O'Kelly B.C Oven-drying characteristics of soils of different origins. Drying Technology, Vol. 23, No. 5,

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1 Citation: O'Kelly B.C Oven-drying characteristics of soils of different origins. Drying Technology, Vol. 23, No. 5, OVEN DRYING CHARACTERISTICS OF SOILS OF DIFFERENT ORIGINS Brendan C. O Kelly BE, MEngSc, PhD, CEng, CEnv MICE, FGS Departent of Civil, Structural & Environental Engineering Museu Building, University of Dublin, Trinity College, Dublin 2 IRELAND and forerly Scott Wilson Consulting Engineers, UK e-ail: bokelly@tcd.ie First subission: 21 June Resubitted: 24 January of 9

2 ABSTRACT The accurate easureent of the oisture content of a soil is an iportant step in characterizing its engineering behavior. Hoever, the oven-drying ethod can cause soe cheical reaction (oxidation or loss of ater of crystallization) to occur in certain soil types. The level of oxidation of the solid particles as studied over the drying teperature range of 60 to 140 o C for different soils. The period of oven drying necessary for the specien ass to equilibrate as also exained. The standard practice of oven drying the soil speciens at 110±5 o C or 105±5 o C over a period of 24 hours as confired as giving accurate oisture content values for inorganic soils. Oven drying of peat and other highly organic soils over a period of 24 hours at 80 o C produced siilar levels of accuracy in the oisture content easureents as that for inorganic soils at the standard oven drying teperature of 105 or 110 o C. Soe oxidation of the organic fraction coenced at beteen 80 and 90 o C. Key Words: Oven drying; Moisture content; Soil; Organic; oxidation INTRODUCTION The accurate easureent of the oisture content of a soil is an iportant step in characterizing its engineering behavior. In geotechnical literature, the oisture content ( ) is defined as the ratio of the ass of the pore ater ( ), to the ass of the dry solid particles ( s ), and is expressed as a percentage. s ( x 100) (%) (1) The oven-drying ethod (ASTM D2216 (1998) and BS (1990)) is the standard laboratory ethod used for the deterination of the oisture content of a soil. For any soils, the ass of the dry solid particles is equal to the equilibriu dry ass ( D ), corresponding to an oven drying teperature of slightly above 100 o C, hich ensures coplete evaporation of the pore ater. ASTM D2216 (1998) recoends an oven drying teperature of 110±5 o C hile BSI [2] recoends a teperature of 105±5 o C. The equilibriu dry ass is usually taken as the ass of the test specien recorded after an oven drying period of 24 hours. The ass of the pore ater is equal to the reduction in the ass of the et specien, and the oisture content can be calculated as: D ( x 100 ) (%) (2) Hoever, oven drying at these teperatures ay cause soe cheical reaction (oxidation or loss of ater of crystallization) to occur in certain soil types. For exaple, MacFarlane and Allen (1963) and O Kelly (2004) reported that peat and other organic clays experienced soe oxidation at oven drying teperatures greater than about 85 o C. Water of crystallization, 2 of 9

3 hich is deeed part of the solid particles theselves for the purposes of oisture content deterinations, ay also be driven off. Reductions in the specien dry ass ( D ) due to cheical reactions, cause the actual oisture content value of the soil to be overestiated (Eq. 3). D ( x 100) (%) (3) D D Hence, the standards recoend that a loer teperature, typically 60 or 80 o C, be used for oven drying of peat and other organic soils. The purpose of the current study as to exaine the effect of the oven teperature on the drying characteristics of a range of different soils. The first stage exained the period of oven drying necessary for the specien ass to equilibrate. The second stage exained the susceptibility of the solid particles to oxidation for a range of oven teperatures, focusing in particular on the standard oven drying teperatures. INDEX PROPERTIES OF SOILS Eight soils that coprised different particle size distributions and different proportions of organic aterial, ere tested. Soe index properties of the soils are listed in Table 1. The soil plasticity characteristics ere assessed using the liquid liit and plastic liit tests (ASTM D4318 (2000)), and the soils ere classified using the Unified Soil Classification Syste. The bulk of the solid particles in the silt and arl aterials (soils 1, 3 6), ere to 0.06 in size, hereas the bulk of the solid particles in the clay aterial (soil 2), ere finer than Loss in ass on ignition (LOI) tests ere conducted by heating dry, podered speciens of the soils at 440 o C for a period of 24 hours. The soils are labeled 1 to 8, and are listed in order of increasing LOI values, in Table 1. The LOI values recorded for the arl and peat aterials (soils 3 8) give reasonably accurate easures of the organic content (Skepton and Petley, 1970). The level of biodegradation of the peat aterial as quantified using the von Post classification syste (Head, 1992). The scale of von Post ranges fro H 1 to H 10, in order of increasing levels of degradation. The peat-1 aterial as only very slightly degraded (H 3 ), and the peat-2 aterial as slightly degraded (H 4 ). The arl aterial (soils 3 6) originated fro the dissolution of arine shells, fragents of hich ere still evident in the shell arl. The specific gravity of the solid particles as easured using the sall pycnoeter ethod (ASTM D854, 2002). Kerosene, ith a density of 0.78 g/l, as used instead of distilled ater as the fluid in the pycnoeters for specific gravity easureents on soils of 9

4 Table 1. Index properties of the soils. Soil Soil Class- Liquid Plastic Specific Loss on label type ification liit liit gravity ignition (USCS) (%) (%) ( ) (%) 1 Silt MH Clay CH Marl-1 MH Marl-2 MH Shell arl MH Organic arl OH Peat-1 Pt Peat-2 Pt OVEN DRYING PERIOD TO ACHIEVE CONSTANT DRY MASS Speciens of the soils ere oven dried to constant ass at 60, 80 and 105 o C. These are the different oven drying teperatures recoended by the standards. Three sets of speciens, one set specific to each test teperature, ere dried using a precision oven (anufactured by Meert ), ith the oven chaber teperature aintained ithin 1.5 o C of the set teperature value. The oven chaber had a capacity of 14 liters, hich as ore than adequate to accoodate all of the speciens belonging to a particular set. The et ass of each specien as about 40 g. The speciens ere oven dried over a period of 96 hours during hich the specien asses ere recorded at regular intervals. The speciens ere cooled in a vacuu desiccator container to abient laboratory teperature of 21 o C before being eighed to an accuracy of 0.01 g. Figure 1 shos the specien asses plotted against the period of oven drying at 60, 80 and 105 o C. Since the asses of the solid particles in each of the speciens ere slightly different, the specien asses ere noralized on the basis of their initial et asses (scale ), and equilibriu dry asses (scale ) recorded after a drying period of 96 hours. Figure 2 gives a direct coparison of the drying characteristics of the peat-1 aterial at the different oven teperatures. The folloing points are concluded in relation to the period of drying fro inspection of Figures 1 and 2: The standard practice of oven drying at 105 or 110 o C over a period of 24 hours as ore than adequate for the specien asses of the different soils to equilibrate (Figure 1c). Longer drying periods, although still generally less than 24 hours, ere required for the specien asses to equilibrate at the loer oven teperatures of 60 and 80 o C. Hoever, the dry asses of the arl-1 and peat-1 aterials only equilibrated after about 36 hours for oven drying at 60 o C (Figure 1a). 4 of 9

5 Noralized specien ass Noralized specien ass Noralized specien ass LEVEL OF OXIDATION OF SOLIDS AT DIFFERENT DRYING TEMPERATURES The second batch of drying tests exained the susceptibility of the solid particles to oxidation for oven drying teperatures in the range of 60 to 140 o C. The drying tests coenced at 60 o C since this is the loest drying teperature recoended by the standards for oisture content deterinations. Speciens of the soils ere oven dried to constant ass at a series of oven teperatures hich increased in steps over the range of 60 to 140 o C. The equilibriu condition for each oven teperature as established by recording the specien asses at regular intervals. For exaple, Figure 3 shos the dry ass of the peat-2 specien recorded for the different oven drying teperatures. After each eighing, the speciens ere replaced in the sae locations in the drying oven. Figure 1. Specien ass vs. period of oven drying. (a) Oven drying at 60 o C 1.1 Wet specien [1] Silt [2] Clay [3] Marl-1 [4] Marl-2 [5] Shell arl [6] Organic arl [7] Peat-1 (H3) [8] Peat-2 (H4) Equilibriu dry ass (b) Oven drying at 80 o C 1.1 Wet specien [1] Silt [2] Clay [3] Marl-1 [4] Marl-2 [5] Shell arl [6] Organic arl [7] Peat-1 (H3) [8] Peat-2 (H4) Equilibriu dry ass (c) Oven drying at 105 o C 1.1 Wet specien [1] Silt [2] Clay [3] Marl-1 [4] Marl-2 [5] Shell arl [6] Organic arl [7] Peat-1 (H3) [8] Peat-2 (H4) Equilibriu dry ass of 9

6 Specien dry ass (g) Noralized specien ass Figure 2. Drying of peat-1 at different oven teperatures. Wet specien o C 80 o C 60 o C Equilibriu dry ass Figure 3: Drying characteristics of peat-2 vs. oven drying teperature Drying teperature ( o C) The level of susceptibility to oxidation as assessed in ters of the reductions in the equilibriu dry asses, expressed as percentages of the equilibriu dry asses initially recorded at 60 o C. Figure 4 shos the reductions in the equilibriu dry asses ith increasing oven drying teperature for the different soils. The folloing points are noted in relation to the level of oxidation over the drying teperature range of 60 to 140 o C fro inspection of Figure 4: For the silt, clay and arl aterials (Figure 4a), the overall reductions in the equilibriu dry asses ere less than 1.5 %, and for practical purposes these reductions do not affect the accuracy of the oisture content values. In general, the gradients of the different drying curves increased beteen 80 and 90 o C (Figure 4a, b). O Kelly (2004) shoed that the greater reductions in the specien dry asses over this teperature range coincided ith the coenceent of soe oxidation of the organic fraction. Sall reductions in the equilibriu dry asses recorded beteen 60 and 80 o C ere principally due to the evaporation of sall residual aounts of loosely bound pore ater fro the void space ith increasing oven teperature. 6 of 9

7 Reduction equilibriu ass (%) Start of oxidation Reduction equilibriu ass (%) Start of oxidation For the peat aterial (Figure 4b), the overall reductions in the equilibriu dry asses of 6.8 % (peat-1) and 4.8 % (peat-2) are ore than quadruple the values recorded for the other soils. The reductions in the equilibriu dry asses of 1.7 % (peat-1) and 0.4 % (peat-2) corresponding to a drying teperature of 80 o C are siilar to the overall reductions easured for the silt, clay and arl aterials. Hence, the ater content values of peat and other organic aterials, calculated on the basis of the equilibriu dry ass at 80 o C, are likely to have siilar levels of accuracy as those for inorganic soils, calculated on the basis of the standard oven dying teperatures of 105 o C or 110 o C. The greater levels of susceptibility of the peat aterial as consistent ith their higher LOI values, and the fact that the natural aterial as only very slightly-to-slightly decoposed (H 3 to H 4 on the scale of von Post). Figure 4: Reduction in specien dry ass vs. drying teperature. (a) Silt, clay and arl aterials 1.5 [1] Silt [2] Clay [3] Marl-1 [4] Marl-2 [5] Shell arl [6] Organic arl Oven drying teperature ( o C) (b) Peat aterial [7] Peat-1 (H3) [8] Peat-2 (H4) Oven drying teperature ( o C) CONCLUSIONS The standard practice of oven drying soil speciens at 110±5 o C or 105±5 o C over a period of 24 hours for oisture content deterinations as confired as giving accurate oisture content values for inorganic soils. 7 of 9

8 The reductions in the equilibriu dry ass recorded for the peat aterial at 80 o C ere siilar to those recorded for the inorganic soils at the standard oven drying teperatures of 105 or 110 o C. Hence, it is recoended that oisture content calculations for peat and other organic aterials are based on the specien ass recorded after a period of 24 hour oven drying at 80 o C. There as no apparent benefit in oven drying the soil speciens at 60 o C for oisture content deterinations, in fact the oisture content values ere less accurate since sall residual aounts of pore ater reained in the void space. Longer oven drying periods of up to 36 hours ere also required for the specien asses to equilibrate. NOMENCLATURE LOI D s D Loss in dry ass on ignition Equilibriu dry ass Mass of dry solid particles Mass of the pore ater Moisture content Reduction in dry ass ACKNOWLEDGEMENTS The laboratory test ere carried out by Martin Carney and Mairead Sayers (University of Dublin, Trinity College) and their efforts are gratefully acknoledged. REFERENCES ASTM D854, 2002, Standard Test Method for Specific Gravity of Soil Solids by Water Pycnoeter, Aerican Society for Testing and Materials, Philadelphia. ASTM D2216, 1998, Standard Test Method for Laboratory Deterination of Water (Moisture) Content of Soil and Rock by Mass, Aerican Society for Testing and Materials, Philadelphia. ASTM D4318, 2000, Standard Test Methods for Liquid Liit, Plastic Liit, and Plasticity Index of Soils, Aerican Society for Testing and Materials, Philadelphia. BS1377 2, 1990, Methods of Test for Soils for Civil Engineering Purposes, Classification Tests, British Standards Institution, London. Head, K.H., 1992, Manual of Soil Laboratory Testing: Soil Classification and Copaction Tests, Pentech Press, London, 388 pp. MacFarlane, I.C. and Allen, C.M., 1963, An Exaination of Soe Index Test Procedures for Peat, Proc. Ninth Muskeg Research Conference, National Research Council of Canada. 8 of 9

9 O Kelly, B.C., 2004, Accurate Deterination of Moisture Content of Organic Soils using the Oven Drying Method, Drying Technology, 22(7) pp Skepton, A.W. and Petley, D.J., 1970, Ignition Loss and other Properties of Peats and Clays fro Avonouth, King s Lynn and Cranberry Moss, Geotechnique, 20(4) pp of 9