Influence of Weathering and Correlations Between Wave Propagation Velocity and Durability with Physical and Mechanical Parameters in Phyllites

. Ultrasonic longitudinal wave velocity ( Vp ) and durability are characteristics largely determined in geotechnical evaluations. These properties are useful in the analysis of weathered rocks because they usually have good relationships with physical and mechanical properties of rocks. Additionally, the tests to obtain these parameters are easy to perform. However, few studies have been performed specifically on low degree metamorphic rocks. The purpose of this study is to evaluate the influence of weathering on these parameters, as to propose correlations between Vp and some mechanical and physical parameters. This study was carried out using phyllites from three weathering profiles located along the highway BR-356, Minas Gerais, Brazil. The results reveal a clear influence of weathering on Vp and durability mainly due to increased porosity. However, as the variation of this parameter is heterogeneous, the relationship between Vp , durability, and weathering is not uniform, although they present general trends. The behavior shown by porosity reflects the complex role of chemical weathering on fractures. The non-uniform pattern of Vp and durability throughout the weathering classes is also a consequence of the heterogeneity and anisotropy intrinsic to these phyllites. The empirical relationships established exhibit good correlation coefficients.


Introduction
Ultrasonic wave propagation test is non-destructive, easy to apply, and thus has been increasingly employed in the characterization of rocks for engineering purposes, including the determination of effects of weathering on rocky materials.
Weathering is known to lead to rapid changes in the properties of rock materials. Alterations of a particular rock type and their rate of occurrence (alterability) are often described by a durability parameter (Crosta, 1998) known as the durability index (I d ) and obtained through slake durability test.
The slake durability test is used to evaluate the resistance to rock alteration as a function of the disintegration resulting from a standard wetting and drying cycle (Franklin & Chandra, 1972). Durability is a characteristic largely affected by mineralogy, texture, porosity, and water content and is strongly related to the history of alteration after its formation (Koncagül & Santi, 1999;Cetin et al., 2000;Dhakal et al., 2002;Yilmaz & Karacan, 2005;Gupta & Ahmed, 2007;Ioanna et al., 2009).
In this sense, this study shows the effects of weathering on Vp and durability, as well as the correlation between these properties and physical and mechanical parameters in phyllites from the region of Minas Gerais, Brazil.
Phyllites were collected in road slopes located in the state of Minas Gerais, Brazil. These slopes are inserted in the geological context of the Iron Quadrangle, an important mineral province of Brazil, consisting of five main lithostratigraphic units (Alkmim & Marshak, 1998): crystalline basement, metamorphic complexes of Archean age; Rio das Velhas Supergroup, consisting of an Archean volcanosedimentary sequence (greenstone belt); Minas Supergroup, formed by Proterozoic, metamorphosed, and folded sediments; post-Minas intrusions; Itacolomi Group, consisting of Proterozoic clastic metasediments. As shown in Fig. 1, the studied phyllites belong to the Minas Supergroup.

Methodology
Phyllite samples at different degrees of weathering were taken from three road slopes located along BR-356 in the state of Minas Gerais, Brazil (Table 1). The degrees of weathering were identified in each profile from the classification proposed by the International Society of Rock Mechanics (ISRM, 2015).
These samples were taken for laboratory tests to obtain physical indices, wave propagation velocity, point load strength index, durability, and mineralogical analysis. It is part of the scope of this paper to present and discuss Vp and durability data for materials at different levels of weathering, as well as correlate the properties Vp and durability, Vp and physical indices, and Vp and the point load strength (Is (50) ). Tests were performed according to the suggestions of ISRM (2007).
Least-squares regression analyses were applied between Vp and durability, Vp and physical indices, and Vp and Is (50) to verify the possible relationships between these properties. The best-fit line and the correlation coefficients were determined. Different relationships (linear, exponential, logarithmic, and power) were tested in order to obtain the highest correlation coefficient values.

Main characteristics of the studied phyllites
The studied phyllites are composed of muscovite, quartz, opaque minerals and chlorite. The main products of weathering on primary minerals are iron oxide/hydroxide and kaolinite. Foliation is penetrative, millimeter-spaced, formed by the alternation of thin micaceous bands, quartzmicaceous bands, and quartz veins. There are also veins consisting of chlorite and opaque minerals, parallel to metamorphic foliation.
One of the main effects of the exposure of these rocks to atmosphere conditions is the formation of fractures parallel to the metamorphic foliation, which intensifies with the advancement of weathering (Fig. 2). There is also increased openness of these discontinuities that become more degraded in the most altered members. This striking characteristic observed in the studied profiles is also noticeable in the petrographic slide due to the presence of microfractures, especially transgranular and intragranular (Fig. 3). As physical weathering is processed, creating preferential spaces for water to enter the rock, opaque minerals are dissolved and iron oxides are precipitated. These results are in agreement with studies carried out in phyllites under tropical  and subtropical Marques et al., 2020;Robert & Hack, 2020) climates. Figures 4, 5, and 6 show the main implications of weathering on the matrix of this rock.

Physical indices
Dry and saturated apparent specific gravity, apparent porosity, and apparent water absorption capacity are shown in Table 2. Physical parameters are clearly influenced by weathering processes. However, the variation of these parameters is not homogeneous in soil-rock transition materials (W3, W3/W4, and W4). It is mainly due to the increased number of micro-fractures and the widening of pre-existing discontinuities parallel to metamorphic foliation. This increase in porosity with the advancement of weathering is also accompanied by higher precipitation of iron oxide/hydroxide rich solutions, as previously discussed. These characteristics occur most sharply from phyllite W3. Sousa et al. (2005) observed that, unlike what happened in intact or slightly altered granites, the porosity variation was not homogeneous in a very altered granite. This is due, according to the authors, to the action of weathering on micro-fractures, making their performance increasingly complex and reflecting on the physical characteristics of the rock.

Durability
The lowest obtained durability index corresponds to phyllite W3 from profile Phy01 (Table 3). It can be explained by the reduced internal cohesion of its matrix, which was also pointed out by Leão et al. (2017) in altered sericite phyllites from the same site. These phyllites are characterized by levels of mafic and siliceous minerals that fill foliation (Fig. 4). Leão (2017) performed durability studies on phyllites located in profile Phy01 and obtained nonlinear behavior of I d2 values among phyllites with different degrees of weathering. The author attributed this behavior to the presence of cemented levels, whose origin would not be the result of the weathering action. These levels are unevenly distributed throughout the profile, causing heterogeneities in the rock. Thus, the conclusion reached Soils and Rocks, São Paulo, 43 (2)    by this author is in agreement with the results from the present study, as these same levels were also identified in this research, which would explain the increase in durability found in phyllite W4 in relation to W3 from profile Phy01. In profile Phy02, I d2 values obtained for slightly altered phyllites (W1/W2, W2, and W2/W3) are above 90 %. Similar values were found by Ramamurthy et al. (1993)     The durability obtained from phyllites W3 is quite variable. Except for phyllite W3 from profile Phy01, which has a very low internal cohesion (a characteristic of this profile), I d2 ranged from 91.81 (profile Phy02) to 44.08 % (profile Phy03). High variations in moderately weathered phyllites were also reported by Andrade & Saraiva (2010) (between 59.6 and 98.2 %) and Silva & Lana (2012) (between 86.18 and 54.32 %).
The variation in durability found for phyllites W3 may be related to the following changes imposed by weathering, which increase significantly in this degree of alteration: increase in pore volume and significant presence of altered opaque minerals, i.e., which had their crystalline form modified (Fig. 3d). Similarly to the present study, Dhakal et al. (2002) found a close relationship between durability and weathering in clastic rocks since it provided changes in pore volume and distribution and caused crystalline changes in smectites, which coincides with the results of the present study.
Graphs of Figs. 7 and 8 compare the average values of I d2 and porosity and I d2 and amount of oxides, respectively, in the different weathering classes, being possible to observe the influence of these factors on durability. The increased porosity, associated with permeability, provides a larger surface area of the rock available for contact with water (specific surface), which acts to decrease durability, by weakening chemical bonds, changing properties of mineral constituents and causing instability along weakness planes due to pore water pressure (Koncagül & Santi, 1999). Figures 9 and 10 show, respectively, I d2 values with weathering progression and phyllite samples after being subjected to two wetting and drying cycles. As can be observed, although I d2 does not uniformly follow the evolution of the alteration, there is a general trend to reduce this parameter with weathering.
The variability found in durability between phyllites of the same alteration class is also attributed to the heterogeneity of the rock, including the same profile. It happened, for instance, in phyllites W3 collected at distinct points in profile Phy03 (Fig. 2). Depending on profile position, the rock may contain more or less opaque and hence, oxide content, as well as variations in quartz content. Thus, durability is influenced not only by secondary processes derived from weathering but also by the heterogeneity itself intrinsic to the mineralogical composition variation of these phyllites.

Wave propagation velocity
Some samples showed no detection of Vp signal measured perpendicularly to foliation, while in others (the majority), the intensity signal detected during tests was very low, rarely 100 %. The attenuation of Vp in the perpendicular direction is caused by the presence of fractures parallel to foliation that make the rock discontinuous, affecting it significantly. There are often reports in the literature of dif-    ferences in Vp in the direction perpendicular and parallel to foliation in rocks with anisotropic properties (Ramamurthy et al., 1993;Marques et al., 2010;Kurtulus et al., 2012;Nikrouz et al., 2016). Jensen & Elming (2013) reported as potential causes of seismic anisotropy the micro-fissures, joints, faults, macroscopic foliation, stratification, and weathering. These authors found anisotropy in Vp in metariolites with microfoliation, obtaining the highest values in the direction parallel to it. The values of Vp obtained in the parallel direction for slightly altered phyllites (W1/W2, W2, and W2/W3) are in the range of 4601.33-5382.42 (m/s), being compatible with the results of phyllites with similar mineralogical composition considered intact or little altered by other researchers (Ramamurthy et al., 1993;Andrade & Saraiva, 2010).
As shown in Fig. 11, moderately to very altered phyllites (W3, W3/W4, and W4) transmit the ultrasonic waves with less efficiency. Vp is sensitive to the state of alteration of the studied rocks, showing a general trend of reduction as weathering increases.

Correlation between I d1 and I d2 with Vp
The values of Vp measured in the direction perpendicular to foliation were not considered, due to the low intensity signal detected during tests. Figure 12 shows the correlations between I d1 and Vp and I d2 and Vp. In Figs. 12a and 12b, correlations were performed between the mean values of each alteration class, while in Figs. 12c and 12d, the individual values were used, obtaining a much lower correlation coefficient. The dispersed values shown in Figs. 12c and 12d are the result of the influence of weathering and anisotropy of the studied phyllites.
Based on the obtained results, the use of mean values was adequate to correlate durability and wave propagation velocity. Khandelwal (2013) obtained a good correlation between Vp and I d1 for different rock types, including shales, for Vp values in the range from 1682 to 4657 m/s. Sharma & Singh (2008) also found a good correlation between these two parameters when evaluating seven different types of rock, including phyllites and shales. Figure 13 shows that all the physical indices have a strong relationship with Vp, with a reduction in Vp as density decreases (increased porosity and water absorption). These results are in agreement with Gaviglio (1989), Tur-grul & Zarif (1999, Yasar & Erdogan (2004), Kurtulus et al. (2010), Diamantis et al. (2011), Sarkar et al. (2012, Khandelwal (2013), andAzimian &Ajalloeian (2015), who studied the relationship between physical properties and Vp and obtained good correlations between these parameters.

Correlations between Vp and Is
Graphs in Fig. 14 show the relationship between Vp and Is (50) . As expected, the lower the Is (50) is, the lower the Vp. There is a good relationship between Vp and Is (50) measured perpendicular to foliation, with a correlation coefficient of 0.7779 (Table 4), while the correlation coefficient between Vp and Is (50) measured parallel to foliation is lower (R 2 = 0.6139), unlike expected. Thus, despite showing influence on Vp and Is (50) , metamorphic foliation is not sufficient to explain the good relationship obtained between these properties.
On the other hand, porosity is closely related to Vp (Fig. 13) and influences the resistance of weathered rocks (Marques et al., 2010;Andrade & Saraiva, 2010;Leão et al., 2017) since the presence of pores makes the rock less continuous, decreasing Vp and increasing the fragility of the matrix when subjected to compression (Sousa et al., 2005). Thus, the increased porosity due to weathering processes may play a preponderant role in the variation of these properties (Vp and Is (50) ) in the studied phyllites. 304 Soils and Rocks, São Paulo, 43 (2): 297-308, April-June, 2020.
Carvalho et al.

Conclusions
The slake durability test performed in the studied phyllites showed that weathering has a clear influence on this geotechnical property, affecting the texture and mineralogical composition of these rocks. As expected, the progressive increase in porosity and mineral alteration due to weathering was accompanied by a reduction in durability, but the intensity in which this process occurs was established.
Vp decrease with weathering is closely related to variation of physical indices, as proved by the significant correlation coefficients found. Intact rock strength and Vp are influenced by metamorphic foliation. Also, the very good correlation found between Vp and Is (50) may be influenced by the effects of weathering, which cause a higher increase in porosity when compared to the anisotropy intrinsic to phyllites resulting from mineral alignment.
Vp and durability, although presenting general trends, do not evolve uniformly with the weathering progression since porosity, the main physical parameter that influences strength has heterogeneous behavior. This behavior attributed to porosity is probably associated with physical weathering, due to the increase in spacing of pre-existing discontinuities and formation of a micro-fracture network. Therefore, the action of weathering on fractures and micro-fractures makes their effects increasingly complex, reflecting on the physical characteristics of the rock.
In addition to the influence of secondary weathering processes on the variability of physical and mechanical parameters, there is also the mineralogical heterogeneity intrinsic to these phyllites, observed between samples with the same degree of alteration in the same profile, but taken from different points. Both heterogeneity and anisotropy are also revealed in the correlation established between Vp and I d1 and Vp and I d2 when all values for each sample from all three slopes are incorporated. The use of mean values of each alteration class improves the correlation between these properties and allows concluding that the evaluated physical parameters should be used prudently in geotechnical projects, being important that they are associated with a field control, contemplating descriptions of the rocky massif (including discontinuity characteristics), petrographic and micropetrographic analyses, as well as other types of physical and mechanical tests.
Good correlations were found for Vp and slake durability index, for Vp and all physical indices, and for Vp and point load strength measured perpendicular to foliation.
Finally, the authors suggest that similar studies, with the same tests/analyses, should be performed on similar rock types (and also with other lithotypes) from other regions from Brazil.