RUS:
Крашенинников В.С. Методика оценки карстоопасности в пределах локального участка изысканий // Экологическая безопасность и строительство в карстовых районах. Материалы международного симпозиума. Россия, Пермь. 2015. С. 183-187.

ENG:
Krasheninnikov V.S. Methodology for assessing karst hazard within a local survey area // Environmental safety and construction in karst area. Proceedings of the International Symposium. Russia, Perm. 2015. Pp. 183-187.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Methodology for assessing karst hazard within a local survey area

Abstract
This report presents a methodology for assessing a local area (e.g., a construction site) in terms of the risk of karst sinkhole formation. Currently, karst hazard forecasting is based on: 1) the ratio of sinkholes occurring annually per unit area; 2) the presence of soluble rocks, their degree of karstification, and the thickness of the overlying aquitard. In the first case, the forecast applies to a large area or an entire region, while in the second case, it applies to a specific survey site or district. However, within a single survey site, zones with varying degrees of karst hazard can be identified. This methodology is based on analyzing the properties of the overburden and detecting early indicators of potential sinkhole formation.

Key words: karst, sinkholes, overburden, static probing, suffosion, buried sinkholes, karst hazard forecast

Intro

As an example of applying this methodology, a site located in the area of Khoroshevskoye Highway in Moscow was selected. In the 1960s–1970s, several karst-related sinkholes occurred there, including within the studied site. In 1969, two karst-suffosion sinkholes formed. By the time surveys were conducted in 1975, these sinkholes had been filled and covered with soil.

The upper part of the geological section consists of Quaternary, Jurassic, and Carboniferous deposits. The site's geological structure is as follows:

• At the surface, there are anthropogenic deposits (t-QIV), predominantly sandy with inclusions of debris, ranging in thickness from 1.2 to 5.8 m.

• Below this, extending to depths of 24.8–26.5 m, there are unclassified Middle and Upper Quaternary alluvial-fluvioglacial deposits (a, f-QII-III), composed of sands ranging from silty to gravelly.

• Under the Quaternary deposits, Upper Jurassic (J3) clays occur, with a thickness of 0.7 to 3.8 m.

• Beneath the Jurassic clays, Upper Carboniferous (C3) deposits are present, consisting of limestones and dolomites with interlayers of clay and marl. In these Carboniferous formations, karst cavities and voids up to 0.9 m in size, as well as highly fractured limestone zones ranging from 0.5 to 5.5 m, have been encountered at depths of 48.5–81.0 m.

The hydrogeological conditions of the upper part of the section are characterized by two aquifers. The first aquifer is a shallow, unconfined groundwater table, located in alluvial-fluvioglacial sands above the Upper Jurassic aquitard, at depths of 19.7–21.0 m (absolute elevations 128.75–130.57 m), with thickness varying from 4.7 to 6.6 m. The second aquifer is a confined, karst-fractured water-bearing horizon, located in carbonate rocks beneath the Jurassic clays. The piezometric levels range from 21.0 to 25.1 m (absolute elevations 125.30–129.22 m), with hydraulic head values between 3.9 and 7.5 m.

The methodology is based on the concept of early indicators of sinkhole formation. The concept includes the following provisions:

1. Presence of buried surface karst deformations (e.g., sinkholes, depressions, etc.).

2. Changes in the granulometric composition of non-cohesive overburden soils, associated with suffosional removal of soil particles into karst cavities.

3. Presence of weakened zones in the overburden soils.

Figure 1. Maps of buried layer surfaces: 1) roof of Upper Quaternary deposits;

2) roof of Upper Jurassic deposits; 3) roof of Upper Carboniferous deposits.

Identification of Buried Karst Sinkholes (Paleogeomorphological Analysis)

To conduct the analysis, at least two marker surfaces are required, one of which is the base of the overburden. A paleogeomorphological analysis of the bedrock and Upper Quaternary deposits identified two areas characterized as buried karst sinkholes (Figure 1). The first area is located in the southwestern part of the study site, while the second is situated at the northern boundary near borehole No. 9.

Depressions in the southwestern part of the site are associated with relatively recent sinkholes from 1969. The center of the depression is located near borehole No. 14 and can be traced in the roof of the Upper Carboniferous and Upper Jurassic deposits. The depth of the sinkholes is approximately 1.0–1.5 m relative to the Carboniferous surface and about 0.5 m relative to the Jurassic clay roof. No depression is observed in the Upper Quaternary deposits at borehole No. 14. However, a depression is noted further north, near boreholes No. 16 and 11. This is likely due to geological features of the overburden formation and its subsequent artificial regrading.

The depression in the Carboniferous roof near borehole No. 9 is also evident in the overlying Jurassic clay and Quaternary sands. The depression amplitudes are 0.6–0.8 m in the Carboniferous deposits, ~0.5 m in the Jurassic clays, and ~1.5 m in the Quaternary sands. Based on the surface morphology of the alluvial sands, a karst sinkhole likely existed at this location but was filled during site development, with no recorded documentation.

Thus, two areas with signs of buried karst deformations can be identified within the study site:

1. The area near boreholes No. 14, 15, and 16 in the southwestern part of the site.

2. The area near borehole No. 9 at the northern site boundary.

Identification of Areas with Signs of Suffosional Changes in Granulometric Composition of Overburden Sandy Soils

Suffosional soils exhibit a rubble structure, consisting of coarse (skeleton) and fine (filler) particles. The proportions of skeleton and filler particles in the soil, as well as its suffosion susceptibility, are determined by granulometric analysis and the construction of differential and integral curves (Kondratyev, 1958). Suffosion results in the removal of filler particles from the soil skeleton. According to research by V.P. Khomenko (Khomenko, 1976), changes in the granulometric composition of sandy soils near a karst cavity occur unevenly. The overburden can be conditionally divided into three zones based on filler content:

1. Zone 1 – Farthest from the karst cavity, consisting of soil largely unaffected by suffosion. The filler content matches background values throughout the thickness.

2. Zone 2 – Located near the karst cavity, significantly altered by suffosion. The filler content is much lower than in Zone 1.

3. Zone 3 – Immediately adjacent to the cavity, where suffosion has removed the filler completely, rendering the soil non-suffosional.

Thus, the presence of suffosion processes can be inferred from the filler content in soil samples.

For analysis, sandy soil samples were collected from the saturation zone at depths of 20.0–25.5 m. This layer consists of two distinct sublayers differing in granulometric composition:

• Layer 1 (20.0–24.0 m) – Composed of fine sands, occasionally silty.

• Layer 2 (24.0–25.5 m and deeper) – Composed of medium to coarse sands, occasionally gravelly.

Models reflecting the distribution of filler content in the collected samples were created for each layer.

• In the first layer (fine sands), filler content ranged from 0.4% to 2.6%. A distinct distribution trend was observed: minimal values were found near boreholes No. 14 and 16 (1.2% and 0.4%, respectively), as well as near boreholes No. 7 and 12 (1.0% and 1.5%, respectively). All other values (1.6–2.6%) are considered background.

• Layer 2, forming the base of the Quaternary deposits, is composed of sands ranging from medium to gravelly. The filler content in this layer varied from 0.4% to 2.8%, with minimal values (0.4% and 1.2%) recorded at boreholes No. 1 and 16.

Several areas showing signs of active suffosion were identified at boreholes No. 1, 14, 16, 7, and 12 (Figure 2).

Figure 2. Areas with signs of suffosional soil changes.

Identification of Weakened Zones in Overburden Soils

Above karst cavities, so-called "weakened zones" form in the overburden soils. These zones are characterized by reduced density or strength compared to background values. Such anomalous zones are effectively detected using static or dynamic probing, indicated by a characteristic drop in cone penetration resistance (Krasheninnikov, 2012).

To analyze the probing data, the soil thickness within the depth interval 3.8–10.8 m was selected. The upper boundary is determined by the thickness of fill soils, which are not comparable due to their heterogeneity. The lower boundary is set by the minimum probing depth reached in some test locations. Based on probing data, a distribution map of cone penetration resistance (q, MPa) was constructed for the site (Figure 3).

Figure 3. Distribution of cone penetration resistance (q, MPa) in the depth interval 3.8–10.8 m.

The analysis of static probing data at the survey site enabled the identification of weakened zones. These zones are most pronounced in the area of probing points No. 14 and 15. Lesser degrees of soil weakening and loosening are observed in the areas of probing points No. 3, 9, and 17.

Figure 4. Synthetic map of areas with karst hazard indicators.

As a result of the conducted analysis, a synthetic map (Figure 4) was created, indicating areas where karst hazard indicators were identified. The final karst hazard assessment of the survey site is presented as a forecast map (Figure 5), marking the boundaries of "hazardous" and "potentially hazardous" zones.

• If all three indicators are present in a single location, the area is classified as "hazardous".

• If two or one indicators are present, the area is classified as "potentially hazardous".

Figure 5. Forecast map of site hazard.

Conclusions

The karst hazard assessment methodology presented here is proposed for the first time for a specific survey site. It allows for the identification of zones where sinkhole formation is likely. The methodology relies exclusively on standard tools used in engineering and geological investigations and is applicable to all types of construction in covered karst areas.

References

Kondratyev V.N. Filtration and Mechanical Suffosion in Unconsolidated Soils. Simferopol: Krymizdat, 1958. 76 p.
Krasheninnikov V.S. Static Probing as a Tool for Karst Hazard Assessment // Proceedings of the Scientific Conference The Role of Engineering Geology and Surveys at the Pre-Design Stages of Territorial Development. 14th Sergeev Readings. Moscow. RAEN. 2012. pp. 22-23.
Khomenko V.P. Suffosional Properties of Water-Saturated Sands Near Karst-Suffosion Collapse Zones // Ref. Coll. Engineering Surveys in Construction. Series 2. 1976. Issue 11 (52). pp. 15-22.

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