Subsidence Properties of Loess Deposits in the Tomsk Ob Region in Relation to Large-Panel Construction

G. A. Sulakshina

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

Large-panel buildings are the most sensitive to uneven settlements, which quite often lead to accidents. Therefore, when drawing up long-range plans for the placement of large-panel construction, the need for strict consideration of the characteristics of the soils at the base of buildings—characteristics that ultimately strongly affect the cost and quality of construction—acquires especially great importance. The basis for drawing up long-range plans for the placement of large-panel buildings should be maps of engineering-geological zoning, with areas of varying degrees of suitability for these purposes distinguished on them. In particular, for regions where subsiding soils are developed, it is recommended to distinguish (I. M. Litvinov, 1961):

1. Areas fully suitable for large-panel construction according to ordinary standard designs, composed of non-subsiding loess soils.

2. Areas where subsiding soils of category I are developed, suitable for large-panel construction according to designs of series 1-480-P and 1-480-G.

3. Areas unsuitable for large-panel construction, represented by subsiding soils of categories II and III.

The problems of large-panel construction are urgent for a number of regions of Western Siberia, including the city of Tomsk.

In the environs of the city of Tomsk and in the adjoining districts, loess macroporous deposits are widely developed and almost everywhere serve as bearing soils. In the general thickness of the loess cover of the interfluve areas, two horizons are clearly distinguished, separated by a buried soil.

1. The upper horizon \((Q_3)\) consists of Upper Quaternary pale loess-like heavy loams and clays, with a thickness from 0.0 to 9.0 m on slopes and up to 13.0 m on elevated parts of interfluves. This horizon possesses all the characteristic features of loess soils—siltiness, macroporosity, the presence of calcareous concretions, and columnar jointing. As a rule, the upper part of the loess deposits is decalcified. The thickness of the leached horizon varies from zero on elevated parts of interfluves to 7–10 m on slopes.

2. The lower horizon \((Q_{2-3})\) consists of Middle–Upper Quaternary gray and grayish-brown sandy, dense, predominantly heavy and more rarely medium loams, with less distinctly expressed macroporosity. The total thickness of this horizon is 5–10 m.

The distinguished loess horizons are characterized by a set of distinctive features of mineralogical and chemical composition, described in the work of B. F. Mikhalchenko. The most important of these, determining the engineering-geological features of these horizons and, in particular, their tendency toward subsidence, are:

1. The composition of the clay fraction, represented in the upper horizon mainly by hydromicas with an admixture of kaolinite, and in the lower horizon by hydromicas and beidellite.

2. Structural features, in particular the increased density of the loams of the lower horizon.

The noted differences in external appearance, composition, and structural features also determine the different behavior of the two horizons upon wetting. Thus, the deposits of the lower horizon (Fig. 1) are non-subsiding

Fig. 1. Dependence of the coefficient of relative subsidence on the porosity coefficient and degree of moisture.

Legend:
x — $i_m > 0.02$; o, ● — $i_m < 0.02$
x, o — deposits of the upper loess horizon $(Q_3)$;
x, ● — deposits of the lower loess horizon $(Q_{2-3})$.

and are not prone to additional settlements when soaked under a load of 3 kg/cm². The upper loess-like loams in a number of cases show a tendency toward deformation when soaked under loads. According to data from extensive investigations of subsidence with soaking at 3 kg/cm², the coefficients of relative subsidence of the loams of the upper horizon vary within wide limits—from 0.0 to 0.04–0.08.

In order to identify indirect indicators characterizing the tendency of the loess rocks of our region toward subsidence, the following were carried out:

1) comparison of the coefficient of relative subsidence with indirect indicators of the properties of the rocks;

2) determination of the dependence of the coefficient of relative subsidence on the load under which soaking is performed.

Considering that the upper loess horizon is quite homogeneous in composition, the porosity and moisture indicators were used as the principal indirect indicators of subsidence. As shown—

As Fig. 1 shows, subsidence properties were found only in the carbonate rocks of the upper horizon, with high porosity (porosity coefficient greater than 0.8) and a low degree of moisture (less than 0.67).

According to the data obtained, and also using the materials presented in our work (G. A. Sulakshina, 1961), the following principal indirect indicators of subsidence can be outlined for the Tomsk area:

1) carbonate content;
2) porosity coefficient (0.8–1.0);
3) degree of moisture (less than 0.7).

Mass determination of these indicators in the course of integrated engineering-geological surveys at medium scales makes it possible to outline on maps areas of rocks prone to subsidence.

It should be noted, however, that the subdivision of areas according to their suitability for large-panel construction in the Tomsk Ob region should be less detailed than is recommended by I. M. Litvinov.

Table 1

As Table 1 shows, the loess deposits of our areas are not typical collapsible soils: they do not exhibit subsidence deformations under ordinary loads, i.e., within the range of 0.4–0.9 kg/cm². Additional settlements in them are observed only when wetted under loads exceeding 2 kg/cm². In the load interval from 2.0 to 3.0 kg/cm², the coefficients of relative subsidence change little. Rocks prone to additional settlements occur only to a depth of 5.5 m; consequently, the possible total magnitude of additional settlement for our area, with foundation depths of 2.2–4.0 m and an average value of the coefficient of relative subsidence of 0.04, will be, respectively, 6–14 cm. Only at individual points, where \(i_m = 0.08\), can the magnitude of additional settlement reach 20–26 cm.

Taking into account the predominantly insignificant magnitudes of additional settlements, as well as the recommendations of the All-Union Conference on Construction on Loess Rocks (Voronezh, 1962), for our areas, as a basis for prospective plans for the placement of large-panel buildings, it is possible to recommend engineering-geological maps with the designation on them of two categories of areas:

1. Fully suitable for large-panel construction according to standard typical designs are areas composed of non-subsiding loessial soils of the upper and lower loessial horizons.

2. Areas suitable for large-panel construction according to designs of series 1-480-II and 1-480-Г, composed of loessial soils of the upper horizon, are prone to additional settlements upon soaking under loads exceeding 2 kg/cm².

References

1. Litvinov I. M. Construction of large-panel buildings on subsiding soils and above mine workings. Construction and Architecture, No. 11, 1961.

2. Sulakshina G. A. Lithological-mineralogical and engineering-geological characteristics of loessial soils in Tomsk. Proceedings of the Voronezh Conference “Construction on Loessial Soils,” vol. 1, 1961.