Alternative ground engineering solutions, such as those incorporating geogrids, can play a vital part in meeting the UK’s future housing needs. Jonathan Cook of Tensar explains.
The UK Government is committed to building hundreds of thousands of homes a year to meet the country’s housing crisis. To ensure they can meet this target, it is crucial for housebuilders to maximise land use, minimise construction costs and deliver new homes as quickly as possible, while maintaining profitability.
This essentially boils down to mitigating risk – a large proportion of which is in the ground. It is incredibly difficult to predict every possible situation arising during construction, but designing to actual ground conditions can deliver more appropriate solutions, mitigate risks and ultimately add more value. This does not necessarily mean spending more money, but requires a change of approach from traditional solutions to acceptance of ‘alter- native’ value engineered designs.
Geogrids, for example, have been used on thousands of residential schemes around the world and on hundreds of projects in the UK, allowing developers to build new homes and associated infrastructure over a wide range of ground conditions, particularly weak and saturated soils.
However, designs incorporating geogrids are sometimes viewed as ‘new’ or ‘alternative’ solutions and, as a result, are sometimes rejected in favour of more traditional approaches. This is despite three decades of empirical evidence and independently-verified research demonstrating how geogrids have helped improve construction efficiency, mitigate risk and reduce delays.
Geogrids can be, and have been, used at every stage of a housing project. They can control differential settlement in capping layers, maximise development space by creating steep slopes, deliver thinner and better-performing temporary access roads and working platforms, and can help build long-lasting and low-maintenance permanent roads.
Maximising development space
With a shortage of suitable land for housing across the UK, housebuilders need to maximise available development space. This can mean creating a level area for construction, supported by perimeter retaining walls, or steepening slopes on the edge of the site to extend the site’s useable area.
The conventional approach is to build reinforced concrete walls, but these can be expensive to build. Those using geogrid to reinforce soil behind block, gabion or vegetated facing, however, can deliver the same level of performance (and slope heights and angles). They are also faster and more economical to construct, with the added benefit that they provide a more aesthetically-pleasing finish. Reinforced soil structures can also serve as noise and visual barriers to both homeowners and those living next to the new development.
Providing safe working areas
Providing safe working areas is critical on any construction project, particularly on weak and variable ground. Geogrids improve the performance of granular fill used for working platforms, increasing bearing capacity so the ground can support heavy loads from cranes and other construc- tion plant.
Load-spread designs incorporating geogrids, in line with ‘BR470: Use of ‘structural geosynthetic reinforcement’ – A BRE review seven years on’, backed by performance testing, have been proven to deliver thinner platforms, shorten construction times and reduce materials use, compared with conventional approaches.
Unpaved access roads & permanent roads
Much in the same way as working platforms, geogrids can be used to mechanically stabilise aggregate layers of unsurfaced access roads, improving their performance and enabling them to support construction traffic.
Once homes are finished, temporary roads can form the foundations of permanent infrastructure, including highways adopted under Section 38 agreements and car parking areas. This can speed up these important aspects of residential developments, ensuring projects are completed on time and as economically as possible.
Because the trafficking capacity of pavements incorporating geogrids can be up to six times greater than traditionally- built roads, they can help make roads safer, for longer. This improved performance also reduces longer-term maintenance and repair requirements, reducing disruption to road-users and local residents.
Because geogrids can reduce the overall thickness of working platforms, capping layers and road pavements, they can contribute to the green credentials of a scheme.
Fewer materials are needed and weak and unstable soils can be left in the ground. Geogrid solutions can also use site-won material, including non-standard fill that would otherwise have to be disposed of and replaced with imported material.
Not only does this mean construction costs can be lower, but it can also result in fewer lorry movements, reduced embodied energy and lower CO2 emissions. An added bonus is that there is less disruption to local residents and businesses – a great selling point to both housebuilders, developers and local authorities.
Geogrids bring a wealth of benefits to housing schemes. They have been shown to deliver up to 50 per cent cost savings in road construction and up to 75 per cent savings in reinforced soil wall construction. They can halve aggregate thickness in temporary access roads and working platforms, and cut permanent road pavement thickness by up to 20 per cent.
All of these benefits can be realised by housebuilders if they are willing to engage with the supply chain and are open to alternative ground engineering solutions. These products were ‘innovative’ 30 years ago, but are now well-established ways of reducing ground risk, maximising the returns on land, and delivering much- needed housing faster and more economically than ever before.
Jonathan Cook is senior product manager at Tensar.