ESTIMATION OF THE WETTING FRONT DEPTH IN A LOESS SOIL DEPOSIT IN RESPONSE TO ENVIRONMENTAL FACTORS

Ping Li, Sai Vanapalli, Tonglu Li

In the proceedings of: GeoQuébec 2015: 68th Canadian Geotechnical Conference & 7th Canadian Permafrost Conference

Session: Problematic Soils and Ground Improvement III / Sols problématiques et amélioration du comportement III

ABSTRACT: The extent of wetting depth in loess soil deposits associated with environmental factors is the key information that is required for reliable prediction or estimation of collapse settlement. This information is also required for better understanding of rainfall-induced loess landslides. In this paper, case study results of a site in Northwestern China are discussed in greater detail. The moisture content variation of soil within 10 m below the surface was monitored for a period of one year. The wetting front extension was determined graphically by understanding the progressive trends of soil moisture in response to environmental changes at various levels of depth. In addition, VADOSE/W was used to simulate the flow behaviour in loess deposits to predict the variation of the soil water content profile due to the influence of environmental factors. The VADOSE/W is found to be a useful tool for reasonably simulating the flow behaviour corresponding to changes of environmental factors in loess soils.

RÉSUMÉ: L™étendue de la profondeur humide dans les dépôts de sol de lœss associé à des facteurs environnementaux est une information clé qui est nécessaire pour la prédiction fiable ou pour l™estimation des tassements à l'effondrement. Cette information est également nécessaire pour mieux comprendre l™effet des précipitations sur les glissements de terrain dans les sols de lœss. Dans cet article, les résultats de l'étude d'un site au nord-ouest de la Chine sont discutés plus en détail. La variation de la teneur en eau du sol, sur une profondeur de 10m, a été surveillée pendant un an. L™avancement du front humide a été déterminé graphiquement avec la compréhension des tendances de l™humidité du sol en fonction des changements environnementaux à différentes profondeurs. En outre, Vadose/W a été utilisé pour simuler le comportement d'écoulement dans les dépôts de lœss afin de prédire le profil de variation de la teneur en eau due à l™influence des facteurs environnementaux. Le Vadose/W se trouve à être un outil utile pour simuler raisonnablement le comportement d™écoulement correspondant aux variations des facteurs environnementaux dans les sols de lœss. 1. INTRODUCTION Loess is one of the widely distributed soils in semi-arid and arid regions of the world. China, U.S.A., Russia, U.K., and New Zealand are some of countries where these deposits are extensively found (Taylor, 1983). These soils are typically collapsible (i.e. a sudden decrease in soil volume) due to an increase in natural water content under a practically unchanged vertical stress (Barden, 1973; Lawton, 1989). It is widely acknowledged that the engineering properties of loess soils deposited during the Quaternary period (more than 1.0 million years ago) are significantly influenced by environmental factors. Chinese researchers classified the loess soils developed in early Quaternary (1.2-0.7 million years ago), middle Quaternary (0.7-0.1 million years ago) and late Quaternary (0.1-0.05 million years ago) as "Wucheng", "Lishi" and "Malan" loess soils, respectively, after the regions where they are available (Liu, 1962, 1978, 1985). Loss deposits thickness varies from 200 to 500 m in Northwestern Loess Plateau in China (Zhang, 1973; Sun, 1986; Ding, 1989; Yue, 1996; Sun, 1997). However, only the upper Lishi loess and Malan loess show the characteristics of collapsibility on soaking; loess soil deposits always become more stable with depth as well as with geological age (Zhang, 1973; Liu, 1994; Gao, 1994). Although all collapsible loess soils, especially in the top 10 to 20 m thick depth zone are assumed to collapse in practice (Ni, 2006; GB50025-2004), significant change in soil properties and volume change behaviour is commonly confined within the extent of wetting front depth (Houston, 1988; Fredlund, 1994). Therefore, reliable procedures for the estimation of depth of the wetted zone along with the collapse settlement would be useful for suggesting reliable design procedures for structures in collapsible soils which contributes to considerably reducing the engineering costs (Ehwany, 1990). On the other hand, landslides in loess deposits induced by natural agents and other human activities are not completely understood. The high water sensitivity of loess soils makes rainfall infiltration as one of the primary factors that trigger landslides in loess deposits. Statistically, 70% of the total loess landslides are attributed to rainfall (Zhang, 1995), and 95% of these landslides occur in wet seasons, from July to September (Zhang, 2011). The shear strength and soil volume decrease dramatically when there is an increase in soil moisture content; this behaviour is attributed to the material composition of loess soils and the influence of

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