CHARACTERISTICS OF HORIZONTAL DEFORMATION OF SOME FOREST SOILS

Mariusz Kormanek¹, Maria Walczykova^2
1 Department of Forest Work Mechanization, Faculty of Forestry, University of Agriculture in Krakow, Poland
² Department of Machinery Management, Ergonomics and Production Processes, Faculty of Production and Power Engineering, University of Agriculture in Krakow, Poland

ABSTRACT

The paper presents results of measurements of horizontal shearing of soil, made by the help of wheel bevameter on four types of forest sites, i.e. fresh mixed coniferous forest (fMCF), boggy mixed pine forest (bMPF), moist mixed broadleaved forest (mMBF) and moist broadleaved forest (mBF), with consideration of different moisture variants. On the basis of obtained data two relationships were determined: between soil horizontal deformation and the circumferential (driving) force as well as between slip and the coefficient of adhesion. Grouping of the investigated soil according to their shearing stresses was made by Ward’s method of agglomerative cluster analysis. For each of the obtained agglomeration new data of slip vs. coefficient of adhesion were calculated.

Key words: forest soils, coefficient of adhesion, slip, shearing stresses, cluster analysis.

INTRODUCTION

Environmentally friendly forest development is considered to be inevitable with regard to restraining soil erosion processes, preservation of water resources as well as protection of the landscape and natural environment [12]. Timber harvesting that enables using wood resources has at the same time negative influence on the environment. As it requires great physical effort of men it must be mechanized [14, 16, 19] and mechanized felling, bucking and skidding directly affect the forest stand and soil. Relocation of machines under forest conditions takes place both on skidding roads and forest sites [6, 9, 18], and it can be considered in two aspects. The first, it is effect of the vehicle wheels that are of prime importance as far as soil damage is concerned [1, 8, 11, 21]. The second is related to the machines traction properties. The wheel ability to transmit driving force on a given soil is decisive with regard to the vehicles traction properties such as pulling force and slip properties, and in fact determines the vehicle mobility under given soil and terrain conditions [1, 7, 17]. Knowledge of traction parameters in relation to the forest soils makes possible to determine the effects that the vehicle used under given conditions can have on the performed job as well as to foresee them [1].

On the basis of available literature review dealing with forest machinery management, it can be stated that only very limited number of research attempts were made towards improving knowledge of forest soil traction problems.

In the presented paper problems of traction properties of four forest sites were undertaken and its scope included:

• determination of the pneumatic wheel slip in relation to coefficient of adhesion,

• grouping the investigated forest soils according to their shearing stresses.

MATERIALS AND METHODS

Variables that determine mutual influence of pneumatic wheel and the ground are basically traction forces making the vehicle movement possible, as well as slips generated by these forces [1]. In classical theory of mobility the coefficient of adhesion is applied in assessment of mechanical vehicles traction properties. This approach is commonly used in approximate calculations of agricultural and forest tractors operating parameters [7, 10].

In method based on coefficient of adhesion µ, the calculated force of adhesion F_µ is understood as a maximum force that wheel can transmit on the ground. The force is proportional to the wheel loading Q_k and its value depends on coefficient of adhesion µ characteristic of the given ground (substrate) (eq. 1) [1, 2, 5, 15, 20].

Fµ = Q_k µ     (1)

The driving wheel will move, when its circumferential (driving) force will be less or equal to the adhesion force (eq. 2).

F_k ≤ F_μ     (2)

Otherwise slipping of the wheel occurs. Values of coefficient of adhesion of a pneumatic tyre depend on type of ground and its mechanical and physical properties as well tyre properties. The above mentioned brings about considerable difficulties in obtaining accurate values of coefficient of adhesion. In calculation only approximate values obtained in measurements are used [5]. Those available in literature are basically related to agricultural soils. Appropriate data for forest soils are in fact non existing.

Layout of measurements

Terrain measurements of horizontal shearing of forest soils were made by the help of wheel bevameter [18] on four types of forest sites, i.e. fresh mixed coniferous forest (fMCF), boggy mixed pine forest (bMPF), moist mixed broadleaved forest (mmBF) and moist broadleaved forest (mBF). With consideration of different soil moisture contents altogether 8 variants were investigated.

Occurrence of features characteristic of the given forest site was used as a criterion of its choice. Open canopy of the forest stands that would not restrain relocation of the measuring devices, and as far as possible flat surface, were taken into consideration as additional factors during laying out the experimental site.

On each site four blocks of ca 30 m long were laid out. Each block consisted of 18 repetitions, which gave 72 replicates for one variant of measurement. Each wheel rut, formed as a result of the soil shearing process, was subject to measurement of its dimensions.

Physical properties of soils were characterized by dry volumetric bulk density, moisture content by weight and penetration resistance. Measurements of the latter were made by cone penetrometer with an electronic data logger, in 120 replicates for each experimental site. Volumetric cylinders were used for density and moisture determination, collecting 5 samples from 3 depth, up to 0.3 m.

The bevameter soil shearing wheel was equipped with tyre of 165/80-R13 dimensions, inflated to p_i = 240 kPa and loaded by Q_k = 4777 N, according to the catalogue data.

Processing of measurement data and calculation of the soil horizontal deformation

Quantity obtained in terrain measurements was torque M_k, measured on the bevameter soil shearing wheel and recorded at every 1.5° of the wheel turning angle.

In order to get the relationship between horizontal displacement of soil j and the circumferential shearing (driving) force F_k, the following calculations were made. Values of the turning torque M_k were divided by the wheel dynamic radius R_d, determined on the site during measurements (eq. 3).

(3)

Conversion of the wheel turning angle into soil horizontal displacement was done by determination of distance (arch) O, along which wheel circumferential points were relocated during the wheel turn by angleb (eq. 4).

(4)

In order to get coefficients of adhesion µ, measured circumferential shearing forces F_k were divided by the wheel vertical loading (eq. 5).

(5)

The next step consisted of determination of slip value s for each value of coefficient of adhesion µ. According to the literature [3] it was assumed that the slip reaches its maximum (i.e 100%), when the wheel turns by an angle corresponding to relocation of its circumferential points along the full length of the soil-wheel contact area. Thus, division of the 100% value by the wheel turning angle embracing the length of the contact area resulted in slip value per 1° of the wheel turn, for which corresponding coefficient of adhesion was determined (eq. 5).

It is known that on deformable surfaces the ability of driving wheels to transmit circumferential forces depends on relation between shearing stresses τ and the soil horizontal displacement j. The above mentioned relations F_k = f(j) were than converted into relations τ = f(j) through dividing the shearing (driving) force by the wheel-soil contact area (eq. 6).

(6)

Dimensions for calculation of the contact area (eq. 7) were determined under terrain measurements.

(7)

Width of the wheel rut b corresponded in fact with the width of tyre, the length was calculated from eq. 8 [13]. Depth of the rut z and tyre deformation D were measured for the given terrain conditions.

L= [[D (z + Δ) – (z + Δ)² ] + (D Δ – Δ² )]²     (8)

RESULTS

Characteristics of the forest site properties

On both sites of fresh mixed coniferous forest (fMCF) (TABLE 1) podsolic soils occured from sandy soils textural group [4]. Upper organic layer present on both compartments was made up of fresh organic matter. On compartment No 188 it was raw humus richly covered by reedgrass, while on compartment No 244 the organic matter was covered by bilberry.

Soil on boggy mixed pine forest (bMPF) was classified as boggy rot on light loamy sand [4]. This place differed from the rest of investigated sites by its low bulk density and high moisture content (TABLE 1). Average penetration resistance there was lowest from among all investigated sites (FIGURE 1).

Gley soils characterized the moist mixed broadleaved forest (mMBF) site. Texturally they were numbered among strong loamy sands (compartment No 244), and sands layered by non skeleton light loamy sand (compartment No 291) [4]. On both compartments the layer up to 0.1 m was made of raw organic matter.

Soil physical properties measured on the moist broadleaved forest (mBF) were characterized by lowest coefficient of variability (TABLE 1). This was mainly caused by lack of soil cover. Up to 0.35 m the soil on this site was classified as heavy silty loam. Under dry conditions it formed dense structure, while at higher moisture it became plastic.

Average penetration resistance concerning all sites and their moisture variants were presented in FIGURE 1.

Shearing forces and slips

Horizontal shearing of soils was carried out on the above described sites and for each research variant results were drew up in form of charts. For each considered situation two charts were presented – relation between shearing strength (wheel driving force) vs. soil horizontal displacement and slip vs. coefficient of adhesion.

On the soil of fresh mixed coniferous forest site (fMCF) maximum value of shearing strength occurred at 0.11m and 0.125 m soil displacement, which corresponds with lower and higher moisture respectively (FIGURE 2). Charts in FIGURE 3 and data included in TABLE 2 show that higher coefficient of adhesion amounting to 0.73 was obtained on weaker soil of fMCF_244_w₂ (TABLE 1). This phenomenon was caused by presence of the above-ground part of bilberry and its root system on that site. Maximum driving force that can be transmitted on the considered soils of fresh mixed coniferous forest site (fMCF) would cause 40.2% and 55.3% of slippage on denser (188_w₁) and weaker (244_w₂) variant respectively (TABLE 2).

In the case of boggy mixed pine forest (fMCF) (FIGURE 4) the soil horizontal displacement at maximum shearing strength, corresponding with maximum driving force, came to 0.081m and beyond that point its value was slowly decreasing along the wheel rut. Boggy rot soil of low bulk density and high moisture content (TABLE 1) as well as lack of living plants cover contributed to forming the longest wheel rut (0.29 m) from among all investigated sites and their moisture variants. Curve of slip dependence on coefficient of adhesion (FIGURE 5) showed characteristic bending at the moment when maximum value of coefficient of adhesion was reached. It happened at 30% slip (TABLE 2), after which values of the driving force on that soil were dropping.

Results of measurements on moist mixed broadleaved forest (mMBF) in three moisture variants depict charts in FIGURE 6. Shape of the curves is characteristic for soils revealing both cohesion and friction. Values of coefficient of adhesion were decreasing with increasing moisture content (FIGURE 7) and were located between 0.73-0.77 (TABLE 2).

Curves of shearing stresses (wheel driving forces) on soil of moist broadleaved forest (mBF) showed asymptotic character without revealing maximum value. It happened at both moisture variants. At lower moisture content lugs hardly sunk into the hard soil and the shearing tyre gave worse grip. At higher moisture soil became plastic and yielded without showing clear shearing maximum. Lack of plant cover considerably contributed to the above described phenomenon. Obtained coefficient of adhesion amounted to 0.52 and 0.60 for dry and moist soil of moist broadleaved forest (mBF) respectively (TABLE 2).

Grouping of the investigated substrates

Shearing strengths (FIGURES 2, 4, 6, 8) were converted into shearing stresses (FIGURE 10) according to equation 6 and agglomerative cluster analysis by Ward’s method was carried out. Results of the procedure were shown in FIGURE 11.

From among 8 research variants differing from each other in type of soil, moisture and plant cover, four groups (clusters) emerged.

Cluster 1: fMCF_w₂; mMBF_w₁
Cluster 2: fMCF_w₁; mMBF_w₂; mMBF_w₃
Cluster 3: mBF_w₁
Cluster 4: bMPF; mBF_w

Order in which groups of substrates were listed corresponds with decrease of their resistance to shearing.

Cluster 1 includes forest soils characterized by ability to transmit high shearing stresses. Here can be found sites with sandy soils at low and medium moisture content. Nevertheless, their resistance to shearing results mainly from the plant covers. That’s why substrate of fresh mixed coniferous forest (fMCF_w₁) at lower moisture (w₂) turned up here, in spite of less favorable soil physical properties than of its equivalent fMCF_w₂.

Cluster 2 includes more resistant sandy soil substrates. Moist mixed broadleaved forest (mMBF) at the considered moisture contents showed best traction ability.

In the third group (cluster 3) substrate of moist broadleaved forest (mBF_w₁) was found. As it was already mentioned, hard surface, bad grip of tyre and lack of plant cover can be named as reasons for reaching rather small shearing stresses.

The last group (cluster 4) is made up of two weakest substrates – moist broadleaved forest (mBF) at plastic conditions and boggy mixed pine forest (bMPF).

For each group of substrates agglomerative curves of shearing stresses vs. soil horizontal displacement (FIGURE 12) and slip vs. coefficient of adhesion (FIGURE 13) were drawn up. In TABLE 3 values of slip for determined clusters were included.

CONCLUSIONS

1. Relationships between slips and coefficients of adhesion determined for some soils make possible rough assessment of the vehicle traction properties working under comparable conditions.

2. Agglomeration of the investigated soils with consideration of their moisture variants showed that type of the forest site does not condition to the full extent the soil resistance to horizontal shear. Heavy soil of moist broadleaved forest (mBF) at plastic state behaved with regards to traction similarly as weak soil of boggy mixed pine forest (bMPF).

3. Plant cover of the forest site soils had considerable and positive influence on their traction properties.

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List of symbols

A – soil-wheel contact area (m²)
b – width of the tyre (m)
D – wheel diameter (m)
F_k – driving force (kN)
F_µ – adhesion force (kN)
j – soil horizontal displacement (m)
L – length of soil-wheel contact area (m)
M_k – wheel driving torque (N·m, kN·m)
O – length of arc (m)
p_i – inflation pressure (kPa)
Q_k – wheel loading (kN)
R_d – dynamic radius (m)
s – slip (%)
z – depth of wheel rut (m)
β – wheel turning angle (°)
Δ – tyre deformation (m)
µ –coefficient of adhesion (–)
µ_max –maximum coefficient of adhesion (–)
τ – shearing stresses (kPa)

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