Engineering Geological Characteristics of Freeze-Thaw Weathered Gneiss in the Wonju Area, Korea

Abstract

We present the results of an experimental physical weathering study that focuses on fresh and slightly weathered gneiss samples from the Wonju area of Korea. The study investigated changes in the physico-mechanical properties of these samples during accelerated laboratory-based weathering, including analyses of microfracture formation. The deteriorated samples used in the study were subjected to 100-150 freeze-thaw cycles, with index properties and microfracture geometries measured between each cycle. Each complete freeze-thaw cycle lasted 24 hours, and consisted of 2 hours of saturation in a vacuum chamber, 8 hours of freezing at $-21^{\circ}C{\pm}1^{\circ}C$, and 14 hours of thawing at room temperature. Specific gravity and seismic velocity values were negatively correlated with the number of freeze-thaw cycles, whereas absorption values tended to increase. The amount of deterioration of the rock samples was dependent on the degree of weathering of the rock prior to the start of the analysis. Absorption, specific gravity, and seismic velocity values can be used to infer the amount of physical weathering experienced by a gneiss in the study area. The sizes and density of microfracture in the rock specimens varied with the number of freeze-thaw cycles. We found that box fractal dimensions can be used to quantify the formation and propagation of microfracture in the samples. In addition, these box fractal dimensions can be used as a weathering index for the mid-and long-term prediction of rock weathering. The present results indicate that accelerated-weathering analysis can provide a detailed overview of the weathering characteristics of deteriorated rocks.

Introduction

Geological materials disintegrate as a result of changes in their physical and chemical properties caused by weathering, eventually resulting in the formation of soil. However, the continuum of weathering between freshly exposed rocks and soil can result in an extensive range in the physical and chemical characteristics of a rock, indicating that the determination of a weathered geological material as a rock or as soil can be difficult; this suggests that the physical properties of materials may be easily over- or under-estimated.

Generally, weathering is controlled by climatic conditions such as temperature, humidity, and precipitation. These factors control the amount and type of physical and chemical processes that affect rock masses at the Earth’s surface. Weathering is the result of physical processes that cause mechanical fracturing in rocks, and of temperature-controlled chemical processes that are especially influenced by the presence of water. Although weathering is a complex combination of both processes, physical weathering is generally confined to near-surface areas, whereas chemical weathering can occur tens or even hundreds of meters below the surface (Chorley, 1969). In Korea, rock weathering is dominated by complex nearsurface weathering processes, primarily as a result of significant summer rainfall and large winter diurnal temperature ranges.

The physical weathering of rocks in Korea is dominated by freeze-thaw processes, and the durability of rocks under severe temperature conditions may be a key factor in the stability of rock masses (Jang et al., 2004). Weathering of intact rock materials that contain water-filled pore spaces and microfractures is caused by periodic freezing and thawing, where the volumetric change associated with the expansion of water during freezing causes internal stresses within the rock material. This stress induced by water expansion during freezing can exceed the tensile strength of the rock, causing the development of new microfracture and the propagation and widening of existing microfracture. Water can also flow through newly developed microfracture after thawing, such that recurrent freeze-thaw cycles will cause further weakening of the rock mass (Yavuz et al., 2006). These observations indicate the importance of understanding the mechanisms of weathering that result from recurring freeze-thaw conditions, and of predicting the rate at which these processes occur in various materials, especially as weathering is an important control on the stability and engineering characteristics of exposed rock masses in excavations.

Previous research on the engineering geology of weathered rocks has focused on the development of weathering grade classifications, and relating these classifications to engineering geological characteristics (Ruxton and Berry, 1957; Dearman, 1974; Fookes et al., 1988, Lee and De Freitas, 1989; Kim and Hong, 1990; Woo and Park, 2004; Lee and Cho, 2005). Recent research in Korea has also indicated that rocks affected by repeated freeze-thaw cycles undergo significant changes in their physical properties (Park et al., 2003; Jang et al., 2004; Um and Shin, 2009; Um et al., 2009; Kang et al., 2011).

Here, we present the results of an analysis of fresh and slightly weathered gneiss samples from the Wonju area of Kangwon-Do, Korea, and discuss changes in their physico-mechanical properties during accelerated freezethaw weathering in a laboratory setting, including changes in microfracture characteristics. We compare the results of these analyses with those obtained during previous research into the weathering of granites and mudstones (Um and Shin, 2009; Um et al., 2009), and discuss the general process of physical deterioration of isotropic and anisotropic rock materials subjected to freeze–thaw weathering.

 

Freeze-thaw testing

Sample preparation

The study area is located near Wonju and includes outcropping units of the Gyeonggi metamorphic complex. Porphyroblastic gneiss samples for the laboratory-based accelerated-weathering experiments were obtained from 54 mm diameter drillcore samples. Photographs of representative drillcore samples are shown in Fig. 1. Initial weathering grade was evaluated from direct observations of core specimens and from comparison with standard literature descriptions (Dearman, 1974; IAEG, 1979; ISRM, 1981). Prior to weathering, the absorption, specific gravity, and seismic velocity index properties of these samples were determined by non-destructive testing, and these properties were used to group samples by their initial degree of weathering, assuming that rock samples with similar properties had undergone the same degree of weathering (Table 1). The gneiss samples from the study area classified as either fresh (F) or slightly weathered (SW).

Fig. 1.Photograph of representative rock cores used in this study.

Table 1.Note: A = absorption ratio, Gs = specific gravity, Vp = P-wave velocity, Vs = S-wave velocity

Experimental setting

The approach used in the freeze-thaw testing of gneiss samples undertaken in this study is the same as that used in previous research on the weathering of mudstone and granite (Um and Shin, 2009; Um et al., 2009). To obtain the best results during freeze-thaw testing, it is necessary that pore spaces and pre-existing microfracture be filled with water. The samples used in the study were fully saturated rock samples that were analyzed in a vacuum chamber. The weathering of the samples was accelerated by subjecting them to a freezing temperature of –21℃ ± 1℃, which is approximately three times below the average winter temperature in the study area. Each sample was subjected to 100-150 freeze-thaw cycles, and the deterioration in the samples was characterized by repeated analyses of the index properties outlined above and the recording of microfracture geometries. Each complete freeze–thaw cycle lasted 24 hours and consisted of 2 hours of sample saturation in a vacuum chamber, 8 hours of freezing, and 14 hours of thawing at room temperature before the cycle was started again.

 

Index property variations

Repeated freezing and thawing of rock samples can trigger separation along preferentially oriented planes and the opening of microfracture related to frost wedging and changes to the volume of water in a sample. This indicates that identifying changes in the physical properties of rock with increasing number of freeze-thaw cycles could be used to accurately quantify rock weathering. Figs. 2-4 show variations in rock sample index properties during the course of the freeze-thaw experiment.

Fig. 2 shows changes in the absorption property of the rock samples with increasing number of freeze-thaw cycles. Absorption tends to increase with the number of freeze-thaw cycles for both F and SW samples, with F grade samples having an average absorption value that increased from 0.10% before analysis to 0.13% after 100 freeze-thaw cycles. A similar increase was observed in the SW samples, where the average absorption increased from 0.20% to 0.23%. These results indicate that absorption values may be unsuitable for use as a mid- to long-term index of physical weathering of F and SW grade gneisses within the study area.

Fig. 2.Relationship between absorption (A) values and number of freeze-thaw cycles for samples analyzed in this study.

Fig. 3 shows the relationship between specific gravity values and number of freeze-thaw cycles. Specific gravity values tend to decrease with increasing number of cycles for all samples, with the specific gravity of F and SW samples decreasing by about 0.004 and 0.006, respectively. This indicates that freezing and thawing has an almost negligible effect on the specific gravity of both F and SW gneisses, suggesting in turn that physical weathering has an insignificant effect on the specific gravity of gneisses in the study area, especially when compared with previous research on the weathering of mudstones in the Gyeongsang Basin (Um and Shin, 2009)

Fig. 3.Relationship between specific gravity values and number of freeze-thaw cycles for samples analyzed in this study.

The seismic velocity of a rock is related to its density and elastic properties, and could also potentially be used as a weathering index. The seismic velocity correlates well with the rock strength and therefore gives an indication of intact rock quality. It is well known that the P-wave velocity of F grade rocks may be > 6000 m/s, but these velocities can often decrease to < 900 m/s, depending on the degree of weathering of the rock (De Vallejo and Ferrer, 2011).

For the gneiss sample analyzed here, changes in P-wave (Vp) and S-wave (Vs) velocities with increasing number of freeze-thaw cycles are shown in Fig. 4. On average, F and SW samples have initial P-wave velocities of 4493 and 4052 m/s, respectively, which decreased to an average of 3899 and 3691 m/s after 100 freeze-thaw cycles. In addition, these F and SW samples had S-wave velocities that decreased from an average of 3212 and 3154 m/s to an average of 2854 and 2701 m/s, respectively. The accelerated-weathering experiments undertaken in this study also indicated a direct correlation between changes in seismic velocity and microfracture formation and propagation. The size and density of microfracture traces on sample surfaces continuously changed during the accelerated weathering, indicating that the weathering caused an increase in the visibility and size of microfracture within the samples.

Fig. 4.Relationship between seismic velocity (V) and number of freeze-thaw cycles for fresh (a) and slightly weathered (b) samples analyzed in this study.

 

Microfracture growth

The effect of microfractures on rock failure was first identified by the theory of failure mechanics (Griffith, 1920). The brittle failure of rock is influenced by tensional stress that is concentrated within flaws inside a rock; these internal rock flaws include microshear deformation structures, pores, microfracture, and microcavities generated during mineral dissolution (Um and Shin, 2009). These flaws are likely to initiate the propagation of cracks that decrease the strength of a rock. McGreevy and Whalley (1985) suggested that the most significant damage caused by freezing would be located around microfracture with moisture contents higher than the intact rock material and suggested that microfracture propagation was caused by lower stresses than the theoretical failure stress of the rock. This indicates that a clear understanding of the initiation and propagation of microfracture and the development of a fundamental theory of microfracture propagation is a necessary step in assessing both the strength characteristics and the weathering of rocks.

Mineral crystals that have undergone deformationrelated extension are aligned to define foliations within the gneisses in the study area. These gneisses are dominated by alkali feldspar and contain minor perthite. Although freeze-thaw-related flaws can be qualitatively assessed, quantitative assessment is difficult, primarily as a result of scale effects. Here, we examine the changing visibility of microfracture with ongoing freeze-thaw weathering of gneisses in the study area.

A 40 × 40 × 10 mm slab specimen was prepared for each sample group assessed in this study and is assumed to be representative of the gneisses in the study area in terms of the quantitative microfracture analysis (Fig. 5). A 30 × 30 mm square outline was marked on the prepared slab specimen after polishing; the specimen was then photographed using a 50 mm 1 : 1 macro lens to obtain a highresolution image of the visibility and propagation of microfracture caused by freeze-thaw weathering.

Fig. 5.Photographs of (a) fresh (F) and (b) slightly weathered (SW) slab specimens analyzed in this study.

Freeze-thaw testing of the slab specimens was undertaken at the same time as the weathering of the drillcore samples described above. The appearance, propagation, and pattern of microfracture within the surface of the slab specimens was mapped using high-resolution digital images (3.6 MP) taken every 20 freeze-thaw cycles up to a maximum of 150 cycles. Microfracture coordinates were then acquired from the digital images to analyze the characteristics of microfracture initiation and propagation.

Fig. 6 shows the pattern of microfracture growth associated with repeated freeze-thawing of the slab specimens. In general, any increase in freeze-thaw weathering is qualitatively associated with an increase in the size and density of the microfracture traces in the surface of the slab. This relationship corroborates the changes in sample seismic velocity, indicating that changes in seismic velocity values most likely relate to microfracture development.

Fig. 6.Microfracture growth within (a) fresh (F) and (b) slightly weathered (SW) 3 × 3 cm slab specimens during progressive weathering caused by laboratory freeze-thaw cycles.

Microfracture generally initiate at and propagate from a foliation plane or free surface (e.g., corners and edges) of the specimen. This is evident in the changing microfracture pattern shown in Fig. 6a, where ongoing freezing and thawing of an F sample causes the sporadic appearance of microfracture and an increase in the density. Although this specimen contains an increasing abundance of microfracture, the microfracture do not propagate over significant distances, indicating that this sample has low crack connectivity.

The SW specimen shown in Fig. 6b records an increase in microfracture density with increasing number of freezethaw cycles. These microfracture also increase in both continuity and connectedness as they propagate parallel to the foliation within the rock, indicating that freezing and thawing of more weathered rocks increases both the initiation and propagation of microfracture. In addition, and as discussed above, it is thought that changes in seismic velocity values and the development of microfracture are closely related. However, although seismic velocities may be useful as a quantitative index of weathering, these data need to be approached cautiously, as obtaining precise and accurate data is often hampered by analytical error.

This study aimed to investigate the relationship between freeze-thaw weathering cycles and the statistical parameters of microfracture density and size values determined using digital images, thereby quantifying the changes in microfracture characteristics during repeated freezing and thawing. This relationship was examined using a box fractal dimension (D; Um et al., 2006) approach that simultaneously incorporates microfracture density and size. This approach uses a box counting method (Feder, 1988; Um and Shin, 2009) that can be used to estimate the box fractal dimension that is subject to change relating to microfracture density and size distribution. Defining r as to the size of a box and N(r) as the number of boxes that contain microfracture traces, we have the following equation:

where B is a proportional factor. The relationship between ln(N(r)) and ln(1/r) in equation (1) can be shown in an X–Y graph, and D values are determined from the linearity of the data using linear regression analysis. The 30 × 30 mm square box described above was divided into four 15 × 15 mm square boxes during the initial stage of determining D values for each slab; this was followed by a second stage where each box was divided again by four to produce a total of 16 boxes of size 7.5 × 7.5 mm. This exponential reduction of the boxe size continued until the seventh stage of subdivision into boxes containing microfracture traces; this was followed by an examination of the linearity of the ln(N(r)) vs. ln(1/r) relationship to calculate a value for D. The reliability of the calculated D value is determined by the linearity of the ln(N(r)) vs. ln(1/r) relationship, where a coefficient of determination (R2) value of > 0.81 indicates a reliable calculated D value. In theory, the calculated D values range between 1 and 2 according to the density and the size distribution of microfracture traces (Um et al., 2006).

Fig. 7 shows the variations in calculated D values for microfracture traces that propagate on the surface of specimens during ongoing freeze-thaw weathering. The F specimen analyzed during this study has a D value that continuously increases from D = 1.04 to D = 1.58 with ongoing freezing and thawing up to the final 150th freezethaw cycle. In comparison, the SW specimen had an initial D value of 1.18 and a final D value of 1.63 after the 150th freeze-thaw cycle, with these data plotting logarithmically. The D value of the SW specimen is also slightly higher than that of a similarly weathered granite specimen (Um et al., 2009), most probably as a result of freeze-thaw-related microfracture developing more quickly within ferromagnesian minerals in the gneiss than in the more felsic minerals in the granite.

Fig. 7.Relationship between estimated box fractal dimension (D) values and the number of freeze-thaw cycles for fresh (F) and slightly weathered (SW) samples.

The fact that D values increase with a simultaneous increase in both the frequency and size of microfracture demonstrates that quantitative engineering geological information on the weathering state of a rock can be generated. Rocks are likely to undergo changes in both physical properties and strength during weathering, with the latter decreasing over both mid- and long-term weathering. This is clearly evidenced by the freezing and thawing of exposed gneisses within the study area. In particular, a significantly weathered rock may undergo physical weathering that is also associated with disintegration and separation, with both chemical and physical weathering occurring in parallel in nature, indicating that internal weathering of a rock is likely to accelerate over time.

Research into the weathering of mudstones (Um and Shin, 2009) indicates that repetitive freezing and thawing causes the rapid development and propagation of microfracture, suggesting in turn that physical weathering is the main mechanism involved in rock weathering. In addition, Um and Shin (2009) reported that box fractal analysis effectively quantifies the variations in microfracture traces and, in the case of the mudstone, the extension and propagation of microfracture could lead to disintegration of the rock specimen at D values of ~1.40. The gneisses within the present study area are thought to disintegrate and separate at much higher D values (~1.63), primarily due to the crystalline nature of these rocks, which means that flaws such as mineral crystal boundaries, pores, and microcavities generated during mineral dissolution are the main controls on microfracture formation and propagation. This sharply contrasts with mudstone, which generally becomes incoherent as a result of flaws that include anisotropic laminations and microshear deformation structures.

The data presented here indicate that the relationship between D values and increasing numbers of freeze.thaw cycles is similar to the changes in the index physical properties described above, meaning that D values can be effectively used to quantify the formation and propagation of microfracture inside a rock. In addition, D values can be used as a new weathering index for predicting the extent of the mid- to long-term weathering of rocks, although further investigations of the physical and chemical weathering of various types of rocks are needed.

 

Conclusions

Predicting the changes in rock characteristics caused by natural weathering is difficult, although weathering tests that quantify the physical and mechanical characteristics of rocks and predict changes in rock characteristics caused by natural weathering can be developed using artificially extreme environments that accelerate natural weathering. This paper presents the results of freeze.thaw testing of gneisses from the Wonju area of Kangwon-do, Korea and reached the following conclusions.

Repeated freezing and thawing leads to a decrease in rock specific gravity and seismic velocity values, and an increase in absorption within the rock. In addition, the size and frequency of microfracture traces continuously increases as a result of progressive freezing and thawing, corroborating the observed decrease in the seismic velocity of rock samples during weathering. In addition, seismic velocity can be effectively used as a quantitative weathering index. Box fractal analysis simultaneously considers the frequency and size of microfracture, and enables the calculation of box fractal dimension values that can also effectively quantify variations in microfracture traces associated with freeze-thaw weathering.

The present results indicate that laboratory-based accelerated weathering can reveal variations in the characteristics of deteriorating rocks during weathering. This type of weathering investigation can be used in engineering practice to determine optimum designs for earth structures, and is necessary to characterize the longterm effects of physical and chemical weathering of rock materials in the field.