RUS:
Крашенинников В.С. Опыт использования динамического зондирования при обследовании провала грунта, вызванного техногенными факторами // Инженерные изыскания в строительстве. Сборник трудов VII Научно-практической конференции молодых специалистов. ПНИИИС. М.: ПНИИИС, 2011. С. 88-92.

ENG:
Krasheninnikov V.S. Experience with dynamic probing in investigating a ground collapse induced by anthropogenic factors // Engineering Surveys in Construction. Proceedings of the VII Scientific and Practical Conference of Young Specialists. PNIIIS. Moscow: PNIIIS, 2011. pp. 88-92.

Krasheninnikov Vadim

Department of Engineering Geology and Geoecology

Moscow State University of Civil Engineering

(MGSU)

Experience with dynamic probing in investigating a ground collapse induced by anthropogenic factors

Here, the experience of using dynamic probing to investigate a ground subsidence caused by anthropogenic factors during tunnel construction in an urban setting is described

In the spring of 2010, in the Western Administrative District of Moscow, underground collector construction was underway on Krylatskaya Street using the micro-tunneling method with an AVN 1500 tunnel boring machine. The machine consists of a metallic structure shaped like a tube (the shield head), with one end (the working face) equipped with rock-breaking tools. On the opposite end, reinforced concrete sections (linings) are added, and the excavated soil is hydraulically transported from the face to the surface. As the soil mass is broken down, the shield is pushed forward along the direction of movement using powerful hydraulic jacks, which also adjust the position of the shield head in space. This process simultaneously constructs the tunnel and installs the collector. The inner diameter of the tunnel was 1.5 meters, with the diameter of the cutting tool at the face being slightly larger—about 1.8 meters. The top of the tunnel at this section was approximately 12.0 meters below the ground surface.

During the work, surface subsidence occurred above the tunneled section, approximately 50.0 meters from the shield's cutting head. The subsidence affected an area of 1.5 x 1.5 meters and reached a depth of up to 0.2 meters. Beneath the exposed asphalt, a cavity measuring 2.0 x 2.0 meters and up to 0.8 meters deep was discovered (Figure 1), which could be attributed to the subsidence that occurred at the depth of the shield’s operation. The situation was further complicated by the fact that the shield was advancing alongside an existing pressurized collector, which was installed approximately 3.0 meters below the ground surface, in close proximity (3.0 - 5.0 meters) to it. The formation and development of the subsidence could have led to damage to the communication infrastructure.

Fig. 1. Test pit at the site of the subsidence that occurred in 2010, Krylatskaya Street, Western Administrative District of Moscow. Photo by V. Krasheninnikov.

The "applied" task of the survey was to identify the dimensions of the zone of subsidence, as well as to quantitatively compare the characteristics of soils at the site of the subsidence and beyond it. Additionally, the author aimed to achieve a "research" goal of analyzing the conditions of subsidence in anthropogenic settings and comparing them with karst-suffosion processes occurring under natural conditions. To study this problem, in addition to drilling control wells and conducting geophysical surveys, soil testing was carried out using dynamic probing. A lightweight portable dynamic probing rig with a 10 kg hammer, designed by the Experimental Plant of TsNIIS and approved by the USSR Ministry of Transport Construction in 1983 [1], was used in this work.

Geomorphologically, the site is located within the second (Mnevnikovskaya) floodplain terrace of the Moscow River. The geological structure of the site (Fig. 2) is characterized by inconsistency in the thickness and extent of lithological layers and their rather heterogeneous composition. In general, the section down to a depth of 20.0 meters consists of:
- technogenic (tQIV) soils (mainly sands with inclusions of coarse-fragment material);
- alluvial (aQIII) silty sands, occasionally fine sands, and flowable sandy loams;
- Lower Cretaceous (K1) silty sands;
- and Upper Jurassic (J3) sandy loams and clays.

The shield tunneling at this site was carried out in Lower Cretaceous sands and Upper Jurassic sandy loams. Groundwater in April 2010 was found at a depth of 4.8-5.0 meters.

Fig. 2. Schematic longitudinal geological cross-section.

On the site, six tests were conducted with probing depths of 9-10 meters. One of the dynamic probing points (DP 1) was located directly at the site of the ground collapse. The test conducted at this location showed that the specific dynamic resistance values of the soil (Rd) in the investigated depth range from 0 to 9.5 meters were between 0.6-1.5 MPa, which are anomalously low compared to other test points.

Fig. 3. Survey Area Scheme

In the other five points (DP 2 - DP 6), these values ranged from 4.0 to 8.0 MPa (Fig. 4). Thus, the significant soil loosening identified at the first probing point (DP 1) is consistent with the observed process of ground subsidence.

Fig. 4. Dynamic Probing Graphs: a) At the point of ground subsidence (DP 1); b) At the point located 30 meters from the subsidence (DP 5).

Let us consider the mechanism by which this subsidence developed. It was found that the tunnel boring shield's operation deviated from the proper technology and trajectory in some areas. The excavation face was flushed with water instead of the required bentonite mixture, and no preventative measures were taken to stabilize the ground in problematic zones. Furthermore, due to the insufficient qualifications of the personnel handling the shield, there were significant deviations (up to 1.0 - 2.0 meters) from the planned trajectory. As mentioned earlier, the geological cross-section contains water-saturated silty sands, essentially quicksand. During the tunneling process, the shield encountered the quicksand zone and began intensively removing the soil, leading to increased extraction of sandy soil from the face area into the shield and causing face collapse. Consequently, the shield's advancement was minimal. In this situation, the shield acted as a cavity-receiver. Thus, a certain space filled with water and collapsed material formed above the tunnel.

Subsequently, the shield's advancement resumed, but the ground collapse continued, and the cavity expanded upward, reaching the surface. Due to the presence of relatively cohesive soil layers (sandy loams) in the section, the collapse diameter increased slightly, unlike in a situation where the cross-section consisted solely of sandy soils. From the level of the shield's path to the groundwater surface, the cavity took on a shape similar to a highly elongated inverted parabola. Above the groundwater level, the collapse occurred in a cylindrical shape, with a diameter observed to be 1.5-2.0 meters.

Therefore, it can be concluded that concerns about the subsidence widening were unfounded, and the ground settlement did not affect the nearby pressurized collector, at least at the time of the study. Based on the fact that the collapse diameter increases from the bottom up under conditions of a non-pressurized descending water flow , it is likely that the subsidence contours correspond to the size of the surface settlement, i.e., approximately 1.5-2.0 meters in diameter.

Analyzing this example and understanding the manifestations of classical karst, we can state:

a) The mechanisms of subsidence formation above underground mine workings and karst cavities are largely similar. The differences lie only in the speed of these processes.

b) By observing subsidence processes occurring in anthropogenic conditions, we can, with a certain degree of accuracy, determine the primary cause of their formation, as well as how and when they developed.

Unlike natural cavities and voids, we almost always know the location, depth, size, conditions, and timing of the underground workings.

Thus, the information obtained from studying anthropogenic conditions of subsidence formation comes from the "source" and is undoubtedly valuable and important for understanding processes occurring in natural conditions.

References

1. Prigoda, V.Y. Manual on Electrocontact Dynamic Soil Penetration Testing. Moscow: TsNIIS, 1983.

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

Page updated

Google Sites

Report abuse