Route Planning

Mega projects consist of many mechanisms that follow and influence each other from beginning to end. In long-distance projects such as highways and railways, construction is carried out on a very large area, passing through districts, cities and even regions. In this case, unlike a classical building approach, it requires evaluating very different disciplines multiple times at different locations. On this point, one of the most important stages of railway design is the determination of the route. Locating the route in the most effective corridor possible not only provides convenience during the construction phase but also ensures that the investment is served efficiently. Many academic studies have been conducted on how to determine routes for various transportation modes. Within this framework, the route is determined according to many factors in high-speed train projects but the region's topography, geological features and current situation stand out.

Before detailing the route, it is important to talk about how to bring "railway investment" to a region. The number of local and international passengers between the 2 points (cities) where the project will be built is analyzed and this analysis creates the passenger potential. Passenger potential is evaluated by looking at a city's population, development, transportation master plans, livelihood, business opportunity, statistics - and sometimes politics. In addition to passenger potential, the geological-geotechnical condition of the region, its topographic structure, the precence of an existing conventional line, the land value of the region, the sociocultural situation of the people, other habitats, the climate and nature that will affect the maintenance and repair costs of the railway after it becomes operational, and interaction with other transportation modes is important in bringing investment to a region. As a result of the feasibility report in which all these are evaluated, a decision is made on whether the new railway investment will be made. Following this stage, an Environmental Impact Assessment (EID) report is prepared, focusing on the habitats, wildlife, agricultural lands and cultural assets that the project will affect, and this report is expected to be approved by the relevant institutions. Afterwards, the process of determining the route can begin.

When we examine the subject in textbooks, it tells us that a "guide" route will be based on the "zero polygon" resulting from combining the contour lines between two points with the same slope. The route created with the help of the zero polygon is basically a route where there is no cutting or filling work, sounds like an optimum route but this is not a possible option in practice, and the route determination process is a little more complicated.

In the very beginning, considering the factors mentioned before, alternative corridors are created for the railway project, whose starting and ending points are almost determined, and these alternative corridors are evaluated among themselves. Some alternatives are eliminated before detailed analysis due to reasons such as excessive extension of the line, increasing number of structures, or passing through registered lands etc. As for other options, both techincal and cost-based criterias are examined and the route with the lowest bad score or the highest good score is tried to be obtained. So how are these candidate corridors created?

During the route determination phase, candidate corridors are drawn on 1/25000 maps. Numerical models to be created by using data such as the length of the line, applicable railway cross-sections, number, length, and foundation types of art structures, amount of earthworks, type of soil where the tunnels will be constructed and mechanical equipment to be used, type of soil and whether improvement is required, superstructure productions to be made for electrification, signaling and safety measures are also effective in the selection of alternative routes over each other. Although one alternative seems better than the other, deeper perspectives are required to determine the most appropriate one. This is where design criteria come into play.

Like all engineering projects, railway projects are built within the framework of various geometric and technical features. The most important of these technical features are the type of train operation (passenger, freight, conventional or high-speed train), the train sets to be used, the desired minimum and maximum operating speed* and the minimum curve radius to be used depending on this speed. All these parameters are significantly interconnected, and as a result, this interaction directly affects the comfort of the line. While choosing a high radius curve in mountainous and rugged terrains increases the need for tunnels and viaducts, a lower curve radius may reduce the speed criterion, resulting in more disturbing but more economical results in terms of cost. Lets give an example, we can see that the train serving between Madrid and Seville slows down from 250 km/h to 200 km/h at one point (See figure below).

*In Turkiye, 200 km/h speed and over railway lines are constructed for only passenger trains.

One of the reasons for this is that in order to reduce costs, it is preferred to reduce the operating speed and provide service at lower curve radii. If the speed of a line suddenly drops significantly, it can be interpreted that this is related to geometric features. Let’s give a numerical example. According to HS2 data, the minimum curve radius on a line where a train will be operated at 200 km/h should be 1800 m, while a radius of 7200 m is required for operating a train at 400 km/h. As can be easily understood from here, the curve radius, which is proportional to the square of the speed, may pose various geometrical difficulties due to extending the distance. Although the standards used by each country use different radius limits for different speed values, it is clear that high speed requires high radius as an approach. When its come to make a choice, either you choose to keep comfort of the line high and accept increasing the cost or decrease not only the speed but also number of tunnels, viaducts etc. In another significant example, in the project I am currently working on, due to the route coincides with a 3rd Degree protected area, various alternatives were studied to how to find the best solution. Since the archaeological excavation procedure is long, and tunnel and viaduct alternatives increase the cost unexpectedly, the solution was found to be rehabilitating the existing railway line and operating on that line, and in turn reducing the speed. This stabilized to the route not only by reducing the speed but also by reducing the number of art structures. In summary, the geometric features of the line and the route determination significantly affect both the construction phase and the comfort of the line.

Another important point in determining the route is to get opinions from other institutions and organizations. It is necessary to work in an integrated manner with relevant institutions and organizations to confirm infrastructure constructions, planned investments for the future, historical findings from past civilizations and current constructions of the region, etc. During construction, many infrastructure works will need to be evacuated, especially at city crossings. Some of these infrastructure works may be of irreplaceable importance (oil lines) or long-term works. It is very important to discuss and determine these situations during the route determination phase. Ignoring this process may have negative consequences during the construction phase, both in terms of time and the cost of additional work to be done in the future. It will also assist in the process of adequacy of funds, obtaining necessary permits, and establishing good communication with local municipalities and institutions.

Last but not least, another issue is that after the corridor is determined and construction begins, the land cannot be used for its former purpose. This situation requires the expropriation of the right parcels - aforementioned issue will be examined in detail later. Whether the land is publicly or privately owned, different costs and time will arise during expropriation. Procedures are initiated after the optimum route is determined, but while the construction activities continue, the emergence of new parcels due to changes in the route for various reasons will cause the same procedure to be repeated again. This situation will both cause the previously expropriated lands to be wasted and will cause the newly expropriated areas to take time. In this respect, it is critical to determine the route before construction activities begin.

Although there will be changes for various reasons after the project starts, determining the route by evaluating all possible scenarios that may be encountered will be useful in preventing many unforeseen problems.

Arifcan Yilmaz, MSc

Civil Engineer

Bibliography

1. Route Selection in the New High-Speed Railway Lines & Design Criteria, TCDD

2. Railway Planning and Design Technical Principles, DLH