SOME RESULTS OF ENGINEERING-GEOLOGICAL STUDIES AT THE SECOND SITE OF THE UROP DEPOSIT

V. E. OLKHOVATENKO, N. S. ROGOVA

(Presented by the Department of Hydrogeology and Engineering Geology of Tomsk Polytechnic Institute)

At present, in Kuzbass the open-pit method of developing coal deposits is widely used; it requires consideration of all natural factors and especially of engineering-geological conditions. Insufficient study of engineering-geological conditions in the course of deposit exploration often leads to incorrect design decisions and, during the construction of open pits, to the development of deformations of pit walls and dumps. Engineering-geological studies acquire particular importance at the Urop deposit, where open pits more than 200 m deep are being designed. Such studies at the second site are being carried out simultaneously with exploration of the deposit by the Department of Hydrogeology and Engineering Geology of Tomsk Polytechnic Institute, under contract with the Kuzbass Geological Expedition. Below, on the basis of the results of preliminary studies, a brief characterization is given of the engineering-geological conditions for the construction of an open pit at the Second Urop site.

The Urop coal deposit is located in the central part of the Kuznetsk coal basin and is confined to two mutually conjugate folds: the Dunaevka syncline and the Urop anticline. The area of the deposit occupies the watershed spaces of the Inya and Urop rivers and is characterized by a gently hilly relief favorable for the construction of open pits.

Deposits of Paleozoic, Mesozoic, and Cenozoic age participate in the geological structure of the site.

Of greatest interest among them are the deposits of the Erunakovo suite of the Permian, the conglomerate suite of the Jurassic, and Quaternary formations, which will occur in the walls and dumps of the planned open pits.

The deposits of the Erunakovo suite of the Permian are represented mainly by sandstones and, to a lesser extent, by siltstones, argillites, and hard coals.

The conglomerate suite of the Jurassic is widely distributed at the site, overlying Paleozoic rocks with a pronounced angular unconformity. The suite is composed of sandstones, siltstones, argillites, and beds of brown coals; conglomerates are sometimes encountered.

Quaternary deposits are widely developed within the site and are represented mainly by loams, more rarely by clays. The thickness of the Quaternary deposits reaches 50 m.

One of the most important factors influencing the development of coal deposits by the open-pit method is hydrogeological conditions. Within the deposit, aquifer complexes of Quaternary, Jurassic, and Permian deposits are distinguished; moreover, higher inflows into the pits should be expected from Jurassic groundwater.

Below is a brief characterization of the properties of the rock types listed above.

Loams, in terms of granulometric composition, are characterized by a high content of silty particles, up to 83.55%. The content of the sandy and clay fractions reaches, respectively, 48.73% and 29.08%. The bulk density of the loams varies from 1.84 to 2.10 t/m³, and the specific gravity from 2.62 to 2.75 t/m³. Arithmetic mean values of the physico-mechanical properties of the rocks are shown in Table 1.

Clays are characterized by a higher content of the clay fraction (up to 54.70%), whereas the content of silty particles is not more than 67.10%, and that of sandy particles is 6.20%. The specific gravity and bulk density of the clays differ little from the corresponding values for the loams (Table 1). The porosity and porosity coefficient of clays and loams also proved to be close. The degree of moisture of these rocks is 0.93, which indicates complete saturation of the pores with water under natural conditions. As can be seen from Table 1, the angles of internal friction and cohesion of loams and clays differ insignificantly, which is explained by the slight difference in the compactness of bedding and in the composition of these rocks. Data on the rocks of the Conglomerate and Erunakovskaya suites are given below.

Among the rocks of the Conglomerate suite, sandstones are the most widespread; siltstones and argillites occur to a lesser extent.

In granulometric composition, the sandstones belong to sandy-loamy rocks with a clay-fraction content of up to 18.35%. The specific gravity of the sandstones, as a rule, varies from 2.60 to 2.72 t/m³; the bulk density is 1.98–2.38 t/m³; low values of specific gravity and bulk density are characteristic of samples with carbonaceous inclusions. In terms of moisture content (Table 1), the sandstones may be classified as moist and water-saturated rocks. In strength, the sandstones of the Conglomerate suite are divided into weak, medium-strength, and strong. The weak sandstones include sandstones of the weathering zone, mainly with clay cement. They weather easily in air and possess extremely low strength and stability. These rocks have not yet been fully studied and are not described here.

Medium-strength sandstones are most widespread in the section of the suite. Studies of their shear resistance showed an angle of internal friction of 32–43°, and cohesion of 2.5–17.5 t/m². Dense sandstones with siliceous cement, assigned to the strong category, are encountered. They occur as separate lenses of insignificant thickness. The bulk density of the strong rocks reaches 2.62 t/m³, the angle of internal friction is 34–55°, and cohesion, allowing for the coefficient of structural weakening, is 27.60 t/m².

Jurassic siltstones and argillites are characterized by a high content of silty particles (46.99–67.99%). The bulk densities of siltstones and argillites are close and on average equal to 2.24–2.26 t/m³. The porosity of siltstones ranges from 19.12 to 33.65%; for argillites it proved to be 19.5–30.0%. The porosity coefficient correspondingly varies for siltstones within the interval 0.24–0.50, and for argillites 0.24–0.43. The angle of internal friction for siltstones varies from 18 to 32°, cohesion from 3.1 to 13.75 t/m². For argillites, the angle of internal friction is 13–33°, and cohesion is 10.6–21.9 t/m².

Average statistical values of the physico-mechanical properties of rocks
of the Second Urop site

Table 1

The sandstones of the Erunakovskaya formation are represented by sandy and silty fractions, the average contents of which are, respectively, 59.98% and 35.09%. Among the Permian sandstones, medium-strength, strong, and especially strong varieties are observed.

The medium-strength Permian sandstones do not differ from the Jurassic ones in specific gravity, but their density is considerably higher (Table 1). The unit weight of the rocks is 2.33–2.42 t/m³. Porosity is 16.85–23.20%, and the porosity coefficient is 0.20–0.26. In terms of moisture content, they belong to moist rocks and to rocks fully saturated with water. The angle of internal friction of medium-strength sandstones with disturbed structure varies within the range 29–37°.

Strong and especially strong sandstones are less widespread and occur as separate lenses. The temporary resistance of sandstones to compression is 974–2440 t/m², and to tension 152–770 t/m². For especially strong rocks, the temporary resistance to crushing is 3770–9050 t/m², and to tension 500–868 t/m². The angle of internal friction of strong sandstones, obtained from strength certificates, proved to be 33–48°, and that of especially strong sandstones 40–49°. The cohesion, calculated with allowance for the coefficient of structural weakening, is considerably higher in especially strong sandstones, as is evident from Table 1.

The engineering-geological investigations carried out made it possible to select design parameters and to perform preliminary calculations of the stability of the slope of the Second Urop open pit.

As the calculations showed, the overall angle of inclination of the slope at an open-pit depth of 160 m, under conditions of preliminary dewatering and with a safety factor of 1.3, should not exceed 32°.

It follows from the brief characterization given above that the construction site of the Second Urop open pit has gently dissected relief, generally favorable for the construction of the open pit and access roads. In hydrogeological terms, the waters of the Jurassic aquifer complex will have the most unfavorable effect on the operation of the open pit; consequently, development of the deposit should be carried out with preliminary dewatering of the rocks. The latter may have a substantial negative effect on the operation of the water intake, the construction of which has begun in the northern part of the site.

Among the rocks of the deposit, the Quaternary deposits and Jurassic rocks in the weathering zone will prove less stable in the slopes of the open pits. The substantially clayey composition of these rocks, together with their low density and strength, will largely determine their behavior in the quarry slopes. To ensure normal operation of the future open pits, the overall angle of inclination of the slopes should be taken as no more than 32°, provided that the rocks are fully dewatered and the safety factor is 1.3.