Evaluation of frost heave and the temperature-moisture migration relationship using a modified laboratory method

Transcription

1 Evaluation of frost heave and the temperature-moisture migration relationship using a modified laboratory method Évaluation de soulèvement par le gel et la relation de migration température - humidité en utilisant une méthode de laboratoire modifié A.S. Sarsembayeva *1, P.E.F. Collins 1 1 Brunel University London, London, UK * Corresponding Author ABSTRACT Seasonal freezing of highway sub soils and systematic maintenance by de-icing chemicals steadily affect their thermal regime and moisture distribution. As the result, it leads to significant change of sub soils from the designed engineering characteristics including their bearing capacity. In order to better understand the process and to evaluate the correlation between temperature field and frost heave, and also to simultaneously supply with de-icing chemicals, the modified laboratory method was developed in this research. The current test method is upgraded by increasing height of soil samples up to 1 meter. More soil columns being able to be tested simultaneously in the modified test, proposed in this study, allows better control of each stage of testing. The modified freezing regime in this new test method is closer to the scenario in cold regions in terms of temperature. The obtained results of this research are going to be presented for the first time in scientific literature. RÉSUMÉ Gel saisonnier de la route sous les sols et l'entretien systématique de produits chimiques de dégivrage affectent progressivement leur régime thermique et la distribution de l'humidité. En résultat, cela conduit à des changements importants de sous-sols des caractéristiques techniques visant notamment leur capacité portante. Afin de mieux comprendre le processus et d'évaluer la corrélation entre le champ de température et soulèvement par le gel, et aussi d'alimenter simultanément avec des produits chimiques de dégivrage, la méthode de laboratoire modifié a été développé dans cette recherche. La méthode d'essai est en cours de mise à niveau de la hauteur des échantillons de sol de plus en plus jusqu'à 1 mètre. Plus de colonnes de sol pouvant être testés simultanément dans l'essai modifié, proposé dans la présente étude, permet un meilleur contrôle de chaque étape du test. Le régime de congélation modifié dans cette nouvelle méthode d'essai est plus proche du scénario dans les régions froides en termes de température. Les résultats obtenus de cette étude vont être présentés pour la première fois dans la littérature scientifique. 1 INTRODUCTION Temperature drops in highway sub soils can be significantly greater than those in adjacent unpaved soils. Greater thermal conductivity of pavement and sub-base layers leads to significant thermal gradients in sub soils. This induces a greater moisture migration and increased frost heave potential under highway soils in winter period. In central Asian countries like Kazakhstan, the typical snow-cover period is 111 days or more per year. During this period, air temperatures often reach -3 C to - C and the sub-soil moisture steadily migrates towards the freezing front of the pavement layers. Removal of snow from highway pavements, together with the spread of deicing chemicals reduces the freezing point of water in adjacent soils and allows it to continue to migrate.

2 This migration may influence soil frost heave near to and beneath the highway (Figure 1). In spring time extensive thawing may produce a substantial reduction in the bearing capacity of sub soils and further disruption of upper layers. In order to better understand these processes, a modified laboratory method was developed to simulate long term natural freeze-thawing cycles. + C C Figure 1. Ground thermal and moisture migration context for a highway in winter. From the 193s, research has examined moisturemigration under thermal gradients, (Hoekstra 19, Loch, Kay 197, Taber 193). More recently, considerable attempts to monitor the frost heave in cohesive soils by cyclical freezing-thawing have been implemented (Wang, Cruse et al. 1, Bi Guiquan 1, Bing, H,He, P 11, Dagesse 13, Konrad, Samson, Kværnø, Øygarden, Nagare, Schincariol et al. 1). Further investigations have examined the mechanical properties of soils affected by cyclical freezing-thawing (Wang, Cruse et al. 1, Konrad, Samson, Wang, Ma et al. 7, Qi, Vermeer et al., Qi, Ma et al. ). There are currently several standard methods applied in different countries to observe freezingthawing under laboratory conditions: BS 1-1:9, ASTM D 91-, GOST -1 (used in Russia and some similar tests in post-soviet countries), and BS EN 1997-:7, section,..1 Frost susceptibility. Notably, most existing standards use test samples that are no more than 1 cm thick, with a fixed gradient between thermal plates, usually -3 C to +3 C. The number of samples in a test chamber is usually set at. This provides a relatively uniform approach, but is not always suitable for research purposes. In some cases simultaneous frost heave simulation and mechanical testing has been performed in an adapted triaxial cell (Ishikawa, Tokoro et al. 1). In that case, only one sample could be tested at a time. In most tests to date, the limited size of samples and short period of freezing in a cycle induce freezing at a faster rate than in the real world. Existing chilling equipment and techniques used for soil freezing do not include daily temperature fluctuations and provide relatively small temperature gradients in the soil. The temperature set at the top and bottom thermo-regulated plates remains fixed to create a stable gradient. Such conditioning of the freezing regime in the test significantly differs from the natural freezing of soil, where the temperature of soils drops slowly. In the long term, this allows the gradual migration of ground water upwards the cooling surface of the soil. To address this, a modified laboratory experimental method has been developed that simulates natural freezing conditions and water migration in highway materials. TESTING METHOD.1 The construction of the freeze-thaw model The model was developed from the ASTM D91- Standard. The height of the tested samples was increased to 1 m and the number of samples tested simultaneously was set at 9. Columns are assembled using 1 rings, each 1 cm height and 1 cm diameter (Figure ). The chilling of the columns is designed to provide the cold from the top downwards, in order to simulate the progressive freeze of the highway sub soils from the pavement. The copper pipes are connected to the temperature bath at one end and rolled around the metal chilling collars on the top of the sample columns at the other. The soil inside each column is loaded from the top by a 3. kpa disk surcharge. This permits frost heave to occur in a vertical direction. The high heat conductivity of the surcharges allows the uniform cold distribution from the surface to the soil to be maintained. Equal length chilling hoses and a system of manifolds and valves ensures identical chilling for each sample. 9 thermocouples are inserted in each mold ring to monitor and record the temperature. A supply of water kept in a refrigerated Mariotte bottle at a stable temperature feeds each test sample from the base. A cm thick layer of fine sand enables free drainage for the supplied liquid. The lin-

3 Dry density, g/cm³ ear movement of each soil sample is monitored separately and recorded daily to analyze the volumetric change. Temperature, C Time, days Figure 3. The test procedure showing temperature changes..3 Soil modelling Soil samples were modelled using geological data from Kazakhstan, with the properties similar to the natural soil of the left bank of Ishim River in Astana Figure. The experimental apparatus showing the soil sample columns, chilling systems and thermocouples placed within the insulated chamber.. Freezing-thawing cycles The oscillating thermal regime includes progressive and very slow freezing from the top, similar to natural conditions in cold countries, where it is associated with heave. At the start of the test, the temperature was stabilized at C over a hr period. During this, consolidation within 1.-. mm took place. The temperature was reduced by C per day, to a minimum of -1 C after 11 days. After a day of thawing at approximately 1 ºC, the second freezing cycle was run, using the same rate as the first. After each cycle of freezing and thawing, 3 columns of soil were removed to test the soil s engineering properties. The thermal regime of the test procedure is represented in Figure Moisture content, % % air voids % air voids 1% air voids Dry density, g/cm3 Bulk density, g/cm3 9% of maximum dry density Figure. Compaction tests results. The soil samples were made from sandy clay: % of sharp sand less than mm and % of Kaolinite mixture. Moisture content was defined as a val-

4 Height level from the top, dm Depth from the column top, cm ue of 9% of maximum dry density in. kg proctor curve, which corresponds to 17% (Figure ). The weight of soil proctored in each column was 1.7 ±.1 kg. Average dry density before the test was set as 11 ± 1 g/cm³. Initially bulk density was 17 ± g/cm³. As shown in Figure, the air voids content at such dry density and moisture content is close to %, which meets the requirements of compaction for highway sub soils. Some variability in temperature profile was caused by uneven cold distribution over time and delaying of the starting point of freezing. 3 OBTAINED RESULTS AND DISCUSSIONS Column No #1 # #3 # # # #7 # #9 3.1 Change in temperature field The average freezing front rate of the top cm was 1 mm per hour. This accelerated to. mm per hour in the next 1 cm, and to between -1 mm/hour and mm/hour at cm depth. The middle sections of the 1m column were subject of crystallization at the same time, i.e. middle 3 cm reached C immediately. From cm depth downwards, migration of the freezing front slows from mm/hour to mm/hour. A proposed explanation for freezing rate inequality is the effect of moisture redistribution during heaving. A higher content of moisture required more latent heat loss to cool down and hence a longer freezing period. Temperature distribution by height of the 9 columns at the end of the first freezing period is presented in Figure. Here, the temperature sensor at cm depth decreased to between -7 C and - C, while the soil surface was -1 C. The reduction in thermal gradients with depth may be explained by lower moisture content and hence less heat conductivity respectively. The deepest soil in the columns featured higher moisture contents captured from the water table and showed greater temperature gradients Temperature,C Figure. Temperature distribution by height for each column. 3. Change in moisture distribution From the initial W=17%, moisture migrated upwards and reached up to % in soil that had been heaved at the end of the first freezing period. In contrast, the middle layers dried to W=1.%. In the transition zone with temperature close to C, at about 7 cm depth, significant moisture accumulated up to W=1% (Figure ) Moisture content W, % 1 3 #7 # #9 Figure. Moisture distribution in columns at the end of first freezing cycle.

5 Frost Heave, mm Height level from the top, dm Temperature, C After the second freezing cycle, moisture curves were less variable (Figure 7). The maximum moisture content at the top reached up to 1% and gradually reduced to 17-% by 1 cm depth. The moisture reduces to W=1% at cm depth and then a slight increase with depth below cm Moisture content W, % #1 # #3 1 - Time, hours #1 # #3 # # # #7 # #9Ti Figure. Frost heave in terms of testing time. Figure 7. Moisture distribution at the end of the second freezing cycle. 3.3 Volume and density change During the first freezing cycle an average of approximately g of water was drawn into the soil-water system per column, while around 7 g had been drawn in by the end of the second cycle at each column of soil. The frost heaving level is plotted in Figure. The onset of freezing varied between sample columns due to minor variations in the experimental apparatus. On Figure 9 frost heave is represented in terms of temperature relationship. A sudden small heave occurred in all samples at the start of the thaw period. This was possibly caused by melting of ice within the column which could then migrate, but this requires further research. The average trend line of frost heave can be expressed by a third order polynomial equation: H = -.T³ -.17T -.13T.19 R² =.997 Where H frost heave in mm and T surface temperature in C H = -.T³ -.17T² -.13T #1 # Temperature, C #3 # # # #7 # #9 Average Poly. (Average) Figure 9. Frost heave and freezing temperature relationship. Black line shows the average polynomial relationship. An example of the relationship between frost heave and temperature over time is presented in Figure 1. Frost heave began earlier during the second

6 Frost Heave, mm Temperature, C freezing cycle. This is probably due to a higher moisture content at the top of the soil column as a legacy of the first cycle. to use solutions with different concentrations of de -icing agents. 1 REFERENCES Frost Heave Temperature at cm depth Figure 1. Temperature and frost heave relationship on the example of column #1. RESULTS AND DISCUSSIONS Frost heave in the second freezing-thawing cycle was achieved faster and at a higher temperature than in the first cycle. At the start of the thawing period, an increase in frost heave was observed. Water was drawn up into the system - g after first freeze-thaw cycle and 7 g per column after the second cycle. The highest frost heave rates are associated with the slowest rates of freezing front migration. In the transition zone from thawed to frozen soil, a reduced temperature gradient occurred. In the heaved top layers the moisture content distribution was vertically variable within the soil due to ice lens formation. In order to analyze the effect of de-icing chemicals on the engineering properties of soil, the first test used distilled water, while further tests are planned Bi, Guiquan, 1. Study on influence of freeze-thaw cycles on the physical-mechanical properties of loess. Smart Materials and Intelligent Systems,, pp. -9. Bing, H. and He, P., 11. Experimental investigations on the influence of cyclical freezing and thawing on physical and mechanical properties of saline soil. Environmental Earth Sciences, (), pp Dagesse, D.F., 13. Freezing cycle effects on water stability of soil aggregates. Canadian Journal of Soil Science, 93(; SI), pp. 73; 73-3; 3. Hoekstra, P., 19. Moisture movement in soils under temperature gradients with the cold-side temperature below freezing. Water Resources Research, (), pp. 1-. Ishikawa, T., Tokoro, T., Ito, K. and Miura, S., 1. Testing Methods for Hydro-Mechanical Characteristics of Unsaturated Soils Subjected to One-Dimensional Freeze-Thaw Action. Soils and Foundations, (3), pp Konrad, J.-M. and Samson, M.,. Hydraulic conductivity of kaolinite-silt mixtures subjected to closed-system freezing and thaw consolidation. Canadian Geotechnical Journal, 37(), pp Kværnø, S.H. and Øygarden, L.,. The influence of freeze thaw cycles and soil moisture on aggregate stability of three soils in Norway. Catena, 7(3), pp Loch, J.P.G. and Kay, B.D., 197. Water redistribution in partially frozen, saturated silt under several temperautre gradient and overburden loads. Soil Science Society of America Journal, (3), pp. -. Nagare, R., Schincariol, R., Quinton, W. and Hayashi, M., 1. Effects of freezing on soil temperature, freezing front propagation and moisture redistribution in peat: laboratory investigations. Hydrology and Earth System Sciences, 1(), pp Qi, J., Ma, W. and Song, C.,. Influence of freeze thaw on engineering properties of a silty soil. Cold Regions Science and Technology, 3(3), pp Qi, J., Vermeer, P.A. and Cheng, G.,. A review of the influence of freeze-thaw cycles on soil geotechnical properties. Permafrost and Periglacial Processes, 17(3), pp. -. Taber, S., 193. The Mechanics of Frost Heaving. The Journal of geology, 3(), pp Wang, D., Ma, W., Niu, Y., Chang, X. and Wen, Z., 7. Effects of cyclic freezing and thawing on mechanical properties of Qinghai Tibet clay. Cold Regions Science and Technology, (1), pp Wang, E., Cruse, R.M., Chen, X. and Daigh, A., 1. Effects of moisture condition and freeze/thaw cycles on surface soil aggregate size distribution and stability. Canadian Journal of Soil Science, 9(3), pp. 9-3.