The authers are conducting a series of research studies on the effect of earth pressure on closely bored tunnels. At the first stage centrifuge tests were carried out with a pair of tunnelling models. In these experiments the characteristics of the earth pressure in sandy soil were investigated when the ground surrounding tunnels had been yielded by tunnel boring work. At the same time simulation analysis by DEM model was carried out and the mechanism of increasing bending moment of first tunnel by occurance of tail void of second tunnel is clarified.