Ground Investigation for Commercial Development

Located in the City of London, the 50 Fenchurch Street development will provide a modern new development of over 62,0000 sqm of flexible office space incorporating innovative vertical green landscaping including a public roof garden and a winter garden.

The development will also include a new public realm situated around All Hallow’s Staining (a restored Grade I church tower) and Lambe’s Chapel Crypt, which will have improved public accessibility.

The ground investigation was required to provide geotechnical and geo-environmental data of the ground conditions to aid in the design of foundations, and to mitigate the risk of site contamination hazards.

Arup were engaged to fulfil the role as the Consultant Engineer on the project. Geotechnical Engineering Limited were appointed directly by BNP Paribas AM to undertake a programme of cable percussion boreholes up to 50m depth, hand dug pits with shoring, vertical and horizontal concrete coring through the basement floors and walls for sample recovery, and pitting/trenching to expose existing foundations in order to establish their arrangement and extent.

Dual Vibrating Wire Piezometers were installed to understand the pore water pressure profile and variability between the different strata within the boreholes. Understanding the pore water pressures would help determine the design for the piles. Gas/groundwater monitoring wells were also installed.

The geology of the site includes the Langley Silt over River Terrace Deposits, overlain by the London Clay Formation. The purpose of the deeper 50m boreholes was to determine the location and the nature of the Lambeth Group.

As would be expected with a large development in Central London, a major challenge of the project was the limited available space impacting the accessibility of both plant and materials. We were solely confined by the footprint of the site without any nearby parking or vehicle storage, so planning the works each day in an orderly manner was essential. As no works were being undertaken at ground level, all equipment needed to be navigated inside, through narrow corridors and down steps.

All investigation locations needed coring of the basement floor slab in order to penetrate the concrete, which was up to 2m thick in some places. Boreholes required just a single 300mm diameter core, but trial pits needed multiple cores stitch drilled in order to open up a sufficient area.

In addition to the main basement works, an additional borehole was requested inside the All Hallow’s Staining church tower to confirm the ground conditions for underpinning and stabilisation works. Given the exceptional national significance of the Grade I listed church, every stage of the borehole construction was planned and executed with conservation-led methodology, including precise skilled assembly of the rig in order to complete the borehole in a safe and efficient manner. The enabling works went far beyond basic protection measures and were designed to ensure that no part of the historic fabric, whether structural, decorative, or archaeological, was exposed to risk.

Fig 1: Operation of a core‑drilling rig undertaking precision stitch‑drilling to form an opening through a reinforced concrete floor in preparation for borehole construction.

A further challenge was the need for careful co-ordination with other parties present on site. At the time of the GI, there were three Principal Contractors already on site undertaking initial demolition works in various areas of the site, including a significant number of archaeologists undertaking careful archaeological investigation around the church tower and crypt.

To ensure the safe execution of the GI, GEL worked closely with the other contractors to align the works in a way that allowed all working crews to carry out their tasks safely and with minimal disruption. We were able to gain efficiency by working together to solve the logistical matters that were presented by being in a basement; For example, we had help from one of the other contractors as they drilled additional horizontal cores through the basement walls to allow us to move equipment into adjacent rooms to lower the manual handling risks of tackling the stairs.

Fig 2: Borehole drilling inside a Grade I listed church tower.

There was a great opportunity on this project for our senior engineering geologists to provide mentoring of the careful on-site logging of the London Clay and Lambeth Group to ensure all relevant detail was captured. The nature of samples retained using cable percussive methods meant samples were not continuous, so accurate logging of fine details needed to be emphasised. This precision logging assisted the Principal Engineer in being confident with their instructions to install vibrating wire piezometers at optimal depths to capture the required ground data.

The makeup of the trenches was different in every part of the building. Detailed logging, drawings and accurate dimensional measurements were required so that Arup were able to understand which areas could be reused post-development and maximise the reuse of the existing foundations.