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Geosynthetics: Meeting the demand

maccaferri

Changing demand placed on transport assets is creating a need to change the solutions used to deliver this infrastructure, and reinforcement geosynthetics are an ideal option. 

Population growth and the demands of modern economies are key contributors to the need for infrastructure investment. When infrastructure encounters poor ground, traditional improvement works such as piled embankments and void-spanning slabs have long been staple solutions. However, growing consideration for long term performance and overall cost means that alternatives, such as reinforcement geosynthetics, have greater potential than ever before.

The use of geosynthetics inconstruction is not new, but is becoming increasingly accepted inpractice. However, a clear distinction has to be drawn between soilstabilisation and the soil reinforcement discussed.

Soil stabilisation is the short to medium term superficial stabilisation of materials such as access tracks or roads by using geotextiles and, typically extruded, geogrids, with the goal of improving the ability to distribute loads. Soil stabilisation design follow one of a range of accepted, broadly similar procedures posited by experienced practitioners in the field.

maccaferri 1

maccaferri 1

Moroccan High Speed railway embankment with Paralink 1000

Soil reinforcement, on the other hand, is the long term structural reinforcement of soils where the ongoing performance of the geosynthetic is required right up to the end of the design life of the structure. Design follows internationally accepted standards, such as BS8006.

First manufactured in Yorkshire in the 1970s by Linear Composites, the “Para” range of geosynthetic products and geogrids were developed by ICI Fibres. Since then, ongoing improvements in manufacturing, fibre and protective coatings have continually improved the products.

In 2006, Linear Composites was purchased by Maccaferri. This gave ParaLink a wider global platform and itis now recognised as a high strength geosynthetic reinforcement for the construction of embankments over soft soils, piles and voids with successful applications around the world.

Soft soil 

Rather than replacing large volumes of soft material beneath a proposed embankment or proceed with extensive ground improvement, ParaLink is often used to provide an alternative and cost effective solution for embankments over soft soils.

It has been extensively used and specified in the UK. Some of the most recent high profile examples are reclamation works in Belfast Harbour and the construction of the new Mersey Gateway bridge. ParaLink was also used for the Blue Water Island project in Dubai.

Partial factors given in BS8006 are used to design structures using ParaLink over soft soil. Its strength is selected to protect the embankment against shear and circular internal failures, sliding at the base and ensuring global stability.

Over piles

ParaLink reinforcement enables the vertical loading of an embankment to be transferred more efficiently onto piles and provide lateral restraint. Compared to a ParaLink reinforced solution, conventional construction would require larger diameter pile caps and a tighter pile spacing, including additional piles along the sides of the embankment.

Recent applications in the UK include the new Tinsley Link Road bypass in Sheffield, approach ramps to the Olympic Stadium in London and an elevated railway viaduct at Reading. Recent overseas projects on pile sinclude the new high speed railway line from Kenitra to Tangier in Morocco.

Spanning voids

Voids can result from the presence of underground caverns caused by natural processes, such as erosion in karstic or chalk areas; or from man made processes such as underground mining.

In such areas, ParaLink can be used at the base of an embankment to prevent catastrophic collapse into the void and to limit the unacceptable surface deformations which may occur during the design life of a structure. The geogrid is required to perform for the full 120 year design life of the infrastructure it is supporting.

maccaferri

maccaferri

20,000m² of Paralink 1350 spans voids at this site

Design analysis using BS8006 indicates that the optimal design strain is usually in the range of 3% to 6%, indicating that a geogrid reinforcement with a short-term ultimate strain of between 8% and 14% suits this application best. Manufacture of a very stiff geogrid, with a low ultimate strain – less than 8% – is technically possible, but unfortunately this does not exhibit strain compatibility with the soil it is reinforcing and results in the soil contributing less, and an even stronger geogrid is required to offer the same support.

The maximum void width that can be spanned is governed by the ultimate short-term tensile strength of the geosynthetic, and the maximum allowable strain in the geosynthetic to control surface deformations. An optimum ratio of “embankment height to void width” is generally close to unity. It is always recommended for these types of applications to rely on material extensively tested and certified by independent national authorities.

In earthquake-prone countries, ParaLink is used also as a liquefaction countermeasure. Local design standards have been developed to deliver disaster mitigation measures such as securing emergency transportation routes.

In reality, it is often impossible to adopt conventional ground improvement methodologies to address this phenomenon as the cost is significant. However, a simple liquefaction countermeasure using ParaLink sandwiched between gravels has been developed and it can be implemented with limited cost. Numerous project examples exist,especially in Japan.

While many of these applications could have been solved using traditional methods, the reinforced geosynthetic solution offered benefits in terms of costs, time and sustainability.

Software Solution

Maccaferri’s MacBars software is a popular tool for the design of basal reinforcement inaccordance with BS8006-1:2016. The user can determine the reinforcement load using Maston’s formula or the Hewlett and Randolph method.

The program also determines the required bond lengths of the reinforcement and estimates the vertical load on the piles. Material characteristics and partial material factors for the ParaLink range of reinforcementare taken from the new 2018 BBA Certificate Number 03/4065.

With a design life of 120 years, ParaLink has the lowest material partial factors within the geosynthetics market. This is a function of its ultra-tough sheathing which protects the polymerfibres providing the reinforcement. The lower material partial factors, such a sinstallation damage, chemical resistance and creep, means that when the ultimate tensile strength of the reinforcement (UTS) is factored to calculate the long term design strength (LTDS) in the reinforcement, ParaLink achieves higher LTDS than other products with equivalent UTS.

For example, a ParaLink 1500 grade(UTS=1,508kN/m) placed in gravel at 20°C design temperature has a LTDS of 999.59kN/m. Ageosynthetic with a similar UTS of 1500kN/m bu twith less favourable material partial factors ofsafety could only achieve a LTDS of 777.11kN/m.This 22% difference in the long term design strength compels the designer to select a product with a higher UTS to achieve the satisfactory LTDS for the structure with additional costs.

Finally, care should be taken to ensure that the product selected can accommodate the environmental conditions expected to be encountered in use, for example, some polymertypes perform differently when wet, compared to others. 

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