Peng S., Zhang J., Engineering Geology for Underground Rocks

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5.3 Chemical components and mechanical properties of mudstones 115

100

150

200

250

300

350

01020304050

Confining pressure (MPa)

Maximum stress (MPa)

Sandstone

Sandy mudstone

Mudstone

Coal

Fig. 5.10. Experimental results showing that clastic rocks fit the linear Mohr-

Coulomb strength criterion in terms of the maximum and minimum principal

stresses for fine- and medium-grained sandstone, sandy mudstone, mudstone, and

coal obtained from the triaxial compression tests. Note that the symbols are the

experimental results and the lines represent linear regressions for the different

rocks.

5.3 Chemical components and mechanical properties of

mudstones

5.3.1 Mineral and chemical components

Since many problems in rock engineering are caused by shales or mud-

stones, the experiments of mineral composition, chemical components and

mechanical properties of mudstone in mudstones are particularly impor-

tant. Excavation stability in soft rocks such as mudstone and shale has

been plaguing the coal mines in the Huainan coalfield, China. The mud-

stone samples taken in the roof and floor strata of coal seams in the

Huainan coalfield are analyzed and tested. The analyzed results using X-

ray diffraction show that the main mineral components of mudstone in this

studied area are clay minerals. The secondary minerals are quartz and side-

rite. There are also some potassium feldspars in sandy mudstones. In the

116 5 Sedimentary rock masses and discontinuities

mudstone, clay minerals occupy 50-90%, with an average of 69.3%.

Among these clay minerals, kaolinite is the essential component, while il-

lite and chlorite are the secondary components. Quartz in the mudstone oc-

cupies 10-30%, with an average of 21.5%.

From the quantitative analyses using the fluorescence spectrum, the

chemical constitution of the mudstone has the following characteristics

(Peng and Meng 2002):

1. The main chemical constitution is SiO

, and the secondary is Al

In sandy mudstone the content of SiO

is up to 65.03%.

2. The contents of ferric oxide (especially Fe

) in granophyric mud-

stone and light gray mudstone are relatively high. The contents of

in dark and carbonaceous mudstone and dark gray mudstone

are low. Hence the former two mudstones were formed in the oxida-

tion environments, and the latter two were in the deoxidization envi-

ronments.

3. The ignition loss in the immediate roof of coal seam # 13 is rela-

tively high. The main ignited substances include carbon, sulfur, and

other organic substances.

5.3.2 Chemical component effect on mechanical properties

There is a close relationship between the mechanical properties of mud-

stone and its chemical composition, particularly the content of SiO

and

ignition loss. Figures 5.11 and 5.12 present the uniaxial compressive

strength and elastic modulus and the content of SiO

relationships in a

mudstone sample (Peng and Meng 2002).

It can be seen from Figs. 5.11 and 5.12 that the uniaxial compressive

strength and elastic modulus of mudstone increase as the content of SiO

increases. When the content of SiO

is less than 40%, the compressive

strength and elastic modulus of mudstone increase slightly. However,

when the content of SiO

is greater than 40%, the mechanical strength and

elastic modulus increase significantly as the content of SiO

increases. This

is because SiO

has a very high uniaxial compressive strength and elastic

modulus. The mechanical properties and the contents of SiO

have the fol-

lowing exponential relations:

eUCS

025.0

85.6

(5.4)

048.0

873.0

(5.5)

where UCS is the uniaxial compressive strength (MPa); S is the content of

SiO

(%); E is the elastic modulus (GPa).

5.3 Chemical components and mechanical properties of mudstones 117

020406080

Content of SiO2 (%)

Compressive strength (MPa)

Fig. 5.11. Uniaxial compressive strength versus the content of SiO

in the mud-

stone.

0 20406080

Content of SiO2 (%)

Elastic modulus (GPa)

Fig. 5.12. Elastic modulus versus the content of SiO

in the mudstone.

Figures 5.13 and 5.14 plot the uniaxial compressive strength and elastic

modulus versus the ignition loss obtained by calculating the loss of mass

after heating the mudstone to 1000

C. It can be found that the compressive

strength and elastic modulus decrease as the ignition loss increases. When

the ignition loss is greater than 10%, the compressive strength or elastic

modulus of mudstone decreases sharply and then tends to have a very low

value. This indicates that the mudstone with a high content of carbon, such

as carbonaceous mudstone, has much lower strength and elastic modulus.

118 5 Sedimentary rock masses and discontinuities

The mechanical properties and the ignition loss have the following nega-

tive power relations:

73.0

18.180



IUCS

(5.6)

45.1

65.491



(5.7)

where I is the ignition loss (%).

01020304050

Ignition loss(%)

Compressive strength (MPa)

Fig. 5.13. Uniaxial compressive strength versus the ignition loss in the mudstone.

0 10203040

Ignition loss (%)

Elastic modulus (GPa)

Fig. 5.14. Elastic modulus of mudstone versus the ignition loss in the mudstone.

5.4 Mechanical behaviors of discontinuities 119

5.4 Mechanical behaviors of discontinuities

Bedding is one of the most common geologic phenomena in sedimentary

rocks. Generally a bedding plane is a weak plane, compared to rock

blocks, it has prominently lower strength and higher compressibility (Peng

1998). Therefore, the bedding plane has much lower compression resis-

tance, shear resistance, and tension resistance. In rock engineering, the

rock is likely to fail in a bedding plane.

There are two basic parameters to describe rock bedding/discontinuity

mechanical behaviors, normal and shear stiffnesses.

The normal stiffness of a rock is defined as the slope in the normal

stress and deformation curve when a discontinuity is loaded under a nor-

mal stress perpendicular to the direction of the discontinuity, i.e.:

(5.8)

where k

is the normal stiffness of the discontinuity; ı

is the normal stress

applied perpendicularly to the discontinuity; u is the normal displacement

of the discontinuity.

When a normal compressive stress exerts on a discontinuity, the normal

stress and discontinuity displacement have a hyperbolic relation (Goodman

1976, Hudson and Harrison 1997):



(5.9)

where u is the normal displacement of the discontinuity; ı

is the normal

compressive stress; and c and d are the experimental constants.

Brandis et al. (1983, 1985) proposed an empirical model for an inter-

locked joint:

Vbc

'

(5.10)

where c, b are constants, and c = 1/k

, c/d = V

; k

is the initial stiffness;

is the maximum closure of the joint; ǻV

is the joint closure; ǻı is the

normal compressive stress; and

JCS

CJRCBAV )()(



(5.11)

120 5 Sedimentary rock masses and discontinuities

15.775.1)(02.0

 JRC

JCS

(5.12)

where A, B, C, and D are constants determined from cyclic loading tests;

JRC is the joint roughness coefficient; JCS is the joint wall compression

strength, and a

is the initial mechanical aperture or initial true aperture.

For a dislocated joint, the normal stress and displacement has the fol-

lowing relationship:

jnin

VMk '



log

(5.13)

where M is a constant.

Zhang et al. (1999) proposed the following exponential relationship for

normal stress and fracture displacement based on the concrete block tests:

)1(

ebu



(5.14)

where b

is the initial fracture aperture; a is the experimental constant.

The shear stress and displacement characteristic of a rough discontinuity

depend on the normal stress and the discontinuity surface characteristics,

because the shear deformation involves dilation and fracture asperities. A

hyperbolic function is frequently used to describe the shear-displacement

relationship for a discontinuity in the pre-peak stress regime:



(5.15)

where v is the shear displacement of the discontinuity; Ĳ is the shear stress;

and c and d are the constants.

The shear stiffness is the slope in the shear stress and shear displacement

curve and can be expressed as:

(5.16)

Table 5.5 presents the mechanical properties of some interfaces of sedi-

mentary rocks in China (Yu 1983). It shows that the shear stiffness in the

discontinuity is much smaller than the normal stiffness. In other words, the

discontinuity is much weaker to resist shear stress.

5.5 Mechanical behaviors of rock masses 121

Table 5.5. Mechanical properties of the interfaces in some rocks and coal seam

Rocks above

and beneath the

interface

Shear stiffness

(MPa/m)

Normal stiff-

ness

(MPa/m)

Cohesion

c (MPa)

Internal fric-

tion angle

(q)

Limestone and

coal seam

24.5 981 0.2 30

Coal seam and

shale

14.7 588 0.1 30

Shale and sand-

stone

14.7 588 0.1 30

5.5 Mechanical behaviors of rock masses

Research shows that the deformation behaviors of a rock matrix and a rock

mass are different, For a simple case, if a set of structure planes (such as

fractures or bedding planes) are parallel and equally spaced in the rock

mass, the Young’s moduli of rock mass and rock matrix can be expressed

as:

skEE

nrm

111



(5.17)

where E

is the Young’s modulus of the rock mass; E

is the Young’s

modulus of the rock matrix; k

is the normal stiffness of the structure

plane; and s is the spacing of structure plane.

The existence of structure planes in a rock mass makes its compres-

sive strength far less than that of the rock matrix. The statistical results

from 16 worldwide rock engineering projects show that for the intact rock

mass (RQD  90%) the ratio of the uniaxial compressive strength of the

rock mass to rock matrix ranges from 0.16 to 0.82, and the average ratio is

0.47. For blocky and fissured rocks, the ratio of uniaxial compressive

strength of the rock mass to rock matrix is lower, ranging from 0.16 to

0.48 with an average of 0.32.

The existence of bedding or other joints also makes the rock mass

anisotropic in both mechanical properties and behaviors, for example rock

mass strength and deformation are different in the parallel and perpendicu-

lar directions of bedding. Lab results show that the bedded rock specimens

have different mechanical properties when they are loaded in different di-

rections. Figures 5.15 and 5.16 show that the uniaxial compressive strength

(UCS) and cohesion are higher when the loading direction is perpendicular

to the bedding plane.

122 5 Sedimentary rock masses and discontinuities

100

150

200

024681012

Grain diameter (Phi)

UCS (MPa)

Parallel

Perpendicular

Fig. 5.15. Uniaxial compressive strength versus grain size (phi) for loading paral-

lel and perpendicular to the bedding plane.

024681012

Grain diameter (Phi)

Cohesion (MPa)

Parallel

Perpendicular

Fig. 5.16. Cohesion versus grain size (phi) for loading parallel and perpendicular

to the bedding plane.

The Young’s modulus, however, is lower when the loading direction is

perpendicular to the bedding plane (Fig. 5.17). This is because the rock

mass has a larger displacement in the bedding plane. Table 5.6 presents

some experimental results of rock uniaxial compressive strength, cohesion,

and Young’s modulus for loading parallel and perpendicular to the bedding

plane in China’s eastern coal measures. It shows that the rock compressive

strength is much lower when loading parallel to bedding direction, and that

the ratio varies from 0.42 to 0.87.

5.5 Mechanical behaviors of rock masses 123

Table 5.7 lists experimental results of bedding effects on rock properties

in China’s coal measures. It shows that bedding plane and loading direc-

tion have impacts on rock tensile strength, sonic compressional velocity,

and dynamic Young’s modulus.

024681012

Grain diameter (Phi)

Young's modulus (GPa)

Parallel

Perpendicular

Fig. 5.17. Young’s modulus versus grain size (phi) for loading parallel and per-

pendicular to the bedding plane.

Table 5.6. Ratios of rock strength and Young’s modulus for loading parallel and

perpendicular to the bedding plane

Properties Ratio of parallel to and perpendicular to bedding plane

UCS 0.42 – 0.87

Cohesion 0.57 – 0.96

Young’s modulus 1.27 – 2.45

Table 5.7. Rock properties and loading directions of parallel and perpendicular to

the bedding plane

Tensile strength

(MPa)

Compressional

velocity (m/s)

Dynamic Young’s

modulus (GPa)

Rocks

Parallel Perpen-

dicular

Parallel Perpen-

dicular

Parallel Perpen-

dicular

Medium-

grained

sandstone

7.55 5.1 4834 4197

Fine-

grained

sandstone

10.67 6.66 5116 4487

Mudstone 0.86-2.1 2167 1696 10.90 6.68

Coal seam 2151 1790 6.35 4.21

124 5 Sedimentary rock masses and discontinuities

5.6 Water impact on sedimentary rock strength

Natural rocks have experienced many tectonic activities and have been ex-

tensively stressed and deformed. Consequently, they are naturally fractured

porous media. Most of oil reservoirs and some aquifers fall into this cate-

gory. These rocks usually contain water or other fluid in the rock pores

and fractures. When rock engineering is constructed in such media, the wa-

ter or fluid effect needs to be considered. Figure 5.18 illustrates the impor-

tance of water pressure on rock strength and rock failure. The figure shows

that if the Mohr-Coulomb strength envelope is assumed to stay unchanged,

then without water effects, the rock is stable. However, the Mohr circle

moves toward the opposite direction of the x-axis due to water pressure. As

a result, the circle exceeds the strength envelope, causing rock failure.

Therefore, in rock engineering design and practice the fluid effect is not

negligible.

-pV

-p

MVW

tan c

Fig. 5.18. Mohr circles with and without water pressure effects, assuming Biot’s

constant of 1.

Zhang et al. (1997) proposed that rock strength reduces due to water

pressure, and rock strengths in dry rock and in rock with a pore pressure of

p have the following relationship (Zhang et al. 1997):

sin1

sin2



UCSUCS

(5.18)

where UCS

is the uniaxial compressive strength in the rock with pore

pressure of p; Į is the Biot’s constant; ĳ is the internal friction angle.