Soil-water retention behaviour of fine/coarse soil mixture with varying coarse grain contents and fine soil dry densities

The interlayer soil identified in conventional French rail track corresponds to a mixture of fine soil and coarse grains. To investigate the role of fines in the soil-water retention property of such a mixture, different coarse grain contents [f.sub.v] and dry densities of fine soil [[rho].sub.d-f] were considered. The filter paper method was applied to measure matric suction. Mercury intrusion porosimetry tests were performed to observe the microstructure of the fine soil. In terms of the gravimetric water content of fine soil [w.sub.f] with matric suction [psi], the soil-water retention curve (SWRC) was significantly affected by [[rho].sub.d-f] for [psi] < 715 kPa, while independent of [[rho].sub.d-f] for [psi] > 715 kPa. Interestingly, this threshold [psi] corresponded to a delimiting pore diameter of the bi-modal microstructure of fine soil, which separated micropores from macropores. In terms of degree of saturation [S.sub.r] with [psi], the SWRC was significantly affected by [[rho].sub.d-f] in the full suction range, while it was independent of [f.sub.v]. These findings help to better understand the results for samples where dry density of mixture [[rho].sub.d] is kept constant: an increase in [f.sub.v] resulted in a decrease in [[rho].sub.d-f] and the suction changed accordingly. In this case, both [f.sub.v] and [psi] affected the mechanical behavior. Key words: soil-water retention, matric suction, pore size distribution, dry density, coarse grain content. La presence d'un sol intercouche identifie dans la voie ferree conventionnelle franqaise represente un melange de sol fin et de gros grains. En vue d'etudier le role des fines dans la propriete de retention d'eau d'un tel melange, differentes teneurs en gros grains [f.sub.v] et densites seches de sol fin [[rho].sub.d-f] ont ete considerees. On a applique la methode du papier filtre pour mesurer la succion matricielle. On a effectue des tests de porosimetrie par intrusion de mercure pour l'observation de la microstructure du sol fin. La courbe de retention d'eau dans le sol (SWRC) a ete significativement affectee par [[rho].sub.d-f] pour [psi] < 715 kPa, alors qu'elle etait independante de [[rho].sub.d-f] pour [psi] > 715 kPa, en termes de teneur en eau gravimetrique du sol fin [w.sub.f] avec la succion matricielle [psi]. Il est interessant de noter que ce seuil [psi] correspondait a un diametre de pore delimitant la microstructure bimodale du sol fin, qui separait les micro-pores des macro- pores. En termes de degre de saturation [S.sub.r] avec C, le CRSW a ete significativement affecte par [[rho].sub.d-f] dans la gamme complete de succion, tout en etant independant de [f.sub.v]. Ces constatations permettent de mieux comprendre les resultats obtenus sur des echantillons dont la densite seche du melange [[rho].sub.d] est maintenue constante : une augmentation de [f.sub.v] a entraine une diminution de [[rho].sub.d-f] et la succion a change en consequence. Dans ce cas, [f.sub.v] et [psi] ont tous deux affecte le comportement mecanique. [Traduit par la Redaction] Mots-cles: retention d'eau dans le sol, succion matricielle, distribution de la taille des pores, densite seche, teneur en gros grains.
Introduction In conventional French rail track, an interlayer was naturally created in the substructure under long-term traffic loading, which corresponded to a mixture of ballast grains and subgrade fine soil. [...]