Fabric formed concrete mattresses are a form of in-situ concrete construction using engineered fabric formwork systems. These systems are employed extensively in marine and hydraulic engineering due to their ability to achieve controlled, uniform concrete sections underwater while preventing mix washout and segregation. Fabric formed systems offer adaptability to complex bed geometries, while the permeability of the formwork enhances concrete strength by promoting bleed water escape during placement.
This article outlines the principal variants of fabric formed concrete mattresses, describes their construction characteristics, and discusses their technical suitability and limitations in various applications.
| Mattress Type | Permeability | Installed Cost Range | Scour Force Resistance | Installation Ease | Filling Speed | Durability | Primary Applications |
|---|---|---|---|---|---|---|---|
| Constant Thickness | Low | Medium | High | Moderate | Fast | Very High | Quay walls, berth scour, bridge abutments |
| Filter Mattress | Moderate (via filter points) | Medium | Moderate | High | Moderate | High | Revetments, riverbank protection |
| Flexible Mattress | Low | Medium - High | Moderate - High | Moderate | Moderate | High | Settling slopes, piled jetty scour zones |
| Bulk Mattress | Low | High | Very High | Low | Slow | Very High | Breakwater toes, high energy flow channels |
| Open Hole Mattress | Moderate (engineered porosity) | Medium - High | High | Moderate | Moderate | High | Revetment wave zones, under jetties, sloped beds |
| Natural Fibre Mattress | Moderate - High | Low | Low - Moderate | High | Moderate | Medium - Low | Riverbank protection (low energy), developing regions |
1. Constant Thickness Mattress
Constant thickness mattresses are formed using a twin-layer fabric envelope with internal spacer ties designed to maintain a specified thickness during pump filling. These internal ties are engineered to resist movement during the filling process, ensuring dimensional stability and limiting surface undulation. The formwork is typically fabricated in panels up to 5 m wide, with thicknesses ranging from 100 mm to over 600 mm depending on loading and uplift resistance requirements.
Design of constant thickness mattresses focuses on deadweight resistance to hydraulic uplift (from propeller wash, suction forces, or tidal flows), and overall sliding stability. These systems are most commonly used at quay walls, under berths, and along bridge abutments, where a uniformly thick, dense concrete apron is required. Panel joints are connected using zipped interfaces, forming a shear interlock that provides continuity between units.
From a structural point of view, the design approach is based on computing the uplift pressure and selecting a mattress thickness and concrete density to counteract it, factoring in a safety margin. The low permeability of the formwork promotes high-strength surface development through water bleed, contributing to long-term abrasion resistance. If used on revetments porosity is required to prevent uplift and cracking over time.
The open hole mattress is a permeable variant of the constant thickness system, designed specifically for use in wave zones where rapid water drainage during wave run-down is essential. It features engineered holes through the fabric formwork, which remain as open voids in the cured concrete. These holes allow water to exit through the mattress body, reducing uplift pressure and enhancing slope stability during cyclical loading.
Open hole mattresses are often used on harbour slopes, under jetties, and in areas subject to long-period swell waves. Their porosity must be balanced to provide adequate drainage without compromising the structural cohesion of the concrete panel. Where slope settlement is expected, an open hole flexible mattress variant can be applied.
2. Filter Mattress
Filter mattresses are structurally different constant thickness mattresses with porous filter points formed by connecting the upper and lower fabric layers at intervals. These points allow the dissipation of pore water pressures from beneath the mattress, functioning similarly to a filter geotextile. This permeability makes them appropriate for applications where groundwater relief is necessary, such as on revetment slopes or in tidal embankments.
Filter point mattress is more advantageous for low wave heights Hs < 1.0–1.5 m where Open Hole Mattress is used for significant wave heights up to 2–3 m, depending on slope angle, mattress thickness, and porosity. Filter mattresses are not suitable for areas with high-energy jetting or suction forces (e.g., direct propeller wash), where the porosity may permit fine loss or reduce resistance to localised uplift.
The primary design consideration for filter mattresses is ensuring compatibility with underlying soils and filter layers. They are typically laid on top of a geotextile or granular filter to prevent soil migration into the filter points. Design parameters include the spacing and size of the filter points, the permeability contrast between mattress and subgrade, and the structural stability of the mattress under flow and wave loading.
Filter Point Mattress will have a larger shrinkage factor when compared to other mattress types due to the undulations, often making it challenging to installed between fixed bays, but this is not a factor when on an open revetment.
Generally all types of fabric formed concrete mattress relay upon the cast concrete for the performance, FP Matt has a need for the filters to remaining in place for performance, these degrade overtime so a geotextile should be place underneath to ensure material cannot escape through the filters over time.
3. Flexible Mattress
Flexible mattresses are developed for conditions involving anticipated differential settlement. Their construction is similar to that of constant thickness mattresses but incorporates engineered zones of reduced thickness. These zones act as pre-defined crack lines, allowing the hardened concrete to articulate under settlement while maintaining inter-panel shear resistance through residual concrete ties and underlying fabric continuity.
Design of flexible mattresses includes estimating the range and distribution of expected settlement and determining the required mattress articulation. The concrete thickness in flex zones is reduced to allow controlled cracking, with residual capacity retained via fabric and mechanical interlock. When applied in wave zones, these mattresses can incorporate porosity holes for drainage, combining flexibility with uplift resistance becoming a variant of an open-hole mattress.
Proserve design FLEX Mattress without filters to eliminate the need for a geotextile and just relay on the concrete crack lines. FLEX Mattress is design to manage slope settlement of up to 130mm over a 3m section. The purpose of the mattress is not to fall into scour holes, rather Hinge Edges are required to prevent underscoring at the edge of protection.
4. Bulk Mattress
Bulk mattresses are characterised by significantly greater thickness (typically ≥ 700 mm) and are designed for mass. These systems are used where very high resistance to hydraulic forces is required — such as breakwater toes, fast-flowing channels, or high wave energy zones. The primary function is to provide a continuous, heavy concrete apron that resists uplift, suction, and lateral displacement forces.
From a design standpoint, the bulk mattress is dimensioned based on uplift pressures, wave rundown forces, or slope stability considerations. The increased filling volume and formwork size require careful planning of pump logistics and form stability under hydrostatic loading.
Due to the higher concrete volume and slower installation rates, bulk mattresses are typically reserved for projects where structural demand outweighs logistical constraints. Their performance in wave action is enhanced when combined with embedded toe details or falling edge aprons to manage underscour.
5. Natural Fibre Mattress
Natural fibre mattresses use biodegradable fabrics such as jute instead of synthetic woven formwork. These systems are suited to regions where access to geosynthetics is limited or where environmental degradation of formwork is desired. Once the concrete cures, the fibre degrades, leaving behind a concrete slab with minimal residual material.
Design considerations include the timing of fabric degradation relative to concrete strength development, the impact of ambient temperature and moisture on the fabric during placement, and the initial structural stability of the form during filling. These systems are typically used in low-energy environments such as riverbanks or protected floodplain areas, and are often combined with vegetation-based solutions.
Given the reduced long-term durability of the formwork, these systems are not suitable for high flow velocities or zones subjected to direct mechanical impact.
Conclusion
Fabric formed concrete mattresses provide a versatile and technically rigorous solution for underwater and slope concrete construction. Their suitability depends on a clear understanding of the hydraulic and geotechnical conditions at the site, along with the interaction between the formwork, concrete mix, and subgrade.
Selection among mattress types should be driven by the dominant design requirement — whether it is uplift resistance, drainage, settlement tolerance, or local material availability. While constant thickness and bulk mattresses provide structural robustness, filter and flexible types offer performance advantages in specific hydraulic or settlement scenarios. Natural fibre systems offer a viable alternative for low-cost, low-impact applications, particularly in developing regions.
Each type requires a tailored approach to design, including appropriate formwork selection, concrete mix development, and compatibility with filter and subgrade systems.
