This work makes some considerations on the present knowledge of the deformability properties of weak rock masses together with comments on the adequate modeling of engineering problems in these materials. An analysis of a hypothetical foundation consisting of a fractured weak rock loaded on the surface was carried out by various techniques and the results are presented.
Ce travail presente des considerations sur l'etat actuel deconnaissancedes proprietes de deformabilite des roches faibles et sur la modelization numerique des problèmes du genie des ces materiaux. On presente encore les resultats de l'analyse numerique d'une fondation hypothetique que consiste de roche faible fracturee"chargee à la surface. Cette analyse a ete effectuee en utilisant plusieurs techniques numeriques.
In diesem Beitrag werden einige ueberlegungen gemacht ueber den stand der Forschung betreffend der Verformungseigenschaften weicher Felsen und die entsprechenden Modell-idealisierungen fuer die Anwendung auf dem Gebiet der Ingenieurwissenchaft. Ein hypothetischer, aus weichem Felsen in gerissenen Zustand bestehenden Grundbau wird an der Oberflache geladen und nach unterschiedlichen Verfahren untersucht.
Recently, a growing number of engineering works are being built or designed in weak rocks. Such undertakings require a proper knowledge of the properties of the rock mass comprising the intact rock itself and possible existing features such as faults, fractures, bedding planes, etc. Sections 2 and 3 of this paper present some available data concerning the deformability of some weak rocks together with a discussion on influencing factors. In section 4 comments are made on the problems associated with the numerical modeling of fractured rocks with relevance to fractured weak rocks and results are presented for loads on a hypothetical foundation. In this analysis, various analytical techniques modeling the fractured medium were used either by means of an equivalent continuum or a medium composed of discrete blocks.
Special relevance is made in this section on the properties of weak sandstones. Typical results of the deformability moduli of weak sandstones are shown in Fig. 1 (Dobereiner and de Freitas, 1986). Each value represents the average of at least five tests and they were obtained on saturated samples tested in uniaxial compression. The range of the tangent modulus of deformability (E) measured before the onset of dilatancy varied between 100 and 3000 MPa. On average, the value obtained for the correlation between the deformability modulus and the saturated uniaxial compressive strength (σc) was E =140 σc This value is lower than the 200 proposed c for weak rocks in general by Rocha(197S). It also produces a lower modulus ratio than the one defined for weak rocks by Deere and Miller (1966). The Poisson's ratio obtained also for weak sandstones is in the range. 27 -. 40, calculated for stress levels below the onset of dilatancy.
The lateral, axial and volumetric strain typical of the weak sandstones tested in uniaxial compression is illustrated in Fig. 2. The plot of volumetric strain records an initial decrease in
(Figure in full paper)
volume followed by an increase. This increase in the volume of the sample starts at low levels of stress, often at about one third of the peak compressive strength. If those stress-strain relations are compared with those for strong rocks as described by Goodman (1980) and Jaeger and Cook (1976), some discrepances can be observed. There is no marked linear relationship between stress and strain even in the pre-peak stage. Typically an inelastic concave upwards stress-strain section is observed, and also that the rate of lateral strain begins to increase, relative to the rate of axial strain, at very low stresses.
The dilatancy observed in the volumetric strain curve of Fig. 2, is probably caused by the beginning of the formation of microcracks inside the most critically stressed portions of the specimens.
It is clear that at stress levels above dilatancy (sometimes at less than one third of peak strength), the strains are inelastic and non recoverable. Therefore it is advisable to use volumetric strain curves as a standard procedure indeformability tests, and to determine the deformability modulus at stress levels below dilatancy (Dobereiner and Oliveira, 1986 or Dobereiner and Dyke, 1986). At least the deformability modulus should not be obtained at high stress levels as there is the certainty of non-recoverable and inelastic strains. At stress levels below the onset of dilatancy the strains are probably a combination of elastic and non-elastic strains due to the closing of pores and fissures, but they should at least be partially recoverable.
