MODEL TESTS ON RESPONSE OF CONCRETE BLOCK ON SEABED SUBJECTED TO UPWARD SEEPAGE FLOW

Abstract

Under stormy weather conditions, offshore structures have been severely damaged by heavy sea wave loading. Many researchers have conducted investigations on damaged structures and revealed that the damage to the structures is emphasized by the disintegration of the seabed caused by sea wave loading on the seabed ground. In this study we conducted model sea-wave loading tests on sand ground and revealed that the disintegration of the seabed and associated scouring were caused by the upward seepage flow as a response to sea-wave loading, which reduced the effective stress in the seabed. Breakwaters consisting of concrete blocks have been monitored for years, and it has been reported that they sink every year by a fraction of a meter in accordance with stormy wave conditions. We conducted a series of model tests on a block equipped with a three-axial accelerometer to detect its attitude when rested on a seabed with generated upward seepage flow. The sinking behavior of the block in the seabed during the gradual increase in the hydraulic gradient of seepage flow was carefully observed to clarify the fundamental mechanism of the sinking of the block. Consequently, this study found the critical condition of the hydraulic gradient under which the sinking of the block is initiated is independent of the density of the seabed material. In the scheme of model tests, the applicability of the three-axis accelerometer for the detection of the block, even under the critical hydraulic gradient attained, was demonstrated.