Artificially ground freezing method is increasingly applied in formations with high permeability. The groundwater seepage flow should be considered because an excessive groundwater seepage flow would make the merging of the frozen wall challenging. Therefore, in this study, we investigate the temperature field and frozen wall merging characteristics at varying groundwater seepage flow rates in gravel formation. Results show that the heat exchange between the seepage flow and freezing pipes delays the merging of the frozen wall and reduces its total thickness. The groundwater seepage flow restricts the freezing of the upstream zone and accelerates the freezing of the downstream zone. The upstream and downstream temperature fields are symmetrical in nonseepage flow conditions but are asymmetrical in the presence of seepage flow. The merged frozen wall presents an arched shape and shifts to the downstream zone. The “scouring effect” and “water barrier effect” simultaneously act on the merging process of the frozen wall. The total thickness of the frozen wall decreases by more than 30% when the flow rate increases from 0 to 5.0 m/d. Optimising the layout of the freezing pipes in gravel formations is a reasonable solution for a safe and economical design.

Ensuring shaft safety is critical in the process of coal mining. During coal mining in the Guotun coal mine, Shandong Province, China, issues, such as shaft deflection, tank channel and shaft deformation, and tank jamming, which did not appear in the shaft before, brought huge hidden dangers to safety. In this study, a set of three-dimensional analog simulation systems for assessing the influence of mining on shaft deflection was designed by using penetration force simulations of gravity fields. In this system, electromagnetic displacement meter is used to collect surface settlement data, which verifies the feasibility of using hydraulic pillow to simulate formation disturbance caused by coal mining. the strain of the shaft is measured by optical fibers and strain gauges, and the amount that mining influenced the deflection of the shaft is obtained. The results show that there is an approximately positive linear correlation between the horizontal displacement of the shaft caused by mining and the mining thickness of the coal. The horizontal displacement of shaft caused by coal mining is inversely proportional to the moving Angle within a reasonable range. The horizontal displacement of shaft is affected by mining mode, and symmetrical mining can significantly reduce the horizontal displacement of shaft. The horizontal displacement of the shaft at different depths caused by mining decreases with increasing shaft depth, which is the main cause of shaft deflection.

Using the coda wave interferometry (CWI) method to obtain the ultrasonic coda wave characteristics of loaded concrete is an important method to evaluate the mechanical response of concrete. In this paper, the ultrasonic coda wave characteristics of C40–C70 concrete specimens (four strengths of concrete) under uniaxial compression were tested by laboratory experiments. Furthermore, to clarify the relationship between the internal damage process of concrete and the change rate of coda wave velocity, an ultrasonic coda wave discrete element simulation model combined with digital image processing technology was established. The results show that the coda wave is very sensitive to small changes in the compressive damage to concrete, and the change in coda wave velocity can correspond to the development process of concrete damage. This research is conducive to a better understanding of the complex material behavior of compressive concrete and proves the feasibility of ultrasonic field simulation and processing by using numerical simulation images of concrete damage.

PurposeThis paper aims to study the impacts of groundwater seepage on artificial freezing process of gravel strata, the temperature field characteristics of the strata, and the strata process, closure time and thickness evolution mechanism of the frozen wall.Design/methodology/approachIn this paper several laboratory model tests were conducted, considering different groundwater seepage rate.FindingsThe results show that there is a significant coupling effect between the cold diffusion of artificial freezing pipes and groundwater seepage; when there is no seepage, temperature fields upstream and downstream of the gravel strata are symmetrically distributed, and the thickness of the frozen soil column/frozen wall is consistent during artificial freezing; groundwater seepage causes significant asymmetry in the temperature fields upstream and downstream of the gravel strata, and the greater the seepage rate, the more obvious the asymmetry; the frozen wall closure time increases linearly with the increase in the groundwater seepage rate, and specifically, the time length under seepage rate of 5.00 m d−1 is 3.2 times longer than that under no seepage; due to the erosion from groundwater seepage, the thickness of the upstream frozen wall decreases linearly with the seepage velocity, while that of the downstream frozen wall increases linearly, resulting in a saddle-shaped frozen wall.Originality/valueThe research results are beneficial to the optimum design and risk control of artificial freezing process in gravel strata.

An artificial ground freezing method is often applied to highly permeable gravel formations. Seepage flow increases energy consumption and engineering accidents under this condition. Based on the physical modeling tests, the numerical simulations were conducted. The physical modeling test examined the development of the temperature and frozen wall. Numerical modeling integrated with the ACO algorithm was established to optimize the layout parameters of the freezing pipes. The results indicate that the flowing water prolongs the closure time of the frozen wall. Meanwhile, the total thickness of the frozen wall is also reduced by the flowing water. There are significant differences in the development rates of the frozen wall in different zones. The thickness of the entire frozen wall is nonuniform owing to the seepage flow. Following optimization using the proposed algorithm, the closure time was shortened from 82.4 d to 56.9 d for the frozen wall. Moreover, the freezing efficiency increased by 30.95% after optimization, and consequently, the entire frozen wall was more uniform with a nonweak zone. A case study showed that this optimization system is an effective method for artificial ground freezing operations in geotechnical engineering.