Correlation between point load strength index and uniaxial compressive strength for hard crystalline rock

Abstract

Large scale constructions such as highways, bridges, high rise buildings, jetties etc. are designed to withstand heavy loads depending on the dead loads and imposed loads on the structure. For this purpose, loads from these super structures are to be transferred to competent rock by adopting pile foundations. For end bearing piles, rock socketing has to be done to the competent rock. To identify competent rock, generally, the Point Load Strength (PLS) index (Is(50)) of a rock sample collected during the rock drilling work is determined and the Unconfined Compressive Strength value of the same sample is estimated. If the Unconfined Compressive Strength value is acceptable, the pile can be terminated. For estimating the Unconfined Compressive Strength value from the Point Load Strength value, there are some correlations which are generally used in the Sri Lankan construction practice. All of these correlations are outcomes of research work done in overseas countries. Therefore, this research is an effort of finding an acceptable correlation between the Point Load Strength value and the Unconfined Compressive Strength value for crystalline metamorphic rocks found in Sri Lanka to be used as a guideline for pile termination criteria. More than 130 rock samples were collected from different rock types and both the Unconfined Compressive Strength value and the Point Load Strength index of the same rock were determined by conducting laboratory tests. In addition to these two tests, the Specific Gravity (SG) value of the same rock was determined and the rock type was also identified visually to group/ classify the data according to the rock type. After analyzing the test data gathered, a correlation between the Unconfined Compressive Strength value and the Point Load Strength index was established for different rock types. Few samples from each rock type were used for validating the correlations established.