تغییرات در برخی از خواص مکانیکی خاک لومی تحت تاثیر بهره برداری مکانیزه از جنگل ها در جنگل راش بلژیک مرکزی

Modification of some soil mechanical properties (penetration resistance and consolidation pressure) induced by vehicle compaction during mechanized forest exploitation was studied in an acid and loamy leached forest soil of the loessic belt of central Belgium. In situ penetration tests and laboratory Bishop–Wesley cell tests were undertaken for the two main soil horizons of a beech high-forest, i.e. the eluvial E horizon (5–30 cm depth) and the underlying clay-enriched Bt horizon (30–60 cm depth). Both undisturbed and wheel-rutted soil areas were studied (E and Bt horizons vs. Eg and Btg horizons). Results show that: The experimental overconsolidation pressure of the eluvial reference horizon (E) is about 50 kPa higher than the value calculated from soil overburden pressure; this probably results from suction action during dry periods. The clay-enriched reference horizon (Bt) shows the same trends. In wheel-rutted areas, seven years after logging operations, the Eg horizon memorizes only 14.5% of the wheel induced stress due to forest machinery. In the compacted Btg horizon, the experimental overconsolidation pressure represents 96% of the exerted theoretical stresses due to harvesting actions. The good recording of the exerted stresses, after seven years, can be explained by: (1) The Btg depth which keeps it from seasonal variations i.e. from desiccation–moistening or freeze–thaw cycling; (2) amorphous and free iron accumulation inducing a “glue” effect of the Btg soil matrix, which could stabilize the soil structure and prevent recovery to initial conditions. These results provide clear evidence that on loessic materials, soil compaction due to logging operations leads to modifications in both physical (bulk density, total porosity) and mechanical (penetration resistance and consolidation pressure) soil properties.

In forest ecosystems, the increase in size, power and weight of forest machinery is one of the main causes of soil degradation (soil compaction and related effects) [1] and [2], the soil being subjected to severe stresses due to mechanical forces exerted by tractor tyres and skidding. Results of soil compaction include the increase in bulk density, the reduction in macroporosity, and, consequently, reduced soil aeration and water holding capacity [3], [4], [5] and [6]. This results in a poor rooting, inducing lower uptake of nutrients and water, and possibly a decrease in tree growth [2], [7] and [8] or an increase in sensitivity of radial growth [9]. Moreover, soil compaction and root damage increase the risks for windthrow and infection by root pathogens [10] and [11]. Soil susceptibility to compaction is mainly determined by particle-size composition, organic matter content and biological activity level. Biological activity is particularly important for soil structural regeneration. Therefore, more attention should be given to degradation in fine-textured and acid forest soils, with low levels of biological activity [12]. These soils are known to be very sensitive to compaction especially where natural soil remediation is very low or even non-existent [3], [13] and [14]. In the loessic belt of central Belgium, the silvicultural management of loamy soils developed under beech stands frequently results in severe soil physical degradation, which causes surface waterlogging (gleization) in the areas compacted by wheel-ruts [5] and [15]. In such conditions, natural forest regeneration is impeded and costly soil improvements may be necessary before planting trees. Moreover, soil compaction due to machine traffic is often referred to as an important aggravating factor in beech decline [16]. In spite of their importance for a sustainable forest management, few analytical data have been published in regard to the degradation of soil physical properties under the influence of repeated wheeled traffic during logging operations and, to our knowledge, changes in soil mechanical properties have been appraised in a forest environment only recently [37] and [38]. The aim of this study is therefore to use mechanical laboratory tests (Bishop–Wesley cell tests) to determine to what extent the mechanical properties and mainly compressibility of a loamy soil developed under a beech forest are affected by logging operations. Previous results had shown that a “cementation” effect as defined by Biarez et al. [17] was detectable in the mid-depth horizon of the wheel-rutted areas [6]. To assess the degree of compaction sustained due to tire and the stress state still memorized by soil horizons in undisturbed and adjacent wheel-rutted areas, the overconsolidation pressure (pc) of these horizons was measured using experimental tests and theoretical calculations. The comparison of the measured and the calculated overconsolidation pressures may give useful information about natural recovery of the forest soil compacted by forestry machines. This also allows estimating the maximum pressure not to be exceeded in the future for wheel-rutted areas, in order to avoid “re-compacting” the soil.

This paper is a contribution to a better understanding of the long time effects influence of mechanized exploitation on forest soils. It is now a well established fact in geotechnics that an overconsolidated soil has an elastic behaviour for stress states lower than the highest one undergone in his history. For forest soils after logging it is important to know the overconsolidation stress-state because it gives information about the tire contact pressure that may not be exceeded in the future to prevent further compaction. Geotechnics, do not take in account any time or climatic effect on overconsolidation. Tire induced stresses affect only the superficial soil layers, whose structure is also influenced by climatic effects as freeze–thaw and moistening–drying. The question is thus to know if and how much the later can affect the “soil memory” of the logging stresses. Although limited to one kind of loamy soil, an answer based on the comparison of calculated and experimental measured stresses was given for a delay of seven years after harvesting. It pointed out that for the most upper layer (10–30 cm) all the memory of the tire induced overconsolidation has been erased and replaced by the stresses induced during very dry periods. On the other hand, in the layer 30–60 cm, the measured consolidation pressure is still 96% of the exerted theoretical pressure during harvesting. This can be explained by: (1) The localization in a moderate depth keeps it from seasonal variations and from desiccation–moistening or frost–thaw phases. (2) Amorphous and free iron accumulation in the Btg horizon induces probably cementation effects of the soil matrix [6]. This “glue” effect could stabilize the soil structure and prevent it to recover its initial characteristics. Thus without intervention by man, forest soil recovery after logging is a long time evolution in loamy soils.