Balfour Beatty Civil Engineering Ltd

Sackler Crossing - Kew Gardens, London

Introduction

In 2004 the Royal Botanic Gardens at Kew commissioned the latest in a series of architectural interventions - a bridge across the lake. The aim was to create a contemporary landmark in its own right and an integral component of a new route through the gardens that takes visitors through previously under-used areas. The Sackler Crossing is the result, spanning the section of the route across water.

Design Concept

The Sackler Crossing plots a serpentine path over the water, the constantly curving route yielding its secrets gradually. The deck is set the minimum possible distance from the lake's surface, allowing visitors to feel they are taking a walk across the water. This sense is enhanced by glimpsed views of the lake between the deck treads and by the near invisibility of the supporting structure.

Clear visual connections are established between the bridge and the repeating natural forms of its setting - the gently rounded contours of the land, the smooth expanse of water and the powerful vertical lines of the trees.

A spare but exemplary material palette of granite and bronze reinforces the elemental character of the design. Rhythmic bands of dark granite form the deck, while bronze uprights emerge from the gaps between them to act as simple balusters, the top of each slender piece contoured to fit comfortably in the hand.

Viewed end on, the balusters appear solid, but from the side this solidity fragments, allowing views through and affording the structure a pleasing material ambiguity. Lamps in the deck preserve this transparency after nightfall.

Design Approach

The structure supporting the 70m-long crossing was designed to be sleek and unobtrusive by using a slender steel framework. The longitudinal hollow sections were curved in plan to enhance interest while enabling standardisation of components. Driven tubular piles in the lake bed are extended as columns above water level, acting as discreet central supports to the framework at 8m intervals. Each deck section was hot dip galvanized and painted for long term durability.

The 564 granite sleepers forming the walkway rest on pads above the longitudinal members and the 990 bronze balusters are bolted to each side using proprietary fixings.

The uprights forming the balusters are a key component and considerable care was taken to refine their design in order to satisfy safety and aesthetic requirements.

The finished quality was dependent upon achievement of accuracy in off-site manufacture and installation. An important feature of this project has been the workshops instigated to engage the whole design and construction team in accomplishing this.

Method of Construction

The contractor constructed the Sackler Crossing from a temporary causeway across the drained lake. This was prepared and surcharged in the pre-construction period to ensure it could support the piling rig and mobile crane.

The hollow steel piles were delivered in short sections so that delivery vehicles could negotiate the route to the crossing site without damaging any trees at Kew. The opportunity to reduce the length of the piles was realised through the involvement of the piling subcontractor and was subsequently confirmed by dynamic testing.

All the steel, bronze and granite components were fabricated off-site so the contractor co-ordinated deliveries to minimise disruption to the regular business of the gardens.

The bronze balusters were installed using a lifting beam suspended from a rough terrain forklift operating from the causeway. The 140kg granite sleepers were transported on a bogey across temporary plywood decking, laid to protect the superstructure. The sleepers were positioned using a bespoke lifting frame.

Removal of the causeway has to be timed to coincide with high tide so the lake can be replenished from the River Thames.

The completed work has achieved the very tight geometrical tolerances between the various materials, as detailed in the specification. The client is delighted with the completed structure, fulfilling as it does the architect's vision.

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