The thermo-poro-mechanical or thermo-hydro-mechanical (THM) coupling in clay related geomaterials is one of the most important issues in sustainable geotechnics. Clay soils have complex mineral composition and microstructure; thus, the study of thermo-poro-mechanical is complicated. Previous studies shed little light on the difference between a thermal plastic strain and thermally induced dehydration behaviors. In this study, we propose a thermodynamic-based constitutive model for describing the thermo-poro-elastoplastic behavior of saturated clay. The proposed model considers the effect of temperature variation, mechanical loading, and rate of loading on elastoplastic strains and dehydrations. The thermo-mechanical behavior is captured by using the thermodynamics laws and subloading surface plasticity. The hardening rule is established by using laws of physical conservation, energy dissipation and plastic flow. Dehydration behavior is considered using the laws of thermodynamics for chemical processes. A comparison between model predictions and experimental data for some clay soils with different geological origins is presented and a reasonable result is achieved.

Keywords: Thermodynamics, Thermo-elastoplastic model, Subloading surface, Clay bound water, Dehydration

The present study was conducted to develop a sustainable inorganic geopolymeric binder for subgrade stabilization, based on a non-hazardous wood-based fly ash generated in the pulp manufacturing mills. The alkali-silicate activation method at ambient temperature (⁓22°C) was employed for synthesizing pulp mill fly ash (PFA)-based geopolymer, by using sodium hydroxide (SH) and sodium silicate (SS) compounds. Initially, the optimal content of virgin PFA required to achieve the desired strength for weak silty soil subgrade was identified based on 28-day unconfined compressive strength (UCS) analysis. Further, the optimum formulation of PFA geopolymer was determined in terms of the UCS variations with mainly considering the different influencing parameters such as activator to ash ratio, SH to SS ratio, and SH molarity. The compacted mixtures of soil and PFA, with varying proportions of SH and SS were subjected to UCS test after 7 days, 14 days and 28 days of curing. In order to evaluate the underlying mechanisms of stabilization in the soil and PFA-geopolymer systems, additional microstructural observation of untreated and treated soils was carried out using SEM-EDS, FTIR and XRD tools.

The experimental results demonstrated substantial strength improvement of subgrade with 20% virgin PFA; moreover, the strength increment was proportional to PFA content and extent of curing. The soil treated with PFA-geopolymer exhibited relatively higher strength when compared with virgin PFA-treated counterpart. The optimum conditions for achieving higher rate of PFA geopolmerization was determined to be 5 molar SH solution, with activator to ash ratio of 1:1, and SH to SS ratio of 1:1. The microstructural observations also revealed the formation of an inorganic PFA-based geopolymer network on the soil surface with high concentrations of calcium and silica, which resulted in the significant strength enhancement of subgrade.

Post-fire soils are vulnerable to higher rates of erosion and surface runoff primarily due to loss of protective surface cover combined with degradation of soil structure and increased water repellency. This study aims to develop a sustainable, eco-friendly and cost-effective strategy for post-fire soil restoration by utilizing a non-hazardous industrial waste product called pulp mill fly ash (PFA). Initially, this study will assess the impacts of wildfire severity on the critical forest soil properties located in the Okanagan region of British Columbia, by conducting comparative physico-chemical, mineralogical, and microstructural analysis of burnt and unburnt soils. Further, the feasibility of utilizing PFA for improving the mechanical and hydrological properties of burnt soil will be evaluated with various laboratory tests conducted on burnt soils treated with different PFA contents. The mechanical improvement will be determined in terms of variations in the soil aggregate stability, while the hydrological properties include soil water characteristics, infiltration rate and hydraulic conductivity. The underlying mechanisms which caused improvement following PFA treatment will be investigated through detailed microstructural analysis using characterization tools and techniques such as FTIR, SEM-EDS and XRD. The results revealed that PFA treatment improved soil aggregate stability by 6.4% and 14.9% with 5% and 10% PFA content, respectively, over 14 days of curing period. The optimum PFA content of 10% caused significant improvement of soil water holding capacity, as well as it reduced percentage of macropores and restored hydraulic characteristics. An increased aggregate stability in PFA treated soil suggested formation of new cementitious compounds predominantly consisting of calcium-silicate bridges, which was also revealed in microstructural analysis. Based on this study, PFA can be considered to be a potential candidate for simple, energy-efficient and environmentally-friendly treatment method to improve and restore fire-burnt soil properties to pre-fire levels.

Wastewater treatment lagoons in permafrost rich areas can be complicated and typically use frozen ground as a containment system for the waste and leachate, particularly where importing competent borrow material may be too costly. KGS Group has been working on developing the concept of using conventional closed loop geothermal systems in the sediments below the base of the lagoon to allow a natural winter air refrigerated solution of calcium chloride (anti-freeze) and water to be circulated below the lagoon to freeze the soil below the lagoon. This freeze-back of thawing permafrost below the lagoon base would extend the life of the lagoon. This concept could be implemented post initial construction of a lagoon using well-developed horizontal directional drilling techniques. Similar systems have been implemented for the addition of heat to the subsurface in the summer for winter heating. This paper assesses the effectiveness of the geothermal freeze back system using a heat transfer model developed in COMSOL Multiphysics, a finite element software. The model evaluates variable HDPE pipe spacings, pipe diameters and flow velocities with the circulation of the calcium chloride solution to temperatures as low as -30°C. Piping may be sized to have very low friction losses and therefore, low energy consumption for limited additional operational costs. The results of this assessment will be valuable in understanding the design and costs of a conventional geothermal freeze-back system for large developments over permafrost.