1 preface
Due to the complexity and diversity of highway geological conditions in mountainous areas, the design of bridge foundation in mountainous areas is quite difficult, and adverse geological conditions such as karst and landslide are often encountered in mountainous areas, which increases the complexity of foundation design.
In recent years, more and more attention has been paid to highway construction and environmental protection. Some rough foundation designs often induce geological diseases. Foundation treatment increases the project cost and destroys the original natural environment. All signs are urging engineering design to put environmental protection in the first place as much as possible to reduce the damage to the natural environment. The design needs to design the infrastructure more carefully and choose the most suitable one according to local conditions.
2 the concept of basic design
Stratigraphic structure and stratigraphic contact are the basis of establishing structure. Design structure and construction quality play a vital role. Foundation pit engineering is a hidden project. If there are defects, it is difficult to find and repair. The existence of these defects often directly affects the whole structure and even the safety. According to the progress of the project, the construction progress is often controlled by the foundation project. On the basis of engineering cost, the cost of the whole structure usually accounts for a considerable proportion, especially in the environment with complex geological conditions. Foundation engineering affects the quality, duration and cost of the whole project and plays an important role in engineering construction. Therefore, the designers of bridge engineering should have professional knowledge, fully grasp the bridge foundation, understand the characteristics and application scope of various bridge foundations, especially in the design of foundation pit engineering under complex geological conditions, so as to better ensure the construction of bridge engineering.
3 Classification and characteristics of highway bridge foundation engineering in mountainous areas
Shallow foundation and deep foundation are distinguished according to buried depth. Shallow foundation refers to shallow burial depth and easy construction; Deep foundation refers to the foundation that needs to be buried deeply and is not easy to construct because of the influence of bad shallow soil.
3. 1 shallow foundation and its characteristics
Shallow foundation has the characteristics of simple structure, no need for large equipment, economy and practicality, and is widely used in small and medium-sized bridges. The rigidity and flexibility of shallow foundation are divided according to the stress and structure. A rigid foundation means that there are no steel bars. It has the characteristics of good stability, convenient construction and can bear large loads, and is the primary basic form of bridge and culvert structure. However, it requires the bearing capacity of the bearing layer. If the bearing capacity does not meet the requirements, it can be used after foundation treatment and reinforcement. In flexible foundations, such as cross foundations and box foundations, a certain amount of steel bars is necessary. It is characterized by good overall performance, high bending stiffness and small additional stress on the superstructure, and is suitable for conditions with poor foundation and high requirements on the superstructure.
3.2 Deep foundation and its characteristics
Deep foundation has the characteristics of strong bearing capacity, good stability, large buried depth and short construction period, and is suitable for large-scale long-span bridges. Common pile foundation, open caisson foundation, pneumatic caisson foundation, etc. The open caisson foundation is excavated in the well, and sinks to the design elevation by its own gravity to overcome the friction of the wall, and then passes through the concrete protective layer to fill the hole, which becomes the foundation of the pier or other structures. The open caisson foundation is a bottomless and covered structure, which sinks into the studio by its own gravity to dig.
These two kinds of foundations have the characteristics of strong integrity, good stability and can bear large loads. But pile foundation is the most commonly used in highway bridges.
3.3 Pile foundation and its characteristics
The pile foundation consists of root piles and pile caps. One or more rows of piles are arranged on the bearing platform, and all piles are connected into a whole to transfer loads. Pile foundation means that the structure above the pile cap passes through the pile cap, and the piles in the foundation bearing layer are connected with the pile cap to form a whole * * * to bear the force together. The pile side and pile end transfer the friction of soil to the pile load downward to the soil layer around each pile. Pile foundation is characterized by high bearing capacity, good stability, small and uniform settlement of bearing capacity and good adaptability. Pile foundation is divided into driving pile, digging pile and bored pile according to construction methods. According to the mechanical characteristics of pile foundation, it can be divided into rock-socketed pile and friction pile. Rock-socketed pile refers to the pile passing through loose soft soil, or embedded in hard rock soil layer or actual incompressible soil layer, and the soil at the bottom of the pile bears vertical load, which is called rock-socketed pile. Friction pile refers to the friction force that passes through and is supported in various compressible soil layers, which mainly depends on the vertical load of the soil next to the pile. It should be noted that most of the vertical load of rock-socketed piles is borne by the ground resistance at the bottom of the pile, but when the compression load of the pile body is large, the pile side friction will occur, which should be considered in the design. For friction piles, in general, the friction and resistance of the soil at the bottom of the pile bear the vertical load vertically.
4. Forms and characteristics of road and bridge foundation engineering in Gongshan area
There are two basic forms of bridge piers and abutments in mountainous areas: bored piles (rock-socketed piles or friction piles) and open-cut foundations.
In the design, the selection should be made according to the specific conditions of the foundation. Generally speaking, if the engineering geological conditions of the bridge are good, the bearing stratum of the rock stratum or foundation is shallow, generally less than 5 meters, and there is a stable bedrock, and there is a certain safe distance from the slope to the edge of the foundation, the open-cut foundation should be selected first. Small structures such as culverts and passages are generally designed with shallow foundations in mind. If the bearing layer of foundation can't meet the requirements of bearing capacity, replacement or reinforcement can be adopted when treating foundation. For the case of large load and weak bearing capacity of foundation soil, pile foundation can be considered when the appropriate bearing layer is in a deep position. The core of pile foundation design is to take the length of friction pile as the control index on the premise of meeting the bearing capacity of single pile; Rock-socketed piles generally choose two control indexes: rock-socketed depth and bedrock strength.
The design of bridges in mountainous areas covers almost all types of bridges, and the safety of bridge structures depends on reliable structural analysis and reasonable treatment measures. For some areas with unfavorable terrain conditions, the requirements are more stringent, and stability analysis is an indispensable part of bridge design in mountainous areas. The mountainous terrain is complex and ravines are criss-crossing, so the layout of the construction site is very difficult, especially the use of large-scale construction equipment is very inconvenient. It is almost impossible to provide a large-scale construction site on the construction site. Coupled with the winding mountain road and poor traffic conditions, the design of prefabricated components in mountain bridges is limited. In addition, it is worth mentioning that the rational use and positioning accuracy of a large number of small structures also play a key role in the quality of highway design in complex mountain roads.
5 concluding remarks
Therefore, the rationality of structural layout is particularly important for complex mountain roads. The road into the mountain leads to more structures. Compared with the plain area, the cost of highway construction in mountainous area is much higher than that in plain area due to a large number of structures. For most economically underdeveloped mountainous areas, the rationality of bridge cost is a very realistic problem, so the feasibility of bridge design in mountainous areas should not only consider technology, but also consider that the selected types of economic indicators are in the best range. Bridges in mountainous areas should save project cost and have good economic indicators. Bridges in mountainous areas should be built reasonably based on environmental awareness, avoid large-scale filling and excavation, prevent adverse geological disasters, minimize damage to vegetation and reduce river pollution.
In a word, the general principles of bridge design in mountainous areas can be summarized as: structural safety, comfortable use, economy, convenient construction and maintenance, decorative appearance and harmony with nature.
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