Features of Adjusting the Frozen Soil Properties Using Borehole Temperature Measurements

The paper examines the theoretical issues of using borehole temperature survey data to control a frozen wall formed around the sinking mine shafts of the Nezhinsk mining and processing plant potash mine. We consider adjusting the parameters of the mathematical model of the frozen soil based on temperature measurements in boreholes. Adjustment of the parameters of the mathematical model (thermophysical properties of the soil) is usually carried out by minimizing the discrepancy functional between the experimentally measured and model temperatures in the temperature control boreholes. An important question about the form of this functional and the existence of minima remained after the previous studies. The study aimed at this question included analysis of heat transfer in two horizontal layers (sand and chalk) for two shafts under construction using artificial ground freezing. It was shown that the discrepancy functional minimum under certain conditions moves over time or is nonunique. This phenomenon results in ambiguity in adjusting the mathematical model parameters in the frozen soil to fit the borehole temperature survey data. At the stage of the frozen wall growth, the effective thermal conductivity in the frozen zone can be determined ambiguously from the temperature measurements in the boreholes—its value can change over time. At the stage of maintaining the frozen wall, the solution turns out to be dependent on the ratio of effective thermal conductivities in the frozen and unfrozen zones.

Variations on Reservoir Parameters of Oil Shale Deposits under Periodic Freeze-Thaw Cycles: Laboratory Tests

As one of the most important unconventional hydrocarbon resources, the oil shale has been extracted with a frozen wall to successfully increase the shale oil production and reduce environmental pollution, which results from the harmful liquids in the in situ conversion processing of oil shale. Thereby, the strength and permeability of the frozen wall are extremely critical to reduce the harmful chemicals leaching into the groundwater. However, the permeability and strength of the frozen wall can be influenced by periodic freeze-thaw cycles. In order to investigate the damage and deterioration characteristics of oil shale samples after various periodic freeze-thaw cycles, the oil shale samples were periodically frozen and thawed as many as 48 times, after which the sample mass, stress-strain, freeze-thaw coefficient, uniaxial compressive strength, elastic modulus, and longitudinal wave velocity of the oil shale samples were separately measured. According to the measured results, the number of freeze-thaw cycles greatly influenced the physical and mechanical properties of oil shale samples. The uniaxial compressive strength and elastic modulus of the oil shale samples were changed with maximum variation rates of 64% and 65%, respectively. Meanwhile, the freeze-thaw coefficient of measured oil shale samples exponentially decreased with the increased number of freeze-thaw cycles, whereas the longitudinal wave velocity of tested samples ranged from 1602 m/s to 2464 m/s as a result of the new micropores inside the oil shale sample. Research results have enormous significance to the efficient and safe in situ exploitation of oil shale deposits.

Molecular Dynamics Study on Water Flow Behaviour inside Planar Nanochannel Using Different Temperature Control Strategies

In the present paper, molecular dynamics simulations were performed to study the influence of two temperature control strategies on water flow behaviour inside planar nanochannel. In the simulations, the flow was induced by the force acting on each water molecule in the channel. Two temperature control strategies were considered: (a) frozen wall simulations, in which the dynamics of confining wall atoms was not solved and the thermostat was applied to the water, and (b) dynamic wall simulations, in which the dynamics of confining wall atoms was solved, and the thermostat was applied to walls while water was simulated in the microcanonical ensemble. The simulation results show that the considered temperature control strategies has no effect on the shape of the water flow profile, and flow behaviour in the channel is well described by the Navier–Stokes equation solution with added slip velocity. Meanwhile, the slip velocity occurring at the boundaries of the channel is linearly dependent on the magnitude of the flow inducing force in both frozen wall and dynamic wall simulations. However, the slip velocity is considerably greater in simulations when the wall dynamics are solved in contrast to the frozen wall simulations.

Improved prediction method for ground surface thawing settlement caused by the melting of tunnel horizontal frozen wall

With the rapid development of urban subway tunnel, artificial ground freezing technology is becoming more and more mature. With the natural thawing of horizontal frozen wall, thawing settlement will occur on stratum due to the thawing of frozen soil and the consolidation of thawed soil, which will inevitably bring adverse impact on the surrounding environment of subway tunnel. Therefore, the establishment of a reasonable ground surface thawing settlement prediction method will provide a favorable theoretical support for predicting the ground surface deformation in advance and taking active thawing settlement control measures. In the paper, the time functions of ground surface thawing settlement and consolidation settlement of tunnel horizontal frozen wall are established based on the stochastic medium theory during natural thawing period, and the calculation methods of thawing front radius, inner radius of thawing shrinkage region and inner radius of consolidation region are proposed. The results show that the cumulative ground surface thawing settlement is larger than that of Cai et al. after considering the consolidation of the thawed soil, which fully indicates that the ground surface settlement caused by the drainage and consolidation of the thawed soil cannot be ignored. In addition, the thawing displacement rate of frozen soil is greater than the consolidation displacement rate of thawed soil during the natural thawing and the thawed soil will be consolidated at a lower settlement rate for a long time after the natural thawing period.

Analysis of Temperature Anomalies during Thermal Monitoring of Frozen Wall Formation

The paper describes a distributed temperature sensing system that was used to monitor the artificial freezing of soils during the construction of a potash mine shaft. The technique of reconstructing the temperature field by solving the inverse problem in the entire volume of frozen soils using the measured temperatures in four thermal monitoring (TM) wells is described. Two local anomalies in temperature distributions in TM wells are described and analyzed theoretically using thermo-hydraulic modeling. The first anomaly concerns the asymmetric temperature distribution in one of the soil layers and is associated with the influence of natural groundwater flow in the horizontal direction. The second anomaly consists of a sharp decrease in water temperature in the section of the TM well located inside the freezing contour. Calculations showed that it is most likely associated with the entry of cold groundwater from the overlying layers of soils through a well filter at a depth of 160 m and the subsequent movement of the water up the well.

Modeling Seepage Flow and Spatial Variability of Soil Thermal Conductivity during Artificial Ground Freezing for Tunnel Excavation

Artificial ground freezing (AGF) technology has been commonly applied in tunnel construction. Its primary goal is to create a frozen wall around the tunnel profile as a hydraulic barrier and temporary support, but it is inevitably affected by two natural factors. Firstly, seepage flows provide large and continuous heat energy to prevent the soil from freezing. Secondly, as a key soil parameter in heat transfer, the soil thermal conductivity shows inherent spatial variability, binging uncertainties in freezing effects and efficiency. However, few studies have explored the influence of spatially varied soil thermal conductivity on AGF. In this study, a coupled hydro-thermal numerical model was developed to examine the effects of seepage on the formation of frozen wall. The soil thermal conductivity is simulated as a lognormal random field and analyzed by groups of Monte-Carlo simulations. The results confirmed the adverse effect of groundwater flow on the formation of frozen wall, including the uneven development of frozen body towards the downstream side and the higher risk of water leakage on the upstream face of the tunnel. Based on random finite element analysis, this study quantitively tabulated the required additional freezing time above the deterministic scenario. Two levels of the additional freezing time are provided, namely the average level and conservative level, which aim to facilitate practitioners in making a rule-of-thumb estimation in the design of comparable situations. The findings can offer practitioners a rule of thumb for estimating the additional freezing times needed in artificial ground freezing, accounting for the seepage flow and spatial variation in soil thermal conductivity.