The relationship between organic content and compressibility of peat and clay soils

Peat soils typically have a high void ratio and compressibility. As a result, land subsidence is a common threat to peatlands. Geotechnical models are used to predict subsidence and the spatial variation therein to assess the potential for land subsidence and the effect of mitigation measures. In these models compaction is often calculated using parameters describing the consolidation and creep behaviour of the soil. Consolidation, a result of excess pore pressure dissipation, is calculated using the compression (CR) and recompression ratios (RR) together with the effective stress and preconsolidation stress. Creep, viscous compression unrelated to dissipation of excess pore pressure, is calculated using an independent secondary compression parameter (C_α) and the preconsolidation stress which are derived from laboratory compression tests. Pristine peat soils have a much higher high organic content (OC), a higher compressibility and are more susceptible to creep than clay soils. This study explores the relation between compressibility and OC by looking at the OC end members, peat and clay, and specifically looking for patterns in mixtures of the two.

A large collection of compression test data is used to evaluate the relation between the compressibility of peat and its organic content. Holocene peat and fluvial and marine clay samples from the Netherlands are analysed. The composition of the samples is described by parameters such as wet unit weight, water content and organic content. A relation between water content and loss-on-ignition was determined by fitting a trend to a different extensive collection of Holocene and lake-infill peat samples from the Netherlands. This relation was used to acquire OC for all compression test samples. Analysis results show that for low OC (<30%) samples values of CR, RR as well as C_α increase with increasing OC following a significant trend. The high OC (>30%) samples show relatively high values for all three parameters compared low OC, about twice as high on average, but due to high variability in these values no trend can be discerned.

These differences between the low and high OC results point out that compressibility is related to the OC for clay rich samples, whereas the compressibility of organic rich samples is not dictated solely by the OC. Using this outcome a distinction was made between clay-dominated and peat-dominated compaction behaviour. This analysis enables an estimation of the three compaction model parameters for clay-rich samples based on the OC. Hence, for example, an OC-dependent C_α can improve subsidence modelling compared to a static C_α which does not incorporate changes in soil composition over time: loss of OC of peat soils and organic clays via oxidation or anoxic decomposition will influence the compressibility of the soil rather than or besides the volume loss directly linked to the loss of organic content. This will alter the way we calculate not only compaction but might also impact the calculation of greenhouse gas emission as a result of decomposition, which is a current topic within land subsidence.