SYNOPSIS: In this paper the characteristics of strength deformation and rupture of red sandstone under polyaxial compression were studied and the feature of the theoretical surface of the strength of rock analysed. The coefficient α, which represents the effect of intermediate principal stress σ2 on the ultimate compressive strength on the rock, was determined. The relation between the stress-strain curves along the directions of three principal stresses and, the lateral pressure was discussed. The rigid and flexible contact modes were adopted during the experiments, and comparisons made between the test results for the two kinds of tests.
RESUME: Les caracteristiques de resistance de deformation et de rupture de grès rouge sous compression polyaxiale sont etudiees et le comportement de la surface theorique de resistance de la roche est analyse dans cet article. Le coefficient α representant l'effet de la principale contrainte intermediaire σ2 sur la resistance à la compression a ete determine et les relations entre les courbes de contrainte-deformation dans les directions des trois principales contraintes et la contrainte laterale sont discutees. Des contacts rigides et flexibles ont ete adoptes durant les experiences et on a effectue des comparaisons des resultats obtenus pour ces deux types d'essai.
ZUSAMMENFASSUNG: In dieser Arbeit wurden die Festigkeit, die Deformation und die Bruch-Charakteristik des roten Sandsteins unter polyaxialer Kompression untersucht. Die Eigenschaft der theoretischen Flache der Gesteinsfestigkeit wurde analysiert. Der Einfluβkoeffizient α, der den Einfluβ der mittleren Hauptspannung σ2 auf die Druckfestigkeit des roten Sandsteins bezeichnet, wurde bestimmt. Die Abhangigkeit zwischen dem Seitendruck und den Spannung-Verformungskurven auf die drei Hauptspannungsrichtungen wurde diskutiert. Beim Versuch wurden die weichen und harten Beruehrungsmethoden angewandt. Ein Kontrast zwischen beiden Versuchsergebnissen wurde gemacht.
Red sandstone is a sedimentary rock of Cretaceous Period, Mesozoic era. It contains about 40% quartz, 25% feldspar, 15–20% detritus, and small amount of muscovite and chlorite etc. with mainly ferruginous cementing material. The physical indexes are: d=2.57, n=4.95%. w=1.52%, γ =24.33 KN/m. The experiments were carried out in a triaxial frame with stress strain curve measuring installations. In rigid contact test thin sheets, of PTFE were laid on all the contact surfaces in order to reduce the frictional effect. 99 specimens were tested for their compressive strength and 45 were tested for their deformation. To approach a flexible contact condition emulsified rubber bags filled with vaseline quartz sand mixture were used. 94 specimens were tested to determine their strength and deformation under this contact condition. A number of tests were also carried out in a conventional (σ2=σ3) 500T triaxial machine. The strength criterion of a rock can be expressed by one of the following equations: F(σ1,σ2 ,σ3)=0, G(τ oct, σ oct,λ)=0 and H(R, Q, λ)=0. In these equations R=(σ1 - σ3)/2, Q= (σ1 + σ3)/2, λ=(2σ2-σ1-σ3)/(σ1-σ3),-1«λ«1, and have the relations as shown in Eq (1). When γ=const. Eq. F(σ1, σ2,σ3)=0 becomes an equation with two variables σ1 and σ3, and in Cartesian coordinates 0σ1σ2σ3, it represents a curve on the plane σ2=σ1(1 +λ)/2 +σ3(1-λ)/2. These curves are distributed within the curves with λ= ±1 and irtersect at a point of equal tension σ1=02=σ3. For the same reason the space curve surfaces for the other two equations could be looked upon as composed of curves with different values of λ. By cutting these curve surfaces with planes corresponding to different values of σ3 and λ, the intersection curves will be scattered regularly on the 0σ1,σ2, ORQ and o τoct σoct coordinate planes respectively. On the ORQ coordinate plane the curves are so close that shows the characteristics of strength of rock under polyaxial compression. On the other coordinate planes, the concentration of curves is less. So it is convenient for us to utilize the characteristics of curves on ORQ coordinate plane to determine the value of. The relations between σ1 and σ2 from the test results are as shown in Fig.1 and Fig.2. Fig.4 shows the results from different contact experiments. The figure shows that for the flexible tests, the strength values are highest, and the results from conventional triaxial tests are less, and the test results from rigid contact are. the smallest. Fig.5 shows the curves from the test results and that from calculations according to several strength criterions. The test data are both from rigid and flexible contacts with σ2= σ3. In the figure curves 1, 2 and 4 show respectively the linear σ1/σc + σ3/σt = 1, hyperbolic and parabolic Mohr criterion. Curves 3 and 5 show respectively the revised Griffith criterion and the Griffith criterion. Curves 6 and 7 show lines obtained from equations (4) and (5) with λ=-1. The stress-strain curves from biaxial and triaxial compressive tests are shown in Fig.6, 7 and 8. In Fig.6 and 7, as σ2 is increased, ε, decreases, and curve σ1- ε, is ascending.