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Technical: Olympic Soil

STEPHEN FLOYD
Creating a park on a polluted site called for large-scale remediation and the creation of some carefully targeted manufactured soils.

Some 2.2 million cubic metres of soil were excavated during the construction of the Olympic park. onsidering the wide ranging contaminants on site, it was very impressive that 80 per cent of this material was reused in the park construction, along with 98 per cent of the building rubble from demolition of the former works. 

Given that landfill charges for contaminated spoil may run at rates exceeding £100 per tonne plus haulage, and that this figure equates to maybe 150,000 truck loads of soil, the pragmatic value of keeping the material on site is overwhelming. The value is both financial and environmental, one of those happy occasions when doing the right thing brings monetary rewards.

This wide availability of cut and fill has been exploited to rich effect across the park, with the new rolling topography offering rich and dynamic environments. The early involvement of landscape design in the engineering solutions of the park was vital. It contrasts with the occurrence too often of landscape architects being detached from engineering and soil science, and so missing opportunities in the design. Too often we work at the surface; the contamination at the Olympic site forces us to dig deeper.

CLEANING UP
This site has a long and dirty history. Heavy industry, illegal tipping, chemical works and the attentions of the Luftwaffe had left a truly brownfield site. Contamination was extensive in both the soils and groundwater. Some materials could be reused immediately, such as crushed concrete in the gabion walls. Most required some treatment and careful management.

The result was Britain’s largest soil washing operation. ‘Soil hospitals’ were set up on site. Using physical separation techniques, these plants processed 40% of the material excavated. The soil hospital produced three main products: a sand, a gravel and a filter cake. The filter cake was the residual fine-grained fraction of the soil matrix and it is these fine particles that tend to hold contaminates within the soil.

The soil hospital therefore had two functions. Firstly it removed a high percentage of contaminants, and secondly it provided graded aggregates that were a useful and known product for use in further construction. The sands and gravels could then be blended to spec and used by contractors for varied construction tasks.

After much testing, analysis and experimenting with the aggregates available, it was realised that due to a variety of factors, and despite the cleaning, the aggregates from site would not be suitable components for use in the manufacture of topsoil or soils in contact with the surface layer. One of the major problems was caused by asbestos,since its fibres are nearly impossible to remove from soils completely.

In addition, many of the aggregates were simply not suitable for the planting. The industrial history meant that much of the ‘soil’ in fact consisted of construction rubble and would therefore have had an unsuitably high pH value and lime content for planting. The solution to this was to lay a marker layer above the landscape base layer and to import soils for all planted and trafficked areas. This was known as the living layer or human health layer.

MANUFACTURED SOILS
Because of the constraints, the entire surface layer at the Olympic park consists of manufactured soils. Soil specialist Tim O’Hare Associates designed nine soil specifications specifically for the various planting habitats and soil functions in the Olympic park. Tim O’Hare has written about this in some detail as part of the Olympic learning legacy. See www.learninglegacy.london2012.com 

‘People often think about soil specification only in terms of suitability for planting but soil needs to provide so many other functions,’ said O’Hare. As part of the Olympic Park’s Sustainable Urban Drainage System (SUDS) design, the landscape soils (topsoil and subsoil) function as drainage routes, filters and slow-release reservoirs, largely replacing conventional land drains on site. The soils form a capping layer over contaminated ground. ‘If the landscape soils had not been introduced the remediated land beneath would not have had a suitable specification and would have had to be removed at great expense,’ O’Hare said.

The subsoils are also an essential part of the soil profile for plant establishment providing support and water attenuation. Finally the manufactured subsoil provided an ideal working surface for vehicles and plant during the construction progress.

Manufactured soils can also have impressive green credentials as they can be manufactured from waste products from other industries. Here the general purpose topsoil is a blend of quarry overburden and green compost. The quarry overburden comes from a site in Kent, and provides the mineral component of most of the soils. It is a waste product from the quarrying process and has a suitable range of particle sizes to act as a base for the soil. It was screened to remove unsuitable sizes.

Green compost is the magic ingredient for many of the manufactured soils. It not only provides nutrients but also contributes to particle aggregation, water retention and provides a whole wealth of living soil biota which, with the rich organic matter, provide a true soil ecosystem. Whereas fertilisers provide temporary doses of nutrients, compost-based manufactured soils provide a complex soil matrix and a sustainable soil environment that should look after itself without future maintenance requirements. ‘It was a real pleasure returning to the site after a growing season and smelling the soil,’ said O’Hare. ‘The soils are already darker and producing their own humic acids. It is then you can see that what you’ve started is working.’

The grassed areas and structural tree soils were constructed with a high content of narrowly graded sands. The consistent particle size ensures a soil structure that can be compacted without removing its ability to translocate air and water, therefore allowing the growth of roots in areas requiring compaction for excess foot-trafficking or hardstanding.

Another interesting soil element is fibreised compost. This is produced from the same stream as green waste but from oversize woody material. ‘It is specially processed into a matrix of organic fibres,’ O’Hare said. ‘The material demonstrates a good available water-holding capacity, high air-filled porosity, high organic matter content and low levels of plant available nutrients.’ This is particularly useful for areas such as the species-rich meadow as it allows the creation of a functioning but low nutrient topsoil without reliance on peat.

It is common with manufactured soils to mix aggregates on site but on the Olympic park all soil was manufactured off site. The main advantage was that it allowed a very constant product to be created. Anyone who has stood with a digger driver between two huge piles of brown material in a field will understand the difficulties of getting a proper mix on site.

While soil specification was freed from the limitations of site materials this is not to say that they were used arbitrarily. The park’s design is an example of form following function and the soils follow the functions, opportunities and restrictions of the site. Meadow planting provides an attractive, diverse and low maintenance ground cover. Wetlands respond to water management, structural soils support spectator areas. The landform seamlessly holds together these functions managing water flow, framing views creating loose amphitheatre and providing the basis of this original landscape.

Such damaged sites provide the designer with a unique mix of problems and opportunities. Contamination forces us to dig and this in turn forces the consideration of landforms and water systems. As we have started with no soils and a derelict site there is a certain freedom. We can choose the ground, shape it to provide services and an amenity, take full advantage of the possibilities at no additional expense. These magical trade offs are however only available to those who know of the options and who can see the possibilities. Green infrastructure increasingly justifies landscape design and rightly so. The designer must be informed of technologies to be able to take advantage of them.

Stephen Floyd is a landscape architect whose practice, Forestopolis, was responsible for the landscape of the Pleasure Gardens.

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