RUS:

Крашенинников В.С., Хоменко В.П. Изучение покрывающей толщи, как один из важнейших компонентов инженерных изысканий в районах покрытого карста // Вестник МГСУ. 2011. № 5. С. 113-119.

ENG:

Krasheninnikov V.S., Khomenko V.P. The research of an overburden as one of important parts of site investigations in covered karst areas

// Bulletin of MGSU. 2011. No. 5, pp. 113-119.

Krasheninnikov Vadim

Khomenko Victor P.

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

The research of an overburden as one of important parts of site investigations in covered karst areas

The article discusses the possibility of assessing the hazard of karst sinkhole formation based on the analysis of the structure and composition of the strata of dispersed sediments that overlie soluble rocks in covered karst areas.

According to V.I. Dublyansky and G.N. Dublyanskaya [1], covered karst areas are territories where soluble rocks are covered by unconsolidated sedimentary deposits of various genesis with a thickness of more than 2 meters, usually referred to as the "overburden" ("overlying strata, overlying deposits"). These territories occupy 17.6% of the area of the Russian Federation. In the European part of Russia, and to some extent in Eastern Siberia, covered karst areas often have relatively high population densities and experience intensive construction development. Notably, major cities such as Moscow, Nizhny Novgorod, Kazan, Ufa, Samara, and significant industrial facilities like the Kama Hydroelectric Station, Kalinin Nuclear Power Plant, and the Gorky Railway are located in these areas.

Apart from purely economic aspects, engineering and geological surveys and studies conducted in covered karst areas are significant because the processes of karst formation, development, and progression have a deep-seated nature and are hidden from direct observation. Surface manifestations of covered karst, such as sinkholes, subsidence, and depressions, are also crucial for study, especially at the initial stages of site examination, as they clearly indicate the presence of deep karst processes in the area. Meanwhile, for engineering karstology, a probabilistic forecasting approach to the problem is essential, even in cases where covered karst has not yet manifested on the surface. Studying the dispersed sediments of the overlying strata, its structure, properties, and characteristics, when approached correctly, can provide necessary information about deep processes. Ignoring this information, or its insufficiency or absence, can lead to incorrect assessments and, ultimately, to negative consequences, up to catastrophic destruction of buildings and structures.

The necessity of studying the overburden during engineering surveys in covered karst areas is now evident: this is particularly mentioned in clause 6.16 of SNiP 11-02-96 [3] and more detailed in clause 5.1.7 of Part II of SP 11-106-97 [4]. However, such indications present in various regulatory and methodological documents require additional clarifications.

First, it should be noted that geophysical methods of investigating the overlying strata in covered karst areas, which appear very effective at first glance, have serious limitations in terms of solving the posed task. Firstly, they are based on the principles of studying wave propagation and reflection (seismic, electromagnetic, etc.), and the data obtained with their help are indirect, not direct. Secondly, due to the integral nature of geophysical survey results, the genesis of anomalies detected by them is challenging to establish. This applies not least to underground karst manifestations [10]. In general, the interpretation of geophysical data significantly involves the so-called "human factor," meaning the quality of the final material entirely depends on the specialist processing it.

Speaking about the methods and technical means of studying the overlying deposits in covered karst areas, which allow obtaining direct information about it (drilling, static and dynamic probing, determining soil properties from samples taken), their use does not exclude the parallel application of geophysical methods for this purpose. In principle, the sequence of actions in both cases is similar: 1) during engineering and geological surveys, corresponding work is carried out in field and laboratory conditions; 2) the information obtained is purposefully analyzed based on certain conceptual models; 3) conclusions are drawn about the presence of karst process manifestations in the overburden, characteristic of specific development stages; 4) a hazard assessment is given, usually having a prognostic nature.

Based on the experience accumulated by the authors of this article, it can be concluded that currently, there are three most promising approaches to studying the overlburden in covered karst areas, which respectively solve three different tasks related to karst hazard assessment. These approaches use direct information about the structure of the overlying strata and the properties of the dispersed sediments that compose it. Each of them is considered separately on specific examples.

Search for buried karst sinkholes

When assessing karst hazard by the criterion of the presence of karst sinkholes at the survey site, it does not matter whether they are present on the surface, filled in, or buried under younger sediments, being elements of paleorelief (paleo landscape) [2]. The closer an existing or planned building is to a buried karst sinkhole, the more dangerous the situation.

Typically, the detection and identification of buried karst sinkholes based on drilling data are relatively simple using morphostructural analysis when there are at least two subhorizontal contact marking surfaces in the overlying strata, one of which represents the base of the overlying strata. Difficulties arise in delineating detected depressions, and they are easiest to overcome using a combination of various methods included in engineering and geological surveys, including geophysical methods.

A characteristic example in this regard can be the analysis of the results of engineering and geological surveys conducted in 2009 in the vicinity of Ufa for the construction of a five-story residential building (Fig. 1), which included drilling, static probing, and ground and borehole geophysical surveys. It was hypothesized that a large buried karst sinkhole was present in the eastern part of the site, which formed during the Mesozoic continental hiatus, was filled with Cenozoic sediments, and "revived" in the late Pliocene; this phenomenon can repeat in the present. To confirm this conclusion, a special prognostic method was additionally used [11].

Fig. 1. Buried karst sinkhole detected by studying the structure of the overburden.

1 - projected building; 2 - deep borehole that uncovered a karst cavity, and its number; 3 - deep borehole that did not uncover any karst cavity, and its number; 4 - static probing point and its number; 5 - vertical electrical sounding point and its number; 6 - zone within which, according to the forecast, the center of the sinkhole will be located if it forms; 7 - presumed outline of the buried pre-Cenozoic karst sinkhole;

8 - presumed outline of the pre-Quaternary "revival" of the buried pre-Cenozoic karst sinkhole.

Searching for weakened zones in the overburden

Clause 5.2.8 of Part II of SP 11-106-97 [4] mandates the identification and delineation of weakened and loosened zones within the overlying sediments during engineering and geological surveys, regardless of conditions, not just in covered karst areas. While the specific "karst" nature of these operations is not mentioned, it is essential. The formation of an underground karst cavity (or any other type of cavity) leads to a release of normal stresses in the overlying soils. This occurs not only along the cavity contour but also at the ground surface, which can be easily detected using static or dynamic probing through a corresponding decrease in soil resistance to the penetration of a probe cone [8].

Therefore, a special analysis of the results of these types of field soil studies becomes highly significant when investigating the overburden in covered karst areas. In principle, it allows for determining the location of karst cavities even when they are not exposed by boreholes. This involves not only primary dissolution cavities arising in karst-forming rocks but also secondary suffusion cavities or collapse cavities genetically related to the karst process, forming within the overlying strata.

The effectiveness of this approach to studying the overlying strata can be illustrated by the following example (Fig. 2). In 1992, static probing was conducted at a site of an industrial enterprise in Dzerzhinsk, Nizhny Novgorod Region, in alluvial Quaternary sandy soils forming part of the dispersed sediments overlaying karstified Permian carbonate and sulfate rocks. Analysis of the spatial distribution of soil resistance under the probe cone revealed a weakened zone, indicating the potential presence of a collapse cavity in the sandy soils comprising the aeration zone. Drilling within this zone subsequently uncovered the cavity as a sudden drop of the drilling tool between depths of 8.8-10.0 meters.

Searching for loosened zones in the overburden

Returning to clause 5.2.8 of Part II of SP 11-106-97 [4], which mentions both weakened and loosened zones, it is necessary to explain what this may imply regarding dispersed sediments. If such loosening is not purely anthropogenic, it may primarily be associated with the development of suffosion processes [7]. If it concerns chemical suffosion, there is no sense in linking the loosening of saline dispersed sediments with the karst process; this situation is addressed separately in clause 6.12 of SNiP 11-02-96 [3]. However, if it refers to a type of mechanical suffosion, such as the free removal of fine particles from the pores of unconsolidated sediments (partial filtration destruction), this phenomenon can indeed be observed in the overburden at the initial stage of a very dangerous paragenetic combination known as karst-suffosion processes.

Special experimental studies have shown [6] that the presence of certain "washed out" loosened zones in unconsolidated sediments with rubble texture, composing the overlying strata and contacting with fractures and cavities in the underlying soluble rocks, can indicate the onset of karst-suffosion sinkhole formation. These zones are composed of so-called non-suffosion unconsolidated sediments. They can be identified through special processing of data on the granulometric composition of soils according to criteria proposed by V.N. Kondratyev [5].

Fig. 2. Weakened zone and collapse cavity discovered in the overlying strata as a result of static probing data analysis (A - plan; B - cross-section).

1. Isoline of soil resistance under the probe cone, MPa. 2. Static probing point performed in 1992 and its number on the plan (a) and on the cross-section (b). 3. Borehole drilled in 1993, revealing a cavity in the sands of the aeration zone. 4. Presumed contours of the cavity on the plan (a) and the cross-section (b).

Figure 3 illustrates a typical outcome of such treatment concerning unconsolidated soils forming the overburden at the site of an already constructed office-commercial center in Moscow. Engineering-geological surveys conducted here from 2003 to 2005 indicated that the building is situated in an area potentially susceptible to karst-suffosion processes [9]. The presence of unconsolidated zones composed of "non-suffosion" unconnected rocks and areas where Quaternary overburden contacts with underlying disrupted and fractured Upper Carboniferous carbonate rocks confirms this conclusion.

Fig. 3. Assessment of "suffosion" of sandy rocks in a sample taken from the bottom of the construction pit, the results of which indicate the presence of a loosened zone in the overburden, presumably caused by the development of karst-suffosion processes.

References

1. Dublyansky V.N., Dublyanskaya G.N. Karstology. Part 2. Regional karstology. Perm, Perm State University, 2008.

2. Savarenskiy I.A., Mironov N.A. Guidelines on engineering-geological surveys in karst areas. Moscow, PNIIS Minstroya Rossii, 1995.

3. SNiP 11-02-96. Engineering surveys for construction. Basic provisions. Moscow, Ministry of Construction of Russia, 1997.

4. SP 11-105-97. Engineering-geological surveys for construction. Part II. Rules for conducting work in areas prone to hazardous geological and engineering-geological processes. Moscow, State Construction of Russia, 2000.

5. Kondratyev V.N. Filtration and mechanical suffosion in unconsolidated soils. Simferopol, Krymizdat, 195.

6. Khomenko V.P. Suffosion properties of water-saturated sands near karst-suffosion sinkhole zones // Engineering surveys in construction. Abstract collection. Series II. 1976. Issue 11(52), pp. 15-22.

7. Khomenko V.P. Patterns and forecast of suffosion processes. Moscow, GEOS, 2003.

8. Khomenko V.P., Kolomenskiy E.N. Influence of underground voids on the condition of overlying dispersed rocks // Industrial and civil construction. 2000. No. 8, pp. 39-41.

9. Khositashvili G.R., Khomenko V.P., Zemskoy G.V., Vorobyev E.A. Experience of engineering-geological investigations of foundations of critical structures // Industrial and civil construction. 2005. No. 6, pp. 18-21.

10. Chervinskaya O.P., Avezova K.R., Khomenko V.P. Karstological interpretation of results of electrical exploration at the site of a planned industrial enterprise // Industrial and civil construction. 2009. No. 11, pp. 16-17.

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