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Section 2. Geology
https://doi.org/10.29013/ESR-21-1.2-10-13
Zakirov M. M., Ochilov G. E., Tashkent State Technical University E-mail: [email protected]
CHARACTERISTICS OF ENGINEERING, GEOLOGICAL AND SEISMIC PROPERTIES OF PALEOGENE CLAYS IN THE MODERN HYPERGENESIS ZONE
Abstract. The article examines the formation and development of engineering and geological properties of the Paleogene clays of North Tamdytau. By examining the features of the geological and tectonic development of the North Tamdytau, we have defined the formation and development of clays within the arid lithogenesis, distribution, thickness, depth of occurrence of lithology and properties. Their study has an important theoretical and practical importance, as these sediments are most often the foundations for various engineering structures.
Keywords: shrinkage, swelling, foundations of structures, wetting, durability, seismic properties, weathering, salt accumulation, weathering agents, hypergenesis, arid lithogenesis, modern hypergenesis.
Introduction. The practice of geotechnical atmospheric precipitation, leaks from water utilities, engineering shows that underestimating the swell- but also due to the accumulation of moisture under ing and shrinkage of clay soils in the foundations the watertight screen and changes in the water table of structures leads to their premature damage. The due to elevated temperatures at the foundation of the problem of damage to the foundations of structures, thermal structure. In this regard, we set a research especially thermal ones, occurs when the clay soil goal - the formation of engineering-geological and under the building is subjected to moisture, begins seismic properties of Paleogene clays in the zone of to swell because of reduced strength properties, in- modern hypergenesis.
creases the pressure on the foundation of the struc- Modern hypergenesis is a global process of physi-ture. During dry periods of the year, this soil can dry cal and chemical transformation of rocks in both out and settle due to decreased moisture content, subaerial and mainly subaerial conditions [1-3, 4]. which also leads to a reduction in strength and bear- The weathering process affects practically all rocks ing capacity. The cyclical repetition of this kind leads composing the upper shell of the Earth, i.e. erup-to a reduction in both strength properties and salini- tive, metamorphic and sedimentary, including car-sation and physico-chemical transformation. At the bonate and chemogenic varieties of the latter. The same time, soil moistening may occur not only from most clear regularities of this process are observed
in the weathering profiles of aluminosilicate rocks (especially erupted ones), some differences ofwhich are characterized by a pronounced contrast in both chemical composition and mineralogical features. However, to elucidate some particular issues of the mineralogy of the weathering crust, with a comparative analysis of different types of residual products, very important data can be obtained by studying the crust on sedimentary rocks as well. The mechanism of hypergenic transformation of various hypogenic minerals is determined by their structural features.
The upper part of the Earth's crust, where hypergenic processes (soil formation, weathering, salt accumulation, geochemical activity of groundwater) take place, is referred to as a hypergenesis zone. It usually includes only natural phenomena, not including technogenesis [8]. Under specific geological conditions, hypergenic and technogenic processes proceed in the same thermodynamic environment. Studying them is of great theoretical and practical importance, since these sediments most often form the base of various constructions.
Discussion ofthe results. In this regard, of interest are the foothills of the North Tamdytau mountains, on the example of which we will consider the peculiarities of engineering-geological and seismic properties of the Eocene clays. Peculiarities of geological and tectonic development of the North Tamdytau determined the formation and development within their boundaries arid lithogenesis clays, characterized by different areas of outcrops to the surface, distribution, thickness, lith-ological properties, depth ofoccurrence. The intensity of the processes of hypergenesis is also determined by the current heat and moisture supply.
The study of changes in the material composition, engineering-geological and seismic properties of clays under the influence of modern hypergenesis made it possible to identify 3 zones.
In the first - strongly changed zone, besides the soil-vegetation layer with thickness up to 0.2 m, enriched with plant remains, the top layers of weathered bentonite clays are attributed. The total thick-
ness is up to 0.2 m. The properties of deposits in this zone are closely connected with the intensity of denudation processes, solar insolation, and are determined by the grain size and mineral composition, and largely depend on the workability of outcrops.
Clays of montmorillonite-hydrosludite composition, covered with a small layer of Quaternary deposits, were under the influence of dry arid climate with deficit of atmospheric precipitation in the last geological periods [6; 7]. Therefore the processes of salt accumulation (CaSO4 up to 10 and more%; CaCO3 up to 20-25%; NaCl up to 0.7-0.8%; Na2SO4 up to 2.5%) actively proceeded here.
As a result of aggregation of clay particles, mechanical fragmentation associated with various weathering agents (sharp fluctuations in daily and annual temperatures, constant winds, root systems ofplants - saxaul, and many others). Sometimes there are plastic dikes filled with sand and fine gravel - products of weathering of rocks composing the Tamdytau mountains. This zone has comparatively low humidity (from 6 to 12%), skeleton density (to 1,38 g/cm3), low plasticity (to 28%), swellability (to 1.6 times or 3,4%), and decreased seismic wave velocity (to 920 m/s).
The second, moderately altered zone includes clays of montmorillonite-hydrosludite composition. It should be emphasized that the change ofengineering and geological properties and material composition is clearly evident along the vertical section of clays. Increase of skeleton density (up to 1.45 g/cm3), water content (up to 20%), clay fraction content (up to 65%), swellability (by 3.2 times, or 9.4%), plasticity (up to 42.8%) and longitudinal speed (up to 1600 m/s) is observed with increasing depth. This is due to an increase in compaction gravity load. However, even at these depths (up to 3.5 m) hypergenetic changes, related to aridity (salinization of CaCO3 up to 10-15%, CaSO4 up to 5-10%, Na2 SO4 - to 0.5% and less, etc.) and appearance ofdesiccation cracks on surface are reflected. Consequently, the influence ofhypergenesis processes on Eocene clays gradually weakens with depth and from about 3.5-4 m depth is weak.
General geological and lithological cross-section
Strength in m.
Engineering and seismological characteristics
(N
O
o
CO
m
CTl
o
-M
O
CTl
M
I - heavily altered zone
1 - soil layer loosened, not exceeding 0.2. m, sparse vegetation.
2 - montmorillonite clay, predominantly hydrous, strongly fractured, filled with secondary floury gypsum, jarosite, iron hydroxides. There are occasional clastic dikes filled with sand or fine gravel. Clay fraction (up to 42%), humidity (up to 12%), plasticity (up to 28%) and swellability (up to 1.2 times of initial volume) change with depth. Longitudinal wave velocity up to 920 m/sec.
II - moderately altered zone
Clay of montmorillonite-hydrosludite composition. Medium-slit, fractures filled with fine sand, «rubble» or clay laminae. The massif is fractured by large fractures. Limonification, formation of fine-crystalline gypsum and jarosite is observed along the fractures, mostly present on the vertical fractures. An increase in humidity (up to 20.8%), plasticity (up to 42.8%), swellability (3.2 times), longitudinal wave velocity (up to 1600 m/s), and electrical resistance (up to 6.8 ohm.m) can be observed
III - slightly altered zone
The clay is massive, fractured by major fractures and tectonic zones. Small weathering zones can be formed, the zones are filled with gaps, secondary newly formed minerals - jarosite, gypsus iron hydroxides, alunite, etc. can be found in the clay. Only pyrite and rare limonite occur macroscopically. They are mainly formed in the lobes of major fractures and zones. There is no definite boundary between zones, presumably from 3-3.5 to 4.0 m. This can be observed in changes in clay fraction (up to 84%), moisture content (up to 30%), plasticity (up to 50%), and swellability (up to 10.5 times of original volume). No further change in their depth is observed. The longitudinal wave velocity is up to 2000 m/s. Specific electrical resistance up to 11.0 ohm.m.
1
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Figure 1. General outline of the effect of hypergenesis on the Paleogene clays of the Central Kyzylkum. 1 - cracks; 2 - small cracks with jarosite; 3 - cracks with gypsum, 4 - cracks with powder limonite; 5 - clastic dikes; 6 - soil-vegetation layer
The third, slightly altered zone, includes clays of montmorillonite composition. In connection with depth, due to pressure of overlying formations, clays of this zone are little changed and are characterized by high values of clay fraction (to 84%), humidity (to 30%), plasticity (to 50%), swellability (to 10.5 times, or to 19.7%), P-wave speed (to 1800 m/s), and on the other hand by lower water soluble salt content and fracturing.
Conclusion. Therefore, an important role in changing properties of Eocene clays belongs to the speed and duration of tectonic movements, which is reflected in acceleration or weakening of the described processes. Comparative analysis of the above-mentioned zones shows that, along with mineralogical peculiarities of source rocks and hy-
drogeochemical conditions of the environment, the following three factors are important in formation of engineering-geological and seismic properties of Pa-leogene clays. The first one is the degree of structural ordering of primary minerals. The second is the inheritance of these properties by the newly emerging phases. The third one is the universality of the process of transformation of hypogenic and formation ofhypergenic minerals in the zones of hypergenesis, developed at different depths of clays. In further studies, the use of the three formulated above additional provisions for the objective identification of the regularities of the zonal structure of hypergenesis in the clay sequence, will be based only on the study of Paleogene clays complex optical-electron-microscopic and structural-crystallochemical studies.
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