Comparison between plate jacking and large flat jack test results of rock mass deformation modulus

•Comparison includes 16 PJTs and 12 LFJTs carried out at Karun 3 project in Iran.•PJT moduli in depth are higher but well correlated with corresponding LFJ moduli.•The results of the comparison facilitate and improve interpretation of the PJT.•A new empirical relationship for indirect estimates of the modulus is suggested.

The results of two types of loading tests for the determination of the in situ rock mass deformation modulus (Em), namely, large flat jack (LFJ) and plate jacking tests (PJT), which were carried out side by side during the exploratory phase of the Karun 3 hydro-electric project in Iran, have been compared, correlated and analysed. The comparison includes sixteen PJTs and twelve LFJ tests. PJT moduli obtained from displacements measured in depth are much higher than the LFJ moduli and are sometimes even higher than the modulus of the intact rock material. However, this investigation has proved that they can be well correlated with the corresponding LFJ results and reduced by factors established in this comparison. The analyses showed that uncertainties connected to PJT results are caused by the restraining effects of the adjoining rock, and the disturbing effects of blast damage, and also that the scattering of results occurs due to insufficient volumes of rock mass tested. A new empirical relationship for indirect estimates of Em based on the RMR classification scheme is made from field data presented in this investigation and other reliable published data.