Bump at bridge head is a common quality problem of highway bridges. This paper analyzes the causes of vehicle bump at the bridge head and puts forward several solutions for your reference.

Keywords: bump at bridge head; Foundation settlement; Compaction; Preventive and control measures

Bump at bridgehead is a common quality problem on highway bridges, which is the phenomenon that high-speed vehicles bump there because of bump at bridgehead. Bumping at the bridge head refers to the significant change of longitudinal slope and stiffness of pavement due to the large uneven settlement at the junction of bridge or passage and embankment fill. Bumping at the bridge head not only affects the comfort of drivers and passengers, but also leads to the loss of control of vehicles, endangering the service life of bridge passages and increasing the cost of bridge maintenance. In the practice of highway construction in China, due to various reasons, the problem of bump at the bridge head has not been well solved, which hinders the normal high-speed driving of vehicles, reduces driving comfort, and even causes traffic accidents, resulting in huge economic losses.

First, the analysis of the causes of vehicle bump at the bridgehead of expressway

Bump at bridge head is caused by uneven settlement between abutment and subgrade behind it, resulting in poor settlement between abutment and subgrade top surface. When the settlement difference exceeds 2cm, the pavement here will be broken, which will cause obvious bumps and discomfort. The causes of settlement fracture are analyzed, mainly because the abutment foundation generally adopts rigid foundation, and the foundation falls to the bearing layer, and its settlement is very small, while the abutment backfill has a large settlement due to the foundation settlement and the compression deformation of the fill behind the abutment, which leads to uneven settlement of the abutment and subgrade, sudden change of the elevation of the pavement and abutment, resulting in bump at the bridgehead.

It can be seen that the cause of bump at the bridge head is mainly caused by the settlement difference between bridges; The main cause of poor settlement of road and bridge is the settlement of subgrade behind abutment. The causes of subgrade settlement mainly include the following aspects:

1, foundation settlement. Because bridges and culverts are usually located in ravines, the groundwater level is high, and most of them belong to soft foundations. Under the action of subgrade running load, the foundation will be compressed and deformed, which will lead to foundation settlement. Soft soil foundation, in particular, generally has the characteristics of high natural water content, large void ratio, high compressibility, low shear strength and low permeability coefficient, and the foundation settlement is serious, which takes a long time to stabilize. At the same time, because most high-grade highways are fully enclosed and interconnected, in order to meet the clearance requirements of roads and waterways, the height of subgrade behind abutment is generally high. Therefore, the foundation stress is relatively large, which is more likely to cause foundation settlement, thus causing the subgrade behind the abutment to gradually sink with the foundation in a certain period of time.

2. Compaction of packing behind abutment. Although the compaction of abutment backfill adopts heavy compaction standard, the compaction degree is required to be above 95%. However, in some projects, abutment backfill is made of sand or permeable materials, which have large porosity and contain a lot of water. Especially under the action of self-weight and vehicle load, the porosity gradually decreases, and the abutment backfill produces compression deformation in a certain period of time. According to the test and related research data, the compression deformation of subgrade filler itself is 65438+ of subgrade filling height. As we all know, the height of abutment backfill is generally high, so the compression deformation of abutment backfill is correspondingly large. What's more, when the abutment is backfilled, due to the weak quality consciousness of some construction units, the required compaction times are often not reached, and it is difficult to ensure the degree of compaction above 95%, leaving a lot of room for the subsequent compaction deformation, which is also the main reason for the settlement of abutment back due to the large compaction deformation of abutment back backfill. In addition, influenced by some objective factors such as time limit for a project, demolition, etc., the abutment backfill of some projects can only be carried out after the abutment construction is completed. Therefore, the roller is limited by the abutment, and it is difficult to crush the fill immediately behind the abutment, especially the ribbed abutment and the U-shaped abutment. Due to the size limitation, some compaction machines can't get in at all, resulting in pressure leakage and compaction, which makes the compactness of the fill behind the abutment seriously insufficient. Although the small rammer is used to supplement the tamping, it is difficult to meet the specified requirements, thus increasing the compression deformation of the packing behind the abutment.

Second, the prevention and control measures of bump at the bridge head

1, pay attention to the foundation reinforcement at the bridgehead, and the abutment back at the bridgehead adopts advanced filling construction technology; Improve foundation performance, do a good job of surface cleaning, do a good job of rolling before filling, improve foundation bearing capacity and reduce differential settlement. General foundation can be reinforced with soil (cement soil, lime soil, etc.). ), for special foundations such as soft soil, collapsible loess, unfrozen soil, and river flood accumulation, special foundation treatment methods suitable for their own characteristics should be adopted, such as replacement of gravel, consolidation, riprap, and silt squeezing. In order to improve the foundation, improve the bearing capacity and reduce the post-construction settlement.

2. Set the transition section at the bridge head. Set a slab with a certain length at the bridge head, and it is best to set a sleeper beam under the slab. There should be a certain thickness of stable soil under the slab, generally 0. Semi-rigid base course over 5m.

3, targeted selection of abutment back packing, design and construction, abutment back packing should be selected on site and combined with local conditions. Embankments at both ends of bridges and culverts are filled with coarse-grained materials, or a stable soil structure layer with a certain thickness is set. This is also an important measure to reduce backfill settlement.

4. Do a good job of backfilling and rolling abutment to improve the compactness of subgrade at the bridge head. In the process of construction, we should expand the construction site as much as possible, give full play to the use of general large-scale fill compaction machinery, do a good job of backfilling and compacting the abutment back, and fully compact it. In order to facilitate the use of large compaction machinery, when limited by the site, the method of transverse rolling can be adopted to make the roller roll as close as possible to the back of abutment. When the large roller can't cling to the abutment back, the small roller can be used for rolling with manual compaction, and the thickness of the rolling layer (15 ~ 20 cm) can be controlled to improve the degree of compaction, so that the degree of compaction can finally meet the design requirements. For places that cannot be rolled, it should be tamped manually until it meets the requirements.

5, set up and improve the drainage facilities and waterproof engineering, to do a good job of drainage of bridge embankment. Drainage measures are very important to the stability of fill, especially the fill near the back of the structure, which is easy to accumulate water and sink during and after construction. Therefore, during the design and construction, the drainage gradient during the construction period should be ensured, and the necessary underground drainage facilities should be set up. In addition, cushion can be set at the junction of abutment and fill section and under the pavement of transition section to prevent water seepage from entering the fill body. At the joint between the filling body with gravel filler in the middle and soil filler on both sides and the reinforced foundation, a longitudinal water collecting pipe of 30 ~ 50m and a transverse drainage pipe every 5 ~ 10m are set to eliminate the leakage between the filling body and the reinforced foundation.

6, optimize the design scheme, from the construction technology to the mechanical combination, the test section and test data should be in line with the actual situation, and a satisfactory construction scheme can be determined to a new level.

7, the implementation of professional management, not only should have the construction team, machinery and equipment, materials that meet the technical indicators, but also have a special quality controller and test personnel for quality control, commonly known as "four-specialized management".

8. Strengthen project acceptance. Check and accept the selection of filling materials, roller equipment and filling thickness in the backfilling construction of abutment back, check the drainage situation, and strictly implement the process acceptance system. Formulate supervision implementation rules, define technical specifications and supervision procedures, and ensure that abutment backfill construction is in the whole process and all-round controlled state. It is necessary to implement on-site supervision and strictly control the construction quality. If the process is unqualified, the next process is not allowed.

Third, some experience.

1. Through many years of highway engineering practice, the author thinks that the compaction effect is the best when backfilling behind abutment and subgrade filling are synchronized. In order to ensure the stability of subgrade settlement after construction, it is necessary to fill the subgrade first, and then excavate the abutment structure after ensuring the settlement stability of surcharge preloading for a certain period of time.

2. Strictly control the quality of the background filler, which must meet the design requirements and be qualified after inspection. If other materials or materials are found to contain stones, sundries, humus and other materials, they shall not be used.

3. Different compaction machines should be selected for different compaction and site conditions. Generally speaking, the abutment slope requires high quality of subgrade compaction, so it is advisable to choose rolling machinery with high compaction effect, such as heavy tire roller and vibratory roller.

4, in order to ensure the quality of abutment backfill, should appoint someone who's in charge, from material approach to paving and rolling, process inspection and other whole process on-site supervision, find problems at any time, put all hidden dangers in the bud.

Solving the problem of bump at the bridge head is a complex system engineering, and bump at the bridge head has always been a common subgrade disease in highway construction. Since the reform and opening up, the highway construction in China is developing at an unprecedented speed, and the automobile industry in China is also showing a high-grade and high-speed development trend. With the rapid and sustained growth of China's economy, the prevention and control of bump at the bridge head is a long-term and arduous task, which requires the joint efforts of design, construction, supervision, management and other units, not only verbally, but also practically.

References:

JTG D63—2007 Code for Design of Highway Bridge and Culvert Foundations

JTG F 10—2006 Technical Specification for Highway Subgrade Construction

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