S. Alveolata is a tube-dwelling polychaete which can be found all the way from the coast of Norway all the way down to Dakhla in the Western Sahara. It is able to build large reefs structures from sand filtered from the water column. It is known to have importance for biodiversity from animals feeding on the animal to its reefs providing a safe haven for fish. At the same time, a move is happening to using habitat protection and recreation for coastal protection. Several soft habitats such as mangroves, saltmarshes and seagrasses have been studied for their wave damping potential and S. Alveolata holds promise in this aspect. Given these functions, the S. Alveolata biogenic reefs need to be protected from outside influences, but little is known about their (mechanical) behaviour. As such, this study investigates the reef structure, erodibility, compressive and shear strengths of these reef materials and compares these to the commonly encountered loads from walking humans and wave loading. The static load from a person can reach up to 150 kPa while loading from 1.8 m waves can exert 36 kPa.  In agreement with Fournier-Sowinski (2013) and Vovelle (1965) but in contrast to Sanfilippo et al. (2019), the structure of the reefs to consist of three-layered tubes, with an organic sheath which partly contains a layer of flat grains placed next to each other on the inside and a scale-like layer of generally flat grains. These grains are glued together with proteinaceous glue spots showing a particular, bubbly structure. New is the description of bubbles radiating from the center of the glue spots. The grain size distribution shows a bi- or trimodal distribution, which has not been described before. S. Alveolata favours flat grains (in contrast to (Gruet & Bodeur, 1994; Lisco et al., 2020)) of about 1.0 mm size to build their reefs, which is a larger grain size than previously found (Gruet & Bodeur, 1994; Lisco et al., 2020; Naylor & Viles, 2000). As previously observed (Gruet, 1972), the reefs are made up of many tubes curving upward from a solid substrate. However, the first 3D mCT-scans of a whole reef block show enough voids so that small tubes wind their way through the reefs, probably made by juvenile worms. The grain size and mCT data suggest that tubes are reconstructed during the year.  The mechanical properties of these reefs show a very high compressibility with total strains between 10-20% under loads of 604 kPa. The reefs show a yield stress varying from 80-185 kPa. This is about the same stress as a person exerts while standing, the dynamic loads of walking are much higher but the limited time these loads are applied means little damage is done. Even though these yield stresses are far exceeded during testing, the samples hold some amount of cohesion due to interlocking and some unbroken bonds. Loose tubes weather from all sides at the same rate during slake durability tests. Shear box testing was conducted in a stepped manner with four loading steps from 48-380 kPa normal pressure, such that the first two steps are below yield stress at 48 and 96 kPa normal load. These show that the reefs have a cohesion of at least 23 kPa and a friction angle of 21.3°, explaining their resistance to waves. During testing, the reef samples compress strongly with their void ratios decreasing from 1.01 to 0.85. As a result of shearing, the internal tube structure of the sample was strongly deformed and shear bands or planes were formed during shearing. Some voids are also opened up in this process. After shearing samples show an apparent cohesion due deformed tube parts in which some bonds remain intact interlocking with each other. Compared to other cemented sands the S. Alveolata reefs exhibit many of the same features as these sands, such as the applicability of the Mohr-Coulomb criteria and high compressibility. However, unlike other cemented sands their open, tubular structure showed lower friction angles due to the alignment of sand particles in the linings. Also, the S. Alveolata lack the filling of void spaces often found on cemented sands leading to compressive behaviour.  Compared to the pressures to which the reefs are subjected in nature, the reef materials probably hold up well. The results provide a basis for more detailed investigation and testing and highlight key behavioural aspects of the material. It shows the response of the material to mechanical stresses and provides a starting point for using these reefs or solutions inspired upon them in different ways.