The role of forced convection in thawing a sloped frozen layer

Joel Steeves, Lee Barbour, Grant Ferguson, Sean Carey

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Cold Regions Geotechnics

ABSTRACT: f forced convection in thawing a sloped frozen layer Joel Steeves, Lee Barbour & Grant Ferguson Department of Civil and Geological Engineering Ð University of Saskatchewan, Saskatoon, Saskatchewan, Canada Sean Carey School of Geography and Earth Science Ð McMaster University, Hamilton, Ontario, Canada ABSTRACT The contribution of heat convection by water flowing down-slope in a frozen slope was investigated as a potential failure mode for a proposed mine waste cover system design. Numerical models of two slopes within the Wolf Creek basin, Yukon, were simulated and thermal Peclet numbers were determined for a variety of water fluxes. The thermal Peclet number is the ratio between convection and conduction and can be used to determine the relative importance of convection. The calculated Peclet numbers reveal that horizontal convection within a surficial, high hydraulic conductivity layer, such as an organic layer, can result in sufficient water flow to initiate convective heat transfer. In the case of a lower hydraulic conductivity mineral soil, convection can likely be neglected. Under realistic snowmelt conditions, conduction dominates in both soils. R…SUM… La contribution de chaleur par convection par lÕeau qui coule en pente descendante dans une pente gel”e a ”t” investigu”e comme mode de d”faillance potentiel pour une conception de syst‘me propos” dÕun couvercle de d”chets miniers. Des mod‘les num”riques de deux pentes dans le basin de Wolf Creek, Yukon, ont ”t” simul”s et des nombres de P”clet thermaux ont ”t” d”termin”s pour une vari”t” de fluxes dÕeau. Le nombre de P”clet thermal est le rapport entre convection et conduction et peut ’tre utilis” pour d”terminer lÕimportance relative de la convection. Les nombres de P”clet calcul”s r”v‘lent que la convection horizontale dans une couche superficielle de haute conductivit” hydraulique, comme une couche organique, peut r”sult” en un montant suffisant dÕ”coulement dÕeau et de transfert de chaleur par convection. Dans le cas de sol min”ral ‹ conductivit” hydraulique plus base, cÕest probable que la convection peut ’tre n”glig”e. Sous des conditions de fonte de neiges r”alistes, la conduction domine dans les deux types de sol. 1 INTRODUCTION Mine closure designs in Northern Canada and other cold regions face several challenges that will require a re-evaluation of the traditional methods of cover design developed primarily for more temperate environments. The soils in northern regions are often relatively immature, with soil development delayed by the cold climate, frost action and slow weathering (Tedrow and Cantlon 1958, Munroe and Bockheim 2001). Large volumes of highly weathered fine-grained material are scarce. Where this material does exist, it is heavily disturbed by cryoperturbation (Rykaart and Hockley 2010). Typical mine closure cover designs rely on finer-grained soil to limit water movement or to store water for release back to the atmosphere by transpiration, thereby limiting the ingress of water into the underlying mine waste (MEND 1.61.5c 2012). The majority of precipitation in northern climates and other cold regions falls as snow, which accumulates over the winter and is released by snow melt. Melt occurs over a relatively short period of time in the spring, releasing large amounts of water from storage (Hayashi 2013). This spring freshet is often the most significant hydrological event in cold climates, especially for cover design. This large influx can overwhelm the cover system, causing breakthrough and ultimately, failure (MEND 1.61.5c 2012). A new cover design has been proposed to utilize the cold climate and the available coarser textured materials to maximize snow melt runoff. The proposed design, a seasonally frozen capillary barrier diversion (SFCBD) cover (MEND 1.61.5c 2012), is made up of a high hydraulic conductivity surface layer overlying a finer textured layer, which itself overlies a coarser layer (Figure 1). The SFCBD cover system integrates the elevated levels of saturation associated with traditional Ôcapillary barrierÕ cover designs (Morris and Stormont 1997, Stormont and Morris 1997) with the seasonal soil freezing that occurs in cold regions. A capillary barrier cover uses a textural difference between an overlying ÔfinerÕ soil layer and an underlying ÔcoarserÕ soil layer to create elevated levels of water saturation within the finer soil layer. A frozen, saturated soil has a lower hydraulic conductivity, as the pores are blocked by ice (Kane 1980). The creation of a frozen, high saturation layer will act as a barrier to vertical flow, diverting infiltrating water downslope. The ability of a frozen layer to divert water has been well documented in natural settings (Kane and Stein 1983, Kane and Chacho 1990, Hayashi et al. 2003, Bayard et al. 2005). The frozen barrier would only be active during the spring freshet. Upon thaw in the early summer, the barrier would revert to a typical capillary barrier store and release cover (MEND 1.61.5c 2012). A potential failure mechanism for the proposed design would be thawing of the frozen finer textured layer prior to diversion of the snow melt water. This thaw may

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Cite this article:
Joel Steeves; Lee Barbour; Grant Ferguson; Sean Carey (2014) The role of forced convection in thawing a sloped frozen layer in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper261,author = Joel Steeves; Lee Barbour; Grant Ferguson; Sean Carey,title = The role of forced convection in thawing a sloped frozen layer,year = 2014}