Geosynthetics on the move

The use of geosynthetics decreases initial construction cost and maintenance cost, increases the life and strength of the roads, which in turn decreases traffic congestion & saves fuel and reduces the carbon footprint.

Even in present times of economic turmoil, as one of the BRICS countries, India still stands out as one of the outstanding emerging markets. It is in the stage where all forms of infrastructure need construction of new systems and rehabilitation and upgradation of the existing. Such infrastructure would essentially include transportation systems, in particular, roads.

As per 2011 statistics, India has a road network of over 42,36,000 kilometres, the third-largest road network in the world. This works out to a quantitative density of 0.66 km of roads per square kilometre of landmass. This is similar to that of the United States (0.65 km) and far higher than that of China (0.16 km) or Brazil (0.20 km).

Yet qualitatively, Indian roads are way behind these countries. These roads are a mix of modern multi-lane expressways and highways and narrow, unpaved roads. However, considering its population, India has less than 4 km of roads per 1,000 people, which includes roads of all qualities, paved and unpaved. If one considers in terms of quality, the country has less than 0.07 kilometres of all season, four or more-lane highways per 1000 people as of 2010. These statistics are some of the lowest road and highway densities globally. Comparatively, United States has 21 kilometres of roads per 1000 people, while France about 15 kilometres per 1000 people, predominantly paved and high quality in both cases. Countries like United States and France have a highway density per 1000 people that is over 15 times as that of India.

At one point of time, the quality of roads in India was undergoing drastic improvement. As of 2008, about 2.1 million kilometres (about 49 per cent) of roads were paved. Today there is a slowdown in road development even on awarded contracts since 2012. Concessionaires have preferred to pay off penalties and escape harsh requirements of the contracts. Environment constraints and restrictions on mining soil and rock for aggregates have rendered scarcity of basic raw materials that go into making of roads. However, judicious use of geosynthetics in the various components of the pavement system, in particular the flexible pavement, would go a long way in incurring economies in the cost (both capex and opex), improving the quality of the roads as a whole and gaining carbon credits, a matter which will be vital in the coming years. Besides, no value can be attached to safety, which by any reckoning, is of prime concern.

India is yet to catch up in the use of geosynthetics as compared to other developed countries and emerging markets, and even as compared to developing countries such as Ghana and other African States.

Geosynthetics

There are basically five functions of geosynthetics:

a) Separation;
b) Reinforcement;
c) Filtration;
d) Drainage;
e) Containment.

Geosynthetics normally includes all polymeric (also known as plastics) geotechnical solutions to be used along with various categories of soil and rock. It may be noted that natural fibres as well as steel are also used as soil reinforcements and protection works. The terminology "Geosynthetics" no longer reflects its literal connotation of strictly synthetic material used for geotechnical systems. For the sake of completeness, the terminology "Geosynthetics" includes various polymers, natural fibres such as jute, coir and hemp, and steel.

Geosynthetics for roads and highway includes:

Geotextiles, also called geofabrics

These form one of the two largest groups of geosynthetics being used. They are in the form of woven and nonwoven fabrics, planar and flat. They are generally made of synthetic fibres in the literal sense, made into flexible porous fabrics by standard weaving machines or by being matted together in a random nonwoven manner. Geotextiles are also thermally (heat) bonded and knitted. The major characteristic is that geotextiles are porous and allow liquid flow both across their plane and within their thickness.

Geogrids

Geogrids are planar material with very open grid-like uniform apertures. The key feature of geogrids is the apertures/openings between the adjacent machine/longitudinal direction and cross/transverse direction ribs. These openings should be large enough for soil-to-soil interaction above and below the geogrids with due consideration to the representative soil particle sizes.

Geostrips

Strips are essentially bands of either galvanised steel or coated polymer, generally PET. These are essentially used as soil reinforcement. Where steel straps are concerned, cost is a major issue.

Geonets

These are extruded polyethylene (PE) geosynthetics and similar to geogrids, except that geonet openings are finer and the material is essentially used for its in-plane drainage capability rather than for reinforcement.

Geomembranes

These along with geotextiles are largely used. Geomembrane are impervious sheets of polymeric materials, generally High Density Polyethylene (HDPE) or Linear Low Density Polyethylene (LLDPE) and PVC.

Geocells

Geocells are essentially three-dimensional honeycomb-like cellular confinement systems. Modern geocells, invented by the US Army, are fabricated from ultrasonically-welded HDPE strips that are expandable at site to form the honeycomb cellular structure. The cells of a geocells system are filled essentially with cohesion less soil for rigidity. The cell walls are perforated for pore water pressure relief and soil-to-soil interaction. The walls are also textured within for better soil-cell wall interaction.

Geocomposites

Geocomposites would consist of a judicious and innovative combination of any of the above geosynthetics, assembled within a factory unit. The combination may include soil such as bentonite clay sandwiched between two layers of nonwoven geotextiles to create an impervious layer. The application areas would be numerous and would encompass the entire range of functions listed above.

3. Functions of geosynthetics

Functions of geosynthetics can be summarised in the form of a Function Matrix as under:

4. Justification for the usage of geosynthetics in roads:

There are four key essentials that make it prudent to adopt geosynthetics in highway engineering practice:

• Environment
• Quality
• Economics
• Safety

Ease on the environment

Development and improvement of roads require natural resources, particularly water, soil and stone. A massive demand for these resources would carry the burden of a major environmental impact and leave behind a major carbon footprint which in the present context is unacceptable. To a large extent, geosynthetics would help in conserving these natural resources.

Quality

Use of geosynthetics within a road section not only reduces the material requirements but also manifests in a marked improvement in the quality of the road. The maintenance cycle is over an extended period. Geosynthetics add to strength of the various courses and improve containment characteristics. This also reflects upon the quality of ride which is superior and facilitates higher speeds with greater safety.

Economics

The other issue that would make use of geosynthetics essential in highway engineering is cost. Cost heads that could be optimised with judicious use of geosynthetics would broadly include:

• Cost of property; with the high cost of property and non-availability of land particularly along highways which require multi-laning, the limited width of Right of Way (RoW) could be optimised with the use of geosynthetics, for example by use of reinforced soil technology for earth-retaining structures instead of embankments;
• Design costs and other back-office costs including procurement;
• Material costs;
• Cost of installation/construction which would include equipment, labour, energy, incidentals and site overheads.

Safety

Geosynthetics has special applications for border roads in mountain terrain and deserts where new roads may be required impromptu. Geosynthetics can be used extensively and innovatively as protection systems against landslides and rockfalls, etc.

When viewed holistically, designs with geosynthetics provide safer solutions, considering lesser activities at the worksite, at the quarries, simplicity of construction processes, less need for specialised skills in execution at the sites, lesser chances of human error, greater speed of construction without compromising basic site safety, and lesser risks in implementation.

Areas of applications :

• Containment of courses of a road section;
• Strengthening of a road section;
• Improving the overall deformation modulus of the formation;
• Use of separator material between the two courses to prevent inter-mingling;
• Distribution of loads over a larger area;
• Basal soil reinforcements for embankment supports particularly over soft ground;
• Pre-consolidation of soft sub-strata;
• Reinforced soil structures to support the formations, bridge approaches and bridge abutments;
• Improving sub-surface drainages;
• Landslide and rock-fall protection systems;
• Erosion control, etc.

Cost effectiveness

Globally, over the last thirty years, using geosynthetics in roads to stabilise weak subgrades has been a well-accepted practice. However, from an Indian standpoint, it is important to appreciate the economics of using geosynthetics in such road applications. A complete life-cycle cost analysis (LCCA), which includes not only costs to agencies but also to users, is immediately needed to understand the advantages of using geosynthetics in various road applications.

The design and construction cement concrete pavement for roads involves the selection, specification and construction of a number of concrete pavement features. Among these features are foundation support, concrete slab thickness, concrete strength, etc. There are a variety of options available for each of these features resulting in several combinations of concrete pavement design possibilities. The road agencies select the best combination of features based on experience, preference, perceived performance, perceived constructability and estimated cost.

The two criteria of performance and cost are usually interrelated. Road agencies must balance these two criteria when selecting concrete pavement design features.

Specifying a feature which adds to construction cost without a suitable increase in performance is a misuse of funds. Less-than-adequate performance leads to early repairs, rehabilitation or reconstruction. Such procedures are costly and can cost more than the savings achieved from the lower construction costs. Additionally, the roadway users are inconvenienced sooner and more often.

Thus, there are two categories of costs which can be considered in the economic evaluation of alternate pavement-Agencies costs and User costs.

Agency costs include:

• Initial construction costs
• Future construction or rehabilitation costs
• Maintenance costs recurring throughout the design period
• Salvage or residual value at the end of the design period (a negative cost)
• Engineering and administrative costs
• Traffic control costs.

User costs include:

• Travel time
• Vehicle operation
• Discomforts
• Time delay and extra vehicle operating costs during resurfacing or major maintenance.

The Path ahead

Currently, Indias annual expenditure on the road sector is around Rs 20,000-30,000 crore. It is said that about 70 per cent of funds are spent for the maintenance of roads. The magnanimity of the expenditure incurred in order to repair roads is alarming. The preferred strategy for long-term road & pavement performance is to build in safeguards during initial construction. These performance safeguards include stabilising the subgrade against moisture intrusion and associated weakening; strengthening road based aggregate without preventing efficient drainage of infiltrated water; and as a last resort, enhancing the stress absorption and moisture proofing capabilities of selected maintenance treatments. From the environmental point of view, the need of the hour is to adopt sustainable design solutions/construction practices which minimise the use of natural resources and energy consumption at all phases of a construction project.

Use of innovative materials like geosynthetics helps in coping with such requirements. Geosynthetics usage decreases the life-cycle cost of the road i.e., initial construction cost & maintenance cost, increases the life and strength of the roads which in turn decreases traffic congestion & saves fuel and reduces the carbon footprint. The work of incorporating geosynthetics in the Indian standard code, IRC-37, for major district roads/State highways has been initiated.

The increased use of geosynthetics will be a huge boon to the Indian infrastructure and the exchequer.

This article has been authored by CII