Étude hydrogéophysique de l'infiltration dans un remblai hétérogène urbain

Abstract

The goal of this study is to assess the contribution of geophysical methods coupled with flow modeling to understand the hydrodynamics of highly heterogeneous anlhropogenic soils, namely urban fills, in the context of groundwater protection. Urban fiUs are generally made of a heterogeneous mixture of varions wastes and soil and they are usually considered as contaminated land in most législation. Moreover, the heterogeneous structure of these fills, and hence its influence on water flow inside them, is difficult to characterize using conventional methods based on borehole drilling. In contrast, geophysical methods can sample a larger volume while producing a high density of data, and bridge the data gaps left by conventional methods. To characterize the influence of structure heterogeneous on flow dynamics. a large-scale controUed infiltration experiment was conducted and monitored using electrical resistivity tomography (ERT). The ERT apparatus was a Syscal Pro (IRIS instruments).The study area was 5x4 m2 and the thickness of the fill at the site was 2.5 to 3m. A controUed irrigation of the test surface was performed with a rainfall Simulator at an intensity of 31 mm/h. Resistivity nieasurements were made by pushing in to the ground a set of 9x7 électrodes with a 1 ni spacing. Thirty others électrodes were installed in five boreholes drilled at the corners and the center of the test area. Measurements of water content were taken in 4 points at différent depths to confirm the observed variations in resistivity. Folowing the infiltration, the site was excavated to contlrm the geophysical model. Samples were taken from différent materials identified and their retention properties determined using the Arya and Paris (1981) model for modeling the unsaturated part of the fill. The inversion of the resistivity was performed with a four-dimensional imaging algorithm. Data are considered to be a function of x. y, z and time t. All data sets from monitoring are inverted simultaneously to obtain subsurface models in one inversion process. The results show the usefulness of the TRE to characterize the infiltration in thèse very heterogeneous environments. The resistivity maps show that the flow is controUed by the distribution of the différent materials in the fill allowing the identification of zones of preferential flow. A resistivity modeling was made with Res3dmod software in order to validate the field measurements and to improve data interprétation. The resistivities of the model were estimated using Mualem and Friedman's model (1991). Based on this model of resistivity and field observations made after the excavation, hydrogeologic model was developed to study the flow in the fill.

Date	17 Dec 2010
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Jean-Sébastien Dubé (Supervisor) & Michel Chouteau (Co-supervisor)