Obtaining the Physical Properties of Soft Rocks

8/12/2019 Obtaining the Physical Properties of Soft Rocks

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Obtaining the Physical Properties of Soft Rocks

D. Wanatowski, A.M. Marshall, L. R. Stace, Y. Geng and R Aslam.

Nottingham Centre for Geomechanics, University of Nottingham

1. Introduction

European coal mines experience more roof closure, floor heave and side squeeze in roadways

than most other coal mines from around the world due to the necessity to mine at great depths

and also the low material strength properties of the strata in which mining takes place. Not

only is there a high stress environment, but the weak strata surrounding the coal seams

become increasingly vulnerable to degradation over time, especially when they are exposed

to weathering by air and water. The consequences of this are the high costs of providing

adequate support and significant increases in the risk and uncertainty of mining.

Between 2010 and 2013, a research project entitled Geomechanics and Control of Soft MineFloors and Sides (acronym GEOSOFT) funded by the EU Research Fund for Coal and Steel

(RFCS) has been concentrating upon mitigating the effects of mining within soft rock strata.

The research partners have come from the UK, Spain and Poland and include industry

representatives, consultants and research bodies.

As a partner on this project, the University of Nottingham took the task of examining ways in

which the physical properties of soft coal measure rocks could be quantified, especially when

conventional rock testing methods were not possible due to the friability of the material

samples obtained. These physical properties are required as input data for numerical

modelling, allowing more accurate assessments to be made of the potential mine opening size

and shape and the support elements that need to be used within them to achieve stability overthe time period in which active mining takes place.

The paper examines the alternative strategies for testing available when Uniaxial

Compressive Strength (UCS) and triaxial testing of intact weak strata samples to obtain

strength and stiffness parameters were not possible. These include:

UCS and triaxial testing of reconstructed weak strata samples

Assessment of the strength properties of graded, crushed materials using soil mechanics

testing equipment such as direct shear and triaxial test rigs.

Large scale triaxial soil mechanics type tests performed on larger sizes of graded

particulate Coal Measure rock.

Index testing (point load and cone indenter) of the weakest samples to obtain UCS values.

The work progressed to examine the effects that time dependency might play on the

behaviour of soft coal measure rock types.

2. Investigation of Effects of Sample Reconstruction on Tested Rock Strength

The strength of the intact rock block material governs, partially, the strength of a rock mass.

Standard determination of the intact rock strength is by means of the UCS Test and Triaxial

Single Stage Compressive Strength Test. Nottingham tends to core its test samples to a 37mm

diameter by 75mm long specimen. However, triaxial tests are also conducted on larger

diameter samples (42mm and 50mm), also with a 2:1 aspect ratio where this is possible,

which is a problem with softer rocks (ISRM 1981).


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One technique that has been considered to enable a range of normal rock mechanics tests to

be conducted on weak or soft rock materials is the testing of reconstructed broken cores. This

can be applied where conventional cores of suitable size for uniaxial or triaxial testing cannot

be obtained either from cores drilled in the mine or from cores drilled in the laboratory where

wet drilling is applied. In both these circumstances, cracking of the cores along stratified lines

is quite common in softer materials preventing 2:1 aspect ratio samples being obtained. Thisfracturing is normally perpendicular to the sample axis. The technique consists of

reconstructing the core by gluing the cored pieces together using a cyanoacrilite adhesive

(better known as Superglue).

A series of tests were conducted to assess the effect of reconstruction using adhesives on

cores of the same material that were both intact and reconstructed (glued), see Figure 1.

These included uniaxial compressive and tri-axial tests. Initially, cores were obtained from

adjacent boreholes drilled into the roof measures of a UK coal mine. Although the material

chosen was quite a strong silty mudstone (which did not qualify as a soft rock) it was chosen

because of its consistency but also because of the bedding planes being perpendicular to the

core axis. Samples of rock including those that were intact and those that had broken and thenglued were tested.

Figure 1 Example of (a) an intact sample and (b) a reconstructed (glued) sample

Figure 2 shows an example of UCS test data obtained on samples of intact and reconstructed

rock (note that the reconstructed samples were all glued along bedding planes perpendicular

to the axis of loading). The results in this graph which are illustrative of a much larger

number of tests indicate that the intact samples have a slightly higher UCS than the glued

samples and that there is more variability in the strength of the glued samples compared to

the intact samples. Figure 3 shows results of triaxial single stage compressive strength tests

on intact and glued samples. This example of test data was from triaxial tests at a confining

stress of 5 MPa. The strength and stiffness of broken samples appears to be slightly lower

than for the intact samples, however the spread of results may be within the normal

variability of results from these tests.


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Figure 2 UCS test data comparing intact samples and glued samples

Load(kN)

Figure 3 Single stage triaxial test data comparing intact samples and glued samples

Reconstructed samples of a much weaker material, coal, were also selected to conduct UCS

and triaxial tests. Some comparison results of the UCS tests for both intact and glued samples

are shown in Figure 4. It can be seen that the strength obtained for the intact sample falls

within the range of strength results from the reconstructed samples. Figure 5 shows a glued

sample after a triaxial test; it can be seen that the glued plane remained intact after testing and

that failure occurred elsewhere in the specimen. These test results suggest that samples glued

in a direction horizontal to the loading axis do not have a significant effect on the assessment

of sample strength by UCS testing.


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Overall results obtained by this approach indicate that if the supply of a particular material is

limited or if it is very difficult to obtain an intact sample, samples glued in the direction

perpendicular to the axis of loading can be used instead of the intact samples to obtain some

reasonable properties for the material. The test data do, however, show considerable

variation and a full statistical analysis of the data is required before drawing definitive

conclusions.

I

G

G

G

G

Axial displacement (mm)

I = Intact sample

G = Glued sample

0

10

20

30

40

50

60

70

80

0.0 0.5 1.0 1.5 2.0 2.5

Figure 4 UCS results comparison between intact and glued samples.

Figure 5 Glued sample (a) before and (b) after triaxial testing.

3. Testing using Soil Mechanics Techniques

The next step in this research was to take broken samples of soft coal measure rock types for

which it was not possible to use reconstruction techniques. These were crushed and sized, and

tested using a similar methodology to that used when soils are tested in the Geotechnics

laboratory using standard soil mechanics testing apparatus. Tests conducted were direct shear

tests and triaxial tests with a range of particle sizes.


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3.1 Direct Shear Testing

The direct shear (shear box) test was used to investigate the properties of broken materials. A

square container with plan dimensions of 100100 mm and 30mm height was used. A

specimen of particles placed in a horizontally split box was sheared by moving the bottom

half of the box relative to the top half at a constant rate of approximately 0.5mm/min. Avertical normal stress was applied during shearing and both the vertical displacement of the

top of the specimen and the shear displacement were measured. Failure was prescribed on the

plane separating the two halves of the box. Four different normal stresses (10, 25, 50, and

100 kPa) were applied for each sample consisting of different particle sizes and three tests

were carried out for each normal stress. Mohr-Coulomb properties (Cohesion cand friction

angle) were obtained from the normal stress-shear stress curves. All tests were performed

on dry samples.

Initially, coal was used for this study and five ranges of particle sizes (6.3-10mm, 5-6.3mm,

3.35-5mm, 2-3.35mm, 1.18-2mm) were obtained. Since these samples consisted of graded

small particles, a particle size effect on the sample shear strength was demonstrated. The sizeof the direct shear box is also expected to have had an effect on test results given that a shear

band of approximately 10 times the average grain size is expected to develop (Roscoe 1970).

ASTM specifies that the thickness of the direct shear specimen should be six times the

maximum particle diameter and that the container width should me a minimum of 10 times

the maximum particle diameter. This requirement is not met for all of the particle size

samples. Large-cell Triaxial testing (described in the subsequent section) was therefore

undertaken on some of the larger particle size samples.

Figure 6 plots the data of shear stress versus horizontal shear displacement from the direct

shear tests for all sample sizes. As expected, the tests show that the maximum shear force

increases with an increase of the normal force for the samples of each of the particle sizes. Itappears from the data that the effect of particle size was greatest for the tests with the higher

normal force applied (50 kPa and especially 100 kPa). Figure 7a shows the maximum value

of shear stress (max) versus the normal stress (n) for all of the data shown in Figure 6.

Similar to Figure 6, the data shows more variation at higher normal stress. Figure 7b plots the

maximum friction angle (max) based on the maximum shear stress data plotted in Figure 7a.

These data were calculated using the Mohr-Coulomb yield criterion max= c+ n tan(max)

and assuming that the cohesion intercept c= 0. Figure 7b shows that the larger particle sizes

resulted in higher estimates of friction angle compared to the smaller samples. However, as

mentioned previously, the larger particle size results would have been affected by the

relatively small size of the direct shear container so these results may not be entirely

representative.

Seatearth materials from a UK coal mine which were difficult to core to obtain intact samples

were also used to conduct shear box tests. As had previously been found when testing coal,

particle size affected the values of shear strength when normal stress was high.


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Figure 6 Direct shear test results for coal: load versus horizontal displacement for all particle

sizes

0

20

40

60

80

100

120

0 20 40 60 80 100Normal stress (kPa)

Maximumshearstress(kPa)

39

40

41

42

43

44

45

46

47

1.18-2 2-3.35 3.35-5 5-6.3 6.3-10Particle size range (mm)

Frictionangle(degrees)

1.18-2mm2-3.35mm

3.35-5mm

5-6.3mm

6.3-10mm

Friction angle calculated based

on assumption that c= 0

(a) (b)

Figure 7 Direct shear results for coal: (a) maximum shear stress versus normal stress and (b)

maximum friction angle for all particle sizes

The shear box test was also used for intact, reconstructed (glued) and differing broken

particle size samples of weak soft coal measure clay, as shown in Figure 8. This represented

an opportunity to compare test results for different sample types of the same material. Direct

shear tests were conducted at a normal stress of 35 and 105 kPa. The Mohr Coulomb strength

parameters of friction angle and cohesion were calculated based on the maximum and

ultimate shear stresses for each sample, as shown in Table 1. The reconstructed samples were

spot glued at several places and sheared along the pre-existing failure plane. The


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maximum strength parameters determined therefore relate to the strength of the glue and

not the rock itself and have been omitted from Table 1. The intact samples show a peak

strength at small displacements, followed by a sharp reduction and then a gradual trend

towards an ultimate value. The particulate samples show a gradual increase in strength

(strain-hardening) with shear displacement. As with critical states in soil mechanics, the

ultimate strength (at high values of shear displacement) is a function of the nature of theparticular rock material and it is therefore not surprising that the data from all tests appears to

converge at higher displacements (unfortunately the larger displacement data for the intact

and glued samples was not obtained and the ultimate state friction angle shows considerable

variation between test samples). The inherent bond strength of the intact samples results in

higher values of maximum shear stress cohesion and friction angle than the particulate

samples and makes a direct link between Mohr-Coulomb parameters from intact and

particulate samples difficult.

Table 1 Comparison of Mohr-Coulomb properties from direct shear testing of intact,

reconstructed and particulate samples of coal measure clay

Maximum shear stress Ultimate shear stress

Sample type Friction angle

(degrees)

Cohesion

(kPa)

Friction angle

(degrees)

Cohesion

(kPa)

Intact 65 219 69 34Reconstructed N/A N/A 50 452 3.35mm particle size 28 42 46 211 - 2 mm particle size 29 31 33 27

.

ShearStress(kPa)

ShearStress(kPa)

Figure 8Direct shear testing of intact, reconstructed, and particulate samples of soft coal measure

clay


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Therefore, the results obtained from Direct shear testing are difficult to compare with those

obtained from intact samples, not only because so little intact material can be available for

testing with softer rocks but largely because of the considerable differential between intact

bonded rock and crushed material. However, the values obtained from Direct shear testing of

particulate samples do have application when considering the behaviour of critical state

fractured soft rocks in a high stress environment.

3.2 Triaxial Testing

One problem with Direct shear box testing mentioned in the previous section was a limitation

on the use of larger particle graded samples. In order to investigate the effect of larger

particle sizes that could not be tested in the shear box, triaxial tests of particulate samples

using soils triaxial equipment were carried out. Triaxial testing is a common laboratory test

used for obtaining strength parameters for soils in Civil Engineering. This research project

proposed this method to obtain strength properties for the crushed and graded rock samples.

The principle of the triaxial test involves applying a confining stress (pressure) 3, around a

cylindrical test sample while the axial stress, 1, or axial displacement (strain) is controlled.

The sample is enclosed in a suitable rubber membrane and its diameter is dictated by the

original core diameter, which for these tests was approximately 40mm. From the triaxial test

results, it is possible to deduce the shear strength parameters, namely friction angle, , and

cohesion, c. For the results presented below, which is for broken granular material, the

cohesion intercept was taken as zero. In addition, the friction angles reported are the peak

friction angles. All tests were performed on dry samples.

Triaxial tests on coal with particle ranges 3.35-5mm, 6.3-10mm were carried out to compare

results with those obtained from the shear box tests. The comparison of results between shear

box and triaxial tests is shown in Table 2, based on maximum shear stresses recorded during

tests. The values of friction angle from the shear box test are noted to be higher than those

from the standard triaxial tests. The range of normal stress in the direct shear tests was,

however, less than the confining stress in the triaxial tests. This may explain some of the

difference between the values obtained from the two tests (a lower confining stress will result

in a higher value of friction angle based on maximum shear stresses).

Tests of samples with an even larger particle size (10-40mm) were conducted using a large

diameter triaxial cell (Figure 9). As the name suggests, the large triaxial test apparatus, which

was designed for testing railway ballast, is significantly larger than a conventional triaxial

apparatus and can accommodate cylindrical samples of 300mm diameter and 450mm height.

An issue with the large scale test is that the amount of material required is considerable.

Therefore, the standard testing approach was thought a good option to conduct triaxial testsas the material supply required is normally limited.

The value of maximum friction angle obtained from the large triaxial test was similar to that

from the standard triaxial cell, as shown in Table 2. The stiffness of the larger particle sample

was, however, less than the smaller particle samples, as shown in Figure 10.


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Table 2 Comparison of results from triaxial and shear box tests on broken coal

Test Particle size

range (mm)

Normal/Confining

stress range (kPa)

Friction angle

(degrees)

Shear box 3.35 5 10-100 45.8

Shear box 6.3 10 10-100 45.1Standard Triaxial 3.35 5 100-200 34.6Standard Triaxial 6.3 10 100-200 36.1Large Triaxial 10 40 100-200 33.9

Note: all values obtained assuming c= 0.

Figure 9 Large diameter triaxial testing apparatus

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.05 0.1 0.15 0.2

axial strain (%)

3.35-5mm

Standard

6.5-10mm

Standard

10-40mm

Large

q = '- '

p' = ( '+ 2 ')/3

Fig 10 Comparison of results for triaxial testing with differing particle sizes.

4. Index testing using Point Load and Cone Indenter apparatus

Index testing of small rock samples, both prepared and unprepared in the field and in the

laboratory, has been regarded as a simple way for obtaining a value for uniaxial compressive

strength. The researchers turned to these techniques as they seemed appropriate for the

particular problem of testing soft rocks. Two pieces of equipment, the Point Load Tester andthe NCB Cone Indenter were used (NCB 1977, Taylor and Spears 1981). The Point Load


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Tester was developed for measuring indirect compressive strength of rock specimens in the

field. A Point Load Strength Index I(50) is obtained and can be used for rock strength

classification. Samples of soft mudstone were selected for this test. The NCB Cone Indenter

is a portable static hardness tester developed by the UK National Coal Board (NCB) to

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

indentation in a small specimen of rock. A correlations between the standard cone indenternumber, Is, and uniaxial compressive strength can then be determined. Mudstone samples

from two adjacent roof cores from the same UK mine were tested using the point load

apparatus and cone indenter and the results compared with intact UCS tests (Table 3).

Table 3 Comparison of UCS values given by intact sample testing, cone indenter and

point load test for a coal measure mudstone

Test Number of

Samples tested

Average, UCS

(MPa)

Standard

Deviation, StDev

(MPa)

Coefficient of

Variation (=

StDev / Avg)

UCS test 6 81 6 0.08

Cone indenter 29 74 21 0.28

Point load 13 91 57 0.63

It can be seen that there was a reasonable level of agreement between the UCS sample results

and those of the cone indenter tests. There was an increased variability in the cone indenter

results but this could also have been due to the additional number of samples included within

the data set. It is also important to consider that samples tested intact would have been the

strongest samples in the core as they had not broken up during drilling whereas the lowervalue for the cone indenter tests could be due to the potentially weaker sample material

tested. The point load tests gave a slightly higher UCS value with a considerably increased

variability over those of the other two test methods and seems on examination to be a less

reliable test method.

5. Time Dependency Testing (Creep)

Although coal measure rock types are not normally thought of as demonstrating time

dependent (creep) properties, it is known that some roadways driven in soft coal measure

rocks do demonstrate increasing and continuous closure despite there being no apparent

changes to stress conditions in their vicinity once driven. As a result, attempts were madeusing a variety of techniques to measure whether time dependent properties could be

identified in both intact and broken samples.

In these tests the objective was to choose an applied stress that was a significant proportion of

the UCS, normally approximately 75%. This was not easy to achieve as there were limited

intact samples of weaker material and the need to identify the UCS meant breaking one of

these samples, reducing further the availability of samples for creep testing. The authors also

found that the value of UCS recorded was not necessarily consistent across the batch,

meaning that when the applied load was increased to 75% of UCS, premature failure of the

sample could occur at the commencement of the test.

Several intact mudstone samples tested over a period of 60 days showed axial strains of lessthan 0.5%. It was therefore concluded that the degree of creep occurring in the intact sample


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was insignificant. Tests were then concentrated on particulate material. For soil like

samples (particulate material) the Hollow Cylinder Apparatus, HCA, (being used as a triaxial

cell) was used to test the material strength and creep properties as this apparatus is able to test

larger sample sizes and also apply higher loadings. The crushed/broken samples required a

sample diameter of at least 100mm and were tested with a sample height of 200mm. Figure

11 shows data collected from the tests of a crushed particle sample of mudstone. The strainsrecorded at the end of the test (at time = 66 hr or just under 3 days) is far in excess of that

experienced with intact samples of similar material.

A similar test was conducted on a sample of crushed coal and it yielded a similar profile of

increasing displacement with fixed load but yielding higher strains than those experienced

with the crushed mudstone (Figure 11). This profile would suggest a degree of time

dependency which when translated to the sides of a roadway driven largely or wholly in seam

and in which the high loading had fractured the coal immediately adjacent to the roadway,

may offer an explanation for continued closure of that roadway under relatively constant

strata loading.

Figure 11 Time dependent behaviour of particulate coal measure mudstone and crushed coal

6. Conclusions.

This paper has discussed some of the testing programmes that were conducted at theUniversity of Nottingham as part of the RFCS GEOSOFT project. It explains the difficulty in

obtaining test data due to problems associated with obtaining samples of soft coal measure

material for conventional rock mechanics tests. The alternative approach has been either to

reconstruct samples when they fracture along bedding planes, or to crush and grade the

material and to use testing equipment and techniques taken from soil mechanics. Data

obtained from these alternative approaches have been compared wherever possible across

differing techniques and a better understanding of the value of such data has been obtained.

Finally the researchers examined the possibility that coal measure materials might exhibit

time dependent behaviour. What was clear from the preliminary results of this testing is that

whilst intact sedimentary rocks such as mudstone appear not to have any time dependency,

crushed coal appears to have such properties. This may go some way to explaining the


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mechanisms that see some roadways closing over the longer term even though external strata

loading is not apparently changing.

References

ISRM (1981). Rock characterisation, testing and monitoring ISRM Suggested Methods

Editor E T Brown

NCB (1977) NCB Cone Indenter MRDE Handbook No.5.

Roscoe, K.H. (1970). The Influence of Strains in Soil Mechanics. Geotechnique, 20(2), 129-

170.

Taylor, R. K. and Spears, D. A. Laboratory investigation of mudrocks Q. J. Eng. Geology

vol. 14 (1981), pp 291 - 309

Obtaining the Physical Properties of Soft Rocks

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