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Case study: Paddington Central, London

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One of the most difficult aspects of the Paddington Central site in London was access.

Sandwiched between the Westway, Regents Canal and railway lines emanating from Paddington Station, the old goods yard was destined to have an introverted nature that focused on a high-quality public realm design.

Through dialogue with the architect, Sidell Gibson Partnership, the central square evolved from a tranquil London square into a curved arena, an active social hub with retail and leisure facilities, and opportunities for organised events and impromptu street theatre.

Ron Sidell had worked at Brindleyplace and recognised the importance of making a major investment in the public realm at the outset. A feature avenue of mature lime trees, imported from Von Ehrens Nursery in Germany, was to become a major element of the scheme, with the intention that pedestrian access would not be hindered by raised planters or mounding to achieve soil depths. The landscape would flow between the buildings, as if on natural ground. The levels beneath the podium deck were to contain shops and restaurants, cafes and a leisure complex.

Services, plant and storage facilities were slotted into the voids underneath and between the large tree pits.

With the trees costing about £1,000 each, it was important that adequate preparation went into the design and construction of tree pits. The tree pits themselves were cast into the roof deck slab, each pair of trees sharing a single pit running perpendicular to the avenue. Due to the size of the imported trees (60-70cm girth, with root balls measuring up to 1m across), the tree pits were constructed 2.0m wide and 2.0m deep. This size accommodated a drainage layer of expanded clay aggregate (Optiroc), which was enclosed in a geotextile filter wrap. The surface finish of the avenue was to be laid to Cedec gravel, a self-binding path gravel which forms a cohesive surface when compacted and rolled.

This is normally laid on a load-bearing surface such as MOT Type 1. In order to lay the Cedec gravel bed beneath the tree avenue, the tree pits were backfilled with Metro-Sand. This material is laid and compacted in 150mm layers in order to achieve an even consolidation throughout the tree-pit depth. In doing so, considerable care had to be exercised so as not to damage the waterproofing membrane on the tree-pit walls.

In order to provide optimal growing conditions for the amenity turf laid in the amphitheatre and on the terraces, turf soil developed by the United States Golfing Association (USGA) was manufactured and laid to a minimum depth of 300mm for all turf areas. Poor drainage in turf areas rapidly leads to fungal diseases and a deterioration in the lawn quality, and the high sand content of the USGA mix ensures an optimal drainage regime, which encourages deep rooting. The use of high-performance turf soils requires a high level of irrigation to ensure year-round performance of the turf, and specialist advice was sought from Heron Irrigation Consultants.

A multi-region irrigation system was implemented at Paddington Central with fittings by Rainbird that provided drip irrigation to trees and shrubs as well as pop-up sprinkler heads for the lawn arena and terraces. Solenoid valve boxes were hidden from view beneath the Cedec gravel where they can be accessed easily by brushing away the surface and lifting the lid for inspection.

An automatic rain sensor was located on the roof of the cafe, where it is neither obtrusive, rain-shadowed nor vandal-prone.

One particular problem of providing irrigation in areas of hard water is limescale build up. Apart from affecting the performance of drip emitters, limescale can render geotextiles impermeable, resulting in the complete failure of drainage layers.

Historically, acid dosing was the only viable method of neutralising the effect of limescale, but the associated health and safety risk of delivering, handling and storing concentrated nitric acid on commercial schemes has seen a gradual decline in their implementation. Alternative systems for water softening, such as the base exchange unit supplied by Ocas and installed at Paddington Central, use natural salts to neutralise the effects of limescale in the header tank prior to being pumped through the system.

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