Bridges are structures built to span rivers, valleys, or other transportation lines, with their main function being to enable the smooth passage of vehicles and pedestrians. The main components of a bridge include the bridge span structure, bearing system, piers, abutments, and pier and abutment foundations. There are also many small components in the bridge structure, mainly related to the bridge's service functions, such as deck pavement, waterproof and drainage systems, railings, expansion joints, and lighting. Currently, the common types of bridge substructures are as follows:

1. Thin Walled Light Type Abutments

To save construction costs and avoid compressing the riverbed, thin-walled light-type abutments are often used for single-span small bridges. A support beam is installed under the two abutments, forming the entire bridge into a frame structure system. The passive earth pressure behind the two ends of the abutments is utilized to maintain stability.

2. Flexible Bent Abutments

Most bridges in urban modern transportation engineering construction adopt this type. The foundation piles of the piers and abutments are mostly pre - fabricated and driven. This bridge structure is commonly seen in bridge construction in areas with gentle water flow, which ensures the stability of the bridge substructure and avoids the adverse effects caused by water erosion.

3. Buried Pile Columns Abutments

This type of abutment is located on the bank and the abutment body is buried in the conical slope. There are two types: single row pile column and double row pile frame. When selecting this type of abutment, considering the maintenance of the subgrade's stability, the bridge length should not be compressed too much. The design should be strictly adhered to to meet the performance requirements.

4. Column Type Piers

Soft soil foundations are one of the challenges in bridge construction. They can significantly harm the bridge's structural performance and pose a risk to future transportation. Column - type piers not only have a simple structure but also a more advanced construction process, making them the best choice for bridge structures on soft soil foundations.

1. Defects in Bridge Foundations

1.1 Defects in Bridge Foundations

Bridge foundation defects generally refer to the settlement and uneven settlement of pile foundations. Foundation settlement caused by compaction and subsidence of the foundation soil is a common occurrence within a certain range. However, if it exceeds a certain limit, it will have harmful effects on the bridge. Bridge foundations constructed on soft soil foundations often experience uneven settlement due to soil compaction and subsidence, as well as fluctuations in groundwater levels.

1.2 Sliding of Bridge Foundations

The foundations of bridges often slide due to frequent flooding. For bridges on gravel riverbeds, during floods, the water flows around the piers and abutments significantly because of the obstruction of the piers and abutments. The water flow stabilizes when it reaches a certain depth where it can no longer move the gravel. Furthermore, when the abutment foundation is built on soft soil, if the height of the back fills soil behind the abutment exceeds a certain level and the foundation structure is not properly treated, the horizontal force acting on the back of the abutment will increase. This will lead to instability of the foundation, causing plastic flow and causing the abutment to move forward. When the forces on the upper and lower parts of the foundation are uneven, the abutment body also experiences uneven sliding, resulting in the foundation tilting. Abutment foundations that slide or tilt are mostly gravity - type abutments and inverted T - shaped abutments built on soft soil foundations.

2. Defects in Pier Bodies

Piers are the key structures of the bridge sub-part. The stress-bearing conditions of the piers directly impact the bridge's performance. When analyzing the stress calculation of piers, designers should focus on the calculation of vertical forces and horizontal forces at the top of the pier piles, as well as the mechanical effects of the basic bridge structure on the bridge sub-structure. For instance, when calculating the stress of piers, cracks caused by the destructive effects of seasonal water scouring are taken into account. Common cracks include horizontal cracks, vertical cracks, and network-shaped cracks. Due to external forces such as ship impacts and floating objects, piers and abutments will be damaged locally and concrete piers and abutments will detach and separate. Additionally, the materials of the piers and abutments will age over time. Generally, the defects of bridge piers and abutments include cracks, detachment, exposed steel bars, corrosion, and aging.

3. Insufficient Bearing Capacity of Piers, Abutments, and Foundations

Piers in the water, due to directly blocking the water, will not only experience general scouring but also local scouring, forming a local funnel-shaped riverbed. When the riverbed is a thick layer of gravel and cobblestones, the bored piles will be severely worn, and in more serious cases, the steel bars in the piles will be exposed. According to relevant literature, under low water levels, above the freezing line or near the scouring line, there is often zonal corrosion on the pier body or foundation. Around the foundation, the surface is relatively loose. In severe cases, concrete cavities may form.

4. Cracks Caused by Bridge Substructures Shrinkage

4.1 Temperature shrinkage

The performance of concrete materials will be affected by the external environment. When the temperature difference reaches a certain point, temperature difference shrinkage will occur, causing cracks in multiple positions of the bridge structure. The temperature change of the concrete itself can also cause temperature - difference shrinkage and deformation. For instance, due to the excessive concentration of heat of hydration inside the concrete, the temperature inside the material increases significantly while the external heat dissipates too quickly. The inconsistent internal and external temperatures create tensile stress, which easily leads to cracks.

4.2 Plastic shrinkage

When the surface water loss rate is too fast and combined with the air - drying effect of the external air, the concrete material is prone to plastic shrinkage, which is common before concrete solidifies. From the perspective of bridge construction, plastic shrinkage is greatly affected by weather, especially in dry, hot, and windy areas where the incidence of plastic shrinkage cracks in buildings is relatively high. If the strength of the concrete material does not meet the standard, its ability to resist air - drying and water seepage is weakened, and plastic shrinkage will be caused after damage.

4.3 Settlement shrinkage

Settlement shrinkage is closely related to the bearing capacity of the bridge foundation. Settlement shrinkage will occur when the gravity of the ground building structure exceeds the bearing range of the foundation. For example, uneven, soft soil, or uncompacted back - filled soil, or water immersion in the bridge's structural foundation, are all inducing factors for settlement shrinkage. This type of crack will change as the foundation structure changes. Especially for bridge buildings on soft soil foundations, the destructive power of the cracks will continue to increase. The cracks will only stabilize when the foundation deformation reaches a stable state.

To Wrap Up

In conclusion, as a crucial component of the bridge, the bridge sub structure faces numerous defects, including settlement and sliding of the bridge foundation, cracks, detachment, and aging of the pier (abutment) body, insufficient bearing capacity of the piers, abutments, and foundations, and various cracks caused by material shrinkage. These defects not only pose a threat to the structural safety of the bridge but also pose potential risks to the lives and property of passing vehicles and pedestrians. Therefore, strengthening the regular inspection and maintenance of the bridge sub - structure, promptly detecting and addressing defects, using advanced technical means and scientific management , and continuously optimizing the bridge design and construction technology are the keys to ensuring the long - term stable operation of the bridge and the smooth flow of traffic. In the next article, we will guide you through the daily maintenance and reinforcement of the bridge sub structure.