Soil compressive behavior: a global assessment of research outputs

Understanding the compressive behavior of soils is essential for establishing management strategies to reduce the risk of soil compaction. Soil compressive properties such as precompression stress, compression index, and swelling index are used to estimate the stress-strain relationship of soil, i.e., the changes of soil volume as a function of applied stress. However, there is no consensus regarding the influence of basic soil physical properties and conditions, such as soil texture, organic carbon content, clay mineralogy, water content, and bulk density on soil compressive properties. Moreover, soil compressive behavior has been measured following non-standardized methods, for example regarding sample size, loading time, methods to obtain the compressive properties from the stress-strain curve, and stress components and packing state of the soil by which the soil compressive behavior can be expressed. These differences in methodology influence the obtained values of soil compressive properties, make comparisons difficult, and limit our understanding of the soil’s stress-strain relationship. We conducted a comprehensive literature study in search of quantifications of compressive properties of agricultural and forest soils, such as precompression stress, compression index, and swelling index, in peer-reviewed articles from the Web of Science and Scopus databases, which currently includes more than 200 articles. We systematically collected the compressive properties as well as information on the soil, soil conditions, methodologies, and other relevant information for each of the published studies. A large part of data originates from a limited number of laboratories in Brazil, Denmark, Germany, Iran, and Sweden, while other parts of the world are less or not represented. We find large variability in soil mechanical properties, that is associated both with variability in soil texture and land use but also with methodological issues. Initial soil moisture was identified as a key driver of soil mechanical properties. Our database allows compiling, synthesizing, and analyzing the data in favor of a comprehensive establishment of relationships between basic soil physical attributes and compressive properties. At the same time, the database is used to identify knowledge gaps and future directions for studies. These findings help the potential development of pedotransfer functions to improve estimations of the soil response to compaction, and to provide a research agenda for a more unified approach for the study of soil compressive properties.