When mining-induced subsidence is strong and concentrated, such as is the case with salt solution or coal mining, there is a non-negligible horizontal component to the associated above-the-ground movement. With traditional levelling, this component was not measured, but with modern techniques such as PS-InSAR from space, we can observe and quantify its effect on buildings. An integral assessment of effects of subsidence on buildings is needed to justify risk assessment based on relatively simple subsidence modelling observation I and II).
Observation I. An example of a single subsidence bowl, showing up slightly dislocated in the east-west direction in two different viewing geometries (left). By geometrically combining the two observations, a rough estimate of horizontal vs ‘real’ vertical deformation is constructed (right) along the 4 km long dashed line. Does this additional movement induce risks that may be neglected when only vertical movement is considered? And how is the total strain on buildings and associated damage risk to be assessed from large-scale subsidence modelling?
Observation II. An example of the measured vertical (top) vs. the horizontal (bottom) deformation velocity derived from publicly available InSAR measurements (EGMS, 2023). The inset picture on the right shows an area west of Veendam where a relatively large difference in horizontal movement is detected: blue is moving towards the East and red is moving towards the West. This movement must be related to salt mining.
The available guidelines by Boscardin and Cording (1989) on when subsidence-induced strain may lead to building damage are rather simplistic and have mostly been developed in the context of tunnel excavation a relatively shallow source of subsidence. In Q2 2024 the TU Delft will publish results of research carried out for IMG and CM within the Gemma program. In this research TU Delft created FEM models and combined this with results of lab tests on masonry walls. The goal was to develop probabilistic fragility curves for representative masonry wall lay outs under horizontal strain and curvature in combination with the factor Psi as an indicator of the extent of damage. With the results it becomes possible to refine the classification of damage according to Boscardin and Cording's criteria. Furthermore, the TU Delft has explored the transfer of strains from the shallow subsurface to buildings involving various soil and building models. We inquire at KEM for advice on scientific approaches that may validate the applicability of these fragility curves. The topics that require investigation include the horizontal strain and angular distortion on buildings caused by both vertical and horizontal movements that can be measured at the surface.
The research questions to be addressed are: (I) to quantify more systematically the observed horizontal deformation for appropriate cases in the Netherlands; (II) to quantify the development of horizontal deformation as a function of time for those cases identified in (I). (III) to review the existing literature on the effects of (differential) ground movement, also related to shallow effects: where in (or near) a subsidence bowl may stresses occur, and how do directions of movement interact? (IV) to validate, sharpen, or define a simple set of rules by which the total strain on buildings due to ground deformations can be assessed from predicted vertical motions alone (possibly combined with background knowledge on underground characteristics of the location); and (V) to define, based on the above calculated total strain, in combination with fragility curves from the TU Delft, a classification system by which probability of different building damage categories due to direct deformation of the underground can be evaluated.
The project is commissioned to TNO and partners and started in 2025.