Abstract：Apparently the ideal apparatus used to simulate soil behavior under principal stress rotation is hollow cylinder apparatus(HCA). The 5 Hz HCA developed by Zhejiang University and GDS instruments Ltd. in England has a combined axial-torsional loading system，which can be used in static and dynamic(at frequency of 0–5 Hz) tests. Accurately measuring hollow specimen¢s deformation is the basis of strain measurement and stress control mode，which is often used in HCA testing. However，after isotropic consolidation，it is not easy to get accurate measurement due to the disturbances in the process of setting-up the sample and the air bubbles between the inner wall and the inner membrane. The formulae for outer and inner radii after consolidation are derived through analyzing the changes of inner cell volume，back cell volume and axial strain by iteration algorithm. Based on the analyses，Tastuoka¢s hypothesis is modified. Comparison between the results with those obtained from Kaman sensors used for local measurement indicates that it is feasible to calculate outer radius by volumetric changes and axial strain. While the test using the amended inner radius can highly improve measuring accuracy，thus the reliability of stress control mode can be also guaranteed.
