Using Of Synthetical Thermal Insulators For Conversation Of Frozen Soil Conditions In The Base Of Railway Embankment

E.S. Ashpiz, L.N. Khrustalev, L.V. Emelyanova, M.A. Vedernikova

Dans les comptes rendus d’articles de la conférence: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: T2-D

ABSTRACT: The design of railway embankment with syntactical thermal insulator on permafrost soil is considered. It is shown that thermal insulator in embankment body allows one to save soils under embankment in frozen condition. It is recommended to put the thermal insulator near the embankment foot in low embankments. The result of analytical calculation and mathematical modeling on the determination of the thermal insulator thickness are given. 1 INTRODUCTION Railroad embankment significantly changes the conditions of heat exchange between soil and air. The upper part of the embankment surface (subgrade) intensifies the cooling of the ground due to cleaning snow off the surface, and the escarpments, on the contrary, weaken this cooling due to accumulation of snow cleaned off the main area or blown off by wind. In the summer time escarpments also intensify the warming of the soil due to radiant energy which increases because of lower reflecting power of their surface and their angle in relation to the horizon. In case of a particular correlation between the surfaces of the subgrade and the escarpment, the body of the embankment receives more warmth than 'cold,' and the upper layer of the permafrost soil lowers down at the base of the road bed and more often than not it leads to deformation of the embankment. It is obvious that the warming effect of the escarpment can be neutralized by using thermal insulators which should be laid at a depth of 15-20 cm directly under the paving of the escarpment. It is advisable to use synthetic thermal insulator (for example, Penoplex), which has low thermal conductivity, hydrophobic properties and small mass; apart from that, it is very easy to apply. When the synthetic insulator has certain thickness, the thermal balance becomes negative, the escarpment begins to have some cooling effect on the soil and the foundation remains in the frozen state. Let us designate the amount of thermal energy outgoing through the subgrade surface as Q1 and the amount of thermal energy coming in through the escarpment surface as Q2. The condition Q1+Q2 >0 means warming of the escarpment foundation, and the condition Q1+Q2<0 means its cooling. With strict formulation these conditions can be evaluated only in terms of numbers by means of mathematical modeling of thermal interaction between the embankment and the foundation soil, however with some assumptions analytical evaluation becomes possible. Let us take a closer look at these issues. 2 THEORETICAL CONCEPTS OF THERMAL INTERACTION BETWEEN THE EMBANKMENT AND PERMAFROST FOUNDATION Let us assume that the intensity of the annual inflow and outflow of warmth through the escarpment surface depends on the difference of potential depths (i.e. the depths which can be reached in the case when the whole body of the soil is either melted or thawed) and the actual depths of seasonal freezing-thawing. q1=(dth,1-d*f,z)Lv [1] q2=(d*th,2-d*f,2)Lv [2] Where dth,1, d*th,2 are the actual and the potential thawing depths on the subgrade and the escarpment respectively, m; d*f,1, df,2 are the potential and actual freezing depth on the subgrade and the escarpment respectively; Lv is the specific heat of freezing-thawing of the soil, Wh/m3. Since in practice the width of the subgrade and the length of the escarpment are more often than not much greater than the depth of seasonal freezing-thawing, the task of freezing-thawing of soil under the middle of the subgrade or escarpment can be considered as linear and apply Stefan's formula. With this approach inaccuracy is possible only at the edges of these planes (the subgrade and the escarpment) and it will not have a significant effect on the general evaluation of the thermal process and it is confirmed by the results of the numerical solution of the two-dimensional problem using computer (see below). 22()thssththsthsvTtdRRL=+- [3] 22()fwwffwfwvTtdRRL=+- [4] Where th, f are the thermal-conductivity coefficients for the embankment soil in thawed and frozen state W/(m.°C); Ts, Tw are the average summer or average winter temperatures of the soil surface within the boundaries of the subgrade or the escarpment , °C; ts, tw are the length of the 55

RÉSUMÉ: of synthetical thermal insulators for conversation of frozen soil conditions in the

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