“Raschel” describes how the mesh is made, and it is the single construction decision that most affects how a rock bag behaves when something damages it.
Woven versus knitted
Woven fabric has two sets of yarns crossing at right angles, one passing over and under the other. Simple, strong in the yarn directions, and dimensionally stable.
Its weakness is what happens at a break. Cut a yarn in a woven structure and the yarns around it are held only by friction at the crossings. Under load, that opening can enlarge as adjacent yarns slip: the break travels, and a small cut becomes a large hole.
Warp knitting, of which raschel is a type, builds the fabric from yarns that interloop. Each yarn passes through loops formed by its neighbours. The structure is mechanically interlocked rather than held by friction.
Cut one yarn in an interlooped structure and the loops around it are still engaged with each other. The damage is contained near where it happened.
Why that matters here
Consider what a rock bag actually experiences:
- Filled with angular rock, which bears on the mesh from the inside at points
- Lifted, so the whole mass is carried by the fabric
- Placed onto an irregular bed, often dropped the last part of the way
- Sitting against other units and against rock for decades
- Struck by debris, ice, and occasionally a vessel
- Abraded by bedload moving past
Damage is not a possibility, it is a certainty over a long service life. The question is not whether the mesh gets cut but what happens when it does.
If damage propagates, one cut eventually empties a unit and the armor at that location is gone. If damage stays local, the unit loses a little capability and the installation carries on, and the repair is replacing individual units rather than a section.
This is the same distinction that separates rock bags from sand-filled geotextile tubes, where a breach releases fill that flows. See rock bags vs geotextile tubes.
The other properties the weave gives
Permeability. An open knitted mesh lets water through freely. That matters for two reasons: hydrostatic pressure does not build behind a unit, and a permeable armor face behaves differently from an impermeable wall for drainage, habitat, and energy absorption. See environment and habitat.
Conformability. A knitted structure has more give than a woven one, so a filled unit deforms against the bed and against its neighbours rather than holding a rigid shape. That is the conforming behaviour that lets bagged armor follow a bed that lowers, discussed in why scour protection fails.
Aperture control. The knit sets the opening size, which determines the minimum fill stone that will stay in. The fill has to be comfortably larger than the aperture. See when local stone changes the economics.
The method behind the claim
The property is tear strength, measured by a tongue tear procedure under ASTM and ISO methods. It is one of three mechanical methods in the programme, alongside wide-width tensile and static puncture (CBR).
The programme, the methods, and the laboratories that run them are set out on test data and standards. Consistent with the policy on that page, we publish which method underwrites which claim and not the measured result. Reports are issued per project on request.
So the accurate statement of what raschel construction buys is: resistance to tear propagation from a cut strand, evidenced by tongue tear testing. Not a percentage, not a comparison figure.
What the weave does not fix
Being complete about the limits:
It is still a fabric. Abrasion works on it, and ice abrasion in particular is a mechanism we hold no citable data for — see ice damage to bank protection.
It is still UV-sensitive. Polyester degrades under sunlight, which is why the on-land service life position depends on the unit being covered within a month. See EN 12224 and weathering.
It does not make the unit indestructible. A sufficiently severe cut, a sustained abrasion regime, or a large enough impact will damage a unit. The design assumption is that individual units may need replacing over a long life, which is why maintenance is inspection plus unit replacement rather than a repair of a continuous structure.