Investigation of the ultimate compressive and tensile strength of certain rock types from the Uropskoye deposit
Abstract Generated abstract
This paper examines the physical and mechanical properties of sandstones and siltstones from the Uropskoye coal deposit in Kuzbass, with attention to their relevance for projected open-pit quarry wall stability. Cylindrical specimens of Jurassic and Permian sandstones and Permian siltstones were tested for uniaxial compressive strength, while tensile strength was determined by diametral compression and wedge splitting, alongside measurements of density, moisture, porosity, internal friction, and cohesion. The results show wide variation in compressive and tensile resistance, but indicate a close empirical relationship in which tensile strength is approximately one tenth of compressive strength. This relationship is proposed as a practical means to estimate tensile resistance and derive parameters needed for quarry slope stability calculations, with allowance for structural weakening in the rock mass.
Full Text
STUDY OF THE ULTIMATE COMPRESSIVE AND TENSILE STRENGTH OF CERTAIN ROCK TYPES FROM THE UROPSKOYE DEPOSIT
V. E. OLKHOVATENKO, B. L. PETRI, N. S. ROGOVA
(Presented by the Department of Hydrogeology and Engineering Geology)
The Uropskoye coal deposit of the Kuzbass is one of the promising deposits for open-pit development. The geological structure of the deposit includes rocks of Middle Permian, Lower–Upper Jurassic, and Quaternary age, which will occur in the walls of the projected open pits.
The Quaternary deposits are represented by clays and loams, and along river valleys by alluvial sands and pebbles. Among the Permian and Jurassic rocks, sandstones and siltstones are the most widespread in the section, while argillites are considerably less common. The strength of clastic cemented rocks depends substantially on the composition of the cement. Within the deposit, along with medium-strength rocks, strong sandstones with carbonate and siliceous cement are widely distributed. Important indices in assessing the strength and stability of rock and semirock masses are the ultimate compressive and tensile strengths. The ultimate compressive strength of sandstones and siltstones from the Uropskoye deposit was determined on specimens of regular cylindrical shape with a height-to-diameter ratio equal to unity [1, 3, 4].
At present, various methods are used to determine the ultimate tensile strength of rocks; a detailed characterization of these methods is given in the work of I. A. Turchaninov, R. V. Medvedev, and V. I. Panin [4]. In our case, the ultimate tensile strength was determined by the method of diametral compression of specimens and splitting by wedges. The results of the investigations are given in Table 1 and in Fig. 1. Below is a characterization of the physico-mechanical properties of the sandstones and siltstones of the Uropskoye deposit.
Jurassic and Permian sandstones are characterized by close values of specific gravities, varying within the range of 2.49–2.78 g/cm³. Bulk densities
Table 1
Engineering-geological properties of certain rock types of the Uropskoye deposit
| No. | Characteristics | Dimension | Jurassic sandstones | Permian sandstones | Permian siltstones |
|---|---|---|---|---|---|
| 1 | Specific gravity | g/cm³ | 2.49—2.75 2.68 |
2.63—2.78 2.70 |
2.65—2.73 2.70 |
| 2 | Bulk density | g/cm³ | 1.88—2.62 2.35 |
2.20—2.54 2.35 |
2.32—2.46 2.38 |
| 3 | Moisture content | % | 1.11—15.60 5.96 |
3.06—13.20 5.73 |
4.82—7.78 6.14 |
| 4 | Porosity | % | 3.84—34.10 22.45 |
10.44—23.71 16.77 |
14.28—19.70 17.08 |
| 5 | Porosity coefficient | 0.04—0.72 0.27 |
0.13—0.52 0.22 |
0.11—0.24 0.19 |
|
| 6 | Degree of moisture | 0.61—1.0 0.90 |
0.62—1.0 0.84 |
0.77—1.0 0.85 |
|
| 7 | Ultimate compressive strength | kg/cm² | 16.1—905.0 255.7 |
32.7—773.0 273.3 |
68.5—168.0 120.08 |
| 8 | Ultimate tensile strength | kg/cm² | 7.25—86.8 34.57 |
1.05—68.9 31.24 |
1.45—15.5 7.81 |
| 9 | Angle of internal friction | degrees | 25°—41° 36° |
26°—49° 41° |
— |
| 10 | Cohesion | kg/cm² | 5.1—165.5 64.2 |
5—125 60.8 |
12.5—41.0 26.0 |
Note. Investigated: 26 Jurassic sandstones, 42 Permian sandstones, 6 Permian siltstones.
Fig. 1. Relationship between the ultimate compressive and tensile strength of rocks of the Uropskoye deposit: 1 — Jurassic sandstones, 2 — Permian sandstones, 3 — Permian siltstones.
the unit weights of the rocks described also coincide closely and on average are equal to 2.35 g/cm^3. The moisture content of the Jurassic sandstones ranges from 1.11 to 15.6%, and that of the Permian sandstones from 3.06 to 13.20%, with mean degrees of moisture content of 0.90 and 0.84, respectively. The porosity of the rocks varies from 3.84 to 34.10% (Table 1). The physical properties of the siltstones, as can be seen from Table 1, do not differ substantially from the properties of the sandstones. The specific gravity of the siltstones is 2.65–2.73 g/cm^3, the unit weight 2.32–2.46 g/cm^3, the moisture content 4.82–7.78%, and the porosity 14.28–19.70.
The study of strength characteristics showed that the temporary compressive resistance of the rocks varies from 16.1 to 905 kg/cm^2, while the tensile resistance ranges from 1.05 to 86.8 kg/cm^2.
On the basis of the results of studies in uniaxial compression and tension, Mohr circle envelopes were calculated, strength passports were constructed, and the angles of internal friction and cohesion of the rocks were determined. As can be seen from Table 1, the angles of internal friction of the sandstones range from 25° to 49°, and cohesion in monolithic specimens is 5.0–165.5 kg/cm^2. The obtained values of cohesion in monolithic specimens will differ from the cohesion of rocks in the massif. To account for the possible reduction of cohesion in quarry walls, a coefficient of structural weakening is usually determined. For the rocks of the Uropskoye deposit, this coefficient, calculated by the procedure of the All-Union Scientific Research Institute of Mining Geomechanics and Mine Surveying (VNIMI), proved to be 0.03. The latter must be taken into account when evaluating the stability of the walls of projected quarries.
Analysis of the studies performed showed that between the temporary compressive resistance ($\sigma_{\mathrm{comp}}$) and the tensile resistance ($\sigma_{\mathrm{t}}$) there exists a close relationship described by the equation $\sigma_{\mathrm{t}} = 0.1\sigma_{\mathrm{comp}}$.
This makes it possible to determine the temporary tensile resistance from the results of uniaxial compression tests. The latter makes it possible, comparatively quickly and with sufficient accuracy, to determine reliable values of the angle of internal friction and cohesion of rocks, which are necessary for calculating the stability of quarry walls.
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