Full flow penetrometers have been shown to overcome problems experienced with the cone penetrometer measuring resistance in very soft peat and organic soil, and give a much more uniform measure of resistance than the cone in fibrous peat. However, at present there is no guidance on the interpretation of strength parameters in these soils using the T-bar and ball. This paper examines the results of tests using these devices at two research sites in the Netherlands in conjunction with high quality Sherbrooke sampling for laboratory testing. In fibrous peat, the T-bar and ball provided a more uniform measure of resistance with a lower degree of scatter than the cone. The in-situ testing results have been compared to the laboratory tests to assess the range of resistance factors relating penetration resistance to the undrained shear strength (s u ) and have been shown to occupy a lower range of values than the cone penetrometer.However, penetration tests in these soils are likely to be influenced by partial drainage effects and this should be considered during testing and the subsequent interpretation of results. Recommendations are made for the use of full flow penetrometers to obtain strength parameters in these soils.

For the saturated case with only one pore fluid, either water or air, the roles of both the intergranular stress tensor and the pore fluid stress can be distinguished easily. In the unsaturated case, the capillary water is recognized to induce capillary suction in the pores and capillary-suction-induced interparticle forces. At the macroscale, volume averaging of these forces would lead to the capillary-suction-induced intergranular stress tensor. In its approximate formulation, the concept of the fabric stress tensor is applied, enabling the effect of the spatial distribution of the intergranular fabric on the capillary water bridges as occurring in the drier pendular saturation phase to be accounted for. Subsequently, the combined intergranular stress tensor and the combined pore fluid stress tensor can be derived directly. The constitutive relation of a granular skeleton, composed of elastic particles with mainly frictional interaction, like quartz sands and silts, is considered to remain independent of the degree of saturation. Under such restrictive conditions, only the additional physical parameters of the capillary-suction-induced intergranular stress tensor need to be determined, which can be achieved by means of inverse modeling, taking advantage of all macroscale experimental data and physical modeling for the whole unsaturated range. For clays and peats, with potential physicochemical and biochemical actions and double porosity and/or fibrous microstructures, the constitutive models can be expected to be physically more complicated, thus involving more physically relevant parameters. Hence, clays and peats must be considered to fall outside the scope of the proposed model framework.Abbreviations: DEM, discrete element method.The physical relevance of the continuum mechanical measures of stress, deformation, and flow of the pore fluids and stress and deformation of the solid skeleton forms the basis of any constitutive modeling and subsequent application for predictions in geomechanics.The physical relevance of the applied continuum measures of stress, deformation, and flow is a reflection of the physical concepts as applied in their descriptions. For instance, for the fluid-saturated case, the calculation of the deformation of the solid skeleton due to a changing pore fluid stress can be achieved by applying the macroscale isotropic pore fluid stress tensor pI, at least if the substance composing the particles remains elastic. For the solid skeleton of granular materials, the intergranular stress tensor s* and a potential microstructure tensor are derived using micromechanics in combination with volume averaging, irrespective of the degree of saturation. For the unsaturated case, the effects of the two simultaneous pore fluids on the solid skeleton are limited by the conditions for the granular skeleton that its deformation remains identically dependent on the tensors of the intergranular stress and a combined measure of both pore fluid stresses, as for both saturated cases. In fact, these conditions ...

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