As shown in Figs. 2, 3a and 4a, the thickness of subsoil was 30 cm. The subsoil and backfill soil were
produced by pluviating air-dried Silica No.7 sand (D50=0.25 mm; Uc=2.2) to obtain a relative density
about 90%. To capture the deformation of the backfill throughout the shaking tests, thin horizontal
layers with a width of about 0.5 cm of black-dyed Silica sand were placed at a vertical spacing of 10
cm immediately behind the front transparent glass window of sand box. In addition, a number of rivets
made of aluminum with a black circular flat edge were used as targets set in the backfill soil adjacent
to the transparent glass window at a horizontal spacing of 5 cm and a vertical spacing of 5 cm for the
purpose of measuring deformation of the backfill on plane strain condition. In order to ensure
permanent contact between the glass and the targets, thereby following the surrounding sand
movement, silicon grease was smeared between the targets and the glass.
After filling the sand, a surcharge of 1 kPa was applied by placing lead shots on the surface of the
backfill to simulate the weight of the road base for structures or traffic.