Author: Bentil, Obed Takyi
Title: Small strain stiffness of a compacted clay with different initial structures under cyclic thermo-hydro-mechanical loads
Advisors: Zhou, Chao (CEE)
Yin, Jianhua (CEE)
Degree: Ph.D.
Year: 2023
Subject: Laterite
Soil mechanics
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xix, 232 pages : color illustrations
Language: English
Abstract: Lateritic soils are chemically weathered materials extensively distributed in tropical and subtropical regions like Asia (India and China), West Africa (Ghana and Nigeria) and South America (Brazil). They have a high sesquioxide (iron and aluminium oxides) content that leads to considerable particle aggregations and affect soil microstructure. The aggregated structures have been shown to significantly affect the mechanical properties of lateritic soils differently from other soils, including their shear strength and compressibility. For instance, due to mechanical and thermal action, the possible loosely bonded particle aggregations may disintegrate into smaller particles. This feature makes it challenging to use their particle size distribution, index properties (e.g., plasticity index) and soil classification to solely predict their geotechnical engineering performance, giving it unique engineering qualities compared to other soils. Compacted lateritic soils are commonly used backfill material in the parts of the world mentioned above due to their favourable material properties. In the past studies of lateritic soils, researchers focused on their mechanical behaviour at large strains (above 1%), such as shear strength and dilatancy. The small strain stiffness has not been well studied especially when the soil is unsaturated and anisotropic, which is crucial for accurately predicting ground movement and geostructures’ serviceability limit state. Furthermore, lateritic soils in many geostructures (e.g., pavements, railway embankments and energy geostructures) experience complex working conditions, like daily and seasonal variations of temperature and suction. Therefore, there is an urgent need to improve the fundamental understanding of the small strain stiffness behaviour of unsaturated lateritic soils, considering the influence of compaction-induced anisotropy, soil microstructure and thermo-hydro-mechanical loading.
This study aims to reveal the small strain stiffness characteristics of compacted lateritic soils, which are widely used in geotechnical construction, along various thermo-hydro­mechanical paths. The principal objectives are to (1) obtain the complete set of cross-anisotropic elastic parameters and reveal the influence of initial microstructure on the evolution of stiffness anisotropy at saturated and isothermal conditions; (2) investigate the coupled effects of suction and initial microstructure on the stiffness characteristics of unsaturated specimens over a wide suction range; (3) reveal the influence of temperature and thermal cycles on the elastic shear modulus of saturated and unsaturated specimens; (4) develop a semi-empirical thermo-hydro-mechanical model for the small strain stiffness and apply it in the pavement analysis for investigating the influence of suction and thermal effects on the pavement performance.
To meet these objectives, this study tested a lateritic clay at saturated and unsaturated conditions. The specimens were compacted at different water contents and densities to produce various as-compacted microstructures and then tested along various thermo-hydro-mechanical paths. Both vertically and horizontally cut specimens were used to investigate the anisotropy of stiffness. Several advanced apparatuses were used in the testing, including a new temperature-controlled oedometer equipped with bender elements, a new suction and temperature-controlled triaxial apparatus with local strain measurements, bender elements and double-cell total volume change measurements. Soil suctions in the low (below 300 kPa) and high (above 4 MPa) ranges were controlled using the axis translation and vapour equilibrium techniques, respectively. In addition, a temperature range of 5 to 60°C was considered. The ranges of suction and temperature were selected with reference to some field monitoring studies on subgrade soils, which showed that temperature and suction fluctuate widely up to a depth of 2 meters. Moreover, microstructural analysis was used to interpret experimental results providing a linkage between micro and macro soil behaviour.
The experimental results at saturated conditions reveal that in comparison to other clays, the lateritic clay exhibits a lower dependency of shear modulus to confining pressure, a higher shear modulus at the same confining pressure and its stiffness degrades rapidly with strain. Due to its large-size aggregated microstructure caused by cladding of particles, the lateritic clay behaves like granular material, and the commonly utilized relationships between stiffness parameters and plasticity index does not fit well for lateritic clay. Additionally, the elastic shear modulus in the vertical plane is fairly higher than in the horizontal plane, and the anisotropy evolves during shearing. A complete set of cross-anisotropic stiffness parameters for effective and total stress analysis were obtained.
At unsaturated conditions, the shear stiffness increases non-linearly with increasing suction. The incremental rate reduces when suction increases. The non-linear response of the increment in stiffness and suction was observed for all different mean net stresses. This non­-linearity occurs because of the effects of particle aggregation and water retention behaviour. More importantly, the degree of anisotropy during shearing is affected by suction and net stress.
Temperature-controlled tests at saturated and unsaturated conditions consistently reveal that soil shear modulus is smaller after heating at a given stress and suction. The thermal effects can be attributed to the reduction of interparticle force while heating saturated clay, according to the double layer theories. The reduction in the air-water surface tension between particles and aggregates is another mechanism for unsaturated soil. Moreover, the reduction in shear modulus upon heating varies in magnitude and depends on the measuring plane, suggesting a potential thermally induced anisotropy. Furthermore, the shear modulus increases by 12% and 16% after four thermal cycles for overconsolidated and normally consolidated specimens, respectively, probably due to soil densification and particle rearrangement during the heating-cooling cycles.
The new results from the experimental study are useful for calibrating constitutive models and evaluating the serviceability of earth structures made of lateritic soils. Taking pavement as one example, a series of numerical simulations were carried out using KENPAVE software to investigate the influence of suction and temperature on rutting prediction and fatigue cracking. An increase in subgrade modulus due to suction causes was found to reduce rutting more than twice the damage caused by fatigue cracking. Similarly, the damage resulting from thermal effects on subgrade modulus is relatively higher for rutting than fatigue cracking.
Rights: All rights reserved
Access: open access

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