Proceedings
of the Tomsk Order of the October Revolution
and Order of the Red Banner of Labor
Polytechnic Institute named after S. M. Kirov

Volume 287

1977

SOME REGULARITIES IN CHANGES IN THE PHYSICOMECHANICAL PROPERTIES OF ROCKS OF THE OVERBURDEN STRATA OF THE BEREZOVSKY LIGNITE DEPOSIT

N. S. ROGOVA, V. N. PULYAEV

(PRESENTED BY THE SCIENTIFIC SEMINAR OF THE DEPARTMENT OF HYDROGEOLOGY)

The study of regularities in changes in the indicators of the physicomechanical properties of rocks is often necessary for understanding the conditions under which rock strength is formed, for predicting their stability and deformability in the foundations of structures, for choosing more economical methods of investigation, etc. All these questions require solution in engineering-geological investigations of deposits of solid minerals being prepared for open-pit mining. In particular, within the area of the Kansk-Achinsk lignite basin, the construction of a whole series of large open-pit coal mines is planned, as well as further preparation of sites and deposits for quarry development. Many deposits are similar: they are situated in brachysynclinal structures composed of Jurassic and Lower Cretaceous semirocky rocks and overlain by Quaternary deposits. The overburden rocks are most often aleurolites, sandstones, and argillites. In this connection, it seems useful to consider certain regularities in changes in the physicomechanical properties of the rocks of the largest deposits. These include, first of all, the Berezovsky deposit, with lignite reserves of more than 32 billion tons.

Detailed investigations of the physicomechanical properties of the rocks made it possible to reveal a number of regularities in the change of these properties. To assess the influence of individual factors and the degree of variability of the physicomechanical properties of the rocks, we used the method of correlation analysis. The principal tasks of correlation analysis as applied to questions of the variability of physicomechanical properties and the dependence between them are the determination of the closeness of the relationship between variables and of the form of the dependence, i.e., the determination of the function \(y=f(x)\).

To establish the closeness of the relationship between variables, the correlation coefficient and the correlation ratio were used. All calculations connected with determining the latter were programmed and performed with the aid of the Minsk-1 electronic computer. Processing of the available factual material using correlation analysis made it possible to establish the qualitative and quantitative dependence between certain indicators.

In the rocks of the Berezovsky and other deposits of the basin, the cement of many semirocky rocks consists of clayey, carbonate, and more rarely ferruginous substances, which is confirmed by chemical, electron-microscopic, thermal, and X-ray structural studies of these rocks. It has been observed that an increase in strength is associated with an increase in the carbonate content of the rocks. As a result of investigations for po-

rocks of the Berezovsky deposit yielded a linear relationship between the temporary compressive strength of the rocks and carbonate content. Its mathematical expression is \(\sigma_{\mathrm{comp}} = 125K\), where \(\sigma_{\mathrm{comp}}\) is the temporary compressive strength in \(\mathrm{t}/\mathrm{m}^{2}\), and \(K\) is carbonate content.

The effect of gravitational compaction on rock properties is often assessed from the change in density, and more rarely in strength, with depth. In our case, when studying the dependence of density (bulk unit weight of the skeleton) on the depth of occurrence, very low values of the correlation coefficient were obtained. Thus, for sandstones it is equal to 0.17, and for siltstones and argillites, respectively, to 0.14 and 0.06 (Table 1), i.e., within the investigated depths, present-day gravitational compaction naturally does not exert any significant influence on the density of the rocks of the deposit.

Table 1

Correlation table for rocks of the Berezovsky deposit

Note. \(N\) is the number of observation points;
\(\delta\) is the bulk unit weight of the rock skeleton;
\(C\) is cohesion;
\(W\) is moisture content;
\(r\) is the correlation coefficient;
\(H\) is the depth of occurrence of the rock;
\(\eta_{x/y}\), \(\eta_{y/x}\) are correlation ratios.

For the Berezovsky deposit, a clear regularity is observed: as the density of the rocks increases, their moisture content decreases. Moreover, as should have been expected, the strongest relationship between these indices is observed in clayey rocks. The dependences obtained for each rock type have the following mathematical expression:

for sandstones, \(\delta = 1.95 - 0.01W\), with \(r=-0.49\);
for siltstones, \(\delta = 1.99 - 0.01W\), with \(r=-0.64\);
for argillites, \(\delta = 2.19 - 0.025W\), with \(r=-0.77\),

where \(\delta\) is rock density, \(\mathrm{t}/\mathrm{m}^{2}\), and \(W\) is moisture content, %.

The moisture content of the rocks of the deposit has a major effect on their strength. As moisture content increases, their strength decreases.

A rectilinear relationship is also observed between rock cohesion and moisture content. The mathematical expression of these dependences has the form:

for sandstones, \(\quad C = 9.52 - 0.31W\), with \(r=-0.30\);
for siltstones, \(\quad C = 8.40 - 0.20W\), with \(r=-0.21\);
for argillites, \(\quad C = 11.66 - 0.34W\), with \(r=-0.26\),

where \(C\) is cohesion in \(\mathrm{t}/\mathrm{m}^{2}\), and \(W\) is moisture content. As moisture content decreases, the cohesion of the rocks increases.

At the same time, a regularity was found in the change of cohesion with the density of clayey rocks. As the density of the rocks increases, cohesion increases. The mathematical expression of the dependence has the following form:

for siltstones \(C = 0.96—1.0\) at \(r = 0.30\);

for argillites \(C = 12.56—16.1\) at \(r = 0.37\),

where \(C\) is cohesion, t/m\(^2\); \(r\) is the correlation coefficient.

For sandstones, no linear dependence between these variables is observed; however, it is not excluded that it has a curvilinear character.

In determining the ultimate compressive strength of the rocks, it was established that rocks with high values of \(\sigma_{\text{comp}}\) are characterized, as a rule, by low moisture values. The dependence between these variables is curvilinear in character; for sandstones it is expressed as \(\sigma_{\text{comp}} = \dfrac{3668}{W} - 168\), for siltstones \(\sigma_{\text{comp}} = \dfrac{6226}{W} - 204.4\), for argillites \(\sigma_{\text{comp}} = \dfrac{5500}{W} + 50\), where \(\sigma_{\text{comp}}\) is the ultimate compressive strength of the rocks, in t/m\(^2\), and \(W\) is moisture content, %.

Thus, using correlation analysis, we obtained an understanding of the nature of the relationship between the indicators of individual physicomechanical properties. The established dependences are of practical interest, since they make it possible, with sufficient accuracy and with small labor expenditures, to predict physicomechanical properties by calculation.