Medway Valley Crossing, Holborough

Key Information

Owner: Trenport
Engineer: WSP
General contractor: BAM Nuttall
Period of works: September 2014 – January 2016

Project description

The works consisted of the construction of a new access road and bridge across the River Medway east of Holborough, to allow development of the land. The scheme involved construction of the roads on embankments up to 9.0m high to allow access to the new bridge but also to elevate the road above the marsh land surrounding the river Medway.

Ground conditions

The soils beneath the site consist of essentially granular made ground overlying alluvium, consisting of very soft to soft organic clays and peat. This alluvium extends to depths of 8.0m and overlies compact sands and gravels to depths of 10m below which lies very stiff Gault Clay.

Solution

The solution involved a two stage process. Stage one involved the placement of vertical drains beneath the proposed embankments either side of the new bridge. The vertical drains were installed through the alluvium soils into the sands and gravels at depths of approximately 8.0 to 10.0m. Following installation of the vertical drains, the first phase of the embankment construction was initiated. This first phase embankment was built to heights of up to 5.5m and extended a significant distance beyond the proposed footprint of the road. The first phase embankment was monitored until the rates of settlements reached an agreed rate. After 12 months of surcharge, the rate of settlement reached an acceptable rate, following which CMC’s were installed to depths of 15m, beneath the footprint of the second phase of the embankment construction. Following installation of CMC’s the second phase embankment construction commenced up to heights of up to 2.5m.

Main Figures

Controlled Modulus Columns

Vibro Menard installed 1044 CMC’s to depths ranging from 11.5 to 14.5m.

Vertical Drains

Vibro Menard installed 18,511 vertical drains to an average depth of 7.3m.

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