###### Abstract
For the same soil type, machine and local conditions, the severity and extent of soil compaction depend on the machines' traffic characteristics such as the number of passes over the same trail and the size of the transported payloads [13; 14; 15]. From this point of view, it is common sense that heavy machines are likely to produce a higher severity in terms of soil compaction in fewer passes as compared to lighter machines or manual extraction means. In turn, information about the machine type and size, size of the extracted payload, number of passes and their joint effect as measured in terms of soil compaction may provide useful information for planning timber harvesting at several decision-making levels, supporting the acceptance of practices or setting planning thresholds that are related to changes that occur in soils. There are many ways and methods used to estimate the soil compaction severity [16; 17; 18], of which measuring the penetration resistance has become a widely accepted and used method in forest operations research (e.g., Refs. [19; 20; 21; 22; 23]) due to the simplicity of setting and implementing the experimental designs and affordability in terms of money, time and other resources spent to collect, process and analyze the required data. However, the method only provides indicative figures on soil compaction state, since it measures the strength of the soil instead of actual bulk density. Nevertheless, penetration resistance was found to be strongly related to parameters such as bulk density and water content (e.g., Refs. [24; 25]), which makes it useful for comparing undisturbed soils against those trafficked by machines; it also provides an important advantage, namely the possibility to readily evaluate the changes that may appear through the soil's depth in a relatively continuous depth profile, while bulk density sampling is rather discrete. Changes in soil compaction are evaluated based on comparison of data; control points are used to take reference data on the undisturbed soil, which are then compared to the data coming from trafficked soil [26; 27; 28; 29; 30]. Unfortunately, one of the main limitations in such experimental designs is related to the heterogeneity of forest soils. Structure and texture, for instance, may vary along the depth for the same sampling point, as well as between the studied sampling points. To accommodate this, and based on informed guesses, experimental comparison studies on soil compaction have been based on pre-determined soil layer depths [31; 32; 33; 34; 35], usually taking a step of 10 cm. Data resulting from these in terms of bulk density and/or penetration resistance were pooled to make the samples for comparison. Then, the interaction between the loaded machines and the soils was frequently examined by observational designs, meaning that there was little control over the payload size extracted at different machine passes. Among other limitations related to the sampling timing and other important characteristics of the soil such as variation in moisture as a function of depth, the results on soil compaction changes in such designs would be only indicative, preventing a detailed planning of operations with the aim to control soil compaction. In Romania, for instance, ground-based extraction systems dominate by their share in timber harvesting. More specifically, skidding, forwarding and animal logging may account for more than 95% of the harvested timber. Of these, skidding by domestic machines is dominant due to a well-established producer that manufactures and sells them at competitive prices [36]. Nevertheless, due to efficiency reasons, forwarding has gradually been introduced in operations and may account for a more important share in the near future as compared with the latest period and statistics [37]. For ground-based machine timber extraction systems, a well-designed network of skid trails and/or skid roads is required [4].