Study On The Mechanism Of Lime Stabilized Low Embankment And Its Application In Seasonal Frozen Region On Song-nen Plain

Relying on the program ?Research on the Key Technologies for Construction of Road Low Embankment in Seasonal Frozen Region‘ and actual project, this study investigated physical indexes, mechanical indexes, pavement performance and mechanism of lime stabilized soil, aiming to improve the construction technology for low embankment of over-wet soil road sections in seasonal frozen region on Song-Nen Plain where sandstone materials are insufficient. The achievements of this study included:1. Using the plain soil obtained from the representative road sections and lime stabilized soil, laboratory experiments were conducted to investigate the influence of the mixing ratio of lime on physical and mechanical indices of lime stabilized soil. The results showed that the subgrade soil of Changchun-Shuangliao Highway is the common silty clay in Jilin province, consisting of mainly small-size particles. Because this type of soil contains a great amount of silt and clay particles with the former as a dominator and consists of a high content of illite/smectite(I/S) mixed layer, the subgrade soil in the study area has a large amount of frost heaving and poor freeze-thaw stability. The addition of lime into the soil improved the optimum water content, water content range for construction, specific gravity of soil, and particle size, resulting in reductions of soil plasticity index(PI), maximum dry density(MDD), and clay particles content. The main failure of lime stabilized soil was brittle failure. The compressive strength could be enhanced by increasing the lime mixing ratio and prolonging the curing time. With the increasing lime mixing ratio, the peak strength, shear strength and cohesion force also increased. However, from the perspective of the shear strength, lime should not be added at a level higher than 7%. The subgrade soil added with lime had a significantly higher California Bearing Ratio(CBR) and large reductions in the amounts of soil swelling and water absorption.2. Freeze-thaw tests were performed to analyze the changing patterns of frost heaving amount and strength of lime stabilized soil under cyclic freeze-thaw conditions and identify the optimal range of the lime mixing ratio. The results showed that, the frost heaving rate first increased and then decreased with the increase of the freeze-thaw cycles, peaking at 4-6 cycles, and the rate first declined and then remained stable with the increasing lime mixing ratio, indicating that the addition of lime effectively inhibited the frost heave of subgrade soil. In addition, the unconfined compressive strength decreased with the increasing freeze-thaw cycles and tended to be stable after 5-7 cycles. Thus, the mechanical indices of lime stabilized soil after 5-7 freeze-thaw cycles could be taken as design parameters. The analysis of shear strength and CBR of lime stabilized soil before and after cyclic freeze-thaw treatment showed that the optimal lime mixing ratio for low embankment construction of Changchun-Shuangliao Highway was 5-7%. The resilient modulus test confirmed that, even at the least favorable condition(freezing-thawingâ†'24 h of water soakingâ†'freezing-thawing), the recommended mixing ratio could still meet the standards required in the design guidelines for low embankment infilling materials.3. To validate the technical feasibility of using lime stabilized soil for construction of low embankment, the General Packet Radio Service(GPRS) wireless public network technology and remote automatic acquisition system were adopted to monitor dynamic variations of subgrade settlement, frost heaving amount, temperature and volumetric water content for both lime stabilized soil and hill-skill soil low embankments in the testing road section of Changchun-Shuangliao Highway under conditions of sandstone material insufficiency. The results indicated that the lime stabilized soil low embankment had a slightly greater amount of frost heaving and superior thermal and water insulation performance than that of hill-skill soil low embankment. In addition, the daily fluctuations of temperature and volumetric water content were relatively slight in both two types of low embankments. Based on the decreasing pattern of the volumetric water content in subgrade soil, we deduced that the strength of lime stabilized soil continuously increased over time. The lime stabilized soil subgrade had a maximal frozen depth of 1.6 m, smaller than that of 1.8 m for hill-skill soil subgrade. The above findings indicated that, in seasonal frozen region on Song-Nen Plain where sandstone material is insufficient, using lime stabilized soil as a substitute for low embankment construction is feasible.4. The concept of the strengthened story and typical pavement structure of low embankment were proposed. Based on the related theories of multi-layer elastic layered systems, this study investigated the influence of the strengthened story on the mechanical response of subgrade pavement structure and provided the parameter range for the design of the strengthened story. The results demonstrated that, with the increasing thickness and modulus of the strengthened story, the vertical compressive stress on the subgrade surface, the x-direction horizontal stress, y-direction horizontal stress, and dynamic deviator stress in the subgrade, the subgrade working area depth, and subgrade surface deflection all decreased, but the subgrade strength increased. In addition, as the thickness and modulus of this layer increased, the pavement surface deflection, base bottom tensile stress, subbase bottom tensile stress, base bottom tensile strain, and subbase bottom tensile strain all decreased, resulting in a ?multiplied‘ fatigue life of the pavement structure. Therefore, base on comprehensive analysis, this study recommends that the thickness of the strengthened story should be within 20-60 cm, and the modulus should be range from 200 to 400 MPa.5. Through micro-structural qualitative and quantitative analyses of lime stabilized soil, this study elucidated the micro-structural changing pattern of lime stabilized soil with freeze-thaw cycles, thereby verifying the feasibility of using lime stabilized soil in low embankment construction in seasonal frozen region. The scanning electron microscopic(SEM) results showed that, lime stabilized soil with 5-7% lime had the highest compactness under cyclic freeze-thaw conditions, which validated the optimal mixing ratio of lime suggested by results of macro-mechanical tests. Observation of micro-structure(5000× magnification) of the lime stabilized soil revealed the enwrapping of Ca(OH)2 flaky hexagonal structure and meshy gelatinous mass on soil particles and the presence of meshy substances suspected to be hydrated calcium silicate. The gelatinous mass between particles formed bridge-like connections. Under an upright universal microscope, large gelatinous ribbon-like matters and transparent gelatinous substance on soil particles were observable. Quantitative analysis on the micro-structure of lime stabilized soil after cyclic freeze-thaw treatment using Image-Pro Plus 6.0 revealed the changing pattern of structural units(sizes, orientability, and average circularity), pores(sizes, orientability, and average circularity), and area ratio, thereby clarifying the mechanisms underlying the superior compactness and freeze-thaw stability of lime stabilized soil than plain soil.6. Through particle size, specific surface area, and pore structure analyses, this study elucidated micro-mechanisms underlying freeze-thaw stability improvement of lime stabilized soil. The addition of lime increases distribution range of soil particles, leading to a decrease of slit and clay particles in soil. Moreover, the lime addition decreases the soil particle specific surface area and therefore lowers surface adsorption capacity and cationic exchange ability. As a result, water-holding capacity of soil decreases, which suppresses the formation and migration of film water. In addition, the cumulative pore volume of soil particles in lime stabilized soil reduces, resulting in a decrease of the volume percentage of micro-pores with a size under 2 nm, which reduces adhesion of film water.7. Composition analysis revealed that lime stabilized soil is mainly comprised of quartz, albite, anorthite, vermiculite, muscovite, etc. The diffraction peak intensities of quartz(Q) and illite/smectite(I/S) mixed layer decrease with the increasing amount of lime added into soil. It is suspected that hydrated silicates or hydrated aluminates are formed in the lime stabilized soil containing 7% of lime. The reaction between lime and soil can generate new substances, such as Ca(OH)2 and Ca CO3, the majority of which is not well crystallized and thus presented in the form of non-crystalline or poorly crystallized gelatin. These new substances contribute to the strength improvement of lime stabilized soil.