Comparison of unconfined compressive strengths of cast versus cored samples of rock-like materials with large voids

Highly porous rocks have received little attention and their properties, while understood on a general level, have not been intensively researched. To better recognize the effect of porosity on unconfined compressive strength (UCS), analog models of plaster of Paris with Styrofoam inclusions were developed and tested. By casting large blocks of plaster with Styrofoam inclusions and coring, samples should be more representative of rock than cylindrical cast specimens with no edge-intersecting voids. Surprisingly, the relationships of UCS and porosity are very similar between the cast and cored specimens, indicating that voids on the surface of a core have little impact on UCS. The correlation coefficient, or R², of the relationship of normalized UCS with porosity for the cast and cored specimens together is 0.85. The R² values for the cast and cored samples alone are 0.9523 and 0.8115, respectively. The relationships of the cast and cored specimens were compared with data from previous studies of tuff. The R² of the tuff group is 0.82. The correlation coefficients of each grouping were tested against each other in a test for equality with a level of significance of 0.05. This statistical analysis proves that cored samples provide better estimates of actual rock, with a p-value of 0.335 versus 0.181.