土体在长期的季节性干湿交替环境下，内部结构受到损伤，进而降低土体强度，危害边坡稳定性。而植物根系可以有效固持边坡表层土壤，起到防止浅层边坡破坏的作用。为研究干湿循环作用下根土复合体强度特性与其微观结构损伤机制研究，本文选取草本植物紫花苜蓿为研究对象，采用PVC管人工种植获取紫花苜蓿根系-黄土复合体试样，通过开展干湿循环试验、三轴试验、CT扫描及核磁共振测试，研究了干湿循环作用下不同生长期根土复合体抗剪强度与微观结构的演化规律；并分析了干湿循环作用下根土复合体抗剪强度时间效应机理。本研究得出的主要成果如下：

（1）干湿循环作用影响下，无论是素土还是根土复合体，其有效黏聚力均与干湿循环次数呈显著负相关，且相对于素土，根土复合体有效黏聚力衰减幅度相对较低。随生长期增加，根土复合体有效黏聚力呈明显增加趋势，说明植物根系对土体强度的提高具有显著的正向作用，当植物生长至一定程度时，即可弱化干湿循环对土体的劣化作用。

（2）未经历干湿循环时的素土与根土复合体试样，其内部主要为小孔径且相互独立的孔隙，大孔径的、连通性好的孔隙数量占比较小。在经历不断的干湿循环过程后，微小孔隙逐渐减少，中大孔隙及裂隙逐渐增多；且随生长期增加，植物根系逐渐发育，也使得试样内部微小孔隙逐渐联通并转变为较大的、连通性更好的孔隙或裂隙。

（3）分析了干湿循环作用下根土复合体强度特性及微观结构的关系，干湿循环过程引起了微观结构的变化，进而导致不同生长期根土复合体有效黏聚力明显劣化，随生长期增加，劣化程度相对减弱，即干湿循环下根土复合体抗剪强度具有明显的时间效应；在此基础上提出了干湿循环下根土复合体抗剪强度时间效应预测模型。

Under the long-term seasonal dry-wet alternating environment, the internal structure of soil is damaged, which further reduces the strength of soil and endangers the stability of slope. The plant roots can effectively hold the surface soil of the slope and prevent the shallow slope from being destroyed. In order to study the strength characteristics and microstructure damage mechanism of root-soil complex under the action of dry-wet cycle, alfalfa was selected as the research object, and the root-loess complex sample of alfalfa was obtained by artificial planting with PVC pipe. Through dry-wet cycle test, triaxial test, CT scanning and nuclear magnetic resonance test, the evolution law of shear strength and micropores of root-soil complex at different growth stages under the action of dry-wet cycle was studied. Considering the influence of dry-wet cycles and growth period, a prediction model of shear strength of root-soil composite under dry-wet cycles with time is proposed. The main results of this study are as follows:

(1) Under the influence of dry-wet cycle, the effective cohesion of both plain soil and root-soil complex is negatively correlated with the number of dry-wet cycles, and the attenuation range of effective cohesion of root-soil complex is relatively low compared with plain soil. With the increase of growth period, the effective cohesion of root-soil complex shows an obvious increasing trend, which shows that plant roots have a significant positive effect on the improvement of soil strength. When plants grow to a certain extent, the deterioration of soil by dry-wet cycle can be weakened.

(2) In the composite sample of plain soil and root soil, which has not experienced the dry-wet cycle, there are mainly independent pores with small pore size, and the number of pores with large pore size and good connectivity is relatively small. After the continuous dry-wet cycle, the micro-pores gradually decrease, and the large-sized pores and cracks gradually increase; And with the increase of growth period, the root system of the plant gradually develops, which also makes the tiny pores in the sample gradually communicate and transform into larger pores or cracks with better connectivity.

(3) The relationship between the strength characteristics and microstructure of root-soil composite under the action of dry-wet cycle is analyzed. The dry-wet cycle causes the change of microstructure, which leads to the obvious deterioration of the effective cohesion of root-soil composite in different growth periods. With the increase of growth period, the deterioration degree is relatively weakened, that is, the shear strength of root-soil composite under dry-wet cycle has obvious time effect. On this basis, a prediction model of time effect of shear strength of root-soil composite under dry-wet cycle is proposed.

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