Abstract
The investigation of the effect of petrography and diagenetic features on the geomechanical properties of the sandstone and their relationship to rock failure are of vital importance for different construction projects. The present study involves analyzing multi-vertical lithofacies profiles around the region of Wadi Halfa, North Sudan. The sandstone is dominantly composed of monocrystalline quartz grains (60%) accompanied by some polycrystalline quartz, feldspars, lithic fragments, micas, and heavy minerals. Iron oxides are the main type of cementing materials (14%), with some (2%) of carbonates and clay minerals. The average porosity of all studied samples is 12%. The compressive strength ranges widely, influenced by weathering, grain size, cementing materials, and bedding planes. The uniaxial compressive strength is more influenced by wetting when the load is parallel to bedding planes. Sandstone anisotropy is suggested by a U-shaped curve, with lower values at 45° and higher values at 90° and 0°. The geomechanical behavior of rocks masses in Wadi Halfa was evaluated through a combination of field and laboratory analyses which revealed a variable Rock Mass Rating (RMR) ranging from 58 to 92 and a Geological Strength Index (GSI) ranging from 33 to 61.
1 Introduction
Sedimentary rocks naturally occur in different textures and structures, this heterogeneity comprising mineral composition, grain size, shape, orientation, and anisotropy highlights the significance of considering the impact of petrography on the geomechanical behavior which is vital for civil and construction engineering projects, so the composition and petrographic properties of sedimentary rocks especially sandstone are essential for assessing important information such as strength-deformation characteristics, failure mechanisms, slope instability, and other engineering purposes [1–4].
The behavior of petrographical characteristics of the sandstone such as different quartz grains, cementing materials, matrix, and intergranular and oversized pores are crucial for various physical and geomechanical properties like elastic properties (γ), ultrasonic pulse velocity (UPV), and uniaxial compressive strength (UCS), understanding these factors is essential, as they can affect the stability of large engineering projects and potentially lead to hazardous situations like rockfalls, causing risks to human safety [5–9].
Bedding planes, joints, and fractures, contribute to weakening and significantly influence overall strength response leading to decreased strength properties of sandstone [10–13].
The selected study area presents dissected Wadi Halfa Oolitic Ironstone Formation (WHOI) outcrops potentially valuable for engineering and construction purposes, such as building materials, ornamental stones, slope stability, foundations, and road bases. While regional and project-specific geotechnical data for Nubian Sandstone Formation (NSF) rocks are available, there is a lack of information on the unique WHOI formation geomechanical properties and rock failures in the study area [14].
The purpose of the present study is to conduct a comprehensive examination of the sandstone within the WHOI formation, with the goal of understanding the relationship between its petrographical characteristics on geomechanical parameters to evaluate the strength of both intact rock and rock mass by utilizing Rock Mass Rating (RMR) and Geological Strength Index (GSI) methods in accordance to petrography and diagenesis affecting these geomechanical properties and to investigate the different types of rock failure occurring around the region of Wadi Halfa, northern Sudan.
2 Study area & geology
Wadi Halfa town is situated near the Egyptian-Sudanese border, close to the Nubian Lake (Fig. 1a), characterized by Paleozoic and Mesozoic sedimentary rocks overlaying the Pre-Cambrian basement complex rocks. The prominent sedimentary formation, known as NSF or recently WHOI formation, exhibiting loose to highly consolidated sandstones, siltstones, mudstones, barite concretions and oolitic ironstone [15, 16].
Location map (a) of the study area; (b) satellite image showing locations of the selected profiles
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
The Precambrian basement complex found in the southern part consisting different lithological units of high-grade metamorphic rocks followed by syn to late tectonic intrusives, ophiolites and immature sediments [17].
The term ‘WHOI’ has been assigned to the NSF around Wadi Halfa with a variety geological history ranging from Upper Carboniferous to Lower Jurassic age [16] as indicated in Fig. 1b. The lithofacies of this formation encompass different deposition environment type including fluvial, glacial, fluvioglacial, and shallow marine sediments, with primary sedimentary structures of sandstone facies (trough and planner cross bedded, rippled sandstone, horizontally stratified sandstone) and mudstone facies (fine and massive mudstone), along with barite concretion facies [15, 16, 18] as shown in Fig. 2.
General geological map showing the WHOI Formation compiled by using own field observations and data published by [16]
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
The selected profiles generally consist of fine- to coarse-grained yellowish, brownish, whitish, and reddish ferruginous sandstone. However, some profiles exhibit three layers of oolitic ironstone with varying thicknesses, while others contain features large, silicified tree trunks embedded in different parts. Some profiles are characterized by intercalations of siltstone, sandstone, and siltstone with approximately the same sizes and quantities.
3 Data & methodology
3.1 Field observations
Identification and selection of fifteen representative vertical profiles, each representing distinct lithofacies associations within the WHOI formation. These profiles underwent field description, sampling, and reclassified into six main profiles representing as the major constituent of other profiles (Fig. 3).
Representative lithofacies profile around the region of Wadi Halfa
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
- a)Measuring of RQD by scan line methods of all selected profiles.
- b)Elaboration of measurement of primary structures including cross bedding for describing their thickness and inclination.
- c)Study of surface condition of discontinuities which include the dip direction and angles of each bed, discontinuities spacing and weathering and alteration of discontinuity surface for GSI classification.
- d)Description of block size and shape bounded by discontinuities of rock masses.
3.2 Laboratory analysis
Twenty-one samples were systematically collected from all the previously described lithological units to facilitate laboratory investigations for the detailed mineralogical and geomechanical properties including:
3.2.1 Mineralogy
The selected representative samples were petrographically analyzed according to the methods suggested by [23]. The petrographic characteristics were determined by using a Leitz Laborlux 12 microscope (PL) to determine the rock constituents and the different porosity types. All selected samples were point-counted (300–500 points per thin section) by point counting stage and J Micro Vision program were used to classify the clastic sediments according to Folk's classification. This technique is standardized in geology as described by [24].
Representative twenty samples were prepared at Electron Microscope Unit, Faculty of Sciences Alexandria University for the Scanning Electron Microscopy (SEM) to study micro-textures using a JEOL-JSM-5300.
According to results of textural, petrographic analysis and scanning electron microscope analysis ten representative samples were carefully selected from all profiles and prepared for two steps of analysis; the first being the bulk mineralogical composition and second step is prepared for fractions less than 2 μm for clay mineralogy. XRD study was carried out using X- ray diffraction unit Philips PW-3710 with generator PW-1830, Cr target and Ni at 40 kV and 30 mA. Nuclear Materials Authority, Egypt.
3.2.2 Physical and mechanical properties
Various physical tests were conducted on all samples, encompassing unit weight (ρ), specific gravity (Gs), porosity (n), void ratio (e), and moisture content (Mc), these tests were performed using standard methods outlined in ASTM standards and procedures [25–30].
Rock samples were cored in three directions: parallel (0°), inclined (45°), and vertical (90°) to bedding planes for measuring geomechanical properties of intact rock including UCS and UPV.
Elastic modulus (E) of intact rock and rock masses was estimated and calculated according to given UCS, UPV, ρ, and gravity (g).
As the water absorption test (W) is the most important factor that effect on the geomechanical properties of rocks [31–33], all samples tested for compressive strength underwent water absorption test.
4 Results
4.1 Field observations of rock masses
Geological and geomechanical data were gathered through various methods to describe the rock masses in the field. Parameters such as color, structure, degree of weathering, fabrics, bedding characteristics, block size and shape, roundness, discontinuity spacing, and rock strength were assessed for all selected profiles.
Wadi Halfa Oolitic Ironstone Formation exhibited distinct structures, including bedding planes (planar, cross bedding, and trough crossbedding), weathering-induced structures (caves and honeycomb formations), and vertical/horizontal joints. The degree of weathering varied across profiles, with 50% slightly weathered, 33.3% moderately weathered, and 16.75% highly weathered.
Profiles 1, 3, and 5 showed slight weathering, indicating no significant loss of strength or discoloration. Profiles 2, 4, and 6 exhibited increased rock joints and falling rocks due to high weathering. Bedding inclinations were predominantly sub-horizontal [0–5°], with some gently inclined layers. All profiles had thick rock beds (>2 m) with large block sizes (>2 m) (Fig. 4).
A massive sandstone showing vertical and horizontal joints with extremely closed spacing of joints, found in Profile 5
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
Block masses were characterized by dominant parallel bedding planes, non-continuous discontinuities, and three orthogonal sets of discontinuities with irregularities. Discontinuity spacing varied, with extremely close spacing (<20 mm) in some profiles and closely to very closely spaced (20–200 mm) in others as shown in (Fig. 5).
Vertical, oblique, and horizontal open joints, indicated by red arrows. These discontinuities are filled with loose sand, as seen in Profile 2
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
A comprehensive summary of field descriptions and observations for all six profiles is presented in Table 1.
The summary of field observations and descriptions of rock masses for each profile
| Rock mass description | Profile1 | Profile2 | Profile3 | Profile4 | Profile5 | Profile6 |
| Structure | Bedding, cross bedding, trough cross bedding, joints caves and honeycombs | |||||
| Color | Yellowish, brownish, reddish, pinkish, greyish, and whitish | |||||
| Weathering | Slightly (SW) | Highly (HW) | Slightly (SW) | Moderately (MW) | Slightly (SW) | Moderately (MW) |
| Fabrics | Massive | Fine | Massive | Coarse | Medium | Fine |
| Bedding inclination | Sub horizontal and gently inclined. | |||||
| Bedding thickness | Very thick (>2m), thick and moderately thick (0.2 – 0.6 mm) | |||||
| Rock strength (geological hummer) | Strong | weak | Extremely strong | Very strong | Strong | Moderately strong |
| Roundness | Planer rough (VII) | Planer rough (VII) | Smooth (VIII) | Rough (VII) | Planer rough (VII) | Rough (VII) |
| Block size | Very large, Medium, and Large | |||||
| Block shape | Tabular | Equidimensional | Tabular | Prismatic | Equidimensional | Polyhedral |
| Discontinuity spacing | Extremely closed | Closely to very spaced | Extremely closed | Extremely closed | Extremely closed | Closely to Very closely spaced |
| Filling materials | Soft filling including sand, silt, and clay | |||||
| Rock Quality Designation (RQD %) | 60 | 66.3 | 100 | 86.8 | 85.4 | 45.7 |
Profiles one and three show little numbers of discontinuities more than 10 cm length and give values of RQD more than 60%, that means the RQD values of 60% refer to fair qualitative description. Profile six shows low values of RQD due to high joints and intercalations of sandstone, siltstone, and claystone layers with similar amounts.
In accordance with the structure and composition section of the GSI classification chart and referenced in the field observations, the profiles can be categorized into different types (A, B, C, and D):
Profiles 1, 3, and 5 fall into the type categories A and B. These profiles exhibit thick bedded, very blocky sandstone, and sandstone with interlayers of siltstone. In some cases, the bedding planes of the sandstone may contribute to structurally controlled instability. The discontinuity surface conditions are good, characterized by a rough, slightly weathered surface. These profiles consist of fine- to medium- to coarse-grained sandstone layers in shades of yellow, red-brown, and white-grey. They also feature well-developed bedding planes, along with thin laminated mudstone and siltstone layers.
Profiles 2 and 4 are classified under type category B and C, characterized by sandstone with thin layers of claystone and siltstone in similar amounts. In profile two, there is also sandstone with oolitic ironstone in comparable proportions. The surface conditions of the discontinuity are considered fair, displaying a smooth, moderately weathered, and altered surface.
Profile 6 falls into type category D, featuring sandstone with silty shale and claystone layers in similar amounts. The surface conditions of the discontinuity in this profile are also considered fair, displaying a smooth, moderately weathered, and altered surface (see Fig. 6).
Different types of rock falling have been detected and observed via detailed field investigations which are mainly related to bedding planes of sandstone, include plane failure and toppling failure.
The similarity amounts of intercalation layers corresponding to type (D) type category in the Geological Strength Index determination (Profile 6)
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
4.2 Mineralogy
The grains of sandstone from WHOI Formation consist of monocrystalline and polycrystalline quartz, along with feldspars, lithic fragments, opaques (iron oxides), clays, carbonates, micas, and heavy minerals as described in Table 2.
The petrographic point counting results of the studied samples from Wadi Halfa
| Number | Samples | Textural Data | Detrital Grains | Cement/Matrix | Pores | |||||||||||||
| Roundness | G. Contact | Quartz | Feldspars | Rock Frag. | Heavy M. | Micas | Iron Oxides | Carbonates | Clays | Others | Intergranular | Oversized | ||||||
| Point C. | Conc. C. | Over G. | Straight | |||||||||||||||
| Mono | Poly | |||||||||||||||||
| 1 | P1S1 | A-SA | 4 | – | – | – | 43 | 5 | 0 | 2 | 0 | 0 | 16 | 0 | 4 | 0 | 25 | 5 |
| 2 | P1S2 | SA-SR | 1 | – | 1 | – | 69 | 1 | 1 | 4 | 0 | 0 | 8 | 3 | 4 | 0 | 9 | 2 |
| 3 | P1S3 | SA-SR | 2 | 1 | – | – | 64 | 0 | 0 | 0 | 0 | 0 | 6 | 0 | 16 | 0 | 12 | 2 |
| 4 | P1S4 | SA-SR | 6 | 2 | – | 1 | 63 | 1 | 0 | 1 | 0 | 0 | 12 | 0 | 0 | 0 | 12 | 0 |
| 5 | P1S5 | A-SA | 7 | 8 | 1 | – | 64 | 0 | 0 | 0 | 1 | 0 | 10 | 0 | 14 | 0 | 12 | 0 |
| 6 | P2S1 | SA-SR | 3 | 1 | 2 | – | 67 | 0 | 2 | 1 | 1 | 0 | 19 | 6 | 0 | 0 | 4 | 0 |
| 7 | P2S2 | R-SR | 4 | 1 | 0 | 0 | 61 | 0 | 2 | 1 | 1 | 1 | 16 | 2 | 2 | 0 | 11 | 3 |
| 8 | P3S1 | A-SA | 5 | 0 | 3 | – | 63 | 4 | 1 | 1 | 1 | 0 | 5 | 0 | 13 | 1 | 11 | 0 |
| 9 | P3S2 | SA-SR | 6 | 4 | – | – | 67 | 2 | 4 | 1 | 0 | 0 | 14 | 0 | 6 | 0 | 5 | 0 |
| 10 | P3S3 | A-SA | 6 | 3 | 1 | – | 59 | 3 | 3 | 1 | 0 | 0 | 14 | 0 | 16 | 0 | 5 | 0 |
| 11 | P4S1 | A-SA | 2 | 2 | 4 | 1 | 50 | 8 | 2 | 1 | 0 | 0 | 5 | 9 | 1 | 0 | 18 | 7 |
| 12 | P4S2 | SA-SR | 5 | 3 | – | – | 61 | 0 | 2 | 4 | 1 | 1 | 0 | 6 | 2 | 0 | 23 | 0 |
| 13 | P4S3 | A-SA | 2 | – | – | – | 40 | 3 | 1 | 2 | 0 | 0 | 37 | 0 | 1 | 0 | 16 | 0 |
| 14 | P4S4 | A-SA | 21 | 4 | – | – | 57 | 1 | 1 | 1 | 0 | 0 | 17 | 1 | 1 | 0 | 15 | 6 |
| 15 | P5S1 | R-SR | 4 | 1 | 1 | 1 | 56 | 10 | 0 | 2 | 0 | 0 | 5 | 0 | 4 | 1 | 16 | 6 |
| 16 | P5S2 | R-SR | 3 | 1 | 1 | 1 | 60 | 4 | 1 | 1 | 1 | 1 | 29 | 3 | 0 | 0 | 0 | 0 |
| 17 | P5S3 | SA-SR | 5 | – | – | – | 50 | 0 | 1 | 0 | 0 | 0 | 49 | 0 | 0 | 0 | 0 | 0 |
| 18 | P5S4 | SA-SR | 8 | – | – | – | 62 | 0 | 0 | 3 | 1 | 0 | 3 | 0 | 3 | 0 | 21 | 7 |
| 19 | P6S1 | A-SA | 5 | – | – | – | 69 | 1 | 3 | 0 | 1 | 0 | 7 | 2 | 8 | 0 | 8 | 0 |
| 20 | P6S2 | SA-SR | 3 | – | – | 1 | 65 | 1 | 3 | 1 | 0 | 0 | 8 | 0 | 11 | 0 | 10 | 1 |
| 21 | P6S3 | SA-SR | 1 | – | – | – | 61 | 2 | 2 | 0 | 0 | 0 | 19 | 11 | 2 | 0 | 2 | 0 |
G. contact: grain contact; Point C.: Point contact, Conc. C.: Concavo-convex; Over G: Overgrowth; A-SA: Angular to sub angular; R-SR: Rounded to sub rounded; SA–SR: Sub angular to sub rounded; Heavy M: minerals.
4.2.1 Detrital framework grains
The detrital quartz grains exhibit angular to subangular quartz (62%) and rounded to subrounded shapes (38 %). Quartz grain sorting ranges from poorly to moderately sorted, with the majority being medium- to coarse-grained and minor amounts being medium- to fine-grained. The contacts between grains include point contacts, straight contacts, quartz overgrowth, concavo-convex contacts, and sutured contacts (Fig. 7a and b).
Photomicrograph of detrital framework grains (a) angular to sub-angular monocrystalline (QM) polycrystalline (QP) quartz grains with straight and point contact between grains (red arrows) pores occurred between grains (P); (b) two types of cementing materials occurred iron oxides (IO) and carbonates (Ca) twined microcline occurred (F); (c) rounded to sub-rounded monocrystalline quartz grains with iron oxide occur as a main cementing material with clay matrix between grains; (d) iron oxide (IO) cemented quartz grains heavy minerals occurred (HM). OP refers to opaque minerals; (e) perfect crystal of quartz overgrowth observed under scanning electron microscope; (f) authigenic kaolinite well developed pseudo-hexagonal plates with rosette shape of chlorite; (g) iron oxides and carbonate within pores of sandstone; (h) elongated shape of hematite cement.
(All photomicrographs were taken under crossed Nicols)
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
The Folk's classification of sandstone from WHOI Formation based on mineral composition (quartz (Qz), lithic fragments (L), and feldspars (F)) reveal that most of the samples are classified as quartz-arenite 67%, with less amounts of sub-lithic arenite 20% and sub-arkose 13%.
4.2.2 Cement and matrix
Authigenic minerals resulting from the alteration of detrital material are present as matrix or grain replacements and cement in the sandstone from the study area. These minerals include carbonates, iron oxides, barite, pseudo matrix, clays, and opaque minerals. The percentage of iron oxides in the studied samples varies from 0. 3%–49%, with an average value of 14 %, primarily acting as cement in most samples (Fig. 7c and d).
The study area exhibits more than six layers of clays, commonly found in sandstones as matrix or cement constituents. Optical, X-ray, and SEM analyses reveal a clay mineral assemblage dominated by kaolinite, with minor amounts of illite, montmorillonite, and chlorite (Fig. 8). The diverse cementation materials significantly impact the geomechanical properties of the sandstone.
Oriented (heated, oriented, and glycolate) XRD samples showing the dominance of kaolinite
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
4.2.3 Pores
The studied samples display various types of pores, categorized as intergranular pores (between sand grains), oversized pores (larger than normal intergranular pores, comparable to sand grain size), and intra-granular pores (within grains). Most of the studied samples show medium to high porosity. The porosity of the selected samples ranges from 2% to 30%, with an average of 12% (refer to Table 1).
4.3 Geomechanical properties
The initial steps in studying the geomechanical properties of rocks involve examining their physical properties. These properties provide essential information for designing stable structures and constructions and establish a strong relationship with geomechanical characteristics.
Summarized results of the determined physical and engineering properties for dry intact samples are presented in Table 3.
Geomechanical properties of the selected intact sandstone samples
| Number Number | Samples | Physical | UCS (MPa) | UPV (m s−1) | Water Absorption | Young's Modulus (E) | |||||||
| Porosity (n) | Specific Gravity (Gs) | Unit Weight (γ) g cm−3 | Void Ratio (e) | Moisture Content (%) | Anisotropy (ß) | Vertical (90°) | Parallel (0°) | ||||||
| Vertical (90°) | Inclined (45°) | Parallel (0°) | |||||||||||
| 1 | P1S1 | 25.1 | 2.67 | 2.24 | 0.34 | 0.12 | 25.5 | 20.1 | 24.7 | 2,750 | 2,430 | 8.0 | 15.3 |
| 2 | P1S2 | 18.7 | 2.65 | 2.38 | 0.23 | 0.10 | 112 | 78.7 | 97.3 | 3,500 | 3,110 | 7.1 | 26.6 |
| 3 | P2S1 | 20.7 | 2.71 | 2.23 | 0.26 | 0.13 | 154 | 87.4 | 121 | 2,660 | 2,880 | 9.5 | 38.2 |
| 4 | P2S2 | 13.1 | 2.74 | 2.20 | 0.49 | 0.12 | 116 | 66.1 | 93.6 | 3,100 | 2,970 | 6.2 | 20.7 |
| 5 | P3S1 | 30.6 | 2.72 | 2.20 | 0.29 | 0.20 | 53.7 | 37.1 | 47.3 | 2,630 | 2,560 | 5.3 | 15.1 |
| 6 | P3S2 | 8.2 | 2.66 | 2.76 | 0.34 | 0.15 | 66.4 | 42.7 | 50.0 | 2,650 | 2,550 | 8.2 | 19.2 |
| 7 | P4S1 | 24.8 | 2.69 | 2.30 | 0.33 | 0.12 | 83.7 | 30.4 | 50.0 | 2,380 | 2,280 | 9.0 | 12.7 |
| 8 | P4S2 | 19.4 | 2.66 | 2.20 | 0.24 | 0.11 | 86.4 | 40.5 | 55.5 | 2,840 | 2,800 | 8.9 | 17.8 |
| 9 | P5S1 | 23.7 | 2.66 | 2.27 | 0.31 | 0.15 | 28.2 | 11.2 | 13.6 | 1830 | 1,570 | 10.3 | 26.8 |
| 10 | P5S3 | 30.6 | 2.67 | 2.13 | 0.44 | 0.16 | 80.9 | 39.4 | 59.1 | 2060 | 1860 | 9.3 | 42.6 |
| 11 | P6S1 | 19.4 | 2.67 | 2.36 | 0.24 | 0.10 | 82.8 | 53.3 | 79.1 | 2050 | 1960 | 9.6 | 48.2 |
| 12 | P6S4 | 29.1 | 2.66 | 2.33 | 0.41 | 0.24 | 58.2 | 36.1 | 49.1 | 2,170 | 1940 | 12.2 | 48.8 |
| 13 | Max. | 30.6 | 2.72 | 2.76 | 0.49 | 0.24 | 154 | 87.4 | 121 | 3,500 | 3,110 | 12.2 | 48.8 |
| 14 | Min. | 8.2 | 2.65 | 2.13 | 0.23 | 0.10 | 28.2 | 11.2 | 13.6 | 1830 | 1,570 | 5.3 | 12.7 |
| 15 | Av. | 19.4 | 2.69 | 2.45 | 0.36 | 0.17 | 91.4 | 49.3 | 67.3 | 2,293 | 2,340 | 8.8 | 30.8 |
| 16 | SE. | 7.44 | 1.14 | 0.21 | 0.49 | 0.05 | 38.25 | 22.43 | 30.39 | 494.6 | 460.0 | 1.86 | 13.38 |
Max: Maximum values, Min: Minimum values, Av: Average values, SE. Standard Error, CS: Compressive strength, UV: Ultrasonic velocity, P1S1, P2S1, P3S1…, refer to profile 1 sample 1, profile 2 sample 2, etc.
The unit weight of sandstone is influenced by texture, mineral composition, and diagenetic processes in the rock's environment. The unit weight of the dry studied samples ranges from 2.13 to 2.76 g cm−3, with an average of 2.45 g cm−3.
The specific gravity of the samples varies from 2.65 to 2.72, with an average of 2.69%. Some samples (P1S2 and P3S1) exhibit elevated specific gravity values due to their high content of clays, iron oxides, and micas.
Compressive strength tests were conducted based on the angle between applied loads and bedding planes (anisotropy, angle ß), samples were cored in three directions: parallel (0°), inclined (45°), and vertical (90°) to bedding planes.
Ultrasonic velocity tests were carried out on dry sandstone specimens cored parallel and vertical to bedding planes. The transit time for ultrasonic waves to traverse the sample was recorded, and UPV values were obtained.
Uniaxial compressive strength of wet and dry samples (MPA) perpendicular (90°) and parallel (0°) to bedding planes and softness coefficient calculation
| No. | Samples | CSWet (MPa) | CSDry (MPa) | Sc | |||
| 90° | 0° | 90° | 0° | 90° | 0° | ||
| 1 | P1S1 | 15.1 | 14.4 | 25.47 | 24.56 | 0.59 | 0.59 |
| 2 | P2S1 | 33.6 | 28.9 | 111.87 | 97.32 | 0.30 | 0.30 |
| 3 | P3S1 | 14.8 | 12.2 | 53.66 | 47.29 | 0.28 | 0.26 |
| 4 | P4S1 | 26.5 | 22.6 | 83.67 | 50.02 | 0.32 | 0.45 |
| 5 | P5S1 | 7.6 | 5.1 | 13.64 | 28.19 | 0.56 | 0.18 |
| 6 | P6S4 | 21.0 | 24.3 | 58.21 | 49.11 | 0.36 | 0.49 |
| 7 | Maximum | 33.6 | 28.9 | 111.87 | 97.32 | 0.59 | 0.59 |
| 8 | Minimum | 7.6 | 5.1 | 13.64 | 28.19 | 0.28 | 0.18 |
| 9 | Average | 20.6 | 17.0 | 62.76 | 62.76 | 0.44 | 0.39 |
| 10 | St. Error | 8.53 | 8.14 | 33.56 | 27.19 | 0.13 | 0.14 |
Average values of ultrasonic velocity in the vertical and parallel directions to bedding planes are 2,293 m s−1 and 2,340 m s−1, respectively. Lower ultrasonic pulse velocity values in certain samples (P5S1, P5S3, and P6S1) correspond to high pore values and weak carbonate cementation. Additionally, the direction of bedding planes significantly influences ultrasonic velocity.
The strength varies between profiles of the studied samples from the Wadi Halfa due to geological and petrographic characteristics (e.g., bedding planes, sorting, types of cementing materials, roundness of quartz grains, clay content, and porosity) and their degree of weathering. The measured compressive strength ranges from weak (P5) to very strong (P2) [26].
When we subject sandstone samples to the uniaxial compressive strength test, different types of failures occurr, samples exhibit brittle tensile failures. The direction of bedding planes to the applied load plays a crucial role in the behavior of rock failure.
Different types of rock failure occurred when examining the uniaxial compressive strength, including multiple fracturing, double fractures, shear fractures along bedding planes at 0°, and axial splitting fractures or extension failures at 90° similar to rock failures in the field (Fig. 9).
Schematic shows different types of failure under uniaxial compressive strength recorded in most of studied samples, (a) axial splitting failure at 90°; (b) shearing along bedding planes at 0°; (c) double shearing; (d) multiple fracturing; (e) failure along bedding planes at 45°; and (f) Y shaped failure corresponding to toppling failure
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
5 Discussion
Field observations of the WHOI formation highlight different primary structures influencing geomechanical parameters, including bedding planes (planar, crossbedding, and trough crossbedding), weathering-induced features (caves and honeycomb), and vertical and horizontal joints. Profiles 1, 3, 4 and 5 exhibit slight to moderate weathering, showing no significant loss of strength or discoloration, while Profile 2 displays increased rock joints and falling due to higher weathering.
Bedding inclination measurements indicate mostly sub-horizontal layers (0–5°), with some gently inclined layers exceeding (5°). All profiles feature thick beds or layers exceeding 2.0 m in thickness. Geomechanical properties of rock samples are influenced by factors such as grain sizes and distribution, grain shapes and contacts, types, and amounts of cementing materials, and pore sizes.
Our results reveal that coarse-grained sandstone samples generally exhibit lower compressive strength compared to medium- and fine-grained samples. The angularity of quartz grains also affects compressive strength, with samples P1S2, P2S1, and P2S2 showing high strength attributed to their iron oxide cement content, while P1S1 and P5S1 exhibit lower strength due to weathering, high porosity, and weak cementing material (carbonates).
The petrographic and geomechanical study results are depicted in scattered plots to illustrate the relationships between geotechnical and petrographic parameters. Both linear and polynomial analysis between compressive strength and cementing materials of sandstone suggests that an increase in cementing materials and matrix corresponds to an increase in the strength of the sandstone (Fig. 10a).
Scatter plot showing (a) the relationship between compressive strength and cementing materials and matrix of studied sandstone samples (red and blue curves refers to polynomial and linear relations respectively); (b) the variation between the values of compressive strength depending on the direction of loading relative to the direction of bedding planes anisotropy angle (β = 0°, 45°, and 90°) showing U-shape curve of anisotropy
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
Bedding planes in sedimentary rocks, being planes of weakness, influence compressive strength variations based on the angles between loading directions and bedding planes. Results in (Table 3, Fig. 10b) indicate higher compressive strength when the load direction is perpendicular (β = 90°) or parallel (β = 0°) to bedding planes, and lower values when inclined (β = 45°), dependent on sandstone anisotropy.
The average compressive strength of wet samples in applying load to bedding plane direction indicates a moderately strong to weak classification (20.6 and 17.0 MPa, respectively). The coefficient of softness varies from 0.28 to 0.59, averaging 0.44 when the load is perpendicular to bedding planes, and ranges from 0.18 to 0.59, averaging 0.39 when the load is parallel to bedding planes as dedicated in Table 4. Observations suggest that the uniaxial compressive strength is more affected by wetting when the direction of loading is parallel to bedding planes (0°), compared to the case of perpendicular or vertical direction of load to bedding planes. A good linear relationship (R2 = 0.7801) is evident between the compressive strength of dry and wet samples (UCSdry and UCSwet) (Fig. 11a).
Scatter plot showing (a) the relationship between compressive strength of dry and wet samples; (b) compressive strength vs pores of studied samples (red curve polynomial relation); (c) water absorption versus porosity of the studied samples excluding oolitic sample; (d) porosity versus ultrasonic pulse velocity
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
The relationship between compressive strength and porosity obtained from point counting in petrographic studies indicates that an increase in pore volume results in a decrease in compressive strength as illustrated in (Fig. 11b) in both simple and complex relationship as porosity increases, the strength of the sandstone samples decreases with (R2 = 0.6) indicating a moderate in linear and polynomial relationship. Additionally, good linear relationships are observed between water absorption and porosity (Fig. 11c) and between ultrasonic pulse velocity and porosity (Fig. 11d).
Moisture content is inversely related to ultrasonic velocity, compressive strength, cement, pores, and unit weight. Direct relationships are identified between cement and uniaxial compressive strength, as well as between cement and ultrasonic pulse velocity. Similarly, there are direct relationships between uniaxial compressive strength and ultrasonic pulse velocity, porosity and cement, specific gravity and compressive strength, cement, and porosity. Furthermore, a direct relationship is observed between unit weight and ultrasonic velocity, compressive strength, and pores. Void ratio is directly related to cement, while porosity is directly related to specific gravity. Moisture content exhibits a direct relationship with porosity and specific gravity, and an inverse relationship with void ratio as illustrated in Fig. 12.
Correlation matrix between different studied petrographical and geomechanical parameters
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
Inverse relationships are identified between pores and ultrasonic velocity, compressive strength, and cement. Porosity shows an inverse relationship with ultrasonic velocity, compressive strength, and pores. Specific gravity is inversely related to ultrasonic velocity and pores. Void ratio exhibits inverse relationships with ultrasonic velocity, compressive strength, porosity, and unit weight. Finally, pores are inversely related to specific gravity, moisture content, void ratio, porosity, compressive strength, ultrasonic velocity, and cement.
The combination of field observations and the laboratory tests can be used for geomechanical classification of the vertical lithofacies associations of the Wadi Halfa which can help for several future engineering projects such as allowable bearing pressure for foundations, analyze the stability of rock slopes, assessment of rock mass strength parameters (cohesion, friction angle, deformation, etc.), and cut slope angle along hill for roads and rail lines.
In applying rock mass classification system such as Rock Mass Rating (RMR) and Geological Strength Index (GSI) followed by Hoek-Brown Criterion [34] in studied six vertical lithofacies of Wadi Halfa; the rock masses divided into a number of geomechanical units according to [20, 35] which is the most recent version of the RMR system of all six parameters. On the basis of RMR values for a given engineering structure, the rock mass is sorted into five classes: very good (100–81), good (80–61), fair (60–41), poor (40–21), and very poor (<20) [35].
For example, selection of Profile 1 and Profile 6 because they were found near the construction builds and roads; for profile 1 the average values of uniaxial compressive strength are 68.22 MPa, the Rock Quality Designation (RQD) 60%, the spacing of discontinuities found to be extremely closed spacing of less than 20 mm, the condition of discontinuities slightly rough surface separation of less than 1 mm and the degree of weathering slightly to unweathered as shown in Table 5 and Fig. 13a.
Geomechanical rock mass classification (RMR and GSI) of all profiles
| Profiles | R1 | R2 | R3 | R4 | R5 | RMR* | GSI | Classification | Cohesion (C) MPa | Friction Angle (Ø0) |
| 1 | 7 | 13 | 20 | 28 | 15 | 83 | 58 | Very good | 0.95 | 58.2 |
| 2 | 12 | 13 | 10 | 21 | 15 | 71 | 41 | Good | 0.84 | 59.2 |
| 3 | 7 | 20 | 20 | 30 | 15 | 92 | 61 | Very good | 0.95 | 57.8 |
| 4 | 7 | 17 | 20 | 25 | 15 | 84 | 50 | Very good | 0.65 | 55.4 |
| 5 | 7 | 17 | 15 | 25 | 15 | 79 | 57 | Good | 1.1 | 59.2 |
| 6 | 4 | 8 | 8 | 23 | 15 | 58 | 33 | Fair | 0.40 | 49.6 |
R1 UCS rating in (MPa); R2 RQD rating in (%); R3 discontinuity spacing; R4 condition of discontinuities which include degree of weathering, persistency, aperture, roughness, and infilling materials; R5 ground water condition.
* Total rating refers to values of RMR <21 very poor rock; 21–40 poor rock; 41–60 fair rock; 61–80 good rock; 81–100 very good rock [35].
Hoek-Brown Criterion and classification of selected profiles (a) Profile 1; and (b) Profile 6 around Wadi Halfa
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
Profile 6 of the average UCS 65.93 MPa, 45.7% RQD, the spacing of discontinuities found to be close to very close spacing (20–200 mm), slightly rough surface separation of less than 1 mm and slightly weathered.
The combination of RMR and GSI using Hoek-Brown Criterion and Classification has yielded favorable results for the estimation of the overall rock mass strength of the selected profiles (Fig. 13b).
Useful parameters, such as global strength, cohesion, friction angle, and major deformation of rock masses, have been obtained through a comprehensive analysis of rock strength using Hoek-Brown criteria and Mohr-Colom strength parameters, as detailed in Table 6. These parameters are critical for a wide range of engineering projects, providing essential insights for applications across various fields. Their applicability spans from geotechnical and structural engineering to mining and tunneling, making them invaluable tools for ensuring the stability and safety of constructions in different geological settings specially in sedimentary rocks. Our study provides a valuable framework for assessing the quality of the sandstone rock mass by considering the typical GSI Hoek's classification ranges (33–61) which are impacted mainly by petrographical factors such as grains, cement, matrix and pores (Fig. 14).
Analysis of rock mass parameters using Hoek Brown classification and MOHR-coulomb fit
| Profiles | Uniaxial Compressive Strength (MPa) | Tensile Strength (MPa) | Global Strength (MPa) | Modulus of Deformation (MPa) |
| 1 | 6.949 | −0.169 | 18.090 | 13069.36 |
| 2 | 5.276 | −0.103 | 27.909 | 5956.62 |
| 3 | 6.455 | −0.177 | 16.169 | 14221.24 |
| 4 | 3.312 | −0.075 | 13.376 | 7416.20 |
| 5 | 7.667 | −0.195 | 22.139 | 13794.61 |
| 6 | 0.971 | −0.017 | 7.140 | 2549.05 |
Ranges of GSI for Wadi Halfa sandstone, original chart from [20, 34]
Citation: International Review of Applied Sciences and Engineering 2025; 10.1556/1848.2024.00883
6 Conclusion
The following conclusions have been drawn based on the results obtained from the detailed field observations and laboratory analyses:
Petrographic assessments display different sandstone facies with different types of cementing materials, grain size and shape, that influence the geomechanical properties.
Anisotropy played an important role, with a U-shaped curve in uniaxial compressive strength. Samples highly cemented by iron oxides exhibited higher strength, while weathered and porous samples with carbonate cementation showed lower strength. The softness coefficient highlighted the role of porosity in strength reduction due to wetting.
The post-test specimen of sandstone showed multiple fracturing, double fractures, shear fractures along bedding planes at 0°, and axial splitting fractures or extension failures at 90°, similar to real-field rock failures.
The combination of field and laboratory analyses provides a robust framework for the geomechanical classification (RMR and GSI) of the vertical lithofacies associations in Wadi Halfa. This classification is essential for future engineering projects such as determining allowable bearing pressure for foundations, analyzing rock slope stability, assessing shear strength parameters, and designing cut slope angles for roads and railways.
Deep examination of petrographic effects on the geomechanical and engineering properties by comprehensive field observations and laboratory analyses of the sandstone can be widely used for different construction engineering projects in the region of Wadi Halfa and other similar regions.
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