Artificial liquid nitrogen freezing technology is widely used in urban underground engineering due to its technical advantages, such as simple freezing system, high freezing speed, low freezing temperature, high strength of frozen soil, and absence of pollution. However, technical difficulties such as undefined range of liquid nitrogen freezing and thickness of frozen wall gradually emerge during the application process. Thus, the analytical solution of the freezing-temperature field of a single pipe is established considering the freezing temperature of soil and the constant temperature of freezing pipe wall. This solution is then applied in a liquid nitrogen freezing project. Calculation results show that the radius of freezing front of liquid nitrogen is proportional to the square root of freezing time. The radius of the freezing front also decreases with decreased the freezing temperature, and the temperature gradient of soil decreases with increased distance from the freezing pipe. The radius of cooling zone in the unfrozen area is approximately four times the radius of the freezing front. Meanwhile, the numerical simulation of the liquid nitrogen freezing-temperature field of a single pipe is conducted using the Abaqus finite-element program. Results show that the numerical simulation of soil temperature distribution law well agrees with the analytical solution, further verifies the reliability of the established analytical solution of the liquid nitrogen freezing-temperature field of a single pipe.

This study aims to solve the problems of the high cost, heavy pollution and poor performance of traditional engineered cementitious composites (ECC) by adding modified Polyvinyl chloride (PVC) aggregate, Polypropylene (PP)–Polyvinyl alcohol (PVA) hybrid fiber and large amount of fly ash. The PVC aggregate is modified by pre-coating silica fume with a PP fiber volume content of 0.5%, PVA fiber volume contents of 1%, 1.5%, and 2%, PVC aggregate contents of 10%, 20%, and 30%, and fly ash volume content of 69%. Different properties and microstructures were studied by carrying out cube compression tests, splitting tensile tests, water absorption tests, drop hammer impact tests, scanning electron microscopy and nuclear magnetic resonance tests. According to the test results, under the same content of PVC aggregate, the use of modified PVC aggregate can, not only effectively avoid the decrease in strength and increase of water absorption, but also improve brittleness and impact failure energy. Regardless of the kind and content of fiber, the compressive strength and brittleness will decrease, while the splitting tensile strength, water absorption, and impact failure energy will increase. After adding 0.5% PP and 1.5% PVA fiber, the performance is ordinary and a negative mixing effect occurs. As more modified PVC aggregate is added, the strength of the ECC concrete with PP–PVA hybrid fiber and modified PVC aggregate added slowly decrease, while the water absorption and impact failure energy increase. Based on a comprehensive analysis of the test data, the reinforcement method of adding 1.5% PVA-0.5% PP hybrid fiber-30% modified PVC aggregate is superior to adding 1.5% PVA fiber, but slightly inferior to adding 2% PVA fiber. This study argues that the reinforcement method is of great significance for the promotion and application of ECC.

Targeting the challenging analysis of the temperature field of the artificial freezing shaft sinking projects in Cretaceous strata, this study used the Hongqinghe air shaft freezing project as an example to perform a numerical simulation and measurement analysis of the temperature field of shaft sinking via the artificial freezing method in Cretaceous strata. First, the thermal physical parameters and mechanical properties of frozen Cretaceous rock were measured in the lab. Based on the engineering geological conditions and test parameters, the Hongqinghe air shaft freezing parameters were designed, and the preliminary scheme of the freezing project was obtained, in which a single ring of freezing holes was arranged in Cretaceous strata, the freezing wall had a design thickness of 3.7 m, and the average temperature was -12°C. Next, a numerical model for the temperature field of the freezing project was established to predict and analyze the effective thickness, average temperature, and shaft wall temperature of the frozen wall at different depths of shaft excavation. Finally, a comparison and analysis of field temperatures measured at the inspection holes and shaft walls showed that the numerical simulation results generally agreed with the field measurements. The field measurements showed that at the initial stage of freezing, the cooling rates of fine-, medium- and coarse-grained sandstones were 0.364 °C/d, 0.397 °C/d and 0.440 °C/d, respectively. Informatized construction enabled the air shaft freezing project in the Hongqinghe coal mine to be completed successfully.