Chen Nuo
China Railway Second Institute Engineering Group Co., Ltd
Abstract: The riverside line is one of the most commonly used route setting methods for mountain railways, and the low-elevation scheme can usually reduce the bridge and tunnel project and reduce the construction investment. However, in the debris flow development section, the solid deposits carried by the branch ditch will change the main channel and cause the riverbed to rise, endangering the safety of the line. Based on this, taking the Dongchuan branch railway as an example, this paper analyzes its route selection design ideas and disaster causes, compares the technical and economic plans of the new line with the old line, and summarizes the key points of railway alignment of the main weak and strong river sections of debris flow development for the reference of similar projects.
Keywords: railway alignment; Mudslide; Dongchuan Railway; The main is weak and the support is strong;
About author:CHEN Nuo (1990—), male, from Chengdu, Sichuan, master's degree, engineer, engaged in railway engineering survey and design related work.
1 The geographical location and natural environment of the Dongchuan branch line
Dongchuan branch line is located in the northeast of Yunnan Province, the starting point of the railway is located in Tangzi Town, Kunming City, and the end point is located in Langtianba, Dongchuan County.
The region has a subtropical low-latitude plateau mountain monsoon climate, and the annual precipitation in the region is obviously dry and rainy in time. The rainy season is May ~ October every year, and the precipitation can reach about 85% of the year, and most of them are concentrated local heavy rains and torrential rains, which are important hydrodynamic factors for the formation of debris flows[1]. It mainly develops Quaternary alluvial and slope residual cohesive soils. Tertiary claystone and sandstone; Jurassic sandstone and sandy shale; Triassic sandstone interbedded with dolomite and limestone; Carboniferous sand shale interbedded coal seams; Cambrian siltstone, shale, argillaceous limestone; Sinian slate, phyllite and dolomite. The area belongs to the Jinsha River water system, the surface water is mainly the Dabai River, its water volume fluctuates with the rainfall, November ~ April of the following year is the dry season, May ~ October is the wet season. Groundwater is mainly bedrock fissure water and karst water, and the reserves are relatively abundant [2]. The basic intensity of the earthquake is 8~10 degrees, the peak acceleration of strong ground motion is ≥0.4g, and the peak acceleration of dynamic motion is 0.40s. The Dongchuan branch line is basically parallel to the Xiaojiang fault, and under its influence, the seismic activities along the line are more frequent, the structure is developed, the rock mass is broken, and the weathering is strong, especially along the Dabai River section, the broken rock mass leads to the development of geological disasters, among which the debris flow is the largest. Dozens of alluvial fans of debris flow are distributed along the Dabai River section of the Dongchuan branch line, which are large in number and scale, and the silted debris flow even forces the main stream to be diverted, which is the most serious debris flow disease section of the national railway network [3].
2 Overview of the old line of the Dongchuan branch line
The original Dongchuan branch line was built in 1958, and the project is positioned as a branch railway serving the Dongchuan copper mine. The line starts from the Tangzi Town Station of Guikun Railway, passes through Xundian, crosses the Tiansheng Bridge Pass, goes down the Dabai River to the north, crosses Xincun and Sanjiangkou, and passes through Xiaojiang Bridge to Langtian Dam. The length of the line is 97km. In the initial design, the line was chosen as a bottom-elevation line scheme along the terraces of the Dabai River valley, mainly in the form of a roadbed. Because the railway runs through the fault zone, the rock mass is broken, the geological structure is very developed, and the mountains along the line are high and the valleys are deep, and the cross slope is steep, so that the mountain disasters such as collapses, landslides, and debris flows are densely distributed. There are nearly 100 debris flow ditches of various sizes in the 48km stretch from Xincun to Langtianba, and during the rainy season, mudslides are frequent, and a large amount of accumulation even forces the main river to be diverted, posing a huge threat to the railways laid along the river. The Dongchuan branch line has been repeatedly affected during the construction and operation period, which is closely related to the frequent debris flow disasters.
The design of the Dongchuan branch line was completed and construction began in 1958, but in the rainy season of the following year, a number of mudslides broke out in the Dabai River, destroying a large number of bridges and roadbeds under construction. In 1959 and 1961, the design was revised twice to avoid the main river, and finally completed and opened to traffic in 1970, but the line continued to suffer for the next 10 years, and finally the old line was severely damaged and completely replaced by a new line built in 1985.
3 Considerations for the design of the old line
Dongchuan branch line starts from Tangzi Station of Guikun Railway, passes through Xundian, Tianshengqiao, Xincun to Langtianba, in addition to Tangzi Town to Xundian section, Xundian to Langtianba about 50km are along the river line, the railway mainly travels along the Yanglin River, Dabai River and Xiaojiang. The main geological problems faced by the line along the river are debris flow erosion, impact on the roadbed, bridge and culvert foundation, siltation and pressure on the main river diversion, etc. Among them, the most serious is the section from Mobodu to Langtianba along the Dabai River, which is about 20km long, and there are nearly 50 debris flow ditches.
In this case, when the line was first designed in 1958, the design unit compared the low elevation on the left bank, the high elevation on the right bank and the low elevation on the right bank of the section of the line along the Dabai River. The low elevation scheme of the left bank was excluded from the consideration of plane circumvention debris flow, and the high elevation scheme of the right bank was excluded from the consideration of saving investment, and finally the low elevation scheme of the subgrade project along the river terrace was finally adopted. However, it is precisely because the elevation of the line is too low and the impact of debris flow siltation on the riverbed of the main river is ignored, which leads to repeated disasters in the construction and operation stages of the railway, and finally abandonment and realignment [4].
It is precisely because the right bank low-line scheme underestimates the severity of the riverbed rise of the Dabai River, and the design elevation of the line is too low, making this section the most severely affected section of the Dongchuan branch line.
4 The situation and causes of old line diseases
After the construction of the old line started in 1958, it continued to suffer disasters, and in the 10 years from 1960 to 1970, the line along the Dabai River was always in a cycle of construction, repair, and realignment, and the engineering design life was reduced from 100 years to 30 years, but it was finally forced to change the line in 1985. In the end, the construction cost of this section was about 48 million yuan, which was four times the initial investment in 1958 [5]. In the 15 years since its opening in 1970, there have been frequent mudslides that have repeatedly invaded the railway, and the siltation of tributaries in this section of the river has forced the main river to rise and divert its course. Due to the low elevation of the old line design, the main riverbed gradually approached or exceeded the railway elevation in the process of rising, washing away the railway tracks, roadbeds, bridges, tunnels and other facilities, which brought a huge threat to the safe operation of the railway. When the O&M costs of the old line gradually exceed its benefits, it can only be abandoned. The main reason for the abandonment of the old line is that the following points were not considered in the design:
(1) Misjudgment of the evolution and development of debris flow. For example, the old gangou tunnel of this line, because there has been no debris flow for many years, it is considered in the design that it is in a period of decline and has no threat to the railway. However, after the construction of the temporary road and the dumping of spoil above the mountain, the soil balance of the slope was destroyed, and the flash flood infiltrated the soil in the rainy season, which finally revived the ancient debris flow ditch. The design of the old line was carried out under the understanding that the old gangou debris flow was in the declining period, and its development trend was misjudged. Initially, a middle bridge with a span of 36m was used to cross the old dry ditch, but it was quickly washed away, and then the scale of the old dry ditch debris flow gradually expanded, and it was changed to a middle bridge with a main span of 84m, and the line was still destroyed; Randomly changed the route many times, and successively used the internal line as the roadbed and tunnel scheme, but it was still frequently affected. It was not until the 80s of the 20th century that a mudslide broke out in the rainy season, which rushed directly into the tunnel and completely destroyed the line [6].
(2) The elevation of the line is too low, and the impact of debris flow siltation and elevation of the river bed is not considered. The old line runs along the Dabai River from Mobodu to Langtianba and is about 20km long. In the initial design, in order to save one-time construction investment, the right bank low line scheme was adopted, and the line was mainly in the form of subgrade engineering along the floodplain. According to preliminary statistics, in the 20 years from 1960 to 1980, the average annual rise of the main channel was about 1 m[7]. For example, the small stone cave bridge that crosses the small stone cave debris flow ditch in 1958 showed that the loose material in the ditch was only 300,000 square meters, so it was designed as a small bridge with a span of 10 meters, and was later washed away. After that, during the great earthquake of 1966, the loose deposits on the slopes along the banks of the river increased dramatically. In the 70s of the 20th century, it was changed to a 32m span bridge, which was soon buried again. Finally, when the line was changed in the 80s of the 20th century, the survey data showed that the loose material in the ditch reached more than 1,200,000 square meters, which was about 40 times that of the original design. In the end, the elevation of the line was raised by about 30 meters compared to the old line, and the bridge was changed to a span of 160 meters to pass safely.
5 Comparison between the new line of Dongchuan branch railway and the old line
According to the above analysis, for the main weak and strong river section of debris flow development, not only should the plane circumvent debris flow ditch be considered in the route design, but also sufficient main river lifting space should be reserved on the elevation of the whole section of the line, that is, a reasonable three-dimensional bypass method.
After fully absorbing the lessons learned during the construction and operation of the old line, the designer rebuilt the Dongchuan branch line in 1985, and the design point was to move the line inward and raise the elevation of the line, so as to avoid the debris flow disaster point with the bridge and tunnel project, and the schematic diagram of the new and old line scheme is shown in Figure 2.
In order to compare the design elevations of the new and old line schemes, the elevation of the Dabai River bed of the Dongchuan branch line in the interval of about 20 km from Mobodu to Langtianba was extracted and compared with the track surface elevation of the new and old line schemes, and the results are shown in Figure 3.
Fig.1 Schematic diagram of the plan along the Dabai River Download the original map
Figure 2 Schematic diagram of the new and old line schemes Download the original diagram
Fig.3 Comparison between the old and new lines and the elevation of the riverbed Download the original map
As can be seen from Figure 3, the design elevation of the old line is too low, and it is already lower than the elevation of the riverbed in the K77-K90 section, and the lowest point is about 10m below it, and some of the ones slightly higher than the riverbed will also be eroded by floods in the rainy season. The new line has significantly increased the elevation of the line, which is already about 25m higher than the old line on average, and the highest point is 50m. The above facts show that it is very effective to adopt three-dimensional avoidance for the main weak and strong river sections.
For the main weak and strong rivers in the mountainous areas developed by debris flow, the accumulation of the main channel will lead to the continuous siltation of the main channel due to the strong sediment-carrying capacity of the branch ditch than the sediment carrying capacity of the main river, and eventually a large amount of solid matter will accumulate in the riverbed, resulting in the rapid rise of the riverbed, thereby reducing the relative elevation of the riverline. This is a very dangerous and irreversible disaster along the river route. The rapid rise of the riverbed in the Dabai River and Xiaojiang basins was underestimated during the design of the Dongchuan branch line, which was the root cause of the old line being abandoned due to the disaster.
Table 1 compares the engineering, technical and economic schemes of the new and old lines from Mobodu to Langtianba.
After the realignment in 1985, the ratio of bridges and tunnels increased by about 13%, and the one-time construction investment increased by about 38% compared with the old line. The total length of the bridge and tunnel has increased by 2.7km, and the elevation of the line has increased by 25m on average compared with the old line. From the perspective of the whole life cycle of the railway, the one-time construction investment has increased, but the safety of the line has been greatly improved. On the other hand, if the low-elevation and old line scheme is adopted, although the initial investment is saved, it is estimated that the operation and maintenance cost will be close to four times the initial construction investment in the 20 years after delivery and opening [8]. Not only is the operating cost of the conversion project higher than that of the new line, but the design life of the railway is only more than 20 years, and it was abandoned in the 80s.
Table 1 The number of major projects and investment estimates of the new and old lines Download the original map
Note: The comparison range is the section from Mobodu to Langtianba.
6 Conclusion
In mountainous and hilly areas, in order to make the line smooth, the project is simple, and to create better conditions for the operation and regional economic development, the line is often selected along the larger river valley in the mountains. Generally, the lower the elevation along the riverline, the more economical the construction investment. In the early days of the founding of the People's Republic of China, the low-elevation scheme was often adopted along the riverside line, and the subgrade project was mostly used to run along the river terrace, and the flood level was used to control the elevation of the line. According to this line of thought, the alignment strategy of the old line scheme of the Dongchuan branch line is reasonable. However, due to the low elevation and frequent debris flows, the cumulative maintenance cost has reached four times the initial construction investment in about 20 years of construction and operation, and the design life of the railway has also been greatly shortened. Compared with the new line plan, although the initial investment of the new line plan is 38% higher, the operating cost of the conversion project has been greatly reduced, and the safety and reliability are also more guaranteed. Therefore, in the debris flow development of the main weak and strong river section of the mountain railway, not only the plane avoidance means, elevation avoidance is also essential, in the technical and economic comparison of the scheme, the conversion project operating cost of the whole life cycle of the line should be considered.
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