Assessing the compaction susceptibility of South African forestry soils .1. The effect of soil type, water content and applied pressure on uni-axial compaction

The widespread use of heavy machinery during harvesting and extraction operations in South African timber plantations has led to concern that soil compaction is causing declines in long-term site productivity and environmental damage. In this paper compaction behaviour is described and compared for a wide range of forestry soils. A description of compaction behaviour was obtained by the use of a simple uni-axial compression technique which enabled the compaction behaviour of typical South African forestry soils to be described and compared. Water-pressure-density (W-P-D) diagrams and semi-log plots of bulk density against applied pressure (compression curves) describe the compaction behaviour of a wide range of forestry soils and provide a means with which to establish the relative importance of applied pressure and water content on the soil compaction process. The range of applied pressures and water contents commonly encountered during timber harvesting and how they effect soil compaction are described by a model utilising applied pressure, water content and initial bulk density as independent variables. The coefficients in the model were related to commonly measured soil physical properties such as clay plus silt percentage or loss-on-ignition (LOI). Soils with between 5 and 10% LOI and between 50 and 70% clay plus silt underwent the greatest increase in compactin as measured by the compression index. Similarly, the role of water content in the compaction process was most important for soils having between 4 and 7% LOI and between 45 and 65% clay pins silt, Inasmuch as local geology affects particle size distribution, clay mineralogy and organic carbon content, it is an effective way of establishing a first approximation of the likely compression behaviour of forestry soils.