Flatjack tests of modulus and stress of claystone

As part of a program of site investigation for the Cimanuk multi-purpose project, Java, in situ measurements of modulus and stress in soft rocks, comprising claystones of three grades and a weak breccia, were made by the flatjack method. The stresses ranged from 0.5 to 1.5 times the pressure of the overburden, of 8–40 m. The moduli were higher than from plate bearing tests done in the same areas : 3 to 6 times for the claystones and up to 2 times for the breccia. It is suggested that rock disturbance was less in the flat- jack than the plate bearing tests, but also that compacting yield may have occurred in the plate bearing tests. Moduli at flat- jack pressures above site stress, found by a suggested method, ranged from 1 to 0.5 times the modulus at lower pressures. Interpolated moduli were proposed for design considerations.