OWNER:
Highways England
ENGINEER:
Arup/Mott MacDonald
GENERAL CONTRACTOR:
Galliford Try
MAIN FIGURES:
c.9,350 CMC & 11,300 PVDs
Project Details
Menard was asked by Galliford Try, working on behalf of Highways England, to provide a robust ground improvement solution as an alternative to piling, to support the embankments of a proposed new motorway junction on the M49 at Avonmouth. The new junction is intended to provide economic growth to this area of Bristol. The project involved the construction of 4 new approach embankments and a gyratory including a new bridge over the M49 and the use of the existing Farm Lane bridge. The new embankments ranged in height from 1m to 10m from existing ground level at a unit weight of 20kN/m3 plus carriageway loading. The new embankments covered an area of approximately 29,250m2.
Ground Conditions
The site investigation showed that ground conditions consisted of cohesive made ground fill material down to 1-1.5m deep, this was underlain by soft alluvium with pockets of peat (CPT cone resistance of as low as 0.35MPa) down to depths ranging from 8.5m to 13.5m with mudstone below.
Solution
Menard provided a solution comprising Controlled Modulus Columns (CMCs) in combination with Prefabricated Vertical Drains (PVDs) as an alternative to piling, allowing the new embankments to be constructed as reinforced earth embankments with a slope angle of 70 degrees with no need for a traditional Load Transfer Platform. A load distribution mattress was installed between the CMCs and underside of embankment, which was made of 600mm of granular material; the working platform for the CMC rigs was reused for this.
Menard designed the CMCs using finite element modelling to limit settlements to 25mm directly behind the bridge abutments and 100mm over the remainder of the area with differential settlements limited to 1/250.
A total of c.9,350no. 360mm diameter CMCs were installed at an average depth of 9.5m (max 13.5m) with an anchorage of 0.5m in the weathered mudstone layer. The columns were installed over two visits to site using up to 2no CMC rigs. In addition to this c.11,300 PVDs were installed over the two visits using a single PVD mast. Over the project, extremely high production rates were achieved. Due to the use of CMCs the project benefitted from substantial cost and programme savings.
