The Xilin-Tianlin Expressway, currently under construction, is located in the northwest of the Guangxi Province where the topographic and geological conditions are extremely complex.
The total length of the highway is about 74km, the design speed is 80km/h, and the roadbed width is 25.5m. The ground elevation of the alignment ranges from 600m-1580m, with the maximum height difference at 980m through a geography that is mainly mountains and hills.
In the initial stage of the feasibility study, the geometry standards (determined using the traditional manual method) included:
- General minimum radius: 420m
- Limit radius: 400m
- Limit maximum longitudinal slope: 4.5%
- General maximum longitudinal slope: 4%
- Average slope: 3%
The Guangxi Communication Design Group Co., Ltd. relied on Quantm E100 to optimize the route per investment and technical criteria. The Quantm alignment planning tool is designed to help road and rail planners select and generate 3D corridors and alignments with specialized route optimization technology.
The following outlines the high level results of the optimization analysis.
Optimization with Quantm E100
1. Free Corridor Optimization
The study team set the geometry parameters in Quantm as follows:
- 450m minimum radius
- 4% maximum slope
- 3% average slope
Figure 1-1 Plan View - Corridor Results
Using the efficient analysis function of Quantm, the project was analyzed in different sections and in different traffic corridors as shown in Figure 1-1.
The longitudinal section of the route plan includes a tunnel crossing ridge and a subgrade (roadbed) crossing ridge. In the first half of the route, there are several corridors in the route plan. In the second half, the route plan follows the mountain-shaped alignment.
According to the results in Quantm, the team determined that the entire route will be divided into two sections at the cross-ridge position as shown in Figures 1-2 and 1-3.
2. Corridor Analysis
2.1 Start Point – Crossing Ridge Point (West Corridor)
According to the results of early stage macro optimization of the complex terrain conditions of the west corridor, the route would likely follow the minimum radius of 400m.
In the optimization with Quantm, the engineering team opted to assess two minimum radius parameters (400m and 550m) with a maximum longitudinal slope of 4% and an average slope of 3%.
The comparison chart (Figure 2-1-1) shows the solution with radius of 550m is only about $70 million more investment than the solution with radius of 400m (Figure 2-1-2). But the 400m radius plan is about 3km longer than the 550m radius route plan (42km versus 39km).
Considering the total investment, technical indicators and social comprehensive costs, the feasibility study team recommended a minimum radius of 550m in this corridor.
According to the results of the sensitivity analysis of the corridor, combined with the requirements of planning and intercommunication settings, the team also prepared a detailed study of the route scheme of this corridor with Quantm.
The horizontal and vertical section of the West Corridor scheme is as follows:
The total length of this section is 39.514km with a bridge to tunnel ratio of 58.13% and a total investment of about $4.77 billion.
The total length of bridges is 13,131m for46 bridges. Of those 46, 8 are bridges with a pier height of more than 80m.
The total length of tunnels is 9839m with 16 tunnels of which 9 are less than 500m, 5 tunnels between 500m and 1,000m, and 2 tunnels between 1,000m and 2,000m.
2.2 Start Point – Crossing Ridge Point (East Corridor)
From the results of broad-scale macro optimization, the terrain conditions of the East Corridor are better than the West Corridor. To best analyze the relationship between the route plan and the project investment under different geometric standards, the team evaluated a minimum radius of 700m and a minimum radius of 550m.
Compared with the 550m radius plan, the 700m radius plan increased by about $220 million, an increase of about 5.7% (Figure 2-2-1).
If compared with the east-west traffic corridor, the route plan of the east corridor with a radius of 700m is about $560 million less than the total investment of the route plan of the west corridor with a radius of 550m—and the saving ratio is about 12.2%.
Considering the terrain, later operation and driving comfort, it is recommended to use a minimum radius of 700m for scheme refinement on the East Corridor.
The team then used Quantm for refinement research of the routing scheme in the East Corridor.
The total length of this section is 40.981km and the total investment is about $4.08 billion. The bridge and tunnel ratio is about 49.97% The total length of bridges is 12,965m for 43 bridges, and the pier is about 80m high. The total length of the tunnel is about 7430 m for 14 tunnels. Seven tunnels are less than 500m and 7 tunnels are between 500m and 1,000m.
2.3 Crossing Ridge Point – End Point
There is little difference in the plane position in the section from crossing the mountain to the end point. There are two ways to cross the ridge: tunnel crossing and subgrade crossing.
The method of tunnel crossing will reduce the design elevation of the crossing position, which is theoretically beneficial to shorten the mileage of the route or reduce the average slope of the long longitudinal slope section, and improve the safety of driving.
Therefore, the team used Quantm macro-optimization to analyze the adaptability of the route plan and terrain over the ridge to determine the optimal combination with the following parameters: under the radius of (1) 700m, (2) 550m and (3) 400m.
The route scheme analysis for the crossing tunnel is shown in 2-3-1, 2-3-2 and 2-3-3. On the whole, the 550m radius is shown to better adapt to the terrain, and can achieve a good balance between design standards and engineering investment on this terrain.
Adopting the method of subgrade crossing ridge will raise the design elevation of the crossing ridge position and increase the mileage of the route. Combined with the research results of the terrain of the road section by the tunnel crossing method, in the optimization study of the subgrade crossing the ridge, the focus is on the balance of the route plan with the terrain and the total project investment under the radius of 550m and 400m.
Comparing and analyzing the route plans with a radius of 550m and a radius of 400m, the 400m radius can better adapt to the terrain and reduce the bridge & tunnel ratio of the entire line, especially the total length of the entire line of tunnels. A 400m radius study route plan of subgrade is recommended.
Using Quantm for refinement research the routing scheme, the route results are as follows:
- Total length of the route: 35.164km
- Bridge-to-tunnel ratio: 59.4%
- Total investment: ~$4.56 billion
- Total length of bridges: 14,725m (51 total; 7 bridges with pier height of more than 80m)
- Total length of tunnels is 6,161m (11 total; 7 tunnels less than 500m, 3 tunnels between 500-1000m, and 1 tunnel 1000-2000m)
The use of the Quantm 3D corridor alignment planning tool helped resolve a common defect in single geometric standards common in traditional route optimization tools. The team was able to use Quantm to assess the impact of bridge and tunnel structures at different positions on the entire route plan, thereby helping alignment designers to scientifically and objectively optimize the comprehensive optimal route plan. Quantm was able to adopt different horizontal and vertical parameters in different sections of the entire project in combination with the terrain conditions, so as to achieve a good balance between the total project investment and design standards.
The Xilin-Tianlin Expressway is on track for opening by the end of 2022.