Scientists have developed an experimental approach to determining the forces produced during the impact of a wave on a seawall.
Designing strong and stable seawalls is of utmost importance in times of rising sea levels. However, what happens when a wave hits such a seawall with great force? Scientists so far have been working only with formulas for calculating force and pressure, but no experimental data has been available.
Scientists at Polytechnic University of Catalunya in Barcelona, Spain, have for the first time developed an experimental approach to determining the forces produced during the impact of a wave.
Pressure and force transducers as well as high-quality measuring amplifiers from HBM have played a pivotal role in this experiment.
The impact of a wave breaking on a vertical wall is a fast and very powerful phenomenon that is very hard to measure. It is important to measure the total force and its application point generated by the impact of the wave in order to predict the design loads and properly design the seawall. The pressure also is important in the design of the armor of reinforced concrete.
For these purposes, a physical model was built in the Maritime Engineering Laboratory at Polytechnic University of Catalunya (LIM-UPC). The model of the seawall was equipped with 6 P8AP pressure transducers positioned in the impact zone to measure the vertical pressure distribution.
The structure is supported by two Z6C3 beam load cells to measure the total force and the momentum generated by the wave's impact.
The P8AP is an absolute pressure transducer based on a strain gauge sensor with a measuring span of 10 bars and an accuracy class 0.3. The P8AP are IP67, meaning they are weatherproof but not waterproof. For this reason, a box that isolates the sensors from the water is needed.
Twenty holes were added to the front wall to enable to try different patterns of pressure sensor positions to be tried. Six holes were plugged with the pressure sensors, and the others were plugged with screws so that the front wall will be waterproof and continuous.
After tests of the pressure sensors in various positions, the definitive pattern was defined with the six pressure sensors placed on the same vertical, with a distance between each sensor of 25 mm.
The Z6 by HBM is a bending beam load cell with a nominal load of 50 kg and an accuracy of 0.009% of maximum capacity. The load cells were mechanically fixed to the box protecting the pressure sensors, and were fixed at the reticular structure previously described.
It was very important that the mechanical connection be very rigid in order not to absorb any force and affect the measurement. A reticular structure fixed on a super structure of the wave flume (independent from the wave flume itself) was used because in a reticular structure, there are normal forces and the nodes are fixed. The stiffness of the load cells should be much lower than the stiffness of the reticular structure in order to deform itself and perform the right measure of force.
The coupling of the two load cells in two different positions was needed in order to obtain time series of the total force, the momentum and the application point of the force.
As the phenomenon is very fast (a few milliseconds), HBM’s QuantumX MX840 8-channel amplifier with a maximum sample frequency of 19,200 Hz for all channels. Both the pressure transducers and the load cells were connected and recorded with the QuantumX controlled with catman software.
The ability to connect the six pressure transducers and the two load cells in the same data acquisition system and sample at a very high speed was crucial in order to compare the results obtained from both.
To verify the results, a comparison between the results of the pressure transducers and the load cells was made, integrating on the vertical section the pressure distribution determined with the P8AP.
The preliminary results showed a good agreement between the results of the pressure transducers and the load cells at low sample frequency. Increasing the sample frequency the agreement diminishes. Some sensors were positioned in a wet/dry zone in which the presence of a mixture of air and water during the impact, and the geometry of the P8AP, can lead to problems during measurement resulting from the compressibility of the air.
In order to solve the problem, the opening of the sensors was filled with glycerin and a vacuum was generated to avoid any vibration of the vacuum membrane and any loss of glycerin.
One of the next steps in the experimental campaign will be to improve the resolution in terms of pressure sensors through the acquisition of eight more sensors and another QuantumX MX840.