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Access road design project needs a staged approach

16th July 2019

In the role of consulting engineers, it’s important to remember that when tasked with a project that may seem simple at first glance, we have to look at every aspect in detail, even when the client has suggested a possible solution.

It requires years of experience and confidence in our abilities to suggest alternatives that may cost a client more at the outset, but we know will deliver long term benefits and cost-savings.

This was the situation we recently found ourselves in when designing an access road to a major site. We believe our work on the project was worthy of a case study, which you can read here, but one particular aspect of the design which required careful consideration was the need to protect the mature trees lining the route of the finished access road in this largely rural setting.

Getting to the root of the problem

The road was designed with three distinct stages to account for the different conditions along the length of the road. Where the road meets the highway, the access is built on the verge which belongs to the local county council, so the road has to be built to meet their standards.

Following California Bearing Ratio (CBR) tests undertaken to allow us to design the road for the in-situ strength of the sub-grade, we specified a road construction more in line with permanent roadways than temporary haul roads – final road of 200mm of blacktop over 460mm of stone.

The third stage was the need to protect the roots of the trees, whilst allowing water to filter down to where it was needed. The access road is adjacent to the boundary of the site and the neighbour’s mature trees. Clearly we had to avoid damaging the roots of the trees during construction of the road.

Cellular confinement system is the answer

The root systems of even tall mature trees are generally much shallower, but more widespread than most people think, making damage possible if no consideration is given during the road design process.

Recognising what was needed was a lightweight, permeable solution that was easy to install we specified TERRAM GEOCELL for areas where the road would traverse tree roots.

Ideal for ‘no-dig’ situations, this 3D cellular flat-pack system ensures the roots are protected from vehicle loads by confining the sub-base and stabilising the ground.

When the permeable plastic honeycomb is filled with a porous, free-flowing aggregate, this not only ensures downward forces are spread laterally to reduce pressure on the sub-grade, but it allows passage of essential air and water to provide nutrients to the roots.

We often specify geotextiles when designing solutions for retaining walls, retention bunds, culvert head walls or even sound barriers. They provide practical solutions that requires specialist installation, but once in place will deliver years of trouble free service.

The changing face of construction

The team here at PDMA can call on more years of experience than we care to add up, but we have all noticed in recent years a determination on the part of construction firms to consider every option and the long-term benefits, rather than the easy or traditional options.

It is unlikely that in the past so much time and energy would be devoted to a temporary access road that will be removed in a couple of years. But firms are increasingly recognising that the additional cost at the outset will pay off in the long run, with low maintenance costs and uninterrupted access to sites.