Li Tianji
Qianxinan Highway Construction and Maintenance Co., Ltd
Abstract: It is an important part of the reconstruction and expansion of low-grade highways in mountainous areas to upgrade the grade of third-class highways, fourth-class highways, other highways or secondary roads with a design speed of 40 km/h. Because the road grade has the corresponding flat and longitudinal standards, if the available resources of the original line are very small, the quality of such low-grade roads is upgraded, and the economic effect of the reconstruction and expansion method is not good, the new plan should be adopted. Based on this, this paper takes a mountainous area as an example, discusses the necessity of reconstruction and expansion, route design, and the layout of traffic safety facilities, and analyzes and discusses the roadbed widening technology and highway municipalization in mountainous areas in combination with the technical regulations for general highway reconstruction and expansion in mountainous areas, aiming to provide reference for similar participants.
Keywords: mountain highway project; reconstruction and expansion of low-grade highways; roadbed widening;
About author:LI Tianyu (1987—), male, bachelor's degree, assistant engineer, engaged in highway maintenance management.
This paper takes the relevant issues of highway reconstruction and expansion in mountainous areas as the starting point, and focuses on the theoretical and practical research on the adaptability, availability, subgrade widening, and municipalization of existing highways. According to the engineering practice, some common technical problems existing on low-level roads in mountainous areas are summarized and discussed, in order to bring some reference to relevant technical management personnel and apply them to the construction of similar projects.
1. The necessity of reconstruction and expansion of low-grade highways in mountainous areas
Road construction plays a very important role in promoting regional economic exchanges, a province decided to increase investment in highway construction, the implementation of "county-to-county expressway", to this day, the province's highway network has been initially complete. However, there are still many roads built in the early stage, but due to factors such as investment scale limitations, functional limitations, and unscientific planning and design, most of them can no longer meet the needs of road driving with the changes of the times, and traffic safety cannot be guaranteed [1].
According to the statistics in the local "basic situation of highway routes", the province in the past 20 years of new reconstruction and expansion of various levels of road data, to 2013 as a node, national, provincial and county three roads as an example, before 2013, mainly to the four-level road as the main body, and since 2013, in response to the national vigorously promote the call for low-grade highway reconstruction and expansion, the province's low-grade highway large-scale upgrading and reconstruction projects have been launched. In August 2021, the provincial Department of Transport issued the "Township Road Construction Plan for Level 3 and Above". It clearly states that by the end of 2023, more than 45% of all townships and towns in the province should be opened to traffic on second-level and above roads, more than 57% of third-level and above roads should be opened to traffic, and the remaining townships should be at least single-lane on fourth-level and below roads [2]. Due to the low level of road technical specifications in the early days, the mountainous area, the lack of economic capacity, the complex terrain and other factors, the existing road grade is low, the traffic conditions are relatively lagging behind, and it has been unable to meet the needs of economic development. To sum up, it is imperative to upgrade low-grade roads in the province.
2. The design of the reconstruction and expansion route of low-grade highways in mountainous areas
2.1 Principles of route design
In most cases, the original county roads, township roads, and village roads are used for reconstruction and expansion, and the principles of the reconstruction and expansion route design mainly include the following:
(1) The principle of using the original line. In the planning of road routes for reconstruction and expansion, it is necessary to comprehensively consider the direction of the original road routes, carry out a detailed investigation of the foundation and pavement quality of the old roads, and combine them with technical standards and specifications to scientifically evaluate the current quality standards of the roads, so as to ensure that the utilization value of the existing roads can be fully utilized when the road route design is renovated, thereby reducing the consumption of resources [3]. At the same time, methods such as lengthening routes and setting up connecting lines can also be used to improve the utilization efficiency of old roads, so as to reduce construction costs and save resources more scientifically and reasonably.
(2) The principle of attaching equal importance to economic benefits and social value. The route design of the secondary highway reconstruction and expansion project in mountainous areas should fully consider the important role played by the secondary highway in connecting the high-level highway, important towns and industrial areas, and maximize the economic and social value of the secondary highway. To carry out route alignment and planning, it is necessary to conduct a detailed investigation of important road sections, and connect the main nodes on the road as much as possible under the condition of ensuring road safety, so as to make travel safer, faster and more efficient, so as to drive urban planning, tourism and resource development in the surrounding areas.
(3) The principle of ecological environmental protection. The planning and design of the mountain highway reconstruction and expansion project should be carried out based on the principles of saving land, maintaining the continuity of the original ecosystem, protecting natural vegetation, and maintaining the stability of the road ecosystem. Specifically, the following methods can be used to protect the ecological environment around the road: (1) reasonable selection of road wiring, (2) conforming to the construction standards of topography and geology, and (3) reducing the height of the roadbed, rational use of resources, ecological restoration, etc., so as to achieve natural harmony between road construction and the ecological environment [4].
2.2 Main precautions for route design
The route design of the mountain highway reconstruction and expansion project should meet the requirements of technical standards and specifications, and at the same time, it is necessary to carefully consider whether the roads along the route have elements such as municipal roads, connecting cities and towns, and connecting various grades of highways, so as to maximize the comprehensive value of economy, society and environment along the route. The road planning of secondary highways in mountainous areas requires comprehensive analysis and discussion by multiple disciplines and disciplines to find the best road planning scheme [5]. The planning of mountain highway routes is a more tedious work, and it is necessary to find the best route scheme according to the actual situation. The following issues should also be considered in the reconstruction of secondary roads in mountainous areas:
(1) Basic line planning work. In the planning and design of the road reconstruction and expansion project, in order to realize the optimal design of the route, the transportation infrastructure around the project, such as municipal roads, interchanges, bus stations, railway stations and other highways, should be investigated in detail, and the design should be carried out based on the local geology, hydrological conditions, ecological environment, and original road design.
(2) Scientific planning of routes. The design of highway reconstruction and expansion routes should be scientific and reasonable in multiple links such as plane, longitudinal plane, cross section, longitudinal section, circular curve, etc., so as to ensure that the reconstruction and expansion of the highway meets the actual transportation needs in terms of carrying capacity, width, and diversion capacity.
3. The reconstruction and expansion of low-grade highways in mountainous areas and the improvement of traffic safety facilities
3.1 Improve the principle of traffic safety facilities
When reconstructing and upgrading low-level roads, road safety construction design should be carried out in accordance with the following aspects:
(1) Security. Review the selected improvement methods for new adverse effects, including safety, traffic and environment.
(2) Prevention is better than prevention. The identification, elimination and control of hazards and potential safety hazards are the key points of prevention.
(3) Reduce the probability of accidents. For hidden dangers that cannot be completely eliminated in a short period of time, control, protection and shielding measures should be taken to spread the risk factors in the system and minimize the probability of accidents.
(4) Rationality of investment decisions. When it is not possible to open a new route due to financial constraints, it is necessary to try to improve various safety facilities on the road surface to ensure the safety of road driving.
(5) Acceptability. whether the improved approach can achieve the objective of preventing traffic accidents and whether it can be recognised and accepted by road users.
3.2 Key points for improving traffic safety facilities
The entrances, high embankments and sharp bends of the low-grade highway reconstruction and expansion project in mountainous areas are the key contents of the optimization design of traffic safety facilities, and the corresponding engineering improvement measures are proposed based on their safety characteristics and existing problems, which are as follows:
(1) There should be a sign of "slow down" or "stop" to give way at the branch intersection. If the connected branch road has a certain longitudinal slope, it is necessary to set up a speed brake on it, set up warning signs here, and match a speed reduction platform marker to remind the driver to slow down.
(2) Warning signs should be set up on both sides of the entry branch intersection to remind drivers of the main road that there is a branch road entering, and ask the driver to slow down when entering the branch road.
(3) Trees within the field of view should be cleared to ensure that the view of the branch intersection is maintained.
(4) Increase protection. The high embankment section of the mountain highway is prone to cars driving out of the lane, causing traffic accidents, in order to ensure the safety of vehicles, protective fences should be set up on the side of the highway. The length of the guardrail must meet the design and specification requirements in order for it to function as it should.
(5) The end of the anti-collision guardrail should have deflection measures, that is, at the end of the railing, it should be deployed to the outside side of the highway, so as to prevent the accident of death caused by the end of the railing piercing the body when the car hits the railing.
(6) High-strength protective fences should be set up at the cliff edge of the roadbed. Reinforced steel plate guardrails can be used in high embankment sections to improve the strength and protection effect of guardrails, so as to reduce the safety accident rate of slopes near cliffs.
(7) Make a sharp turn alone. In the reconstruction and expansion project of the low-level highway in the mountains, when a single lane turns, it is often due to factors such as poor sight and excessive speed, resulting in vehicle collision accidents. In view of the above problems, straight line guidance signs, contour signs, reflective road studs or vibration markings can be added to the ordinary curve warning signs.
(8) Continuous sharp turns. Drivers drive more carefully in places with successive sharp bends, so their accident rate is lower than in sharp bends alone. However, in places with successive sharp corners, brake failure is likely to occur. Therefore, it is necessary to reasonably set up avoidance lanes in continuous sharp bends where traffic accidents occur frequently due to brake failure [6].
4. Technical problems of roadbed widening for the reconstruction and expansion of low-grade highways in mountainous areas
The selection of road widening mode and the treatment of the problem of uneven settlement of new and old roadbed are the technical problems to be considered for the widening of the roadbed of low-grade roads in mountainous areas.
There are three main methods of highway subgrade widening: unilateral widening, both sides widening, separate widening, etc., in view of the advantages of unilateral widening, it is often recommended to use the method of unilateral widening. When it is in some township markets, there are a large number of structures on both sides of the road in this section of the road, and the method of widening on both sides should be adopted with reference to the practice of urban roads. For example: a provincial highway reconstruction and expansion project, the original road is a four-level highway, its horizontal and longitudinal indicators are relatively low, the pavement grade is low, which limits its passing capacity and service level, and the roadbed width of most of the old road sections is 7.5 m, and the width in some places is 6.5 m, or even smaller. When the existing highway is renovated and upgraded, when the urbanization on both sides of the highway is serious, and there are some structures that cannot be demolished, or facilities that need to be demolished at high cost, it is necessary to connect with the specific situation of the site and widen the roadbed by adding vehicular cover plates to the ditches on both sides of the original road surface [7]. Referring to the past experience, this method not only avoids the shortcomings of large amount of unilateral widening and demolition and high demolition amount, but also ensures the safety of the drainage project and cover structure of the side ditch. See Figure 1 for details.
After the road widening method is selected, it is important to consider the overlapping scheme of the old and new roads to deal with the problem of uneven settlement. In practical application, finite elements can be used for simulation, because the compressive stress on the new foundation and the compressive modulus of the new foundation soil are important reasons for the differential settlement of the road. The road docking link should be carried out at the junction of the new and old roads, and the scheme of setting up steps should be adopted, and the width of the steps should be greater than 100 cm, and it should be designed as an inward cross slope of 2%~4%. If the width of the puzzle is relatively small, methods such as ultra-wide filling or excavation of the existing roadbed can be adopted, and the area of not less than 2 m outside the joint surface should be treated with strong pressure to ensure that the docking is compacted, and the addition of geosynthetics to strengthen the connection between the old and new roadbed is also a way to widen the subgrade [8].
Fig.1 Standard cross-sectional view of the roadbed with side ditch and car cover plate (cm) Download the original drawing
5. The problem of municipalization of existing highways in the reconstruction and expansion of low-grade highways in mountainous areas
At present, the mainland is in a period of rapid development of urbanization, the withdrawal of counties and districts, the establishment of economic development zones and other measures, all show that the towns are extending outward, and more roads are also included in the scope of urban and rural planning, which makes the road in addition to meeting the traffic functions between cities and regions, but also must become a part of the urban planning network, and have certain municipal functions, in order to meet the needs of the city's future development.
At present, there are a lot of relevant studies on road urbanization. In the reconstruction and expansion of highways, the key to the problems existing in the urbanization of existing roads lies in the cross-sectional design of road municipalization. In the current highway design code, it is specifically pointed out that the design of the subgrade width as an urban road must be carried out within the width of the red line of the urban road plan, and the form corresponding to the urban road can be selected. Therefore, in the process of municipalization of low-level highways in mountainous areas, we can refer to the existing experience of highway municipalization. In addition, considering the specific situation of the municipalization of low-level highways in mountainous areas, it is suggested that in the process of municipalization of low-level highways in mountainous areas, it is recommended to refer to the existing successful experience of highway municipalization, pay attention to the characteristics of land shortage in mountainous areas and the generally small urban area, and carry out the scientific design of low-level highway municipalization [9].
6. Other technical problems in the reconstruction and expansion of mountain roads
Low-grade roads in mountainous areas usually have no tunnels, and bridges are scarce, or small and medium-sized bridges, in most cases, in the process of reconstruction and expansion, they cannot be reused and need to be demolished or rebuilt. In addition, culverts also generally have problems such as too small aperture and insufficient flow rate, which greatly affects the use function of culverts, so in the construction process, the technical problems of tunnels, bridges, and culverts in low-grade road reconstruction and expansion projects in mountainous areas should be referred to the corresponding level of new road standards [10].
7 Conclusion
At present, the mainland's high-grade highway transportation system has been basically completed, in view of the current situation of "strong at both ends and weak in the middle" in the construction of low-level highways, the reconstruction and expansion of low-grade highways has very important practical significance. The central content of the road route design is to evaluate the adaptability of the old road reconstruction and expansion, and to evaluate the availability of the old road. In terms of highway widening technology, the main concern is how to select the widening method and how to deal with the problem of uneven settlement of new and old foundations.
According to the reconstruction idea proposed in this paper, the road widening method is generally unilateral widening, and combined with the actual situation of the project, in the case of many large structures on both sides of the low-grade highway, it is recommended to adopt the two-sided widening method, that is, to cover the road by adding traffic cover plates on the ditches on both sides of the original roadbed to widen the road. The key to road urbanization is cross-sectional design, so in the process of urbanization of mountain roads, we should fully learn from the practical experience of existing road urbanization. In addition, the design of low-level highways in mountainous areas, in terms of tunnels, bridges, culverts, as well as signs, markings, guardrails, etc., can refer to the specification requirements of the corresponding level of highways when they are newly built.
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