THE EFFECT OF WATER CONTENT ON THE MECHANICAL
BEHAVIOUR OF FINE-GRAINED SEDIMENTARY ROCKS
G. R. Lashkaripour
Assistant professor, Department of Geology, University of Sistan and Baluchestan, Zahedan,
Iran.
R. Ajalloeian
Assistant professor, Department of Geology, University of Esfahan, Esfahan, Iran.
ABSTRACT
To investigate the effect of water content on the mechanical behavior of fine-grained
sedimentary rocks three different types of shales; clayshale, mudshale and mudstone, from
different locations were studied. The samples were tested at different water contents ranging
from oven-dried to saturated condition. Preparing samples of standard size for testing was a
troublesome task. Many samples break during coring, cutting, grinding and storing because of
disintegration of these rocks when subject to change in water content. This nondurable
behavior of these rocks is responsible for numerous slope instability problems, underground
excavation problems and embankment failures.
Water content has been demonstrated to have a marked influence on the strength and
deformation properties. The uniaxial compressive strength and modulus of elasticity decrease
significantly as the water content increases. The results show a reduction of more than 90% in
uniaxial compressive strength from oven-dried to saturated condition. This strength reduction
due to moisture content for the rocks studied is significantly higher than the values reported
by other researchers. A general equation was developed for this type of fine-grained
sedimentary rocks that may be used for predicting uniaxial compressive strength from the
available information on water content.
Modulus of elasticity was found to be influenced by anisotropy, but the most significant
influence, regarding reduction of the modulus of elasticity, was the change from dry to
saturated condition. Test results show a significant reduction in the static modulus of
elasticity from oven-dried condition to saturated condition. The results of these tests show an
average reduction in modulus of elasticity, from that in dry state, of about 84% for the shales
studied.
INTRODUCTION
1
Water content in fine-grained sedimentary rocks is an important consideration in many
engineering projects. It has been well established that water content of these rocks
significantly changes most aspects of physical and mechanical properties. Fine-grained
sedimentary rock in this study includes mudrock or shale. This type of rock is formed in both
marine and nonmarine environments and it is one of the most abundant sedimentary rocks.
Tucker (1991) reported that mudrock is the most abundant of all lithologies, constituting
some 45-55% of sedimentary rock sequences. Because of its abundance, mudrock is the most
frequently encountered rock type in engineering construction. King et al. (1994) reported that
shales are found in many large civil engineering structures in United Kingdom and elsewhere.
Moreover this rock is one of the most problematic materials for geotechnical and petroleum
engineers.
Many investigators have studied the behavior of fine-grained sedimentary rock under
different water content conditions. (Van Eeckhout, 1976; Bauer, 1984; Tandanand, 1985;
Steiger & Leung, 1990; Zhiyl & Jinfeng, 1993; Hsu & Nelson, 1993; Ghafoori, 1995;
Lashkaripour & Ghafoori, 1999). Analysis of their results has shown that water content of
these rocks significantly changes most aspects of physical and mechanical properties.
Van Eeckhout (1976) studied the effect of water content on the strength of different rocks.
He found a significant reduction in the strength of shale due to an increase in water content
from dry condition to saturated condition. He lists five processes of strength loss in shales due
to increased moisture content: fracture surface energy reduction, capillary tension decrease
(equivalent to the air-breakage of Terzaghi & Peck, 1967), pore pressure increase, frictional
reduction, and chemical deterioration.
Hsu and Nelson (1993) reported a strong correlation between compressive strength and
water content for Cretaceous clay shales of north America. Steiger and Leung (1990) reported
that, in shales, unconfined compressive strengths measured with dry samples can be 2 to 10
times higher than for wet samples. Colback and Wiid (1965) found that the compressive
strength of quartzitic shale under saturated condition was about 50% of that under dry
condition.
EXPERIMENTAL RESULTS AND DISCUSSION
To investigate the effect of moisture content on the mechanical behavior of fine-grained
sedimentary rock, mudrocks from different locations were tested. The main aim of these
series of tests was to establish a correlation between the results of uniaxial compression tests
and values of water content.
Preparing samples of standard size (NX=54mm) for testing was a troublesome task. These
samples were obtained from the blocks of mudrocks from fresh excavation. Many samples
break during coring, cutting, grinding and storing because of disintegration of this rock when
subject to change in water content and an inherent weakness of bedding planes. This causes
great difficulties in sample preparation. All specimen for this research were prepared in the
direction perpendicular to the laminations.
Effect of Water Content on the Strength
To study the effect of water content on the strength of mudrock, three types of mudrocks
were tested at different water contents varying from oven-dried to saturated condition. Table
1 shows the results of compression tests of mudrocks at different water content. This table
2
presents a reduction of more than 90% in compressive strength from oven-dried to saturated
condition. This strength reduction due to moisture content for the mudrocks studied is
significantly higher than the values reported by other researchers.
Table 1. Results of compression tests of mudrocks at different water contents.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Mudstone
W
(%)
0.025
0.05
0.12
0.98
1.03
1.11
1.11
1.12
1.35
1.75
2.12
2.14
2.22
2.39
4.61
5.77
6.23
UCS
(MPa)
92.039
93.987
81.998
61.293
62.428
54.161
47.581
53.032
39.672
38.660
37.517
38.633
32.199
29.044
11.199
6.513
5.970
Mudshale
W
(%)
0.05
0.75
0.80
0.96
0.98
0.98
1.17
1.22
1.36
1.38
1.48
2.28
3.47
3.85
3.96
Clayshale
UCS
(MPa)
100.802
47.912
56.435
53.032
56.972
54.781
44.795
37.698
32.932
46.099
46.248
26.229
15.306
10.382
9.748
W
(%)
0.05
1.68
1.70
1.94
2.48
3.03
3.17
3.22
4.01
6.72
UCS
(MPa)
76.230
38.139
40.966
31.075
31.596
28.208
26.301
23.645
14.931
4.650
The results of these tests have been presented graphically in figures 1(a), 1(b) and 1(c).
These figures show a strong correlation to an exponential relationship between uniaxial
compressive strength and water content.
Equations for the plots in Figure 1 are as follows:
c = 88.087 e-0.544w
c = 88.781 e-0.44w
c = 82.279 e-0.411w
where c = uniaxial compressive strength in MPa
(1a)
(1b)
(1c)
w = water content in %.
The same trend of decreasing compressive strength with increasing water content was
found for all three types of mudrock tested as shown by overlaying the results in Figure 2. A
correlation was made for all tested mudrocks in Figure 3 and a general equation was drived
for mudrocks (including clayshale, mudshale and mudstone) as follows:
c = 83.592 e-0.443w
(2)
3
Compressive strength (MPa)
As shown in equation 2, the water content significantly affects the uniaxial compressive
strength of the mudrocks. From equation 2 a general equation was developed to use for the
mudrocks as follows:
120
-0.5443x
y = 88.087e
2
R = 0.9617
100
80
60
40
20
0
0
1
2
3
4
Water content (%)
Compressive strength (MPa)
(a)
100
90
80
70
60
50
40
30
20
10
0
-0.445x
y = 88.781e
2
R = 0.9892
0
2
4
Water content (%)
(b)
4
6
8
Compressive strength (MPa)
90
80
70
y = 82.279e -0.4111x
R2 = 0.9748
60
50
40
30
20
10
0
0
2
4
6
8
Water content (%)
(c)
Figure 1. Effect of water content on the uniaxial compressive strength of tested mudrocks; (a)
mudshale, (b) mudstone, (c) clayshale.
Compressive strength (MPa)
120
100
80
60
40
20
0
0
2
4
6
8
Water content (%)
Figure 2. Trend of decreasing compressive strength with increasing water content for all tested
mudrocks.
5
120
Compressive strength (MPa)
-0.4433x
y = 83.592e
2
R = 0.9557
100
80
clayshale
mudstone
60
mudshale
all samples
40
20
0
0
1
2
3
4
5
6
7
Water content (%)
Figure 3. Correlation between uniaxial compressive strength and water content for tested mudrocks.
c = dry e-0.443w
where c = uniaxial compressive strength at variable water content in MPa,
dry = uniaxial compressive strength at dry condition in MPa.
(3)
It seems that different parameters control strength loss in mudrocks due to increasing
moisture content as noted by Van Eeckhout (1976). Also, mudrock is more affected by the
addition of water, especially in the case of montmorillonite clayshale.
Effect of Water Content on the Deformation Properties
Modulus of elasticity was found to be influenced by anisotropy but the most significant
influence, regarding reduction of the modulus of elasticity, was the change from dry
condition to saturated condition. A comparison between calculated values of static modulus
of elasticity for oven-dried, air-dried, and saturated conditions for three types of fine-grained
sedimentary (clayshale, mudshale and mudstone) is presented in Figure 4. This figure shows a
significant reduction in the static modulus of elasticity from oven-dried condition to
saturated condition. The results show an average reduction in modulus of elasticity in
relation to that of the dry state of about 84% for these samples. The highest value of static
modulus of elasticity reduction was associated with the mudshale samples.
6
7
clayshale
Modulus of elasticity (GPa)
6
mudshale
5
mudstone
4
3
2
1
0
oven-dried
air-dried
saturated
Figure 4. Comparison of static modulus of elasticity for oven-dried, air-dried and saturated
conditions for tested mudrocks.
The test results show that water content can significantly affect the mechanical behavior
(such as strength and deformation properties) of mudrocks.
CONCLUSIONS
Sample preparation for testing was a difficult task in this study. A large number of the
uniaxial compressive strength test samples disintegrated upon contact with water or cracked
during drying in oven.
Water content has been demonstrated to have a marked influence on the mechanical
behavior, such as strength and deformation properties, of these rocks. Testing has shown
significant decrease in the uniaxial compressive strength and modulus of elasticity as the
water content increases. These strength and modulus of elasticity reductions for the mudrocks
studied are significantly higher than the values reported by other researchers.
A general equation was developed for the mudrock that may be used for predicting
uniaxial compressive strength from the available information on water content and dry
uniaxial compressive strength for this type of rocks.
REFERENCES
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moisture content indices of highly anisotropic sediments of the Illinois basin. Proc. 25th US
Symp. on Rock Mech., Northwestern University: 398-405.
Colback, P.S.B. & Wiid, B.L. 1965. The influence of moisture content on the compressive
strength of rocks. Proc. 3rd Can. Rock. Mech. Symp., Toronto: 55-83.
7
Ghafoori, M. 1995. Engineering behaviour of Ashfield shale. PhD thesis, University of
Sydney. Australia.
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8