Paddington Integrated Project
Paddington Station, London

The B400 Piling Rig used for the Large Diameter Piles

Crossrail is a major new cross-London rail link project that has been developed to serve London and the southeast of England. Crossrail will support and maintain the status of London as a World City by providing a world class transport system. The project includes the construction of a twin-bore tunnel on a west-east alignment under central London and the upgrading of existing National Rail lines to the east and west of central London.

The Paddington Triangle site is part of the Paddington Integration Project enabling construction of a new LUL station, upgrade of Paddington Central and the relocation of an existing taxi rank from Eastbourne road onto the Red Star Deck.

The Paddington Triangle site owes its name to the very restricted nature of its shape, being sandwiched between the elevated A4206 - Bishop's Bridge Road, the Paddington Basin Canal and Paddington Station.

The piled foundations for the project involved a secant retaining wall to the north of the site adjacent to the Paddington Canal, mini piles to support new structures above existing platform canopies and large diameter piles for the major new structures required by the Crossrail development.

The secant piled retaining wall consisted of 111 No 880mm diameter male and female rotary bored piled drilled to depths of 16.0m below piling platform level. Inclinometers were included in the construction of piles to monitor deflections of the wall.

The mini piles required were 135 No 300mm diameter piles drilled under low headroom conditions on the existing railway platforms and beneath roof canopies. The mini piles were drilled to depths of up to 17.8m and several were constructed with permanent casings to avoid load shedding onto an adjacent sewer. The extensive obstructions encountered were removed by rotary coring.

The large diameter bearing piles consisted of 26 No 1200mm diameter and 15 No 1500mm diameter piles drilled to depths of up to 55.3m below platform level.

As an added complication the large diameter piles also incorporated a requirement for plunge columns, to facilitate 'top down' construction, and full depth geothermal loops, to provide a renewable energy source for the future building.

Where plunge columns were required these were lowered through a special guide frame. The guide frame was fitted with external scissor jacks that permitted exact and rigid adjustment against the inside of the temporary casing. The tight verticality tolerances were achieved with the use of an optical laser plumb.

The requirement for full length geothermal pipework to the full depth of all of the large diameter piles was a major challenge for the piling project. At the time of construction, these were thought to be the deepest geothermal piles to be constructed in the UK.

Since the pile reinforcement was generally only 15m long the 4 No geothermal loops had to extend down another 40.0m to the toe of the piles with no rebar cage to locate them. The piles were ultimately concreted using full length tremmie tubes to protect the geothermal loops from falling concrete. A method of supporting the geothermal loops had to be developed to ensure that no contact between the tremmie and geothermal pipes could take place.

To demonstrate the chosen method of construction a full scale trial took place. Representatives of Crossrail and Mott MacDonald were invited to attend such that a thorough understanding of the process was assured.

A 1500mm diameter hole was bored in excess of 20.0m deep and the following procedure adopted...

1. A 1.0m high 'lobster pot' rebar cage was supported on the top of the temporary casing and the lower ends of the 4 No geothermal loops clamped to it.

2. This was then lowered down the bore and acted as a weight to extend the geothermal pipes down the bore.

3. At intervals of 3.0m internal centralisers were fitted to the geothermal loops to maintain them in position around and close to the bore perimeter.

4. To guide the geothermal loops safely down the bore a special guide frame was developed and positioned around the temporary pile casing. The geothermal loops were fed from 4 No power reelers over the guide frame and down into the pile.

5. This process continued until it was time to introduce the reinforcement cages. The geothermal loops were then bolted securely to the outside of the cages as they were lowered down the bore.

6. Attaching the geothermal loops to the outside of the reinforcement cages positioned them as close as possible to the perimeter of the pile and increased the thermal energy transfer efficiency to and from the ground.

7. The full scale trial ended with the geothermal loops being filled with water and successfully pressurised.

8. Once refined, this method was introduced into the construction of all working piles on site.

In addition, to validate the geothermal design of the piles a programme of on site thermal response testing was specified.

The principle of the test was to determine the amount of ground source energy available over a 10 month period.

The test involved one of the working piles becoming an instrumented test pile and 2 No thermal response boreholes being drilled immediately adjacent to it.

For the test pile, temperature sensors (thermistors) were attached to the pile reinforcement cages at 3 No locations around the perimeter of the pile and spaced at approx 6.0m centres down the length of the pile.

A fully automated thermal response test rig was used to monitor and record data over the test programme.

For further photos relating to this contract, download the complete case study PDF.