Effect of Soil Moisture and Number of Skidder Passes on Rutting in Skid Trails (Case Study: Kheyrud Forest)

Impacts of skidding operations on forest soils can be divided into three major categories: soil profile disturbance، soil compaction and soil puddling and rutting. The aim of the study was to assess the effect of number of machine passes and soil moisture content on skid trail rutting، and to quantify these effects. The study was designed as a complete factorial experiment in the Kheyrud Forest following Timberjack cable skidder operations. The effect of 25، 35 and 45% soil moisture content and different levels of compaction were assessed by varying number of skidding cycles: 1، 5، 10، 15، 20، 30 passes. The results showed that the rut depths increased with increasing number of machine passes، but most of the rutting occurred after the initial few passes. Rut depth at 45% soil moisture was higher than rut depths at 35 and 25% of moisture content. The average rut depths in soil with 25، 35 and 45 moisture were 43، 101 and 172 mm، respectively. Rut depths significantly increased with soil moisture and number of machine passes. Skidding operations should be planned when soil conditions are dry to minimize rutting، but if skidding must be done under wet conditions، the operations should be stopped when machine traffic creates deep ruts.