A torsional shear box for determining the shear strength of agricultural soils

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Abstract
A hand-held torsional shear box, suitable for direct measurements of the in situ soil strength (ôo) under field conditions and for laboratory determinations of shear strength parameters, is described. A field procedure is given, and the errors in the method are discussed. Under field conditions, it is simple to operate and requires a single measurement to determine ôo. Field studies indicate that it is suitable for a wide range of soil conditions which occur in agricultural soils. Results are presented, which show that both moisture status and structure influence the value of ôo, as well as the nature of the failure plane. Agricultural soils have a number of characteristics which make it difficult to design methods for determining soil strength parameters or to interpret empirical estimates of soil strength. Chief amongst these are: (1) marked changes in most soil physical properties with depth; (2) a polymodal distribution of pore sizes. Because the measurement is on a surface which is nearly horizontal and is of an area to allow for all forms of failure, the described torsional shear box largely overcomes these problems, and provides a value of ôo which is physically interpretable and relevant to studies concerned with soil failure.

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