Shear behaviour of idealized infilled joints under constant normal stiffness

Abstract

The shear behaviour of soft joints containing infill materials was investigated in the laboratory under constant normal stiffness (CNS) conditions. Tests were conducted on joints with asperities having inclinations of 9·5° (type 1) and 18·5° (type II), under a given range of initial normal stresses (σno) 0·30 to 1·10 MPa, and at a constant normal stiffness of 8·5, Kn/mm. It was found that the shear strength of joints decreases considerably even with the addition of a thin layer of infill. Results also show that the effect of asperities on shear strength is significant up to an asperity height to infill thickness (t/a) ratio of 1·4—1·8, whereas the shear behaviour is controlled by the infill alone beyond this critical ratio. The shear displacement corresponding to the peak shear stress is considerably reduced once the infill starts to govern the shear behaviour of the joint. In this study, the drop in peak shear stress under CNS conditions has been modelled by a hyperbolic relationship. In relation to ‘clean’ joints, it is verified that the proposed equation can predict the drop in shear stress as a function of the infill thickness, with good agreement with the measured data. In order to predict the dilatancy behaviour of ‘clean’ joints, a Fourier transform method is introduced, which can be used to predict the shear strength of joints under CNS conditions. Nous avons examiné en laboratoire le comportement de cisaillement de joints tendres contenant des mat&acuteriaux de remplissage dans des conditions de rigidit&acute normale constante. Nous avons mené des tests sur des joints qui pr&acuute;sentaient des aspérit&acutes ayant des inclinaisons de 9·5° (type I) et 18·5° (type Il) sous une gamme donnée de contraintes normales initiales (σno.) variant entre et 1,10 MPa et ´ une rigidit&cute normale constante de 8·5 kN/mm. Nous avons constaté que la r&acutesistance des joints au cisaillement diminuait consid&acuterablement méme lors de I'adjonction d'une mince couche de remplissag. Les résultats montrent aussi que I'effet des aspérit&acutes sur la résistance au cisaillement est significatif jusqua'´ un rapport hauteur d'asperite — &acutepaisseur de remplissage (tla) de 1,4—1,8, alors que le comportement au cisaillement est contrôlé par le remplissage seul au-del´ du rapport critique. Le deplacement de cisaillement correspondant ´ la contrainte tangentielle de pointe est considérablement réduit une fois que le remplissage commence ´ gouverner le comportement du joint au cisaillement. Dans cette étude, nous avons mis en maquette la baisse de contrainte tangentielle sous condition de rigidité normale constante au moyen d'une relation hyperbolique. En cc qui concerne les joints ‘propres’, nous vérifions que I'équation proposée permet de prévoir la baisse de contrainte tangentielle comme fonction de I'&acite;paisseur de remplissage, avec un bon accord entre les données mesurées. Alin de prédire le comportement de dilatance des joints ‘propres’, nous utilisons la méthode de transformation de Fourier qui pent étre utilisée pour prédlre la resistance au cisaillement des joints en conditions de rigidité nor-male constante.

Shear strength model for overconsolidated clay-infilled idealised rock joints

“…Under these conditions the dilation is expected to be less than that associated with the initial asperity angle. This field condition was simulated by Indraratna et al (1999) using CNS test conditions. They used a Fourier function coupled with the energy consideration proposed by Seidel & Haberfield (1995) to model the shear strength of a clean joint as given by…”

“…Indraratna et al (1999) investigated the effect of infill focusing on the shear strength of clean joints and the drop in shear strength (˜ô p ) caused by the infill. They fitted the reduction in the normalised shear strength of infilled joints to a hyperbolic decay curve for a given t/a ratio, as shown by equation (4).…”

“…For smaller t/a ratios the shearing of asperities is influenced by the normal stress exerted on the joint. It is likely that for higher normal stress, when dilation will be restricted, shearing may occur through the asperities (Indraratna et al, 1999. In this case, some breakage of asperities will be inevitable.…”

“…In this case, some breakage of asperities will be inevitable. Different types of infill will give different critical t/a ratios, which in general exceed unity (Phien-wej et al, 1990;Papaliangas et al, 1993;Indraratna et al, 1999. It is also possible that the critical t/a ratio of a particular infilled joint will be influenced by its degree of overconsolidation.…”

“…Kanji, 1974;Lama, 1978;Barla et al, 1985;Barton, 1974;Bertacchi et al, 1986;Pereira, 1990;Phien-wej et al, 1990;de Toledo & de Freitas, 1993) or constant normal stiffness (CNS) conditions (e.g. Ohnishi & Dharmaratne, 1990;Skinas et al, 1990;Haberfield & Johnston, 1994;Indraratna et al, 1999. The infill materials used in all of these investigations may be categorised as being either cohesive (clay) or frictional (silt and sand).…”