5th Interntionl Conference on Civil, Architecturl nd Hydrulic Engineering (ICCAHE 2016) Effects of dry density on soil wter chrcteristic curve of cly Hu Mengling, byo Hilin, cren Jinxi School of Architecture nd Civil Engineering, Xi n University of Science nd Technology, Xi n, Chin b Institute of Rock nd Soil Mechnics, Chinese Acdemy of Sciences, Wuhn, Chin c School of Architecture nd Civil Engineering, Xi n University of Science nd Technology, Xi n, Chin KEYWORD: dry density; unsturted soil; soil wter chrcteristic curve; VG model ABSTRACT: The soil wter chrcteristic curve is importnt prmeter tht expresses the unsturted chrcteristic of wter holding nd moisture migrtion. With filter pper method, the mtric suctions of soil smples in six wter contents nd three dry densities re mesured, nd the soil wter chrcteristic curves in different dry densities re obtined. Then the soil wter chrcteristic curves re fitted with VG model. The effects of dry density on soil wter chrcteristic curve nd the prmeters of VG model re discussed. The results show tht for the cly in different dry densities the prmeter - the residul mss wter content in VG model of soil wter chrcteristic curve is constnt. The prmeter is linerly correltion with the reciprocl of ir entry vlue. The dry density hs gret influence on soil wter chrcteristic curve, so the influence must be considered in the nlysis of moisture migrtion with hydro-mechnicl coupling. INTRODUCTION The embnkment filling is typicl unsturted soil. It s engineering properties re significntly influenced by the vrition of moisture condition. When filled, the wter content of filling is controlled in the rnge of optiml moisture content ±2% (The Ministry of Communictions of the People's Republic of Chin. 2006). In this cse, the soil hs high strength nd gret stbility. As the embnkment is in the nturl environment, its moisture content re influenced with the moisture migrtion csed by the rinfll infiltrtion nd the vrition of groundwter level. When the wter content of soil increses, the mtric suction reduces, the sher strength decreses, nd the bility of resisting the deformtion wekens which results in wetting deformtion. Tht is one of the primry resons tht in mny res the embnkments were spoiled by wter nd the slopes slid. Accordingly, it is quite necessry to nlysis the lw of moisture migrtion from the perspective of unsturted soil. The soil wter chrcteristic curve is key prmeter of soil property in the reserch of moisture migrtion of unsturted soil. The curve includes the informtion tht concerning the pore size distribution nd the chrcteristic of wter holding of soil. The soil wter chrcteristic curve plys n importnt role in unsturted soil mechnics by which the relted prmeters such s the permebility coefficient nd sher strength of unsturted soil cn be obtined. In view its importnce role, mny reserchers hve been devoted to this spect. The current reserches show tht the soil wter chrcteristic curve is influenced by the dense condition, stress condition nd temperture etc (Vnplli, S.K. et l. 1998 & Miller, C.J. et l. 2002 & Zhou J. 2005 & Wng T.H. et l. 2008 & Liu F.Y. et l. 2011 & Zhng X.D. et l. 2010). In which, the influence of dense condition is most significnt. And the stress condition tkes effect through its impct on dense condition. So, the soil wter chrcteristic curve considered the influence of dry density is necessry prmeter in the reserch of moisture migrtion with hydro-mechnicl coupling. Therefore, study on the influence of dry density on soil wter chrcteristic curve is meningful for crrying on the reserch of the lw of moisture migrtion of sbugrde with hydro-mechnicl coupling. 2016. The uthors - Published by Atlntis Press 761
THE SOIL WATER CHARACTERISTIC CURVE TEST WITH FILTER PAPER METHOD Tken the cly in Xiohongshn, Wuhn, Chin s experimentl soil. The depth of soil is bout 3~ 5m. Its bsic physicl property indexes re shown in tble 1. The soil is low liquid limit cly. Tble 1. The bsic physicl property indexes of the cly in Xiohongshn physicl property in- unit vlue nturl wter content % 21.8 nturl wet density g/cm 3 1.69 specific grvity / 2.762 liquid limit % 44.41 plstic limit % 21.11 plsticity index / 23.30 The test uses shungqun NO.203 filter pper mde in Hngzhou Xinhu pper mill to test the mtric suctions of soil smples in different wter contents. It tests the mtric suctions of soil smples in six wter contents(0.14, 0.16, 0.18, 0.20, 0.22 nd 0.24 respectively) nd three dry densities(1.70, 1.75 nd 1.80g/cm 3 respectively) to nlysis the influence of dry density on soil wter chrcteristic curve. The soil wter chrcteristic curves of soil in three dry densities re obtined with the experiment dt. As the soil wter chrcteristic curve is importnt prmeter tht expresses the unsturted chrcteristic of wter holding nd moisture migrtion, in recent yers lrge number of scholrs mde deep reserches on it, nd more nd more mthemticl models were dopted to express the soil wter chrcteristic curve. So fr the widely used models re Brooks-Corey model (Brooks, R.H. et l. 1964), Vn Genuchten model (Genuchten M.T.V. 1980), Fredlund-Xing model (Fredlund D.G. et l. 1994), frctl model (Xu Yongfu et l. 2002) nd logrithmic eqution model (Qi Guoqing et l. 2004) etc. A lrge number of reserches showed tht Vn Genuchten model hd universl pplicbility in the rnge of quite wide wter potentil or wter content for most soil, nd ws the most commonly used model for soil wter chrcteristic curve. So, the reltionship of volumetric moisture content nd mtric suction in three dry densities, those re 1.70g/cm 3, 1.75g/cm 3 nd 1.80g/cm 3, re fitted with Vn Genuchten model, tht is θs θr θ = θr + (1) n (1 1 n) 1 + ( ϕ) In which, θ r is residul volumetric moisture content, θ s is sturted volumetric moisture content,, n re fitting prmeters. The fitting curves re shown in fig. 1. The figure shows tht the dt points gree well with the fitting curves in these three dry densities. Fig. 1 shows the fitting curves of soil wter chrcteristic curves in 1.70g/cm 3, 1.75g/cm 3 nd 1.80g/cm 3. As depicted in the figure, it cn be seem tht the dry density hve gret influence on soil wter chrcteristic curve. The lrger the dry density, the smller the sturted wter content, the lrger the ir entry vlue, the greter the desorption rte when 762
Figure 1. The fitting curves of soil wter chrcteristic curves in three dry densities mtric suction go beyond the ir entry vlue, nd the stronger the chrcteristic of wter holding. The reson cn be ttributed to tht the lrger the dry density, the more the soil prticles in soil per unit volume, the less the pore in soil, the smller the pore rtio, then the smller the sturted volumetric wter content. The lrger the dry density, the smller the pore rtio. The most likely porosity reduction method is diminish the lrge pore in soil. The smller the mximl pore in soil, the lrger the ir entry vlue, tht is the mtric suction when the ir begin to enter into the soil. Besides, when the pore rtio decreses, the verge pore size reduces, the chrcteristic of wter holding boosts up, then the desorption rte slows down. INFLUENCE OF DRY DENSITY ON THE PARAMETERS OF VG MODEL Tble 2. The fitting prmeters of SWCCs in three dry densities prmetesrs dry density/ g/cm 3 1.70 1.75 1.80 θs 0.3845 0.3665 0.3483 θr 0.0576 0.0593 0.0610 2.768e-3 3.981e-4 2.838e-4 n 1.2282 1.7925 2.4503 Tble 2 shows the fitting results of VG model for soil wter chrcteristic curve in three dry densities. It cn be seem tht the dry density hve gret impct on the prmeters of VG model of soil wter chrcteristic curve. The effects will be described below one by one. 763
Figure 2. The vrition of sturted volumetric wter content with dry density There is the following reltionship between the sturted volumetric wter content nd dry density, Gsρw ρd ρd θs = = 1 (2) Gsρw Gsρw From eqution (2), it cn be seem tht the sturted volumetric wter content decreses linerly with dry density. Fig. 2 verifies the reltionship between sturted volumetric wter content nd dry density. The residul wter content refers to the criticl wter content when the increse of soil suction do not result in significnt chnge in wter content. It mens tht the the desorption rte in desturtion zone is much lrger thn tht in the zone of residul sturtion, nd the sorption of soil on wter in the zone of residul sturtion is much lrger thn tht in desturtion zone. So, it cn be ssumed tht under mtric suction the dischrged wter in soil is the free wter in desturtion zone nd is bound wter in the zone of residul sturtion which is tightly dsorbed on the surfce of soil prticles(sillers W S et l. 2001). Therefore, the volume of bound wter in unit volume is directly proportionl to the surfce re of soil prticles. When the soil is homogeneous medium, the surfce re of soil prticles is proportionl to the qulity of soil prticles. Then the following eqution cn be obtined m w-resρ w = wres = const m (3) s m is the qulity of soil pr- In which, m w-res is the qulity of residul wter in soil per unit volume, s ticles in unit volume, w res is residul mss wter content. Bsed on the reltionship between volumetric wter content nd mss wter content, the following eqution cn be obtined θres = wres. ρd (4) In which, θ is residul volumetric wter content. res 764
Figure 3. The vrition of residul volumetric wter content with dry density From eqution (4), it cn be seem tht the residul volumetric wter content hve liner correltion with dry density. Fitted the experimentl dt in tble 2 with eqution (4), the following results cn be received (5) θres = 0.03389ρd The fitting results is shown in fig. 3. It cn be seem tht the experimentl dt re gree well with the fitting curve. It proves the nlysis of residul wter content bove nd the reltionship between residul volumetric wter content nd dry density. In VG model, the prmeter is relted to the ir entry vlue ψ. The ir entry vlue refers to the mtric suction of the intersection between the symptote of cpilliry sturtion zone nd the symptote of desturtion zone (Fredlund D.G. et l. 1994). From the vlues of in tble 2, it cn be seem tht the vlue in dry density=1.70g/cm3 is much greter thn those in dry density=1.75g/cm3 nd dry density=1.80g/cm3. Bsed on the drwing method, the ir entry vlues of these three soil wter chrcteristic curves re 168.13kP, 1134.66kP nd 1787.07kP respectively. It indictes tht the more the dry density, the more the ir entry vlue. By the mens of dt nlysis, the liner correltion coefficient of prmeter nd the reciprocl of ir entry vlue is 0.99, which revels the sound liner reltion between them. By dt fitting, the reltionship between the prmeter nd the ir entry vlue is obtined, tht is 0.4635 = (6) ψ The fitting curve is shown in fig. 4. 765
Figure 4. The vrition of prmeter with ir entry vlue Figure 5. The vrition of prmeter n with dry density Fig. 5 shows the vrition of prmeter n with dry density. In VG model, the prmeter n is relted to the slope of desturtion zone in soil wter chrcteristic curve. Fig. 5 indictes tht the prmeter n increses with dry density nd there re stisfied liner reltion between them. With dt fitting, the reltionship of prmeter n nd dry density is received, tht is n= 12.2217 ρ d 19.5642 (7) CONCLUSION (1)The dry density hs gret influence on soil wter chrcteristic curve. The more the dry density, the smller the sturtion wter content, the bigger the ir entry vlue, the lrge the desorption rte in desturtion zone, the stronger the chrcteristic of wter holding of soil. (2)The prmeter-residul mss wter content in VG model of soil wter chrcteristic is constnt, which refers to the wter content when the soil prticles only dsorb bound wter fter missing free wter under mtric suction. (3)The prmeter in VG model is liner correltion with the reciprocl of ir entry vlue. 766
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