In Geofabrics v Fiberweb, the English Court of Appeal has provided a useful reminder on the appropriate approach to purposive construction of patents.


In March 2020, Deputy High Court Judge David Stone found Geofabrics’ patent, EP (UK) 2 430 238, to be valid and infringed by Fiberweb. The patent was directed to the problem of pumping erosion. This occurs where a train passes over a track which has been laid over clay subgrade. A short-lived heavy load is transmitted downwards via the ballast to the subgrade causing the pore water to be squeezed out of the clay. As the pore water is forced out of the subgrade, it carries with it fine particles of clay and silt, causing erosion to the trackbed, and settling of the track. The patent taught the use of a synthetic trackbed liner made of a "filtration layer" which restricts the passage of solids material through the liner sandwiched between two "support layers" as a solution to the problem. Fiberweb’s Hydrotex 2.0 product was found to infringe the patent.


Permission to appeal the decision was granted by Lord Justice Floyd on two grounds, one being that the judge had erred in his construction of claim 1 of the patent.


Claim 1 is worth considering in full. It read:

"1. A trackbed liner comprising:

- an upper support layer;

- a lower support layer; and

- at least one filtration layer of a material having a plurality of pores

- and which is normally impermeable to liquid water, that is in the absence of the load of a vehicle acting on the trackbed,

- the filtration layer located between the upper and lower support layers;

- in which the pores of the filtration layer are dimensioned so that, in use and under load of a vehicle acting on the trackbed, the filtration layer permits passage of liquid water upwardly therethrough but restricts the passage of solids materials, so as to restrict pumping erosion of material located beneath the liner."

The issue which fell to be determined was how “normally impermeable” should be construed.


Typically, a patent specification is its own dictionary, defining terms subsequently used in the claims. In this instance, the patent explained that "normally impermeable to liquid water … should be taken to mean that the material is impermeable to liquid water in the absence of the load of a vehicle acting on the trackbed". However, it was common ground that the skilled addressee would not understand the term to mean impermeability in the absence of a vehicle “at all times and in all circumstances”.

This begged the question what did it mean? Fiberweb argued that "normally impermeable" meant "impermeable under a water pressure equivalent to that of the load of track, sleepers and ballast". This was described in an embodiment of the patent as being typically about 2.9kN/m2. This was relevant to the question of infringement because experiments showed that the water entry pressure of Hydrotex 2.0 was somewhere between 0.5kN/m2 and 2.7kN/m2. This meant that water would not pass through Hydrotex 2.0 at a pressure corresponding to a 50mm head of water, but would pass through Hydrotex 2.0 at a pressure corresponding to a 280mm head of water.

The trial judge construed the patent as follows concluding that there was no infringement: “Considering the [p]atent through the eyes of the skilled addressee, and applying a purposive construction, in my judgment 'normally impermeable to liquid water, that is in the absence of the load of a vehicle' means what it says – in 'normal' conditions, the filtration layer will not allow water to pass through it. At [0011], the [p]atent sets out the reason for this: so that rainwater, and water which has passed up through the filtration layer, will run off to the sides rather than passing (back) down through the filtration layer into the subgrade. The skilled addressee will appreciate that the product is to be used on a railway track. Those 'normal' conditions are therefore when in use as a geotextile under ballast and above subgrade as part of a railway track. The 'normal' conditions include the weight of the ballast, sleepers and rails (but not a train). They also include the presence of rainwater (including any water that has been permitted to pass upwardly through the filtration layer), but not a flood … the experts agreed that ... a stand of water of greater than 50mm would not be normal, because stands of water would only arise in unusual (ie, not 'normal') flood conditions.”


Lord Justice Arnold, giving the lead judgment of the Court of Appeal, endorsed the trial judge’s findings (although he noted that more weight was placed on the word “normally” than he considered warranted). In doing so, he held, among other things:

  • It was rarely appropriate to read a limitation into a claim from a comment made in relation to a specific embodiment (in this instance, 2.9kN/m2).
  • The static load due to the presence of the track, sleepers and ballast is a load applied during the construction of the track which generates an equal and opposite reaction exerted by the subgrade. Thereafter the system is in equilibrium. Although the result is compression of the liner, this would have no bearing on the hydrostatic pressure of any water present;
  • “The ballast cannot contain water during and after rainfall because of its open structure, and so the water will run off. Thus (except under flood conditions) the hydrostatic pressure on the liner, and hence the filtration layer, from rainfall will be minimal. By contrast, the passage of a train will cause load dynamically to be applied to the subgrade, causing a transient increase in hydrostatic pressure in water present."

The judgment provides the reader with a useful guide as to the latitute which purposive construction gives to the patentee when seeking to establish infringement.