A method for the assessment of the stability of slopes in closely jointed rock masses is presented. A lower bound on the possible rock mass shear strength is obtained. As an example of the method data on slope height-angle relations for the greywacke slopes at various locations in the North Island of New Zealand are back analysed. The mobilised strength curves obtained for the greywacke rock masses are compared with the shear strength envelopes derived from the modified (1992) Hoek-Brown failure criterion.


Une methode d'evaluation de la stabilite de talus de masses rocheuses fracturees serrees fait l'objet de cette presentation. Une limite inferieure de resistance possible au cisaillement de la masse rocheuse est obtenue. A titre d'exemple de la methode presentee, des donnees sur les rapports hauteur/angle du talus sont identifies par back analysis pour des talus de grauwacke situes dans differents endroits de I'île Nord de la Nouvelle-Zelande, Les courbes de resistance mobilisee obtenues pour les masses rocheuses de grauwacke sont ensuite comparees avec les enveloppes de resistance au cisaillement derivees du critère modifie de rupture Hoek- Brown (1992).


Eine Methode zur Bewertung der Stabilitat von Abhangen eng verfugter Felsmassen wird vorgestellt. Eine untere Grenze der möglichen Scherkraft in der Felsmasse wird erhalten. Als Beispiel fur diese Methode werden Daten zurueckgerechnet, die die Beziehung zwischen Abhangshöhe und Winkel von Grauwacke Abhangen an mehreren Orten auf der Nordinsel Neuseelands angeben. Die mobilisierten Krafkurven, die fuer die Grauwacke Felsmassen erhhalten Warden, werden mit den Scherkraft Mantelkurven verglichen, die sich aus dem modifizierten (1992) Hoek-Brown Versagenskriterium ergeben.


Much of the international rock mechanics literature is concerned with two types of rock mass. Firstly there is the so-called intact rock, which is material that, to the naked eye, is without defect. Secondly there is the situation where the behaviour of the rock mass is controlled by a few well defined and widely spaced joints. Until recently scant attention has been given to a third class of rock mass in which the joint spacing is very close but without any particular joint, or joint direction, having a dominant effect. This seems to be the characteristic of many rock engineering situations in New Zealand. Rock masses of this type occur in other parts of the world, notably the west coast of the United States, so it is not a problem that is unique to New Zealand. It is more likely that the difficulties are just so great that no line of attack has been readily apparent. In this paper three separate threads are developed, and then drawn together in an example application. Firstly the difficulties associated with closely jointed rock masses are discussed in more detail. Secondly a method for the assessment of existing stable slopes is presented which gives a lower bound on the possible rock mass shear strength. As an example of the method existing data about slope height-angle relations in the greywacke slopes at various locations in the North Island are back analysed. Thirdly there is some discussion of the significance of a curved failure envelope and the effect of earthquake loading on a closely jointed rock mass. The basic philosophy of the method recognises that, as it is unlikely that an accurate assessment of the true strength parameters for a given rock mass will ever be available, an initial approach to designing any alteration to the slope profile should aim to ensure that the final state of the slope is no worse than the initial state.

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