RUS:
Крашенинников В.С. Формирование ослабленных зон над карстовой полостью (экспериментальное подтверждение) // Промышленное и гражданское строительство. 2015. № 1. С. 42-45.

ENG:
Krasheninnikov V.S. Formation of weakened zones over the karst cavity (experimental confirmation) // Promyshlennoe i grazhdanskoe stroitel'stvo [Industial and Civil Engineering]. 2015, no. 1. Pp. 42-45.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Formation of weakened zones over the karst cavity (experimental confirmation)

Abstract
In the overburden soil above the karst cavities, some areas, characterized by abnormally low values of soil strength in comparison with the surrounding array, so-called «weakened» zones, come into being. In the article, devoted to the actual theme of ensuring the safety of buildings and structures constructed in karst-dangerous areas, the possibility to forecast the sinkhole formation under conditions of the covered karst is considered. In this article, the author gives a description of an experiment performed by physical modeling of the «karst cavity – overburden soil» system, followed by an imitation of the collapse of the cavity roof and the formation of weakened zone over it. Detection of these zones is possible with the help of static (CPT) or dynamic probing of soils. For the first time the author presents the experimental confirmation that above the karst cavities in overburden soils, the areas with abnormally low strength characteristics come into being that makes it possible to detect the dangerous areas before the beginning of the sinkhole development.

Key words: karst, overburden soil, karst cavity, weakened zone, softening of soil, stress-strain state of soil, dynamic probing, forecast of soil collapse.

In conditions of covered karst, so-called weakened zones form above a cavity. These zones appear long before the formation of an actual sinkhole. Weakened zones are characterized by abnormally low soil strength compared to "background" values. The existence of such zones has been known in engineering survey practice for quite some time [1, 2]. Their formation is due to a release of normal stress above the cavity [3], which eventually leads to collapse. Weakened zones are easily detectable through static (CPT) or dynamic sounding [4]. However, in situ, confirmation of the karst origin of these “weakened” zones has not yet been possible, as the formation of a karst cavity and the weakened zone above it occurs at significant depths and is hidden from direct observation. Nevertheless, studying this phenomenon is possible through physical modeling.

The purpose of the experiment was to confirm this phenomenon, specifically that the formation of a weakened zone in the overburden occurs above the karst cavity long before the collapse begins. To achieve this, conditions simulating the process in covered karst—collapse of the karst cavity—were created, and changes in the overburden soils following the collapse were recorded.

The goal of the experiment was to measure the stress state of the soil before and after simulating the collapse of the cavity roof. Indirectly, the stress state of the soil mass, according to G. K. Bondarik's theory of geological field variability [5], can be expressed through the soil's resistance to cone penetration during dynamic sounding. Soil resistance is determined by the number of hammer blows needed to drive the probe to each specified depth interval. Determining the absolute strength of the soil is irrelevant in this case, as the experiment is qualitative and does not require quantitative indicators.

The experiment used a setup designed by V. P. Khomenko to study the suffusion stability of soils (see Fig. 1). This setup also allows for solving other tasks, such as modeling the karst-collapse process.

Fig.1. Experimental equipment
a) Setup: 1- main chamber filled with soil; 2 - side chambers; 3 - microprobe; 4 - conductor; 5 - permeable partitions; 6 - lower chamber; 7 - lower gates; 8 - upper gates; 9 - falling block; 10 - screws adjusting gate opening

b) Microprobe: 11 - hammer; 12 - hammer stroke limiters; 13 - probe’s working section, marked at 2.5 cm intervals

The setup consists of a main rectangular chamber measuring 20×32×17 cm, filled with a material simulating the overburden soils, which in this case is medium-grained moist sand. The initial physical properties of the sand were determined using the ring shear method and are as follows:

- Soil density: 1.9 g/cm³

- Soil particle density: 2.65 g/cm³

- Soil skeleton density: 1.63 g/cm³

- Natural moisture content: 16.4%

- Degree of saturation: 0.7

- Porosity coefficient: 0.62

The main chamber is separated from the lower chamber by sliding gates. A rectangular block 36 mm wide is placed between the gates, which freely falls into the lower chamber when the gates are opened.

Dynamic micro-sounding was used to determine the relative soil strength. The probe (see Fig. 1b) is a metal rod 6 mm in diameter and 50 cm long, with a cone-shaped tip at a 60° angle. The probe’s working section is marked at 2.5 cm intervals. The upper part of the probe is fitted with a cylindrical hammer weighing 100 g. The hammer’s drop height (3 cm) is limited by adjustable stops. Relative soil strength is expressed by the number of hammer blows required to drive the probe cone for each depth increment. The hammer drop height was selected to deliver a relatively small impulse, allowing the detection of even minor changes in soil strength.

The experiment began with layered placement of soil in the main chamber. To ensure cohesion, the soil was pre-moistened and lightly compacted during placement.

To establish initial indicators of soil strength, the first set of probing points was created (see Fig. 2). Points 1-5 were located at a distance from the chamber walls to minimize wall influence on probing results.

Fig. 2. Layout of micro-sounding points. 1-5 – probing points performed before modeling the collapse of the karst cavity roof; 6-12 – after modeling the collapse.

During sounding, the number of hammer blows was recorded for each 2.5 cm increment of probe penetration. The collected data was used to create graphs of soil resistance to the probe cone (determined by hammer blows) and a digital 2D model of stress distribution fields in the soil, indirectly represented through sounding indicators (see Fig. 3). As shown in the graphs (see Fig. 3a, b), the number of hammer blows increases with depth, from 20-30 to 130-150.

Fig. 3. Stress distribution in soil mass
(a, b) before cavity roof collapse simulation; (c, d) after collapse, expressed in the number of hammer blows per depth increment

After establishing the initial soil strength parameters, a simulation of the karst cavity roof collapse was conducted. To do this, the gap between the gates separating the main and lower chambers was widened with adjusting screws to allow the block to fall freely. Notably, the sand did not fall into the lower chamber, and its structure remained unchanged. Thus, a condition was created where the karst cavity roof collapsed, but soil subsidence above the cavity had not yet occurred.

After the block fell, a 30-minute pause was observed to allow stress redistribution in the soil mass. Then, at a distance from the first row of points of probing, a second row consisting of seven probing points was created (see Fig. 2). Graphs and a digital model of stress field distribution were generated from the data (see Fig. 3c, d).

The second cycle of probing clearly identified a weakened zone above the formed cavity. The soil itself retained its original structure and did not collapse into the cavity, meaning no sinkhole formed, as required by the experiment conditions. The weakened zone is characterized by abnormally low probing values: the number of blows above the cavity ranged from 30 to 80, while “background” values at these depths were 100-180 hammer blows per depth increment.

The experiment fully agrees with the results of computer modeling of sinkhole formation at the pre-collapse stage (see Fig. 4) by E. N. Kolomensky [6]. It also fully confirms the hypothesis [7] that changes in the stress state of overburden soils above a karst cavity occur long before a sinkhole forms.

Fig. 4. Density Distribution of Dispersed Rocks Above the Cavity

(in a vertical plane), in arbitrary units at various time points "t", obtained through cellular automaton-based computer modeling

Conclusions

The experiment demonstrated that sounding can assess the potential for and location of future sinkholes at an early stage, before reaching the ground surface. The presence of weakened zones and the location of potential sinkholes can also be identified during pile driving for foundation construction. The obtained data can be used at all stages of construction in karst-affected areas, during design, and in engineering-geological surveys.

References

1. Kutepov V. М., Kozhevnikova V. N. Ustojchivost' zakarstovannyh ferritorij [The stability of the karst areas]. Moscow, Nauka Publ., 1989. 151 р. (ln Russian).

2. Savarenskij I. А., Mironov N. А. Rukovodstvo ро inzhenerno-geologicheskim izyskanijam v rajonah razvitija karsta [Guidance on geotechnical investigations on the karst ares]. Moscow, PNIIIS Рubl., 1995. 167 p. (ln Russian).

3. Anikeev А. V., Kalinin Je. V., Tarakanov S. 1. Determination of the stress state of a ground layer of karst cavity. lnzhenernaya geologija, 1991, no. 5, pp. 64-70. (ln Russian).

4. Krasheninnikov V. S. Staticheskoe zondirovanie kak odin iz instrumentov ocenki karstovoj opasnosti [The Cone Penetration Test as a tool for assessment of karst hazard]. Sbornik trudov nauchnoj konterencii «Rol' inzhenernoj geologii i izyskanij na predproektnyh jetapah stroitel'nogo osvoenija territorij». Moscow, RAEN PubI., 2012, рр. 22-23. (ln Russian).

5. Bondarik G. К. Osnovy teorii izmenchivosti inzhenerno-geologicheskih svojstv gornyh porod [Foundations of the theory of variation of geotechnical properties of rocks]. Moscow, Nedra Рubl., 1971. 272 р. (ln Russian).

6. Homenko V. Р., Kolomenskij Е. N. lnfluence of underground cavities on the state of the overlying soils. Promyshlennoe i grazhdanskoe sfroitelstvo, 2000, no. 8, pp. 39-41. (ln Russian).

7. Homenko V. Р. Zakonomernosti i prognoz suffozionnyh processov [The regularities and prediction of suffusion processes]. Moscow, GEOS Рubl., 2003. 216 р. (ln Russian).

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