Runoff and infiltration estimates based on pore pressure changes in shallow aquitards

Amirhossein Shafaei, François Duhaime, Michel Baraër, Vahid Marefat

Dans les comptes rendus d’articles de la conférence: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Mining

ABSTRACT: The pore pressure time series that are recorded by piezometers installed in clay aquitards can be used to study the local water budget. For deep aquitards, pore pressure time series corrected for barometric pressure changes and earth tides can be related to the moisture loading at the top of the soil profile. When compared with precipitation and evapotranspiration, the moisture loading allows runoff and infiltration to be calculated. For shallow aquitards, the pore pressure record can be more complex as pore pressure changes due to seasonal fluctuations of the water table are superimposed on the total stress changes due to moisture loading. In this study, a finite element model was developed with COMSOL Multiphysics and its MATLAB LiveLink interface to calculate the moisture loading from complex pore pressure record in shallow aquitards. The MATLAB script discretizes the simulation in a series of time steps. The script iterates on the total stress boundary condition for each time step to replicate the pore pressure measured in the field. The paper presents a case study for the Sainte-Marthe test site in near Montreal, Canada.

RÉSUMÉ: Les séries temporelles de pression interstitielle enregistrées par les piézomètres installés dans les argile étudier le bilan hydrique local. Pour les aquitards profonds, les séries temporelles de pression interstitielle corrigées pour les variations de la pression atmosphérique et les marées terrestres peuvent être utilisées pour évaluer les humidité qui est stockée au sommet du profil de sol. Avec les précipitations et être utilisées pour calculer le ruissellement et l'infiltration. Pour les aquitards peu profonds, les séries temporelles de pression interstitielle peuvent être plus complexes en raison des fluctuations saisonnières de la nappe phréatique qui se superposent aux variations de la contrainte totale dues à la charge en humidité. Dans ce projet, un modèle d'éléments finis a été développé avec COMSOL Multiphysics et son interface MATLAB LiveLink pour calculer la charge en humidité pour les aquitards peu profonds à partir de séries temporelles de pression interstitielle complexes. Le script MATLAB discrétise la simulation en une série de pas de temps. Le script itère sur la condition à la frontière sur la contrainte totale pour chaque pas de temps afin de reproduire la pression interstitielle mesurée sur le terrain. -Marthe, près de Montréal, Canada. 1. INTRODUCTION Micro-scale studies of soil behaviour show that soil stability is the result of a balance between various internal forces. A change in any of these forces will lead to alteration of other forces for compensation and to maintain balance. For instance, a change in the total vertical stress will be followed by a change in pore pressure and effective stress resulting in a high correlation between mechanical loads on the ground surface and hydraulic head fluctuations in confined aquifers or aquitards. Hydraulic head or pore pressure time series can be used to analyze the soil water balance (van der Kamp and Schmidt 2017). The idea of using pore pressure records for estimating the water budget in the soil profile comes from an approach proposed by van der Kamp and Gale (1983) which itself was derived from the Biot (1941) theory by considering some simplifications and assumptions. Based on this approach, if the tidal and barometric effects are subtracted from the observed hydraulic heads, the remaining fluctuations are strongly correlated to the changes in soil moisture loading induced by precipitations, runoff and evapotranspiration (van der Kamp and Maathuis 1991). The ideal situation for implanting this approach is within thick aquitards or within aquifers fully confined by thick aquitards where the impact of transient flow on hydraulic head changes in the soil is minimum. The low permeability of aquitards isolates them from the pore pressure changes at their top and bottom boundaries (Barr et al. 2000). Pore pressure changes in thick aquitards are most likely to reflect only the mechanical loading on the formation (van der Kamp and Schmidt 2017). Anochikwa et al. (2012) considered this characteristic of aquitards and simulated the pore pressure variation induced by surface loading and groundwater level fluctuations utilizing a coupled stress-flow code. Based on a superposition technique, they applied a new approach based on the van der Kamp and Gale (1983) theory to replicate separately the pore pressure response to mechanical loading or water table fluctuations in a stiff clay layer below a surficial layer of gravel and sand. Their study also showed that by increasing the aquitard thickness, the impact of water-table transients is less significant and hydraulic head changes within the aquitard reflect more accurately the mechanical loading induced by soil moisture fluctuations. Other studies have analyzed the pore pressure variations in soil and rock for the assessment of soil moisture balance (e.g., Marin et

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