Coastal & River Defence

Marine erosion can occur when the sea is energised by wind and gravity to produce waves, tides, and currents: it is caused by by corrosion, abrasion, and hydraulic processes. When these impinge on the shoreline, they cause coastal attack through the process of beach and toe erosion leading to over steepening of soil or rock lopes.  A similar phenomenon can occur in rivers, where the agents are corrosion, hydraulic lifting, scouring, cavitation and mechanical erosion by wind or running water charged with detritus.

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HPS-C for Coastal and Marine Applications

The principle of mitigating the effects of these processes involve sealing, strengthening and/or retaining.

The benefits of using geosynthetics are often undervalued in their impact to the stability of a hydraulic defence structure, partly because the unit cost is so small compared with armourstone.  The consequences of not designing and specifying them correctly can be catastrophic, potentially threatening the stability of the whole structure.

When correctly specified and installed the use of a geotextile can offer huge savings on a project and increase the life of the structure significantly.

Geofabrics HPS-C geotextiles are designed to be used as filter/separators in hydraulic defence structures: they are placed on lower permeability beach materials to prevent the escape of fine particles while permitting the free passage of water, and provide a stable and consistent bedding layer, often relinquishing the need for one or more layers of armourstone resulting in huge cost savings.

Geofabrics HPS-C Geotextiles are used as a cost-effective alternative to traditional underlayers and provide benefits for toe design. Our HPS-C range has been designed specifically to resist critical construction loads with high strength and high elongation to prevent damage and can often be installed without the need for a smaller granular layer.

Advantages:

  • Geofabrics HPS-C can be used as an effective replacement for underlayers, saving in materials, transport, and placement.
  • The amount of ‘lost’ material at the toe is minimised by preventing stones burying themselves into a soft subsoil.
  • Differential settlement is reduced, helping with long-term maintenance of alignment of a revetment or breakwater.
HPS11C Coastal Protection Geotextile
Filtration & Separation

The inclusion of an appropriately specified geotextile at the interface with the external medium will contain fine particles and preclude them from being carried away while permitting the free circulation of the water. In a geotechnical application a filtration system must function throughout its complete deign life without clogging, unlike many other filter applications the system cannot merely be replaced.

Rather than trapping all the particles in the filter, the system should prevent the complete soil structure from moving. The geotextile filter must retain the entire soil structure by allowing some smaller particles to pass. The structure is then able to stabilise by holding the larger particles in a framework which when in contact with each other, retain the smaller particles.

A properly specified and installed geotextile will:

  • Maintain the complete soil structure
  • Allow unstable fine particles to pass to prevent clogging
  • Maintain permeability in the short and long term

There are two potential failure mechanisms which must be considered when specifying a geotextile filter in a reverse flow application:

  • Inadequate soil retention - this may be due to a failure to retain the soil skeleton, possibly because of a poorly specified pore size or an inadequate contact between the soil and the filter. It could also be because of damage to the filter which has occurred during installation or from abrasion in service.
  • Inadequate permeability - this can destabilise the entire structure by creating excessive pore water pressures. This could be due to poor initial permeability, excessive soiling during installation or clogging.

There are two primary properties that are critical within any filter specification:

1. Geotextile Opening Size - the opening size of a geotextile is measured using EN 12956, by determining the particle size distribution of a graded granular material which is washed through the geotextile filter.

While the size of the largest particle collected in the sieve is theoretically the maximum opening size, this value cannot be practically measured. The standard therefore reports an O90 value, defined as the average diameter of soil particles, 90% of which is retained by the product tested in the sieve.

To facilitate filtration, a geotextile must have an opening size that is smaller than d50, which is defined as the median diameter of the particle size distribution. Established design rules for reversing flow applications and a non-cohesive (granular) soil state that the geotextiles O90 should be less than the d50 of the soil being filtered. For a cohesive soil O90 < 10 x d50.

2. Permeability - to ensure the free circulation of water and prevent an increase in internal pressure, classic filter rules are that each layer of a filter system must be more permeable than the layer beneath.

Kn filtration system >> Kn soil

Similar rules for developed for geotextiles suggest a coefficient of permeability 10-100 times greater than that of the filtered soil: it is important that the geotextile should maintain or exceed its index permeability whilst under load. i.e. any reorientation of the fibres should not increase/decrease permeability. Other properties which have a bearing on the geotextiles ability to function as an effective filter relate to the materials ability to maintain an intimate contact with the underlying surface without spanning, the materials extensibility (elongation) and thickness under load is key to this.

Survivability
Durability
The Design Mechanism

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