RSA Geotechnics was commissioned by the National Trust to carry out an investigation at Hatfield Forest in Essex, in connection with upgrading an existing access road along part of its length.
Hatfield Forest is the best surviving example in Britain of an ancient royal hunting forest. It represents a managed landscape created by the woodland management techniques of coppicing, pollarding and grazing which have been practised for centuries. Many ancient trees are present in the forest some of which are 1200 years old.
The proposed upgrade comprised reconstruction and widening of the access road, to accommodate two-way traffic. The road ran from a junction with a minor local authority road at its eastern end, through the grounds of the Forest for approximately 1.5km to Shell House, a National Trust property, at its western end. The road passed close to a large ancient oak tree and at this location a ‘No Dig’ zone had been defined, to avoid damaging the tree roots and protect the tree.
At the site of the oak tree the client was considering widening the road and constructing a rigid road pavement (continually reinforced concrete) on piles to accommodate the ‘No Dig’ zone restrictions. Two cable percussion boreholes were drilled near to the oak to depths of 20m for the purpose of providing information for pile design. Careful hand excavation was carried out prior to drilling to ensure that there was no damage to the roots.
The Lowestoft Formation (glacial till) was encountered below the topsoil to the base of each of the boreholes. It typically comprised stiff/very stiff, high/very high strength, orange/brown or brown, slightly sandy, silty clay of low to high plasticity with some subrounded, fine to coarse chalk gravel and a little subangular to subrounded, fine to medium flint gravel. A suite of laboratory testing was carried out on the till and parameters were provided for pile design.
It was considered that the limited space beneath the tree branches and possible risk of damage to the tree roots due to ground vibration generated by the driving process, would restrict the use of driven pile equipment. Bored piles were considered to be more appropriate as installation would not generate excessive ground vibration compared to driven piles and low headroom rigs are available that are capable of working close to the tree canopy. For CFA piles smaller rigs utilising a flight auger that could be broken down to short sections would be required. Preliminary safe working pile loads were provided.