44 research outputs found
New analytical solution for the study of hydraulic interaction between Alpine tunnels and groundwater
La solution analytique dite du « puits artésien » développée par Jacob et Lohman [1952] est appliquée au
contexte des tunnels profonds en haute montagne pour modéliser la décroissance en fonction du temps du débit des venues
dâeau. La comparaison de la solution avec un modĂšle numĂ©rique dĂ©montre sa validitĂ© durant les premiers instants
aprĂšs le percement du tunnel. Ensuite, les conditions aux limites supposĂ©es dans la rĂ©solution du cas analytique â milieu
infini â ne sont plus conformes Ă la rĂ©alitĂ©. Les simulations numĂ©riques effectuĂ©es montrent que la durĂ©e de validitĂ© de
la solution de Jacob et Lohman est notamment inversement proportionnelle à la diffusivité hydraulique du massif. Les
rĂ©sultats de ces simulations permettent de scinder la dĂ©croissance du dĂ©bit des venues dâeau en deux phases distinctes :
la premiĂšre phase correspond Ă la pĂ©riode de validitĂ© de la formule de Jacob et Lohman. Elle est caractĂ©risĂ©e par lâextension
dâun cĂŽne de rabattement autour de lâouvrage aprĂšs percement. Durant la seconde phase, le cĂŽne de rabattement
atteint les limites physiques de lâaquifĂšre (latĂ©rales et supĂ©rieure) et les dĂ©bits dĂ©croissent plus rapidement suite Ă la dĂ©croissance
des potentiels hydrauliques aux limites. Sous réserve du peu de données disponibles, ce type de comportement
a Ă©tĂ© mis en Ă©vidence dans les Alpes aux tunnels du Simplon et du Mont-Blanc. Dans la seconde partie de lâarticle,
un abaque paramĂ©trique est prĂ©sentĂ©, permettant dâĂ©valuer, aprĂšs le percement dâun tunnel, le temps nĂ©cessaire pour
quâun rabattement donnĂ© soit atteint en un point dĂ©fini du massif. Bien que nĂ©gligeant les effets de limites mis en Ă©vidence
dans la premiĂšre partie de lâarticle, cet abaque permet nĂ©anmoins de mettre en Ă©vidence la grande inertie, Ă cette
Ă©chelle dâobservation, des aquifĂšres en massifs montagneux. Il peut ĂȘtre appliquĂ© Ă la dĂ©termination du temps nĂ©cessaire
pour que lâeffet hydraulique du tunnel soit mesurable en surface et dĂšs lors Ă©tendu au dimensionnement des campagnes
de surveillance des sources lors du percement dâun ouvrage souterrain. Cet abaque dĂ©montre lâinfluence de la gĂ©omĂ©trie
du tunnel (rayon, profondeur) et des caractĂ©ristiques hydrauliques de lâaquifĂšre (diffusivitĂ©, hauteur de la nappe) sur la
durĂ©e nĂ©cessaire des campagnes de surveillance. Celles-ci devraient ĂȘtre ajustĂ©es au cas par cas en fonction du contexte
de lâouvrage souterrain
The Merti aquifer (Kenya), a sustainable water resource for the Dadaab refugee camps and local communities?
The Merti aquifer (Kenya) is the only permanent water resource for the supply of the Dadaab refugee camps and local community. Risk of well salinisation and water level depletion has led to the concerns regarding sustainability of groundwater management. A methodology was developed to quantify groundwater recharge, based on hydrogeological data analysis, linking river discharge rates to groundwater recharge by combining remote sensing techniques with surface/groundwater numerical modelling. Also, the risk of salinisation of wells was assessed by means of numerical modelling combined with data provided from a groundwater monitoring network. Groundwater recharge estimates yielded 50 to 100 times higher values than previous studies, suggesting a multi-layered aquifer system. Since abstractions are concentrated within only one aquifer horizon of the system, they were found to be close to recharge rates. Hence, groundwater prospection of deeper horizons could reveal a potential for new groundwater resources in the area
Change of biological soil quality in organic and conventional farming systems of the DOK trial
The DOK trial has started in the 1970ies, when first reports warned us on the consequences of our actions and the limits to growth. Even though farmers and a huge research community know better, we are still not managing our soils in a sustainable way. It seems inevitable that the mainstream agriculture wants to go beyond natural frontiers. Soils have an enormous buffering capacity, but this ends, when ecosystems are collapsing not only at the local, but also at the global level
Modelling Discharge Rates and Ground Settlement Induced by Tunnel Excavation
Interception of aquifers by tunnel excavation results in water inflow and leads to drawdown of the water table which may induce ground settlement. In this work, analytical and numerical models are presented which specifically address these groundwater related processes in tunnel excavation. These developed models are compared and their performance as predictive tools is evaluated. Firstly, the water inflow in deep tunnels is treated. It is shown that introducing a reduction factor accounting for the effect of effective stress on hydrodynamic parameters avoids overestimation. This effect can be considered in numerical models using effective stress-dependent parameters. Then, quantification of ground settlement is addressed by a transient analytical solution. These solutions are then successfully applied to the data obtained during the excavation of the La Praz exploratory tunnel in the Western Alps (France), validating their usefulness as predictive tools
Improved computation of nonâlinear advection in porous media using slightly modified basic finite element algorithms
The numerical stability of standard finite element schemes applied to the advectionâdiffusion equation is evaluated using a spaceâtime eigenvalue analysis. Unlike the usual approaches which only consider temporal aspects of stability, this analysis also describes the spatial stability of the solutions. To this end, the oneâdimensional advectionâdiffusion equation is put into an alternative semiâdiscrete form which allows the derivation of a very practical stability condition. In multidimensional flow situations the latter is applied along the streamlines by means of a tensorial corrective function that prevents excessive numerical smearing of fronts or phase interfaces. The efficiency of the procedure is illustrated by an example which successfully simulates the coupling of two low miscible fluid phases in a variably saturated porous medium. Copyrigh