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Getting civil with structures

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To AHA the process of design and construction is a partnership between architect, structural engineer and civil and services engineer. The nine-strong team of civil engineers at the Cirencester headquarters originally evolved from a need to give support to structural projects in terms of roads, drainage and external works, but some projects have developed architectural/ structural/civil collaborations that have been very creative.

One such scheme is the Eden Project, the botanical garden and educational centre in a former china-clay quarry near St Austell in Cornwall. The drama of its structure contributed to its huge popular success - it had queues of more than 3,500 visitors a day even when under construction. Nicholas Grimshaw & Partners and AHA were originally approached by Eden on the basis of its glass roof at Waterloo International Terminal, but the need to hug the contours of the quarry and to consider environmental issues resulted in a very different solution.

The first phase consisted of two enormous 'biomes' - climate-controlled greenhouses - one of which recreates a rainforest environment and one the Mediterranean. Flanking the steep-sided walls of the quarry, each biome is made up of four joined geodesic domes, of which the largest is 110m across and 45m high. The domes are constructed of lightweight tubular steel hexagonal frames interlinked with arches. The largest of the hexagonal planes spans 11m on a framework of steel tubes only 193mm in diameter, creating a light 'spider's web' structure. The domes are clad with translucent ETFE foil cushions, providing maximum light and UV transmission.

The position of the biomes within the steep-sided quarry walls was determined by AHA civil engineers; a threedimensional model of the site topography was combined with a model mapping the orientation of the sun. The quarry walls slope down to form natural terraces at the internal sides of the biomes, creating a dramatic backdrop to plants. More than 850,000m 2of excavation and fill were required. Three-dimensional modelling techniques were also used in the design of access roads, parking, foul and stormwater drainage, slope stabilisation and visitor access routes.

The Visitor Centre, adjacent to the biomes, consists of a pair of single-storey buildings, linked by a fabric roof, which form a sweeping curve to fit snugly against the side of a quarry bank. One of the walls is made of rammed earth with the shuttering lift lines clearly visible; another is flanked with gabions filled with crushed rock taken from the site - an example of zero embodied energy. The steeper slopes of the quarry have been stabilised with innovative earth walls reinforced with geotextile; other slopes have been stabilised with hydro-seeded grass.

AHA is now designing the next phase of the Eden Project. The Dry Tropics Biome consists of a delicate 110m-diameter roof in the form of a hyperbolic parabola; it rests lightly on top of a series of curved terraces that have been carved out to take the shape of an inverted bowl. These massive earthworks contrast with the light roof - a steel truss and two-way cable-net clad with ETFE cushions. The Educational Resource Centre is similar in concept but the roof is made of a glulam-beam and ply-panel lattice which spirals from the centre in a sequence based on the Fibonacci series.

The new visitor centre at Stonehenge, won in competition by architect Denton Corker Marshall, has involved AHA in the coordination of structural, civil and transport engineering skills.

The building is set below ground level to avoid intruding into the historic landscape and to maximise energy conservation; it is only visible as a series of curved fins lying above a grass-covered roof. A transit link takes visitors from the centre to within walking distance of the stones.

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