Case Study: Clifton Suspension Bridge, Bristol

The Clifton Suspension Bridge was designed by Isambard Kingdom Brunel and is a Grade 1 listed structure. It remains a testament to 19th Century engineering, and one that still retains a few surprises for the 21st.

The sandstone abutment supporting the 26m high tower at the Leigh Woods end of the bridge was found to house a honeycomb of 12 vaulted chambers. Arranged in two tiers – an upper tier of 7 chambers and a lower tier of 5 – they are interlinked by narrow tunnels and shafts just 0.6m in diameter.

The purpose of the chambers is unclear, as original plans for the bridge held by the University of Bristol do not show the design of the abutment. However, with each chamber measuring on average 11m high by 15m long – the largest being 17.25m by 5.6m by 10,8m high – they would have offered a considerable saving in material.

The problem of gaining access to these chambers was solved by Cintec International and its patented structural anchoring technology to stabilise and reinforce the new opening.

Cintec anchors are more commonly used to reinforce arch spans in bridges and viaducts in the form of a dedicated system called Archtec. However, Cintec anchor technology was more than capable of meeting the unusual application at Clifton and it is approved by The National Trust, English Heritage and Welsh heritage body Cadw for the restoration of all types of listed structures.

After engineering surveys confirmed that the abutment was structurally safe, work began on forming a permanent door for maintenance access.

First, an exploratory core was put through the abutment wall in order to establish the form of its construction.

The wall was found to be solid with an overall thickness of 1,800mm. It comprises two sandstone skins – the outer measuring 600mm wide, the inner 300mm wide – with lime mortar in between.

Then, in spring 2003, work commenced to form a door way 12m below the level of the footway, approximately halfway down die abutment where the wall returns to tie into the side of the gorge.

The work began with the stitch drilling of 70 holes to a length of 1,800mm, each with a diameter of 102mm, in order to create an opening approximately 2,000mm high by 830mm wide.

Following this, 20 Cintec stainless steel rebar anchors were used to pin together the external and internal sandstone blocks cut through by the opening. These 16mm diameter solid circular section anchors, measuring 1,500mm long, were installed at an angle and at 300mm centres around the doorway.

The anchors were inserted in 40mm diameter predrilled holes, oversized to accommodate expansion of the anchor sleeve with grout.

In order to maintain the aesthetics of the bridge, the anchors were set into the opening by 200mm to ensure they would not be visible on the external sandstone face. This achieved a sympathetic invisible ‘mend’ on the listed structure.

While pinning the adjacent blocks together, the close spacing of the Cintec anchors effectively created a bond around the new opening. This eliminated the need to bond the sides of the new opening using in-situ concrete, which would have required additional time.

The work was approved by English Heritage as well as the local planning authorities, Bristol City Council and North Somerset County Council.