Static and dynamic loads on the first row of interlocking, single layer armour units

Abstract

Interlocking, single layer concrete armour units are placed in a specific grid depending on the type of armour unit. Within this grid, armour units are placed in horizontal rows. The number of horizontal rows of single layer armour units on a breakwater is limited to 20. This limit is proposed in order to prevent major settlements, which might affect the interlocking of the armour units. The limit on the number of rows is based on experience from prototypes and is not yet confirmed in a systematic study. Then number of rows also might have an effect on the load on the first (bottom) row of armour units, which affects the structural integrity of the armour units. The load on the first row of armour units is however unknown. The research presented in this thesis is a study on the load on the first (bottom) row of concrete armour units placed on a breakwater. Both the static load and the dynamic load were examined. The static load is defined as the load on the bottom row of armour units resulting from the higher positioned rows of armour units during conditions without waves. The dynamic load is defined as the load on the bottom row of armour units during conditions with wave attack minus the static load. These loads were studied by physical model tests. The static load was studied in an experiment in which the down slope force on the bottom rows of armour units (Xbloc units of 366 grams) was continuously measured during the placement of 20 rows of armour units on a slope of 37 degrees (slope of 3:4) in a series of 15 tests. The dynamic load was studied in a physical model test in a wave flume. The first row of armour units was placed on a movable frame which was connected to a load cell. The dynamic load was measured during tests with regular waves of 20% to 100% of the maximum wave height corresponding to the used armour unit (Xbloc units of 61.7 gram which were positioned on a typical breakwater slope of 3:4) and a wave period corresponding to an Iribarren number of 3, 4 and 5 for all of the described wave heights. This static load experiment resulted in a relationship of the measured static load on the first row of armour units with the number of rows applied on the slope of the model. From this relationship appeared that the static load approaches a maximum value after 10 rows. An analytical model was developed and validated against the measured results. This model gives an interpretation of the cause of the maximum value. The measurements of the dynamic load showed two clear phenomena. The dynamic load appeared to be a harmonic load with the same period as the waves imposed on the model. The dynamic load is the result of the flow of water along the armour layer. The maximum dynamic load on the first row of armour units occurred simultaneous with the maximum downwash which is in line with expectations. A relation between the downwash velocity and the amplitude of the dynamic load was found. The second observed phenomenon is the increase of the wave averaged load on the first row of armour units during the test. During the tests the harmonic load oscillated around an equilibrium line which showed a positive trend. The measured load after testing was significant higher than the measured load at the beginning of the tests. A relation was found between the wave characteristics and the increase of the load on the first row of armour units.