~目前寒区桩基多采用钻孔灌注施工工艺，造成桩侧土体形成不同程度的粗糙面，且寒区桩基受冻融影响多产生间歇性失稳、融沉等工程病害，究其原因是土石混合体与混凝土界面受冻融影响抗剪强度损失、黏结劣化所致。因此，为探究冻融循环下不同粗糙度土石混合体-混凝土界面强度劣化机制开展了以下试验研究：（1）基于NMR核磁共振技术开展了不同冻融次数、含石率、粗糙度下土石混合体-混凝土核磁分层测试，探究各参数变化下界面层细观孔隙结构演化特征规律；（2）开展不同工况下土石混合体-混凝土界面室内直剪试验，探究各变量下界面抗剪强度及抗剪强度参数劣化规律，构建含有冻融次数、粗糙度、孔隙度等参数的界面抗剪强度劣化理论模型；（3）基于PFC数值模拟软件构建不同工况下的界面直剪模型，明晰界面剪切带裂隙发育全过程，探究剪切带颗粒旋转、位移特征对剪切带变形破坏特征的影响，从细观角度揭示土石混合体-混凝土试样界面宏观变形破坏机制，为寒区桩基承载力劣化提供理论参考。
针对冻融循环下土石混合体-混凝土界面层细观孔隙结构劣化问题。开展核磁共振分层测试发现：界面层存在3类孔径孔隙且随着冻融次数的增加3个特征T2谱均向右移动，孔隙体积增加使界面整体性被破坏；随含石率增加界面层介孔、大孔孔隙占比逐渐增加，孔隙总体积剧烈增大；受粗糙度影响界面孔隙结构变化复杂多样，由于混凝土对土石混合体凹槽的侵占及水泥浆液的渗入界面层小孔孔隙及大孔孔隙含量随粗糙度的增大而减小，介孔孔隙占比增大，孔隙分布向均一化趋势发展；随着含石率的增加微孔、大孔孔径占比逐渐减少，介孔孔隙占比增加且总体孔隙度增加量在逐渐降低。
针对冻融循环下土石混合体-混凝土界面强度损失特征问题，开展不同冻融次数、粗糙度、含石率、法向应力下土石混合体-混凝土界面室内直剪试验，探究界面力学参数变化特征。发现试样剪切应力-位移曲线均呈应变硬化型，抗剪强度受冻融影响显著呈迅速下降、小幅度反翘、缓慢下降三阶段，界面粗糙度不同产生的“树根桩”效应也不尽相同，体现为抗剪强度随界面粗糙度的增大而增大；黏聚力与界面强度变化趋势大致相同，粗糙度对高含石率内摩擦角影响程度较小。最终构建了含有虑冻融次数、粗糙度、界面孔隙度等参数土石混合体-混凝土界面抗剪强度冻融劣化损伤理论模型，经验证该模型能较好表征其劣化损伤特征。
针对土石混合体-混凝土界面剪切带变形破坏特征问题，基于PFC数值模拟软件开展不同工况下直剪模拟试验，模拟结果发现试样剪切破坏裂纹数量受冻融循环影响，随冻融次数增加最终破坏时剪切带裂缝数量也在逐渐增加，冻融作用对颗粒旋转及位移量影响较小；界面碎石赋存情况不同，剪切带厚度随含石率的增加而变厚，由于界面处碎石与混凝土的互锁作用，剪切带常常出现“绕石”现象；粗糙度为0时试样破坏多为滑移破坏，与其他粗糙度相对剪切破坏时剪切带破坏特征较为简单，破坏时颗粒旋转及位移量很小，随着粗糙度的增加剪切带破坏变得复杂多样。
针对土石混合体-混凝土界面抗剪强度变化特征机制问题，结合宏细观试验研究绘制不同影响因素下强度变化机制示意图。发现低含石试样由于冻融作用使土石混合体孔隙体积增大界面损伤劣化显著，土体与混凝土黏结力减小，抗剪强度降低；高含石试样由于冻融作用使土石混合体整体变得松散，剪切时界面处碎石与混凝土咬合力变小，抗剪强度表现出降低趋势；随着含石率及粗糙度的增加，界面处碎石的咬合、锁固作用强化，剪切面呈现出不规则的剪切带，且剪切带厚度不断增大，界面抗剪强度增大。以上研究成果不仅是对土石混合体-混凝土界面体系的有益补充，更可为寒区桩基工程建设、运营及维护提供有效指导。

~At present, most of the pile foundations in cold regions adopt the drilling and pouring construction technology, which causes the soil on the side of the piles to form rough surfaces of different degrees. Moreover, the pile foundations in the cold regions are affected by freezing and thawing, and many engineering diseases such as intermittent instability and thaw settlement are caused. It is caused by the loss of shear strength and the deterioration of the bond between the soil-rock mixture and the concrete interface affected by freezing and thawing. Therefore, in order to explore the deterioration mechanism of soil-rock mixture-concrete interface strength with different roughness under freeze-thaw cycles, the following experimental studies were carried out: (1) Based on NMR nuclear magnetic resonance technology, soil-rock mixture under different freeze-thaw times, rock content, and roughness was carried out. Body-concrete nuclear magnetic delamination test was carried out to explore the evolution characteristics of the mesoscopic pore structure of the interface layer under the changes of various parameters; (2) Indoor direct shear tests of the soil-rock mixture-concrete interface under different working conditions were carried out to explore the interface shear strength under various variables. and the deterioration law of shear strength parameters, and build a theoretical model of interface shear strength deterioration including freeze-thaw times, roughness, porosity, etc. The whole process of the development of cracks in the interface shear zone, explore the influence of particle rotation and displacement characteristics of the shear zone on the deformation and failure characteristics of the shear zone, and reveal the macroscopic deformation and failure mechanism of the soil-rock mixture-concrete sample interface from a mesoscopic perspective. The base bearing capacity degradation provides a theoretical reference.
Aiming at the deterioration of the mesopore structure of the soil-rock mixture-concrete interface layer under freeze-thaw cycles. Carrying out NMR layering test, it is found that there are three types of pores in the interface layer, and with the increase of freeze-thaw times, the three characteristic T2 spectra all move to the right, and the increase of pore volume destroys the integrity of the interface; The proportion of mesopores and macropores gradually increases, and the total pore volume increases dramatically; the interface pore structure changes complex and diverse due to the influence of roughness. The macroporous pore content decreases with the increase of roughness, the proportion of mesoporous pores increases, and the pore distribution develops toward a uniform trend; The porosity ratio increases and the overall porosity increase gradually decreases.
Aiming at the characteristics of strength loss of soil-rock mixture-concrete interface under freeze-thaw cycles, the indoor direct shear test of soil-rock mixture-concrete interface under different freeze-thaw times, roughness, rock content and normal stress was carried out to explore the change characteristics of interface mechanical parameters. . It is found that the shear stress-displacement curves of the samples are all strain hardening, and the shear strength is significantly reduced by freezing and thawing in three stages: rapid decline, slight inversion, and slow decline. The "root stump" effect caused by different interface roughness also The results show that the shear strength increases with the increase of the interface roughness; the cohesion force and the interface strength change in roughly the same trend, and the roughness has little effect on the internal friction angle with high stone content. Finally, a theoretical model of freeze-thaw deterioration damage of soil-rock mixture-concrete interface shear strength is constructed, which includes parameters such as freeze-thaw times, roughness, and interface porosity. It is verified that the model can better characterize its deterioration damage characteristics.
Aiming at the deformation and failure characteristics of the shear zone of the soil-rock mixture-concrete interface, direct shear simulation tests under different working conditions were carried out based on the PFC numerical simulation software. The number of fractures in the shear zone increases gradually when the final failure increases, and the freeze-thaw effect has little effect on the rotation and displacement of the particles; the occurrence of rubble at the interface is different, and the thickness of the shear zone becomes thicker with the increase of the stone content. The interlocking effect of the crushed stone and the concrete at the interface often causes the phenomenon of "wrap around the stone" in the shear band; when the roughness is 0, the failure of the sample is mostly slip failure, and the shear band failure characteristics are relative to other roughness. Relatively simple, the particle rotation and displacement are small during failure, and the shear band failure becomes complex and diverse with the increase of roughness.
Aiming at the characteristic mechanism of shear strength change at the soil-rock mixture-concrete interface, a schematic diagram of the strength change mechanism under different influencing factors was drawn based on the macro and mesoscopic experimental research. It was found that the pore volume of the soil-rock mixture was increased due to the freezing and thawing of the low rock-containing samples, and the interface damage was significantly deteriorated, the bonding force between the soil and the concrete was reduced, and the shear strength was reduced; The whole becomes loose, the occlusal force between the crushed stone and the concrete at the interface becomes smaller during shearing, and the shear strength shows a decreasing trend; The shear plane presents an irregular shear band, and the thickness of the shear band increases continuously, and the shear strength of the interface increases. The above research results are not only beneficial supplements to the soil-rock mixture-concrete interface system, but also provide effective guidance for the construction, operation and maintenance of pile foundation engineering in cold regions.

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