The relationships between applied stress, deformation and internal pressure of water filled inclusions embedded in soft soils were studied to improve understanding of pore-scale mechanical behavior and towards development of methods for in situ measurement of key mechanical properties. Analytical expressions for inclusion internal fluid pressure as a function of biaxial remote stresses and material properties were developed and tested with Finite Element calculations and experimental results. We found that the applied mean stress, matrix yield stress and Poisson's ratio influence pressure in a water filled inclusion. Internal pressure tends to be higher than applied mean stress, in contrast with established effective stress theory for saturated soils. Changes in inclusion shape is mainly controlled by matrix shear properties (changes in volume are negligible due to water incompressibility). The results of this study provide the framework for development of sensors for in-situ measurement of soil mechanical and rheological properties required for linking mechanical and hydraulic properties.