PHYSICAL PROPERTIES AND SUBSIDENCE DATA OF LOESS-LIKE DEPOSITS IN THE KEMEROVO AND BELOVO DISTRICTS OF KUZBASS

F. P. NIFANTOV

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

Introduction

Within the Quaternary sequence of the Kuznetsk Basin, cover, locally subsiding, loess-like deposits are widely distributed. They cover low flat watersheds, gentle slopes, and river terraces, with the exception of floodplains. These rocks often serve as foundations for industrial and civil structures. In cases where construction is carried out without the corresponding engineering-geological investigations and necessary engineering measures, uneven additional settlements (subsidence) and deformations of structures often occur. The first settlements and deformations of large buildings appeared in Kuzbass in 1929–1932 [1, 2, 3]. Later, despite the progressive growth of construction, subsidence of structures decreased somewhat, which was explained by more comprehensive investigations and the application of special measures. Recently, subsidence of structures has occurred in a number of industrial districts of Kuzbass. Thus, for example, in the postwar years some above-mine and residential buildings at the Chertinskie mines in the Belovo district were deformed. Subsidence and deformation of structures were also observed in the Prokopyevsk, Leninsk, Kemerovo, and other districts of Kuzbass. Taking into account the further development of construction in Kuzbass, the physical properties and subsidence of the cover loess-like rocks of the Belovo and Kemerovo districts of Kuzbass are discussed below.

Principal Features of the Geological Structure of the Loess Sequence

Field investigations in the indicated districts have established the heterogeneous structure of the loess-like rocks. According to lithological features, the cover sequence of loess-like rocks is most often clearly divided into three main horizons. The first horizon, distributed directly beneath chernozem soils, is usually represented by soil loams formed in the process of diagenesis from the underlying loess-like rocks under conditions of a humid climate, evidently simultaneously with the formation of the chernozems. These loams are distinguished by a distinctly developed macroaggregate structure, large macroporosity, locally up to 3 mm in diameter, the presence of microcracks, and extremely weak carbonate saturation. The thickness of this horizon rarely exceeds 1.5–2.5 m. In places the loams are subsiding. Beneath these

loams, both on river terraces and on watersheds, the first horizon of typical loess-like loams and sandy loams is widely distributed. The loams of this horizon are distinguished by a pronounced coarse, dusty composition, uneven saturation with carbonates, intensely developed macroporosity, locally microfissuring, variable moisture content, and often subsidence. The thickness of the horizon of these loams varies widely, from 2–3 to 10–12 m. In places the loam contains lenses of silty sand, and on the terraces of the Tom’ and Bachat rivers, layers of buried soils and brown clays. Beneath these deposits, on river terraces and watersheds, a second horizon of the loess sequence is clearly traceable, often separated from the first by buried soil, sometimes of peaty composition. The loams of this horizon are characterized by poorly developed fine macroporosity, high and more constant moisture content, and very weak subsidence. Their thickness most often ranges up to 5–6 m. As a rule, in zones of constant wetting, especially below the groundwater level, they have acquired a gray color and a silty-clayey composition [5, 6].

Composition of the loess-like rocks

Laboratory studies showed the heterogeneous composition of the loess-like deposits, with a predominance of silty-clayey fractions represented by grains of quartz, feldspars, and other minerals. Soluble and sparingly soluble salts and organic compounds occur in the soil in small quantities and, evidently, do not exert a major influence on the properties of the loam.

Among the mineral grains, as semidispersed analyses showed, the fraction 0.05–0.005 mm predominates, amounting to 56–80%, and as a result it substantially affects the principal properties of these rocks (Table 1).

Table 1

Grain-size composition of loess-like deposits of the Kemerovo and Belovo districts of Kuzbass

Note: Monoliths for analyses were taken from the first horizon at depths of 2–9 m. The analyses were performed using the semidispersed preparation method.

Parallel analyses carried out on the same samples by dispersed and semidispersed preparation methods did not show a large difference in the quantity of clayey and silty particles (Fig. 1). It turned out that silty particles are substantially represented by separate-

…by grains, and not by microaggregates, as some researchers assume. At the same time, the loams, especially of the first horizon, readily disintegrate into large columnar aggregates, which are clearly distinguished by aggregate analyses (Fig. 1).

The sandy particles in the loams are usually represented by fine sand, 0.25–0.05 mm. In the predominant number of samples this fraction does not exceed 7–8%, and only in rare monoliths does it reach 15–18%. With an increase in sandy and coarse-silt particles, the cohesion between particles decreases, and the rate of soaking and subsidence of the loams increases.

The amount of clay particles in both horizons of the loams varies within wide limits and amounts to 13–35% (Table 1). Despite the fairly high percentage, the clay fraction does not provide substantial cohesion, as is observed in non-loesslike clayey soils. This phenomenon can be explained by two reasons:

Fig. 1. Curves of the dispersion (1), semi-dispersion (2), and aggregate (3) composition of loesslike loam of the first horizon.

first of all, by the high content within the clay fraction of coarse clay particles (0.005–0.001 mm), represented substantially by quartz and feldspars, and by the insufficient content of typical clay-colloidal particles and minerals, which make up no more than 9–14% in the soil.

Thus, in terms of grain-size composition, the loesslike loams of the investigated districts belong to typical silty soils having an insignificant amount of fine clay-colloidal particles. Siltiness and the low content of clay-colloidal particles and minerals, together with a loose macroporous and macroaggregate structure, sharply reduce the strength and water resistance of these soils. These properties, moreover, depend on the mineralogical composition of the loams. It turned out that silty and fine-sandy particles are substantially represented by poorly rounded grains of quartz (12–50%), plagioclase (34–44%), orthoclase and microcline (18–30%), and to a lesser…

to a lesser degree magnetite (3–5%), hornblende (up to 2%), calcite (up to 1%), epidote (up to 2%), and individual grains of zircon, tourmaline, apatite, garnet, and other rarer minerals. On the surface, many grains of the listed minerals are coated with continuous or discontinuous crusts of calcium carbonate, and sometimes of clay, iron oxides, and organic matter. Some grains, especially those represented by feldspar, hornblende, and tourmaline, have been rather deeply affected by weathering. In terms of mineralogical composition, the clay fraction is clearly divided into primary minerals (quartz, feldspars, etc.), often represented by coarser particles, and secondary clay minerals, represented chiefly by kaolinite, hydromicas, and beidellite.

Carbonate and other salts, essentially those of calcium and magnesium, whose content according to data from 2.5% hydrochloric-acid extracts ranges from 4.40 to 18.20%, evidently do not exert a great influence on increasing the strength of the loam structure, as is sometimes noted in the literature. In dry loams, salt crusts most often cover the surfaces of silty and sandy particles unevenly, accumulating for the most part in microcracks and along the walls of macropores. In places, the carbonates are gathered into concretions, “cranes,” or are dispersed throughout the entire soil mass as separate crystals weakly bound to the clayey-silty particles.

Microscopic observations did not show a dense concentration of carbonate salts at contacts between particles and aggregates. Under natural conditions, with considerable moisture and the presence of free carbonic acid in the soil, carbonate salts partially pass into soluble bicarbonates, in connection with which the strength of the structure of the loams decreases.

Porosity of the Loams

It was noted above that loess-like loams, especially those of the first horizon, are distinguished by well-developed microfissuring and macroporosity. Under the microscope, a loose accumulation of silty particles, gathered as if in clusters, and a dense network of small pores are clearly observed. The loose accumulation of particles is emphasized by the high total porosity, reaching 43–54% in the first horizon and 44–49% in the second (Table 2). A certain decrease in maximum porosity in the second horizon is explained by compaction and by a decrease in the number of macropores measuring 0.1–0.6 mm in diameter. The macroporosity coefficients, obtained by soaking and compacting naturally moist soils under loads of 3 kg/cm², reach up to 0.09 in the first horizon and do not exceed 0.04 in the second.

Taking into account that, at a total porosity above 46%, soils possess a loose structure, and that, at macroporosity coefficients greater than 0.04, they are collapsible, loess-like loams may be divided into four categories according to total porosity and macroporosity.

The first category consists of loams with high total porosity (above 46%) and high macroporosity coefficients, greater than 0.04. Soils of this category, at a degree of moisture less than 0.7, are usually collapsible and compact well under load in the moist state. The second category comprises loams with total porosity greater than 46% and macroporosity coefficients less than 0.04; they are weakly collapsible, but compact well in the moist state. Loams of the third category, with total porosity less than 46% and macroporosity coefficients greater than 0.04, are collapsible when slightly moist and moderately compressible when wet. Loams of the fourth category, with porosity less than 46% and a macroporosity coefficient less than 0.04, are weakly collapsible-

…subsiding and moderately compressible. The first two categories most often include the loess-like loams of the first horizon; the third and fourth usually include the loams of the second horizon.

Table 2

Specific gravity, bulk density, porosity, and porosity coefficient of loess-like loams

Influence of Moisture on the Stability and Subsidence of Loess-like Loams

The strength of the structure and the stability of loess-like loams change sharply with changes in the amount and state of their natural moisture. In turn, natural moisture—especially in the first horizon of loams located in the aeration zone—does not remain constant. Unfortunately, stationary observations of changes in soil moisture in Kuzbass are not yet being carried out. At the same time, individual observations point to significant changes in the natural moisture of loams in the aeration zone, especially during the development of construction sites, where a large number of pits and trenches are excavated. In such cases, the bearing soils during construction prove to be considerably wetter than is noted in engineering-geological reports. Therefore, data obtained during investigations must necessarily be checked before the foundations of structures are laid, since with a change in natural moisture many properties and the stability of the bearing stratum of loess-like soils change substantially. The natural moisture of the loams of the first horizon, determined mainly in the summer periods of the year, varies in the predominant number of samples from 14 to 26%; in the second horizon, from 17 to 35%; the corresponding degree of moisture is 0.43—0.90 and 0.55—1.0 (Table 3).

Under the noted conditions of wetting, the strength of the structure and the subsidence of loams of the first horizon vary within wide limits, depending on local conditions, depth, and season of the year. In those cases when the degree of moisture rises above 0.7—0.8, these loams, despite the presence of well-developed macroporosity, lose their subsidence property and pass into the category of slightly subsiding, well-compressible soils. Wet and saturated loams of the second horizon proved to be non-subsiding, but compressible soils (Table 3).

Thus, typical subsiding loams are not widely distributed in Kuzbass. They occur only on dry, drained sites, where the degree of moisture does not rise above 0.7 and the groundwater level lies deep below the ground surface.

Fig. 2. Graph of the dependence of the subsidence coefficient on the degree of natural moisture content \(q\) of loams.

Fig. 3. Compression curves of loess-like loam of the first horizon. Belovo district, pit 7, depth 4 m.

It should be particularly emphasized that, with a change in the natural moisture content, the subsidence of loams varies within rather wide limits; therefore the value of relative subsidence, which is a calculated characteristic, does not remain stable for each subsiding soil layer on different geomorphological elements and, moreover, changes over time.

Table 3

Natural moisture content, limits of plasticity, and coefficients of subsidence of loess-like loams for characteristic monoliths

Change in the subsidence of loams at different moisture contents and loads

To determine how the value of the relative subsidence of soil changes as a function of changes in moisture content and load, a series of experiments was carried out with naturally moist soils having an undisturbed structure, taken from the first horizon. As a result of the studies, different values of relative subsidence were obtained for the same soils at different moisture contents and loads (Figs. 2 and 3). The maximum subsidence was shown by slightly moist, very macroporous loams, with a high total porosity of 47–52%. But the same soils, at a degree of moisture content above 0.7, despite their high porosity and macroporosity, proved to be slightly subsiding or showed no subsidence at all. In this case, ordinary compression curves with large coefficients of compressibility were obtained [4, 5]. In addition to the noted dependence, it was found that subsidence increases significantly with an increase in the load under which the soil is soaked.

The observed dependences of subsidence on load and moisture content agree well with data obtained for other regions of our country, and indicate the need for a fundamental restructuring of the methodology for investigating and evaluating the subsidence of loess-like rocks.

LITERATURE

1. Abelev Yu. M. Practice of construction on loess-like soils, based on the experience of Kuznetskstroy. ONTI, 1934.

2. Abelev Yu. M. Fundamentals of design and construction on macroporous soils. Stroyvoenmorizdat, Moscow, 1948.

3. Vorontsev G. A. Experience in the construction of the Kemerovo coke-chemical plant. Koks i Khimiya [Coke and Chemistry], No. 9, 1933.

4. Nifantov F. P. Geology of loose deposits and physical properties of cover loess-like rocks of the Kemerovo district of Kuzbass. Tomsk, 1951.

5. Nifantov F. P. Determination of the subsidence of structures on loess-like loams of Kuzbass. Izvestiya of the Tomsk Polytechnic Institute, Vol. 90, 1958.

6. Nifantov F. P. Engineering-geological characterization of loess-like rocks of the Belovo district of Kuzbass. Proceedings of the Tomsk Civil Engineering Institute, Vol. VII, 1960.