The Effect of Shallow-Water Waves on the Stability and Bearing Capacity of Sea Beds

This paper reports a theoretical and experimental investigation of the influence of shallow water waves on the bearing capacity of foundations in sea beds. The propagation of pressure waves through a porous sea bed is calculated, assuming plane elastic-plastic soil deformation under undrained loading conditions, and including the effect of dilatancy. The effective stresses thus generated are compared with the soil-stability limits. The consequences for both sand and clay deposits are considered individually. Model experiments in a flume demonstrate how prolonged wave action increases the density of an initially very loose sand, whereas it expands an initially dense sand. Because small changes in void ratio have a significant effect on the bearing capacity of a sediment, the bearing capacity of a loose sand bed increases with prolonged wave action, but that of a dense sand decreases. It was found, however, that under all circumstances the ultimate bearing capacity of a sandy sea bottom is largely sufficient for pile-foundation purposes. In fact, stormy weather tends to stabilize the bearing capacity of a sandy sea bottom. In contrast, calculations for clay deposits show that the excess pore pressure caused by wave action on a normally consolidated clay cannot be dissipated within a practical time limit. Waves are thus unable to compact a clay deposit. The shearing forces generated by shallow waves in an underconsolidated clay may therefore cause soil instability during a severe storm. Our calculations and model experiments seem to indicate that shallow foundations subject to strong wave action will settle gradually within a plastic region of the sea bottom as a result of cyclic wave loading. This paper describes a practical method for calculating a safe depth of burial and bearing capacity for shallow foundations subject to wave action. INTRODUCTION Ocean waves are able to generate significant shearing forces in sea-floor sediments up to total (water + soil) depths of approximately half the length of the waves. Waves could thus affect the end bearing capacity of shallow foundations or the lateral resistance at shallow depths of deep foundation piles. We shall first calculate the pressure fluctuations at the sea bottom due to plane irregular waves using the small-amplitude wave theory. We shall then calculate the stresses and pore pressures generated in the sea bottom by such irregular pressure fluctuations at the bed surface, assuming plane elastic - plastic soil deformation and introducing a pore - pressure parameter for the saturated soil under undrained, biaxial loading condition s. We shall compare those stresses with the soil stability limits and consider soil consolidation due to wave action. Then we shall present the results of model experiments performed in a laboratory flume, showing that prolonged wave action increases the density of an initially very loose sand, whereas it expands an initially dense sand. We shall compare theory and experiment. Finally, we shall discuss a method for calculating a safe depth of burial and bearing capacity for shallow foundations subject to wave action.