RUS:

Крашенинников В.С. Статическое зондирование как один из инструментов оценки

карстовой опасности // Сергеевские чтения. Роль инженерной геологии и изысканий на

предпроектных этапах строительного освоения территорий. Выпуск 14. Материалы годичной

сессии Научного совета РАН по проблемам геоэкологии, инженерной геологии и

гидрогеологии (22-23 марта 2012 г.). М.: РУДН, 2012. С. 40-44.

ENG:

Krasheninnikov V.S. Static probing as a tool for assessing karst hazard // Sergeev Readings. The role of engineering geology and surveys at the pre-project stages of construction development of territories. Issue 14. Materials of the annual session of the Scientific Council of the Russian Academy of Sciences on problems of geoecology, engineering geology, and hydrogeology (March 22-23, 2012). Moscow: RUDN, 2012. pp. 40-44.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Static probing (CPT) as a tool for assessing karst hazard

Introduction

In conducting engineering-geological surveys, special attention should be given to hazardous geological processes that occur or may potentially develop in the study area. These processes, including karst phenomena, are addressed in Part II of SP 11-105-97 [3]. Section 5.2.7 of this document emphasizes the necessity of using, in addition to drilling and geophysical surveys, field testing of soils to "identify and delineate weakened and loosened zones within the overlying rock mass, determine soil properties, and study the relief of the roof of karst-prone rocks at depths accessible for probing." For these purposes, static probing is highly suitable and is one of the main and most widespread types of field tests in Russia.

Currently, most surveying organizations, when analyzing data from field soil tests, perform a minimal amount of manipulation. Probing is typically used to determine soil characteristics and identify zones of loosening within the profile, which are displayed on individual probing graphs and engineering-geological sections as separate engineering-geological elements (EGE). The issue is that each test point is considered in isolation. The relationship between individual test points is only traced during statistical data processing to assign EGE characteristics within the survey area. This approach significantly narrows the potential of the static probing method. Meanwhile, static (as well as dynamic) probing, in conjunction with other geological methods, enables the resolution of such an important task as assessing karst hazard, especially in the initial stages of surveys.

As an example of using static probing data for karst hazard assessment, the author examines survey materials conducted at a site in Moscow, located near the "Ulitsa 1905 Goda" metro station.

Processing and analysis of information obtained during engineering-geological surveys

The presence of weakened zones above cavities and voids has been known for quite some time and is mentioned in scientific literature [3]. Such zones are well identified using static and dynamic probing by recording the specific resistance of the soil to penetration by the probe cone under corresponding loads. As noted by V.P. Khomenko [5]: 1) at the same depth, the soil resistance to the probe cone is lower the closer the probing point is to a sinkhole; 2) with the characteristic increase in cone resistance with depth, the gradient of this increase decreases as the probing point approaches the sinkhole. The formation of weakened zones is due to the change in the stress state of the dispersive rocks above the cavity. According to G.K. Bondarik's theory [1], we are dealing with parameters of the geological stress field, which can accordingly be measured. V.P. Khomenko [5] concludes that by analyzing the structure of stress fields, indirectly expressed through the spatial variability of the soil resistance Q to the probe cone, it is possible to predict the locations of expected sinkholes. Additionally, the researcher's work provides a description of the mathematical apparatus for localizing and identifying both the entire anomalous zone and its central part [6].

In the site examined by the author, measuring approximately 50 by 70 meters, in addition to drilling and geophysical surveys, 10 static probing tests were conducted to depths ranging from 13.0 to 25.0 meters. According to drilling data, the geological profile up to 60.0 meters depth includes Upper Carboniferous, Upper Jurassic, and Quaternary deposits. The Carboniferous layer consists of limestones, dolomites, marls, and clays. The upper layer (3.0-4.0 meters) of the Carboniferous sequence comprises fractured dolomites, heavily decomposed, sometimes to rubble, flour, with layers of marls and marly clays. A karst cavity filled with groundwater, at least 1.8 meters high, was encountered in these dolomites by one of the boreholes, confirmed by geophysical data. Two other boreholes revealed smaller cavities, approximately 0.5 meters high, filled with rubble and dolomite flour, in roughly the same depth interval. Above lie Upper Jurassic stiff clays. The clays are not widespread and have a very small thickness, up to 1.4 meters. They are overlain by Quaternary sands, mostly medium and coarse-grained. Near the base, there are layers of gravelly sand. The groundwater level is at a depth of 10.0 meters.

The relief of the Carboniferous roof (Fig. 1) shows a sharp depression (approximately 2.0 meters) in the southwest corner of the site. This trend is also observed in the overlying Upper Jurassic clays. Such a depression may indicate the presence of a relict karst cavity, whose collapse occurred in post-Jurassic time. Determining the exact age is challenging as the ground surface has undergone significant anthropogenic changes, and no visible traces of sinkholes are observed.

Based on the results of static probing, models reflecting the distribution of stress fields were created.

Fig. 1. Map of the Roof of Upper Carboniferous Deposits.

Fig. 2. Map of Average Soil Resistance Q to Probe Cone Penetration in the Depth Interval from 2.6 to 13.0 meters.

In one of the models (Fig. 2), the horizontal plane shows the distribution of average values of parameter Q in the depth interval from 2.6 to 13.0 meters, covering both the aeration zone and the saturation zone. The upper value of the interval, 2.6 meters, corresponds to the maximum depth of the anthropogenic soils, which, due to their heterogeneity, must be excluded from the calculation. The lower value, 13.0 meters, corresponds to the minimum depth of probe penetration. The illustration shows a significant decrease in parameter Q in the southwestern part of the survey area compared to the background values. These patterns are also observed when analyzing other depth intervals, both in the aeration zone and in the fully saturated zone. The position of the groundwater table is given special importance, as groundwater is the main factor influencing the mechanisms of karst processes.

Fig. 3. Distribution of Dispersive Soil Density Based on Soil Resistance Q under the Probe Cone.

The next step involves constructing a model using the same soil resistance parameters but in the vertical plane and across a single cross-section. Naturally, the selected section should pass through the weakened zone identified using the first model (Fig. 2), in this case from west to east. As seen from the cross-section (Fig. 3), the left, western part of the model shows a zone of abnormally low (< 5 MPa) soil resistance values (Q), whereas the background value at this depth is about 10-12 MPa. The fact that the zone of loosening is located below the free surface of the groundwater may indicate the downward filtration of water and the mechanical removal of silt particles into the underlying layers, i.e., suffosion. Such a process is impossible without the presence of a receiving cavity and/or a network of communicating cracks and pores ready to accept the displaced material.

To confirm the fact of particle displacement, the granulometric composition of the soil is analyzed. Differential curves are constructed to reflect the percentage ratio of fractions in each sample taken from different depths. The graphs

(Fig. 4) clearly show an increase in the amount of silt particles and the degree of heterogeneity of the granulometric composition with increasing depth in the coarse-grained sand sample.

Fig. 4. Differential Curves Reflecting the Increase in the Percentage of Dust Particles in the Sample with Depth.

Conclusions

According to the "Map of Karst and Karst-Suffosion Hazard in Moscow" [2], this area is classified as highly hazardous for the potential manifestation of karst processes. The analysis conducted based on static probing confirms the presence of karst-suffosion processes in the southwestern part of the site. Therefore, it can be concluded that the use of static and dynamic probing is appropriate for assessing karst hazards.

References

1. Bondarik G.K. *Fundamentals of the Theory of Variability of Engineering-Geological Properties of Rocks*. Moscow, Nedra, 1971.

2. *Moscow: Geology and the City*, Ed. V.I. Osipov, O.P. Medvedev. Moscow, AO "Moscow Textbooks and Cartolithography", 1997.

3. Svarensky I.A., Mironov N.A. *Guidelines for Engineering-Geological Surveys in Karst Development Areas*. Moscow, PNIIIS Ministry of Construction of Russia, 1995.

4. *SNiP 11-105-97. Engineering-Geological Surveys for Construction. Part II. Rules for Conducting Work in Areas of Dangerous Geological and Engineering-Geological Processes*. Moscow, Gosstroy of Russia, 2000.

5. Khomenko V.P. *Patterns and Forecasting of Suffosion Processes*. Moscow, GEOS, 2003.

6. Khomenko V.P. *Forecast of a Collapse Location: New Approach*. Quarterly Journal of Engineering Geology and Hydrogeology, 2008, Vol. 41, Part 3, pp. 393–401.

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