Novel Fabrication Method for Tafoni-inspired Seawall Blocks, Increasing Complexity and Provision of Marine Microhabitats

Abstract: The aim of this research is to develop fabrication processes for complex seawall modules using biodegradable formwork, which allows for the creation of heterogeneous habitat surfaces and voids not currently achievable using conventional construction practices. By exploring the use of parametric design and additive manufacturing, the intent is to improve the typical block-based revetment strategy and protect the urban environment while trying to minimize the impact of coastal armoring on the flora and fauna. By printing a tafoni-inspired PVA sacrificial formwork, the concrete block becomes lighter as the amount of concrete can be reduced. PVA dissolves in seawater over time and is a proven non-toxic material. The form and materiality of the blocks draw attention to the preservation and enhancement of biodiversity.

1. Introduction
As human populations continue to grow, people are increasingly moving from rural to urban centers [1, 2] and, closer to the coast seeking an elevated style of life [3]. The need to control erosion and protect the intertidal shorelines from wave-surge led to the increased use of coastal infrastructures, such as piers, docks and coastal armoring [4, 5]. The anticipated climate change effects, such as sea level rise and increased storm frequency, will lead to a global aggravation of coastal erosion and flooding [6, 7].

In response to the growing demand for defense mechanisms against sea-level rise and the increased frequency of extreme meteorological events, hard-substrate defense structures are introduced to the coastal landscapes altering the intertidal and shallow sub-tidal habitats [8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. Compared to terrestrial systems, only recently there have been some studies in the impact of coastal infrastructure on marine habitats [18,19,20]. An increasing number of research has shown that epibiota living on structures, such as breakwaters, concrete walls and floating fiberglass pontoons, differ from those on natural reefs [18, 21, 22, 23]. Therefore, it is safe to claim that the extensive use of hard-substrate defense systems, replacing natural shorelines, alter marine environments through the introduction of foreign materials and novel three-dimensional morphologies into ecosystems which have evolved distinct abiotic and biotic characteristics. These characteristics are often different from the constructed infrastructure that replaces them [11, 15, 16, 19, 24, 25]. As a result, urbanized waterfronts present a unique design challenge and that is where the attention has been focused on designing defense systems that try to minimize their ecological impact [27]. Thousands of miles of armored edges and countless artificial marine structures currently exist globally, and much more is in the process of being built or planned, as sea levels rise, storm surges increase, and coastal development hastens. Artificial materials and structures affect marine environments on micro artificial and macro scales, making detailed design and material choices significant beyond specific sites, and potentially impacting larger ecological systems. The marine environment of Sydney Harbor, for instance, is estimated to be 96% urbanized by walls, piers, wharves, jeties, docks and other structures with more than 50% of the shoreline composed of seawalls [12, 28]. In California, America’s most populated state, an astonishing 110 miles of the entire 1100-mile coastline is armored [29]. This urban condition is of course not unique to Sydney or California. Most major harbors and urban waterfront are heavily armored with variable configurations of steel sheet piles, cut stone, riprap, concrete and timber.

The pervasiveness of artificial materials and simplified form of the urban intertidal zone has changed the ecology of marine systems, as it replaces vast, gently sloping sedimentary planes of seagrass, marsh plants, and fine sediment with vertical steel sheet piles or monolithic concrete seawalls [30]. Additionally, the replacement of naturally eroding limestone and extensive mangrove swamps with immovable vertical concrete structures also obliterates sediment and groundwater exchange and stabilizes a once dynamic landscape condition [30]. Vertical structures truncate the intermediated boundary between water and land, altering the edge morphology in horizontal and vertical profile and diminishing habitat areas. Armored edges also restrict the movement and exchange of groundwater, alter salinity gradients and PH, limit sediment transport, replace vegetated slopes and planes, and simply the heterogeneity of microclimates, ultimately altering intertidal habitats [31]. Integration of habitat criteria into urban marine infrastructure may never reverse this damage, but it may help to bridge the ever-expanding schism between urbanization and marine ecology.

Studies conducted in California [32], along the Mediterranean [25], and in China [33] further illustrate that hard-marine structures in areas with naturally soft edges invite invasive species and ultimately reduce biodiversity. Extrapolating to the future, the problem only appears to intensify as sea levels rise and storm surges increase. This, however, can and should be seen as a great opportunity for improving the ecological health of the urban intertidal and subtidal zones through the integration of habitat or new seawalls. The goal of this research is to introduce a novel method for building seawall blocks, that increase complexity and provision of marine microhabitats beyond what is currently available with traditional seawall blocks, while using traditional materials (concrete) and more recent technology (large-scale 3D printing).

2. Context

Background and prior art

Historically, the manmade seawalls are predominately vertical and lack the horizontal rock platform common with the natural rocky seashores in the harbor. The natural rocky seashore also shows varied orientation and slope, both of which are radically simplified on constructed seawalls. The reduction of slope associated with these structures shortens the intertidal zone, and within this zone their surface textures are relatively smoother. Furthermore, seawalls lack the overhangs, pools, and large crevices commonly associated with natural habitat. As a result, hard surfaces and artificial structures, such as seawalls, can be colonized, but they can hardly be considered as analog or surrogate habitats for marine species [5]. For example, recent studies [16, 24, 34, 35] have shown that such defense systems can alter the composition, abundance, size and structure of the hosted marine assemblances.

Scientists attempted to identify factors that contribute to the fouling or colonization of marine structures and vessels by sessile organisms (barnacles, tubeworms, oysters and mussels) and mobile animals (limpets, snails, chitons, gastropods and crabs). Pomerat and Weiss conducted studies [36] on the attachment of marine organisms to plastics, glass, woods, metals, linoleum, and other materials with additional tests on glass surface textures ranging from flat to ribbed and factrolite (patterned and textured type of glass with multifaceted, miniature pyramid geometry). In these early experiments, variable materials and textured surfaces were arrayed in a harbor or bay to determine the rate and density at which organisms (limpets, corticated turfs, barnacles, ephemeral algae etc.) attached to the artificial surfaces. The study concluded that material composition played an important role in the surface fouling dependent on the porosity of the material [36]. Other researchers have advanced the study of artificial marine structures and have concluded that species diversity (micro – and macroscopic algae, sessile animals, particularly barnacles, tubeworms, oysters and mussels, and mobile animals, such as limpets, snails and crabs) increased with the availability of microhabitats such as those provided by deteriorated walls and therefore may be intentionally designed [12, 28]. Lack of habitat heterogeneity has been delineated as the main culprit of the decreased level of epibiotic biodiversity recorded on hard-coastal defense structures [11]. There is now documented evidence that novel engineering interventions that increase the surface complexity and heterogeneity (for instance by including water-retaining features, niches, pits and crevices) can substantially boost the richness of the structure’s colonizing assemblages [26, 27, 37, 38]. The diversity of niches allows for structural overlap and therefore increases the species diversity. For instance, Martins et al. [39] documented, over a period of 4 months, a sharp increase in the population of limpets residing on seawalls where experimental habitat enhancements have been added. These enhancements included the addition of pits of different sizes, at different densities. Rebuilding complexity in habitat is, thus of significant importance to positively impact and preserve biodiversity. One way to achieve this complexity is by creating heterogeneity as far as the surface topography, salinity, light, and sediment are concerned [40, 41].

The topographical heterogeneity (pattern in elevation over an area) can impact the distribution of organisms at smaller scales through the creation of diverse aspects, orientations, textures, rugosity [42], and slopes [43]. It is therefore clear that the topographical heterogeneity provides an opportunity for design innovation, especially with the recent and rapid advancement of software technology that allows for the generation of very intricate heterogenous surfaces [44, 45]. Thus, novel coastal defense systems should be designed to maximize the intertidal and subtidal surface areas subject to colonization and to incorporate complex small-scale topography that enhances biodiversity [45, 46] and provide shelter to different species [5, 16, 41, 44]. (Figures 2, 3, 4)

 

 

 

 

 

The chemical composition of artificial materials is another important factor in creating artificial habitats and results in novel ecosystems [47]. For instance, modified concrete mixtures have been studied and tested for promoting colonization of marine life on infrastructure depending on the pH, density and the addition of fibrous matrices [47]. In this context, diverse materials like concrete, steel, wood, shell, rock, fiberglass have already been tested as part of an ongoing research on the matter by the U.S National Artificial Reef program, while the effective use of artificial materials in habitat creation still continues to develop [48].

Nevertheless, there has been an increased interest towards combining engineering processes with ecological principles to introduce innovative techniques that ameliorate urban environments. Thus, using ecological engineering as a guide, built infrastructure can be designed to reduce the environmental impact and enhance marine biodiversity [16,39,49,50].

Precedent projects for hard habitats of coastal armoring

Pilot projects have already been constructed around the world to test the viability of ecologically engineered seawalls, breakwaters, and bulkheads. These projects are conducive to meeting sustainability goals, but they also provide researchers with real life experiments that help them evaluate further the impact of life supporting design for hard coastal infrastructure.

– Habitat panels, Seattle, Washington

Seattle’s waterfront is in the process of being redesigned with a new masterplan. (Figure 5) The need to repair large sections of dilapidated seawall, which threatened important infrastructure, provided great impetus to incorporate ecological concerns into the design. The new seawall is designed to habitats through a layered sequence of strategies that include the use of textured panels that have shown to support more diverse communities than the existing seawall. The project, led by the City of Seattle Department of Transportation, involved a diverse set of consultants, including Parsons engineering as the prime consultant, and Magnusson Klemencic Associates (civil engineers) who led the overall seawall/public realm design. The University of Washington also conducted experiments and research that led to the design parameters for texture size, depth, and shelf configuration. The study found that the mussel and algae populations were more prominent where features with high-relief elements, like shelves, were included. Furthermore, incorporating texture complexity (use of cobble stone) on the seawall panels, seemed to encourage the settlement of larger populations of mussels and algae which are highly important as they provide food and shelter for small invertebrates that attract fish. That is why they are often referred to as “ecosystem engineers”, modulating the availability of resources and having a large impact on the species diversity [51]. More information about the project can be found here: https://waterfrontseattle.org/waterfront-projects/seawall

– Mussel Beach, East River, NYC

Ken Smith Landscape Architects designed the Mussel Beach ecological habitat demonstration project as part of the East River waterfront in NYC. (Figure 6) The folded slope of the project is composed of specially textured, precast concrete panels embedded with rocks to enhance the marine habitat of the river’s native mussel population. The geometry of the project is intended to increase the intertidal area, while the diversity of the texture of the panels aims to provide refuge for marine species. In this case, the fact that the panels were precast provided a greater design freedom as far as the diversity of the texture is concerned. The Project team included the City of New York (client), Ron Aleveras (ecologist), ARUP (engineering), HDR (engineering), SHoP (architects), and Tillotson (lighting design). More information about the project can be found here: https://www.fast-company.com/90338865/the-hot-new-amenity-in-nycs-newest-park-mussels

 

Apart from a number of projects, there is also a significant number of patents [52, 53] for artificial habitat and shore protection systems that address complex environmental problems, from the dissipation of wave energy to the construction of large ports in open water (Figures 7, 8). Together with the development and application of new research and technology, these patents can also be proven a great source of inspiration moving forward as far as the wave energy dissipation notion is concerned and the incorporation of multiple novel habitat types on the seawall.

3. Materials and Methods

Disclosed Principles and Abstracted Biological Role Model

The seawall armoring block was conceptualized and inspired by the biological investigation of the tafoni rock formations. Groom et al. [54] define tafoni as “cavities or hollows of various sizes in stone surfaces. Cavernous rock decay is a globally occurring phenomenon that has been under scientific exploration for centuries”. Tafoni typically occur in sedimentary rocks that contain silica and they can be developed in all climates, but are most common in deserts and coastal environments. [55, 56, 57]. Similar to the honeycomb weathering effect that occurs when liquids penetrate a porous surface, those cavernous decay features are the result of the penetration of seawater through sandstone in an effort to find the fastest route out as it evaporates. At the same time, the salt remains in the cavities of the sandstone and starts to erode it further, forming these intricate, structural patterns. The underlying principles of this phenomenon of topology optimization were abstracted and transferred into a fabrication strategy for the design and production of a seawall armoring block. The goal was to provide the necessary structural integrity, while maximizing the efficiency of material by creating cavities of various scales and, thus, the potential for habitat formation.

Honeycomb weathering (Figure 9) happens to relatively coarse-grained and porous rocks that absorb water, like sandstone. The geometry of honeycomb weathering is dictated by movement of water from the surface of the material to its interior and vice versa. The flow of the water is divided into numerous streams which are evenly distributed over the surface. (Figure 10) The sandstone’s low permeability, for example, makes the behavior of the trapped water similar to a liquid of much higher viscosity. Thus, it forms streams of a significantly larger diameter than the diameter of the pores of the sandstone. The pits that are formed are basically the locations that the water “uses” as its way in and out of the stone. These pits grow bigger over time as the phenomenon keeps occurring [58].

Mathematically, hexagonal patterns are the “best way of packing as many non-over-lapping, equal sized circles on a plane as possible” [58]. (Figure 12) The bones of birds have exactly the same pattern as the tafoni rock formations and their purpose is to present the largest surface area possible to be structurally stable without obstructing the air flow [58]. Similarly, the water finds the shortest way in and out, forming patterns that provide the least resistance to its flow. In this way, what remains from the rock is what is absolutely necessary for its structural stability (topology optimization), until eventually erodes away due to the continuous weathering process.

The pits of the honeycomb pattern have a limited depth, which is indicated by the reduction in efficiency of the surface of the pit to act as a surface for evaporation or energy dissipation. As the erosion continues and the density of the holes keeps increasing, they gradually start to merge with each other. As Doe describes the process of the pattern development, “by acting as a vent for the internal moisture, the holes deprive the area im-mediately around them of evaporating moisture. By depriving their perimeters of moisture, they also deprive their perimeters of damaging crystallizing salt, and so the holes deepen leaving their perimeters intact” [59].

This process serves as a relevant role model for topology optimization strategies in architectural applications. By mimicking these six-point patterns, and modelling the forms using software for parametric design, a porous block design was developed with the intention to minimize the use of material, while maximizing structural integrity, and potentially enhancing the coastal ecosystem.

Thought Process for System Development

The concept for a prototypical architectural system emerged out of these initial biological investigations. Pre-cast concrete has been extensively used for the fabrication of armoring blocks over the years and, lately, there have been studies [47] that have improved its consistency significantly towards the goal of the ecological enhancement of the marine species in urban environments. More specifically, ECOncrete® mix was used for the design of a high texture seawall units (2014) at the Herzliya Marina (Israel), aiming to provide inviting habitats to the local marine flora and fauna, while complying with structural seawall standards. The experimental habitat was monitored over a period of 22 months, revealing a higher diversity in colonizing assemblages compared to a smooth concrete featureless seawall lacking surface heterogeneity, commonly found in ports and marinas [47] (Figure 13). Based on the aforementioned study, pre-cast concrete is the fabrication method selected as the most suitable for the tafoni-inspired armoring block and ECOncrete® is the selected concrete mix that will be used for the full-scale block. For the purpose of this study, where the fabrication process was being investigated, regular concrete mix has been used.

Once the general form of the block is introduced abstracting the biological role model (Figure 15), its negative form that constitutes the formwork needs to be prepared. A big part of this project was the use of Polyvinyl Alcohol (PVA), a non-toxic, biodegradable synthetic polymer [60, 61]. Ethylene, which constitutes a hormone naturally emitted by plants and leads to fruit mellowing, can also be produced synthetically, and through a chemical reaction it turns into vinyl acetate. If the latter gets polymerized and dissolved in alcohol, it converts to a water-soluble polymer. At this point it is important to mention that PVA should not be confused with Polyvinyl Acetate, a highly toxic polymer that goes by the same acronym (PVA or PVAc).

The PVA formwork ultimately dissolves into the sea water over time, revealing the cavities made of pre-cast concrete. In order to fabricate the intricate tafoni forms, a large-scale 3D printer iteratively applies extruded PVA (Polyvinyl Alcohol) filament onto a planar surface. (Figure 16) The filament layers slowly transition into two separate halves of the sacrificial formwork. These halves combined, constitute the negative form of the armoring block. Once 3D printed, they are filled with concrete and joined together. This phase could potentially involve the addition of beneficial nutrients and minerals into the PVA, such as calcium, strontium, magnesium, carbonate and iodine. (Figure 17) As PVA dissolves over time, it will release these nutrients into the sea attracting and encouraging the growth of coralline algae. Further studies will need to be conducted in order to prove the possible benefits of this step.

 

 

 

The tafoni form is not revealed until it is set in place at the sea edge and the water slowly dissolves the PVA, releasing the beneficial nutrients and revealing habitat cavities. The formwork could also be removed before the installation. However, in this case the dissolution would constitute an additional step, which would make the process more time-consuming and expensive. Although there are other soluble materials on the market that could be 3D printed and form the armoring block formwork, the significant ad-vantage of using PVA is that it is scientifically proven to be non-toxic, thus harmless for the marine environment [60, 61]. There are several commercial PVA filaments currently on the market [62]. (Figure 21)

The tafoni pattern does not continue throughout the entire block however. One quarter of the 2x2x4-meter block remains solid at this early design phase with the intention to prevent the wave energy from eroding the city’s edge. This allows the tafoni structure to act as a scaffolding which supports the seawall while also potentially dissipates the wave energy. Further testing is required to ensure those qualities. Scouring on the toe, overtopping by wave, seeping or piping issues at the base of the structure often lead to damage or even failure of seawalls, revetments and, generally, various types of stone masonry or concrete coastal structures. Run-up and run-down waves on the wall constitute one of the main factors that can result in such damage, as the first will cause overtopping while the run down will cause scour in the toe if it was constructed with no adequate toe protection. Perforated structures can be considered multi-functional if they dissipate wave energy and reduce reflected wave height as well as wave run-up/run-down at the same time. After the concrete is cured, the 2x2x4 meter blocks are transferred to the site, and, using a crane, stacked in a running bond pattern and arranged along the sea edge to assemble the seawall. (Figure 18) The design of the block includes a groove which allows the blocks to fit together. The land edge is then back-filled with porous gravel and the paving material meets the wall to create a seating edge. (Figure 20)

 

Over time the sacrificial formwork starts dissolving revealing multiple cavities varying in shape and size. (Figure 19) Those crevices, cavities and pits could attract local marine species to create novel habitats and potentially enhance biodiversity. The final result is a seawall armoring block (dimensions 2x2x4 meters) that was designed with the use of software for parametric design (Grasshopper, Rhinoceros 3D) in order to apply the aforementioned abstracted bio-inspired principles (Figure 14).

Having defined the fabrication process and the materials that will be used for prototyping it is now time for this process to inform and refine the 3D modeling algorithm. Thus, 3 parameters were set and tested in order to assess the design of the fabricated prototype:

– Parameter 1: Tafoni 6-point rule

Since the geometry of this armoring block is inspired by the complexity of the tafoni rock formation, the first parameter to follow during 3D modeling was the six points rule: A swarm of points are generated inside the overall block volume. Each point connects to the six points closest to it to form a net-work of nodes and “branches” that will later structurally support the overall geometry. (Figure 14)

– Parameter 2: Size range of cavities

Once the basic algorithm is set up based on parameter 1, the size range of the cavities is the next factor to explore, which translates to the distance be-tween the aforementioned nodes.

Recent study regarding the efficacy of artificial new habitats on seawalls has indicated that the incorporation of various habitat typologies (pits and pools of various sizes and depths, crevices, overhangs and ledges) can result not only in attracting marine species [39] but also in a significant increase in species diversity [27]. Firth et al. [27] further explored prototype units including rock pools, pits and crevices which were placed both horizontally and vertically and had a variety of diameters (with a minimum of 15cm diameter and a maximum of 25cm diameter) attracting marine organisms including algae, barnacles, annelids, crabs, shrimps, ctenophores and gastropods. For this research and based on the aforementioned study, all the 3D modeling tests followed a cavity size range of 0 – 25 cm.

– Parameter 3: Density and connectivity of cavities

The density and interconnection of the cavities / voids also constitutes a significant parameter for the form generation process. As it is already mentioned the fabrication process requires for these cavities to form a tunnel-like geometry so that the casted concrete can reach all of the voids and form the final block.

More specifically, since the fabrication process is predicated on the logic of casting concrete into a formwork that is the negative volume of the actual block, the cavities of model A are so small in size and sparsely populated withing the block that it was not be possible to actually 3D print it. On the other hand, models B and C were structurally inadequate, as in the first case the PVA formwork collapsed during the 3D printing process, whereas model C failed during the casting process. (Figure 22)

Finally, model D was successful both in 3D printing the PVA formwork and after casting the concrete mix. Although model D (Figure 22) turned into a suc-cessful prototype in scales 1:64 (Figure 23) and 1:16 (Figure 25, 28), further studies are required to achieve structural optimization while scaling up to a 1:1 scale.

4. Prototyping and Results

The initial proof of concept was tested at a variety of scales:

3.1. Scale 1:64

The smallest scale physical model of this series served to test the relationship between two or more blocks, forming a cluster. (Figure 23) It was initially assumed that no mechanical bonding would be required to lock the blocks together, instead, the system could rely only on gravity. A cluster of nine blocks was tested to mock up the final state of the seawall when the PVA formwork is fully dissolved. The results satisfy the 3 parameters that were set during the design and resolve their reciprocal relationships to a degree that would not be achievable within a top-down design strategy.

3.2. Scale 1:16

For the 1:16 scale demonstrator of the seawall armoring block (Figure 26), it was critical to establish a design method which would follow the exact same process of the production of the full-scale sacrificial formwork. A two-step strategy was employed where a commercial single extruder (1.75mm nozzle) 3D printer printed the two pieces of the formwork using PVA filament. (Figure 24) In a subsequent step, concrete was poured into these two formwork pieces and allowed to cure for 12 hours in order for the viscosity of the concrete mix to increase (Figure 25). Then, the two pieces were aligned and joined together into the prototype. Industrial tape was used to hold the formworks together until concrete is fully cured. The final step was to place the 1:16 prototype into seawater that was collected to emulate the seawater PVA dissolution. (Figure 26) The PVA formwork of the 1:16 scale model started dissolving after 20 min into a container filled with sea water. Figures 26 and 27 show the condition of the PVA after 24 hours, while the PVA formwork was completely dissolved in 48 hours.

As one of the predominant design drivers, the constraints of a commercial 3D printer setup had to be analyzed and integrated into the design process. An important challenge of such a process is that the PVA formworks had to be printed using no support structure to allow for the concrete to fill the void space. Thus, a series of tests were performed to determine the optimized settings for the 3D printer using the slicing software (Slic3r). The challenge was to deposit the material layer by layer as fast as possible while maintaining an acceptable quality for the print without any support structure. Because of the intricate forms of the cavities being evident throughout the printed area, in all directions, printing orientation of the formwork pieces was not critical for the final outcome.

 

 

 

3.3. Scale 1:1

Finally, a 1:1 scale demonstrator was created to examine a small portion of the final product. (Figure 28) The goal here was to test the micro-texture of the block surface. Once the concrete mix is cured, and after the PVA is dissolved, it is apparent that the formwork drives the surface texture. In addition, air is trapped in the concrete mix during the curing process resulting in small bubbles/niches.

Based on existing research, on a micro-scale (<1 cm), the geological origin of building materials and hence their composition and surface roughness have a significant effect on the structure and functioning of the colonizing assemblages [44, 45]. Moreover, according to the same research, varying sizes of void space can have an effect on seawall community composition, as they provide shelter for different kinds of marine wildlife and lead to a more diverse habitat.

3.4. Available Technology in sizing up to 1:1 scale

Furthermore, different criteria have to be matched in order for the seawall armoring block to be mass produced. The initial purpose of 3D printing and additive manufacturing was predicated on the notion of producing with precision complex, small scale objects, which still constitutes its core competency. However, there is currently an unceasing effort within the 3D printing industry to “go large.” More specifically, the first criterion is availability and the printing volume: a 3D printer with a 1x1x4 meter minimum printing capacity is needed in order to print one of the two halves, at a time, completing the sacrificial formwork. A few printers are already available on the market, however, many of them are proprietary or currently purely designed and used for research purposes. Here, the biggest 3D printers in the world currently are lined up: (Table 1)

The additive manufacturing method that would be used during the 3D printing process constitutes the second criterion. There is a great variety of methods for 3D printing, but the process that is most widely known is called Fused Deposition Modeling (FDM). During this process, a physical object is designed using a design computer program (CAD) and then fabricated directly from that program using layer-by-layer deposition of a filament material that is extruded through the printer’s nozzle. Bearing in mind that the selected material is PVA, Fused Deposition Modeling is the ideal printing method for the creation of the sacrificial formwork of the seawall armoring block [64].

3D printing is currently seen as a potential response to limitations in the supply chain and manufacturing operations, since nowadays the demand for complex yet sustainable production is ceaselessly increasing. Thus, we can only expect the technology to react to this demand and grow even more, allowing for larger, faster and cheaper 3D printers that would easily satisfy all the aforementioned criteria and facilitate the pro-duction of the tafoni–inspired armoring block.

5. Discussion
The advantages of using casting materials like ECOncrete for the production of armoring blocks, in terms of structural efficiency and ecological performance have been thoroughly discussed in precedent research projects and are well-established [47]. Similarly, the advantages of varying scales of texture and geometry have already been proven regarding the attraction and diversity of marine life [26, 27, 37, 38]. By investigating geometric, and computational logics, this research-based prototypical fabrication approach aims to introduce the design of one armoring block with an intention to respond to the need for structural stability as well as the need for enhancing the marine ecology. Multiple scaled prototypes were developed to demonstrate that it is possible to create an armoring block of great morphological complexity at varying scales by using a sacrificial formwork of 3D printed, water soluble PVA material. Through careful geometric control, the materials used and the fabrication system employed, are suitable to be used for large-scale applications.

The scale of the prototypes is only an indicator of the achievable size, as the commercial single extruder 3D printer that was used for this research had a bed size of 280mm x 280mm x 250mm and a nozzle of 1.75mm. While the spatial limitations of the prototypes where informed by the constraints of the 3D printer and the lab, the simulation of the whole fabrication process proved to be successful; from printing two halves of the PVA formwork to the assembly of the casts into a single block and the dissolution of the PVA into water that revealed the final tafoni-inspired geometry. The research proved that the size and the characteristics of the specific 3D printer used play a vital role in the design and fabrication process. Thus, in case a larger 3D printer is to be used for the production of a full-size armoring block, a new series of tests would need to be conducted based on that specific printer in order to determine the most suitable speed and temperature set-tings for the successful printing of the PVA formwork on a larger scale.

Additionally, through this study, current economic and design obstacles to get to the final product were also revealed, especially since large scale 3D printers are still very ex-pensive and not yet extensively used in the construction industry. However, the capability to 3D print at full-scale is gaining momentum and is certain to occur. The presented fabrication system was designed with a strong focus on potential scalability, where the digital and the physical infrastructures are theoretically extendible/available. With a variety of 3D printing machinery becoming more relevant and present in the construction industry, the localization and communication of these devices becomes more important when aiming for the more automated fabrication of larger construction systems. The authors foresee the possibility of a tailor-made large-scale 3D printer being assembled for such purposes.

Placing the prototypes into sea water for a certain period of time in order to test how marine species will inhabit the various cavities and microtextured surfaces was not part of this research. In addition, further research is required to test and study the wave energy dissipation efficacy of the current geometry, as well as to structurally test and optimize the block. Therefore, the authors see great opportunities for further refinement in a collaboration with experts from other areas of expertise as a next step, such as marine ecologists, biologists, geologists, concrete experts, engineers, and designers, with the aim to bring this tafoni–inspired armoring block to life and develop it further.

 

References

1. Hammond A. 1992. “World Resources 1992–1993: Towards Sustainable Development.” Oxford University Press, Oxford.
2. Gray J.S. 1997. “Marine biodiversity: patterns, threats and conservation needs.” Biodivers. Conserv. 6, 153–175.
3. Yapp G.A. 1986. “Aspects of population, recreation, and management of the Australian coastal zone.” Coastal Zone Manage. J. 14, 47–66.
4. Chapman M.G., and Underwood A.J., 2008. “Scales of variation of gastropod densities over multiple spatial scales: comparison of common and rare species.” Mar. Ecol. Prog. Ser. 354, 147–160.
5. Chapman M.G., and Underwood, A.J. 2011. “Evaluation of ecological engineering of ‘‘armoured’’ shorelines to improve their value as habitat.” J. Exp. Mar. Biol. Ecol. 400, 302–313.
6. Bray M.J., and Hooke J.M. 1997. “Prediction of soft-cliff retreat with accelerating sea level rise.” Journal of Coastal Research 13, 453–467.
7. National Institute of Coastal and Marine Management of the Netherlands, 2004. “A guide to coastal erosion management practices in Europe.” European Commission Contract nr. B4-3301/2001/ 329175/MAR/B3. Directorate General Environment European Commission, [online] URL: http://www.eurosion.org/shoreline/lessons_learned.pdf
8. Airoldi L., Abbiati M., Beck M.W., Hawkins S.J., Jonsson P.R., Martin D., Moschella P.S., Sundelöf A., Thompson R.C., Åberg P. 2005. “An ecological perspective on the deployment and design of low crested and other coastal defences.” Coast. Eng. 52, 1073–1087.
9. Airoldi, L., and Bulleri, F. 2011. “Anthropogenic disturbance can determine the magnitude of opportunistic species responses on marine urban infrastructures.” PLoS ONE 6, e22985.
10. Bulleri, F., and Chapman, M.G. 2010. “The introduction of coastal infrastructure as a driver of change in marine environments.” J. Appl. Ecol. 47, 26-35.
11. Chapman, M.G. 2006. “Intertidal seawalls as habitats for molluscs.” J. Mollus. Stud. 72, 247–257.
12. Chapman M.G., and Underwood A.J. 2009. “Comparative effects of urbanization in marine and terrestrial habitats.” Ecology of cities and towns: A comparative approach, pp. 51–70.
13. Glasby T.M., and Connell S.D. 1999. “Urban structures as marine habitats.” Ambio, 28, 595–598.
14. Firth, L.B., and Hawkins, S.J., 2011. “Introductory comments e global change in marine ecosystems: patterns, processes and interactions with regional and local scale impacts.” J. Exp. Mar. Biol. Ecol. 400, 1-6.
15. Firth L.B., Thompson R.C., White F.J., Schofield M., Skov M.W., Hoggart S.P.G., Jackson J., Knights A.M., Hawkins S.J. 2013a. “The importance of water-retaining features for biodiversity on artificial intertidal coastal defense structures.” Divers. Distrib. 19, 1275–1283.
16. Moschella P.S., Abbiati M., Åberg P., Airoldi L., Anderson J.M., Bacchiocchi F., Bulleri F., Dinesen G.E., Frost, F., Gaciag E., Granhag L., Jonsson P.R., Satta M.P., Sundelöf A., Thompson R.C., and Hawkins S.J., 2005. “Low-crested coastal defence structures as artificial habitats for marine life: Using ecological criteria in design.” Coast. Eng. 52, 1053–1071.
17. Walker H.J. 1988. “Artificial Structures and Shorelines.” Kluwer Academic Publishers, Los Angeles.
18. Connell S.D., and Glasby T.M. 1999. “Do urban structures influence local abundance and diversity of subtidal epibiota? A case study from Sydney Harbour, Australia.” Mar. Environ. Res. 47, 373–387.
19. Chapman M.G., and Bulleri F. 2003. “Intertidal seawalls—new features of landscape in intertidal environments.” Landscape Urban Plan. 62, 159–172.
20. Sanger D.M., Holland A.F., and Gainey C. 2004. “Cumulative impacts of dock shading on Spartina alterniflora in South Carolina estuaries.” Environmental Management, 33, 741–748.
21. Osman R.W. 1977. “The establishment and development of a marine epifaunal community.” Ecol. Monogr. 47, 37–63.
22. Sutherland J.P., and Karlson R.H. 1977. “Development and stability of the fouling community at Beaufort, North Carolina.” Ecol. Monogr. 47, 425–446.
23. Anderson M.J., and Underwood A.J. 1994. “Effects of Substratum on the Recruitment and Development of an Intertidal Estua-rine Fouling Assemblage.” Journal of Experimental Marine Biology and Ecology 184 (2): 217–36. https://doi.org/10.1016/0022-0981(94)90006-X
24. Chapman, M.G. 2003. “Paucity of mobile species on constructed seawalls: effects of urbanization on biodiversity.” Mar. Ecol. Prog. Ser. 264, 21–29.
25. Vaselli S., Bulleri F., and Benedetti-Cecchi L. 2008. “Hard Coastal-Defence Structures as Habitats for Native and Exotic Rocky-Bottom Species.” Marine Environmental Research 66 (4): 395–403. https://doi.org/10.1016/j.marenvres.2008.06.002
26. Browne MA, and Chapman MG. 2014. “Mitigating against the Loss of Species by Adding Artificial Intertidal Pools to Existing Seawalls.” Marine Ecology Progress Series 497: 119–29.
27. Firth L.B., Thompson R.C., Abbiati M.A., Airoldi L., Bouma T.J., Bozzeda F., Ceccherelli V.U., Colangelo M.A., Evans A., and Ferrario F. 2014. “Between a rock and a hard place: environmental and engineering considerations when designing coastal defence structures.” Coastal Engineering 87: 122–135.
28. Chapman M. G., Blockley D., People J., and Clynick, B. 2009. “Effect of Urban Structures on Diversity of Marine Species.” In Ecology of Cities and Towns, edited by McDonnell M. J., Hahs A. K., and Breuste J. H., 156–76. Cambridge: Cambridge University Press.
29. Griggs G.B. 2009. “The Effects of Armoring Shorelines – the California Experience.” Puget Sounds Shorelines and the Impacts of Armoring – Proceedings of a State of the Science Workshop. N.p. 77-84.
30. McDonnell M., Hahs A., and Breuste J. (Eds.). 2009. “Ecology of Cities and Towns: A Comparative Approach.” Cambridge: Cambridge University Press. https://doi.org/10.1017/CBO9780511609763
31. Bozek C.M., and Burdick D.M. 2005. “Impacts of Seawalls on Saltmarsh Plant Communities in the Great Bay Estuary, New Hampshire USA.” Wetlands Ecology and Management 13(5):553–568.
32. Pister B. 2009. “Urban marine ecology in southern California: the ability of riprap structures to serve as rocky intertidal habitat.” Mar. Biol. 56, 861–873.
33. Dong Y.W., Huang X.W., Wang W., Li Y., and Wang J. 2016. “The Marine ‘great Wall’ of China: Local- and Broad-Scale Ecological Impacts of Coastal Infrastructure on Intertidal Macrobenthic Communities.” Edited by David Schoeman. Diversity and Distributions 22 (7): 731–44. https://doi.org/10.1111/ddi.12443
34. Bulleri F., Chapman M. G., and Underwood A. J. 2005. “Intertidal Assemblages on Seawalls and Vertical Rocky Shores in Sydney Harbour, Australia.” Austral Ecology 30 (6): 655–67. https://doi.org/10.1111/j.1442-9993.2005.01507.x
35. Moreira J., Chapman M.G. and Underwood A.J. 2006. “Seawalls do not sustain viable populations of limpets.” Marine Ecology Progress Series, 322, 179–188.
36. Pomerat C.M., and Weiss C.M. 1946. “The Influence of Texture and Composition of Surface on the Attachment of Sedentary Marine Organisms.” The Biological Bulletin 91 (1): 57–65. https://doi.org/10.2307/1538033
37. Chapman M.G., and Blockley D.J. 2009. “Engineering novel habitats on urban infrastructure to increase intertidal biodiversity.” Oecologia 161, 625–635.
38. Firth L.B., Mieszkowska N., Thompson R.C., and Hawkins S.J. 2013b. “Climate Change and Adaptational Impacts in Coastal Systems: The Case of Sea Defences.” Environmental Science: Processes & Impacts 15 (9): 1665. https://doi.org/10.1039/c3em00313b
39. Martins G.M., Thompson R.C., Neto A.I., Hawkins S.J., Jenkins S.R. 2010. “Enhancing stocks of the exploited limpet Patella candei via modifications in coastal engineering.” Biol Conserv 143: 203–211
40. Kozlovsky R., and Grobman Y.J. 2017. “The Blue Garden: Coastal Infrastructure as Ecologically Enhanced Wave-Scapes.” Land-scape Research 42 (5): 439–54. https://doi.org/10.1080/01426397.2016.1260702
41. Loke, L.H.L., Ladle R.J, Bouma T.J., and Todd P.A. 2015. “Creating Complex Habitats for Restoration and Reconciliation.” Eco-logical Engineering 77 (April): 307–13. https://doi.org/10.1016/j.ecoleng.2015.01.037
42. Miloslav N., Jirát J., Košata B., Jenkins A., and McNaught A., eds. 2009. “IUPAC Compendium of Chemical Terminology: Gold Book.” 2.1.0. Research Triagle Park, NC: IUPAC. http://goldbook.iupac.org
43. Larkin D., Vivian-Smith G., and Zedler J.B., 2006. “Topographic Heterogeneity Theory and Ecological Restoration.” Foundations of restoration ecology: 142-164
44. Loke L.H.L. and Todd P.A. 2016. “Structural Complexity and Component Type Increase Intertidal Biodiversity Independently of Area.” Ecology 97 (2): 383–93. https://doi.org/10.1890/15-0257.1
45. Loke L.H.L., Bouma T.J., and Todd P.A. 2017. “The Effects of Manipulating Microhabitat Size and Variability on Tropical Sea-wall Biodiversity: Field and Flume Experiments.” Journal of Experimental Marine Biology and Ecology 492 (July): 113–20. https://doi.org/10.1016/j.jembe.2017.01.024
46. Loke L.H.L. and Jachowski N. R. 2014. “Complexity for Artificial Substrates CASU): Software for Creating and Visualizing Habitat Complexity” PloS one 9.2 (2014): e87990
47. Ido S., and Perkol-Finkel S. 2015. “Blue Is the New Green – Ecological Enhancement of Concrete Based Coastal and Marine Infrastructure.” Ecological Engineering 84 (November): 260–72. https://doi.org/10.1016/j.ecoleng.2015.09.016
48. Subcommittees, Lukens R.R. and Selberg C. 2004. “Guidelines for Marine Artificial Reef Materials”. 2nd edn
49. Moreira J., Chapman M.G., and Underwood A.J., 2007. “Maintenance of chitons on seawalls using crevices on sandstone blocks as habitat in Sydney Harbour, Australia.” J. Exp. Mar. Biol. Ecol. 347, 134–143.
50. Langhamer O., and Wilhelmsson D. 2009. “Colonization of fish and crabs of wave energy foundations and the effects of manufactured holes – a field experiment.” Mar Environ Res 68: 151–157
51. Cordell J.R., Toft J.D., Munsch S.H. and Godd M. 2017. “Benches, beaches, and Bumps: How Habitat Monitoring and Experimental Science can Inform Urban Design.” In Living Shorelines: The Science and Management of Nature-based Coastal Protection, D.M. Bilikovic et. al (eds). Boca Raton, Florida: CRC Press, 421-438
52. Jarlan, G.E. “Seawall Structure” US Patent 3,387,458 filed March 10, 1965, and issued June 11, 1968. https://pa-tents.google.com/patent/US3387458A/en
53. Verble P.R. “Seawall Construction” US Patent 5,125,765 filed August 28, 1991, and issued June 30, 1992. https://pa-tents.google.com/patent/US5125765A/en?oq=us+patent+1992+5%2c125%2c765+seawall
54. Groom, Kaelin M., Casey D. Allen, Lisa Mol, Thomas R. Paradise, and Kevin Hall. “Defining Tafoni: Re-Examining Terminological Ambiguity for Cavernous Rock Decay Phenomena.” Progress in Physical Geography: Earth and Environment 39, no. 6 (December 2015): 775–93. https://doi.org/10.1177/0309133315605037
55. Blackwelder, E. 1929. “Cavernous rock surfaces of the desert.” American Journal of Science, 17, 393-399.
56. Hejl E. 2005. “A Pictorial Study of Tafoni Development from the 2nd Millennium BC.” Geomorphology, v. 64: 87-95.
57. Smith P. J. 1982. “Why Honeycomb Weathering?” Nature, v. 298: 121-122.
58. Doe N.A. 2004. “What Makes Holes in Sandstone” SHALE 9 (2): 12–40.
59. Doe N.A. 2011. “The Geometry of Honeycomb Weathering of Sandstone” SHALE 26 (1): 31–60.
60. DeMerlis C.C, and Schoneker D.R. 2003. “Review of the Oral Toxicity of Polyvinyl Alcohol (PVA).” Food and Chemical Toxicology 41 (3): 319–26. https://doi.org/10.1016/S0278-6915(02)00258-2
61. Chiellini E., Corti A., D’Antone S., and Solaro R. 2002. “Biodegradation of Poly (Vinyl Alcohol) Based Materials.” Progress in Polymer Science 28 (6): 963–1014. https://doi.org/10.1016/S0079-6700(02)00149-1
62. Gaaz, T.S., Sulong, A.B., Akhtar, M.N., Kadhum, A.A.H., Mohamad, A.B., Al-Amiery, A.A. 2015. “Properties and Applications of Polyvinyl Alcohol, Halloysite Nanotubes and Their Nanocomposites.” Molecules 20 (12): 22833–47. https://doi.org/10.3390/molecules201219884
63. All3DP. n.d. “The World’s Biggest & Most Expensive 3D Printers in 2018” Accessed August 5, 2022. https://all3dp.com/1/big-gest-large-3d-printer-world-most-expensive/#d-shape
64. Mohamed, O.A., Masood, S.H. & Bhowmik, J.L. 2015. “Optimization of Fused Deposition Modeling Process Parameters: A Review of Current Research and Future Prospects.” Advances in Manufacturing 3 (1): 42–53. https://doi.org/10.1007/s40436-
    014- 0097-7
65. Chai W. L, Jing-Dong C., Chien-Chung C., Fu-Sheng C., and Wei-Chuan. 2009. “Evaluation of the Biodegradability of Polyvinyl Alcohol/Starch Blends: A Methodological Comparison of Environmentally Friendly Materials.” Journal of Polymers and the Environment 17 (2): 71–82. https://doi.org/10.1007/s10924-009-0123-1