Abstract
In recent years, many gravity dams are constructed on weak rock foundations. But until now, as to weak rocks, there are few standardized methods to evaluate the engineering properties of rock masses as dam foundations.
In weak rocks, it's very useful to measure the strength of rock specimens for evaluating some engineering properties of rock masses, because they have an intimate relation to those of rock specimens relative to hard rocks.
Needle penetration test is one of the most avaiable methods to evaluate the hardness of weak rocks. As a field evidence, needle penetration gradient (or strength: Np[kgf/mm]) has a well correlation to uniaxial compressive strength. For this reason, it is recognized Np will be a possible indicator of strengh of weak rocks.
But in some cases, needle can't penetrate into rocks. In other words, needle penetration test isn't applicable to following rocks.
1) Th rocks, its grain size is greater than coarse size.
2) The rocks, its uniaxial compressive strength is higher than 100-150kgf/cm2 (≅10-15MPa).
