Soil Shrinkage Characteristics of Peat and Other Organic Soils

Shrinkage is the volume reduction of a soil due to desiccation and decreasing pore pressure. This is important for the determination of volume based physical and hydraulic soil properties in the laboratory e.g., bulk density, volumetric moisture and water retention functions. Furthermore, it leads to changing surface elevation and crack formation at the field scale.

There are two types of shrinking soils, clayey soils and organic soils, which are defined here as soils having a soil organic carbon content (SOC) above 7.5%. Clayey and organic soils differ strongly in their shrinkage behavior.  Furthermore, only few shrinkage studies differentiate between different organic soils. Parameters of existing shrinkage models are fully empiric and not directly linked to soil properties as dry bulk density, SOC, botanical composition and degree of decomposition.

To determine the soil shrinkage characteristics (SSC i.e., relationship between moisture ratio and void ratio) of a variety of organic soils, we determined sample volumes with a three-dimensional (3D) structured light scanner at different moisture states from full saturation to dryness. We sampled 33 horizons (n = 4 replicates each) covering a wide range of botanical composition, development stages and degree of decompositions. Desiccation was performed by suction plates up to pressure heads of -200 hPa, followed by evaporation and oven-drying at 105°C. Volume and height of the 3D models created this way were determined by 3D graphic software and R, respectively. The volumetric moisture was determined by weighing the sample before and after scanning. Afterwards, volume and volumetric moisture were converted to moisture ratio and void ratio with the volume of solid particles. Due to small differences in particle volume between the replicates, both, moisture and void ratio were normalized by dividing them by the value at saturation. This normalization led to congruent results for the replicates.

The shape of the SSCs strongly depended on the botanical composition and degree of decomposition. Peat consisting of slightly decomposed Sphagnum remains showed a supernormal shrinkage phase, i.e., volume reduction exceeds lost water volume at the dry end of the SSC and a relatively large range where volume reduction is (much) smaller than lost water volume, i.e., subnormal or structural shrinkage phase, at the wet end. The latter behavior was also shown by amorphous top soils. With increasing degree of decomposition or complete absence of Sphagnum remains the SSC flattened and tended to show a (near-) normal shrinkage phase, i.e., volume reduction equals lost water volume.

The results showed that rigid Sphagnum remains strongly influence the shrinkage behavior of organic soils by stabilizing the matrix during desiccation until the large pores collapse rapidly when soil moisture and tension undercut a certain level. The SSC of organic soils without rigid fibers were more similar to SSCs of clayey soils.