Since the introduction of the point load test, size effect has been observed in the point load strength index. While considerable research has been undertaken to investigate the size effect in the point load strength index, and the general cause of size effect, there has been limited research applying size effect theories to the size effect observed in the point load strength index. This paper investigates the applicability of size effect models including SEL, MFSL, Brook Model and FFSEL to both axial and diametral point load strength indexes at varying sample length to diameter ratios. In addition, the size effect at varying length to diameter ratios of a point load strength index which incorporates the contact area between the loading pointer and the sample has also been investigated.
Through an experimental investigation of both axial and diametral point load tests involving 374 samples of Gosford sandstone, this investigation found that the point load strength index varies increasingly with sample diameter for all investigated samples at different length to diameter ratios. The SEL and MFSL size effect models best fitted to the size effect trend of Gosford sandstone results obtained through axial point loading. However, when the contact area was included then the point load strength index found to increase with sample diameter for all investigated length to diameter ratios. The FFSEL size effect accurately modelled the increasing trend of Gosford sandstone results under both axial and diametral conditions across all length to diameter ratios.
The point load test is widely used within geotechnical and rock engineering in the classification of rock. Due to the low cost and portability of the test unit, it is commonly used within the mining industry to classify intact rock strength for use in rock mass classification systems such as the Rock Mass Rating (RMR) or Q systems . Knowledge of intact rock strength and the rock mass classification is a fundamental input for geotechnical design and also is a parameter for selecting mining method. Accurate classification of intact rock strength is critical for the accurate classification of rock mass which is necessary for the development of safe geotechnical and mine designs.
Since the introduction of the point load test, a size effect has been observed in the test results where the measured rock strength varies increasingly with sample size. This effect can have a significant impact on the classification of intact rock strength. Considerable research has been conducted to investigate the size effect in the point load test with previous research finding that point load results vary with sample diameter and sample length to diameter ratio [2-6]. This research has led to the introduction of sample size requirements for the point load test.