CALIBRATION OF A SHEARGRAPH

A sheargraph transmits normal and shear stresses to the soil via a spr ing which is compressed and rotated. The compression and rotation are recorded directly on conventional. rectilinear graph paper. However, t he forces involved in compression and torsion of the spring in a shear graph are not independent of one another. As a result, lines of equal normal or shear stress are not parallel to the axes of rectilinear gra ph paper. Calibration equations for the relationship between normal an d shear stress and the compression and rotation of the spring contain interaction terms. This paper describes a general form for these equat ions which is shown to fit experimental calibration data reasonably we ll. For a spring which is rotated during a test in the direction of co iling (i.e. wound up). as is the case for the sheargraph used in this paper, assuming independence of shear and normal stresses leads to an underestimate of the soil shear strength compared with the correct cal ibration. Conversely, a spring which is rotated opposite to the direct ion of coiling (i.e. unwound), assuming independence of shear and norm al stresses leads to an overestimate of the soil shear strength compar ed with the correct calibration. Tests on a sand (which had a linear f ailure envelope) and a silt loam (which had a curved failure envelope) demonstrated a closer agreement with shear box results when the corre ct calibration was used.