METHOD AND RESULTS OF STUDYING SEMI-ROCKY ROCKS WITH A MICROPENETROMETER

V. N. PULYAEV, V. V. KRAVCHENKO, Yu. S. KAZAZAEV

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

Introduction

The search for new rapid and informational methods for studying the properties of rocks is an exceptionally important task of engineering geology. Recently, in the practice of engineering-geological investigations, sounding, penetration testing, and micropenetration have been widely used [1, 2, 5].

In our case, the object of investigation was the semi-rocky rocks of the Mesozoic of the Berezovsky brown-coal deposit of the Kansk-Achinsk basin, occurring at depths down to 200–250 m. According to data from the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute, the rocks have various compositions and properties [3, 4]. Thus, in sections, sandstones and siltstones with clayey cement, argillites, and beds of brown coal, reaching a thickness of 90 m, are most often distinguished. The temporary resistance to crushing of weak rocks ranges from 2.0 to 50.0 t/m² and cohesion is 2.0–3.0 t/m²; for medium-strength rocks, having carbonate-clayey cement, 50–250 t/m² and cohesion 5–10 t/m²; for strong rocks with clayey-carbonate cement, 250–1500 t/m²; and for especially strong rocks with carbonate and siliceous cement, more than 1500 t/m². The latter make up no more than one percent in the sections.

Fig. 1. Micropenetrometer. 1—rod, 2—frame, 3—dynamometer, 4—cone, 5—holder, 6—rock being studied.

Methodology of micropenetration of rocks

The tests were carried out with a micropenetrometer designed at the Department of Hydrogeology and Engineering Geology of the Tomsk Polytechnic Institute by the author of the present article, primarily for the study of semi-rocky rocks. The instrument is simple in design and consists of a pressure screw, a frame-stand, a set of dynamometers, cones with apex angles of 30°, 45°, 17.5°, and a working cylinder loaded with the rock being investigated.

The experiments were performed on monoliths and samples selected during the drilling of boreholes. Penetration was carried out along the bedding and normal to the bedding of the rocks. Under field condi-

…in field conditions micropenetration was carried out at intervals of 0.2–0.5 m, and in the laboratory at intervals of 1–2 m.

A number of researchers (P. A. Rebinder, 1960; V. F. Razorenov, 1961; L. L. Koshlyak, 1964; I. M. Gorkova, 1966) believe that rock testing should most often be carried out with a cone having an apex angle of 30°, which, when it is driven in, causes the least crushing and compaction of the rock structure in the cone’s zone of action. When testing with such a cone, the strength characteristic adopted for semi-rocky rocks was the value of the unit resistance when the cone was driven to a depth of one centimeter. Loads on the cone, through the pressure rod and the dynamometer, were applied uniformly and continuously. By the indicated method, more than 450 tests were performed in the field and in the laboratory, including 195 on sandstones, 155 on siltstones, 50 on argillites, 20 on carbonaceous siltstones and argillites, and 30 on loams.

Fig. 2. Graph of the dependence of temporary crushing resistance on unit resistance to rock micropenetration.

Part of the monoliths tested with the micropenetrometer was carefully removed from the ring and crushed in a press or tested in a shear apparatus in order to obtain the temporary resistance to crushing and shear and to compare the results of micropenetration with the indicated characteristics.

Results of Micropenetration by Rock Type

Loams. The unit resistance of these rocks obtained with the micropenetrometer proved to be 0.2–5 kg/cm². At the same time, a decrease with depth is observed, from 4–5 kg/cm² to 0.2–0.5 kg/cm², which is explained by water saturation of the loams below 3–4 m.

Sandstones have a unit micropenetration resistance of 2–100 kg/cm² and more; most often it is 5–7 kg/cm². The anisotropy coefficient is 1–4, more often 2.

Siltstones showed the same extreme values of unit micropenetration resistance as sandstones, but in sections below the weathering zone it proved to be 10–30 kg/cm² and only sometimes 30–100 kg/cm² and more. The anisotropy coefficient for siltstones reaches 6, more often 2–3.

Argillites under the cone of the micropenetrometer often split along microcracks. In unfractured specimens, the unit micropenetration resistance proved to be 3–100 kg/cm². The anisotropy coefficient for bedded argillites is 3–8.

Carbonaceous siltstones and argillites in the fresh state are characterized by a fairly high unit micropenetration resistance, reaching 20–100 kg/cm². But the strength of these rocks rapidly decreases during weathering and when they are exposed to air. Many examples were obtained in which the rock core, over the course of 2–3 days, completely disintegrated into fine crushed stone.

It has been established that with depth, regular changes in strength…

rocks is not observed, as is noted for loams. Only in the weathering zone are rocks encountered with low specific micropenetration resistances of not more than \(10\ \mathrm{kg/cm^2}\); on the other hand, in the section there are sandstones with clayey cement at depths down to \(150\ \mathrm{m}\), having a specific resistance of \(2\)—\(5\ \mathrm{kg/cm^2}\).

Analysis of the Results

In the work of I. M. Gorkova a summary graph is presented, emphasizing the dependence of micropenetration data and the temporary resistance to crushing for a number of rock types [1]. It was found that for certain varieties of rocks, in the range of strength values below \(30\ \mathrm{kg/cm^2}\), a direct proportionality is observed between resistance to crushing and penetration. In our case, this dependence for the semirocky rocks of the deposit noted above, which has no visible fracturing, was obtained for the strength range from zero to \(90\ \mathrm{kg/cm^2}\). It should be emphasized that, in order to clarify the indicated dependence, monolithic rock samples were subjected to uniaxial compression and micropenetration at fixed moistures. Loads from the stamp and the cone were in both cases transmitted to the rock normal to the bedding.

At the same time it was established that in rocks with an anisotropy coefficient close to unity, the relation between micropenetration data and cohesion is expressed by a curve and is more distinct.

Conclusions

In conclusion, we note that, as applied to semirocky rocks, the micropenetration method must be widely used at all stages of engineering-geological investigations, especially in the development of engineering-geological classifications of rocks. The method is economical and makes it possible rapidly to determine indirect classification indices. Thus, for example, in the case of penetration of core along a borehole \(100\ \mathrm{m}\) deep at intervals of \(0.5\)—\(1\ \mathrm{m}\), it was necessary to spend only 5 hours. The laboratory method of determining the temporary resistance to crushing of these same monoliths required 60—65 hours and the delivery of the monoliths to the laboratory. In addition, application of the micropenetration method can significantly reduce the number of monoliths selected, without substantially reducing the reliability of the information obtained; with a sandstone bed thickness of \(26\ \mathrm{m}\), according to the micropenetration results it was sufficient to select 2 monoliths, whereas in its absence it would have been necessary to select 5—6 monoliths.

LITERATURE

1. I. M. Gorkova. Theoretical foundations for evaluating sedimentary rocks for engineering-geological purposes. “Nauka,” 1966.

2. L. L. Koshlyak. Investigation of the process of destruction of the natural structure of clay in the plastic state. Abstract of Cand. diss. Moscow, 1961.

3. F. P. Nifantov, Yu. A. Usynin, E. A. Pisarev. Engineering-geological characteristics of the first site of the Berezovsk brown-coal deposit. Proceedings of TISI, Tomsk, 1957.

4. F. P. Nifantov, V. N. Pulyaev. Engineering-geological conditions of the Uryup area of the Berezovsk brown-coal deposit. Izvestiya TPI, vol. 185, 1970.

5. V. F. Razarenov. Determination of the friction and cohesion of soils from the results of combined tests by the methods of penetration and rotational shear. In: Rapid methods of engineering-geological investigations for mass-construction purposes. “Nedra,” 1964.