Supersized peat samples for predicting land subsidence

Published: 18 May 2020

Supersized soil samples are being taken at the peat meadow test site KTC Zegveld (in the Dutch municipality of Woerden) with the aim of predicting the extent of land subsidence due to the compression of peat. The samples, which have a diameter of 40 cm, can be used to determine the geotechnical properties of the peat such as its compressibility, density, and amounts of organic matter and carbon.

These geotechnical properties are used as input for land subsidence models. The high compressibility of peat is an important cause of land subsidence in areas with a peaty subsurface and it is therefore important to include this factor in predictions of land subsidence. ‘We can use these models to predict how far and how fast peat land can still subside as a result of peat compression, under various scenarios of load and groundwater level,’ says Sanneke van Asselen, a Deltares land subsidence expert. That information can be used to take more effective steps to combat land subsidence.

Undisturbed soil samples with the Deltares Large Diameter Sampler

In all, five samples with a diameter of 40 cm and 50 cm high will be taken with the Deltares Large Diameter Sampler (DLDS) in collaboration with Wiertsema and Partners. The DLDS was developed specifically for sampling in peat, a soft type of soil with relatively large fibres and a heterogeneous structure. The relatively large volume is needed to study the peat samples in as undisturbed a condition as possible in the Deltares geotechnical laboratory. With this sampling method, the geotechnical properties of peat can be determined better than with the standard methods. To determine compressibility, parts of the samples are subjected to pressure in controlled conditions until they deform. At that point, it is possible to arrive at conclusions about how strong the peat is.

This video gives an impression of how the supersized soil samples are being taken  by the Deltares Large Diameter Sampler  at the peat meadow test site KTC Zegveld.

National network monitors land subsidence structurally and frequently

The test location in Zegveld is one of the five locations in the national network for the structural and frequent monitoring of land subsidence. These locations are part of the National Research Programme for Greenhouse Gas Emissions from Peat Meadows (NOBV). In this programme, a consortium of research organisations is studying current levels of greenhouse gas emissions from peat, the factors that affect emissions and possible measures to reduce emissions. The study has been organised in the context of the Climate Agreement, which also involves monitoring land subsidence. The measurements can be used to unravel the processes that lead to land subsidence in peat meadow areas. Effective technical and policy measures can be developed only if we know which processes cause subsidence. Structural monitoring helps to determine the right approach for each area.