RUS:
Крашенинников В.С. Применение палеоструктурного анализа для изучения условий

формирования карстовых провалов в районе Дзержинской ТЭЦ // Геотехнические проблемы

проектирования зданий и сооружений на карстоопасных территориях. Материалы Российской

конференции с международным участием (22-23 мая 2012 г., г. Уфа). Уфа: БашНИИстрой, 2012.

С. 194-199.

ENG:
Krasheninnikov V.S. Application of paleostructural analysis for studying the conditions of karst sinkhole formation in the area of Dzerzhinskaya Combined Heat and Power Plant (CHPP) // Geotechnical Problems of Designing Buildings and Structures in Karst-Hazardous Areas. Materials of the Russian Conference with International Participation (May 22-23, 2012, Ufa). Ufa: BashNIISstroy, 2012. pp. 194-199.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Application of paleo structural analysis for studying the conditions of karst sinkhole formation in the Dzerzhinskaya Combined Heat and Power Plant (CHPP)

In this report the author aims to examine some aspects of formation conditions of the karst area of Dzerzhinsk CHPP in Nizhny Novgorod region. Structural analysis of paleotopographies is used for this purpose. The corresponding method of prediction of a sinkhole formation is briefly illuminated

The studied area is located in the southeastern, peripheral part of the city of Dzerzhinsk. It encompasses the majority of the Dzerzhinskaya CHPP territory and some adjacent areas (see Fig. 1). The main buildings of the enterprise are concentrated in this part of the CHPP. To the west, there is currently an unused area that can be considered for potential economic development.

As is well known, the karst problem is acute in this region, so researchers dealing with this issue are tasked with predicting and assessing the possibility of karst sinkhole formation. This task concerns both built-up and prospective construction areas. According to GOST R 54257-2010 [2], the Dzerzhinskaya CHPP falls under the category of structures with a "heightened" level of responsibility. Therefore, the karst problem in this area is highly relevant and requires thorough study.

The author of the report aims to use paleostructural analysis to assess the conditions of sinkhole formation and briefly outline the methodology of such an assessment.

The territory of the Dzerzhinskaya CHPP is located on the left bank of the Oka River, a right tributary of the Volga River. Geomorphologically, the area is situated on the first floodplain terrace of the Oka River. The upper part of the geological section is composed of Quaternary and Permian deposits.

Quaternary deposits are mainly represented by alluvial (aQ3) sands, with a thickness ranging from 11.0 to 44.0 meters, and, starting from a depth of 2.0 to 6.0 meters, they are water-saturated.

Beneath them, with a stratigraphic hiatus, lie the Permian system rocks. The upper Permian is represented by the Tatarian (P2t) and Kazanian (P2kz) stages. The Tatarian stage is mainly composed of clays, with a thickness of 1.0 to 19.0 meters, and is characterized in the section as a local, relative aquitard. The Kazanian stage is composed of limestones and dolomites of varying degrees of disintegration and is locally karstified. The thickness of the stage varies from 0.0 to 3.0 meters, rarely up to 6.5 meters. The carbonate layer is water-saturated. The piezometric level is established at depths of 5.0 to 9.0 meters from the ground surface.

The Kazanian stage deposits rest on the eroded surface of the Sakmarian stage (P1s) of the lower Permian system. The upper part of the Sakmarian stage is composed of sulfate rocks, while the lower part consists of carbonate rocks. The Sakmarian stage deposits are locally fractured and karstified. Their total thickness reaches 60.0 to 90.0 meters, they are widespread and act as a regional aquitard.

The area is characterized by a complex hydrogeological situation. In the upper part of the rock mass, there are two aquifers: unconfined groundwater above the karst and confined karst interlayer water. Significant fluctuations in the level of above-karst groundwater of 3.0 to 5.0 meters have been recorded over recent decades, leading to the activation of karst processes. Additionally, the carbonate and sulfate karstified rocks present in the section are sometimes covered by a relatively thin (1.0 to 5.0 meters) aquitard. The hydrogeological conditions and geological structure of this area indicate the possibility of forming karst-suffosion-collapse (mixed) and karst-collapse sinkholes [4, 6].

According to the "Preliminary Scheme of Engineering-Geological Zoning of the Coastal Territory of Dzerzhinsk by Karst Hazard, 2011," compiled by specialists of ZAO (CJSC) "Anti-Karst Protection," Dzerzhinsk (Gantov B.A., Balashova T.A., Khaydarova D.A., and others), this area is in the zone of sulfate karst distribution. Over the past approximately half-century, more than three dozen karst sinkholes have been documented in the area of the Dzerzhinskaya CHPP, along with an even greater number of surface manifestations (funnels, subsidence), whose formation date and exact genesis cannot be determined.

For the analysis, maps of the roof and thickness of deposits of different ages were constructed using the "Surfer 9" computer program. The initial data included materials from exploratory drilling carried out over various years, starting from the mid-20th century.

Fig. 1. Map of the buried natural topography of the earth's surface

(I, II - large sinkhole features dated to the Quaternary period)

Figure 1 shows the natural topography of the earth's surface at the initial stage of land development, that is, before its mechanized leveling. Against the background of the general southward slope of the terrain towards the Oka River valley, large and small local depressions stand out. The largest depressions are located in the western part of the CHPP territory and to the northwest of its boundaries (I and II in Fig. 1). The karst nature of these depressions becomes evident when studying the roof of the underlying layers. Sinkholes can be traced in the buried natural relief from its surface down to the karst-prone rocks (Fig. 3). Thus, sinkhole formation occurred during the Quaternary period. The presence of numerous small sinkholes in the roof of the Tatarian clays indicates that a series of small sequential collapses occurred at the location of the large troughs, rather than a single large collapse. On the surface, due to denudation-accumulation processes, several sinkholes merged into one larger form.

In addition to the sinkholes that formed during the Quaternary period, earlier ones are also distinguished in the area. The most indicative example is the depression in the north of the area, which can be traced only in the Permian deposits (III in Fig. 2). Therefore, its formation time is pre-Quaternary. The aforementioned sinkholes can be considered ancient, as it is impossible to determine their exact formation date.

As mentioned earlier, several dozen collapses have occurred in recent years. In some areas, modern sinkholes coincide with ancient ones. Figure 2 clearly shows that most of these sinkholes are located along certain linear structures of sublatitudinal and submeridional orientation. These lines represent projections of deep tectonic faults onto the surface. The territory of Dzerzhinsk is located at the junction of two major structural units – the Volga-Ural anteclise and the Moscow syneclise, so faulting here is quite natural.

Fig. 2. Map of the Roof of Tatarian Stage Deposits (P2t)

(I, II - large sinkhole features dated to the Quaternary period, and III - pre-Quaternary; (— — —) lines of presumed tectonic faults)

Fig. 3. Map of the Roof of Karst-Prone Rocks of the Kazanian Stage (P2kz) (see previous figures for other designations)

Sinkholes along fault lines occur because weakened zones with increased fracturing form in the sedimentary cover above them, promoting karstification of soluble rocks and increasing the permeability of the aquitard. The permeability of the aquitard is evidenced by the fact that the occurrence of sinkholes on the surface, as seen in Fig. 4, does not necessarily correspond to areas with minimal aquitard thickness. They occur even in areas where the thickness of the Tatarian clays exceeds 10 meters.

Fig. 4. Map of the Aquitard Thickness (P2t) (see previous figures for other designations).

The mechanism of sinkhole formation in the conditions of Dzerzhinsk area [6] follows this scheme: 1) cavity expansion; 2) detachment of clay particles from the base of the aquitard; 3) subsidence of the aquitard into the cavity and gradual downward movement of alluvial sands; 4) catastrophic collapse of the aquitard with simultaneous downward movement of the overlying sands into the cavity and the upward breakthrough of pressurized karst water.

In the stage preceding the collapse, the overlying layer shows a change in the stress state of the soil above the future sinkhole site. Such weakened zones are well identified by static or dynamic probing due to the characteristic decrease in resistance to cone penetration into the soil [5]. By using special data processing of probing results, digital models are constructed that reflect the distribution of stress fields [1] in the soil, both horizontally and vertically.

Based on the above, some conclusions can be drawn:

- The area has both ancient (formed in the Quaternary and pre-Quaternary periods) and modern sinkholes.

- Some modern sinkholes spatially coincide with ancient ones.

- Several groups of sinkholes on the studied territory are associated with tectonic fault lines.

- A significant thickness of the aquitard (more than 10 meters) in areas with tectonic faults does not always prevent sinkhole formation.

- Fluctuations in the level of above-karst groundwater lead to the activation of karst sinkhole formation processes.

- Future sinkhole sites can be identified through static or dynamic probing.

References

1. Bondarik, G.K. *Fundamentals of the Theory of Variability of Engineering-Geological Properties of Rocks*. Moscow, Nedra, 1971.

2. GOST R 54257-2010. *Reliability of Building Structures and Foundations*. Moscow, Standartinform, 2011.

3. Krasheninnikov, V.S., Khomenko, V.P. *Study of the Covering Stratum as One of the Most Important Components of Engineering Surveys in Covered Karst Areas*. Bulletin of MGSU, 2011, No. 5, pp. 113-119.

4. Svarensky, I.A., Mironov, N.A. *Guidelines for Engineering-Geological Surveys in Karst Development Areas*. Moscow, PNIIIS Ministry of Construction of Russia, 1995.

5. Khomenko, V.P. *Patterns and Forecasting of Suffosion Processes*. Moscow, GEOS, 2003.

6. Khomenko, V.P. *Causes and Mechanism of Karst Sinkhole Formation in the Territory of Dzerzhinsk, Nizhny Novgorod Region*. In: *Karst Studies – XXI Century: Theoretical and Practical Significance: Materials of the International Symposium, May 25-30, 2004, Perm*. Perm: Perm State University, 2004. 384 pp.

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