Geotechnical specialist conducts mini piling works for high profile city centre office development

The typical intrusive rock, which makes up most of the geological formation of Edinburgh city centre, has always provided a solid foundation for the city’s buildings. However the combination of a rugged landscape and solid rock formation can sometimes prove to be a challenge in the initial stages of construction. A recent example is at a high profile city centre office development where Bachy Soletanche Limited has recently completed a quarter of a million pound mini piling project on a site with a 20m solid rock drop.

Bachy Soletanche Limited’s Scottish Division Engineer, Paul Doyle explained:

“Although the site looks level from above ground there is in fact an extremely steep rock face beneath ground level. So at one point the rock is just below ground and ideal for normal foundations, but within a few metres the rock has dropped to a depth of 20m, losing its load strength. We were drafted in to create a consistent bearing capacity across the entire site.”

Designed by Kilmartin Property Group, the office development named ‘The Cube’ will provide 80,000sq ft of Grade A office space in a prime city centre location. This is on Leith Street which is just east of Edinburgh’s most famous thoroughfares, Princes Street. Furthermore, the building itself is specially designed to maximise daylight and ventilation, but minimise heat loss which will be managed by integrated, moveable, full height screens - providing the building with an excellent BREEAM rating.

In order to progress with the project, ground works had to be conducted first and foremost. The initial thought and design was to use the Large Diameter Rotary Auger (LDA) technique due to its high torque and performance that would power through the ground and into the toe of the rock. However, when Bachy Soletanche was asked to tender for the project, the design team suggested another solution using the mini piling technique. The main contractor, Sir Robert McAlpine, deemed this an excellent alternative, as it was ideal for the limited, city centre space and quite flexible in regards to where the piles should be placed on site. It was this design that won Bachy Soletanche the ground works contract.

Bachy Soletanche used a mini piling technique quite similar to the LDA process but on a smaller scale. As such, a 273mm diameter temporary casing was installed down to the rock head at 20m. Then a core was drilled out using a 240mm auger just beyond the temporary casing in order to use the rock as the toe of the pile. Next, a reinforcement bar was installed into the pile which was then filled with grout up to the surface. Once the grout was placed, the temporary casing was removed.

The overall project included 50 mini piles all drilled to an average depth of 22m with each holding a load bearing capacity of 1,400kN. Although more piles were required to create the same load capacity as just 20-25 LDA piles, this option proved to be the better alternative. For instance, the rigs were smaller so could move about the site without difficulty, making logistics easier on the already restrictive area. It also proved to be a more flexible option as the load could be spread across the site more readily; unlike LDA piles that have to be placed in set positions or deemed ineffective.

“This flexibility of the mini piles proved vital to the project as there was many underground obstacles undetected within the ground profile of 10m of made ground and 10m of sand before reaching the rock bed. Sometimes the mini piles could bore through these, but other times it was difficult. When this occurred we just moved the pile to a slightly different position which saved a considerable amount of time or redesign.”

Bachy Soletanche was on the site over six weeks throughout May and June. The final completion date of the office is expected to be October 2009.

Email: fiona@manifestcomms.co.uk