Experimental analysis of a thermoactive underground railway station

Little is known about the real energy potential of thermoactive underground infrastructures, such as railway stations, that can act as a heating/cooling provider for the built environment. This study presents the results of thermomechanical full-scale in situ testing and numerical analysis of a thermoactive underground train station. The thermal performance and related geostructural impact of a portion of the new underground energy infrastructure (UEI) installed at the Lancy-Bachet train station in Geneva (Switzerland) are analyzed. Heating and cooling tests simulating real operative geothermal conditions are considered. Particular attention is given to (i) the monitored wall–tunnel hydrothermal interactions, (ii) the thermal response of the UEI to heating/cooling thermal inputs and (iii) the thermomechanical behavior of the energy geostructure. Among the main results of this study, it is shown how the hydrothermal tunnel behavior considerably varies on a seasonal basis, while the train circulation completely drives the airflow in the tunnel. The UEI shows a strong heat storage potential due to the main conductive heat transfers between the geostructure and soil, while lower heat fluxes are detected at the wall–tunnel interface. The extraction potential is of lower magnitude with respect to storage because of the limited range of operative fluid temperatures and of the concurrent action of temperature variations at the tunnel boundaries affecting the materials within the UEI. Preliminary guidelines for the thermal response test execution on underground thermoactive infrastructures are also reported. The monitored thermomechanical behavior suggests different wall behaviors in the vertical and longitudinal directions. Low-magnitude strains are recorded, while the mechanical capacity of the existing geostructure can satisfactorily sustain concurrent thermomechanical actions

Suction induced effects on the fabric of a structured soil

International audienceThis paper presents the mathematical modelling of the modification of the pore space geometry of a structured soil subjected to suction increase. Structured soil concepts are first introduced considering different fabric units, such as aggregates and fissures. The numerical modelling of the structural evolution is based on experimental test results in which the evolution of the structure of the samples subjected to different suctions is determined using the mercury intrusion porosimetry technique. From this information, the macro and micropore volume evolutions are determined. The results show that drying produces a reduction in the soil total porosity which mainly corresponds to a reduction of the macropore volume. Associated with this phenomenon, an increase in micropore volume is also observed. The proposed model divides pore size distribution into three pore classes (micropores, macropores and non-affected areas). Using the concept of a suction-influenced domain, the proposed model is able to reproduce the main observed fabric evolution between the saturated and dry states

Water flow between soil aggregates

Aggregated soils are structured systems susceptible to non-uniform flow. The hydraulic properties depend on the aggregate fabric and the way the aggregates are assembled. We examined the hydraulic behavior of an aggregate packing. We focused on conditions when water mostly flows through the aggregates, leaving the inter-aggregate pore space air-filled. The aggregates were packed in 3mm thick slabs forming a quasi two-dimensional bedding. The larger aggregates were wetted with water and embedded in smaller aggregates equilibrated at a lower water content. The water exchange between wet and drier aggregates was monitored by neutron radiography. The three-dimensional arrangement of the aggregates was reconstructed by neutron tomography. The water flow turned out to be controlled by the contacts between aggregates, bottle-necks that slow down the flow. The bottle-neck effect is due to the narrow flow cross section of the contacts. The water exchange was simulated by considering the contact area between aggregates as the key parameter. In order to match the observed water flow, the contact area must be reduced by one to two orders of magnitude relative to that obtained from image analysis. The narrowness of the contacts is due to air-filled voids within the contact

Water flow between soil aggregates

Aggregated soils are structured systems susceptible to non-uniform flow. The hydraulic properties depend on the aggregate fabric and the way the aggregates are assembled. We examined the hydraulic behavior of an aggregate packing. We focused on conditions when water mostly flows through the aggregates, leaving the inter-aggregate pore space air-filled. The aggregates were packed in 3mm thick slabs forming a quasi two-dimensional bedding. The larger aggregates were wetted with water and embedded in smaller aggregates equilibrated at a lower water content. The water exchange between wet and drier aggregates was monitored by neutron radiography. The three-dimensional arrangement of the aggregates was reconstructed by neutron tomography. The water flow turned out to be controlled by the contacts between aggregates, bottle-necks that slow down the flow. The bottle-neck effect is due to the narrow flow cross section of the contacts. The water exchange was simulated by considering the contact area between aggregates as the key parameter. In order to match the observed water flow, the contact area must be reduced by one to two orders of magnitude relative to that obtained from image analysis. The narrowness of the contacts is due to air-filled voids within the contact