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Theme: landscape and exterior products

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Landscape architects are increasingly required to work with ground conditions that are not 'natural'. But how does one approach a landscape design when the ground is completely man-made, or so heavily contaminated that it must be stripped of its original character? The following case studies, designed by Gustafson Porter, present solutions to those problems and demonstrate how landscapes can be created to live and thrive on man-made ground

THE WESTERGASFABRIEK CULTURE PARK, AMSTERDAM In the Netherlands, much of the landscape has been engineered by man, and constant intervention has produced a dry but habitable surface.

The design for a new landscape for the Westergasfabriek involved working over and remediating contaminated ground to create a Culture Park. In this context, a heavily engineered solution seemed the most reasonable option. The polder system of flat terraced surfaces from which water is pumped from a low to a higher series of water tables, and then eventually out to sea, is the most natural and utilitarian expression of a need to inhabit dry ground.

The initial competition-winning scheme, entitled 'Changement', presumed that the polder aesthetic could not be challenged, and that the creation of the park spaces would be defined through planting and changing use. However, the strict rules that govern the remediation of contaminated ground meant that no polluted soil could leave the site, and that a varying depth of soil would be required to distance the park surfaces from the contamination. Spaces would need to be defined by means of an undulating landform.

With the majority of the landscape's construction budget spent on cleaning and remediation of the polluted ground, our brief was to use existing traits, including soil, materials, resources and water from the surrounding polder, to create a refined, sustainable and flexible landscape.

Soil, sand and greenery To achieve a cut-and-fill balance and ensure that no polluted soil left the site, an undulating, sculpted topography emerged. We chose varying depths of imported soil and sand according to the character of the finished surface. Where planted surfaces defined volumes of open space to either side of the broadway, we used excess polluted soil and greater depths of imported soil to raise the ground away form the saline water table.

Where surfaces link with water in the Water Gardens, on the canal promenade and the events field, soil was removed to the amphitheatre mound that protects the park from the railway to the north.

Breedon limestone and Welsh slate-bound gravel characterise the broadway and Water Gardens pathways. In these areas, a rich variety of plants supplied by Van den Berk Nurseries (based in the Netherlands but with offices across Europe) creates diverse habitats that evolve from the formal picturesque 19th century context of the historic Westerpark to the west, to the more natural plantings of the ecological zone adjacent to the polder in the east. To give the landscape a sense of constructed character in these areas, bands of light-coloured granite define the ridges of high ground. We planted grass to create the events field, sculpture plain and mounded amphitheatre, which was reinforced with Hard surfaces The unstable ground conditions that characterise the reclaimed marsh, on which Amsterdam and the park are built, require paving materials that can adapt to the moving ground. We used smaller-scale reclaimed bricks and basalt setts along the canal promenade and around the perimeter of the existing buildings. In contrast Stelcon, a 2 x 2m concrete paving material laid loose on sand, was reused in the context of the historic brick-built gas works, allowing truck access to the buildings and giving the streetscape its contemporary edge.

We used new materials for hard surfaces to create a contemporary form. Chinese granite paving on a lean mix concrete sub-base was used on the central axis, and Kilkenney Irish limestone paves and clads the beached and vertical edges of the water basin that creates the north western boundary to the events field.

Water water everywhere? Water pervades all areas of the park. Where it appears most natural, in the water gardens and the gasholders, it is in fact laid over a manufactured clay lining to ensure that it does not mix with the contaminated water below or, in the case of the gasholders, the most contaminated soils found on the site.

Here the water circulates through reed beds, creating the correct conditions for marginal and aquatic plants. The water in the events lake and the water required for irrigation purposes (or to top-up the closed systems of the new water features) is diverted from the adjacent polder - a sustainable source that reduces maintenance costs.

The Westergasfabriek has been a large undertaking for all those concerned. Where a budget was not immediately available, it has had to be found during the construction process. The park opened to the public in September 2003 with construction still under way and work is expected to finish later this year.

Netlon fibres to allow truck and pedestrian access to major events.

COURTYARDS OF THE GOVERNMENT OFFICE, GREAT GEORGE STREET, CENTRAL LONDON A much smaller project that involves building a landscape on man-made ground is the newly completed East Courtyard Garden of the Government Offices on Great George Street (GOGGS) in London - the Grade IIlisted building housing the Treasury as well as the offices of Customs and Excise. Within the block sit three internal courtyards connected by a major pedestrian axis linking Whitehall with St James' Park. In 1999, Foster and Partners first approached Gustafson Porter to design the West Courtyard of the Treasury building after their successful collaboration on the National Botanic Garden in Wales. The principal axis, or causeway, connects the grand formality of Whitehall with the less formal St James' Park, and was the starting point for the design of the 30m 2 courtyard. The existing space was a former light well, sitting on top of the Cabinet War Rooms. Formed from slate, the undulation in the ground plane creates a dished surface in the heart of the space, which appears to catch the rain and creates a reflecting pool.

In 2002, Foster and Partners began the renovation of the eastern half of the GOGGS, which included the East Courtyard (to be surrounded by offices for the Inland Revenue and Customs and Excise). Although similar in scale to the West Courtyard, the East Courtyard has a prominent semicircular protrusion into it, housing the main staircase.

The space was originally used as a light well with a roof slab over a basement area located approximately 2m below ground-floor level, allowing light into the sub ground rooms.

The courtyard, although at ground level, is effectively a roof garden.

Sculpting London's Chateau Villandry The conceptual starting point for the East Courtyard was a sense of geometric and architectonic formality, in contrast to the West Courtyard's more informal character.

An image of the geometric classical gardens of the Chateau Villandry in France inspired a contemporary interpretation of the classical parterre. The plan form of the East Courtyard maintains the east-west axis and divides the courtyard into four quadrants with an expressed cross axis. The main axis is flanked by yew hedges, which are sculpted in an arc from 600mm to 1,450mm tall. This sculpting of the hedge allows controlled views down into the more intimate gardens, and forms a dark green border, contrasting with the lower boxwood hedges.

The ground plane is subtly manipulated to control the viewer's perspective as one moves through the various spaces. The boxwood hedges form a geometrical pattern in plan, and maintain a constant datum which exposes the subtle change in ground level. The northwestern corner houses a café that receives good sun exposure, allowing outdoor tables and chairs. This quadrant is paved in slate and is dominated by two semi-mature trees. A library is connected to smaller, enclosed garden spaces with wide benches integrated into the boxwood hedges for reading and sitting on the southern side of the courtyard. The gardens are planted in bands of colourful perennial planting and ornamental trees.

Waterproofi ng challenge Demolishing the existing garden slab and creating a new one allowed for the 2m-deep tree pits, as well as hedge pits and sufficient planting depth for the perennial planting.

We raised the slab levels for the hard paving areas as high as possible to minimise the finished build-up, and therefore cost. The resultant 'ground' level was an undulating slab of varying depths that was then coordinated with the necessary requirements for drainage and irrigation.

Waterproofing the ground slab proved a challenge, especially when building water features over the top of the plant room, which housed not only the mechanical plant for the adjacent offices but also the water storage tank and treatment plant for the water feature. The water rill was designed as a slate-lined rill with a relatively shallow slope, allowing the water to flow gently downhill before dropping into a chamber.

The Treasury renovation is a PFI project and project manager Bovis Lend Lease suggested using GRP (Glass Reinforced Polyester) to simulate slate, rather than build a traditionally waterproofed slate-lined rill.

Initially, both we and Foster and Partners were skeptical about using 'artificial' slate but we were convinced after seeing full-sized mock-ups of cast 'U' sections of the rill, made using natural slate as a mould. The colour and texture of the natural compared to the man-made slate imitation was surprisingly close; apart from the fact that the GRP does not absorb water or turn noticeably darker when wet, the two are almost undistinguishable. The advantage of casting the rill in 2m-long sections was that the whole thing could be completed off site and delivered supported by adjustable steel sections.

'Babbling brook' One of the design ambitions for the East Courtyard was to explore movement and sound in a water feature, which took the form of a 1m-wide water rill. The water enters the courtyard at two 'sources' integrated into the hedge planting, just below existing windowsill height on the eastern side of the building. From here it tumbles into the rill and flows gently along the perimeter of the building, turning at 90° and dropping into a 'sound chamber' which amplifies the sound of falling water.

Visitors entering the courtyard cross a bridge, where the sound of tumbling water adds an aural dimension to the garden. We worked together with fountain specialist Ocmis to develop a sound chamber which would simulate the 'babbling brook'. Ocmis built a full size mock-up of the sound chamber in its Somerset workshop. Water was pumped into a full-size channel and the depth and width of the chamber were tested for the desired sound.

Integrating trees and hedges The tree species are all shade tolerant and are derivative of a woodland-type environment where typically, there would be low-level plant growth on the ground. The trees are all deciduous and have interesting bark textures and autumnal colours, with small and well-defined leaves allowing a dappled light to fall on to the surface of the courtyard. This is in contrast to the West Courtyard, where we chose flowering woodland trees. The planting of the courtyard is a perennial mix of subtle blues and striking white bands of colour which define the edges of the rill and reflect the building, planting beds and sky.

Two rows of tall, sculpted dark green taxus (yew) hedges line the principal eastwest axis walkway, controlling views and directing pedestrians through the space.

Smaller buxus (box) hedges frame the individual garden spaces and create a low level datum against which the subtle undulations in the ground plane are emphasized.

The construction of the courtyard garden, like most landscapes, came at the end of the building construction programme. The courtyard housed the main cranes for lifting heavy material from the street into the building. Because of the programme, there was enough time for the hedges to enjoy one full growing season at the nursery. The sculpted hedges were contract-grown in a field at the Arbor Nursery in Belgium. We identified individual shrubs at the nursery, which were then tagged and grown and pruned to specific sizes one year before being delivered to site. The tallest specimens were more than 1.6m high and the shortest approximately 450mm high. They were pre-shaped so that when they were brought to the courtyard for planting the basic curved shapes were already formed and the effect of an 'instant' architectural hedge was created.

Mary Bowman and Neil Porter are directors of Gustafson Porter

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