A method of stress measurement is proposed whereby epoxy resin is allowed to set in a borehole. The hole is overcored, the overcored section cut into slices and the stress-relieved epoxy inclusion is analyzed for stresses in two-dimensions using the theory for biaxial photoelastic strain gages. The method was verified in a laboratory experimental program by simulating stress relief in the annulus of an overcore. During the laboratory testing suitable techniques were developed which virtually eliminated shrinkage and residual stresses during casting of the epoxy resin. The method is relatively simple and inexpensive. In a borehole no electrical wires or light sources are needed and no additional cementing material is required. It is believed that this technique should be capable of use at any practical drilling depth and in poor rock. Moisture conditions may present difficulties yet to be overcome.
A method of measuring the nu secondary principal stresses in a rock mass is proposed whereby epoxy resin is allowed to set in a 1.5 inch diameter (EX) borehole and then overcored as shown in figure 1. The overcored section is then removed and cut into one-inch thick slices and the strain-relieved epoxy inclusion is analyzed for stresses in two-dimensions using the theory for biaxial photoelastic strain gages as presented by Hawkes and Fellers (1969). The method is relatively simple since no electrical or other connections are needed, and inexpensive since epoxy resin is the only material placed in the hole. These factors should make more measurements possible in a given area than has previously been feasible. It is believed that the technique could be useable at considerable depth and in fractured rock as well. There is also promise that diagonal slicing of the overcore may permit the determination of stresses in three dimensions. As a byproduct of the stress measurement program a complete core sample is obtained because the epoxy will act as reinforcement to bind the entire length of core in an integral piece. There are essentially two types of photoelastic strain gages. The one most studied and successfully applied is the "stiff" or "hard" gage. This type requires a ratio of host rock Young's modulus to gage modulus (Eh/Eg) of 1/5 for successful operation. In other words if the ratio is greater than 1/5 the gage ceases to behave as a hard inclusion and the theory no longer applies. A truly hard inclusion can be termed a "photoelastic stressmeter" since the stress induced in the gage is independent of the host modulus. Interpretation of this type of gage is given by Hawkes(1 968) and Hawkeasn d Fellers (op. cit).
Figure 1. Vertical section of a borehole showing epoxy resin in place in smaller diameter axial hole.(available in full paper)
When Eh/Eg is greater than 10 a "soft" inclusion results. In this case the strain induced in the gage by the deformation of the borehole upon overcoring is independent of the material properties of the inclusion. The borehole deforms as if the inclusion were not there. If the ratio Eh/Eg is between 1/5 and 10 then the gage and the rock overcore interact, complicating the theory.