RUS:
Крашенинников В.С., Хоменко В.П. Покрытый карст: необходимые условия, причины и признаки подготовки провалообразования // Промышленное и гражданское строительство. 2013. № 11. С. 6-8.

ENG:
Krasneninnikov V.S., Khomenko V.P. Collapse sinks formation due to covered karst: conditions, reasons and signs of preparation

// Industrial and Civil Construction. 2013. No. 11. pp. 6-8.

Krasheninnikov Vadim

Khomenko Victor P.

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Collapse sinks formation due to covered karst: conditions, reasons and signs of preparation

The proposed article examines the main genetic types of sinkholes, whose occurrence is influenced by the development of covered karst and anthropogenic factors, and provides a brief description of the early indicators of their formation. The possibility of detecting these indicators during engineering surveys is also assessed

In Russia, all industrial, civil and transportation construction sites that have experienced negative impacts from karst processes are located in regions of covered karst, where soluble karst-forming rocks are overlain by sedimentary unconsolidated deposits more than 2 meters thick [1], forming what is known as the cover layer. In such conditions, five genetic types of karst sinkholes develop (Figs. 1-3). It should be noted that the causes of these sinkholes include dynamic impacts on soils and their open excavation, which are characteristic of construction activities.

Karst-collapse sinkholes (Fig. 1) form where impermeable cohesive soils overlie soluble rocks. However, for "simple" sinkholes [2] to occur, the roof of these soils must reach the surface (Fig. 1a), while for "complex" sinkholes [3], they must be covered by water-saturated sands (Fig. 1b). In both cases, karst waters may have a confined nature, and their level can occupy any vertical position. The formation of karst-collapse sinkholes of both types is initiated by the expansion of cavities in the soluble rocks and the drainage of water from them, but "complex" sinkholes also result from the inflow of water into the cover layer.

Fig. 1. Simple (a) and complex (b) types of karst-collapse sinkholes.
1 - soluble soils; 2 - impermeable cohesive soils; 3 - unconsolidated soils;

4 - karst water level; 5 - epikarst water level

The necessary condition for the formation of karst-suffosion-collapse sinkholes is the presence of water-saturated unconsolidated soils in the cover layer [4]. These soils may either be in contact with the soluble rocks, as shown in Fig. 2, or separated from them by a layer of impermeable cohesive soils. In the latter case, the sinkhole can only form if there are through-going fractures or breaches in the integrity of this layer, and the karst waters must be under pressure, with the piezometric surface above the base of the water-saturated unconsolidated soils. The upper part of the water-saturated unconsolidated soils may be overlain by cohesive or unsaturated unconsolidated soils (Fig. 2). The causes of karst-suffosion-collapse sinkhole formation are often the expansion of cavities in soluble rocks and the inflow of water into them.

Fig. 2. Karst-suffosion sinkhole.

1 - 5 are shown in Fig. 1; 6 - cohesive or non-saturated unconsolidated soils; 7 - level of karst and epikarst waters

To form karst-suffosion sinkholes (Fig. 3), the presence of unconsolidated soils in the cover layer is required [4]. If these soils are water-saturated, phreatic-type sinkholes form (Fig. 3a). This occurs even where water-saturated unconsolidated soils are separated from the soluble rocks by a layer of impermeable cohesive soils with through-going fractures or where such a layer is absent. For phreatic-type karst-suffosion sinkhole formation to occur, the karst water level must drop below the level of the overlying water, due to water drainage from the soluble rocks and/or its inflow into the cover layer. If the unconsolidated soils are unsaturated, then upon water inflow, vadose-type karst-suffosion sinkholes may develop (Fig. 3b).

Fig. 3. Phreatic and vadose types of karst-suffosion sinkholes.

1 - 6 are shown in Fig. 1, 2

Several easily detectable early indicators of karst sinkhole formation related to the considered genetic types exist (Fig. 4). These indicators are closely associated with the development processes in the cover layer during the formation of karst sinkholes, meaning some may only be detected under certain conditions (Table 1). All karst sinkhole formation indicators can be divided into two groups: the first, which includes earlier indicators, is associated with the destructive effects of water on rocks, and the second with gravitational forces.

Fig. 4. Indicators of sinkhole formation due to the development of covered karst.

1 - 7 are shown in Figs. 1, 2; 8 - Unconsolidated soils;

9 - Unsealed cavity in soluble rocks; 10 - Buried sinkhole; 11 - Zone of suffosion-induced loosening of unconsolidated soils; 12 - Zone of weakened rocks in the overlying strata above the cavity; 13 - Cavity in the overlying strata; 14 - Surface subsidence

The earliest indicator in the first group is the presence of unsealed cavities in the karst-forming rocks resulting from their dissolution. This indicator signifies the initiation of karst sinkhole formation of any genetic type, not only in regions of covered karst. The discovery of buried surface manifestations of covered karst (subsidence and sinkholes) should be equally alarming. In cases where an impermeable cohesive layer lies between the soluble rocks and the water-saturated unconsolidated soils, this indicator points to a high probability of the presence of through-going fractures or breaches in the integrity of the impermeable layer at the periphery of the buried subsidence or sinkhole. These fractures create necessary conditions for the development of suffosion, although they cannot be detected due to their small horizontal dimensions. Finally, the third indicator in the first group is the presence of loosened zones in the unconsolidated soils of the cover layer caused by suffosion processes, involving the removal of fine particles from the pores. This process represents the early stage of suffosion development, but only in unconsolidated soils with a rubble texture, limiting the potential for detecting this indicator.

Table 1. The fundamental possibility (+) or impossibility (-) of detecting Indicators of sinkhole formation

The indicators in the second group appear later. The first of these is related to changes in the stress state of the cover layer above the cavity in the soluble rocks, leading to the formation of a weakened zone in the dispersive soils, which, under favorable conditions, can be relatively easily detected. A later indicator is the presence of a cavity in the cover layer, which will appear if the initial stage of collapse in dispersive soils is limited to internal caving. If cohesive soils are present in the cover layer, their sagging over the cavity or weakened soils may lead to surface subsidence. This phenomenon often immediately precedes the formation of a sinkhole.

All these indicators can be detected using geophysical methods, but this often leads to interpretation challenges [5]. Other engineering geological survey and research methods allow for more definitive results, though not all ensure the detection of the sought indicators with 100% certainty (Table 2). Moreover, accurate identification of buried surface manifestations of covered karst, suffosion-loosened zones in unconsolidated soils, and weakened zones in the cover layer above cavities require specific analysis of data obtained during the survey work [6]. Ultimately, all this significantly increases the reliability of karst hazard assessments for industrial, civil, and transportation construction sites.

Table 2. Technical capabilities for detecting indicators of karst sinkhole formation during engineering surveys

References

1. Dublyansky, V.N., Dublyanskaya, G.N. Karst Studies. Part 1. General Karst Studies. Perm: Perm University Press, 2004. 308 p.

2. Khomenko, V.P. Karst-collapse Sinkholes of the "Simple" Type: Field Studies. Engineering Geology. 2009, No. 4, pp. 40–48.

3. Khomenko, V.P. Karst-collapse Sinkholes of the "Complex" Type: Physical Modeling. Engineering Geology. 2009, No. 3, pp. 34–41.

4. Khomenko, V.P. Patterns and Forecast of Suffosion Processes. Moscow: GEOS, 2003. 216 p.

5. Chervinskaya, O.P., Avezova, K.R., Khomenko, V.P. Karstological Interpretation of Electrical Survey Results at the Site of a Projected Industrial Enterprise. Industrial and Civil Construction. 2009, No. 11, pp. 16–17.

6. Krasheninnikov, V.S., Khomenko, V.P. Study of the Cover Layer as One of the Most Important Components of Engineering Surveys in Covered Karst Areas. Bulletin of MGSU. 2011, No. 5, pp. 113–119.

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