D/W, should be between 1/3 and 1, preferable close

to 1. The distance L should be at least 0.5W. Length 2L is the largest dimension, width W

the smallest. The specimen is installed to ensure that the platens of the testing machine

make contact along a minimal cross section, and not along a plane of weakness;

4. Measure and record the dimension of the lumps , smallest width W , distance between

loading points D and maximum width, length L. if the sides are not parallel then take the

mean width;

5. The dimension and desired test orientation of the lumps selected or indicated by marking

color lines on the specimen. These lines are used for centering the specimen in the testing

machine, and to ensure proper orientation during testing. These lines also are used as

reference line for measuring thickness and diameter;

6. Take picture of the lump. Photographs of all the specimens collected have been made

(For example, No S2 of samples ID)

Figure 3.1.3: Specimen Geometry.

3.2 LOADING CONDITION OF POINT LOAD INDEX

There are two types of loading condition which are applied in the point load index

which are defined as below:

Axial type of loading condition.

Diametric type of loading condition.

3.2.1 AXIAL TYPE OF LOADING CONDITION

When load or thrust are applied on the cylindrical body by thickness wise or vertically, this type

of loading condition are known as the axial type of loading condition. In fig: 3.1.1 clearly

mentions that the axial type of loading condition is applied on cylindrical shape of core specimen

rock with the help of point load index test.

Figure 3.2.1: Core Specimen dimensions for an axial point load test.

3.2.2 DIAMETRIC TYPE OF LOADING CONDITION

Diametric type of loading condition is that the load or thrust applied on the body by diametrically

shape. In fig: 3.2.2 the loading condition is the diametrically on the cylindrical core specimen

with the help of the point load index test.

Figure 3.2.2: Core specimen’s dimensions for a diametric point load test.

3.3 DIFFERENT TEST TYPES AFTER BROOK (1985), ISRM (1985) AND

ASTM

Known from the onset of testing, the point load strength is highly dependent on the size of the

specimen as well as the shape.

Using thick instead of tall specimens for the block and the irregular lump test and standardizing

the general shape of the specimens were steps forward Broch and Franklin (1972), Brook 1985.

Specimen shape requirements are to obtain more reliable testing results with a smaller standard

deviation. However, analysis and evaluation were limited by size variation and the lack of a

reliable and easy-to-comprehend method for size correction.

Broch and Franklin (1972) offered a Size Correction Chart with a set of curves to standardize

every value of the point load strength Is to a point load strength index (I(50)) at a diameter of D

= 50 mm. The purpose of the function was to describe the correlation between I and D and to

answer the question, whether this function is uniform for all rock types or if it depends on the

rock type together with grain size, composition of mineral bonds, grain cleavage etc.

Figure 3.3: Specimen Shape Requirements For Different Test Rock Types.

Brook (1985) and the ISRM (1985) suggest three options to evaluate the results of a test set:

1. Testing at D=50 mm only (most reliable after ISRM (1985).

2. Size correction over a range of D or De using a log-log plot. The most reliable method of size

correction is to test the specimen over a range of D or De values and to plot graphically the

relation between P and De. If a log-log plot is used, the relation is a straight line.

Points that deviate substantially from the straight line may be disregarded (although they should

not be deleted). The value of Is(50) corresponding to De =50 mm can be obtained by

interpolation and use of size corrected point load strength index ASTM.

3. When testing single-sized core at a diameter other than 50 mm or if only a few small pieces

are available, size correction may be accomplished using the formula containing the“Size

Correction Factor” f:

3.4 TEST PROCEDURE

Step # 01: Name/mark the samples.

Step # 02: Measure the Diameter of the each specimen along any reference axis, then rotates the

sample at 90o from reference axis and notes another Diameter by means of vernier calliper.

Step # 03: Take an average of the both diameter reading.

Step # 04: Measure two lengths readings of the specimens, in the same the diameter by using

varnier calliper and takes an average.

Step # 05: Note down all the diameters and length of respective sample accordingly.

Step # 06: Load the sample in the point load tester, carefully between the pair of conical loading

platens

Step # 07: Set the equipment to “0” adjustments.

Step # 08: Set the equipment for “Peak” measurements.

Step # 09: Start loading and observe the developments of cracks (If any).

Step # 10: Note the Peak load reading after the failure of specimen has occurred.

Step # 11: The load “P” is used to calculate the point load strength of samples using:

Figure 3.4: Experimental

Apparatus For Point Load Testing.

Is = P/d2

Where;

P = Load at failure (daN).

d = Diametrical distance between conical platens at failure (m).

Is = Point Load index of rocks (MPa).

Is50 = Point load index for 50 mm diameter core (MPa).

3.5 LABORATORY EXPERIMENTS

Sample no. 01

Rock type: “Lime stone”

Loading type: Axial type loading condition.

Diameter: (d)

d1 = 99.53 mm

d2 = 99.70 mm

d = d1+d2/2

d = 99.53+99.70/2

d = 199.23/2

d = 99.615 mm

d = 0.09961 m

Length: (L)

L1 = 23.65 mm

L2 = 24.95 mm

L = L1+L2/2

L = 23.65+24.95/2

L = 48.6/2

L = 24.3 mm

L = 0.0243 m

Load: (P) = 423 daN = 4230 N

1 daN = 10 N

1 pascal = N/m2

Is = P/d2

Is = 4230/(0.09961)2

Is = 4230/0.00992

Is = 426411.29 N/m2

Is = 426411.29 pascals

Is = 426.411 Kpa

Is = 0.426 Mpa

c = 24*Is

c = 24*0.426

c = 10.224 MPa

Sample no. 02

Rock type: Lime stone

Loading type: Diametric type loading condition.

Diameter: d

d1 = 99.59 mm

d2 = 99.90 mm

d = d1+d2/2

d = 99.59+99.90/2

d = 199.49/2

d = 99.745 mm

d = 0.09974 m

Load: (P) = 1590 daN = 15900 N

Is = P/d2

Is = 15900/(0.09974)2

Is = 15900/0.00994

Is = 1599597.58 N/m2

Is = 1599597.58 pascals

Is = 1599.597 Kpa

Is = 1.599 Mpa

c = 24*Is

c = 24*1.599

c = 38.376 MPa

Sample no:03:-

Rock type:-“Clay Stone”

Loading type:- Diametric type loading condition.

Diameter: (d)

d1 = 61.83 mm

d2 = 61.91 mm

d = d1+d2/2

d = 61.83+61.91/2

d = 123.74/2

d = 61.87 mm

d = 0.0618 m

Load: (P) = 25 daN = 250 N

Is = P/d2

Is = 250/(0.0618)2

Is = 250/0.00381

Is = 65616.79 N/m2


Is = 65.616 Kpa

Is = 0.0656 Mpa

c = 24*Is

c = 24*0.0656

c = 1.574 MPa

Sample no:04:-

Rock type:-“Silt Stone”


Diameter: (d)

d1 = 62.26 mm

d2 = 60.94 mm

d = d1+d2/2

d = 62.26+60.94/2

d = 123.2/2

d = 61.6 mm

d = 0.0616 m

Load: (P) = 44 daN = 440 N

Is = P/d2

Is = 440/(0.0616)2

Is = 440/0.003794

Is = 115972.58 N/m2


Is = 115.972 Kpa

Is = 0.115 Mpa

c = 24*Is

c = 24*0.115

c = 2.76 MPa

Sample no:05:-



Diameter: (d)

d1 = 61.32 mm

d2 = 62.80 mm

d = d1+d2/2

d = 61.32+62.80/2

d = 124.12/2

d = 62.06 mm

d = 0.0620 m

Load: (P) = 56 daN = 560 N

Is = P/d2

Is = 560/(0.0620)2

Is = 560/0.00384

Is = 145833.33 N/m2


Is = 145.833 Kpa

Is = 0.145 Mpa

c = 24*Is

c = 24*0.145

c = 3.48 MPa

Sample no:06:-


Loading type:- Axial type loading condition.

Diameter: (d)

d1 = 62.89 mm

d2 = 61.64 mm

d = d1+d2/2

d = 62.89+61.64/2

d = 124.5/2

d = 62.26 mm

d = 0.0622 m

Length: (L)

L1 = 22.22 mm

L2 = 20.74 mm

L = L1+L2/2

L = 22.22+20.74/2

L = 42.96/2

L = 21.48 mm

L = 0.0214 m

Load: (P) = 39 daN = 390 N

Is = P/d2

Is = 390/(0.0622)2

Is = 390/0.00386

Is = 101036.26 N/m2


Is = 101.036 Kpa

Is = 0.101 Mpa

c = 24*Is

c = 24*0.101

c = 2.424 MPa

Sample no:07:-


Loading type:- Axial type loading condition.

Diameter: (d)

d1 = 62.49 mm

d2 = 62.23 mm

d = d1+d2/2

d = 62.49+62.23/2

d = 124.72/2

d = 62.36 mm

d = 0.0623 m

Length: (L)

L1 = 28.19 mm

L2 = 29.54 mm

L = L1+L2/2

L = 28.19+29.54/2

L = 57.73/2

L = 28.86 mm

L = 0.0288 m

Load: (P) = 32 daN = 320 N

Is = P/d2

Is = 320/(0.0623)2

Is = 320/0.00388

Is = 82474.22 N/m2


Is = 82.474 Kpa

Is = 0.0824 Mpa

c = 24*Is

c = 24*0.0824

c = 1.977 MPa

Sample no:08:-

Rock type:-“Lime Stone”


Diameter: (d)

d1 = 48.52 mm

d2 = 48.74 mm

d = d1+d2/2

d = 48.52+48.74/2

d = 97.26/2

d = 48.63 mm

d = 0.0486 m

Load: (P) = 1791 daN = 17910 N

Is50 = P/d2

Is50 = 17910/(0.0486)2

Is50 = 17910/0.00236

Is50 = 7588983.05 N/m2

Is50 = 7588983.05 pascals

Is50 = 7588.983 Kpa

Is50 = 7.588 Mpa

c = 29*Is50

c = 29*7.588

c = 220.05 MPa

Sample no:09:-



Diameter: (d)

d1 = 48.64 mm

d2 = 49.05 mm

d = d1+d2/2

d = 48.64+49.05/2

d = 97.69/2

d = 48.84 mm

d = 0.0488 m

Load: (P) = 542 daN = 5420 N

Is50 = P/d2

Is50 = 5420/(0.0488)2

Is50 = 5420/0.00238

Is50 = 2277310.92 N/m2

Is50 = 2277310.92 pascals

Is50 = 2277.310 Kpa

Is50 = 2.277 Mpa

c = 29*Is50

c = 29*2.277

c = 66.03 MPa

Sample no:10:-

Rock type:-“China Clay”

Loaidng type:- Axial type loading condition.

Diameter: (d)

d1 = 62.23 mm

d2 = 61.59 mm

d = d1+d2/2

d = 62.23+61.59/2

d = 123.82/2

d = 61.91 mm

d = 0.0619 m

Length: (L)

L1 = 28.19 mm

L2 = 29.54 mm

L = L1+L2/2

L = 28.19+29.54/2

L = 57.73/2

L = 28.86 mm

L = 0.0288 m

Load: (P) = 25 daN = 250 N

Is = P/d2

Is = 250/(0.0619)2

Is = 250/0.00383

Is = 65274.15 N/m2


Is = 65.274 Kpa

Is = 0.0652 Mpa

c = 24*Is

c = 24*0.0652

c = 1.564 MPa

Sample no:11:-



Diameter: (d)

d1 = 62.88 mm

d2 = 62.81 mm

d = d1+d2/2

d = 62.88+62.81/2

d = 125.69/2

d = 62.84 mm

d = 0.0628 m

Load: (P) = 47 daN = 470 N

Is = P/d2

Is = 470/(0.0628)2

Is = 470/0.00394

Is = 119289.34 N/m2


Is = 119.289 Kpa

Is = 0.119 Mpa

c = 24*Is

c = 24*0.119

c = 2.856 MPa

Sample no:12:-



Diameter: (d)

d1 = 48.52 mm

d2 = 48.74 mm

d = d1+d2/2

d = 48.52+48.74/2

d = 97.26/2

d = 48.63 mm

d = 0.0486 m

Load: (P) = 2014 daN = 20140 N

Is50 = P/d2

Is50 = 20140/(0.0486)2

Is50 = 20140/0.00236

Is50 = 8533898.30 N/m2

Is50 = 8533898.30 pascals

Is50 = 8533.898 Kpa

Is50 = 8.533 Mpa

c = 29*Is50

c = 29*8.533

c = 247.45 MPa

Sample

Number

Rock type Loading

condition

Diameter(d) Length(L) Load:

(daN) Is Is50

c

MPa

In meter In meter And (N) (MPa) (MPa)

Sample No:01 Lime

stone

Axial 99.615 mm

0.09961 m

24.3 mm

0.0243 m

423 daN

4230 N

0.426 MPa ___ 10.224

MPa

Sample No:02 Lime

stone

Diametric 99.745 mm

0.09974 m

___ 1590 daN

15900 N

1.599 MPa ___ 38.376

MPa

Sample No:03 Clay stone Diametric 61.87 mm

0.0618 m

___ 25 daN

250 N

0.0656

MPa

___ 1.574

MPa

Sample No:04 Silt stone Diametric 61.6 mm

0.0616 m

___ 44 daN

440 N

0.115 MPa ___ 2.76

MPa


0.0620 m

___ 56 daN

560 N

0.145 MPa ___ 3.48

MPa

Sample No:06 Clay stone Axial 62.26 mm

0.0622 m

21.48 mm

0.0214 m

39 daN

390 N

0.101 MPa ___ 2.424

MPa

Sample No:07 Clay stone Axial 62.36 mm

0.0623 m

28.86 mm

0.0288 m

32 daN

320 N

0.0824

MPa

___ 1.977

MPa

Sample No:08 Lime

stone

Diametric 48.63 mm

0.0486 m

___ 1791 daN

17910 N

___ 7.588

MPa

220.05

MPa

Sample No:09 Lime

stone

Diametric 48.84 mm

0.0488 m

___ 542 daN

5420 N

___ 2.277

MPa

66.03

MPa

Sample No:10 China clay Axial 61.91 mm

0.0619 m

28.86 mm

0.0288 m

25 daN

250 N

0.0652

MPa

___ 1.564

MPa


0.0628 m

___ 47 daN

470 N

0.119 MPa ___ 2.856

MPa

Sample No:12 Lime

stone

Diametric 48.63 mm

0.0486 m

___ 2014 daN

20140 N

___ 8.533

MPa

247.45

MPa

3.5.1 Laboratory Experiments Table:

Table 3.5.1 : Readings Of Different Samples On Point Load

Strength

CHAPTER 04

RELATIONSHIP BETWEEN POINT LOAD INDEX AND THE

STRENGTH PARAMETERS OF COAL MEASURE ROCKS

4.1 INTRODUCTION

The point load test (PLT) is an accepted rock mechanics testing procedure used for the

calculation of a rock strength index. This index can be used to estimate other rock strength

parameters. The focus of this thesis is to present the data analysis used to correlate the point load

test index (Is50) with the uniaxial compressive strength (UCS), and to propose appropriate Is50 to

UCS conversion factors for different coal measure rocks. The rock strength determined by the

point load test (PLT), like the load frame strengths that they estimate, is an indication of intact

rock strength and not necessarily the strength of the rock mass.

4.2 UNIAXIAL COMPRESSIVE STRENGTH (UCS)

The UCS is undoubtedly the geotechnical property that is most often quoted in rock engineering

practice. It is widely understood as a rough index which gives a first approximation of the range

of issues that are likely to be encountered in a variety of engineering problems including roof

support, pillar design, and excavation technique (Hoek, 1977). For most coal mine design

problems, a reasonable approximation of the UCS is sufficient. This is due in part to the high

variability of UCS measurements. Moreover, the tests are expensive, primarily because of the

need to carefully prepare the specimens to ensure that their ends are perfectly parallel.

In rock mechanics and engineering geology the boundary between rock and soil is defined in

terms of the uniaxial compressive strength and not in terms of structure, texture or weathering.

Several classifications of the compressive strength of rocks have been presented, In this work a

material with the strength ≤ 0.25 MPa is considered as soil, refer to ISRM (1978) and figure 4.1.

Figure 4.1: Various Strength Classifications For Intact Rock (From Bieniawski, 1984)

The uniaxial compressive strength can be determined directly by uniaxial compressive strength

tests in the laboratory, or indirectly from point-load strength test (see Section 1.4.2). The tests

should be carried out according to the methods recommended by the ISRM (1972)

.

The classification of the uniaxial compressive strength suggested by ISRM is shown in Table

4.1.

---------------------------------------------------------------------------------------------

Soil c < 0.25 MPa

Extremely low strength c = 0.25 - 1 MPa

Very low strength c = 1 - 5 MPa

Low strength c = 5 - 25 MPa

Medium strength c = 25 - 50 MPa

High strength c = 50 - 100 MPa

Very high strength c = 100 - 250 MPa

Extremely high strength c > 250 MPa

---------------------------------------------------------------------------------------------

Table 4.1: Classification Of The Uniaxial Compressive Strength Of The Rocks ( C) From

ISRM (1978).

Figure 4.2: Universal Testing Machine

4.3 POINT LOAD TEST (PLT)

The PLT is an attractive alternative to the UCS because it can provide similar data at a lower

cost. The PLT has been used in geotechnical analysis for over thirty years (ISRM, 1985). The

PLT involves the compressing of a rock sample between conical steel platens until failure

occurs. The apparatus for this test consists of a rigid frame, two point load platens, a

hydraulically activated ram with pressure gauge and a device for measuring the distance between

the loading points. The pressure gauge should be of the type in which the failure pressure can be

recorded. A state of the art point load testing device with sophisticated pressure reading

Instrumentation is shown in Figure 4.3.

----------------------------------------------------------------------------------------------------

Term Bieniawski (1984) Deere (1966)

----------------------------------------------------------------------------------------------------

Very high strength Is > 8 MPa Is > 10 MPa

High Is = 4 - 8 MPa Is = 5 - 10 MPa

Medium Is = 2 - 4 MPa Is = 2.5 - 5 MPa

Low Is = 1 - 2 MPa Is = 1.25 - 2.5 MPa

Very low Is < 1 MPa Is < 1.25 MPa

----------------------------------------------------------------------------------------------------

Table 4.2: Classification Of The Point Load Strength Index (Is).

The International Society of Rock Mechanics (ISRM, 1985) has established the basic procedures

for testing and calculation of the point load strength index. There are three basic types of point

load tests: axial, diametrical, and block or lump. The axial and diametrical tests are conducted on

rock core samples. In the axial test, the core is loaded parallel to the longitudinal axis of the core,

and this test is most comparable to a UCS test.

The point load test allows the determination of the uncorrected point load strength index (Is). It

must be corrected to the standard equivalent diameter (De) of 50 mm. If the core being tested is

"near" 50 mm in diameter (like NX core), the correction is not necessary. The procedure for size

correction can be obtained graphically or mathematically as outlined by the ISRM procedures.

The value for the Is50 (in psi) is determined by the following equation.

Figure 4.3: Point Load Tester

4.4 POINT LOAD STRENGTH TEST MATHEMATICALLY

REPRESENTED AS BELOW

Is50 = P/De2……… (1)

P = Failure Load in lbf (pressure x piston area).

De = Equivalent core diameter (in).

As Hoek (1977) pointed out, the mechanics of the PLT actually causes the rock to fail in tension.

The PLT’s accuracy in predicting the UCS therefore depends on the ratio between the UCS and

the tensile strength. For most brittle rocks, the ratio is approximately 10. For soft mudstones and

claystones, however, the ratio may be closer to 5. This implies that PLT results might have to be

interpreted differently for the weakest rocks.

Early studies (Bieniawski, 1975; Broch and Franklin, 1972) were conducted on hard, strong

rocks, and found that relationship between UCS and the point load strength could be expressed

as:

UCS = (K) Is50 = 24 Is50……….. (2)

Where K is the "conversion factor." Subsequent studies found that K=24 was not as universal as

had been hoped, and that instead there appeared to be a broad range of conversion factors. Table

1 summarizes published results obtained for sedimentary rocks. Most of the estimates place the

conversion in a range between 16 and 24, with even lower values for some shales and

mudstones.

In studies comparing the PLT with the UCS, it is generally assumed the UCS test is the standard.

In reality, however, UCS tests provide an estimate of the “true” UCS of the rock. The accuracy

of the estimate depends on the natural scatter in the UCS test results (indicated by the standard

deviation (SD)) and the number of tests conducted (n). This relationship is captured by the

concept of the “Confidence Interval” (CI). For normally distributed data, the 95% CI of the mean

is expressed as:

CI95% = 1.96 SD/√n…………. (3)

Table 4.3: Published Comparisons Between The Point Load And Uniaxial Compressive

Strength Tests For Sedimentary Rock.

In general, the variability in the PLT-UCS relationship can be attributed to three sources:

1. Inaccuracy in the estimate of the true UCS obtained from UCS tests.

2. Inaccuracy in the estimate of the true PLT obtained from PLT tests.

3. Real differences between the two tests.

Many of the studies summarized in Table 4.2 compared a suite of point load tests to a single

UCS test. With such an experimental design, much of the scatter in the results might actually be

attributable to the inaccuracy of the UCS tests.

4.5 COMPRESSIVE STRENGTH DETERMINED FROM THE POINT

LOAD INDEX

The principle of the point load strength test is that a piece of rock is loaded between two

hardened steel points. Details on the measuring procedure are described by ISRM (1985), and the

method is further dealt with in several textbooks and papers (Lama and Vutukuri, 1978; Hoek

and Brown, 1980, among others).

Both Franklin (1970) and Bieniawski (1984) recommend the use of point load strength index (Is)

for rock strength testing. The reason is that Is can be determined in the field on specimens

without preparation, using simple portable equipment. Also Broch (1983) points out the great

advantage using the point load strength test as it does not require machined specimen. As long as

the influence of specimen size and shape are considered in the calculation of the strength index,

any piece of rock, whether the surface is smooth or rough, can in principle be tested. Although

tests on irregular specimens appear to be crude, Wittke & Louis (1969) have shown that the

results need be no less reproducible than those obtained in uniaxial compression.

4.5.1 RELATIONSHIP BETWEEN POINT LOAD STRENGTH INDEX (IS)

AND UNIAXIAL COMPRESSIVE STRENGTH (C)

Greminger (1982) relates the test results, Is50, to standard 50 mm thick samples; this has also

been applied by ISRM (1985) in a revised edition of the 'Suggested Method for determining

point load strength'. The point load strength test is a form of "indirect tensile" test, but is largely

irrelevant to its primary role in rock classification and strength as a tensile characterization

(ISRM, 1985). Is50 is approximately 0.80 times the uniaxial tensile or Brazilian tensile strength.

Also from the point load strength test a strength anisotropy index (Ia50) can be measured (Broch,

1983) from the maximum and minimum strengths obtained normal to, and parallel to the

weakness planes, respectively of the rock, i.e. bedding, foliation, cleavage, etc.

As shown that point load strength varies with the water content of the specimens. ISRM (1985)

mentions that the variations are particularly pronounced for water saturations below 25%. At

water saturation above 50% the strength is less influenced by small changes in water content, so

that tests in this water content range are recommended unless tests on dry rock are specifically

required.

In the following two tables, different rock types are tested on the Point load test as well as

uniaxial compression test.

Table 4.4 :Rocks Tested On Uniaxial Compression

Table 4.5: Rocks Tested On Point Load Index

4.6 CALCULATION OF UNIAXIAL COMPRESSIVE STRENGTH AND

UNIAXIAL TRAXIAL COMPRESSIVE STRENGTH

In laboratory experiments we are calculate the strength of rock core samples by using

three main parameters of the coal measure rock of different rock core samples in our

laboratory, which are the point load strength index (Is,Is50), Uni-axial compressive

strength and the Uni-axial tensile strength. Following mathematically proved experiments

are resolved by these parameters to find the strength of different rock core samples. There

are 4 types of rock core samples used in our laboratory for experiments of different

length and diameter (d) which are the lime stone, clay stone, silt stone and china clay, and

we calculate the strength of rock core specimen in Mega Pascal’s (MPa). In the last of

this chapter also the total readings of all rock core samples are also mention in table 4.6.

Sample no. 01



Is = 0.426 MPa

c = 24*Is

c = 24*0.426

c = 10.224 MPa

t = 2.28*Is

t = 2.28*0.426

t = 0.971 MPa

Sample no. 02



Is = 1.599 MPa

c = 24*Is

c = 24*1.599

c = 38.376 MPa

t = 2.28*Is

t = 2.28*1.599

t = 3.64 MPa

Sample no. 03

Rock type: Clay Stone


Is = 0.0656 MPa

c = 24*Is

c = 24*0.0656

c = 1.574 MPa

t = 2.28*Is

t = 2.28*0.0656

t = 0.149 MPa

Sample no. 04

Rock type: Silt Stone


Is = 0.115 MPa

c = 24*Is

c = 24*0.115

c = 2.76 MPa

t = 2.28*Is

t = 2.28*0.115

t = 0.262 MPa

Sample no. 05



Is = 0.145 MPa

c = 24*Is

c = 24*0.145

c = 3.48 MPa

t = 2.28*Is

t = 2.28*0.145

t = 0.330 MPa

Sample no. 06



Is = 0.101 MPa

c = 24*Is

c = 24*0.101

c = 2.424 MPa

t = 2.28*Is

t = 2.28*0.101

t = 0.230 MPa

Sample no. 07



Is = 0.0824 MPa

c = 24*Is

c = 24*0.0824

c = 1.977 MPa

t = 2.28*Is

t = 2.28*0.0824

t = 0.187 MPa

Sample no. 08

Rock type: Lime Stone


Is50 = 7.588 MPa

c = 29*Is50

c = 29*7.588

c = 220.05 MPa

t = 2.76*Is50

t = 2.76*7.588

t = 20.94 MPa

Sample no. 09


Loading type: Diametric type loading condition

Is50 = 2.277 MPa

c = 29*Is50

c = 29*2.277

c = 66.03 MPa

t = 2.76*Is50

t = 2.76*2.277

t = 6.28 MPa

Sample no. 10

Rock type: China Clay

Loading type: Axial type loading condition

Is = 0.0652 MPa

c = 24*Is

c = 24*0.0652

c = 1.564 MPa

t = 2.28*Is

t = 2.28*0.0652

t = 0.148 MPa

Sample no. 11



Is = 0.119 MPa

c = 24*Is

c = 24*0.119

c = 2.856 MPa

t = 2.28*Is

t = 2.28*0.119

t = 0.27 MPa

Sample no. 12



Is50 = 8.533 MPa

c = 29*Is50

c = 29*8.533

c = 247.45 MPa

t = 2.76*Is50

t = 2.76*8.533

t = 23.55 MPa

Sample Rock Loading Diameter(d) Length(L) Load: Is Is50 c t

Number type condition In meter In meter (daN)

And (N)(MPa) (MPa)

MPa Mpa

Sample No:01 Lime

stone

Axial 99.615 mm

0.09961 m

24.3 mm

0.0243 m

423 daN

4230 N

0.426 ___ 10.224

0.971

Sample No:02 Lime

stone

Diametric 99.745 mm

0.09974 m

___ 1590 daN

15900 N

1.599 ___ 38.376

3.64

Sample No:03 Clay

stone

Diametric 61.87 mm

0.0618 m

___ 25 daN

250 N

0.0656 ___ 1.574

0.149


0.0616 m

___ 44 daN

440 N

0.115 ___ 2.76

0.262


0.0620 m

___ 56 daN

560 N

0.145 ___ 3.48

0.330

Sample No:06 Clay

stone

Axial 62.26 mm

0.0622 m

21.48 mm

0.0214 m

39 daN

390 N

0.101 ___ 2.424

0.230

Sample No:07 Clay

stone

Axial 62.36 mm

0.0623 m

28.86 mm

0.0288 m

32 daN

320 N

0.0824 ___ 1.977

0.187

Sample No:08 Lime

stone

Diametric 48.63 mm

0.0486 m

___ 1791 daN

17910 N

___ 7.588 220.05

20.94

Sample No:09 Lime

stone

Diametric 48.84 mm

0.0488 m

___ 542 daN

5420 N

___ 2.277 66.03

6.28

Sample No:10 China

clay

Axial 61.91 mm

0.0619 m

28.86 mm

0.0288 m

25 daN

250 N

0.0652 ___ 1.564

0.148


0.0628 m

___ 47 daN

470 N

0.119 ___ 2.856

0.27

Sample No:12 Lime

stone

Diametric 48.63 mm

0.0486 m

___ 2014 daN

20140 N

___ 8.533

MPa

247.45

23.55

Table 4.6: Calculate The Strength Of All Core Rock Samples In Three Rock Strength

Parameters (Is, C, T ) Which Are Performed In Laboratory Experiments Of Point Load

Strength Index.

Co Relation of Pont Load With Uni Axial Compressive Strength

Documents

Transcript of Co Relation of Pont Load With Uni Axial Compressive Strength