Physical and analytical modeling of rhythmic karst springs

Rhythmic Karst Springs (RKSs) are rare geologic features that rhythmically outflow water. A mechanical model for the rhythmic flow with rhythmic spill-over configuration was constructed in this work. The evolution of the RKS was revealed by using geological process analysis. The analytical model can directly explain the existence of RKSs in soluble rock regions and their formation mechanism in nature. Visual observations and flow measurements were performed using a laboratory physical model of RKS. The physical model components included a soluble rock simulation area, karst pipes, cave-reservoir, karst depression terrain, water tank, rhythmic spring, and the outflow measurement system. Groups of tests were carried out to recreate the process of RKS functioning and to confirm the rhythmic cycle duration and the threshold of replenishment intensity. This research helped to interpret the behavior of rhythmic springs using the recharge and evacuation of the subsurface cave-reservoir by means of fluid mechanics and groundwater hydraulics theories.

Remote Sensing Study of Sliding Surface of Slopes nearby a Large Tobacco Growing Plantation

Objectives: A study of remote sensing detection of slip surfaces on man-made slopes in a natural environment nearby a large tobacco growing plantation. Based on the GPR method, the image characteristics of potential slip surfaces on slopes are analyzed based on the change in GPR wave amplitude caused by the water content of the slip surface. The large granularity and permeability of the spoil allows natural precipitation to continue to infiltrate into the bedrock, and the infiltrated water gradually erodes the spoil and forms a water-rich rubble layer with the easily soluble rock mass, which develops into a slip zone and threatens the stability of the slope nearby a large tobacco growing plantation. By analyzing the magnitude of the remote sensing images, the process of water content changes in the soft fracture zone at the bedrock interface of the slope can be efficiently and accurately understood. Ensured the safe operation of the planted tobacco plantation.

Pretreatment for Tunnel Karst Cave during Excavation: A Case Study of Guangxi, China

Karst landscape is a general term for earth surfaces and underground patterns that have been formed by the dissolution of soluble rock. Karst landscapes are widely distributed throughout China—particularly in the Guangxi and Guangdong provinces. The main features of karst landscapes are typically reflected in karst caverns, sinkholes, and other geographical phenomena. During tunnel construction in karst areas, various forms of karst caverns may appear on the construction route, and they can cause hazards—such as water inrush and collapse—during tunnel construction. These hazards affect the tunnel construction process. As such, it is necessary to propose a treatment for karst caverns. In this work, a case study of the tunnels on the Hechi-Baise expressway is presented. A comprehensive pretreatment method suitable for this tunnel is proposed. On the premise of prioritizing the safety and timeline of construction, an optimized treatment scheme for the karst caverns of Hebai tunnel is followed. The optimized treatment scheme primarily includes calculation of safe thickness of tunnel face, strengthening the initial support and increasing the thickness of the second lining, increasing the reserved deformation, and grouting. The proposed scheme achieved favorable results in the treatment of a karst cavern in the Hebai tunnels.

Chemical and isotopic composition of nitrogen thermal waters of the Kuldur Deposit (JAR, Russia)

The article presents the results of the hydrogeochemical study of thermal groundwaters from the Kuldur spa (Jewish Autonomous Region, Russia). The main characteristics of these groundwaters are high temperature (73 ᵒС), low TDS (up to 0,38 g/l), and alkaline (pH 9). The dominated cation is sodium, and the anion is hydrocarbonate. Water is enriched with fluorine, silicon, aluminum, tungsten, molybdenum, and some other anionic elements. This study provides detailed data on the chemical and mineral composition of host rocks and sources of solutes. Isotopic data from bubbling gases show that the main component of the gas phase (N2) comes from an atmogenic source, while CH4 and CO2 are biogenic. Argon and oxygen are also derived from air, while helium is predominantly radiogenic. The ϬD and Ϭ18О in the aqueous phase indicate the undoubtedly meteoric genesis of groundwater with an extended circulation period. Our results suggest that the studied groundwaters are results of the penetration of meteoric waters to 4 km depth and heating up to 100 ᵒC. The solutes come to aquifer via the dissolution of rocks, but since groundwater circulates within the poorly soluble rock (granitoids), respectively, the water TDS remains low.

Study of the Corrosion Characteristics of Tunnel Fissures in a Karst Area in Southwest China

The development of the fissures in soluble rock of karst areas directly affects the construction and operation safety of tunnel engineering. It is thus of theoretical and practical significance to study the characteristics of its corrosion and its influencing factors. Taking the Wulong tunnel as the research object, the numerical model of the study area was established to quantitatively analyze the corrosion range, corrosion ratio, and changes in the permeability and porosity of the fissures in soluble rock of karst areas of the tunnel over the past 100 years, and the simulation results were verified by field experiments. The results show that the main controlling factor of the fissure corrosion of the tunnel in the karst area is the flow rate. The corrosion range and corrosion ratio of the fissures of the tunnels in the karst area increased with temperature because the reaction rate constant increased with temperature, causing the reactions’ equilibrium to move towards the direction of the solution. The larger the initial permeability and the larger the porosity of the fissures, the faster the fissures corrode. In the same time period, the fissures with high permeability and large porosity will lead to the permeability and porosity being more enhanced, thus causing the corrosion of the fissures to exhibit secondary enhancement effects. The opening of the dead-end pores greatly enhanced the permeability and slightly increased the porosity, which caused the differential corrosion of fissures in the karst area. The protection of the tunnel should be strengthened, mainly in strong hydrodynamic conditions and in the fracture development zone.

Dissolution and precipitation of fractures in soluble rock

Soluble rocks such as limestone, anhydrite, and gypsum are characterised by their large secondary permeability, which results from the interaction of water circulating through the rock and dissolving the soluble fracture walls. This highly selective dissolution process enlarges the fractures to voids and eventually cavities, which then carry the majority of flow through an aquifer along preferential flow paths. We employ a numerical model describing the evolution of secondary porosity in a soluble rock to study the evolution of isolated fractures in different rock types. Our main focus is three-fold: The identification of shallow versus deep flow paths and their evolution for different rock types; the effect of precipitation of the dissolved material in the fracture; and finally the complication of fracture enlargement in fractures composed of several different soluble materials. Our results show that the evolution of fractures composed of limestone and gypsum is comparable, but the evolution time scale is drastically different. For anhydrite, owing to its difference in the kinetical rate law describing the removal of soluble rock, when compared to limestone and anhydrite, the evolution is even faster. Precipitation of the dissolved rock due to changes in the hydrochemical conditions can clog fractures fairly fast, thus changing the pattern of preferential pathways in the soluble aquifer, especially with depth. Finally, limestone fracture coated with gypsum, as frequently observed in caves, will result in a substantial increase in fracture enlargement with time, thus giving these fractures a hydraulic advantage over pure limestone fractures in their competition for capturing flow.