Ms. Sneha Bhosale
|Construction of a sustainable and well-connected road network forms a vital part of the infrastructure development for any nation. Conventional pavement design methods and road construction practices present ever-growing challenges to engineers, contractors and owners. Road construction budgets are under increasing pressure to deliver improved value and performance for less money. Further issues include inadequate pavement structures, lower maintenance budgets, availability of stone for road construction, increasing material costs and environmental impact of quarrying of stone required for the road construction.|
These issues have forced roadway designers to seek alternative sustainable and environmentally friendly solutions to conventional strategies. So how can the pavement designs be optimised to get the best performing sustainable roads for the lowest cost and with low carbon footprint?
One of the well-known and well-researched solution is the use of geogrids in pavements – something that is being done since early 80’s. Traditionally the geogrids were used in soil improvement layers – capping or subbase – to help reach some required bearing capacity under the base and asphalt layers. However, the influence of geogrid was not taken into account in the pavement design itself.
In the last years, a new approach to design of pavements incorporating geogrids, called Pavement Optimisation (PO), have been proposed and Indian Road Congress (IRC) has published IRC SP 59 -2019 “Guidelines for use of geosynthetics in Road Pavements and Associated Works” elaborating the use of geogrids in this application and the procedures.
During the course of time geogrids have been constantly developed to create more efficient products, the latest significant development being the introduction of hexagonal structure stabilisation TriAx geogrids in 2007. When a wheel travels on the road surface, the load from wheel is distributed radially through the pavement layers. The radial nature of loads requires radial response from the geogrid used for stabilisation of unbound aggregate layer. Tensar TriAx geogrid, because of its hexagonal geometry, offers near circular 360⁰ stabilisation of the unbound aggregate layers of base and sub-base.
Tensar TriAx geogrid improves aggregate stability by locking the aggregates/rock fill together, the triangular shape of TriAx geogrids ensures excellent shape and rigidity. High aspect ratio of the ribs allows greater vertical contact with the aggregate; these properties allow the geogrid to restrict aggregate movement caused by the dynamic loads from the traffic movement, thereby distributing the load over a larger area.
TriAx geogrid has higher rib profile than conventional biaxial geogrids of the same weight. The shape of the vertical sides of the ribs is slightly concave. The shape and high profile of the ribs provide a very positive interlock with the aggregate particles.
Efficient junctions help to transfer the load applied to a mechanically stabilised layer into the geogrid. In order for the geogrid to confine the aggregates, the junctions need to remain intact. The stiffness and strength of the junctions will have a direct impact on performance of the geogrid.
For TriAx geogrids, rigorous testing has been conducted in line with each of the three rib directions. The junction strength was found to be equal to the rib strength in each direction. Thus, giving a junction efficiency of 100%.
Benefits Of Using Triax Geogrid
- Better performing pavement for the same initial cost as a conventional non-stabilised design.
- Proven performance backed by worldwide research and field validation.
- Comprehensive engineering support services
- AASHTO & ME design methodology compliance
When unbound well graded aggregate is placed and compacted on a layer of stiff geogrid, aggregate particles interlock within the geogrid apertures and are confined by its stiff ribs. A geogrid/aggregate composite is created, in which lateral restraint provided by the geogrid reduces strain and thereby increases the stiffness of the layer compared to the same layer without geogrid. This composite layer is called Mechanically Stabilised Layer. ISO 10318-1 has a definition for the geosynthetic function of ‘Stabilisation’ as distinct from the ‘Reinforcement’ function.
Aggregate layers stabilised with hexagonal geogrid have higher parameters (modulus & layer coefficient) and durability compared to the same layer without geogrid. Improved parameters of one layer affect the life of whole pavement, so it is possible to modify the thickness of other layers (including asphalt) to design thinner pavement, which will still have
the required life. This can also be utilised in other way – it is possible to increase pavement life without changing its thickness. This approach can be utilised both in soft soil and firm soil conditions.
The stabilisation of unbound aggregate layer forms a Tensar Mechanically Stabilised Layer (MSL). Tensar TriAx is certified by the European Organisation for Technical Approvals (EOTA) for stabilisation function (European Technical Approval Certificate ETA 12/0530; ETA 12/0531). Tensar TriAx geogrid are accredited by the IRC as well as many approving authorities worldwide.
Pavement Optimisation Design
After years of extensive research validated by leading experts in the field, Tensar is proud to lead the industry into a new era of pavement design. Pavement optimisation with TriAx Geogrid can be carried out using AASHTO as well as Mechanistic Empirical (ME) design methodology, allowing to precisely quantify the benefit of integrating a MSL into the roadway section. The pavements can be designed for enhanced performance (MSA) and lower design risk, while maximizing the project’s cost-effectiveness.
IRC SP 59 provides alternative design methods for flexible pavement design using geogrids. Modified AASHTO design method following Layer Coefficient Ratio (LCR) and Mechanistic Empirical Pavement Design (MEPD) method following Modulus Improvement Factor (MIF) are the methods elaborated in the code.
The crucial component of the optimised pavement design is the Modulus Improvement Factor (MIF) / Layer Coefficient Ratio (LCR). The modulus of the aggregate layer stabilised with geogrid is increased by multiplying the initial value by a MIF or back calculated from the enhanced structural coefficient obtained by multiplying the initial by LCR.
The values of the MIF/LCR are not constant – they differ depending upon the subgrade CBR, granular layer thickness and the properties of the geogrid. As recommended by the IRC SP 59 the LCR & MIF values used for TriAx geogrid for Tensar have been based on number of full-scale trafficking trials & large-scale laboratory testing and are validated by independent third party.
M-E pavement design software tool (SPECTRA M-E) as well as AASHTO pavement design tool (TENSAR PAVE) has been developed by Tensar. Both software’s have been validated by a third-party independent reviewer (Applied Research Associates (2015)).
Considerations For Pavement Optimisation
Pavement optimisation design for any project will depend on few important parameters as listed below:
- Sub-soil strength (CBR) – This is the prime most important parameter for any pavement design conventional or optimised. The value of the sub-soil strength i.e., CBR majorly affects the pavement design as well as the design approach.
- Sub-soil variability – Typically roads alignments pass through different sub-soil conditions due to which variability of sub-soil strength is usually observed along the length of road; hence this variation needs to be taken into account while designing the pavement. Effective subgrade modulus should be taken into consideration for the pavement design. Proper consideration of the sub-soil strength with careful testing of the parameters shall be done before any pavement design because failure of subgrade is complete failure of the pavement. And in this case of failure complete rehabilitation of the pavement needs to be carried out.
- Cost of materials – Location of the project plays an important role in consideration of pavement optimisation. As it is well known that in India, granular material is scarcely available as well as the bituminous layer costing is high in some regions. Hence the cost of material, transportation leads of this material are governing while carrying out the PO design and influence which layer of the pavement (unbound/bound) is optimised. The project budget and the timeline of project are also economic driving factors for PO designs.
Optimised Pavement Design – Different Options
Depending upon the project requirement and considerations listed above different options of pavement optimisation are possible. Different alternatives either giving maximum life or reduced construction cost, or the optimum design for a fixed budget, can be possible and carried out using the Tensar Software and TriAx geogrids.