Calcutta Road, Tilbury

Central Piling has completed a piling contract to enable essential homes to be built for residents in Tilbury, Essex.

The development at Calcutta Road is made up of four interconnected low-rise buildings that will provide 35 new flats for local older people as well as a shared courtyard and wild gardens and new community rooms for the residents and wider community.

The £9.9 million scheme is being built on a former brownfield site by Essex Developers Group and South Essex Housing Group. Central Piling was employed by main contractor Kind & Co to install 374 load bearing piles to support the residential blocks.

Ground conditions

The variable ground consists of a 14m layer of very soft to soft sandy clay and silt with alluvial peat bands, above a 2.9m layer of medium dense gravel. The underlying chalk bedrock is found at approximately 16.9m below ground level.

The groundwater table on the site is around 2m below ground level.

Solution

The scheme was originally designed to be carried out using continuous flight auger (CFA) piling but, having reviewed the ground conditions, Central Piling advised the client that driven precast piles represented a lower risk and lower cost solution. “Due to the layers of very soft peat in the top 14m, we were concerned about potential structural issues with CFA piles if the in-situ concrete slumped,” explains Central Piling Managing Director Steve Hadley. “The preferred alternative was driven precast piles.”

This change in the design did not affect the total number of piles but, as it is easier to ensure a good end bearing capacity in the gravel with driven piles, it did result in shorter piles compared to the bored pile option. The 374 piles are all founded on the very dense gravel at around 16m depth. An added benefit was that spoil disposal costs associated with this displacement method are minimal compared to a replacement technique.

The square section precast concrete piles were delivered in 8m long sections, and joined in situ. They consist of three different sections – 200mm, 250mm and 275mm – and were designed for compressive loads ranging from 125kN to 650kN.

Central Piling used a Junttan PM16 rig to install the piles during a period of four weeks, finishing well ahead of the agreed contract programme. The main contractor then constructed a combination of piles caps and 600mm section ground beams to support the building construction. Groundworks commenced in each area as soon as piling was completed.

Challenges

The site is very close to the River Thames, and the groundwater table is approximately 2m below ground level. However, it did not pose any problems during piling as the piles were being driven rather than bored.

The site is also in a busy town centre location, close to local residents and businesses. Central Piling installed noise and vibration monitoring equipment to ensure that trigger values agreed with the local authority were not exceeded. This equipment showed that noise and vibration levels were within the agreed limits for the entire duration of the piling contract due, in part, to the very soft clay and peat in the upper soil layer.