Liquefaction of crushable soils

Abstract

Crushable soils when sheared in a dense state have stress paths which are similar to those for loose, hard-grained sands; however, because of their angular nature they do not liquefy as easily under cyclic loading. In the Hyogo-ken Nambu, 1995, earthquake, there was widespread liquefaction of land areas reclaimed from the sea using a crushable, highly angular, decomposed granite soil, Masado, which was previously regarded as a high-quality fill material with little risk of liquefaction in earthquakes. Results are presented from monotonic and cyclic triaxial tests on three types of crushable soil, namely Masado, Shirasu volcanic sand and Dogs Bay carbonate sand, together with standard silica Toyoura sand, to establish liquefaction criteria which can be applied not only to hard-grained silica sands but also to more angular crushable sands. In monotonic triaxial testing the stress paths passed through a point of phase transformation, with each material exhibiting large strains after this point. Normalization of the stress—strain curves and stress paths with respect to the effective consolidation stress brought the phase transformation points closer together. For loose soil, liquefaction failure occurred, while for dense sand cyclic mobility was evident, where the stress path cycled through or close to zero-p' conditions while the cyclic axial strain increased at a steady rate to large values. The liquefaction strength was defined in terms of the number of cycles to cause a double-amplitude strain of 5%, for both types of failure. A relationship was established between the normalized deviator stress required to cause liquefaction failure after 20 cycles and the normalized phase transformation strength obtained from monotonic tests. This relationship was independent of relative density and type of material. Les sols concassables, quand Us sont cisaillés à ľétat dense, ont des parcours de contrainte qui sont similaires à ceux des sables boulants à grains durs mais en raison de leur nature angulaire, Us ne se liquéfient pas aussi facilement souscharge cyclique. Lors du tremblement de terre de Hyogoken Nambu en 1995, it šest produit une liquéfaction très étendue des zones de terre regagnées sur la mer en ut lisant un sol de granite décomposé trèos angulaire et concassable, le Masado, qui était auparavant considéré comme un matériau de remplissage de haute qualité présentant pen de risque de liquéfaction lors ďun séisme. Nous présentons ici les résultats ďessais tri-axaux monotones et cycliques sur trois types de sol concassable: le Masado, le sable volcanique de Shirasu et le sable carbonaté de Dogs Bay ainsi que le sable quartzeux normal de Toyoura afin ďétablir les critères de liquéfaction qui peuvent être appliqués non seulment à des sables quartzeux à grains durs mais aussi à des sables concassables plus angulaires. Dans les essais tri-axaux monotones, les parcours de contrainte passent par un point de transformation de phase après lequel chaque matériau montre de fortes déformations. La normalisation des courbes de déformation dues aux contraintes et des parcours de contrainte par rapport à la contrainte de consolidation effective, a rapproché les points de transformation de phase. Pour les sol boulants, it se produit un défaut de liquéfaction alors que pour les sables denses, la mobilité cyclique est évidente là où le parcours de contrainte approche de zéro pour la valeur p' alors que la déformation cyclique axiale augmente à un taux régulier pour atteindre de hautes valeurs. Nous définissons la force de liqué faction en termes de nombre de cycles provoquant une déformation de 5% de double amplitude pour les deux types de défaillance. Nous établissons une relation entre la contrainte déviatrice normalisée qui cause la défaillance de liquéfaction après 20 cycles et la résistance de transformation de phase normalisée obtenue après les essais monotones. Cette relation ne dépend pas de la densité relative ni du type de matériau.

Particle Crushing and Deformation Behaviour

“…Miura and O-Hara (1979) mention that particle crushing increases the negative dilatancy, which may reduce the peak shear strength. Hyodo et al (1998) note that crushable soils, when sheared in a dense state, trace undrained stress paths similar to those for loose sands of a less crushable nature. Mahboubi et al (1997) suggest that soil samples of diŠerent initial densities tend to the same void ratio owing to shearing but this void ratio continues to fall as breakage continues.…”

“…Stress tensor normalised using the state parameter and their invariants are given in the same way as the original Severn-Trent sand šsij＝ sij 1-kc ＋pd ij (15) where s ij is the deviator stress tensor. Variation in the state parameter c becomes:…”

Particle breakage during cyclic triaxial loading of a carbonate sand

“…The engineering behaviour and properties of the material have been examined in detail by a number of researchers (e.g. Houlsby et al 1988;Golightly 1989;Coop 1990;Coop & Lee 1993;Yasufuku & Hyde 1995;Hyodo et al 1998;Coop et al 2004;Tarantino & Hyde 2005;Qadimi & Coop 2007). There is some variability among the particle size distributions ( Figure 1) and index properties (Table 1) reported in the literature.…”

Undrained monotonic and cyclic simple shear behaviour of carbonate sand. D. PORCINO, G. CARIDI and V. N. GHIONNA (2008). Géotechnique 58, No. 8, 635–644

“…These findings are supported by similar conclusions drawn by Mao & Fahey (2003) from undrained cyclic simple shear tests carried out on three calcareous soils. It is important to remember that the PT mon line represents a key element in determining the observed undrained cyclic response of carbonate soils under both simple shear and triaxial conditions (Hyodo et al, 1998). The results reported in the authors' paper show that, for Quiou sand, the PT mon line is relatively unaffected by initial density state.…”