Rotta, G. V., Consoli, N. C., Prietto, P. D. M., Coop, M. R. & Graham, J. (2003). Ge ´otechnique 53, No. 5, 493–501 493 Isotropic yielding in an artificially cemented soil cured under stress G. V. ROTTA,  N. C. CONSOLI,  P. D. M. PRIETTO,† M. R. COOP { and J. GRAHAM } The work simulates, in the laboratory, the formation of a cemented sedimentary deposit in which cement bonding occurs after burial and under geostatic stresses. Isotropic compression tests were carried out on artificially ce- mented specimens made with variable cement contents. After consolidating the samples to the uncemented nor- mal compression line, the specimens were allowed to cure for 48 h before testing. The curing confining stresses ranged from 50 to 2000 kPa, and were intended to represent soil elements at different depths in the fictitious sedimentary deposit when the cementing occurred. The contribution of the cement bonds to soil compression and the changes in the isotropic yield stress as a function of void ratio and cement content were analysed. The results showed the importance of the void ratio during the formation of cement bonds and also of the degree of cementation for the compressive behaviour of the ce- mented soil, and demonstrated that the variation in yield stress with void ratio and cement content is dependent on the curing stress and independent of the stress history. KEYWORDS: compressibility; laboratory tests; stiffness; struc- ture of soils Nos travaux simulent, en laboratoire, la formation d’un de ´po ˆt se ´dimentaire cimente ´ dans lequel la cimentation se produit apre `s enfouissement et sous certaines contraintes ge ´ostatiques. Des essais de compression isotrope ont e ´te ´ pratique ´s sur des spe ´cimens artificiellement cimente ´s con- tenant diverses quantite ´s de ciment. Apre `s avoir consolide ´ les e ´chantillons jusqu’a ` la ligne de compression normale non cimente ´e, nous avons laisse ´ ces spe ´cimens durcir pendant 48 h avant de pratiquer les essais. Les contraintes confinantes dues au durcissement allaient de 50 a ` 2000 kPa et devaient repre ´senter des e ´le ´ments du sol a ` diffe ´r- entes profondeurs dans le de ´po ˆt se ´dimentaire fictif pen- dant la cimentation. Nous avons analyse ´ le ro ˆle des agglome ´rats cimente ´s dans la compression du sol et dans les changements d’efforts de tension isotrope comme fonc- tion du taux de pores et de la teneur en ciment. Les re ´sultats ont montre ´ l’importance du taux de pores pen- dant la formation des agglome ´rats cimente ´s et e ´galement du degre ´ de cimentation quant au comportement compres- sif du sol cimente ´ ; ils ont de ´montre ´ aussi que la variation des efforts de tension en fonction du taux de pores et de la teneur en ciment de ´pend de la contrainte de durcissement et ne de ´pend pas de l’historique de contrainte. INTRODUCTION AND BACKGROUND For clayey soils, structure has been defined as the combina- tion of bonding and fabric (Burland, 1990; Leroueil & Vaughan, 1990). The effect of structure is to give the soil a strength and stiffness that are greater and to allow the natural soil to exist at a higher volumetric state than that of the same material in a reconstituted state. In sands, structure has often been equated solely with interparticle bonds, although Dusseault & Morgenstern (1979) and Cuccovillo & Coop (1999) have identified the influence that the fabric of geologically aged sands may have on the peak strength. The effect of structure has been observed on a wide range of natural soils and weak rocks of both sedimentary and residual origins, and also for artificially cemented soils, as might be created, for example, by soil improvement. Much of the literature investigating the effect of structure is based on laboratory testing of natural soil specimens retrieved from the field (e.g. Burland, 1989, 1990; Leroueil & Vaughan, 1990; Airey & Fahey, 1991; Smith et al., 1992; Clayton et al., 1992; Airey, 1993; Petley et al., 1993; Cuccovillo & Coop, 1997, 1999; Kavvadas et al., 1993; Lagioia & Nova, 1995; Consoli et al., 1998). This approach, however, presents some difficulties resulting from the dis- turbance to the structure that can occur during the sampling process (Clayton et al., 1992). In natural sands, Stokoe & Santamarina (2000) have seen a loss of stiffness as a result of sampling that they believed resulted from the disturbance of interparticle contacts and breakage of cement bonds. Similarly, Coop & Willson (2003) attributed lower than expected stiffnesses, observed in triaxial tests for oil reser- voir sandstones, to the breakage of the cement bonds as a result of sampling as they were unloaded from in-situ states at several kilometres depth. Fernandez & Santamarina (2001) confirmed experimentally that a sand that had been cemented under pressure could have its interparticle bonding damaged by unloading. The other principal problem associated with the testing of natural sands is that, depending on their geological origin, there can be a high spatial variability, both in the degree of cementation and in the nature of the particles. An alternative is therefore to use artificially cemented specimens made up through the addition to the soil of a cementitious agent, such as Portland cement, gypsum, or lime (e.g. Dupas & Pecker, 1979; Clough et al., 1981; Coop & Atkinson, 1993; Cuccovillo & Coop, 1993; Huang & Airey, 1993, 1998; Zhu et al., 1995; Prietto, 1996; Consoli et al., 2000, 2001; Schnaid et al., 2001). This allows the simulation of natural cemented soils in the laboratory and the qualitative under- standing of the behaviour of structured soils without exces- sive sample variability or any bias due to sample disturbance. Cementation may arise in natural sands through a variety of processes. In some cases, the cement has been deposited soon after deposition when the sand was at a shallow depth. This is typical, for example, of carbonate sands with calcium carbonate deposited from supersaturated pore fluid to form calcarenites (Clough et al., 1981; Airey & Fahey, 1991; Cuccovillo & Coop, 1993), so that Coop & Atkinson (1993) formed their artificially bonded sands at zero confining stress. For these soils, burial is therefore subsequent to Manuscript received 27 March 2002; revised 13 January 2003. Discussion 1 December 2003.  Department of Civil Engineering, Federal University of Rio Grande do Sul, Brazil. † School of Engineering and Architecture, Catholic University of Pelotas, Brazil. { Imperial College of Science, Technology and Medicine, Uni- versity of London, UK. } University of Manitoba, Canada.