RUS:
Крашенинников В.С. Оценка опасности карстового провалообразования на одном из участков Павшинской поймы в Московской области // Строительство-формирование среды жизнедеятельности. Сборник трудов 15-й Межвузовской конференции молодых специалистов. МГСУ. Москва. 2012. С. 627-630.

ENG:
Krasheninnikov V.S. Assessment of the hazard of karst sinkhole formation in one of the sections of Pavshinskaya Floodplain in the Moscow region// Construction - Formation of the Living Environment. Proceedings of the 15th Interuniversity Conference of Young Specialists. Moscow State University of Civil Engineering (MGSU), Moscow, 2012. pp. 627-630.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Assessment of the hazard of karst sinkhole formation in one of the sections of Pavshinskaya Floodplain in the Moscow region

As is known, hazardous geological processes, including karst processes, pose significant challenges both during the economic development of territories and during the operation of structures. Karst processes are particularly dangerous for settlements and linear structures in the European part of Russia, as this region is densely developed and populated.

Moscow and the nearby Moscow region are no exception in this regard. According to the "Map of Karst and Karst-Suffosion Hazard in the City of Moscow" (Kutepov V.M. et al., 1996) [2], approximately 15% of the city's territory is assessed as highly hazardous regarding the potential occurrence of karst. The criterion for assessing karst hazard includes the degree of disintegration, fracturing, and karstification of soluble rocks. In Moscow, the so-called covered karst is prevalent—karst-prone rocks, represented by limestones and dolomites, are overlain by a thickness of terrigenous deposits. Covered karst is particularly dangerous because, being at depth, it is hidden from direct observation and can manifest suddenly, leading to catastrophic consequences. This complexity in predicting karst phenomena arises from the fact that these processes are not easily observed.

The mandatory condition for the development of karst processes is the presence of groundwater flow. The most hazardous areas in this respect are located in the western, northwestern, central, and northeastern parts of Moscow.

Currently, predicting the development of karst processes involves calculating the number of surface karst manifestations (sinkholes, subsidence, pits) per year per unit area. There is also a relatively formal approach to the problem during engineering surveys. Geophysical surveys and drilling "for karst" provide only qualitative assessments: whether there are signs of karst or not. However, geophysical methods: 1) provide only an indirect view of what is happening within the karst-prone rocks since these methods are based on the propagation and reflection of waves of various origins; 2) the quality of the investigation depends on the expertise of the specialist interpreting the geophysical data. Drilling, when it reaches the karst-prone rock layer, can reveal fractures and cavities in the core brought to the surface, but there is also the risk of missing a large underground cavity, leading to an inaccurate assessment of the situation. Thus, both methods do not provide reliable information about the potential development of karst.

The current pace of construction and urban density require a different approach to predicting karst development. Designers need information not only about the possibility of sinkholes on the construction site but also about the potential locations of their formation.

As an example of karst hazard assessment, the author has selected a site adjacent to Moscow in the Pavshinskaya Floodplain area. The material for analysis was provided by OAO "MosTsTISIZ," the organization that conducted engineering and geological surveys on this site in 2011.

Fig. 1a. Map of the thickness of Middle Jurassic clays (J2pr)

Fig. 1b. Map of the average values of soil resistance to cone penetration (Q) in the Middle Jurassic deposits.

The geological structure of the site is characterized by the shallow occurrence (9.0 - 13.0 m) of the Dorogomilovskaya formation deposits of the Upper Carboniferous (C3dr)—water-saturated limestones and dolomites of varying degrees of weathering and disintegration, overlain by thin (less than 3.5 m) Middle Jurassic clays of the Pronsk suite (J2pr). In the northern part of the site, around the static probing points (SP) 17 and 19, the impermeable layer is completely eroded (Fig. 1a). The overlying Quaternary deposits are represented by sands, mostly water-saturated, and loams at the top. The site is ubiquitously covered by fill (anthropogenic) soils.

Regulatory documents governing engineering surveys in karst-prone areas [3] stipulate the necessity of studying the overburden using static and dynamic probing to identify and delineate weakened and loosened zones. However, there is no specific explanation of the karst specificity, as this type of field testing should be conducted in all engineering-geological surveys. It is known that above karst cavities, areas of altered stress state in the soil form [4], preceding the formation of subsidence or karst sinkholes on the surface. These areas are well identified using static or dynamic probing.

To assess karst hazards, the author has created maps of the average soil resistance (Q) to the penetration of the probe cone, reflecting the distribution of stress fields [1] in the soil mass. Notably, the material provided by the surveyors allowed the construction of maps for both the Quaternary sand layer (Fig. 2) and the Middle Jurassic clays (Fig. 1b).

Several areas with abnormally low soil resistance (Q) values have been identified on the investigated site: in the northern part, around static probing points SP17, 19, 20, and 24, and in the central part—SP9, 10, and 28.

Fig. 2. Map of the average values of soil resistance to cone penetration (Q) in the Quaternary deposits.

Despite the apparent similarity, the weakened areas have different origins. For instance, in the vicinity of SP17, 19, and 20, the impermeable layer separating the karst-prone rocks from the overlying sands is absent, creating a direct hydrodynamic connection between them. As a result, suffosional removal of silty soil particles from the overburden layer occurs.

Fig. 3. Distribution of stress fields in the vertical plane.

In the central part of the site, a clay layer is present, albeit with a small thickness of around 2.0 m. Nevertheless, significant soil loosening is observed around static probing points SP9 and 10, which is also evident in the vertical section (Fig. 3) passing through a series of static probing points (Fig. 2). The section reflects the distribution of stress fields in the vertical plane, expressed through the average values of soil resistance to cone penetration (Q). The soil loosening is likely related to the subsidence of the impermeable layer at this location (Fig. 4a), caused by the formation of a cavity in the carbonate layer's roof. A similar situation is observed around SP24 and 28. At the latter point, this assumption is confirmed by the reduced stress in the Jurassic clays, as revealed by probing. It should be noted that, in current engineering-geological survey practices, probing is mainly conducted on Quaternary soils and rarely on Mesozoic deposits.

Particular attention should be paid to SP13. As seen in Figures 4a and 4b, a small depression is clearly visible at this location. This is an ancient karst sinkhole that formed in the early Cenozoic era. It is well-known that ancient karst can also pose a danger by reactivating in the present day.

Fig. 4a. Maps of the roof of Middle Jurassic deposits (J2pr)

Fig. 4b. Maps of the roof of Upper Carboniferous deposits (C3dr).

From the above, it can be concluded that soil testing using both static and dynamic probing, combined with paleorelief analysis, constitutes an effective tool for the predictive assessment of an area with regard to the manifestation of karst processes.

References

1. Bondarik, G.K. Fundamentals of the Theory of Variability of Engineering-Geological Properties of Rock Masses. Moscow: Nedra, 1971.

2. Moscow: Geology and the City / Ed. V.I. Osipov, O.P. Medvedev. Moscow: AO "Moscow Textbooks and Cartolithography", 1997.

3. SP 11-105-97. Engineering-Geological Surveys for Construction. Part II. Rules for Conducting Work in Areas of Hazardous Geological and Engineering-Geological Processes. Moscow: Gosstroy of Russia, 2000.

4. Khomenko, V.P. Patterns and Forecast of Suffosion Processes. Moscow: GEOS, 2003.

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