| The increment of moisture content and deterioration of local compaction degree in the operating subgrades are common issues concerned by the road engineers in China.Especially in the downstream area of the Yellow River(Shandong),the characterized alluvial silt is highly sensitive to the moisture from the environment,leading to a wetting and softening subgrade during the service life.Current subgrade design method and backfilling material modification technology are insufficient to deal with the wetting and softening subgrades,which are subjected to continuous environmental loads such as dry-wet cycles,freeze-thaw cycles and groundwater level change in operating period.Therefore,new ideas,materials and methods are strongly expected to achieve the humidity controlling and performance maintenance of subgrades in the downstream area of Yellow River.Focusing on the technology of humidity control and performance maintenance of silt subgrade in the downstream area of Yellow River,a weather-resistant absorbent polymer(bentonite super absorbent polymer,BT-SAP)for humidity control in silty subgrade is innovatively developed based on the interdisciplinary integration in this thesis.Theoretical analysis and laboratory tests are employed to explain the macro and micro mechanism of BT-SAP mixtures to reduce the environmental sensitivity of silt.Composite subgrade structures employing BT-SAP mixtures with full section are designed in meteorological model tests,and the humidity control mechanism and performance maintenance technology of silt subgrade in the downstream area of Yellow River is innovated combined with the feasibility and reliability analysis.The main conclusions are drawn as follows:(1)Bentonite with high polymer network node strengthening ability and hydroxyethyl methacrylate(HEMA)with excellent self-crosslinking ability were introduced to improve the aging performance and environmental stability of absorbent polymer.Based on the water bath polymerization method,the effects of synthetic environment(including polymerization temperature,and neutralization degree)and raw materials(including crosslinking agent,initiator,HEMA and bentonite)on the absorption rate,weather resistance and reusability of BT-SAP were investigated.The optimum design for the BT-SAP parameters was determined by the orthogonal tests,provided as a new alternative for humidity control in silty subgrades.(2)Physical and mechanical properties of BT-SAP improved soil were obtained through laboratory tests,including the limit moisture content,compaction parameters,strength parameters and soil-water characteristics.A method to evaluating the moisture distribution,volume change and crack characteristics was put forward.The effect of BT-SAP on reducing moisture sensitivity was evaluated based on the crack development and volume change subjected to the drying-wetting cycles.The optimum dosage range of BT-SAP in the silty subgrade is determined.(3)With the employment of micro-focus computed tomography(CT)technology and image processing algorithm,the 3D graphic information analysis of BT-SAP improved soil was achieved.The characteristics of solid particles and fluid clusters of BT-SAP improved soil were analyzed in microscale.Furthermore,the micro-mechanism of BT-SAP hydrogel on controlling the wetting and softening of silty soil was investigated.The evolution of 3D pore structure(including pore size,pore volume ratio and pore connectivity)of BT-SAP improved soil was quantitatively analyzed by using pore network model,and the micro-mechanism of BT-SAP on inhibiting the formation and development of macropores was explained.(4)24 groups of soil column tests were conducted to observe the moisture migration with unidirectional freezing and bidirectional melting.Subjected to a certain temperature gradient,the effects of water supplement method,BT-SAP content and freeze-thaw cycle sequences on moisture migration were revealed.Based on the temporal and spatial distribution of moisture content at the bottom and top of soil column,the critical thickness of BT-SAP in soil was proposed.In addition,according to the moisture state of the frozen surface,the residual ice content in the frozen zone and the freezing condition on the column top,the mechanism of BT-SAP on inhibiting moisture migration in the subgrade under temperature gradient was clarified.(5)1200 groups of dynamic resilient modulus(MR)were obtained through 225 groups of dynamic triaxial tests.The effects of stress state,initial moisture content,BT-SAP content and freeze-thaw cycle sequences on MR were compared and analyzed.An empirical prediction model of MR for BT-SAP improved soil considering freeze-thaw cycle was proposed,and compared with the GEP model,ANN model and classical models.With the quantification of the sensitivity of MR to various influencing factors,a design control index of BT-SAP improved soil strength was proposed.Combining the pore evolution and freezing temperature,the mechanism of BT-SAP on maintaining performance under freeze-thaw cycle was investigated.(6)A meteorological environment simulation test platform,which can simulate the high/low temperature,rainfall/snow and sunlight environment,was designed and employed in a series of largescale model tests.Three structural subgrades(including raw,layered improved and wrapped improved)were considered.The evolution of moisture content,compaction degree on the top of subgrade and deformation modulus of subgrade subjected to full cycle condition was investigated,and the practical application effect of BT-SAP was evaluated.A new full section composite subgrade with humidity control and performance maintenance of BT-SAP was proposed.According to the requirement of highway subgrade construction,the application method of subgrade composite structure is put forward. |
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