RUS:
Крашенинников В.С. Локальная оценка карстоопасности: краткое описание концепции существования признаков - предвестников провалообразования // Инженерные изыскания в строительстве.

Материалы первой научно-практической конференции молодых специалистов. ООО «ИГИИС» (Институт геотехники и инженерных изысканий в строительстве). Россия, Москва. 2017. С. 132-138.

ENG:
Krasheninnikov V.S. Local assessment of karst hazard: a brief description of the concept of predictive indicators of sinkhole formation// Engineering Surveys in Construction. Proceedings of the First Scientific and Practical Conference of Young Specialists. OOO "IGIIS" (Institute of Geotechnics and Engineering Surveys in Construction). Russia, Moscow. 2017. Pp. 132-138.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Local assessment of karst hazard: a brief description of the concept of predictive indicators of sinkhole formation

Intro

In covered karst conditions [3], the main problem in assessing the karst hazard of an area is the fact that karstified rocks are hidden from direct observation.

Currently, the regulatory and instructional documents in force in the Russian Federation contain a list of works that must be performed in areas of karst development. However, these works are general in nature and do not reflect the specifics of karst. Moreover, there are no methodological or other guidelines on how to use the geological information obtained through these works.

Nevertheless, a methodology for assessing karst hazard within a local survey site exists [5]. It uses standard engineering-geological survey tools. The methodology is based on the concept of the existence of early indicators of possible sinkhole formation in covered karst conditions, which states that just before a sinkhole forms, characteristic changes occur in the structure, properties, and composition of the overburden soils [1, 7, 8, 10]. Several predictive indicators of possible sinkhole formation exist: a) presence of buried surface karst manifestations (sinkholes, subsidence); b) changes in the granulometric composition of non-cohesive soils caused by suffosional removal of soil particles into karst cavities; c) weakening of cohesive rocks above the karst cavity.

Search for Buried Surface Karst Manifestations (Paleogeomorphological Analysis)

The search for buried karst formations is carried out by drilling boreholes and, in some cases, using static or dynamic probing. To analyze buried layers, at least two sub-horizontal surfaces (so-called marker horizons) are required, one of which is the roof of the karstified rocks (also serving as the base of the overburden), and the other is the roof of one of the overlying layers within the overburden [4, 9]. It is assumed that deformations experienced by the roof of the karstified rocks are inherited by the overlying layers. The marker layer may undergo various types of deformation – plastic (soil subsidence) or shear (soil collapse) (Figure 1). In both cases, the strength properties or even the integrity of the aquitard layer deteriorate sharply. The danger of buried karst manifestations lies in the fact that ancient karst sinkholes may "reactivate" over time and appear on the surface as a sinkhole or subsidence.

Figure 1. Forms of Marker Surfaces of Buried Karst Deformations.

A – Marker horizon subjected to plastic deformations (soil subsidence);

B – Marker horizon subjected to shear deformations (soil collapse);

C – Deformation of the marker horizon due to karst-suffosion processes.

1. Unconsolidated sandy deposits;

2. Cohesive clayey soils (marker horizon);

3. Cavity in karstified rocks;

4. Borehole.

Search for Areas with Suffosional Changes in the Granulometric Composition of Non-Cohesive Overburden Soils

A suffosional soil is considered one that has the so-called rubble structure, i.e., a soil in which fine filler particles can move between large immobile skeleton particles under the influence of a filtration water flow. Near the entrance to a karst cavity, within the sandy soil mass, three zones can be conventionally distinguished by granulometric composition (Figure 2). The first zone is the original, suffosion-unaffected soil. The second zone is the soil that has undergone suffosional changes and has been partially depleted of filler. The third zone is the soil that has completely lost filler and has ceased to be suffosional. The detection of zones with signs of suffosional changes in non-cohesive soil is carried out using special analysis of sand samples (construction of differential and integral granulometric curves, determination of suffosional properties), taken from boreholes. The author conducted an experiment aimed at simulating the suffosional removal of fine particles of non-cohesive soil by a downward filtration flow into a receiving cavity and studying how soil parameters in the mass change relative to their initial state.

The experiment began with filling the side chambers with water, which then entered the working chamber through mesh filter partitions, saturating the model material (Figure 3). The chambers were filled for 5 minutes to a level of 15 cm, then the level was maintained at a given mark for 3 minutes. During this time, water flowed through a slit from the working chamber to the lower one, carrying soil particles with it.

Figure 2. Schematic Zoning of the Sandy Mass Modified by Karst-Suffosion Processes Based on Granulometric Composition.

a) Fractured karstified rocks;

b) Sandy soils;

c) Karst cavity;

d) Groundwater level;

e) Confined karst water level;

f) Direction of water flow.

Figure 3. Apparatus for Studying the Suffosional Properties of Soils (Design by V.P. Khomenko, E.P. Makhanko, E.I. Isaev).

1. Main chamber (32 × 20 × 17 cm), partially filled with soil;

2. Side chambers filled with water;

3. Lower chamber;

4. Water-permeable partitions separating the side chambers from the main chamber;

5. Bottom of the lower chamber with a slit;

6. 5 mm-wide slit simulating a karst conduit;

7. Overflow tube for gradient pressure regulation;

8. Tubes connecting the side chambers;

9. Drainage tubes of the lower chamber.

After 3 minutes, the water supply was stopped, and free drainage occurred through the slit until all chambers were emptied. After conducting three cycles of "filling – level maintenance – drainage," samples were taken from the model material for granulometric composition analysis. For each sample, two parameters were determined: the percentage of filler content and the degree of heterogeneity.

The results of the experiment were used to analyze the spatial distribution in the vertical plane of both parameters. The filler content was found to be concentrated in the center of the working chamber and reduced in the area of the slit simulating the entrance to the karst cavity (Figure 4).

Figure 4. Spatial Distribution of Filler Content in the Soil Mass After the Experiment.

Figure 5. Spatial Distribution of Soil Heterogeneity Index (Cu) After the Experiment.

For the soil heterogeneity index (Cu), the pattern was more pronounced (Figure 5). It showed an almost linear dependence of this parameter on the distance between the measurement point and the entrance to the receiving cavity. Thus, in the absence of a confining aquitard (or if its integrity is compromised), a certain spatial distribution pattern of filler content in suffosional soil or the heterogeneity of its granulometric composition can serve as a predictive indicator of karst-suffosion sinkhole formation.

Search for Weakening Zones in Overburden Soils Above Karst Cavities

The presence of a karst cavity near the base of the overburden can cause its collapse and the formation of a sinkhole on the surface. This is preceded by changes [2, 7], expressed in the redistribution of the stress state of the overburden soils, which can be easily detected using static or dynamic probing due to the characteristic decrease in soil resistance to the cone of the probe.

To confirm the phenomenon, the following experiment was conducted [6]. The working chamber was filled with model material simulating the overlying strata, and its bottom was equipped with double sliding shutters, creating a gap of the required width (Figure 6). A rectangular block with a width of 3.6 cm was placed in the gap, and its fall simulated the interaction of the karst cavity roof with the base of the overlying strata. The objective of the experiment was to compare the spatial distribution patterns of the resistance of the model material to the penetration of the microprobe cone before and after simulating the collapse of the karst cavity roof.

Figure 6. Installation design a) and diagram of probe point locations b). Installation: 1 - main chamber filled with model soil; 2 - side chambers;

3 - microprobe; 4 - conductor; 5 - permeable partitions; 6 - lower chamber;

7 - lower shutters; 8 - upper shutters; 9 - falling block; 10 - screws adjusting the shutter opening. Microprobe: 11 - hammer; 12 - hammer stroke limiters; 13 - working part of the probe, marked every 2.5 cm.

Points No. 1-5 were performed before modeling the collapse of the cavity roof, points No. 6-12 - after the collapse.

Figure 7. Stress distribution in the soil layer before simulating the cavity roof collapse (left) and after (right), conventionally expressed by the number of blows required for the probe penetration (at the top of the figure – probing graphs, at the bottom – contour lines of probing indicators).

Based on the obtained data, probing graphs and a vertical cross-section along the test points were constructed (Figure 7). A weakened zone of soil above the formed cavity was identified, which was clearly detected by the pattern of a decrease in the number of blows during the probe penetration. At the same time, the "shelf" of the base of the model material remained intact and did not collapse into the lower chamber during dynamic probing. Thus, through a special analysis of the results of static or dynamic probing, it is possible to determine a limited area on the earth's surface within which a future karst sinkhole will occur.

Conclusions

This article discusses some features of the overlying strata structure as indicators of the preparation for karst sinkhole formation even before its occurrence, briefly outlines methods for their detection, and provides experimental confirmation. These features form the basis for a methodology to assess local karst hazard, which also includes the construction of a predictive synthetic map that considers the presence of all three indicators.

References

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