ON THE QUESTION OF THE METHODOLOGY OF ENGINEERING-GEOLOGICAL MAPPING IN AREAS OF LOESS ROCK DEVELOPMENT

G. A. Sulakshina, L. A. Rozhdestvenskaya

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

Numerous areas in the south of the West Siberian Lowland are regions of development of loess rocks. The industrial development of these areas raises the task of timely preparation of engineering-geological maps necessary for the planning and design of construction, and taking into account the specific features of loess rocks.

Experience in developing a methodology for compiling engineering-geological maps for a number of districts of Tomsk Oblast, as well as an analysis of available materials from previously conducted geological surveys, has made it possible to outline the principal questions whose resolution is necessary for solving the assigned task. Among the questions concerning the fundamental principles of engineering-geological mapping in general, it is first of all necessary to note the question of the need for carrying out all survey work in an integrated manner, with the compilation, alongside generally accepted geological maps, of engineering-geological maps as well.

This question, raised in the literature by Professor N. V. Kolomensky, is of especially great importance for the regions of Western Siberia. The universally existing practice of carrying out survey work, in which geological, hydrogeological, and engineering-geological surveys are performed successively on one and the same territory, leads to unjustified duplication of work and lengthening of schedules. Thus, for example, within Tomsk Oblast from 1956 to 1959 geological surveys of medium scales were conducted. In 1959–62, within these same districts, medium-scale engineering-geological surveys were being conducted. The available materials from earlier geological surveys, in a number of cases, did not make it possible to provide even the most general engineering-geological characterization of loose Quaternary rocks, which are widely developed in Western Siberia and everywhere serve as foundation soils. Their geological documentation—especially of cover deposits—from the engineering-geological point of view, as a rule, is carried out very schematically. At the same time, on the basis of the materials of geological surveys, it would undoubtedly have been possible to divide the rocks, including loose rocks developed in the zone of influence of structures, at least into petrographic types in the sense used by Professor I. V. Popov. In this connection, it is undoubtedly correct to raise the question of developing a unified methodology for integrated

of geological-engineering mapping. From our point of view, even now, as a working procedure, the geological documentation used in geological surveys should include a mandatory set of the simplest engineering-geological observations, with determination of a minimum of classification indicators of the physico-technical properties of rocks.

The methodological features of compiling a medium-scale engineering-geological map for areas of loess-rock development, as well as a model of the map, were developed in accordance with the following fundamental provisions: the map must reflect the features of the planned construction; the map must show the factors that determine the conditions for erecting specific structures, taking into account the specific features of loess rocks, with a minimum load on the map and a minimum number of maps. In investigations connected with civil and industrial construction on loess rocks, the map should reflect all factors determining the conditions for structures, the choice of foundation, the design of foundations, and construction methods, with a degree of detail corresponding to the scale of the map.

In analyzing the materials obtained in the process of engineering-geological surveying, it proved that the most important factors determining construction conditions in our case are:

a) geomorphology and relief;
b) the composition and properties of loess deposits, which occur everywhere in the foundations of structures;
c) the conditions of occurrence of the first aquifer from the surface. Physico-geological phenomena, owing to their weak distribution in our case, are of lesser importance.

Geomorphologically, the region proved to be fairly simple—represented by the lacustrine-alluvial plain of the Tom–Chulym interfluve and by the terraces of the Tom and Chulym rivers. The conditions of occurrence of the first aquifer from the surface were determined by the features of the relief and correlated well with the geomorphological structure of the region. Thus, both of the noted factors could be depicted on the map quite simply, by combining them with the engineering-geological zoning map compiled basically according to the method of I. V. Popov. Greater attention had to be given to depicting on the map the composition and properties of the rocks of the bearing stratum. This was connected with the peculiar nature of the loess rocks developed in our regions and with the prospects for the development in Siberia of large-panel construction, which is especially sensitive to uneven settlement and subsidence. Under such conditions, the basis for compiling plans for the placement of large-panel buildings had to be engineering-geological maps with the designation of areas of loess-rock development differing in the degree of their subsidence (I. M. Litvinov, 1961).

In accordance with the intended purpose of the map—for the needs of civil and industrial construction—and also taking into account the usual foundation depth for Tomsk Oblast of 2.5–4.0 m, at which the zone of influence of structures on the rocks (the bearing zone) is 10–12 m, the map had to reflect all features of the composition and properties of the rocks within this entire zone.

The maps of Quaternary deposits often used in engineering-geological mapping practice, in combination with section maps at the depth of foundation emplacement, from our point of view could not satisfactorily solve this problem, since the depth of the section, even under conditions of a single type of construction, is an uncertain or, in any case, variable quantity. This is confirmed by an analysis that was carried out of the use by design and construction organ—

organizations of the city of Tomsk, compiled in 1957, engineering-geological section maps at depths of 2.5 and 4.0 m. It turned out that, in solving a number of problems—in particular, in laying tram lines and heating mains, and in other cases—these maps could not be used. All the more must this be true of small-scale maps.

In the contemporary literature there are individual recommendations on this question (3, 4, 5). Thus, on engineering-geological maps for irrigation purposes, I. V. Popov recommends indicating the composition of the rocks of the upper horizon by hatching, and characterizing the underlying rocks at individual points on the map by a conventional scale-free column. While giving a general idea of the section of the bearing stratum, this method does not make it possible to judge changes in thickness, which in areas where subsidence rocks occur is necessary for calculating the total magnitude of subsidence. On the maps of A. A. Smirnov and V. L. Solonenko, alongside hatching that shows the composition of the surface rocks, the composition of the underlying stratum is given in figures or indices; however, deciphering the figures and indices requires very cumbersome tables and is therefore not very convenient for practical use.

Taking these considerations into account, on the proposed engineering-geological map, areas with a homogeneous structure of the bearing stratum are distinguished by hatching; their detailed characteristics, indicating the age, composition, and thickness of the rocks, are given in the map legend. The age of the rocks is indicated on the map by color according to the generally accepted geological legend. This brings the engineering-geological map closer to a map of Quaternary deposits and, possibly, will in the future make it possible to compile a single map combining both the geological and the engineering-geological maps. Selectively, by figures in the form of a fraction at boreholes along section lines, the thicknesses of the principal horizons are indicated. This makes it possible to judge the section of the bearing stratum at each specific point with sufficient detail and, in addition, makes it possible, when necessary, to calculate the total magnitude of subsidence.

In many districts of Tomsk Oblast, loess rocks are widely developed in the section of the bearing stratum; among them, two horizons of different ages, separated by a buried soil, are clearly distinguished:

1. The upper horizon ($Q_3$)—Upper Quaternary pale loess-like heavy loams and clays with all the characteristic features of loess rocks. Thickness from 0.0 to 13 m.

2. The lower horizon ($Q$ 2—3)—Middle–Upper Quaternary gray and grayish-brown sandy, dense loess-like heavy and medium loams. Thickness—5.0–10.0 m.

The loess-like deposits are underlain by a thick loamy-clayey stratum of Middle and Lower Quaternary age. In macroscopic appearance, mineralogical and chemical composition, and structural features, the stratigraphic horizons distinguished above proved to be fairly clearly separated. This gave grounds for assuming the presence of substantial differences also in their physico-technical properties. In particular, the tendency toward additional settlement upon soaking under loads was found only in the loess rocks of the upper loess horizon. Considering that direct determinations of subsidence in accordance with the scale of the investigations were made only at individual points, it was especially important to identify the simplest features and indicators that, with the least expenditure of effort and with a sufficient degree of reliability, would make it possible to distinguish on maps areas of varying degrees of subsidence. To this end, an analysis was carried out of the relationship between the coefficient of relative subsidence, on the one hand, and granulometric, mineralogi-

composition, swelling, slaking, and indices of porosity, moisture content, and plasticity, on the other. The clearest relationship emerged when identifying the simultaneous dependence between the coefficient of relative collapsibility, porosity, and the degree of moisture content or the moisture content of the rocks. Only undegraded loess rocks of the upper horizon, with high porosity ($\varepsilon$ greater than 0.8) and a low degree of moisture content ($G$ less than 0.7), proved prone to collapse.

Determination of these indices from undisturbed samples taken from boreholes made it possible to outline on the map, within the areas of development of the upper loess horizon, rock areas prone to collapse. Control determinations of collapsibility from undisturbed samples taken from pits confirmed the differentiation carried out. In order that calculation of the total magnitude of collapse could be made at any point on the map, near the pits from which collapsibility was determined, the value of the coefficient of relative collapsibility was indicated numerically.

In accordance with the requirements of SNiP of 1962, the role of direct calculated indices of compressibility and shear resistance of rocks in calculating the stability of structures increases sharply. In order to distinguish on the map engineering-geological rock types by these indices as well, and also so that, without cluttering the map with a large amount of numerical material, it would nevertheless provide designers with the calculation materials necessary for the given scale, the dependence of the shear-resistance and compressibility indices of each stratigraphic horizon on all the simplest indirect indices was analyzed, as were scatter diagrams by depth and area for the latter.

The analysis showed that the upper loess horizon, homogeneous in granulometric composition and plasticity, proved heterogeneous in porosity and moisture content. The latter was clearly linked with relief, microrelief, and the depth of occurrence of perched water and of the first aquifer. The two engineering-geological rock types that had emerged on the basis of collapsibility, as stated above, also proved different in their capacity to swell and, to a lesser degree, in compressibility: with a porosity coefficient of more than 0.8 and a degree of moisture content of less than 0.7, the swelling value of the rocks of the upper loess horizon is, as a rule, more than 5%. With $\varepsilon$ less than 0.7 and $G$ more than 0.8, the swelling value does not exceed 1–3%.

The areas of Upper Quaternary loess rocks with $\varepsilon$ greater than 0.8 and $\varphi$ less than 0.7, outlined on the map in this way, proved to be confined to the highest parts of the watersheds, with groundwater occurring at depths greater than 10.0 m. No clear dependence of the shear-resistance indices on these same indirect indices was discerned. Accordingly, generalized and calculated values of the direct indices were established by methods of mathematical statistics as a whole for each stratigraphic horizon.

The map compiled in this way is sufficiently simple and clear, and at the same time illuminates all features of the composition and properties of the rocks required by designers. In combination with the engineering-geological zoning map, compiled basically according to the methodology of I. V. Popov, it gives a complete characterization of all conditions for civil and industrial construction on loess rocks.

Thus, in the case of engineering-geological investigations in connection with civil and industrial construction under the conditions of Western Siberia, it is possible to manage with two engineering-geological maps: a map of engineering-geological zoning and a map detailing the factors most important under the given specific conditions, which determine construction conditions. In our conditions this proved—

a map characterizing the composition and properties of the rocks. Under other, more complex conditions, the content of this map may, of course, be changed in accordance with the specific situation and the requirements of construction.

Such maps can already be compiled at present for all large cities of Siberia even without carrying out special surveys, but only by using the available materials of scattered engineering-geological investigations. Experience in compiling such a map for the city of Tomsk has shown that in this form the map is most convenient for practical use and is a good basis for developing local design standards in addition to the all-Union standards.

REFERENCES

1. V. A. Dubrovkin, E. G. Chapovskii. Questions of the methodology for compiling engineering-geological maps. Izv. vuzov, No. 10, 1959.

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

3. I. V. Popov. Engineering-geological maps for irrigation purposes. Exploration of Mineral Resources, No. 2, 1953.

4. A. A. Smirnov. Maps of engineering-geological conditions and maps of engineering-geological regions. Soviet Geology, No. 2, 1939.

5. V. N. Solonenko. On the methodology for compiling engineering-geological maps. Proceedings of the East Siberian Branch of the Academy of Sciences of the USSR, issue 10, 1959.