Hydromechanical Simulation of Tunnel Excavation in Rock Considering a Nearby Karst Cave

Tunnel excavation tends to be affected by karst cavities in karst areas. Some cavities that are at low risk of causing safety issues without treatment tend to be ignored in the design and construction of tunnels to reduce costs. It is necessary to gain a better understanding of the effect of such a cavity on the seepage around a tunnel, the deformation of the surrounding rock, and the stress of the tunnel lining. In this paper, a two-dimensional rock-tunnel hydromechanical model with a karst cave was established with FLAC3D finite difference software to simulate the tunnel excavation with the consideration of seepage. Numerical simulations were performed to analyze the deformation of the surrounding rock, the seepage field of the surrounding rock, and the stress of the tunnel lining, and the results were compared for scenarios when the karst cave is at different locations relative to the tunnel. These results can provide a reference for the design and construction of tunnel engineering in rock with karst caves.

Stability analysis of rock slope and calculation of rock lateral pressure in foundation pit with structural plane and cave development

In this study, numerical simulations were carried out to analyze the influence of caves in different positions and shapes, in combination with structural planes, on the stability of the slope and the failure characteristics of a rock slope in a deep foundation pit with high inclination structural planes and cave development. The schemes for substituting a single karst cave for karst caves were constructed. Based on the penetration failure characteristics of karst caves between parallel structural planes, methods for calculating the safety factor of the rock foundation pit and the upper bound of the lateral pressure of the supporting structure under the combined influence of the caves and structural planes were developed, which can be used to assess the safety factor of a rock mass and to calculate the lateral pressure under complex geological conditions.

Comparison of historical and current temperatures in show caves (Slovenia)

Historical air temperatures at three karst caves in Slovenia have been compared to current data time series. In Postojnska Jama (PJ), the most visited show cave in Slovenia, the significant temperature difference between historical and modern measurements at the Pulpito site relates to the months April to November. Mean monthly temperatures measured at the Sepolcro site (PJ) in the modern period (2016–2019) are year round significantly higher than in the historical period (1935–1937). The temperature increase over the last 85 years in PJ is attributed to outside temperature rise and additional heat input from visitors, especially for Sepolcro site. A comparison of current (2017–2019) and historical (1956–1957) temperature data in touristically poorly visited Predjama Cave shows lower increase as in PJ and is completely related to outside cave conditions. In the case of Škocjanske Jame (Tiha Jama), air temperature has not significantly increased since the historical 1928 measurements because the monitoring site looks to be morphologically isolated from significant impacts of outside climate and visitors.

Multi-Step Combined Control Technology for Karst and Fissure Water Inrush Disaster During Shield Tunneling in Spring Areas

Shield tunneling in highly fractured karst water-rich conditions easily results in water inrush disaster or even causes the roof of the karst caves to collapse. Severe water inrush disasters have occurred during the EPB (earth pressure balance) shield machine of the Jinan Metro Line R1 advanced through a karst and fissure groundwater-rich limestone ground in the spring area. To cope with the extreme water inrush risk, a multi-step combined control technology was put forward. First, a detailed geological exploration was carried out by ahead geophysical prospecting using high-density resistivity method, geological radar, etc., and geological borehole drilling was conducted from the ground surface before excavation. As a result, the distribution orientation, size, fissure development degree, and water inflow channel within the surrounding rock of the karst caves were detected. Second, multi-step grouting was performed to reinforce the surrounding rock, including pre-grouting treatment and filling rock blocks to the big karst caves from the ground surface, multiple grouting with a small amount of inert slurry each time inside the tunnel, and secondary circumferential hoop grouting at the shield tail. Third, the tunneling process was optimized, including optimizing the tunneling parameters, making full use of the air-pressurized tunneling technology of the EPB to press bentonite into the fractures around the excavation cabin to seal the fissure water, and using the drainage system of EPB and muck improvement technology to reduce the water inrush disaster. Meanwhile, shield protection slurry technology is applied to cutter inspection and replacement in the pressurized chamber under dynamic water flow environment of the spring terrain. The practice shows that the water inrush on the tunnel face is obviously alleviated after the shield machine advanced into the grouting area. According to statistics, the water inflow on the tunnel face decreases from about 4 m3/h before treatment to less than 0.3 m3/h after the abovementioned control, and the water seepage between the segmental linings reduces to almost zero. The average advance rate increased from 3 m/day without stopping or even zero when the shield machine needs to shut down 2–3 days for drainage to about 6 m/day. In addition, the treatments prevented the shield machine from jamming and the head descending disaster. This study provided a reliable control method for shield tunneling through the karst and fissure water-rich area and played an essential role in protecting the spring water.

Diamicton facies in the vicinity of Duhlata cave, Bosnek karst region, Southwestern Bulgaria

Karst caverns in the Upper Triassic dolostones of the Rusinovdel Formation are filled with allochthonous clastics (brecciaconglomerates with maximum boulder size) and locally with speleothems (flowstones). Deposition of the former (diamicton facies) by debris flows resulted from extreme flood events along the upper reaches of Struma river. The polymict material reflects erosion of various rock types in the source area but is dominated by resedimented Lower Triassic red beds. The diamictons are more or less similar to the few known examples from Quaternary karst caves.

Ventilation Conversion Mechanisms in Karst Caves

Ventilation modes in karst caves are of great significance for exploring issues regarding "carbon sources and sinks" in karst areas. Therefore, this study conducted continuous monitoring of air temperature, humidity, and CO2 concentrations inside and outside the Dafeng cave in Suiyang, Guizhou from August 2015 to July 2020 in order to comprehensively analyze each element using a systematic analysis method. The results revealed that: (1) the Dafeng cave is mainly divided into three ventilation modes: ① during summer and autumn, the inside of the cave is mainly in a restricted ventilation mode, wherein air exchange inside and outside the cave is suppressed, and the CO2 concentration in the cave easily forms a cumulative effect; ②during winter, the inside of the cave is mainly in an active ventilation mode, wherein there is strong air exchange, and the CO2 concentration inside the cave is close to that outside the cave; ③ during spring, due to the variable climate and rising temperature, the inside of the cave gradually transitions from an active to restricted ventilation mode and the air exchange intensity gradually changes. The isotope data change characteristics outside the cave at the Yemingzhu monitoring site verifies the ventilation mode of the tunnel during each season. (2) Due to the influence of tourists, air changes inside and outside the cave, and the structure of the tunnel, there may be multiple ventilation modes within a single season. Tourists and tunnel structure primarily affect the structure of the tunnel. A change in the air environment outside the cave mainly affects the virtual temperature outside the cave, increasing or reducing the virtual temperature difference between the inside and outside of the cave, thereby affecting the ventilation mode. Thus, our study suggests that more consideration should be given to changes in external climate or weather conditions when studying the conversion mechanism of karst cave ventilation modes.