Soils subjected to shearing experience dilation or contraction depending on their initial porosity, and the relative displacement of individual particles determines a soil's unique particle-pore microstructure during volume change. It has been suggested that soil microstructure tends to be stabilized as pores are aligned parallel to the loading direction as particles are mobilized. We explore the evolution of internal pore fabric and directivity during direct shear conditions in which a constrained boundary hampers the full mobilization of particles. Two representative volumetric responses for dense and loose granular soils during direct shear are simulated via the discrete element method. The arbitrarily shaped pore structure in 3D space is quantified using best-fitting ellipsoids to evaluate pore characteristics. Changes in pore fabric are analyzed based on local porosity, pore size distribution, and geometrical configuration of fitted ellipsoids. Results show that initial porosity determines the characteristic pore evolution during shearing. Numerical results also demonstrate that a pore elongation oriented in the direction of the shear manifests under dense packing, while randomly distributed pore directivity is observed under loose packing.