Sinking of concrete modules into a sandy seabed: A case study

Three submerged coastal structures on the sandy seabed at Santa Maria del Mar (SMM) Beach in Southwest Spain were monitored during the six months after installation, starting in November of 2005. The monitoring evaluated the self-burial phenomenon of the 40 precast modular concrete elements of the three structures. Assessment included scouring around the structures, their vertical movement, the sinking velocity of the modules and the resulting beach profiles. Pressure sensors and precision topographies permitted continuous monitoring of the self-burial process for the first time in a full-scale case. Comparison of two bathymetries (one performed immediately prior to installation and another performed six months after installation) indicated slight accretion of the seabed, whereas intermediate topographic surveys indicated extensive scouring around the structures. The three structures began sinking into the sandy bottom platform immediately after placement and continued until they reached the rocky bottom. This study implemented a new methodology using pressure sensors attached to the concrete modules to monitor the structural sinking. Unexpectedly, the average sinking speed was extremely rapid at approximately 3–6 cm/day; 50% of the height of the element was reached in three to six weeks. The results of this study are also compared with the results of other full-scale cases. Among other conclusions, data from this study indicated that the sinking rate of the modules in SMM Beach was one order of magnitude greater than the other values.