The Back Den Bridge, Kildrummy Castle Estate | Restoration Case Study

Our role: Main contractor
Client: Kildrummy Estate
Architect: LDN Architects
Value: £0.5m to £1m
Programme: May 2021 to July 2024
Scope: Spandrel and parapet rebuild, deep packing, pinning, repointing, deck renewal, terrace walls

The Back Den Bridge is a single pointed arch in estate sandstone, 11.5m high at midspan, built around 1900 as a replica of the Brig o' Balgownie in Aberdeen. A section of parapet and spandrel wall had collapsed and the bridge was closed. For Kildrummy Estate we returned to a structure we first repaired in 2009, to make it safe, rebuild it and renew its deck, alongside repairs to the garden terraces at Kildrummy Manor.

Back Den Bridge: drone flyover of the repaired bridge

The challenge

The north east spandrel wall had bulged and shed a large area of facing stone, and the north west face was close to collapse. Once the scaffold was up, the south west face proved to have separated from its core: raking out revealed deep voids behind the ashlar, the outer skin had moved outward by at least 26mm and a matching gap opened in the deck surface over a single weekend. Work stopped while the structure was propped, monitored and a new repair sequence agreed with the engineer.

Our solution

Bulging masonry was held with emergency propping and drilled steel rods and anchor plates before being recorded and taken down. Fallen stones of up to 400kg were recovered by hoist for reuse, and around 130 sqm of spandrel wall and parapet was rebuilt with stainless remedial ties. Voided walls were deep packed in 3 sqm bays to depths of up to 400mm with NHL 5 lime mortar and stone and slate pinnings, then pinned back to the core with 750mm stainless rods resin fixed in mesh sleeves. Every stone was tap tested, with defective stones replaced in estate sandstone dressed in our workshop. The masonry was cleaned, then bedded in St Astier NHL 3.5 and pointed flush in Otterbein NHL 3.5 with local Aberdeenshire sands, following sample panels. The deck was taken back to its concrete slab, given new drainage and waterproofing and finished in Pennant stone setts.

Outcomes

Bridge made safe

A collapsed and moving structure stabilised, rebuilt and returned to the estate.

Walls tied back to their core

Deep packing and stainless pinning bond the ashlar skin back to the rubble hearting.

Original stone reused

Salvaged stones recorded, recovered and rebuilt, with new stone dressed to match.

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The Back Den Bridge