Engineering Behaviors Of Unsaturated Expansive Soils In The Xinqiao International Airport Area Of Hefei

Expansive clay is a typical unsaturated soil, which is sensitive to climate andmoisture. With the change of moisture, it behaves complex characteristics ofmulti-fissured, swelling and shrinkage, and mechanical property. It often results inlarger settlement and cracking damages to roadbed and buildings built on expansiveclays and huge economic losses. Therefore, with the increase of disasters inducedby unsaturated expansive clays, recently, engineering behaviors of expansive clayhas been a topic for the engineers. The runway project of Xinqiao internationalairport in Hefei was founded on expansive clays. To ensure the rational design offoundation treatment, and safe operation, research on the engineering properties ofunsaturated expansive clays in site are of great significance.Based on XRD, SEM and Compaction tests, composition content,microstructures and shrinkage-swelling characteristics are discussed for theunsaturated expansive clays in Xinqiao international airport project area.Settlement and swelling characteristics were further investigated according to theresults from field immersion test. Moreover, series of test were carried out todiscuss soil water characteristic and unsaturated strength property by means ofGDS triaxial stress system, and numerical simulations and comparisons wereconducted for the corresponding soil water characteristic curve (SWCC). Finallysoil water characteristics and mechanical performances of unsaturated expansiveclays in the engineering area were comprehensively analyzed and discussed. Somemain achievements of this dissertation are reached as follows:(1) Expansive clay is of medium or low shrinkage and swelling grade. Thecomposition of clay minerals composed mainly of montmorillonite and illite fromXRD tests.(2) The particles of undisturbed expansive clays contact closely with stackedpolymer from SEM tests. The larger aggregates were observed in plymers. Theedges of layered particles are irregular. This microstructure type is sensitive tomoisture, development of micro porosities and cracks.(3) Unconfined compressive strength, cohesion and internal friction angle of undisturbed expansive soil in project area almost not changed with the increase ofdepth. The optimum water content is16%. Immersion loading test results show thatthe expansion value weakened with the increase of depth on the field, and therebound phenomena of settlement were observed after soaking.(4) SWCC of undisturbed expansive clays in the engineering area is similar totypical SWCC. The intake value is about70kPa. The measured SWCC behaveddifferent characteristic during drying process and wetting process. The numericalsimulation analysis showed that the measured curve is in good agreement with theFredlund model.(5) The results from unsaturated strength tests show that volumetric strain,deviatoric stress and effective principal stress ratio increase with the increase ofaxial strain. Volumetric strains behave dilatancy during shear tests. For dryingprocess, the stress-strain relation shows phenomenon of strain hardening. The shearstrength increases with the increase of suction at the same cell pressure.These results will provide a basis for making a rational foundation treatmentplan for Xinqiao international airport, and also provide some reference value tosummarize the engineering behaviors of expansive clays in Hefei.