Dr SHARAD MHAISKAR, Dean, Faculty of Technology, MPSTME, NMIMS, elaborates upon the design and construction of reinforced soil walls.
Since the beginning of civilisation, man has been attempting to reinforce materials to construct efficient and economic structures. Reinforcing mud with hay and concrete with steel are classical examples of such attempts. The motivation to reinforce materials stems from the need to improve their performance under different types of load.
Why is soil reinforced?
Reinforcing soil is another such example where soil is reinforced to improve its performance and, at the same time, yield a techno-economic earth structure. It is well known that soil, when dumped, forms into a slope with an angle to the vertical, referred to as the angle of repose. The angle of repose can be shown to be equal to the angle of internal friction, which for soils used for embankments ranges from 20°-40° under drained conditions. If left unreinforced, this implies a relatively large width at the base of the unreinforced embankment. In urban settings, such width is not available or the cost of land is so high that it is necessary to consider alternatives where land use can be minimised. By reinforcing soil, an embankment that is almost vertical can be considered. This results in saving invaluable space and creating a right of way for service roads and connecting roads. In addition, such structures are flexible and offer better performance under seismic loading.
A growing trend
Reinforced soil walls have been constructed for the past 40-50 years worldwide while in India, the use of such walls started about 35 years ago. However, in the past 25 years, usage has grown exponentially. The technology received real impetus when approaches for 34 flyovers were constructed by Maharashtra State Road Development Corporation in Mumbai using this technology. Approaches are the paths followed while building the flyover from the ground-zero level and moving higher to the level of the flyover. All these approaches are made of reinforced soil work. Some flyover projects that have been completed using this technology include Mumbai Airport junction flyover, Goregaon flyover and Eastern Express Highway flyovers, to name a few.
Material and manufacture
Initially, soil was reinforced with metallic strips. Later, polymeric materials in the form of geo grids, sheets and straps were used in view of their cost-effectiveness and ease of construction. In earlier applications, reinforcing material was imported as manufacturing facilities were not available in India. However, in the past decade, several indigenous manufacturers have set up world-class manufacturing facilities. But there are no accredited laboratories that can perform all the tests required for testing reinforcing materials and connecting systems. This is a major source of concern to users as well as owners as the reinforcement and connections cannot be tested to ensure compliance to standards.
The design
Preceding the design phase is the step where the properties of the reinforced and retained fill are evaluated along with the other geotechnical investigations to determine the properties of the founding soil and ground improvement measures required to ensure external and internal stability of the reinforced soil wall. Lack of attention to pre-project investigations to evaluate characteristics of the borrow material as well as founding soil has delayed several projects and even resulted in failure in a few cases.
The design of reinforced soil walls is done using BS, FHWA and NCMA codes or guidelines that are now based on limit state principles. Codal provisions along with BIS codes to account for seismic effects are used to ensure internal and external stability. BS code has a long-standing experience on limit state design although it has limited exposure to seismic effects. Software is available to design as well as analyse reinforced soil walls. This proprietary software yields documents to facilitate proof checking and can be used to evaluate global stability. Attempts are on to develop indigenous software that will give the user the option to design a wall using the above mentioned codes.
Ideal vs real
Ideally, the reinforced soil should be free draining and possess high shear strength. It has become increasingly difficult to obtain borrow material owing to mining restrictions and the time-consuming processes of making quarries available. In some instances, the delay has been to the extent of two years. Further, it is difficult to get free draining material in sufficient quantity and quality.
In view of these constraints and the pressure of getting the ideal material in sufficient quantity, there has been increasing demand from users and owners to permit usage of available material like colliery waste, locally available residual soils (murum) and fly-ash (already being used to a limited extent) with adequate drainage provisions. It is worth noting that such materials have been used worldwide with due precautions and provisions. Such demands are being voiced more in other infrastructure projects, more so in road connectivity projects.
Construction stage
The most important phase in reinforced soil wall usage is the construction stage. Performance largely depends on the extent to which quality control and assurance procedures are practised at a site. The emphasis on compaction in construction of reinforced soil walls is not realised or understood by many implementers. In addition, finer points like edge compaction, importance of drainage, connection details to be maintained and corresponding strength to be realised need to be given due importance; failure to do so may result in problems that are difficult to rectify. Testing the reinforced soil, reinforcing material and connection strength is crucial during the various stages of construction. As mentioned earlier, no accredited facilities are available to test the reinforcement and connection. Lack of testing facilities has been the major irritant in the growth of reinforced soil wall technology.
The challenges
Today, reinforced soil wall technology is at a crossroads. Realising the concern of designers, users, owners and manufacturers, the Indian Roads Congress has taken up the task of formulating guidelines for the design and construction of reinforced soil walls. The draft guidelines have tried to address the concerns to the best possible extent to ensure that the technology continues to play a major role in infrastructure projects and results in enhanced growth of important component technical textiles. The major provisions and issues in the draft guidelines are:
The promulgation of these guidelines is expected to address the concerns of stakeholders and ensure that the technology grows in a systematic manner. It is also likely to serve as an important tool for design and construction of reinforced soil walls. With the infrastructure sector poised for further growth, this technology is bound to play a decisive role in development. After all, with cost savings of 15-40 per cent compared to conventional RCC wall, this is an alternative that should not be ignored.