Introduction
The term ‘‘sandstone’’ denotes a large class of sedimen-tary rocks with different mineral compositions, diagenetic process and ages or degree of diagenesis. This diverseness in rock-forming origins results in a great variety of rock properties, including the strength, deformability, perme-ability and resistance to weathering. The mechanical behavior of sandstones and its affecting factors have been explored (Azzoni et al. 1996; Bell and Culshaw 1993,
1998; Bell and Lindsay1999; Bernabe et al.1994; Chigira
and Sone 1991; Clough et al. 1981; David et al. 1998; Handin and Hager1957; Hawking and McConnell1992). Corresponding classification systems or methods of esti-mating strength of sandstone were suggested by Barton et al. (1993), Dick et al. (1994), Erosy and Waller (1995), Fahy and Guccione (1979), Gunsallus and Kulhawy (1984), Howarth and Rowlands (1986), Shakoor and Bonelli (1991) and Ulusay et al. (1994).
How the sandstone was fractured, when subjected to external loading, has been studied (Zhang et al. 1990; Sangha et al.1974; Menenaez et al.1996). In summary,
wetted sandstones may become softer and weaker than dry sandstones (Bell 1978; Turk and Dearman 1986; Dobereiner and De Freitas1986; Dyke and Dobereiner 1991). The degree of wetting softening can be related to porosity (Turk and Dearman1986) and matrix content or mineral composition (Hawkins and McConnell1992). Wetting weakening is of concern when assessing the stability of a dry rock slope that can be wetted due to heavy rainfall, or when predicting the crown settlement of an excavating tunnel that can be wetted by seeping water inside the tunnel.
In Taiwan, tertiary sandstones have a digenetic age of no more than 70 million years and such relatively short rock forming period is insufficient to classify them as hard rocks. The typical strength of tertiary sandstones in Taiwan ranges from 10 MPa to 80 MPa (Jeng and
Hu-ang1998). These tertiary sandstones were often
charac-terized as medium to weak rocks and their mechanical behavior differs from that of many hard rocks.
Among the tertiary sandstones, discrepancies in their mechanical behavior must be recognized. Tertiary sandstones are classified into two types, Type A and Type M. L. Lin
F. S. Jeng L. S. Tsai T. H. Huang
Wetting weakening of tertiary
sandstones—microscopic mechanism
Received: 28 December 2004 Accepted: 19 April 2005 Published online: 18 June 2005 Springer-Verlag 2005
Abstract The micromechanism accounting for wetting weakening of tertiary sandstones was studied. It was found that intragranular frac-ture prevails for all dry sandstones. However, when the sandstone is wet, intergranular fracture occurs for Type B sandstones. Therefore, one sandstone from Type A sandstones, MS1, and another from Type B, TK, were selected to further investigate the nature of the matrix. It was found that (1) for both sandstones, the major mineral components of the
matrix are illite and kaolinite except that the MS1 sandstone has more chlorite; (2) leaching of matrix in-duced an increase of porosity and consequently results in leaching softening; and (3) among the mineral composition, chlorite is easiest to be dissolved and leached out and in-duces a more significant increase of porosity, which, in turn, results in a more significant leaching softening. Keywords Fracture Æ Tertiary sand-stone Æ Matrix Æ Wetting softening
M. L. Lin Æ F. S. Jeng (&) L. S. Tsai Æ T. H. Huang Department of Civil Engineering, National Taiwan University, Taiwan E-mail: [email protected]
Tel.: +886-2-23630530 Fax: +886-2-23645734
B, Fig.1(Jeng et al.2004). The mechanical properties of Type Aare close to those of hard rock except that Type A sandstone has more significant shear dilation. Never-theless, Type B sandstone, compared to the Type A sandstone, is characterized with (Jeng et al.2004): 1. Lower stiffness in bulk modulus and shear modulus. 2. Substantial amount of volumetric deformation can be induced by shearing, namely the so-called shear dilation phenomenon.
3. Wetting will significantly reduce both the strength and the stiffness of the Type B sandstone. A strength reduction ratio due to wetting, R, is accordingly de-fined as
R¼ UCSdry=UCSdry ð1Þ
where UCS is the uniaxial compressive strength. The R of Type B sandstone is defined to be greater than 0.5. Type B sandstone can be prone to tunnel squeezing.
This paper is directed toward finding the microscopic mechanism that accounts for the significant reduction of strength and stiffness. Cycles of leaching tests were also conducted to identify what components of matrix can be dissolved and to determine the consequences.
Set-up of experimental study
A total of 13 samples of sandstones were obtained from eight geological formations of northern Taiwan to be tested. These sandstones were deposited under marine, marine-terrestrial and littoral facies, and their geological ages range from Oligocene to Pliocene. The specimen size
was 5.5 cm in diameter and 12.5 cm in height. The specimen was oven dried (105C) to remove its natural water content. For the uniaxial compression test, the axial load was provided by a servo-controlled high-stiffness machine, which had a maximum load and stiff-ness of 4448 kN and 13.1·109
N/m, respectively. The load is applied at a rate of 5 MPa/min. The longitudinal and transverse types of deformation were separately measured by a full Wheatstone bridge consisted of four strain gages, which were capable of measuring strains up to 2% with an accuracy of ±0.85 (lm/m)/C.
Since some of the tertiary sandstones exhibit wetting softening behavior, experiments were conducted on dry and wet specimens. The sandstones were soaked in water in a vacuum chamber for sufficient length of time (at least 24 h) so that the water content would stop increasing. Water was allowed to fill all the coalescent pores. Specimens, for the calcitic sandstones, should not be submerged in water too long to avoid dissolving and leaching of the minerals.
Petrographic features of the sandstones were accomplished by thin section analysis for better color contrast between grains and to identify grain bound-aries. The size of area is chosen to include 80–150 grains in an image so that representative petrographic features can be observed. With these four types of images, the grain boundary, the matrix, the pore and the mineral composition of grains can be identified by computer and by visual recognition.
To study the relative mineral contents of matrix, both nonquantitative and semiquantitative X-ray diffraction tests (XRD) have also been conducted. Consequently, the relative mineral composition of matrix could be identified.
Scanning electron microscope (SEM) was used to observe the grains on the fracture surface (Oatley1972). At least one piece of rock fragments was chipped from the fracture surface of dry or wet specimens, followed by coating of gold film, as observed under SEM. Broken grains on the fracture surface were thus identified and could reveal how the fracture surface was developed.
Furthermore, dry, thin section slides of rock speci-mens with a dispeci-mension of 2 cm·1 cm·1 cm were frac-tured by applying axial compression and observed under the microscope to notice the development of fracture surface. Comparing the fracture pattern observed by SEM to that of microscope, the validity of using SEM can be justified, provided that the results are consistent. Once the sandstone is fractured by uniaxial compres-sion, the fractured specimen is cemented by epoxy glue and ground to a thin section to be observed under the microscope and grains fractured and the coalesced fracture surface can be observed.
As the properties and the components of matrix could inherently affect the macroscopic property and the wetting softening behavior of sandstones, cycles of
Fig. 1 Geotechnical classification of the studied sandstones in terms of n and GAR. The empirical UCS and R are shown by solid and dashed contour lines. The sandstones are classified into two groups: Type A (R>0.5) and Type B (R£ 0.5). The classification of strength (Type I, II, III and IV) is based on the definition of ISRM (1981)
leaching test were also conducted on these sandstones to identify which components can be dissolved and how the mechanical properties are affected. The dissolved mate-rial was collected after 30 and 60 cycles of dry-and-wetting tests and analyzed by XRD to identify the rel-ative contents of composition.
Basic Properties of tertiary sandstones
As shown in Table 1, the sandstones are mainly com-posed of quartz (greater than 75%) with the minor content of rock fragments and very little of feldspar (less than 5%); accordingly, these sandstones are classified as
lithic graywacke or quartzwacke based on Pettijohn’s definition (Pettijohn et al. 1987). The porosities range from 11% to 25%.
Figure2illustrates some of the petrographic images of the sandstones. In general, the grains have sub-rounded to subangular geometry. Some of the sand-stones have a rather small grain ratio (GAR less than 50%), which implies a great portion of matrix and porosity.
The mechanical properties of sandstones, including uniaxial strength and R are listed in Table2. A fairly wide spectrum of mechanical behavior was obtained, in which the dry and wet uniaxial compressive strengths varied from 7 to 86 MPa and 3 to 45 MPa , respectively.
Table 1 Compositions of sandstones studied (modified after Jeng et al.2004)
Formation n(%) GAR (%) Matrix (%) PD Mineralogy of grains
Quartz (%) Feldspar (%) Rock fragment (%)
WGS1 17.4 65.0 17.6 68.9 90.3 0.0 9.7 WGS2 16.7 25.3 58.0 44.9 85.8 0.0 7.2 MS1 11.5 50.4 38.2 – 88.0 0.2 10.3 MS2 14.1 67.5 18.5 74.2 90.7 0.2 9.0 MS3 13.1 51.0 35.9 65.4 85.0 2.3 12.2 TL1 13.1 36.4 50.5 56.2 86.5 1.7 9.9 TL2 12.8 50.0 37.2 67.0 87.3 0.7 9.7 ST 18.2 40.4 41.4 57.4 77.7 4.4 12.7 NK 14.8 28.6 56.6 55.2 90.0 2.3 5.6 TK 12.8 28.2 59.0 49.4 84.5 0.5 13.0 SFG1 24.6 52.6 22.8 73.6 95.6 0.8 3.0 SFG2 16.9 42.8 40.4 65.4 78.4 1.6 8.9 CL 20.7 39.4 40.0 61.2 83.7 1.0 5.5
Fig. 2 Some of the typical pet-rographic images (crossed nickel) of the studied sand-stones. A white bar shows the scale of each image. The sym-bols MS1, MS2, NK and SFG2 represent the stratum formation where the specimens came from. Figure1defines the rock types
Remarkably, significant wetting softening could be observed, either in strength or in stiffness. The reduction ratio R ranged from 0.85 to as low as 0.14.
The relative mineral composition of the matrix for all sandstones is summarized in Table 3. The matrix is mainly composed of the illite, kaolinite and chlorite. A minor content of montmorillite can be found in some sandstones.
The UCSdrycan be expressed in terms of n and GAR as (Jeng et al.2002):
UCSdry¼ ð133:7e0:107nÞð3:2 0:026GARÞ ð2Þ where the units for UCS, n and GAR are MPa, % and %. If the UCS expressed by Eq. 2 is defined as empirical UCS, it can be compared to the actual UCS, as shown in
Fig.3. In general, the actual UCS and the empirical
UCS are consistent.
Fracture behavior of tertiary sandstones
The fracture surfaces of the studied sandstone are shown in Fig.4. It reveals that fracture grains can often be
Table 2 Mechanical properties of sandstones (modified after Jeng et al.2004)
The strength reduction ratio R due to wetting softening is de-fined as R¼ UCSwet=CSdry
Formation UCSdry(MPa) UCSwet(MPa) R No. specimen
Dry Sat WGS1 34.1 25.4 0.74 8 9 WGS2 47.5 6.7 0.14 10 8 MS1 48.5 28.9 0.60 15 2 MS2 37.1 28.3 0.76 27 23 MS3 82.7 43.3 0.52 3 3 TL1 68.7 23.2 0.34 11 9 TL2 77.5 44.2 0.57 3 3 ST 38.4 7.8 0.20 5 3 NK 86.0 43.2 0.50 4 3 TK 69.0 29.4 0.43 10 2 SFG1 14.5 12.2 0.84 3 3 SFG2 46.4 19.9 0.43 3 3 CL 19.9 3.1 0.16 7 6
Fig. 3 Comparison of the empirical UCS (Eq. 1) and the actual UCS
Table 3 Mineral contents of the matrix for each type of sandstone (modified after Jeng et al.2004)
a
Figure1defines the types of sandstone and the strength ty-pes (I, II, III and IV) are clas-sified based on ISRM (1981)
Formation Illite (%) Kaolinite (%) Chlorite (%) Montmorillite (%) Mixed layer (%) Typea
WGS1 72 14 2 4 8 IIIA WGS2 73 3 0 15 9 IIB MS1 39 50 11 0 – IIA MS2 75 7 1 4 13 IIIA MS3 69 11 19 0 – IIA TL1 32 14 6 6 42 IIB TL2 16 30 54 0 – IIA ST 21 64 15 0 – IIIB NK 46 44 10 0 – IIB TK 60 40 1 0 – IIB SFG1 17 59 23 1 – IVA SFG2 20 56 24 0 – IIIB CL 35 8 3 27 27 IIIB
found on the fracture surface of dry sandstones regardless of Type A or Type B sandstone (Figs. 4a, b, c). That is, the fracture surface is a result of coalescence of intragranular microcracks for all dry sandstones.
When the sandstone is wet, however, all the grains on the fracture surface remain intact for Type B sandstone. As the fracture surface is formed by coalescence of in-tergranular (or trans-matrix) cracks instead of
intra-granular ones (Fig.4f). This phenomenon concurs with the finding by Dobereiner and De Freitas (1986).
For the intergranular type of fracture, the matrix is relatively softer than the grain and grains rotate during fracture process so that stress concentration will not be induced within the grains, fracture surface only tracks through the matrix without passing through the grains and eventually induces no grain breakages.
Fig. 4 Typical SEM images of fractures surfaces studied sand-stones. The classification of sandstones is defined by Fig.2b. The left and the right column of images are fracture surface obtained from dry and wet sandstones, respectively. The geotechnical properties of the sandstone shown in this figure are listed in Tables1,2,3
For Type A sandstone, however, intragranular frac-ture still prevails except for the MS1 sandstone, as listed in Table 4. Therefore, the fracture mechanism of Type A sandstone is not changed whether it is dry or wet.
The above-mentioned phenomena naturally render a scenario interpreting why Type B sandstone exhibits a greater wetting softening behavior than Type A does. When both types of sandstones are dry, the matrix seems to be strong enough to hold the grains in position; these grains are broken during the fracturing process. When sandstone is wetted, the matrix of Type B sandstone becomes much softer than the grains, which results in an trans-matrix fracture and to a much lower compressive strength with a strength reduction ratio R£ 0.5. The wetting of Type A sandstones however, does not seem to induce sufficient matrix softening to transform the fracture type from intragranular to intergranular. Remarkably, some degree of strength reduction still occurs for Type A sandstone; however, the degree of wetting softening is less severe for Type A than Type B, which implies that the softening of matrix could still exist for Type A sandstone.
The observation obtained from tests of slice speci-mens confirms the aforementioned findings. As illus-trated by Figs.5a, b, intergranular fracture and intragranular fracture do actually happened for dry Type B and Type A sandstones, respectively.
Weakening due to leaching
The above-mentioned experimental results reveal that wetting softening of matrix would account for the Type A or Type B behavior. Therefore, one sandstone was selected from each type of sandstone, MS1 from Type A sandstone and TK from Type B sandstone, to study the wetting softening behavior and the effect of leaching.
When these two sandstones were submerged in water over various length of time, the degree of saturation increased with submergence time. Both the strength and the stiffness of the two sandstones decrease upon greater degrees of saturation, as shown by Figs.6 and 7. For MS1 sandstone, the strength (UCS) reduces from 80 MPa to 30 MPa and a 63% decrease of strength, from a dry state to a completely wet state as shown in
Fig.6a. Similarly, a 40% decrease of strength occurs
when the dry TK sandstone is wetted as illustrated by
Fig.6b. The reduction of stiffness is also significant;
about a 60% loss of stiffness (Young’s modulus) occurs for both sandstones, as depicted by Figs.7a, b.
Although both MS1 and TK sandstones exhibit wetting softening behavior, the leaching effects of these two sandstones are somewhat different. The strength of TK sandstone does not seem to be affected by at least 60 cycles of dry–wet process as shown in Fig.8. However, the MS1 sandstone loses 20% of strength after 60 dry–
Fig. 5 Fracture surface ob-tained from tests on thin slice specimens. Thick white lines mark the major fracture sur-face. Intergranular fracture oc-curred for Type B sandstone (a), while intragranular fracture occurred in Type A sandstone (b). In b, microcracks within grains can be seen
Table 4 Fracture type of studied sandstones and corresponding properties Formation Grain (%) Matrix (%) Porosity (%) UCSdry (MPa) UCSwet (MPa) Strength reduction ratio (R) Rock group Fracture type Dry Wet SFG1 52.6 22.79 24.6 14.5 12.2 0.84 A Intragranular Intragranular MS2 67.5 18.5 14.1 37.1 28.3 0.76 A Intragranular Intragranular MS1 50.4 38.2 11.5 48.5 28.9 0.60 A Intragranular Intergranular TL2 50.0 37.20 12.8 77.5 44.2 0.57 A Intragranular Intragranular MS3 51.0 35.9 13.1 82.7 43.3 0.52 A Intragranular – NK 28.6 56.62 14.8 86.0 43.2 0.50 B Intergranular Intergranular TK 28.2 59.04 12.8 69.0 29.4 0.43 B Intragranular Intergranular SFG2 42.8 40.36 16. 9 46.4 19.9 0.43 B Intragranular Intergranular ST 40.4 41.45 18.2 38.4 7.8 0.20 B Intragranular Intergranular
wet cycles. The phenomenon implies that the matrix of MS1 sandstone, which leads to a wet intergranular fracturfe similar to Type B sandstone, of MS1 sand-stones is easier to be ‘‘leached out’’ than the matrix of TK sandstone. A further examination of the porosity before and after leach tests confirmed this assertion. The porosity and density of TK sandstone remain un-changed after 60 cycles of dry–wet process. However, significant reduction of matrix content occurred for MS1 sandstone (Fig. 9a), which led to an obvious increase of porosity as shown in Fig.9b. Apparently, the matrices of the two sandstones have different resistances to cycles of leaching. At this point, what accounts for such dis-crepancy should be further examined.
Before looking into the matrix, the microstructure of these two sandstones should be evaluated. As listed in Table5a, these two sandstones have similar packing, types of contact and fracture feature; however, the grain size of TK sandstone is smaller. It is about 1/5 of the MS1 sandstone grain. The influence of wetting cycles on the two sandstones is summarized in Table 5, panel b, and about 0.13% and 0.09% (in weight) of matrix
material has been leached out from MS1 and TK sandstones, respectively. The leached-out material was dried and tested using XRD to study its mineral content, as shown in Fig.10. Results of XRD tests revealed the mineral content of the dissolved matrix, as shown in
Fig.11. Figure11 indicates that
1. The major mineral components of the matrix are illite and kaolinite for both sandstones. However, the MS1 sandstone appears to have much more chlorite (about 10% before leaching) than the TK sandstone does. 2. After leaching test, the chlorite appears easier to be
washed out than the other two minerals, illite and kaolinite. The relative content increases from 10% and 0.7% to 25.2% and 7% for MS1 and TK sand-stones, respectively.
The mineral content of the matrix highlights that chlorite could be the key factor that makes the matrix different and thus account for the discrepancy in leach-ing effects. Meanwhile, chlorite is more easily affected by
Fig. 7 Variation of Young’s modulus with degree of saturation for MS1 sandstone (a) and TK sandstone (b)
Fig. 6 Variation of UCS with degree of saturation for MS1 sandstone (a) and TK sandstone (b)
Fig. 8 Influence of wetting cycles to UCS obtained from MS1 and TK sandstones
Fig. 9 Influence of wetting cycles to matrix content and the porosity of MS1 sandstone. More number of cycles reduces the
matrix content (a), and increases the porosity (b) Table
5 Comparison of MS1 and TK sandstones MS1 sandstone TK sandstone Comparison of microstructure Typical grain size 0.2 mm 0.04 mm Microstructure Packing Contact 92.0%No contact 8.0% Contact 79.9 %No contact 20.1% Types of grain contact Tangential contact 61.0%Linear and su ture contact 31.0% Tangential contact 66.7% Linear and suture contact 13.2% Fracture feature Dry rock Intragranular fracture prevai ls Wetted rock Intergranular fracture prevails Influences of wetting cycles (1) Compressive strength Decrease 85% Not effected (2) Dry density Before: 2.24 g/cm 3Aafter: 2.19 g/cm 3 Not affected (3) Other changes Significant decrease of ma trix content and increase of porosity Not affected (4) Grain cont act Type of packing Not affected Not affected Type of contact Not affected Not affected (5) Leached-out material Leached-out weight 5.04 g 0.13% 2.71 g 0.09% Clay minerals More chlorite than illite and kaolinite was leached out XRD peak values are not significant enough for identified
Fig. 10 Typical results of XRD for leach-out materials obtained from MS1 and TK formations
water than illite and kaolinite. It could also be the factor that induces intergranular fracture instead of trans-grain fracture of wet Type A sandstone.
Conclusion
The fracture patterns of Type A and Type B sandstones were investigated based on observations of fracture surface under SEM and on tests on thin slices of the sandstones observed under the microscope. It was found that intragranular fracture prevails for all dry sand-stones. However, when the sandstone is wet, intergran-ular fracture occurs primarily in Type B sandstone. Considering the macroscopic mechanical behavior of sandstones, the strength and stiffness of dry sandstone is reduced when it is wet and Type B sandstone tends to have more significant wetting softening than Type A sandstone does. The intergranular fracture possibly en-ables an easier coalescence of microcracks so that Type Bsandstone is more sensitive to wetting.
Study of the mineral composition of matrix found that chlorite is dissolved and leached out easier and that the porosity of sandstone increases and leads to a strength reduction. Special attention should thus be put on chlorite content of the matrix when allocating pos-sible problematic sandstones. In addition to the factors found by previous research, including GAR and n, the nature of matrix appears to have influence on deter-mining the behavior of sandstone to be Type A or Type B.
Acknowledgements The research is supported by the National Science Council of Taiwan, grant no. NSC-89-2211-E-002-152. Fig. 11 Comparison of mineral contents for matrix before
leach-ing, after leaching and the leached-out material. The number on top of each bar represents the relative contents (in weight) during the three phases of experiments. The sum of the relative contents for each color of bars is 100%
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