Scholarly article on topic 'Assessment of physicomechanical parameters in the Lower Red Unit (LRU) of the Wajid Sandstone, Bir Askar, Najran, Saudi Arabia'

Assessment of physicomechanical parameters in the Lower Red Unit (LRU) of the Wajid Sandstone, Bir Askar, Najran, Saudi Arabia Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Ahmed Abd El Aal

Abstract The aim of this study is to determine the mechanical and petrophysical properties of sandstone rocks and using P-wave to correlate among these properties as well as the influence of cement which are intrinsic factors controlling the mechanical properties of clastic rocks. The field study included the description of the Lower Red Unit of the Wajid Sandstone outcrop analog section described previously, lithofacies description, identification, and rock sample collection from the subsurface section. Both X-ray diffraction, SEM and petrographic analyses indicate that the red sandstones are composed of predominantly K-feldspar and quartz grains and an illite matrix. In the laboratory work; porosity (n%), permeability (K md), apparent density g/cc, water absorption (Ab%), void index (e%), Schmidt hammer rebound hardness (SHR), uniaxial compressive strength (σc MPa)), Point load index (Pl) and ultrasonic pulse tests (Vp) were carried out. Then, the results were analyzed statistically to find the correlation coefficient between these measured parameters. The results of Vp with the mechanical and physical properties of the rocks were analyzed using the method of least squares regression. In all cases, the weakly to moderately correlation coefficient (R 2) was determined for each regression. Relatively high correlation values were seen between Vp index and porosity of R 2 0.60. The sandstone in the study can be classified according to the UCS classification for intact rock into very low to low strength (1–20.39MPa) and very low velocity of sound velocity classification.

Academic research paper on topic "Assessment of physicomechanical parameters in the Lower Red Unit (LRU) of the Wajid Sandstone, Bir Askar, Najran, Saudi Arabia"

Egyptian Journal of Petroleum (2016) xxx, xxx-xxx

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Assessment of physicomechanical parameters in the Lower Red Unit (LRU) of the Wajid Sandstone, Bir Askar, Najran, Saudi Arabia

Ahmed Abd El Aal *

Geology Department, Faculty of Science, Al Azhar University, (Assiut Branch), Assiut, Egypt

Civil Engineering Department, Faculty of Engineering, Najran University, P.O. 1988, Najran, Saudi Arabia

Received 19 April 2016; revised 4 June 2016; accepted 22 August 2016

KEYWORDS

Wajid Sandstone; Physicomechanical; P-wave; Strength;

Statistical relationships

Abstract The aim of this study is to determine the mechanical and petrophysical properties of sandstone rocks and using P-wave to correlate among these properties as well as the influence of cement which are intrinsic factors controlling the mechanical properties of clastic rocks. The field study included the description of the Lower Red Unit of the Wajid Sandstone outcrop analog section described previously, lithofacies description, identification, and rock sample collection from the subsurface section. Both X-ray diffraction, SEM and petrographic analyses indicate that the red sandstones are composed of predominantly K-feldspar and quartz grains and an illite matrix. In the laboratory work; porosity (n%), permeability (K md), apparent density g/cc, water absorption (Ab%), void index (e%), Schmidt hammer rebound hardness (SHR), uniaxial compressive strength (rc MPa)), Point load index (Pl) and ultrasonic pulse tests (Vp) were carried out. Then, the results were analyzed statistically to find the correlation coefficient between these measured parameters. The results of Vp with the mechanical and physical properties of the rocks were analyzed using the method of least squares regression. In all cases, the weakly to moderately correlation coefficient (R2) was determined for each regression. Relatively high correlation values were seen between Vp index and porosity of R2 0.60. The sandstone in the study can be classified according to the UCS classification for intact rock into very low to low strength (1-20.39 MPa) and very low velocity of sound velocity classification.

© 2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

* Address: Civil Engineering Department, Faculty of Engineering, Najran University, P.O. 1988, Najran, Saudi Arabia. Fax: +966 175428887.

E-mail address: Ahmed_Aka80@yahoo.com

Peer review under responsibility of Egyptian Petroleum Research Institute.

1. Introduction

Bir Askar center is located in the Najran area at Coordinates 17° 35' 37" N 44° 4' 57" E. It's far about 13 km from the Najran area. Several workers since the 1940s have proposed subdivisions for strata which crop out on the Wajid plateau. The term 'Wajid Sandstone' was first used in an unpublished

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1110-0621 © 2016 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Aramco report for these strata. Powers et al. [1] and Powers [2] formally proposed the term and described a type section. Pallister [3] mapping in the Wajid plateau, proposed two members of the 'Wajid Sandstone' the upper 'Wajid Sandstone' was termed the Ilman Member while the lower was called the Shum. The Wajid Sandstone (Ordovician-Permian) on the Arabian Shield is a medium- to coarse-grained siliciclastic unit forming as outliers on the plain surface of a dominantly Pre-cambrian basement complex. The abundance of iron (in the form of iron-rich horizons) in the Wajid Sandstone has been reported by a number of workers including [4-6]. Hussain et al. [6] observed iron-rich horizons (ironstone) in the basal section of the Wajid Sandstone in the Abha-Khamis Mushayt area, Asir Province (longitudes 42° and 42° 53'E and latitudes 17° 50' and 18° 15'N). Until now, there has been no detailed study of the origin of this ironstone.

Based on detailed fieldwork, geomechanical and petrophys-ical data, the present study explains the impact of ironstone on these characters in Bir Askar, Najran area (Fig. 1a-c).

The main goal of the present study is to evaluate some physical (porosity, permeability, apparent density, water absorption and void index) and mechanical properties (Schmidt hammer rebound hardness, uniaxial compressive strength and Point load index) of Lower Red Unit of Wajid Sandstone and their relationships applying the P wave velocity measured

for some sandstone samples, as well as the effect of iron oxide cement on these properties.

2. Previous Studies

Kellogg et al. [7] proposed the terminology followed by most subsequent workers. They named four members of their 'Wajid Sandstone' based upon lithostratigraphic criteria and field relationships. At the time, no age diagnostic fossils were known from the 'Wajid Sandstone' thought their members were Cambrian through to Ordovician in age [7]. Evans et al. [8] proposed to drop the term Wajid sandstone and replace it with recognized divisions of the Saq and Qasim formations. They also recognized in outcrop the Qusaiba Member of the Qalibah Formation.

The mechanical properties of clastic rocks are not only influenced by external factors, such as stress and surrounding geological environment, but also strongly dependent on the depositional and sedimentary environments, including components, textures, and structures of the rocks. The mechanical properties of clastic formations are important for the energy industry [9]. The relationship of strength and grain size of clastic rocks has been preliminarily investigated [10-12]. Some researchers concluded that peak strength and inverse of the square root of grain size in clastic rocks had a linear relation

Figure 1 (a) Location map for Saudi Arabia indicating the area of study, (b) close up of Najran-Bir Askar road, (c) Detailed Photo image showing the location of study area with borehole and test bits.

[13-17]. Sandstone with a smaller gain size had a higher strength [18].

3. Stratigraphy and general setting

Powers et al. [1,2] briefly described the Wajid sandstone as continuously exposed sequence, from Wadi ad Dwasir (Lat. 20°30'N) south nearly reaching the city of Najran (Lat. 17°35'N), a distance exceeding 300 km. However, the Wajid Sandstone also extends further south, crossing the border of Yemen, where it appears east and north of the Jawf area and north of Sa'dah area, [19]. The lower contact of the Wajid sandstone is non-conformable on the igneous and metamor-phic rocks of the Arabian Shield. The upper contact is drawn disconformably with the basal Khuff (upper Permian) carbonate formation above. The total thickness of Wajid Sandstone is nearly 950 m in southwest Arabia [1].

The Najran and Abha area constitute part of the Asir ter-rane, an oceanic terrane that occupies the southwestern part of the Arabian Shield in Saudi Arabia. During the Precam-brian to Early Cambrian, the basement complex was subjected to compressional stress and subsequent preplanation associated with the Ideas Orogeny [20]. The Wajid Sandstone non-conformably overlies the deeply weathered, preplanned surface of the Proterozoic basement complex [21-23]. Uplift and erosion, related to the Hercynian Orogeny, have also affected the early Paleozoic sequences in the area, as indicated by the presence of incised valleys [5]. During the Cambrian to

Ordovician interval, the Wajid sandstone was unconformably distributed in the area of southwest Arabia and deposited on the southeast margin of the Arabian Shield.

4. Wajid Sandstone and lateral correlation

The Najran area is located near the southeastern margin of the Arabian Shield. Several outliers in the vicinity of the Bir Askar, Bader Al Janoub and Dhahran Al Janoub localities represent the Wajid Sandstone in this area.

Babalola [5] divided the Wajid Sandstone in the Abha-Khamis Mushayt area into informal Red and Gray Units. In the absence of chronostratigraphic markers, [5] tentatively correlated the Red Unit with the Lower Unit of the Dibsiyah Member based on overall lithological similarities, and suggested that the Upper Unit may locally have been removed by erosion. In the Dhahran Janoub - Najran area, the Wajid Sandstone can be correlated with the Upper Unit based on bed similarities and evidence of bioturbation. In addition, the Lower Unit can be tentatively correlated with much of the Wajid Sandstone of [24] in the area near the Yemen border.

The Wajid Sandstone in the study area is represented by an approximately 100 m thick sequence of dominantly coarse to medium-grained massive to cross-bedded sandstone and red hard massive sandstone. Based mainly on the color and distribution of iron-rich horizons, the succession is informally divided into two units; a Lower Red Unit (basal) and an Upper Gray Unit [5,25].

4.1. The Upper Gray Unit (UGU)

This is separated from the overlying Gray Unit by an approximately 50 cm to 1m thick conglomerate bed, comprising quartz and claystone clasts ranging in size from few mm to up to 10 cm. The Gray Soft Unit is composed of a monotonous sequence of white-grayish, pebbly, coarse to medium, occasionally fine-grained, massive to cross-bedded sandstone (Fig. 2a and b). Furthermore, the Lower and Upper unites of sandstone beds generally contain diverse sedimentary structures such as graded bedding, planner and trough cross-stratification, horizontal and parallel laminations, cross-lamination, horizontally bedded to low angle-bedded sandstone and some biogenic disturbances. No fauna were observed in these beds (Fig. 2c and d).

4.2. The Lower Red Unit (LRU)

In outcrops, the iron-rich beds in the Wajid Sandstone show four distinct modes of occurrence: (i) thin beds ranging from a few centimeters to ~2 m thick, (ii) parts of the forests of cross-bedded units, (iii) fractures infill, and (iv) concretions and nodules (Fig. 3a-e) reflect differential cementation and matrix strength. The Lower Red Unit (LRU) composed of alternating successions of dominantly red-pink, occasionally brick red, gray to white, massive to cross-bedded sandstone with ironstone The Wajid sandstone (Cambrian-Ordovician) generally appears with different colors of purple-red to pale pink, brown to light yellow, and gray to pale white (Fig. 4a and b). The sequence is commonly friable except for some ferruginous cemented beds which are generally seen cast

Figure 3 (a) General view of Wajid sandstone, (b) the iron-rich beds in the LRU, (c)-(e) thin beds with few centimeters thick.

Figure 4 (a) Vertical distribution of the upper and lower units in representative boreholes (the horizontal distance is un-scaled), (b) Field photograph showing the lower and upper unites.

Figure 5 (a) Test bit in the site, (b) core samples used in the present study.

Figure 6 (a) Ultrasonic wave (pulse) test of samples, (b) uniaxial Compressive Strength machine.

with various sizes of iron concretions observed at several stratigraphic levels.

4.3. Cement in LRU of Wajid Sandstone

Ferruginous Sandstones Deposits belong to the red beds sediments that possess red coloration due to the presence of finely divided ferric oxides, chiefly hematite. The detrital quartz grains of Wajid Sandstone are seen cemented with ferruginous material and show either point grain-to-grain contacts or no other type of contacts at all. In the Lower Red Unit, all concretionary beds are very rich in ferruginous cement. These concretionary beds are very hard and massive on the crust and the amount of the ferruginous cement decreases inwards, leaving in a few cases the core material either poorly cemented or completely uncemented. The increasing of iron oxide cement leads to increase the Ultrasonic wave (pulse) test, Schmidt Hammer rebound test, Uniaxial Compressive Strength (UCS), Point load index test IS (50) and Apparent Density measurements but decreases the Porosity values.

Also, the increasing of iron oxide cement leads to increase the hardness properties of Lower Red Unit. The source of

ferruginous cements observed in the Wajid sandstone was formed by diagenetic haematite in sandstones in situ after deposition through intrastratal alteration of iron bearing detrital grains.

5. Materials and methods

To study the physical and mechanical properties of LHRU of Wajid Sandstone at Bir Askar center, Najran area, samples were collected from the subsurface. The subsurface samples are collected from 5 boreholes and 2 trail pits (see Fig. 1b). The depths of the boreholes are 10.5m while the depths of the test pits are 2:3 m (see Figs. 4a, and 5a, b). The investigations of physical and mechanical properties of Wajid Sandstone at Bir Askar Center consist of integrated field work and laboratory. The field study included the description of the Wajid Sandstone outcrop analog section described previously, lithofacies description, identification, and rock sample collection from the subsurface section. Moreover, the tested samples of red sandstone of LRU are subjected to X-ray diffraction (XRD) to determine the minerals composition in sandstone specimens (Mineralogical studies). Thin sections were also made for petrographic study under the microscope. The scanning electron microscope is a valuable instrument when heterogeneous materials are to be characterized on a micro-scale. The area investigated is scanned by a thin focused electron beam. The beam can either be stationary or scanning a raster over the sample surface. When the focused beam hits the sample surface, it generates signals that can be detected. These signals can give information regarding chemical composition, topography, crystallography, average atomic number etc. Scanning Electron Microscope (SEM) microanalysis was used to identify the composition of individual ferruginous sandstone grains and to obtain more details about the mineral content. In the laboratory work, 65 cylindrical specimens were prepared; porosity (using saturation method) and permeability (using Hassler Core holder Assembly) measurements were taken. A Schmidt hammer rebound hardness test was carried out on hand specimens. Point load index, uniaxial compressive strength and ultrasonic pulse tests were carried out. Then, the results were analysed statistically to find the correlation coefficient between these measured parameters. The details of these methods are provided here below.

5.1. Schmidt Hammer rebound test

Schmidt hammer has been used in rock mechanical practice since the sixties as a non-destructive index test for a quick characterization of rock strength and hardness [26]. The Schmidt hammer is characterized by its rapidity, portability, and simplicity. It is usually used during the initial stages of design. The resulting reliability is affected by many factors such as rebound value, normalization, type of hammer, smoothness of surface, specimen dimension, moisture content, and weathering. When Schmidt hammers are perpendicularly pressed on the surface, the piston is released toward the surface, and part of the piston impact energy is absorbed and the other part is transformed (sound and heat). The remaining part is the degree to which the surface resists the penetration or the surface hardness. This energy causes the rebound of the piston. The harder the surface and the shorter the depth of penetra-

tion, the greater the rebound is. The ratio between the traveled distances by the piston after the rebound to the original extension is the rebound value. For this test, 15 readings were recorded and the average was taken [27] to reduce the limitation and enhance the reliability of the obtained results.

5.2. Ultrasonic wave (pulse) test

In these methods, P-wave velocity (Vp) measurements are correlated with the porosity increase due to fabric damages in rocks. Such correlations provide a method of measuring the degree of structural damage due to weathering and deterioration using indirect measurements. Damage prevention and repair planning can be made on the basis of such a Vp/structural damage classification.

The ultrasonic wave velocities are determined from the length of the travel pathway and the travel time. The velocity of the P-wave is always higher than that of the S-wave. As they travel through the rock sample the different wave types also experience different attenuation. During the experimental measurements the S-waves are more difficult to handle, so that for most applications P-waves are used.

A cylindrical specimen is prepared by trimming the ends. Before the test, sample length is measured. An ultrasonic instrument consists of a digital indicator, generator of pulse, transmitter, and receiver transducer. Both transmitter and receiver are placed at the ends of the sample (Fig. 6a). The travel time of wave pulse is measured. Finally, the velocity is measured through dividing the sample length by the time. According to ISRM [27], the ultrasonic wave velocity through any solid materials depends upon material elastic properties and density. Thus, an ultrasonic wave test can be conducted to indicate quality of the material.

5.3. Uniaxial Compressive Strength (UCS)

Uniaxial compressive strength (UCS) of construction material is probably the most important index properties for the evaluation of mechanical behavior of rocks [28]. The UCS provides a clear perception regarding the selection of material for appropriate civil structure. UCS is also used to calculate the energy required to cut the sample [29,30]. Furthermore, it helps in selection of appropriate excavation technique. In the present study, sample from boreholes are prepared to measure UCS as per ASTM [31] specifications (Fig. 6b). The strength of rock decreases in the presence of joints and veins [32]. For the sandstones, Mosch [33] documented that the majority of the values are in the range between 1.86 km/s and 3.62 km/s, with only a few outliers. The methods selected (Schmidt Hammer and point load index testing) were utilized for predicting compressive strength; they have many advantages over the other methods because they do not require complicated preparation of rock samples and most of them are portable [34]. Many authors have reported empirical equations through which uniaxial compres-sive strength can be predicted from point load index and Schmidt hammer number particularly for carbonate rocks [35-37].

5.4. Point load index test IS (50)

Point load index test IS (50) has been developed by Broch and Franklin [38]. Many authors suggested linear rather proposed

Figure 7 (a) Typical thin-section view of red sandstone in crosspolarised light, Qz = quartz, f = feldspar, yellow arrows indicate iron cement, (b) abundant iron cement (mainly hematite ''Yellow Arrows") impacts a floating grain (Qz) texture of the LRU in Wajid Sandstone, (c) X-ray diffraction pattern of a typical red sandstone sample, showing reflections of quartz (Q), K-feldspar (F), illite (I), and hematite (H). Intensities of reflections are related to abundance of minerals.

Figure 8 (a) Field photograph of high iron oxide cement (Hematite), (b) SEM image of spherical hematite particles (yellow arrow), p = some pores in the natural sample (c), close-up of (a) q = quartz, f = feldspar, for hematite particles (yellow arrow). The right middle photo is a close up showing iron cement ''spherular hematite" (DSEI).

power relations [34]. According to Broch and Franklin [38], in conical, spherically truncated platens, and the machine also a point load test, the sample is loaded until failure state consists of manual control handle and dial gages, and the fail-through applying a concentrated load through a pair of ure is achieved within 10-60 s using a manual handle. The cor-

Table 1 Physical properties of some selected representative samples from the LRU.

No. Depth (m) W sat (g) W dray (g) W in water (g) Ab% Density (g/cc) n% e% K (md)

1 1.5 460 411 221 11.92 2.163 20.50 25.79 114

2 3 765 687 344 11.35 1.991 18.53 22.74 112

3 6 824 800 674 3.00 2.137 16.00 19.05 113

4 4.5 651 593 316 9.78 2.149 17.31 20.94 112

5 7.5 632 612 511 3.27 2.243 16.53 19.80 114

6 9 456 435 324 4.83 2.742 15.91 18.92 114

7 4.5 567 523 321 8.41 2.18 17.89 21.78 111

8 7.5 856 823 645 4.01 2.241 15.64 18.54 109

9 9 567 542 401 4.61 2.227 15.06 17.73 108

10 10.5 534 498 276 7.23 2.243 13.95 16.22 107

Maximum 11.92 2.742 20.50 25.79 114

Minimum 3.00 1.991 13.95 16.22 107

Average 6.841 2.2316 16.732 20.151 111.4

W sat = Weight of Saturated surf. Dry sample in air, g. W dray = Weight of i oven dry sample in air, g W in water = Weight of sat. Surf. Dry

sample in water, g, n = Porosity%, e = Void index0/», K = permeability md.

rected size index value is measured at D = 50 mm, the point load index (PLS) index can be represented as follows:

IS (50 mm) = FP/DE2

where IS (50 mm) = corrected PL index for a 50-mm diameter sample.

F = the size correction factor, P = the peak load, and De = diameter of sample.

IS (50) can be used for several shapes (irregular lumps, cylindrical and circular specimens). Regular and irregular lump cuttings were prepared to carry out the point load index test [38].

5.5. Porosity and apparent density measurements

Porosity is defined as the ratio of the pore volume to the bulk volume. In this study, it was measured using a core test Helium porosimeter which is based on the concept of Boyle's law. Grain volume is measured after porosimeter reference volume calibration; then, bulk volume and porosity are measured. Apparent density is the ratio between grain weight and grain volume in gram cubic centimeter. It is significant in differentiating between several types of rocks and useful in determining porosity in the subsurface.

Table 2 Mechanical properties of some selected representative samples from the LRU.

No. Depth (m) Vs RN RQD (%) Pl (MPa) rc (MPa) Rock mass quality

1 1.5 0.5 22 50 0.5 13.1 Weathered rock

2 3 1.2 22 66 0.8 14.3 Moderately weathered rock

3 6 1.1 23 72 1.0 14.8 Moderately weathered rock

4 4.5 1.0 25 65 0.9 15.3 Moderately weathered rock

5 7.5 1.3 23 71 0.8 15.2 Moderately weathered rock

6 9 1.5 27 78 0.6 16.1 Hard rock

7 4.5 1.4 26 62 0.7 12.3 Moderately weathered rock

8 7.5 1.4 30 78 0.9 13.3 Hard rock

9 9 1.6 31 80 1.1 14.5 Hard rock

10 10.5 1.7 33 85 1.2 15.7 Hard rock

Maximum 0.5 22 50 0.5 13.1 -

Minimum 1.7 33 85 1.2 16.1 -

Average 1.27 26.2 70 0.85 14.46 -

V = sound velocity km/s, RN = rebound number, RQD = Rock-quality designation (%), PL = Point load index (MPa), rc = Unconfined Compressive Strength (MPa).

5.6. Rock-quality designation (RQD)

Rock-quality designation (RQD) is a rough measure of the degree of jointing or fractures in a rock mass, measured as a percentage of the drill core in lengths of 10 cm or more. High-quality rock has an RQD of more than 75%, low quality of less than 50%. Rock quality designation (RQD) has several definitions. The most widely used definition was developed in 1964 by Deere [39]. It is the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm in length measured along the centerline of the core. In this respect pieces of core that are not hard and sound should not be counted though they are 100 mm in length. RQD was originally introduced for use with core diameters of 54.7 mm (NX-size core). RQD has considerable value in estimating support of rock tunnels. RQD forms a basic element in some of the most used rock mass classification systems: Rock Mass Rating system (RMR) and Q-system.

5.7. Liquid permeability measurement (K)

The permeability was measured using Hassler Core Holder Assembly. This equipment is composed of the hand pump, air cylinder, wet test meter, and core holder panel. This assembly enables to measure data of flow rate using flow line gages. When the flow reaches stable conditions (steady state), the regression between UCS and point load and part of permeability is measured using an equation which is a function of core area length and cross-sectional area, atmospheric pressure, air viscosity, and flow rate. The gas permeability is converted into liquid permeability using a Klinkenberg equation.

6. Results and discussion

6.1. Petrographical analyses

Many researchers have shown that the mechanical properties of rocks are greatly affected by their fabric and petrography. The effect of grain size and shape, porosity and mineralogical composition on strength and crack propagation was analyzed for many lithologies. Several relationships and good correla-

tions have been established between the porosity and engineering properties for different rock types. Thin-section petrography shows that the Lower Red Unit of Wajid Sandstone of the studied outcrop is a medium to coarse-grained quartz arenite with quartz content of over 91% (Fig. 7a). Among the non-quartz framework grains, lithic fragments are the dominant grain types followed by opaques and feldspars. Except for a few grains of polycrystalline quartz, the framework quartz is exclusively monocrystalline suggesting a felsic igneous source. The grain size of the quartz ranges from less than 100 mm to over 1400 mm with the majority ranging from 500 to 700 mm (Fig 7b).

6.2. Mineralogical studies

X-ray Diffraction (XRD) results revealed that the major minerals are quartz and iron oxides, the minerals in the red sandstone specimens are mainly feldspar and quartz, with some illite, and some quantities of hematite (Fig 7a and).

Based on SEM microscopy results, the iron oxides in the occurrence as matrix coated and impregnated ferruginous sand grains with low crystallinity cement between the sand grains. Fig 8a-c are clear pictures SEM for the presence of the elements associated with the red sandstone and iron oxide cement. In the back scattered electron image (BSE)-mode of SEM, the groundmass of the studied samples consist of fine to medium grained sand (0.1-0.5 mm), and most of the sand is coated and impregnated by iron oxides (Fig 9a and b). The red sandstones contain SiO2 and Al2O3 values ranging from 61 to 71 wt.% and from 6 to 13 wt.%, respectively. Relatively high SiO2 and K2O values are linked to the presence of quartz and K-feldspar grains and the illite matrix. In all the samples, hematite was determined to be an accessory mineral. Hematite content ranges from 6% to 12%. The large hematite crystals are interpreted to be of late diagenetic origin. Whole-rock SEM analyses confirm the presence of feldspar and quartz grains and an illite matrix. Petrographical analyses of the studied samples indicate that the increasing the cement percentage increases the compressive strength, both point load, Wave velocities, apparent density and schmidt hammer rebound test and decreases the porosity and permeability.

Figure 10 Correlation between physical properties: (a) between porosity and absorption, (b) void index and porosity, (c) permeability and porosity and (d) Apparent density (yAPP) and permeability.

6.3. Physical and mechanical properties

Tables 1 and 2 show the test results of physical and mechanical properties of lower Red Units of Wajid Sandstone in Bir Askar Center. The UCS of the samples ranges between 12.3 and 15.7 MPa. According to the UCS classification for intact rock [40], the studied sandstone rocks can be classified into very low to low strength (1-20.39 MPa). This category is matched with the lithologic properties of the corresponding samples. Whereas the most of the hard ferruginous sandstone at the base of the Lower hard unit has low strength, soft sandstone in the upper gray unit has very low strength.

6.3.1. Physical properties

The main physical properties (Apparent density, absorption, porosity, permeability and void index) of the studied sandstone were done and the results of these tests are given in Table 1 for some representative samples.

Apparent density and porosity are the most important factors in assessing the mechanical properties of rock units. The Apparent density of samples may tell us on the compactness of the rock components which will affect the overall properties of the rock [41]. Apparent density and porosity are often related to the strength of rock material. A low-density and high-porosity rock usually has low strength. The porosity of rocks is one of the prime determining factors with respect to rock durability as construction materials [42,43]. Water absorption is an important rock index depending on mineralogy and porosity of the rock.

Porosity is one of the governing factors for the permeability. Porosity provides the void for water to flow through in a rock material. High porosity therefore naturally leads to high permeability [41].

In the present study, the minimum apparent density (yAp) value of the studied samples is 1.99 g/cm3, and the maximum value is 2.74 g/cm3 with average 2.23 g/cm3. The porosity (n %) of the study varies from 13.95% to 20.50% with an average of 16.73%.

The maximum permeability value is 107 md, the minimum value is 114 md, and the median is 111.4 md. The maximum absorption reading is 11.29%, the minimum is 3.00%, and the average is 6.84%. This indicates that the average porosity and permeability are low due to intensive iron oxide cement. The maximum void index reading is 25.79%, the minimum is 16.22%, and the average is 20.15%.

Fig. 9(a, and b, c and d) show the relations between Apparent density (yAPP), porosity, permeability and absorption and void index. The R value (correlation value) is low that is due mainly to the presence of heterogeneous concentrations of iron oxides. In addition, there appears to be a strong significant correlation (R value is 0.99) between porosity and void index of the study samples (Fig. 10a and b).

It is clear that apparent density decreases with increasing the iron oxide cement. Whereas the porosity (n%) of the studied samples increases with increasing permeability Fig. 10(c and d), void index and absorption, as well as the Point load increases with decreasing porosity (n%) of the studied samples. For instance, the dissolution of unstable minerals, packing

Figure 11 (a) Correlation between porosity (n) and point load (Pl), (b) Correlation between wave velocity (V) and density, (c) Correlation between wave velocity (V) and rebound number (RN) porosity, (d) Correlation between wave velocity and point load (Pl), (e) Correlation between P-wave velocity and uniaxial compressive strength, and (f) Correlation between wave velocity and permeability.

inhomogeneity and poor connectivity of the grains contribute significantly to variability in the permeability distribution.

6.4. Wave velocities-mechanical properties and porosity/ permeability

The velocities of porous rocks strongly depend on the porosity and the fluid content inside. The UCS values of the studied sandstone range from 13.1 to 16.1 MPa. The velocity (V%) of the study sandstone varies from 0.5 km/s to 1.2 km/s with an average of 1.27 km/s. The point load (PL) varies from 0.5 to 1.2 MPa with an average of 0.8 MPa. In LRU the highest Schmidt hammer rebound value SHV number (N) was 33, the lowest was 20 with an average of 26.2. The rock quality

designation (RQD) ranges from 50% to 85% with an average of 70%. The values of RQD of the study sample decrease with the increased porosity and void index (Table 2).

The velocity is inversely proportional to porosity and permeability. The velocity decreases with increasing porosity and permeability. The RN increases with an increase in wave velocity (Vp). The same relationship is found in point load index and UCS. This is mainly because the rock samples are strongly compacted, and therefore, waves can pass quickly through the rock specimen. The wave velocity increases with increasing the apparent density of the all studied samples. High wave velocities are due to intensive and heavy compaction and cementation processes which reduce porosity [44].

Figure 12 (a) Correlation between wave velocity (V) and porosity. (b) Field photograph for the iron concentration in the LRU, (c) high wave velocity low porosity sample, and (d) Low wave velocity high porosity sample.

The Classification of rock samples according to Rock mass quality is moderately weathered rock to hard rock, as well as according to the SV classification [45], the measured SV values are in the very Low category because the al sample has V less than 2.5 km/s. Fig. 11a shows that point load index is moderately correlated to porosity; inverse (negative) non-linear correlations are noticed.

The results of SV, uniaxial compressive strength, point load index, Rebound number, porosity, permeability and density of the rocks were analyzed using the method of least squares regression. The equation of the best-fit line and the correlation coefficient (R2) were determined for each regression. The SV values of the rocks were correlated with the physical and mechanical properties for each rock type. In all cases, the relationships between are characterized as non-linear, and the coefficient of determination (R2) is ranging between 0.09 and 0.6, which shows moderate correlation.

Relatively weakly to moderately correlation values between SV index Apparent density R2 = 0.09 (Fig. 11b) and Rebound number of R2 0.44 (Fig. 10c) were found. Also, it is seen that, the correlation values between SV index and uniaxial compres-sive strength R2 = 0.16 (Fig. 11e), and permeability R2 = 0.32 (Fig. 11f).

The graphs of the mean values of the test results between SV index and uniaxial compressive strength; point load index, Rebound number, porosity, permeability and Apparent density are shown in Fig. 11a, c-f.

The variance in physical and mechanical aspects in the rock samples can be attributed to the influence of lithologic variables such as the ratio and distribution of matrix and porosity, grain size variations, and subtle mixing or interlamination of the lithologies present. Physical and mechanical aspects distri-

butions are highly variable due to the spatial distribution of the architectural elements, sedimentary structures and diagenesis.

Relatively high correlation values between SV index and porosity of R2 0.60 (Fig. 12a-d). The SV increases with increases in uniaxial compressive strength; point load index, Rebound number and density. The SV decreases with increases of porosity and permeability.

7. Conclusions

Based on the collected geological and geomechnical information about the measured properties of specimens of the studied LRU of Wajid Sandstone in Bir Askar, Najran area, it is able to formulate the following conclusions:

(1) This study mainly focuses on Lower Red Unit of Wajid sandstone (Cambrian-Ordovician) in Bir Askar center, Najran area. Laboratory testing was carried out on selected samples to investigate physical and mechanical properties of

(2) In the present study, the minimum apparent density (yAp) value of the studied samples is 1.99 g/cm3, and the maximum value is 2.74 g/cm3 with an average of 2.23 g/cm3. The porosity (n%) of the study varies from 13.95% to 20.50% with an average of 16.73%.

(3) The maximum permeability value is 117 md, the minimum value is 107 md, and the median is 111.4 md.

(4) The maximum absorption reading is 11.29%, the minimum is 3.00%, and the average is 6.84%. The UCS values of the studied sandstone range from 13.1 to 16.1 MPa.

(5) The velocity (V%) of the study sandstone varies from 0.5 km/s to 1.2 km/s with an average of 1.27 km/s.

(6) The point load (PL) varies from 0.5 to 1.2 MPa with an average of 0.8 MPa. In LRU the highest Schmidt hammer rebound value SHV number (N) was 33, the lowest was 20 with an average of 26.2.

(7) The rock quality designation (RQD) ranges from 50% to 85% with an average of 70%. The values of RQD of the study sample decrease with the increased porosity and void index.

(8) Strong relationships are observed between compressive strength and both point load strength index IS (50). The high correlation coefficients (R2) were determined for each regression. The correlation equations are as follows:

n = —4.47ln(vp) + 17.593 R2 = 0.6938

UCS = 1.6195ln(vp) + 14.278 R2 = 0.1603

permeability = —4.2ln(v) + 112.21 R2 = 0.3254

RN = 7.5411 ln(Vp) + 24.748 R2 = 0.4474

Vp = 0.7669 ln(PL) + 1.4188 R2 = 0.3571

n = —4.776ln(PL) + 16.053 R2 = 0.363

Void index = 24.817 ln(n) — 49.628 R2 = 0.9919

n = 2.3327 ln(abs) + 12.517 R2 = 0.4054

The equations are practical, simple and accurate enough

to apply and are recommended for use in practice.

(9) The velocity is inversely proportional to porosity and permeability. The velocity decreases with increasing porosity and permeability. The RN increases with an increase in wave velocity (Vp). The same relationship is found in point load index and UCS. This is mainly because the rock samples are strongly compacted, and therefore, waves can pass quickly through the rock specimen. The wave velocity increases with increasing the apparent density of the all studied samples.

8. Recommendations

(1) It is important to perform more studies on different types of sandstone samples that are encountered in engineering projects in order to improve the statistical relationships between rock hardness and rock engineering properties.

(2) It is important to investigate the effect of sample orientation on the relationships between rock hardness and its engineering properties.

Acknowledgements

The author would like to thank the Prof. Alaa Moustafa, Geology Department, Faculty of Science (Assiut Branch); Al-Azhar University provided essential logistic, administrative and scientific help. Also the author is deeply grateful to the editor in Chief Prof. Ahmed Al-Sabagh and to two anonymous reviewers for their helpful comments on the manuscript.

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