Physical experiments of land subsidence within a maar crater: insights for porosity variations and fracture localization

Abstract

Abstract. We present the results of a series of physical models aiming to reproduce rapid subsidence (at least 25 m in 30 years) observed in the sediments of a maar crater caused by extraction of groundwater in the interconnected adjacent aquifer. The model considered plausible variations in the geometry of the crater basement and the measured rate of groundwater extraction (1 m per year in the time interval from 2005 to 2011) in 15 wells located around the structure. The experiments were built within a rigid plastic bowl in which the sediments and rocks of the maar sequence were modeled using different materials: (a) plasticine for the rigid country rock, (b) gravel for the fractured country rock forming the diatreme fill and, (c) water saturated hollow glass microbeads for the lacustrine sedimentary fill of the crater. Water table was maintained initially at the surface of the sediments and then was allowed to flow through a hole made at the base of the rigid bowl. Water extraction provoked a sequence of gentle deformation, fracturing, and faulting of the surface in all the experiments. Vertical as well as lateral displacements were observed in the surface of the experiments. We discuss the results of 2 representative models. The model results reproduced the main geometry of the ring faults affecting the crater sediments and helps to explain the diversity of structures observed in relation with the diatreme geometry. The surface of the models was monitored continuously with an optical interferometric technique called structured light projection. Images collected at nearly constant time intervals were analyzed using the ZEBRA software and the obtained interferometric pairs permitted to analyze the full field subsidence in the model (submilimetric vertical displacements). The experiments were conducted at a continuous flow rate extraction and show a also a linear subsidence rate. Comparison among the results of the physical models and the fault system associated to subsidence in the maar show that fault geometry in the sedimentary sequence imitates closely the geometry of the volcanic basement.

Analogue model of ground ruptures due to groundwater pumping in an aquifer above a bedrock ridge

“…The displacements near the slopes of the ridge decrease significantly because of the small thickness of the compressible material. The experimental results reproduce also some of the general observations made by Budhu (2008), i.e. the surface of the experiment evolved from (1) gentle bending to (2) rupture and (3) shearing.…”

“…Overall our experiment shows that analogue models can successfully simulate the results of previous finite element numerical models and are qualitatively similar to those obtained by Frigo et al (2019). Dynamic scaling of the frictional material is straightforward, but the geometrical similitude is not completely achieved, since the angle of fracture formation is prescribed as vertical in the numerical model.…”

“…From the mechanical point of view, wet microbeads form fractures and faults that follow a Mohr-Coulomb behaviour with an angle of friction around 30 • and negligible cohesion. The use of a Mohr-Coulomb criterion to model ground fissures was proposed previously by Budhu (2008). Lateral walls create a border effect that limits the fracture propagation.…”

“…Understanding of the mechanical processes controlling the formation of ground fractures is needed. Three common geologic configurations encountered in aquifer systems prone to deform and fracture because of extensive groundwater pumping have been recently studied by using an advanced numerical model (Frigo et al, 2019). The three cases correspond to (i) reactivation/displacement of a pre-existing fault, (ii) differential compaction due to variations in thickness of aquifer/aquitard layers constituting the aquifer system and (iii) the development of tensile stresses above a bedrock ridge that forms the base of the aquifer system.…”

“…Scale analogue experiments of land subsidence are rare (e.g. Cerca et al, 2015) but can help engineers to gain insight into the dynamic evolution of aquifer systems through their exploitation. Physical experiments are made with analogue materials that behave mechanically similar to natural materials in scale.…”