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

Our Product Range for Coastal and Fluvial Erosion Control

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.

Filtration & Separation Survivability Durability The Design Mechanism

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

Puncture Resistance

A geotextile must be able to withstand puncture loads imposed during installation and service. The rock weight, its angularity and the drop height all contribute towards puncture load. This can be further intensified if due care is not taken during installation. Ideally, it will also possess isotropic (square) tensile properties to spread the load consistently in all directions.

Extensibility

Rock armour functions by virtue of its dead weight being transmitted over a wide an area as possible to consolidate the underlying soil and minimise particle movement. The load imposed on a geotextile by large rock is not evenly distributed. The highest stress concentration will be at the point of intimate contact which in turn will impose high localised strains. The geotextile needs to be sufficiently extensible to enable it to adapt to point loads without puncturing and without loss of hydraulic properties.

Site Installation Damage Testing

Geofabrics conducted a series of tests on the Cardiff Bay Barrage site where a number of rock sizes and HPS geotextiles were tested: this allowed us to produce the Coastal Rock Drop design curves, enabling engineers to select different grades based on the expected installation damage of their particular site.

For instance, a 3.5 tonne rock dropped from a height of 1.5m has an energy of 3500 x 1.5 = 5250kgm.

The graph shown was developed from these tests enabling the engineer a guide based on experiment to select a suitable grade of HPS geotextile. From the graph this would suggest a 14000N (HPS14C) puncture resistant geotextile would give sufficient resistance to damage.

If such data is unavailable for other types of geotextile or the site has some unique conditions it is advisable for Engineers to require a site specific rock drop test to try to simulate actual site placement conditions as part of the specification and geotextile selection process. Once the true nature of the chosen geotextile is established this leaves the installer free to exploit the characteristics of the textile to its full in the installation method chosen.

Durability

Geotextiles used in hydraulic engineering are expected to carry out one or more functions over a given design life. There are a number of factors that will help to determine the durability of a geotextile; the physical structure of the fabric, the nature of the polymer used, the quality and consistency of the manufacturing process, the physical and chemical environment in which the product is placed, the condition in which the product is stored and installed and the different loads that are supported by the geotextile.

It is essential that a geotextile performs effectively for the required duration of the design (many being more than 100 years), and not just in initial conformance testing.

Raw Material Selection – Super high tenacity polypropylene fibre

Geotextiles are normally manufactured by either woven or nonwoven techniques, the polymers used are generally thermoplastic materials which contain variations of both amorphous and semi-crystalline regions.

Geofabrics HPS-C geotextiles are manufactured from polypropylene super high tenacity fibre. There are several factors relating to fibre selection that must be considered in relation to product durability: the basic polymer from which the product is made, any additives compounded with it, and the fibre morphology.

Geofabrics HPS-C range is manufactured from super high tenacity virgin polypropylene fibre, mechanically drawn to form fibres with higher tensile properties and improved durability. The increased drawing within the manufacturing process re-orientates the molecules within the fibre making it stronger. The increased molecular orientation and associated higher density leads to increased environmental resistance. This is because the level of crystallinity within the fibre has a large effect on the properties relating to durability. The tightly packed molecules result in dense regions with higher inter-molecular cohesion and resistance to penetration by chemicals. An increase in the degree of crystallinity leads directly to an increase in rigidity and yield or tensile strength, hardness, and softening point, and decrease in chemical permeability.

Low cost alternatives

The selection of the right polymer type for the manufacture of textiles for use in civil engineering applications is essential. Geofabrics HPS-C range is manufactured from virgin polypropylene fibres which have a high resistance to acids, alkalis, and most solvents. Polypropylene can be considered as inert to acid and alkali attack and is suitable for most geotextile applications.

Polyester (PET) is often promoted in hydraulic applications and is sometimes promoted as a sinking geotextile due to its specific gravity. This is misleading, as all oil-based polymers have a specific gravity less than 1. Pure water, of course, has an S.G. of 1 and salt water has an S.G. of around 1.03 dependent on salt content. This means that by their nature geotextiles float and therefore gives the installer the basic challenge of holding this material down flat on the seabed.

There are some exceptions with polymers such as polyester which have an S.G. > 1 suggesting that this material will naturally sink, implying an easier time for the installer. The benefit is marginal for several reasons. Firstly in salt water the ability to sink can be reduced to zero and the fabric if not positively placed with a sinking system will “wallow” just beneath the surface and will neither sink or float making the task even more difficult. The thicker needlepunched nonwovens tend to have air trapped in them which gives them extra buoyancy initially and takes some time and agitation to remove the air bubbles. The recommendation for installers is therefore is to treat all polymers as floating, especially in seawater.

PET can offer good mechanical properties and is suitable for some applications; however, the ester group can be hydrolysed in the presence of water, which is accelerated by alkaline conditions. Polyester can also be susceptible to heightened degradation where there is lime treated soil, concrete or cement present.

Hydrolysis in polyester takes two forms. The first form of hydrolysis is alkaline or external hydrolysis which occurs more rapidly in soils above pH 10, and particularly in the presence of calcium, and takes place in the form of surface attack, or etching. Increased caution should be taken with polyester in soils with pH 9 or above. The second type is internal hydrolysis which takes place across the entire cross section of the fibre, this occurs in aqueous solutions or humid soil at all pH levels. This process is slow in mean soil temperatures of <15°C or neutral soils, however this is accelerated in acids and increased soil temperatures.

Although polyester can have advantages over other polymers the alkaline sensitivity of this polymer under long term loadings should be a major concern in many geotextile applications, polyester can be susceptible to damage in high pH applications. An independent study conducted by the University of Leeds showed that ‘If the conditions are slightly alkaline, the combined action of load and alkali could be catastrophic, and the use of polyester would have to be restricted'.

UV Resistance – Weathering

Geosynthetic products can be exposed to weathering and the resulting effect on the performance of products is of importance. The aging of geotextiles in predominately set in motion by the climate effects through the presence of solar radiation, heat, wetting and moisture.

Geofabrics HPS-C range contains fine grade carbon black additive for ultraviolet light stabilisation. This is mixed in the polymer prior to the point of extrusion to allow for homogenous dispersion within the product. Carbon black acts as a strong UV absorber.

For areas with a high radiant exposure (UV) or where the product is to be exposed for extended times we are able to offer specific UV packages to protect the material further.

For further information on Durability please see our report.

Designing for Durability

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The Design Mechanism

The type and the fibre, together with the manufacturing process, define the inherent properties of a geotextile filter. Geofabrics HPS-C needlepunched nonwoven geotextiles are manufactured using specially engineered super high tenacity fibres that are bound together by mechanical entanglement. This means that the balance between hydraulic and mechanical properties are optimised for:

High permeability with fine filtration
High isotropic puncture resistance with high strains to failure
Good cushioning ability and impact resistance
High in-plane flow capacity

Design methodology will consider whether the primary armour is to be placed directly or indirectly on the filter/separator. Armour can be placed directly on HPS-C products so there is no requirement for an intermediate bedding layer of stone.

Geotextile Filter Selection Process

1. Establish the primary armour weight from wave height predictions

2. Establish the type and permeability of the underlying soil (see table for typical figures in the absence of specific data)

3. Provisionally select an HPS-C grade based upon its permeability e.g. If the soil permeability is 10-5 m/s then the grade must have a permeability of >10-4 m/s

4. Check the grade’s O90<d50 of the subsoil. This requirement is satisfied by most of the HPS-C range.

5. Check that the selected grade can withstand installation loadings without puncture.

For armour <100kg: Our standard HPS4 can normally be used.
For armour <1000kg: HPS5, with a bedding layer of stone, may be considered at the discretion of the engineer.
To assess for damage for armour >1000kg placed directly on the geotextile please continue to 5.1.

5.1 Choose a Factor of Safety (FOS) for damage. E.g. 3 for a flood bund where the consequences of failure are costly.

5.2 Establish the maximum weight of the rock likely to be in contact with the geotextile. e.g. 4000kg

5.3 Estimate the maximum drop height dependent on site working conditions e.g. 1.5m

5.4 Calculate rock drop energy level (rock weight x drop height) using the attached document

Coastal Rock Drop Design Curves

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5.5 Plot the selected FOC (5.1) and energy level on the Installation damage graph.

5.6 Check that the selected geotextile is on or above the plotted point. If not then upgrade to a grade that will and check that it still meets the permeability requirements.

5.7 Check that there is sufficient design elongation for the fabrics to function without tearing. Rock diameter is likely to be approximately 1.5m: assuming rock depression into subsoil to a third of its depth i.e. 0.5m, the localised elongation in the geotextile due to surface friction between the subsoil and rough edges of the rock could be as much as 20%. To allow a FOS=3, a minimum tensile extension would therefore be 60%.

Specification

A well written specification for a filter/separator is of paramount importance as there are many geotextile types available with widely varying physical characteristics and production qualities. Testing and quality assurance are as important for geotextiles as it is for the other materials incorporated into the works.

European (EN) and International (ISO) index tests are available to enable engineers to compare one geotextile with another: these tests and quality control schemes need to be referenced in the specification.

The tests should be used to assess the suitability of the proposed geotextile for the works. The manufacturers Quality Control procedures should also be made available.

Prior to the installation, the product should be tested in an ISO 17025 (UKAS in the UK) accredited laboratory to ensure conformance.

A model specification is available for download, where relevant properties can be added in line with the requirements.