Although plane-strain is the preferred method of analysing many rock mecahnics problems, little is known about the behaviour of rock under this loading condition. A simple and inexpensive apparatus has been built for laboratory testing of rocks in plane-strain conditions. A thorough investigation of the specimen geometry and the friction end effects showed that the results can be considered as reliable. The behaviour of a brittle sandstone in the simplest plane-strain condition (s³ = 0) is significantly different than that of the uniaxial compression (s² = s³ = 0). The plane-strain values of the Young's Modulus and the Poisson's Ratio are decreased in comparison to the corresponding uniaxial values. Dilatancy is also reduced in plane-strain, whereas the pre-failure brittleness of the rock increases. The most dramatic effect of the plane-strain condition is that on the strength of the sandstone, which increases by 49 per cent in comparison to its uniaxial compression value.
Plane-strain state in a rock mass is the state of strain in which all strain components normal to a certain plane are zero. This stage is often encountered in mining and civil engineering situations and in many more instances it can closely approximate the reality for design purposes. Its importance has been recognised by those involved in the theoretical analysis of engineering structures, with the result that almost every computer package incorporates a plane-strain analysis option. It is surprising that these theoretical developments have not been followed by any experimental work to assess the rock behaviour in plane-strain conditions. It is recognised that the plane-strain state is brought about by a suitable combination of the principal stresses, which generally are s¹>s²>s³. If the rock behavior were to be assessed in these general stress conditions, it would have involved "multiaxial" stress testing, which requires specially designed and expensive equipment with often questionable results because of ill defined end conditions due to friction. Instead, it was decided to study at this stage only one simple plane-strain case, that of s³ = 0, and concentrate on the thorough investigation of the reliability of the test results. A fine grain sandstone of the Karoo System has been selected as testing material. It is a fairly consistent rock with a uniaxial compressive strength 80,8 MPa.
Testing Procedure and Equipment The plane-strain test described in this paper is essentially one of biaxial compression (s³ = 0) with the intermediate principal stress, s², controlled in such a way as not to allow any deformation in its direction (e² = 0). A configuration of this principle is shown in Figure 1. The implementation of this idea was achieved through a simple apparatus used in conjunction with the stiff compression machine of the Chamber of Mines Research Organisation. A perspective view of the testing set-up is shown in Figure 2. Its main design considerations were the application and checking of the plane-strain state, the elimination of all friction effects as far as possible and the accuracy in recording stresses and strains.