In 2018, a period of bad weather including very heavy rain caused the collapse of the retaining wall adjacent to a public footpath at Holbrook Church in Holbrook, Derbyshire. This Grade II listed heritage wall runs along the boundary of the parish church of St Michael, retaining the approx. 2.3m level difference between the road and the Churchyard.
CAN Geotechnical Ltd was awarded the design and build contract to carry out remedial works by Derbyshire County Council, acting as principle contractor, to repair the collapsed section of wall and strengthen the wall either side of the failed area.
The failed wall material was carefully excavated and the masonry set aside, and CAN then installed L shaped pre-cast concrete units along the line of the wall. The units were anchored into the underlying rock, with three vertical 0.75m length 20mm dia solid bar anchors installed through the base of each unit with 0.3m embedment into bedrock, and a 38mm diameter hollow bar raking tie installed through alternating units declined at a 30 degree angle, embedded 3m into rock. Adjacent units were tied together by steel tie plates and M12 rawl bolts, and the ground behind the units was then backfilled with Type 1 fill.
The masonry was recovered and re-used where possible as a facing to the concrete units, seamlessly blending in the repaired section of wall with the original sections either side. Further 38mm hollow bar raking ties were then drilled and installed through the wall either side of the failed area, ranging from 4m to 9.5m length in order to achieve a 3m embedment into competent rock, tying the wall back and preventing further instability. Bespoke fabricated patress plates were installed on each tie in keeping with the heritage nature of the structure.
Weep holes were also drilled through the wall, ensuring that water build up behind the wall would be avoided in future. Reinstatement of the surrounding area was carried out, including replacing top soil along the top of the wall and within the churchyard to allow a wildflower meadow to be sown.
The majority of the work was completed within a full road closure, providing a safe working area and space the plant required to undertake the work. The works were delivered on time, on budget and to the satisfaction of the client.
Duration: 11 weeks
Client: Derbyshire County Council
Location: Holbrook, Belper