East side story

Shepherd Construction are well underway with The East Street, Leeds project on behalf of developer The Gateway Ltd as an important part of the regeneration of this area of Leeds.

The £135 million 'mixed use' development is made up of various buildings on a four-acre triangular shaped complex within Central East Leeds on the former Howarth Timber Yard site. The 74,765 sq m development consists of 640 apartments, a 215-bed hotel, 2,335 sq m of office space, 1,400 sq m of mixed commercial and retail outlets as well as a basement car park for 500 vehicles.

The heavily reinforced concrete framed buildings are being constructed over a basement. Some of the buildings are thirteen storeys high whilst a majority of the building cores are interconnected at different levels.

The foundation support level for these reinforced concrete framed buildings is located within a high water table zone, the site being only 4 m above the river level. The ground conditions are such that the overburden cannot provide the required support and the underlying rock is variable in both strength and depth.

The high compression, tension and horizontal loads require the buildings to be supported by Continuous Flight Auger (CFA) piles bored through the weak and wet gravely clay and into rock sockets in the deeper, solid mudstone.

Shepherds installed sheet piling to the perimeter of the site prior to a reduced dig thus allowing May Gurney to install their piles from basement level. They are also installing floodwalls to sectionalise the car park as well as drainage holes to allow any floodwater to drain away easily.

May Gurney have already completed the £642,000 Phase 1 piling contract in February 2005, and have recently been awarded the piling for Phase 2 with works ongoing at time of writing.

May Gurney's extensive experience of installing CFA piles in this type of ground was paramount, as alternatives such as pre cast driven piling were impractical within this residential area due to noise and vibration.

The main structural concrete 'Cores' required highly loaded piles; May Gurney installed 79 no. 900mm diameter piles to support a safe working load of 3000kn in these locations on both phases.

In Phase 1 the main structural column bases required groups of 4 no. piles per cap utilizing a total of 275 no. 750mm diameter piles to support a safe working load of 2400kn and the smaller 2 no. piles per cap, a total of 229 no. 600mm diameter piles to support a safe working load of 1700kn.

The site investigation report shows both soil strata and strength varying constantly throughout the site. May Gurney need to found the piles into the different thickness's of banded mudstone and sandstone to achieve the required pile loads. Constant monitoring of the strata encountered is essential, as is probe boring in advance of pile installation.

Generally the perimeter blank bores encountered hard mudstone at shallow depth where as the central bores found soft, deep layers of more weathered mudstone.

Once the founding strata had been identified then a series of Preliminary Piles were installed and tested, to confirm the load carrying capacity of the rock socket within that type of mudstone strata.

From the test pile results the CFA Pile diameters and lengths were determined and prior to installation a pile schedule agreed with the Consulting Engineers Connell Mott Macdonald.

Nick Sharp, May Gurney's Northern Area Manager, said:
'In the process of boring to form the rock sockets at refusal within the banded clay and mudstone a lot of friction and consequently heat was generated. The clay and mudstone became baked on to the augers, which slowed down the construction of the Test piles. This encrusted material had to be regularly removed using air driven jackhammers and a jet wash. Through experimentation this issue was overcome for the main piling by modifying a combination of auger teeth. "Successful piling in ground this variable depends on experience, sound geotechnical knowledge, good instrumentation, sharp eyes and an open mind. Nothing can be assumed," adds Nick Sharp "we are very comfortable in this environment".

The successful procedures set up by May Gurney and Shepherds in the first phase are now being repeated on the second. Albeit the site is smaller, with two large piling rigs working within a confined area. This is one of the main issues to be managed in Phase 2, a landlocked triangular site, boarded on two sides by dual carriage way and one-way system on the other.

As May Gurney progress and install the piles for this phase some 274 no. 750mm and 77 no. 600mm diameter, completed sections of the site are being handed back to Shepherds, allowing capping beams and column bases to be constructed at the same time as the pile foundations, drastically speeding up sub-structure construction

May Gurney's Project Manager, Darren Daniels, and Foreman, Mark Bunting, are working closely with Shepherd Construction's Project Manager, Steve Bulmer, to complete this value-engineered section of the project safely and on time.

Unlike the better known West Side Story there are no warring factions and no bitter disputes involved in this story, in fact Rob Rushworth, Shepherd's Senior Planner complimented May Gurney on how professionally they have performed over the course of the two phases and is confident that ground works will be finished in time, largely due to better outputs then planned.

May Gurney is scheduled to finish Phase II next month, with Shepherd Construction completing by Christmas 2008.