What strange phenomenon could have occurred recently near Douar Laâb in the commune of Beni Hilal, El Jadida province? A hole approximately 8 m in diameter and 15 m deep has opened up in agricultural land, following a land subsidence. The hole could continue to grow, and the new fissures that have appeared are dangerously weakening the ground.
A strange awakening for the inhabitants of Douar Laâb in the commune of Beni Hilal, in the province of El Jadida. An agricultural plot, located in the irrigated perimeters of Sidi Smaïl, about 2 km from the centre of Jamâa Beni Hilal, to the left of provincial road 3426, connecting this centre and the national road, suddenly subsided. A gaping hole more than 15 m deep now spreads in the middle of this land. While the event is not dangerous, it will remain particularly striking for the inhabitants of Douar Laâb, as its understanding goes beyond the analysis of the average citizen. For this state of affairs is intriguing while installing a certain fear in this area, which is highly frequented by farmers and is located a few dozen metres from residential areas, notably Douar Laâb.
It is under these circumstances that specialists from the Faculty of Sciences of El Jadida, and more precisely those from the Laboratory of Geosciences and Environmental Techniques (LGTE), responded positively to this call, which falls within the scope of their expertise and work on outdoor environments, by forming a mission that went to the site of the disaster, thus mobilising the available means to provide the first elements of an answer to the problem. The main objectives of this field trip focused on three levels, notably the location of the subsidence site, the recognition of existing data on the nature of the subsoil, essentially through the analysis of the geological context and existing surveys in the area. In addition to a hydrogeological survey through the diagnosis of the nature and modalities of groundwater circulation in the sector.
The initial diagnoses established by the team of scientists demonstrate, regarding the location and identification of the subsidence site, that this place is located in the irrigated perimeters of Sidi Smaïl, about 2 km from the centre of Jamâa Beni Hilal, to the left of provincial road 3426, connecting this centre and the national road. It occurred in the centre of an agricultural plot located 150 m from a dwelling in Douar Laâb. It is a subsidence forming a hole of approximately 8 m in diameter and 15 m deep, likely caused by a natural phenomenon frequent in the Sahel Doukkala-Abda area. According to Abderrahim El Achheb, a hydrogeologist professor, the classic cases described in the literature show that we are here in the presence of a "Fontis" or "Suffosion" type collapse. Taking into consideration the existence of gypsiferous rocks at depths around 50 m, the main conclusion that emerges regarding the origin of the collapse relates to the location of the subsidence in an area with a high potential for groundwater circulation. The consequences are indeed direct on the degree of alteration and dissolution of rocks at depth. The development of karst cavities is probably accelerated by fluctuations in the water table level and the resulting dissolution activity, as the decrease or increase in internal pressure can lead to collapse above open cavities under poorly coherent rocks. The presence of gypsum in the deep formations can cause the formation of gypsum karst. We are therefore probably in an area where gypsum leaching continues, where there is a strong geochemical imbalance due to lateral and vertical drainage: recharge of the water table by irrigation water and rapid circulation of water in cavities and fractures.
