On Methods of Engineering-Geological Investigations of Brown Coal Deposits in Siberia

ON METHODS OF ENGINEERING-GEOLOGICAL STUDIES OF BROWN-COAL DEPOSITS IN SIBERIA

F. P. Nifantov, V. N. Pulyaev

(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)

Objectives and Stages of the Studies

Siberia is rich in brown coals. In the Kansk-Achinsk basin alone, more than ten large deposits with thick seams of brown coal have been discovered and explored. The simple geological structure and weak lithification of the enclosing rocks predetermine the wide development of open-pit coal mining by quarries with a productivity of up to 30–50 million tons per year. Such large quarries are planned first of all for the Berezovskoye, Itatskoye, Abanskoye, and Nazarovskoye deposits, where in recent years the Department of Hydrogeology and Engineering Geology of Tomsk Polytechnic Institute has been carrying out studies to clarify the engineering-geological conditions, while at the same time developing methods for these studies and for predicting the stability of rocks in quarry walls [1–9].

In the course of this work we have come to the conviction that engineering-geological studies of deposits should rationally begin at the stage of preliminary exploration and continue through the period of quarry operation. In this connection, the general cycle of studies may be divided into four stages: engineering-geological work at the stage of preliminary exploration; the same at the stage of detailed exploration; studies during the preparation of the technical design and working drawings; and studies and observations during the construction and operation of quarries.

At the first stage, engineering-geological work is carried out in conjunction with exploration and should be performed mainly by geological teams and expeditions. In this case, simultaneously with clarification of the geological structure, calculation of reserves, and assessment of the quality of the mineral, the conditions of stripping the deposit are studied in order to obtain the materials necessary for preparing a plan for full development and zoning of the deposit. The most favorable areas are designated for detailed exploration, detailed engineering-geological studies, and priority development. At the same time, sites are indicated for the construction of a town, plants, thermal power stations, and roads, and sources of water supply are recommended.

At the stage of detailed exploration, the engineering-geological conditions for construction of the first quarries are clarified, and an assessment is made of the stability of the rocks that will fall within the zones of influence of these quarries. For this purpose, the geologi-

tectonic structure, water-bearing capacity, and the properties of the rocks of the areas selected for the placement of open pits. The engineering-geological classification of rocks is finalized, their variability is established, generalized indicators are determined, and a preliminary assessment of overall stability is made.

At the stage of open-pit design, the stability of pit walls, benches, external and internal dumps is clarified and evaluated, and other questions are resolved. Investigations at this stage are carried out by design organizations according to special programs, taking into account the adopted method of uncovering the seams and the complexity of the site.

During the period of open-pit operation, stationary observations are conducted chiefly on water inflows, groundwater drainage, and the degree of rock disintegration within the sphere of influence of each open pit. It should be considered that all general questions concerning the forecasting of engineering-geological conditions for uncovering deposits must be resolved at the first two stages of exploration.

Methods of Investigation

Depending on the tasks of the exploration stage, the size and complexity of the deposits, investigations are carried out with varying degrees of detail and include: engineering-geological surveying, engineering-geological sampling of rocks in reference exploration workings, laboratory work, and stationary observations of the regime of water-bearing horizons and the development of modern physicogeological phenomena.

Engineering-geological surveying is usually carried out at the stage of preliminary exploration over an area 1.5–2 times larger than the size of the deposit. During these works, geological, hydrogeological, and engineering-geological mapping is performed at scales of 1:25,000 or 1:50,000. Particular attention is paid to the study of the structure of the deposit, the lithology and water-bearing capacity of the rocks, landforms, and physicogeological phenomena. In describing the relief, the geomorphological elements of the investigated territory are identified, and the relationship of modern landforms with processes of erosion and accumulation, with zones of coal burning, rock collapse, and other factors is clarified. At the same time, the steepness of slopes is determined, including beyond the limits of the deposit. Operating open pits are examined in especially great detail; there the method of mining, the height and inclination of slopes and benches, the behavior of rocks in working and nonworking walls, the influence of water on rock stability, etc., are clarified.

The strength of individual rock types during the survey is assessed by micropenetration, through description of composition and properties. In the areas of the most unstable slopes and pit walls, monoliths are taken and the principal properties of the rocks are determined.

Sampling of rocks is carried out chiefly in engineering-geological boreholes combined with exploration boreholes. The frequency, depth, and diameter of workings depend on the stage, objectives of the investigations, and complexity of the deposit. The distance between reference engineering-geological sections at the stage of preliminary exploration for large deposits is taken by us to be no more than 2–3 km, so that two or three sections fall within the limits of each planned open-pit field.

At the stage of detailed exploration, the distance between reference sections was taken as 1–2 km, and boreholes along the section are placed every 0.3–0.4 km.

Boreholes are drilled with a diameter of not less than 80–90 mm with flushing.

with a clayey solution. Drilling rigs are equipped with apparatus of the IRB-5 type for measuring the rate of penetration and the pressure at the bottomhole, which makes it possible to assess the strength of the rocks. In addition, detailed testing of the properties of rocks and aquifer horizons is carried out with the use of micropenetration, logging, observations of core recovery, and losses of flushing fluid. Such detailed testing, together with surveying, makes it possible quickly and without great expense to determine the general engineering-geological conditions for uncovering deposits.

According to descriptions of drillability and micropenetration, engineering-geological rock types are identified directly in the field, which makes it possible to reduce the number of monoliths sampled. Micropenetration is carried out with simple hand micropenetrometers [8], and the cohesion of semi-rocks in this case is calculated by the formula

\[ C = 9.81 \lg R_m - 2.62, \]

where \(C\) is rock cohesion, t/m\(^2\);

\(R_m\) is the specific resistance to micropenetration, t/m\(^2\).

Radioactive and electrical logging is carried out in all engineering-geological boreholes. From the curves obtained, the density of the rocks is determined quite well, being calculated by the formula:

\[ \Delta = 2.52 - 1.3 \frac{I_{\gamma\gamma}}{I_{0\gamma\gamma}}, \]

where \(I_{\gamma\gamma}\) is the intensity of scattered gamma radiation;

\(I_{0\gamma\gamma}\) is the intensity of the natural background.

The data of micropenetration and geophysical observations are checked by studying the properties of rocks in field and stationary engineering-geological laboratories; for this purpose, monoliths of characteristic rocks are taken from the core every 5–8 m. It has been established that 50–60% of laboratory determinations should be carried out in field laboratories, where bulk density, moisture content, slaking, resistance to micropenetration, and, for weak rocks, shear resistance are determined.

In stationary laboratories, in addition to the characteristics listed, specific gravity, temporary resistance to crushing and rupture, composition, swelling, and the microstructure of each engineering-geological type are determined. Here the greatest attention is given to studying shear resistance, especially of weak rocks. Taking into account the conditions in which a given rock will occur in the walls of the planned open pits, shear tests on each monolith are carried out at the corresponding loads and moisture contents. Shears are directed along the bedding plane, along normal or weakened microzones, taking into account the development of the presumed slip surfaces. Much attention is also given to the preliminary study of disturbed rocks, which is necessary for calculating the stability of dumps. The temporary resistance of rocks to crushing is determined on the same monoliths using the methodology developed at the A. A. Skochinsky Institute. Studies of rock properties of this nature should be performed for all deposits of solid mineral resources that are at the stage of preliminary and detailed exploration.

Processing of Materials

The research materials obtained at the first two stages are processed according to the following plan:

1. Data on the general engineering-geological conditions (relief, climate, hydrography, geology, hydrogeology, and modern geodynamic processes) are analyzed and processed. The composi-

the necessary figures, tables, graphs, sections, and maps are compiled. The basic propositions on the development and influence of each factor on the mining of the deposit are worked out.

1. An engineering-geological classification of the rocks of the deposit is compiled. In accordance with it, the properties of the rocks are described for each engineering-geological type; at the same time, using one of the statistical methods, generalized indicators are determined [6].

2. Engineering-geological sections and an engineering-geological map are compiled, making it possible to carry out zoning of the deposit according to the engineering-geological conditions of mining. As a result, taking geological-economic data into account, the most favorable areas for construction of the first open pits are identified.

3. The design values of the principal properties of the rocks are tentatively determined, and preliminary calculations of the overall stability of the pit walls are carried out along reference engineering-geological lines; this is done mainly at the stage of detailed exploration and during the period of open-pit design.

4. On the basis of all materials obtained in the course of investigations and processing, an engineering-geological report is prepared for the deposit, and, in the case of detailed exploration, for the area of the planned open pit. The report includes the following principal sections: the purpose and stage of the investigations; the program, scope of work, and methodology of the investigations; the influence of engineering-geological conditions on the mining of the deposit; the engineering-geological classification and characteristics of the rocks and coals of the deposit; the nature of the strength of the rocks of the deposit; an analysis of the stability of the walls of operating open pits working under analogous conditions; zoning of the deposit according to engineering-geological conditions; selection of design indicators; and a forecast of the stability of rocks in the zones of influence of the projected open pits.

Thus, during exploration it is necessary to study the engineering-geological conditions of deposits sufficiently fully, which makes it possible to design large open pits correctly, to forecast the conditions of mining coal seams, and to avoid accidents.

LITERATURE

1. F. P. Nifantov. Principal directions of engineering-geological investigations of coal deposits in Siberia. In: Problems of methods of engineering-geological investigations. Vol. 2, TSU Press, Tomsk, 1967.

2. F. P. Nifantov. Certain questions of engineering-geological investigations of brown-coal deposits of the Kansk-Achinsk basin. In: Materials on geology and mineral resources of Western Siberia. TSU Press, Tomsk, 1964.

3. F. P. Nifantov, V. N. Pulyaev, E. A. Pisarev. Engineering-geological conditions of mining of certain brown-coal deposits of Transbaikalia. Proceedings of TISI, TSU Press, 1969.

4. F. P. Nifantov, V. E. Olkhovatenko, E. A. Pisarev. Engineering-geological characterization of the Itat brown-coal deposit. Collection of Scientific Works of TISI. Vol. XII, TSU Press, Tomsk, 1967.

5. F. P. Nifantov, V. N. Pulyaev. Engineering-geological conditions of the Uryup area of the Berezovsky brown-coal deposit. Izvestiya TPI. Vol. 185, TSU Press, Tomsk, 1971.

6. E. A. Pisarev. On the question of methods of statistical processing of indicators of engineering-geological properties of rocks. Proceedings of the Tomsk Civil Engineering Institute. Tomsk, 1967.

7. V. N. Pulyaev, E. A. Pisarev. On the question of the spatial variability of the properties of rocks. Izvestiya TPI. Vol. 196, 1969.

8. V. N. Pulyaev, V. V. Kravchenko, Yu. S. Kazazaev. Method and results of investigations of semi-rocky rocks with a micropenetrometer. Izvestiya TPI. Vol. 201, TSU Press, Tomsk, 1971.

9. V. E. Olkhovatenko, V. N. Pulyaev. Investigation of shear resistance of certain types of rocks of the Itat brown-coal deposit. Izvestiya TPI. Vol. 151, TSU Press, Tomsk, 1966.