Unravelling shallow subsurface deformation processes leading to land subsidence in organic-rich coastal plains

Many low-lying coastal plains worldwide contain abundant organic facies, often peat, in the Holocene subsurface. These facies are very susceptible to soil deformation processes, some of which leading to irreversible land subsidence. Most important irreversible processes are oxidation of organic matter, shrinkage (in the unsaturated soil zone) and compaction (in the saturated soil zone). Reversible soil deformation processes are shrinkage and swell and poro-elastic deformation.

In cultivated areas, deformation processes leading to land subsidence are often driven, and accelerated, by human activities such as drainage for agriculture and loading of the subsurface. To reduce land subsidence, the first step is to quantify subsidence rates in space and time and to identify the relative contribution of soil deformation processes to total subsidence. Next, measures may be developed and applied.

At various locations in the Dutch coastal plain, extensometers specifically designed for soft organic facies are used to measure vertical movement of multiple levels in the Holocene subsurface at high temporal resolution. Resulting multiyear timeseries are used to quantify the amount of irreversible and reversible deformation over time for different soil intervals, which subsequently may be linked to soil deformation processes. Results demonstrate a large variability in the relative contribution of deformation processes to total subsidence, due to spatially variable geological and hydrological circumstances, indicating that site-specific measures are needed to reduce land subsidence. This spatial variability also requires spatially explicit mapping approaches, e.g. models, to predict deformation behaviour in soft soil sequences.