Pores in granular media tend to align parallel to maximum stress upon direct shear and creep conditions. We herein numerically investigate the evolution of pore orientation under biaxial compression using 2D particle assemblies in conjunction with image analysis, in particular for the case of particle clumps bounded by flexible wall. The discrete element method produces the dense and loose packings where two particles are clumped to closely simulate the nonspherical particles followed by biaxial compression. The Delaunay triangulation and polygonization allow construction of unit pore space whose irregular shape is fitted by an ellipse. This helps characterize the orientation of each unit pore and quantify the pore size distribution at each strain regime. The simulated results are analyzed within the framework of local void ratio and elongation factor. The initial void ratio uniquely determines the development of elongation factor with strain and pores aligned toward the direction of maximum compression. Its manifestation is pronounced near the regime of shear band attributed to the formation arching for stabilizing the macroscale particle network.