It's reported that Ensis directus (a species of Bivalves) achieves exceptionally high penetrating efficiency through periodically expanding/contracting its body during burrowing. This paper provides insights into the cyclic expanding/contracting process in cohesionless granules based on a simplified discrete element method (DEM) framework. In this DEM model, Razor clam is simulated by a circular cavity with a radius changing in a sinusoidal fashion. Different expanding frequencies are considered to study the interaction pattern between the cavity and the surrounding granules. Various stages of expanding process including initial movement, progressive expansion and contraction are captured. A static penetration case is included for comparison of penetration performance. Results positively confirm the contribution of dynamic expansion on the reduction of penetration resistance and energy saving. A critical expanding frequency exists, under which energy saving is optimal. Findings from this study can shed light on the design of smart devices for self-boring or site characterization.