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close this bookProceedings of the seminar on environmentally sound forest roads and wood transport (1998)


Zeljko Tomašic1

1 Chief, Transport & Mechanization Nasic, Uprava Suma Nasice, Zoljan- 34500 Nasice, Croatia.


With the construction of skid trails as a third-rank means of communication begins the last development stage in the opening of woods to ensure easier access for forest mechanization and forest products utilization. More developed, stronger and easily movable wood hauling machines, the need to economize on construction operations, the need to encroach less on forest stands and to preserve the natural environment are just some of the reasons why these trails are built with considerably steep longitudinal slopes. It seems that the higher danger of soil erosion on these slopes is a limiting factor could be a limiting factor to their value. Experimental skid trails of various dimensions were built with longitudinal slopes varying from 1.2 percent to 35.8 percent. In addition to haul testing, on some of them research was carried out on soil erosion to obtain reliable scientifically based knowledge about the interaction between the longitudinal slope of the trail, wood hauling and soil erosion. During a four-year period (1992-1996) the course of soil erosion was observed and its values measured on several characteristic longitudinal slopes of the trail. As soil erosion is a complex phenomenon caused by the interaction of numerous different factors, it was necessary to measure and establish indicators of many important environmental and other conditions in situ, in a laboratory, or to collect other indispensable data. The analysis of the measurements gave useful insights on the course and intensity of soil erosion and their correlation with various longitudinal slopes of bladed skid trails on the geological foundation of chlorite slates.

Keywords: Soil erosion, skid trail, longitudinal slope, geological foundation, water runoff (drainage), rut formation gully formation, soil dislocation


It is very difficult to describe separately the factors that create the conditions for soil erosion on the tractor skid trails that were already built (bladed skid trail). It is almost impossible to estimate which factors related to soils erosion and in which moment are the most important or crucial ones. These hypotheses are explained by the fact that none of the causes of soil erosion on skid trails arises and develops alone (independently) but rather in mutual interaction. An individual source often causes the formation or disappearance of one or many of those that directly or indirectly influence the intensity and kind of erosion. So, for example, a longer longitudinal slope of the trail increases the speed and amount of water that scours the soil but, at the same time, reduces the duration of water retention on the trail surface, which proportionally reduces soil erosion. The latter, however, also depends not only on the length and inclination of the longitudinal slope, but also on the transversal slope of the trail, earlier use (smoothness of the surface), water permeability of the geological foundation, etc.

The above-mentioned example shows only a tiny part of the complexity of the whole problem - soil erosion on trails.

Nevertheless, in order to make clear and effective observations, the conditions and factors that cause soil erosion should be divided into three basic groups:

1. Environmental conditions;
2. Climatic conditions and meteorological circumstances;
3. Use of the trail.

Issues and aims

Environmental conditions

The concept environmental conditions refers to all the factors relating to the object of the research (experimental skid trail) and its surroundings from a geological, pedological and vegetational point of view since they, together with the particular properties of the soil (soil constitution and site disposition) can influence the course of soil erosion.

No doubt, the most important factor amongst them is the geological foundation together with a certain developmental stage of the soil (kind/sort of soil) on which the skid trail has been built. The essential components of geological foundation that directly influence its erodibility are water permeability and hardness of which the most important components are resistance to physical and chemical decomposition.

In 1991 Rebula carried out research on soil erosion on trails with calcareous and fliss foundations. The author describes the two geological foundations paying special attention to the properties relating to their tendency towards soil erosion.

Calcareous foundations are naturally hard (larger carrying capacity and resistance to impacts of physical-mechanical disintegration) and less liable to decomposition under the influence of water action (chemical persistence) partially because their better permeability neutralizes the effects of water retention on the trail surface.

The properties of fliss foundations with respect to soil erosion are of a totally opposite nature. They are soft and very liable to physical-mechanical and chemical decomposition while their solid foundation is almost complete water impermeable.

The results of the above-mentioned research clearly show the properties of these foundations. The average values of soil dislocation (loss) on the calcareous foundations were established in the research at approximately 0.10 m3/m of trail, while those on fliss foundations were from 0.15 to 0.20 m3/m of trail.

A total different course of erosion emerged on fliss foundations. On this soft non-skeletal soil, the machine (wood hauling) left wheel ruts that gradually took over the function of drain ditches, growing in size constantly. The foundations are also liable to relatively fast chemical decomposition. Water rapidly dislocates these soft, soluble components scouring them up to the impermeable foundation. The erosion is here slowed down but only temporarily. The hard part of the base is rapidly decomposed and is constantly dislocated by water. The drain ditches (wheel ruts) join together into a water stream and soil erosion does not stop until the stream is diverted from the trail. In the fifth grade of soil erosion on fliss foundation, the loss of soil amounted to 0.96 m3/m of trail, which is very significant compared to the values on calcareous foundations.

Between the two geological foundations described above, there are many transitional forms that can be more or less similar to one or the other with respect to soil erosion.

However, it should not be forgotten that the basic geological foundation is disturbed during trail construction; it is scattered, loosened, mixed or compacted by the machine tracks or wheels. Leaf mould and the surface layer of vegetable mould (humus) are removed and partially mixed with the soil in the trail embankment. Otherwise, the vegetation floor, which with its branched root system fixes the base and reinforces resistance to soil erosion (the so-called "soil armature" effect), is also removed. Thus, plant cover is also a very important environmental condition.

The speed and the way in which the trail is overgrown with vegetation after construction of the trail, the kind of vegetation and its ability to regenerate and survive during the works on the trail (wood hauling), as well as the volume of vegetation growth in the environment play an important role in the prevention or reduction of erosion on the trail. Dense vegetation, however, normally does not allow water runoff by the transversal slope of the trail. This example of inverse effect illustrates very well the complexity of the problem.

In order to attain natural draining of water (water runoff) and to prevent water retention on a trail, the transversal slope of the trail is constructed at 3-5 percent. This can reduce erosion considerably if the trail is not made by cutting. The transversal slope of the trail can have favourable effects due to plant recovery and smaller rut formation since the skid impact directs the water to the longitudinal stream.

One of the most important environmental conditions relating to soil erosion on a skid trail is its longitudinal slope. Its steepness and length influence directly the quantity, speed and duration of water retention on a surface which scours and dislocates soil from the trail. The influence of several different longitudinal slopes on soil erosion intensity was investigated on the experimental skid trail and is described below in detail.

Among the environmental conditions that influence soil erosion we should not forget geographical position, sun exposure, relief and also general and local hydrologic activity.

Sun exposure or position in the shade depends on the width of the light stripe and trail positioning to the roadsides. These largely influence the duration of water retention on the trail surface and particularly the interaction with climatic conditions (temperature, humidity, meteorological circumstances, radiation, evaporation, etc.).

Roughness of the environment (relief) can influence the amount and speed of water action (for example, water streams rushing downhill - the so-called "water cascade" effect).

Hydrological conditions of the environment - general/macro (height of underlying constant water streams) and special/micro (periodic streams combined with rainfall) form surface streams on the trail that are crucial causes of erosion.

Climatic conditions and meteorological circumstances

Temperature and humidity of air and soil (average as well as extreme values) are the climatic factors that influence significantly the creation of conditions for possible dislocation of the soil from the trail. It is clear that the soil will be drained faster at higher temperatures and in dry conditions when the results of water action are reduced. At very low (winter) temperatures, especially in the case of black frost, the surface layer of the soil freezes and, consequently, it frequently crumbles and disintegrates. With the thaw and consequent water accumulation, the soil partially melts and crumbles into smaller pieces so that the water dislocates and scours these fragments increasing in this way the level of erosion. However, if we consider the situation from the point of view of trail utilization, the carrying capacity of the foundation is increased when the soil is frozen and this reduces the impact of wheels and/or loads on the soils while the work is carried out. The final outcome in this case could be partial elimination of the conditions leading to soil erosion.

The effect of temperature on the degree of humidity has to be considered in relation to data on number of cloudy and sunny days in the region, which provides information on the genesis and stage of transition of the water from a liquid to a gaseous state (evaporation).

The most important climatic and meteorological factors that affect directly soil erosion are all kinds of water precipitation in a liquid condition. The average amounts (annual, monthly, daily) and extreme, especially maximum, values are equally important. This is logical because the speed and amount of water (including duration) are crucial factors for soil erosion.

Also the importance of the distribution of rainfall during the year should not be overlooked, because this can also influence considerably erosion, especially if the largest quantities and intensities occur in the spring. The soil is saturated with water (melting snow and ice) so that there is almost no vertical water runoff, surface seepage is more difficult (plenty of water streams and gully formations) and evaporation is almost negligible (high air humidity).

Wind in the area is also important and should not be considered separately from rainfall. Wind direction, intensity and speed are particularly significant. Stormy weather, for example, with strong wind and abundant rainfall can cause torrents whose rushing downhill frequently produce extreme forms: falls, slides, slumps, creeps, etc., and various grades of soil erosion. Such torrents can sometimes cause landslides or induce landslips on some parts of the forest roads.

Trail uses conditions

The way a trail is used can have significant effects on its erodibility. The beginning of soil erosion is usually connected with rut or gully formations from wheels and hauled wood. During these processes, the soil is partially compacted (impression of wheels or load into the soil), the shear is displaced due to the wheels slippage effect, then pushed and turned (rutting and ploughing effect) while the ends of the logs pass. The prerequisites for soils erosion are thus created because the soil is loosened, which enables its faster scouring and dislocation. At the same time sheer and transversal water runoff is caused by soil compaction and roughness of the surface (gully and rut formations). Since permeability is reduced, the water is directed downwards through the wheel ruts resulting in further dislocation of the soil.

When a trail is used in unfavourable weather conditions, with high humidity and lower carrying capacity of the foundation, the above phenomenon appears to a greater or lesser extent, depending on the type of foundation. This can be avoided if the trail is used only in dry conditions or when the soil is frozen. The formation of ruts, gullies and soil erosion also depends largely on the frequency of trail use, i.e. on the intensity of use. Frequent use of a trail, even in favourable weather conditions, will not only cause rut formation, but will also prevent vegetation growth on the surface, which is of great importance for the prevention and reduction of soil erosion risks.

Some of the control measures used to avoid the above risks and further progress of soil erosion on forest roads are: planning of wood harvesting, construction of trails that can prevent water retention on the surface; or design of a system of trails that does not allow water accumulation (skidding directions).

The description of the conditions and factors that cause dislocation of soil grains from the trail surface shows the complexity of the problem.

An experimental skid trail with increasing longitudinal slopes was built in order to establish how much and in which way longitudinal inclination influences soil erosion during a period of several years.

Objective and method of research


The experimental skid trail was built with six different dimensions of longitudinal slopes of an increasing inclination from 1.6 to 35.8 percent.

Most part of the trail follows a natural ditch and this is the reason why it was constructed as a so-called hillside trail. The proportion between cut and embankment is 60:40. It is placed in the area of Forest Management Našice, Forest Office of Durdenovac, in the Forest Region Kerekuš.

The roughness of the surrounding area is considerable and the average inclination is approximately 75 percent with a south/southeast exposure. According to the amount of rainfall, the climate can be described as humid; according to annual distribution of rainfall, this is an area with continental rainfall, while according to the temperature characteristics, it can be described as an area with temperate climate with continental characteristics.

Average annual air temperature in the last decade was 11°C; average annual soil temperature at a depth of 20 cm during the same period was 1.3°C; and average annual amount of rainfall was 735 mm (data from Agrometereological Station Šipovac-Našice in the period 1983-1992). According to the data of the monthly amount of rainfall that rarely is over 160 mm, it would be surprising to find that a survey of soil erosion would show any significant value (Figure 1).

Figure 1. Average annual distribution of rainfall in the surveyed region

The vegetation cover of the environment where the tests were carried out consists of a mixture of beech forests with woodruff (Asperulo fagaetum Prov.)

According to the basic geological map of this area, the geological foundation is metamorphic sedimentary rocks of quartz - silicate origin from the Pre-Cambrian Period with a high presence of chlorite slates (Scose), which with regard to form and properties are similar to phillits. The main elements are quartz, chlorite, albite and muscovite, and minerals of zircon apatite, tourmaline and pyrite (that gradually changes into limonite).

The soil found on the experimental trail is relatively acid, automorphical ranker distric regolitic, together with typical distric cambison and colluvium with preponderance of soil material.

According to dimension and distribution of soil grains (texture) based on Bennet's triangular diagram of soil classification of the US Bureau of Soils (Figure 2), it was established that the soil belong to the dusty loam type.

Figure 2. Determination of kind of soil based on USBS classification

Penetrometrical surveys on the trail and its environment were made before and after the hauling experiment. The purpose was to establish the tendency (resistance) of soil compaction in dry and humid conditions (temporary humidity 24, or 44%). The penetration resistance of the soil decreased in wet conditions by 10-40 percent compared to the values in dry conditions due to changes of the degree of soil humidity in almost all the slopes and measuring points examined. An increase in soil humidity directly produces a decrease of earning capacity.

In the same way, comparing the penetrometrical results before and after the hauling experiment, it was found that alterations of these resistances are influenced not only by the degree of dampness or by the pressure of the wheels and hauling loads, by the shear effects of wheels' slippage and by the ploughing and rutting effect of the hauling load, but also by the increase of the longitudinal slope of the trail due to the disburdening of the vertical component of the tractor and load weight.

The penetrometrical results before and after the hauling experiment showed in some places an unexpected result (decrease after hauling). This can be explained by the fact the firm slate formation that occasionally heap on the surface of the trail was smashed into pieces by the impact of the wheels and by the hauling loads. These formations became part of the skeletal component of the soil and resulted in a reduction of the soil carrying capacity in a particular place.

The physical properties (Škori 1986) showed that the soil has a moderate water retention capacity (35-45%) and belongs to the group of porous soils with 45-60 percent pores in the soil (Table 1).

Table 1



Bulk density

Water retention capacity
vol. (%)

vol. (%)

Air capacity
vol. (%)

Temporary humidity
vol. (%)








The chemical properties of the soil (Škori 1986) indicate that it belongs to the acid soils group (pH in nKCl 4.5-5.5); and in relation to its phosphorous and potassium content it belongs to the group of soils poorly supplied with these chemical elements (Tables 2 and 3)

Table 2


pH in

pH in



mg on 100 g tla

mg on 100 g tla









Table 3. Adsorbic complex










Since the main purpose of the research was to establish whether a longer longitudinal slope influences soils erosion, the intensity of this phenomenon had to be surveyed on several slopes of the experimental trail

Measuring points on the slopes of 1.6 percent, 14.6 percent and 26.1 percent were chosen and marked. The slope of 26.1 percent was divided into two measuring points, below and above the stream that originated from a spring and flowed on the trail after its construction and caused increase in soil humidity and erosion.

Figure 3. Measurement of soil erosion

Dislocation of the soil was surveyed on the entire width of the trail (4 m), every 10 cm (Figure 3). The area of dislocated soil expressed in m2 was determined by the method of trapezium areas sum. The same survey was repeated over one metre of trail length, and the average values of both surveys were calculated. This average section area also represents the volume of dislocated soil expressed in m3 because it refers to one metre of trail length (Figure 4).

Figure 4. Determination of the amount of soil erosion on the trail


The first survey on soil erosion was carried out in November 1992 (60 days after the hauling experiment), the second in November 1993 and the third in March 1996.

The results of these surveys (Figures 5 and 6) show that on the slopes of 1.6 percent and 14.6 percent gradient there was no significant dislocation of soil and it was more the result of tractor wheels and load compaction impact (decrease of pores volume), shear displacement by wheels slippage or partially displacement of the ruts by the machine wheels. Values E1, E2 and E3 represent the first, second and third survey, and R1 and R2 the differences between E2 and E1 and between E3 and E2.

Figure 5. Soil erosion on a skid trail longitudinal slope of 1.6 percent

Figure 6. Solid erosion on a skid trail longitudinal slope of 14.6 percent

The entire loss of soil on the full trail width (4 m) on these two slopes was only 0.15-0.26 m3 in a period of almost four years. That was for the most part the result of the already mentioned skid impact and only to a lesser extent was soil dislocation due to water action.

Surveys on a steeper longitudinal slope of 26.1 percent showed different results. On both measuring points - wet part of the slope exposed to the influence of a new hillside spring that flowed on the trail surface, and the dry part (above the stream flow) - a relatively more intensive soil dislocation was observed than on the more gentle slopes (Figures 7 and 8).

The wet part of the slope of 26.1 percent showed (during the first survey) a soil dislocation of about 0.26 m3/m of trail; during the second survey it was about 0.36 m3/m of trail. In the third survey a soil loss of almost 0.54 m3/m of trail was measured.

The stream flow in this part of trail cut significantly in the waterbed, gradually carrying away the soil grains, especially in former ruts turning them into gullies.

Figure 7. Soil erosion on a skid trail longitudinal slope of 26.1 percent (wet part)

On the dry part of the same slope, the first survey showed no soil dislocation (about 0.03 m3/m of trail). The second survey, after one year. showed a soil loss ten times higher - approximately 0.30 m3/m of trail (Figure 8). Notching into the foundation, the water scours tiny trains of soil, which causes considerably deep gully formations. Also part of large skeletal (slates) emerged from the bottom of these formations.

Figure 8. Soil erosion on a skid trail longitudinal slope of 26.1 percent (dry part)

The results of the third survey show that the soil loss was approximately 0.1 m3/m of trail in about a 2.5 year period, and that is higher than on the more gentle slopes (0.07-0.08 m3/m in a 2.5 year period). However, the erosion on the same slope exposed to stream flow on the trail is almost double.

According to the classification of gullies by depth (US Conservation Service), which was presented in the Watershed management field manual by T.C. Sheng in 1990, the relatively shallow gully formations observed in all our surveys belong to small gullies (less than 0.9 m depth).

According to the modified "Erosion classification for stream" presented by the same author in the same publication (0-4 class range) soil erosion determined for shorter longitudinal slopes (1.6% and 14.6%) can be put in class 0.

In the same way, the soil erosion intensity established for the longer slope (26.1%) belong to class 1.

Compared to these classifications, all the results of our surveys show that soil erosion is not significant as the quantity of rainfall is relatively small in the region.


Research on soil erosion on several slopes of a skid trail in the four-year period 1992-1996 was carried out with the purpose of determining the influence of the longitudinal slope of the trail on the intensity and possible form of erosion.

· We observed that erosion is more intensive on steeper longitudinal slopes.

· This phenomenon is more intensive if, in addition to rainfall, there are additional influences that increase the amount and duration of water retention on the trail (permanent and periodical stream flows, frequent use of the trail, etc.)

· Soil dislocation on shorter longitudinal slopes (1.6% and 14.6%) is slow and gradual and amounts to approximately 0.07 m3/m of trail. This is an annual average (in the 3,5 year period of research) of which about one-half of the loss is due to the decrease of pore volume by compaction of the soil, or shear soil displacement (skid impact).

· The average annual loss of soils on the longer longitudinal slopes in the period of the research was about 0.15 m3/m of trail on the wet part of the trail with an inclination of 26.1 percent and about 0.11 m3/m of trail on the dry part of the same slope, However, erosion was significant in the first year when, on the wet part of the trail, the loss of soil amounted to approximately 0.36 m3/m on the entire width of the trail (4 m wide), or 0.30 m3/m of trail on the dry part.

· Analysing the results of the last and last but one survey during the 2.5-year period, it can be concluded that erosion on the dry part of the slope of 26.1 percent diminished (plant overgrowing) and the average annual loss of soil now amounts to approximately 0.40 m3/m of the 4 m wide trail.

· Soil dislocation in the last 2.5-year period on the wet part of the trail is still relatively intensive (average annual soil loss of approximately 0.07 m3/m of trail), which is due to the above-mentioned additional causes of erosion. The stream flow on the trail keeps on notching in its waterbed, constantly dislocating a certain amount of tiny soil material, while chemical reactions result in further decomposition of the skeletal part. Soil erosion will almost certain continue to advance to such a degree that further use of this part of the trail will become questionable unless the stream flow is diverted from the trail in the near future.


Rebula, E. 1991. Posljedice gradnje vlaka u šumi. Mehanizacija šumarstava, god. 16, br. 1-4, pp. 3- 10.

Sever, S., Horvat, D. & Tomašic, . 1994. A contribution to damage assessment standardization resulting from the research on machine use on variable slopes. Forsistrisk, Interactive Seminar and Workshop "Soil, Tree, Machine Interactions. Feldafing (Germany), 408 July 1994. pp. 1-9.

Sheng, T.C. 1990. Watershed management field manual, watershed survey and planning. FAO Conservation Guide No. 13/6. Rome. 170 pp.

Tomašic, . 1994a. Naiveci uzduni nagib traktorskih prometnica u svezi s projjektiranjem i nacinom njihove izgradnje te svoistvima traktora za privlacenje drva. Magistarski rad. 119 pp.

Tomašic, . 1994b. The influence of skid road longitudinal slope on the values of soil penetration resistance to soil compaction under tractor wheels and wood road. Mehanizacija šumarstava, god. 19, br. 4, pp. 233-246.

Index of values and measuring units



Definitions and descriptions



dislocated soil area of the lower transversal section (A)



dislocated soils area of the higher transversal section (B)



average dislocated soil area determined on every 10 cm of the trail transversal section



average dislocated soil area of the entire trail width M transversal section



dislocated soil volume determined for the entire trail width transversal section (soil erosion)



dislocated soil volume determined at the first soil erosion survey (November 1992)



dislocated soil volume determined at the second soil erosion survey (November 1993)



dislocated soil volume determined at the third soil erosion survey (March



dislocated soil volume determined at the third soil erosion survey (March 1996)



length of the tested trail segment (1 m) on which the soil erosion surveys were carried out



difference between E2 and E1



difference between E3 and E2



entire trail width (4 m)

a, b....


dislocated soil depths (heights) measured on lower trail transversal section (A)

a1, b1...


dislocated soil depths (heights) measured on higher trail transversal section (B)



trapezium height (0.1 m) - measuring points distance along the entire width of the trail