I would like to know what the minimum strength of a rock has to be when I want to conduct a point load test. When rocks or indurated soils have a strength below a certain treshold they fail very fast and the result of the point load test is not evaluable.
Basically the ratio of uniaxial rock UCS to point load index ranges from 20-25. Understanding the implication of this and relating the Benniaskwi 1989 Rock ratings table for rock strength during rock mass rating, the lowest rock UCS value that gives corresponding Point load index is that greater than 25MPa.
Lower strength rock fails instantly understand point load test.
Find an attachment containing the rating table for more clarity.
Wish you the best
Hello
minimum value to of UCS to perform reliable PLT is accompanied by the deformation mode prior to failure. Progressive failure and large indentation with large "footprint" of the cone create problems in correlation. Quasi-brittle rock materials can be tested starting from UCS 15 MPa, while softer materials need at least about UCS 20 MPa. Take care that also coefficient for correlation between the test index and the UCS can vary: for most brittle rocks, the ratio is approximately 10, for soft sedimentary stones the ratio can be lower ( 5). As a consequence, PLT results might have to be interpreted differently.
See also USING THE POINT LOAD TEST TO DETERMINE THE UNIAXIAL COMPRESSIVE STRENGTH OF COAL MEASURE ROCK by Rusnak and Mark.
Hope this can help
Regards
If you need a scientific resource, more than those which are mentioned in answers, see also :
Engineering properties of rocks (Lianyang Zhang).
However having been in labratory and tested a bunch of weak rocks specially across the shelf areas (like loamy shells and mudstones) i have experienced as low as 1.2 Mpa for soft rocks performing point load test.
it's recommended to go through the ISRM for the point load test:
http://www.geoplanning.it/test/wp-content/uploads/2012/02/Suggest-method-for-determining-Point-Load-Strength.pdf
the fast failure is not a big problem in that case. you need to check the magnitude of penetration before failure if the material is soft if the penetration is exceeding the test requirements you may use the bunch block test. I have faced the same problem when testing soft shale please have a look at this research:
Anisotropic index strength behaviour and failure mode validation of weathered shale
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