OWNER:
Sky
ENGINEER:
Fairhurst
GENERAL CONTRACTOR:
BAM
MAIN FIGURES:
5,176 Vibro Stone Columns (VSC) installed
Project Details
This development by Legal & General, Sky and NBCUniversal intends to create the most sustainable film and TV studio in the world. It will comprise 12 sound stages split across 8 buildings and is located in Elstree, just north of London. The structural frame of these buildings is designed to be supported by shallow pad foundations with SLS ground bearing pressure of up to 150 kPa, whilst a uniformly distributed load of 30 kPa will be applied on the ground bearing slabs. Performance criteria of 25 mm residual settlements, with relative differential settlements limited to 1/500, have been specified.
Ground Conditions
The site is underlain by London Clay, however the ground situation has been complicated by previous earthworks and construction, with a made ground area in the southern part of the project. This made ground, of depth up to 4 m, comprises clay matrix with inclusions of some bricks and concrete elements.
In addition, the site will be subject to a significant cut/fill exercise to bring the levels to a suitable development plateau, with up to 3 m of upfill that will generate settlements in the softer top layer of the London Clay. The presence of uncontrolled made ground and upfill requires therefore some ground improvement to ensure a sufficient bearing capacity and reduce the settlements, both total and differential, to acceptable limits. The characteristics of different soil layers, including the recently placed upfill, have been determined precisely thanks to an extensive Cone Penetration Test (CPT) survey on site.
Solution
In the areas of made ground and upfill, Menard designed and installed a solution of Vibro Stone Columns (VSC) to avoid piling and keep the same principle of conventional spread foundations as in the rest of the site. This brought significant benefits to the overall project, by reducing the carbon footprint,
accelerating and simplifying the construction sequence, and reducing costs. The VSC have been installed through the made ground and soft London Clay, at depth between 2 and 4 mbgl.
Beneath the slab, the VSC were arranged in a regular grid with centre-to-centre spacing of maximum 3 m, whilst 1 to 12 nos. of VSC were required beneath the pads, depending on the applied load. The detailed design of the ground improvement solution was based on calculations using the Priebe method, complemented by some Finite Elements calculations. The works have been completed between February and May 2021 with a single VSC
rig.
