OWNER:
Ocado
ENGINEER:
Fairhurst
GENERAL CONTRACTOR:
Winvic Dixon Construction
MAIN FIGURES:
c.3,400 CMCs
Project Details
Winvic approached Menard to find a cost effective ground improvement to the external yard and car park areas of a proposed, large Ocado development which spanned over an existing landfill in Purfleet. With around 25,000m2 of external slabs and car parking spanning over the landfill area a solution was required to ensure total settlements remained below 75mm with differential settlements limited to 1/300. The large amount of drainage and interceptor tanks to be constructed beneath the slab also had to be considered in any ground improvement proposal.
Ground Conditions
The site investigation reports indicated made ground underlain by landfill material with the presence of soft and very soft organic material to depth of up to 8m with dense sands and gravels below. The site investigation included Cone Penetration Tests giving parameters of the landfill material to be used in a detailed ground improvement design.
Solution
Taking into consideration the depth of landfill material and the presence of very soft organic and degradable material, Menard proposed a ground improvement solution using Bi-Modulus Columns to support the external concrete slab and tarmac car park areas catering for UDLs of up to 10.5kPa. The Bi-Modulus Columns were made up of a 280mm diameter Controlled Modulus Column topped off with a Vibro Stone Column head. The incorporation of the flexible Vibro Stone Column heads meant that drainage could be easily installed at a later date by the Principal Contractor as they were able to dig directly through the upper flexible element. The concrete section of the columns were embedded around 0.5m in the dense sands and gravels with the depth of the stone column head generally at 2m deep which was locally varied to suit the different levels of drainage runs on the site. This solution enables the Principal Contractor to construct a regular ground bearing concrete slab and also use flexible tarmac surfacing in the car park areas. The design for the ground improvement was carried out using Finite Element Models to ensure that the settlement criteria was met including checks on differential settlement at the outer limits of the landfill area.
3,400no BMCs from 3m to 9m deep were installed over 3no. visits to site fit in with Winvic’s construction programme.
