A467 Newbridge to Crumlin Rockfall Remediation

Pictured above: Views of site prior to works. Steep earthworks adjacent to carriageway were allowing rockfall events to land in road.

Pictured above: Views of site prior to works. Steep earthworks adjacent to carriageway were allowing rockfall events to land in road.

The A467 is a critical traffic route through the Gwent Valley, running from Brynmawr in the North to Newport in the South. The road is essential for the local population, providing a main travel route between small communities in Southeast Wales and the larger cities of Southwest Britain (incl. Newport, Cardiff, Swansea and Bristol). Running broadly along the toe of the valley throughout its length, this road often encounters steep cutting sides and natural slopes adjacent to the carriageway which can give rise to significant geotechnical hazards.

Throughout the Newbridge to Crumlin section, rockfall occurrence along the A467 was prevalent. With a narrow verge between the toe of the rock slope and the carriageway edge, this meant that any rockfall debris falling from the slope had a high chance of reaching the carriageway. A moderate vegetation coverage was the only means of controlling the trajectory of rockfall and dissipating the energy. Due to the frequency of traffic through this section, the outcome of a rockfall event could’ve included multiple fatalities.

The works comprised the installation of rockfall protection along a 400m length of slope, adjacent to the Southbound carriageway. This consisted of draped passive netting secured by rock bolts. The rock bolts were to be installed primarily along the perimeter of the netting area, with additional profiling and spot bolts required at specific locations as agreed with the Designer on site. Works were let on a build-only basis, therefore Griffiths worked closely with the Client (Caerphilly Council) and the Client’s Designer (WSP) to ensure that the requirements of the remedial system were adequately addressed, and a high-quality solution was delivered.

Traffic Management was installed to safely manage the road interface during construction. This included lane narrowing with a Varioguard barrier throughout the site and reduced traffic speeds. The programme of works was also communicated to Gwent Police to gain their support in enforcing the reduced speed limits through the section by providing a regular presence.

Extensive tactile environmental and ecological inspection undertaken by specialists prior to devegetation.

riffiths geotechnical specialists installing rock bolts using drill-mounted 21t Long Reach excavators.

Vegetation clearance was required throughout the rock face prior to the main construction phase. This was undertaken following a Preliminary Ecological Appraisal and under watching brief from a licenced ecologist to ensure the works caused no harm to any sensitive receptors or species. Vegetation clearance was then completed by Griffiths’ roped access operatives.

Devegetation allowed an opportunity to better inspect the slope face, and to review the design and improve the efficiency of this based on new information. Griffiths engineers have extensive experience in the geological and geotechnical assessment of rock faces to maximise design efficiency and control risk. These engineers were therefore able to assist the Designer in assessing areas of outcrop not previously visible during the design phase, ensuring that any requirements for additional rock bolts and/or amendments to rock bolt spacings could be made during setting out. This would maximise the effectiveness of the system and ensure that the Clients requirements were adequately achieved.

Griffiths control risk to site operatives during geotechnical works through implementing a Permit to Drill system. This requires a competent Griffiths geotechnical engineer to assess each section of site before work starts, identifying areas that may require further control prior to main drilling works. This may include scaling, targeted bolting, drape meshing and other control measures to minimise risk to staff.

All information is then recorded on the Permit and the work activities may be safely undertaken according to the specified sequence and hold points. Various high-risk rock outcrops were observed throughout the site with potential for rock fall during construction. These were safely controlled by the Permit to Drill process, ensuring operatives were kept safe at all times.

All drilling works were undertaken using long reach excavators mounted with pneumatic drilling equipment. Through careful planning of the available working width along the road verge when working behind the Varioguard barrier, Griffiths were able to specify the smallest possible plant required to reach all areas of site. Excavator-mounted drilling works eliminate the requirement for hand drilling methods and thus HAVS exposure to drilling operatives. These also have greater power output and therefore reduce overall drilling time and contract programme, ensuring that disruption to the general public through Traffic Management and Clients costs are minimised.

Griffiths roped access operatives fitting final terminal rock bolt and netting equipment, and tensioning cables on completion of project.

Griffiths implemented a strict quality control procedure for the drilling and netting works. This included setting out all rock bolts in accordance with the Designer, recording all drilling information in digital drill logs, documenting grout batches per rock bolt, undertaking grout testing for each batch, monitoring grout take and undertaking all required bolt testing as outlined in the design. All information was collated effectively for clear handover to the Client in completion of the works in the Health and Safety File.

All works were completed on time, to budget and within the pre-agreed road closure duration, thus ensuring that disturbance to the general public and road users was minimised.