DEEP SOIL MIXING has been used to combat soft soils on the Botlek rail tunnel project in Rotterdam in the Netherlands.
German contractor Bauer Foundation Engineering, part of the consortium of Ballast Nedam, NGT and Visser & Smit carrying out the ground improvement, suggested using the method instead of a three phase jet grouting process.
The Botlek tunnel runs under the Oude Mass channel in the port of Rotterdam. It replaces a lifting bridge in the harbour and forms part of a Netherlands Railways twin track freight line from Rotterdam to Germany.
Soil improvement was needed on a large section of the tunnel as the alignment runs through saturated soft peat and clay.
Engineer Holland Railconsult's design called for a post treatment stiffness modulus of between 20MPa and 40MPa and a maximum compressive strength of 5MPa.
After successful trials of the wet soil mixing method on site, main contractor BTC awarded the subcontract to the consortium, which began installing 4500, 800mm diameter soil-cement columns up to 23m deep in February this year.
Two fully automated MAT mixing plants and three drilling rigs fitted with mixing tools designed by Bauer were used.
Grout volume, mixing time and rate of penetration were monitored and recorded throughout the work and inclinometers used to check verticality of 10% of the columns. Samples were also taken for laboratory testing.
Soon after the rigs started work, heave occurred. To counteract this and to protect the rail tracks and the numerous services within the harbour, construction speed was adjusted to ground behaviour, which was constantly monitored.
Despite this, work was expected to finish ahead of schedule last month.