The control of composition, texture and weathering on the physical and strength properties of selected intrusive igneous rocks from North Pakistan

This work characterizes intrusive igneous rocks from north Pakistan in terms of their mineralogy, texture and weathering grades and their effect on the physical and strength properties. The mac and intermediate rocks showed a low cumulative percentage of quartz, feldspar and plagioclase with high specic gravity, strength (i.e. UCS and R-value) and UPV values compared to the felsic rocks. Likewise, samples with anhedral grain shape, irregular boundaries, ne to medium grain size (UD, ANS, CGN) showed higher strength values, that is, 121, 118 and 91 MPa compressive strength and 11, 9, and 12 MPa tensile strengths, respectively. The weathering grades assigned to the investigated samples, such as fresh (WG-I), slightly weathered (WG-II) and highly weathered (WG-III) corresponded well with the physical and strength properties, that is, as the grade increased from WG-I to WG-III, the porosity and water absorption increased (0.28% and 0.72% respectively), whereas the specic gravity, compressive strength and tensile strength decreased (2.04, 20 MPa and 2.5 MPa, respectively, for CGA). The presence of quartz affects rock strength; however, no signicant correlation was observed for strength and maximum and mean grain sizes of different minerals.


Introduction
Rocks, whether igneous, sedimentary, or metamorphic, have been used as a construction material throughout human history. Strength and durability of rocks are the two key parameters that require evaluation before their selection as construction material and dimension stone. Studies have shown that in addition to the composition, several petrographic features control rock strength and the stress and strain behavior of rock, such as modal abundance, texture and grain size distribution (Arif et al. 1999;Sajid et al. 2016;Sousa 2013).
Moreover, weathering and alteration also in uence the rock strength and durability (Coggan et al. 2013;Tugřul 2004).
The mechanical properties of different textural varieties of Utla granites from north-west Pakistan were investigated (Sajid and Arif 2015). It was observed that reduction in the strength is linked to the extensive recrystallization and associated mineralogical changes. Altered parts of minerals and petrological features such as exsolution in mineral phases can induce compressional fractures under loading (Coggan et al. 2013). As a general trend, ne-grained rocks show higher strength compared to their coarse-grained counterparts. However, with increasing complexity in texture (variation in grain size, shape and boundary), the strength also increases (Åkesson et al. 2003;Lindqvist et al. 2007;Tuğrul and Zarif 1999). Lindqvist et al. (2007) noticed that minerals with euhedral grains (regular boundaries) serve as discontinuities in a rock structure that facilitates fracture growth. The textural variations affect other properties such as resistance to drilling penetration and thermal wear (Howarth and Rowlands 1986). Studies show that micro-textures and fractures affect rock weathering and are primarily responsible for the changes in the physical and mechanical properties (Coggan et al. 2013;Rigopoulos et al. 2010;Sajid and Arif 2015;Sajid et al. 2016).
The northern part of Pakistan consists of Indian Plate, Kohistan Island Arc (KIA) and Eurasian Plate (Coward et al. 1986;Tahirkheli 1979). The Indian Plate consists of several intrusive igneous rocks that can be proposed as quality construction materials such as Mansehra Granite, Malakand Granite, Utla Granite, Ambela Granite, Chakdara Granite, Swat Granite and some dolerites in the form of dykes. The textural and weathering controls on mechanical behavior for a particular rock type have been elaborated in detail (Rigopoulos et al. 2010;Sajid et al. 2016). This study, in contrast, highlights the effects of common textural observation, derived from various rock units, on the mechanical nature of rocks. For this purpose, the geochemically and texturally different intrusive rocks from north Pakistan were investigated and the role of texture and weathering on their physical and strength properties was discussed. Petrography, physical tests (speci c gravity, water absorption, porosity, Ultrasonic Pulse Velocity-UPV) and strength tests (compressive, tensile, Schmidt hammer rebound-R-value) were performed. Finally, the relationships of petrographic and engineering properties of these rocks were also statistically modeled and evaluated.

Materials And Methods
Fieldwork was conducted to collect bulk samples from Utla Dolerite (UD), Nepheline Syenite (ANS), Chilas Gabbronorite (CGN), Ambela Granite (AG), Chakdara Granite (CGB), Swat Granite (SG), and Chakdara Granite (CGA) (Fig. 1). The samples were collected based on texture, weathering-grade and mesoscopic structures ( Table 1). The weathering condition was assessed, based on a careful examination of rocks in terms of the original texture preserved, the color of the fresh and weathered surfaces and impact sound produced by the geologic hammer (Borrelli et al. 2007;Irfan and Dearman 1978). Indicators of weathering, such as fractures, recrystallization and alteration of different minerals were also noticed. Figure 2 shows cylindrical core specimens (50-mm diameter) obtained from the investigated rock types to perform the physical and strength tests. The tests were conducted on three core specimens from each rock sample and average results are reported. Thin-sections were prepared from small chips of cores (size 40 x 20 mm) for the petrographic study. The petrographic examination included both the naked eye and microscopic observations. Thin-sections from each sample were studied under the polarizing microscope (Nikon Eclipse LV100ND). Minerals were identi ed based on their optical properties (MacKenzie and Guilford 2014). The modal abundance was determined (based on visual estimation) and grain shape, size and arrangements were noted for textural identi cation.
The physical property tests such as speci c gravity and water absorption were determined in the laboratory according to standard test methods for absorption and bulk-speci c gravity of dimension stone (ASTM, DC97 / C97M-18). The porosity of the investigated rocks was obtained using the saturation method (Franklin 1979). The UPV was determined using the ultrasonic pulse velocity test instrument from MS CONTROLS Italy. A pitch-catch technique was used that involves a pair of transducers i.e. transmitter and receiver (Aydin 2013).
The frequency of 10s -1 was used to acquire the transit travel time of the core samples at two conditions i.e. saturated surface dry (UPV SSD ) and the oven-dry (UPV OD ) at 110 °C for 24 h. To obtain better correlations, all the tests were conducted on the same specimen that was later used for the strength tests.
The Uncon ned Compressive Strength, (UCS) test was conducted according to the standard test methods for compressive strength and elastic moduli of intact rock core specimens under varying states of stress and temperatures (ASTM, D7012-14e1). Uncon ned Tensile Strength (UTS) test was performed using a Brazilian test method according to the standard test method for splitting tensile strength of intact rock core specimens (ASTM, D3967-16). The Schmidt hammer rebound test (R-value) is a nondestructive method for determining the strength and was conducted according to the standard test method for determination of rock hardness by a rebound hammer method (ASTM, D5873-14). The R-value was obtained using N-type Schmidt Hammer (SH) equipment having impact energy 0.735 Nm 3. Results Figure 3 shows the selected microphotographs of the investigated rocks and Table 1 shows their petrographic description. Table 2 shows the modal mineralogy and average grain size. Based on the modal mineralogy, the rocks are classi ed as ma c (UD, CGN) intermediate (ANS,) and felsic (AG, CGB, SG, CGA). Tables 3 presents weathering grades (WG) assigned to the investigated rocks based on eld and microscopic observations. The rocks were classi ed as fresh, WG-I (UD, ANS and CGN), slightly weathered, WG-II (AG, CGB and SG) and highly weathered, WG-III (CGA). Table 4 enlists the average results of physical properties. Fresh, WG-I samples (UD, ANS and CGN) showed higher speci c gravity, and UPV sat (3.08 and 5573.17 m/sec respectively for UD), and lower WA (0.13% for CGN) and porosity (0.12% for ANS). The highly weathered sample, WG-III, (CGA) showed the minimum speci c gravity (2.04) and UPV dry (1526.26 m/sec), and the maximum water absorption (WA) (0.28%) and porosity (0.72%). The results of UPV displayed a slight decrease from UPV sat to UPV dry , except for CGA, that displayed a slight increase. These results agree with previous studies on weathering effects on granites that showed a material with high porosity can retain more water and thereby results in reduced UPV values (Sousa et al. 2005;Vasconcelos et al. 2008).

Strength Properties
The strength values of the investigated rocks followed the physical properties (Table 4), i.e. fresh, WG-I samples showed higher UCS (120 MPa for UD) and UTS values (12.40 MPa for CGN). Whereas highly weathered, WG-III sample showed lower UCS and UTS values (20 MPa and 2.5 MPa, respectively, for CGA). These results are comparable with those reported in previous studies from Pakistan (Arif et al. 1999;Ra q et al. 1988;Sajid and Arif 2015;Sajid et al. 2016) and from various parts of the world such as (Basu et al. 2009;Sousa 2013;Tugřul 2004). Likewise, high R-value was obtained for fresh, WG-I sample (53.78 for UD) and lowest for highly weathered, WG-III sample (13.67 for CGA).

Petrographic, Physical and Strength Properties
The petrographic, physical and strength properties of the investigated rock varieties are discussed.
Sample UD contained a relatively high number of opaque minerals (5%-8%), that resulted in a higher speci c gravity (3.08). Likewise, the higher porosity (0.37%) was attributed to the slight alteration of pyroxene mineral (Fig. 3a) and intergranular fractures. Among the investigated samples, fresh, WG-I sample, UD showed the highest UCS and UTS values (121 MPa and 11 MPa, respectively).
The sample ANS showed a slight alteration of alkali feldspar and amphibole that resulted in relatively high water absorption and porosity values (0.19% and 0.12%, respectively). However, it also displayed high UCS and UTS values (i.e. 118 MPa and 9 MPa, respectively) and fresh weathering-grade (WG-I). This is attributed to the inequigranular, anhedral grains, irregular grain boundaries mainly of ne-grained feldspar surrounding the nepheline (Fig. 3b).
The sample CGN showed a uniform grain size and regular boundaries. The twinning and slight alteration (Fig. 3c) resulted in moderate values of water absorption and porosity (i.e. 0.13% and 0.28%, respectively) and lower UCS and UTS values (i.e. 91 MPa and 12 MPa, respectively) compared with other fresh, WG-I samples.
In sample AG, the water absorption and porosity were the lowest (0.06% and 0.14% respectively) among the investigated samples. The subhedral grain shape and the presence of large feldspar grains (up to 10 mm) having intra-granular fractures (Fig. 3d) contributed to its moderate UCS and UTS values (i.e., 60 MPa and 6 MPa, respectively) and slightly weathered, WG-II weatheringgrade.
The sample CGB showed relatively fresh grains of alkali feldspar, quartz and mica than CGA (Fig. 3e) and therefore resulted in lower water absorption and porosity (0.17% and 0.44% respectively), higher UCS and UTS values (i.e. 53 MPa and 6 MPa, respectively) and slightly weathered WG-II weathering-grade.
The sample SG showed gneissosity in the form of aligned aky mica (Fig. 3f). Besides, it showed gneissosity and slight weathering (WG-II) and resulted in moderate water absorption and porosity (0.12% and 0.26% respectively). The alteration as sericitization and fractures in alkali feldspar impacted its UCS and UTS values (i.e. 45 MPa and 6 MPa, respectively). Åkesson (2004)also made similar observations on microstructures in granites and marbles from Sweden.
The sample CGA was highly sheared (WG-III) and showed sericitization, alteration and intense fracturing of alkali feldspar (Fig. 3g). As a result, high water absorption and porosity values were obtained (0.28% and 0.62%, respectively). It showed the lowest UCS and UTS values among the investigated samples (20 MPa and 3 MPa).  Figure 4a shows the ma c and intermediate rocks, fresh (WG-I) having a lower cumulative percentage of Q+F+P resulted in a higher speci c gravity (2.68 to 3.08). Whereas, the felsic rocks, slightly weathered to highly weathered (WG-II and WG-III) having a higher cumulative percentage of Q+F+P showed lower speci c gravity (2.0 to 2.67). Rocks having higher speci c gravity indicate heavy and high-strength minerals, which have a signi cant impact on the rock strength (Fig. 4d). These ndings are in agreement to (Sajid et al. 2016), who correlated modal composition of quartz, plagioclase and feldspar against the UCS and found negative correlations for quartz and plagioclase but positive for feldspar. Similarly, compared to felsic rocks, ma c rocks showed higher UPV OD values (Fig. 4e) with R²=0.90, as also reported by (Behn and Kelemen 2003).  (Fig. 6a). The presence of quartz also affects rock strength, which agrees with the results of (Sajid et al. 2016). The rock strength was plotted against maximum and mean grain sizes of different minerals (Fig. 6band c) and no signi cant correlation was observed which is in contrast to the previous studies by Sajid et al. (2016); Tugřul (2004) and many others. It can be inferred from these plots that meaningful relationships derived by the previous workers cannot be generalized for any rock type.

Summary And Conclusions
The details of the eld and petrographic observations, geochemical analyses, physical and strength properties of the selected intrusive rocks from north Pakistan are presented in this study. Based on the detailed laboratory testing, the investigated intrusive igneous rocks were categorized as fresh, WG-I (UD, ANS and CGN), slightly weathered, WG-II (AG, CGB and SG) and highly weathered, WG-III (CGA).
Physical and strength properties showed a strong relationship with weathering grades. Fresh, WG-I samples (UD, ANS and CGN) showed higher speci c gravity, and UPVsat (3.08 and 5573.17 m/sec respectively for UD), and lower WA (0.13% for CGN) and porosity (0.12% for ANS). Whereas the highly weathered sample, WG-III, (CGA) showed the minimum speci c gravity (2.04) and UPV OD (1526.26 m/sec), and the maximum water absorption (WA) (0.28%) and porosity (0.72%). Likewise, fresh, WG-I samples showed higher UCS (120 MPa for UD) and UTS values (12.40 MPa for CGN). While highly weathered, WG-III sample showed lower UCS and UTS values (20 MPa and 2.5 MPa, respectively, for CGA). The ultrasonic pulse velocity of UPV OD slightly decreased compared to UPV SSD . The rock strength decreased as the composition changed from ma c to felsic. The presence of quartz also affects rock strength; however, no signi cant correlation was observed for the rock strength and maximum and mean grain sizes of different minerals. It can be inferred from the results that relationships derived by previous workers cannot be generalized for any other rock type.

Declarations Funding
This research did not receive any speci c grant from funding agencies in the public, commercial, or not-for-pro t sectors.
Con icts of interest/Competing interests: The authors certify that they have NO a liations with or involvement in any organization or entity with any nancial or non-nancial interest in the subject matter or materials discussed in this manuscript.     Ophitic to sub-ophitic texture, with polysynthetic twinning in plagioclase, was mostly fresh but slight alteration was observed in pyroxene.
Fresh I ANS Fine to medium-grained, grey and no discolouration. Purely fresh and produced good sharp sound with a geologic hammer.
Major minerals such as feldspar and nepheline were fresh but a slight alteration of amphibole was observed.
Fresh I CGN Greyish in colour on fresh while brown on the weathered surface, medium-grained. Very hard, having compact sound with a geologic hammer.
A slight alteration was observed in plagioclase and pyroxene at places but overall dominantly consisted of fresh mineral grains.
Fresh I AG Milky white colour with dark greyish phenocryst, medium-grained, original texture was preserved. Produced a compact sound when struck with a geological hammer. The weathered surface colour was brownish-grey.
Minerals with a fresh appearance and no signs of prominent alteration. Some fractures in feldspar and quartz were present.
Slightly weathered II CGB Light brown, ne to medium-grained. Slight discolouration and moderately foliated. Fairly compact sound with a geologic hammer.
Comparatively fresh mineral grains to CGA, however, alteration of feldspar was observed.
Slightly weathered II SG White in colour, moderately gneissose and medium to coarse-grained. Slightly fresh, and produced dull sound with a geological hammer.
Alteration and sericitization were observed in both feldspar and micas. Feldspar was fractured and mica was mostly aligned.
Slightly weathered II CGA Milky white in colour, ne-grained, having discolouration. Extremely sheared and foliated. Produced a dull sound and was easily breakable with a geologic hammer.
Thin sections appearance was dirty.
Major minerals such as feldspar and quartz were highly fractured. Sericitization and alteration were commonly observed in feldspar and amphibole.