Quick Answer
Filter point grouted mattress is a variant of standard GGFM with drainage openings woven into the upper fabric layer. These filter points allow porewater to escape through the mattress, preventing hydrostatic uplift pressure from building behind the grout matrix. The filter point variant is specified wherever the water table fluctuates relative to the mattress surface — tidal coastal revetment, flood retention embankments, artesian ground conditions, and submerged river bank protection. It meets GRI GT16 requirements and provides identical structural performance to standard grouted mattress, with the addition of passive drainage relief.
What Is Filter Point Grouted Mattress and How Does It Prevent Uplift?
The filter point grouted mattress is identical to the standard variant, except that the upper fabric layer includes a regular array of small-diameter weave openings (filter points). These openings are sized to allow porewater to escape through the mattress while blocking sediment migration.
In tidal and submerged applications, the water table fluctuates with tide level. A solid-face mattress traps the water behind the grout matrix, building hydrostatic pressure that can lift the entire mattress. Filter points equalize pressure automatically — no drainage layer, no additional filter blanket required.
- Tidal revetments where water table fluctuates — 0.5 m
- Artesian groundwater conditions (positive porewater pressure)
- Submerged river bank slopes in flood/draw-down conditions
- Reservoir upstream slopes subject to rapid drawdown
Coastal & Marine Installations
Filter Point Specifications
| Parameter | Standard Grade | Heavy Grade |
|---|---|---|
| Thickness | 75–200 mm | 150–200 mm |
| Filter Point Opening Size | 2–5 mm | 2–5 mm |
| Filter Point Spacing | 300 - 300 mm grid | 300 - 300 mm grid |
| Hydraulic Conductivity | 0.01 cm/s | 0.01 cm/s |
| Compressive Strength | ≥17 MPa | 20 MPa |
| Design Wave Height (Hs) | Up to 1.5 m | Up to 3.0 m |
| Design Velocity | 3.5 m/s | 6.0 m/s |
| Standard | GRI GT16 | GRI GT16 |
Where Filter Point Is Specified
Coastal Revetments
Tidal shore protection where wave run-up and draw-down create rapid pressure changes behind the facing.
Seawalls & Breakwaters
Front-face and apron protection where wave impact combines with saturated subgrade drainage demands.
River Bank Slopes
Flood levee slopes subject to rapid draw-down after high water, when hydraulic gradient reverses.
Estuarial Banks
Tidal estuary banks where salinity and tidal flux create challenging porewater conditions.
Reservoir Upstream Slopes
Rapid drawdown conditions in reservoir operation create negative pore pressures — filter points equalize.
Submerged Canal Banks
Below-water canal sides in regions with significant groundwater head differentials.
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Filter Point Mattress Questions
What is the difference between filter point and standard grouted mattress?
The filter point variant incorporates woven drainage openings at regular intervals through the mattress face. These allow two-way water movement — preventing hydrostatic pressure from building up beneath the mattress — while the surrounding grouted area maintains full erosion resistance. The standard mattress face is impermeable.
When should I specify filter point over standard?
Specify filter point when your site has: tidal fluctuation (water level rises and falls across the mattress), artesian pore pressure in the subgrade, submerged or fluctuating groundwater conditions, or any situation where hydrostatic uplift could destabilise an impermeable face.
Does the filter point mattress provide the same structural protection?
Yes. The erosion-resistant concrete matrix between the filter points provides the same hydraulic performance as the standard mattress. The filter openings occupy a small proportion of the total face area and do not reduce the overall armour capacity for the design velocity range.
What is the size and spacing of the filter openings?
Filter point openings are approximately 50 mm - 50 mm at 300 mm centres (standard grid). Custom spacing can be specified for high-artesian-pressure conditions. Contact our engineering team with your groundwater data for a tailored recommendation.
Can filter point mattress be installed underwater?
Yes — this is its most common application. The same pump-and-grout installation method applies. The filter openings allow expelled air and water to escape during grouting, which actually simplifies underwater installation compared to the impermeable standard variant.
Other Grouted Mattress Products
Standard Grouted Mattress
The universal erosion protection system for canals, slopes, and river banks.
Vegetated Grouted Mattress
Open-cell variant for ecological slope protection where vegetation establishment is required.
Technical Specifications
Full product datasheets, GRI GT16 test reports, and BOQ templates.