由于基础竖向不均匀沉降或水平方向位移，使结构中产生附加应力，超出混凝土

Due to the uneven vertical settlement or horizontal displacement of the foundation, additional stresses are generated in the structure, which exceed the tensile capacity of the concrete structure, resulting in structural cracks. The main reasons for uneven foundation settlement are: (1) Insufficient geological survey accuracy and inaccurate test data. Design and construction without fully grasping the geological conditions is the main cause of uneven foundation settlement. For example, for bridges in hilly and mountainous areas, the drill holes are too far apart during the survey, and the foundation rock surface is undulating and large, and the survey report cannot fully reflect the actual geological conditions. (2) The foundation geology is too different. For bridges built in mountain valleys, the geology and slopes of the river ditch vary greatly. There is even a weak foundation in the river ditch, and the foundation soil has uneven settlement due to different compressibility. (3) The structural load difference is too large. Under the condition of relatively consistent geological conditions, when the foundation load of each part is too different, it may cause uneven settlement. For example, the middle part of a box-shaped culvert with high fill is larger than the load on both sides. The settlement will be larger than on both sides, and the box culvert may crack. (4) The types of structural foundations vary greatly. In the same bridge, mixed use of different foundations such as enlarged foundation and pile foundation, or simultaneous use of pile foundation but large difference in pile diameter or length, or simultaneous use of enlarged foundation but large difference in base elevation may also cause uneven foundation settlement . (5) The foundation of phased construction. When new bridges are built near the original bridge foundation, such as the left and right half of the expressway bridge built in stages, the reconsolidation of the foundation soil caused by the new bridge load or foundation treatment may cause large settlement to the original bridge foundation. (6) Frost heave of foundation. The foundation soil with higher moisture content under the condition of lower than zero degrees swells due to freezing; once the temperature rises, the frozen soil melts and the foundation sinks. Therefore, the freezing or melting of the foundation can cause uneven settlement. (7) When the bridge foundation is placed in bad geology such as landslide body, karst cave or active fault, it may cause uneven settlement. (8) After the bridge is completed, the original foundation conditions will change. After most natural foundations and artificial foundations are immersed in water, especially the special foundation soils such as plain fill, loess, and expansive soil, the strength of the soil body decreases in contact with water and the compression deformation increases. In soft soil foundation, because of people<br>Pumping or dry season causes the groundwater level to drop, the soil layer of the foundation reconsolidates and sinks, and the buoyancy to the foundation decreases, the negative friction increases, and the foundation load increases. Some bridge foundations are buried too shallow, and the foundations may be displaced due to flooding and scouring. Changes in ground loading conditions, such as a large amount of waste, sand and gravel piled up near the bridge due to landslides and landslides, the soil layer within the bridge site may be compressed and deformed again. Therefore, changes in the original foundation conditions during use may cause uneven settlement. For arch bridges and other structures that generate horizontal thrust, insufficient knowledge of the geological conditions, unreasonable design and destruction of the original geological conditions during construction are the main reasons for horizontal displacement cracks.

Due to uneven vertical subsidion or horizontal displacement of the foundation, additional stress is generated in the structure, which exceeds the pull resistance of the concrete structure and causes the structure to crack. The main reasons for the uneven subsidon of the foundation are: (1) the accuracy of geological survey is not enough, and the test data are not allowed. Design and construction without fully grasping the geological situation, which is the main reason for uneven subsidon of the foundation. For example, the hilly area into the mountainous area bridge, drilling gap is too far, and the foundation rock surface is undulating and large, the survey report can not fully reflect the actual geological situation. (2) The quality of the ground base is too different. Bridges built in mountain valleys, the geology of the ditches and hillsides vary greatly, and there are even weak foundations in the ditches, which cause uneven subsidon due to different compressions. (3) The structural load difference is too large, under the condition that the geological situation is more consistent, when the difference between the basic loads of each part is too large, it may cause uneven subsidion, such as the middle of the high-filling box-shaped culvert is bigger than the load on both sides, the central subsidion is bigger than the two sides, the culvert may crack. (4) The basic types of structure vary greatly. In the same bridge, the mixed use of different foundations such as the expansion of the foundation and pile foundation, or the use of pile foundation but pile diameter or pile length differences, or the use of expanded foundation but large differences in substrate height, may also cause uneven foundation subsidon. (5) The basis for phased construction. When a new bridge is built near the foundation of the original bridge, such as the left and right half of the highway built in stages, the re-consolidation of the foundation soil caused by the load of the new bridge or the foundation treatment may cause a greater settlement of the original bridge foundation. (6) The foundation freezes and swells. Under conditions below zero, the soil with a high water content expands due to freezing, and once the temperature rises, the permafrost melts and the foundation sinks. Therefore, the freezing or melting of the foundation can cause uneven subsidion. (7) Uneven subsidion may be caused when the bridge foundation is placed in poor geology such as landslides, caves or active faults. (8) After the bridge is completed, the original foundation conditions change. After most natural and artificial foundations are immersed in water, especially special foundation soils such as soil filling, loess and expansion soil, soil strength decreases and compression deformation increases. In the soft land base, because of people<br>The working pumping or dry season causes the water table to drop, the foundation soil layer to re-solidify and sink, and at the same time to reduce the buoyant force of the foundation, the negative resistance increases, the base load increases. Some bridge foundations are over-buried, washed away by floods, dug, and the foundations may shift. Changes in ground load conditions, such as near the bridge due to landslides, landslides and other reasons piled up a large number of waste, sand and stone, the bridge site range soil layer may be compressed and deformed again. Therefore, changes in the original foundation conditions during use may result in uneven subsiding. For structures such as arch bridges that produce horizontal thrust, insufficient knowledge of geological conditions, unreasonable design and destruction of the original geological conditions during construction are the main reasons for horizontal displacement cracks.

Due to the uneven vertical settlement or horizontal displacement of the foundation, the additional stress in the structure exceeds the tensile capacity of the concrete structure, resulting in structural cracking. The main reasons for the uneven settlement of foundation are: (1) the accuracy of geological survey is not enough and the test data are not accurate. It is the main reason for the uneven settlement of foundation to design and construct without fully mastering the geological conditions. For example, for bridges in hilly areas, the distance between boreholes is too far, and the foundation rock surface fluctuates greatly, so the survey report can not fully reflect the actual geological situation. (2) The geological difference of foundation is too big. For bridges built in mountain valleys, the geology and hillside of gullies vary greatly. There are even weak foundations in the gullies. The uneven settlement of foundation soil is caused by different compressibility. (3) If the structural load difference is too big, under the condition of relatively consistent geological conditions, the foundation load difference of each part may cause uneven settlement. For example, the load in the middle of high fill box culvert is larger than that on both sides, and the settlement in the middle is larger than that on both sides, and the box culvert may crack. (4) The types of structural foundation vary greatly. In the same joint bridge beam, when different foundations such as enlarged foundation and pile foundation are mixed, or when pile foundation is used at the same time but the difference of pile diameter or pile length is large, or when the expanded foundation is used at the same time but the difference of base elevation is large, the uneven settlement of foundation may also be caused. (5) The foundation of phased construction. When a new bridge is built near the original bridge foundation, such as the left and right half of the expressway bridge constructed by stages, the foundation soil re consolidation caused by the new bridge load or foundation treatment may cause large settlement of the original bridge foundation. (6) Frost heaving of foundation. Under the condition of below zero, the foundation soil with higher moisture content will expand due to freezing; once the temperature rises, the frozen soil will melt and the foundation will sink. Therefore, the freezing or thawing of the foundation can cause uneven settlement. (7) When the bridge foundation is placed in landslide, karst cave or active fault, uneven settlement may be caused. (8) After the completion of the bridge, the original foundation conditions change. Most of the natural foundation and artificial foundation are immersed in water, especially the special foundation soil such as plain fill, loess and expansive soil. In the soft soil foundation, because of people<br>In the dry season or pumping, the groundwater level drops and the foundation soil layer consolidates and sinks again. At the same time, the buoyancy of the foundation decreases, the negative friction increases, and the foundation load increases. Some bridge foundations are too shallow to be scoured and excavated by flood, and the foundation may be displaced. Due to the change of ground load conditions, such as a large amount of waste rock, sand and gravel piled up near the bridge due to collapse, landslide and other reasons, the soil layer within the bridge site may be compressed and deformed again. Therefore, the change of original foundation conditions during the service period may cause uneven settlement. For arch bridge and other structures that produce horizontal thrust, the main reasons for horizontal displacement cracks are insufficient understanding of geological conditions, unreasonable design and destruction of original geological conditions during construction.