The Temperature Field Evolution and Water Migration Law of Coal under Low-Temperature Freezing Conditions

This study examines the evolution law of the coal temperature field under low-temperature freezing conditions. The temperature inside coal samples with different water contents was measured in real-time at several measurement points in different locations inside the sample under the condition of low-temperature medium (liquid nitrogen) freezing. The temperature change curve was then used to analyse the laws of temperature propagation and the movement of the freezing front of the coal, which revealed the mechanism of internal water migration in the coal under low-temperature freezing conditions. The results indicate that the greater the water content of the coal sample, the greater the temperature propagation rate. The reasons for this are the phase change of ice and water inside the coal during the freezing process; the increase in the contact area of the ice and coal matrix caused by the volume expansion; and the joint action of the two. The process of the movement of the freezing front is due to the greater adsorption force of the ice lens than that of the coal matrix. Thus, the water molecules adsorbed in the unfrozen area of the coal matrix migrate towards the freezing front and form a new ice lens. Considering the temperature gradient and water content of the coal samples, Darcy’s permeation equation and water migration equation for the inside of the coal under freezing conditions were derived, and the segregation potential and matrix potential were analysed. The obtained theoretical and experimental results were found to be consistent. The higher the water content of the coal samples, the smaller the matrix potential for the hindrance of water migration. Furthermore, the larger the temperature gradient, the larger the segregation potential, and the faster the water migration rate.

Aggregate Subgrade Improvements Using Quarry By-products: A Field Investigation

This report presents a case study for constructing aggregate subgrade improvement (ASI) layers using quarry by-product aggregates (QBA), a quarry mix of large primary crushed rocks (PCR) and sand-sized quarry fines. The construction took place at Larry Power Road in Bourbonnais Township in Kankakee County, Illinois, where the Illinois Department of Transportation placed two QBA mixes. The first mix (QBA_M1) consisted of 45% quarry by-products and 55% railroad ballast–sized 3×1 PCR. The second mix (QBA_M2) consisted of 31% and 69% quarry by-products and PCR, respectively. Two conventional ASI sections were also constructed conforming to Illinois Department of Transportation’s CS02 gradation. All sections consisted of a 9 in. (229 mm) QBA/PCR layer topped with a 3 in. (76 mm) dense-graded capping layer. Laboratory studies preceded the construction to recommend optimum quarry by-product content in the QBA materials and construction practice. The Illinois Center for Transportation research team monitored the quality and uniformity of the construction using nondestructive testing techniques such as dynamic cone penetrometer, lightweight deflectometer, and falling weight deflectometer. The segregation potential was monitored by visual inspection and imaging-based techniques. Short-term field evaluation of the constructed QBA layers, particularly QBA_M2 with a 31% quarry by-product content, showed no evidence of abnormal segregation and did not jeopardize the structural integrity of the QBA ASI layers, which had slightly lower but comparable strength and stiffness profiles to the conventional ASI sections. The use of QBA materials in ASI was field validated as a sustainable construction practice to provide stable pavement foundation layers.

Effect of particle trapping on frost heaving soils

A model of freezing soils is developed that accounts for the dependence of the frost heave rate on particle trapping. At sufficiently low cooling rates the soil experiences primary frost heave with a single growing ice lens that rejects all soil particles. At higher cooling rates ice lenses start to engulf the largest soil particles and the rate of segregation heave is reduced. At the highest freezing rates all particles are engulfed by the ice and the pore water freezes in situ. A new kinetic expression for the segregation potential of the soil is obtained that accounts for particle trapping. Using this expression a simple transient frost heave model is developed and compared with experimental data.

Frost heave and thaw consolidation modelling. Part 1: A water flux function for frost heaving

Although much effort has been made to develop various frost heave models in the past decades, a simple yet versatile model is still needed for engineering applications. This paper presents a method to estimate frost heave in frozen soil using a macroscopic water flux function that extends the segregation potential to make it applicable for both steady state and transient freezing and thawing states. The formation of an individual ice lens is modelled by combining previously developed stress and strain criteria. The water flux function, which includes various factors in accordance with the porosity rate function, can describe the growth of both new and old ice lenses. More importantly, every component of the water flux function is physically explained by the theory of pre-melting dynamics, where all the influencing factors are traced back to their impacts on the ice volume distribution. The performance of the model is demonstrated via simulations of one-dimensional freezing and thawing processes after the model is validated by a specific case from previous literature. Although adequate data are not available for a stricter experimental verification of the model, it is observed that the simulations predict the general course of events together with significant specific features that were identified in previous experimental studies.

Application of the Segregation Potential Model to Freezing Soil in a Closed System

Previous studies have shown that an accurate prediction of frost heaves largely depends on the pore water pressure and hydraulic conductivity of frozen fringes, which are difficult to determine. The segregation potential model can avoid this problem; however, the conventional segregation potential is considered to be approximately unchanged at a steady state and only valid in an open system without dehydration in the unfrozen zone. Based on Darcy’s law and the conventional segregation potential, the segregation potential was expressed as a function of the pore water pressure at the base of the ice lens, the pore water pressure at the freezing front, the freezing temperature, the segregation freezing temperature and the hydraulic conductivity of the frozen fringe. This expression indicates that the segregation potential under quasi-steady-state conditions is not a constant in a closed system, since the pore water pressure at the freezing front varies with the freezing time owing to the dehydration of the unfrozen zone, and that when the pore water pressure at the freezing front is equal to that at the base of the ice lens, the water migration and frost heave will be terminated. To analyze the possibility of applying the segregation potential model in a closed system, a series of one-sided frost heave tests under external pressure in a closed system were carried out in a laboratory, and the existing frost heaving test data from the literature were also analyzed. The results indicate that the calculated frost heave was close to the tested data, which shows the applicability of the model in a closed system. In addition, the results show the rationality of calculating the segregation potential from the frost heaving test by comparing the potential with that calculated from the numerical simulation results. This study attempted to extend the segregation potential model to freezing soil in a closed system and is significant to the study of frost heaves.

Deformations and ground temperatures at a road embankment in northern Canada

The paper examines the behaviour of a highway embankment in an area of discontinuous permafrost about 18 km northwest of Thompson, Manitoba. Frequent maintenance has been required. Research involved site investigation, laboratory testing, installing instruments, data collection, and numerical modeling. The paper reports data from almost 3 years of observation. Measurements of ground temperatures suggest that formerly ice-rich foundation soil has thawed under the toe and side-slope. Approximate values of segregation potential have been back-calculated from observations of settlements and temperatures. Results provide insight into the nature and cause of deformations of the embankment.

Stress Analysis of Pipeline Subjected to Differential Frost Heave

Frost heave must be considered in cases where pipelines are laid in permafrost in order to protect the pipelines from overstress and to maintain the safe operation. In this paper, a finite element model for stress/strain analysis in a pipeline subjected to differential frost heave was presented, in which the amount of frost heave is calculated using a segregation potential model and considering creep effects of the frozen soil. In addition, a computational method for the temperature field around a pipeline was proposed so that the frozen depth and temperature variation gradient could be obtained. Using the procedure proposed in this paper, stress/strain can be calculated according to the temperature on the surface of soil and in a pipeline. The result shows the characteristics of deformation and loading of a pipeline subjected to differential frost heave. In general, the methods and results in this paper can provide a reference for the design, construction and operation of pipelines in permafrost areas.