With the development of municipal bridges, the phenomenon of bump at the bridge head has attracted more and more attention of bridge engineers and technicians, and has become the focus of quality control of completed bridges. Bumping at the bridge head not only affects the comfort and safety of driving, but also easily leads to traffic accidents when driving at high speed. Moreover, vehicles traveling at high speed have a strong impact on the bridge head, which will have an additional impact on the bridge and pavement structure, aggravate the damage of the bridge pavement and abutment expansion devices, and directly affect the service life of municipal bridges. Therefore, it is particularly important to solve the problem of bump at the bridge head of municipal bridge reasonably.
1 Cause Analysis of Vehicle Jumping at Bridge Head
The main reasons for bump at bridge head are structural stiffness difference, uneven settlement of embankment, design defects and inadequate construction quality control. First of all, the abutment itself is rigid, and the compression of abutment itself is very small, which belongs to elastic deformation, while the stiffness of embankment itself is very small, inelastic deformation is very large, and the compression of subgrade itself is also very large, so the compression and settlement of subgrade at abutment will cause the dislocation of abutment back road surface and bridge deck, and the settlement of foundation will also cause dislocation, which will inevitably lead to bump at bridge head; Secondly, due to the erosion of road path water and traffic load, cavities are gradually formed at the back of abutment, and the loss of embankment filler at the bridgehead and the lateral displacement of slope soil will lead to bump at the bridgehead; Thirdly, the wrong bridge end transition slab and improper pillow block design, unreasonable subgrade filler and concrete are damaged and worn under the repeated action of traffic load, which will also affect the normal running of vehicles; Finally, poor construction technology quality, unqualified compaction quality and other factors will also cause bump at the bridge head.
The settlement difference caused by stiffness difference is 1. 1
Abutment foundation is generally rigid foundation (such as pile foundation, enlarged foundation, etc.). ), its compression deformation and settlement are very small, so the settlement of abutment after operation can be ignored. However, the subgrade at the bridgehead is plastic, and the back fill of abutment is generally filled with the best water content. No matter what measures are taken in the construction, it is impossible to completely eliminate the gaps between filler particles. Under the repeated action of vehicle driving load, abutment backfill will produce compression settlement, which will take a long time to stabilize after opening to traffic. In engineering practice, even if the construction technology meets the requirements and the degree of compaction meets the requirements, settlement will inevitably occur with the passage of time because of the high fill behind the abutment. Sometimes, the filling load behind the abutment will produce additional pressure on the basement, and in serious cases, it will make the abutment lean backwards, resulting in uneven settlement, which will endanger the driving safety. For example, the approaches of Nanhe Bridge and Xinbeihe Bridge on Zhang Chi Line have been open to traffic for about 10 years, and the maintenance department has to carry out major and medium-sized repairs every year, using asphalt as the leveling layer, but the settlement cannot be ignored.
1.2 subgrade soil loss
Pavement water infiltrates into subgrade, causing subgrade soil to soften, and soil erosion causes abutment subgrade approach to sink; Backfill is not timely, resulting in water accumulation, resulting in insufficient compaction of backfill soil at the bridge head. Abutment is generally equipped with a bridge expansion device, and rainwater will penetrate into the abutment back filler along the expansion device, which will corrode and soften the soil filler on the abutment back, resulting in the deformation of the filler body, but it will not cause obvious damage to the rigid foundation of the abutment. In this way, under the impact of vehicle load, the settlement of bridgehead subgrade becomes inevitable. Furthermore, if the waterproof and drainage works on both sides of embankment soil are not perfect, poor drainage will easily lead to subgrade soil loss, resulting in subgrade soil settlement and bump at the bridge head.
1.3 is poorly designed.
Some joints are designed at the junction of abutment and embankment, while others are designed as continuous pavement. Due to the uneven settlement of subgrade, cracks will inevitably form in the course of use. Therefore, under the condition that the foundation has not been thoroughly treated and the settlement has not been stabilized, the connection between the abutment structure and the approach road should be carefully considered, and the concrete slab cannot be directly poured without quality assurance. For example, the approach roads on both sides of Ronghua Bridge in Jieyang, Guangdong Province, because the two sides of Rongjiang River belong to soft soil foundation, the settlement of the approach roads on both sides is long-term. If the transitional pavement is set first, the original design pavement will be laid after the embankment settlement is basically completed.
Usually, in the design, more attention is paid to the difference of settlement at the bridge head due to the change of stiffness, but less attention is paid to the change of the longitudinal slope of the approach road caused by the settlement after the bridge operation, which leads to the change of the longitudinal slope at the bridge head and the formation of platform drop at the abrupt point, which leads to the occurrence of vehicle jumping.
1.4 construction quality control is lax.
In order to pursue the construction progress, some construction units do not strictly follow the construction regulations and reasonable working procedures. For example; The foundation pre-filling treatment in bridge embankment and cone slope is not complete. The filling speed of abutment back is too fast, and there is not enough time for full consolidation; The thickness of the backfill layer at the back of abutment is too large, and the degree of compaction can not meet the requirements; When mechanical compaction is used, it is difficult for the back side of abutment wall and the inner side of wing wall to meet the design density requirements due to the limitation of topography, working surface and machinery. The plane shape of the fill near the bridge deck is irregular, and the real-time compaction by artificial pressure can not meet the design requirements. The slope protection or retaining wall in front of the platform is not built in time, which leads to soil slip, affects the operation effect of compaction machinery, and even harms the bridge foundation in serious cases; The "three-point method" (layered filling, layered rolling and layered detection) was not strictly followed, the quality of filler was not strictly controlled, drainage measures were not taken, and the compactness did not meet the requirements. These human factors lead to the instability of the fill approach road and the large and uneven settlement after construction, which is one of the main reasons for the bump at the bridge head.
2. Construction quality control of vehicle bump at bridge head
2. 1 Do a good job of foundation treatment
Treating the soft foundation behind the abutment is an important measure to control the bump at the bridge head. At present, the commonly used methods to treat soft foundation in China are replacement filling method, overload preloading, plastic drainage board, powder jet grouting pile composite foundation and so on. As far as the current situation is concerned, the effect of strengthening soft soil with cement powder injection pile composite foundation is obvious, the construction period is short, but the cost is high; Generally, the construction load can be used as the preloading load of soft foundation for overload preloading, but the construction period is long and the residual settlement is large. The plastic drainage board method has the advantages of good reinforcement effect, short construction period, simple construction and mature construction experience, and is a common method for treating soft foundation at present. In the design of South Central Ring Bridge in Taiyuan, in order to ensure the compactness of subgrade, 30 cm thick natural gravel was replaced for foundation treatment. In addition, considering the high fill at the bridge head and poor engineering geological conditions, the local construction method of dynamic compaction combined with plastic drainage board surcharge preloading is adopted.
2.2 Strictly control the quality of filler and the selection of filler materials.
The abutment back filler is selected pertinently to improve the compactness of subgrade at the bridge head. The permeable material with large internal friction angle should be filled behind the abutment to control the compaction quality and reduce the compression settlement of the subgrade, which is beneficial to the rainwater infiltrating into the gap at the back of the abutment to be discharged out of the subgrade along the blind ditch or drainage pipe. For example, sandstone with good consolidation performance and small deformation can be selected. There are two kinds of fillers in this bid section of Taiyuan South Central Bridge: natural gravel borrowed in this bid section and soil excavated in this bid section. The former is mainly used for foundation treatment. For the latter, particle analysis, water content and compactness, liquid limit and plastic limit, organic matter content, bearing ratio (CBR) test and compaction test are carried out in accordance with the method specified in Highway Geotechnical Test Code before filling.
2.3 The pavement at the bridge head should be treated with special structure.
The abutment of subgrade structure is a different system. Considering the differences in structure, material, stiffness, expansion and contraction, generally, measures such as setting sleeper beam, overlapping slab and buried slab with variable thickness can be taken to ensure smooth gradient in vertical and horizontal directions. For example, a reinforced concrete slab is set at the joint between the concrete pavement and the bridge, one end of the slab is placed on the abutment, anti-slip anchoring reinforcement is added, and grouting holes are reserved on the slab.
There are many reasons for uneven settlement at the bridge head, which is difficult to eradicate. Therefore, according to the length of bridge and culvert and the length of line fill, rigid transition layer or asphalt transition layer should be laid within a certain length range at the bridge head to treat the joint of bridge head pavement.
2.4 strictly control the construction
Reasonable arrangement of construction plan and strict implementation of construction procedures are the key to effectively reduce bump at the bridge head. The principle of "early construction, fine compaction and full settlement" should be followed. First of all, we should pay attention to the foundation treatment of the bridge head, adopt advanced construction technology, choose appropriate compaction machines, ensure that there is enough preloading length in front of the bridge, ensure that the abutment back is backfilled in time, and the backfill density meets the design requirements. Secondly, choose appropriate fillers to ensure the quality of fillers. Control the filling thickness and rolling times of each layer, and test the filling quality of each layer, especially the degree of compaction. Backstage joint filling should be coordinated with abutment masonry as far as possible, and the compactness of these places which are not easy to crush should meet the requirements as far as possible. It is best to set a blind drainage ditch behind the abutment, and the bottom of the ditch is compacted with clay to facilitate drainage. Finally, continuous settlement observation is carried out to ensure that the settlement rate of the bridge head meets the specified requirements before unloading, and then the construction is transferred to the next working procedure.
3. Conclusion
The prevention and control of vehicle bump at bridge head is a long-term and arduous task. For bump at the bridge head, prevention should be given priority to. In the process of construction and design, effective prevention can be carried out through reasonable design and improvement of construction quality, so as to minimize the post-construction settlement of subgrade. During the construction of the South Central Ring Bridge in Taiyuan, the construction quality of the project was guaranteed due to the cooperation of the construction, supervision, design and construction units. It has been open to traffic for two years, and there has been no obvious bump.
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