Drained behaviour of sands under simultaneous increase in stress ratio and principal stress rotation is investigated. The hollow cylinder torsional (HCT) device which permits independent control of four stress parameters:​ mean normal stress σ'[subscript]m, stress ratio R, intermediate principal stress parameter b and the inclination σ_σ of σ^'₁ to the vertical, is adopted as the testing device.

In order to conduct complex stress path testing in the HCT device, a new automatic stress path control system is developed. The stress non-uniformities due to the curvature of the HCT specimen is assessed using an incremental elastic representation of sand behaviour, in order to delineate the domain of stress space that could reliably be explored using the HCT device. It is shown that previous assessments of stress non-uniformities assuming linear elastic soil grossly overestimate the stress non-uniformities in a HCT sand specimen. A much larger domain of stress space with acceptable levels of non-uniformities is apparent from the results of incremental elastic analysis. New domain of stress space for reliable exploration using the HCT device is delineated and the testing program is developed so that all stress paths lie within these acceptable limits. Tests are carried out on pluviated sand under saturated drained conditions.

The deformations under increasing R and σ_σ is shown to be path independent, if the final stress state is within the approximate bounds of R ≤ 2 and σ_σ ≤ 45°, regardless of the b or relative density D_r, levels. With increasing stress ratio R and/or principal stress rotation σ_σ, the deformations gradually become path dependent.

Once loaded to a stress state within the domain R ≤ 2 and σ_σ ≤ 45°, the strain response under subsequent principal stress rotation is shown to be independent of the previous loading history. It is demonstrated that the strain response under any general increasing R - α path in the domain of R ≤ 2 and σ_σ ≤ 45° can be predicted using the results of a limited number of tests characterizing that domain. It is shown that these concepts can be extended to loading paths which involve simultaneous increase of three stress parameters.

Strain increment direction α_Δε is shown to be approximately coincident with and totally governed by the stress increment direction α[subscript]Δσ when the stress increment direction α[subscript]Δσ is more inclined towards the vertical deposition direction. When the stress increment direction is inclined closer to the bedding plane, the strain increment direction depends in addition, on other parameters such as R, σ_σ and D_r etc.

Under any stress path involving principal stress rotation, the deformations decrease with increasing density and therefore the principal stress rotation is more crucial in loose sands. Deformations increase with the level of stress ratio R. Level of b parameter does not affect deformations under principal stress rotation, if the rotations are small. However, with increasing σ_σ deformations due to principal stress rotation tend to increase with decreasing b value.