The Effect of Wave Action on Structures with Large Cracks

Abstract

Damages of coastal structures are otten wave induced. A new study revealed that wave impact generated pressures can travel into water filled cracks or joints, thus damaging or destroying the stmctm'e ~om within. A series of experiments was conducted in QUB's wave channel to assess the characteristics of breaking wave impact induced pressure pulse propagation through partially water filled cracks. It was found that the pressure pulses generally travel at very low velocities of 45-70 m/s within completely submerged cracks, whereas velocities in partially submerged cracks are significantly higher. Pulses attenuate fast inside ~ e fldby submerged ~tdc, but slowly in l~lially submerged ones. In partially submerged cracks the pressure pulse was found to travel in the air, propagating fi~t and with little attenuation deep into the structore, signifying that partially filled cracks are probably more dangerons than completely filled cracks. They are probably also the mai~ cause for the seaward removal ofblockwork in coastal engineering stmc~es. This effect however is not limited to biockwork structures, but to every struetta-e where cracks or joints o~ur, including natural rock cliffs. INTRODUCTION Many coastal structures, as well as natural rock cliffs, are subjected to breaking waves, which create very high and short pressure pulses. Especially older structures and soft, chalk-like rock contain cracks orjoims which are exploited during storms. Typical damage mechanisms include the seaward removal o f individual blocks in breakwaters or whole cliff sections. It is assumed that the wave impact pressures travel into the structure/cliffdamaging it from within. Although it has been suspected for a long time it was only recently shown that these impact pressures can enter water filled cracks and that they propagate as compression waves, Mflller (1997), MOiler et al (1999). Although in the last 20 years a lot o f research effort was directed to investigate impact pressures, the mechanism o f impact pressure generation is still disputed, there is still no reliable prediction formulae for such pressures and very little is known about impact pressure propagation and pressure pulse characteristics. I G. Wolters, PhD Student, Queen's University Belfast, Civil Engineering ~ e n t , David Keir Building, ~ i l f i s Road, Belfast BT7 fAD, UK, Tel.: +44 (2990) 274001 2 G. Milller, Lecturer, Queen's University Belfast, Civil Engineering Department, David Keir Building, SWanmiilis Rood, Belfast BT7 5AD, UK, Tel.: +44 (2590) 2745 ! 7, Fax: +44 2890 663754 ~-lllall: ' :!~ ~ y ! :!