Digital Cable Road Planning

Forests must be maintained so that they can provide their valuable services in the long term and sustainably. For the necessary work, the forestry service uses heavy machinery such as tractors and forwarders. However, steep terrain is hardly accessible for these machines. And in the lowlands, care is taken to protect sensitive soils.

In both cases, the cable cranes (also kown as cable yarders) is an alternative: A carriage moving on the skyline is used to transport logs to the collection point (Fig. 1). Such logging methods are currently used on about a quarter of the Swiss forest area. To ensure that the cable road is guided safely and efficiently in each case, the people in charge must carefully plan the system to suit the terrain conditions. This way, the workers on site know where to pull the cable through and where to build intermediate supports.

In the 1960s, two professors independently dealt with such ropeway calculatons. Ernst Pestal in Vienna published a book on ropeways and cable yarders at that time, which is considered a standard work in the forestry industry. The cable mechanical formulas described in it is still widely used today, although it overestimates the skyline sags for cranes. Its advantage is that it can be easily calculated by hand and graphically.

At the same time, Otto Zweifel at the ETH in Zurich developed a method that better corresponded to the conditions of cable cranes. In contrast to Pestal, he did not assume a weight-tensioned skyline, but a fixed skyline anchored on both sides (Fig. 2). The snag: To calculate his computation method, powerful computers were needed, which were only available to a limited extent at the time. As a consequence, the method fell into a deep sleep.

The team of researchers tested Bont's findings on the steep slopes of the Tösstal, Toggenburg and Pilatus. The forestry companies Nüesch & Ammann AG and Abächerli AG worked with them. They measured the built cable roads and determined the skyline sags at different loads. At the anchor tree, they determined the skyline tensile forces using load cells.

Back in the office at WSL, Bont compared the data with those calculated by Seilaplan. And indeed: The calculation method according to Zweifel led to more accurate predictions of the skyline sags and the skyline tensile forces than those according to Pestal. For all span lengths, the Zweifel formulas integrated in Seilaplan provided an accuracy of about +/- one meter for the sag - a very good result. The slight overestimation of the skyline tensile forces is to be seen as a positive factor, as it puts the user on the "safe side" when planning.