Vertical Wick Drains - Red River Floodway
LOCATION: Winnipeg, MB TIMEFRAME: July 2007
SCOPE OF WORK: Installation of vertical wick drains for construction of railway embankment across Red River Floodway.
PROJECT PARTNERS: Contractor: Hugh Munro Construction Ltd. Supplier: Layfield Environmental Systems
Prefabricated Vertical Drains Used to Speed Consolidation of Red River Floodway Embankments
Winnipeg, Manitoba - In July, 2007, Layfield Environmental Systems completed a prefabricated vertical drain project for the Red River Floodway Expansion project. The prefabricated vertical drains (also known as PVDs or wick drains) were installed in preparation for the construction of railway embankments for a new bridge across the Floodway. The drains will accelerate the consolidation of the clay subsoil under the embankment allowing the bridge project to proceed on schedule.
The two embankments, one on either side of the Floodway, are part of a railway bridge embankment that is part of the Red River Floodway Expansion project. The PVD drainage area will eventually be underneath 1,200 meters of railway track and cover an area of about 32,000 m2. At this site there is a 20 m layer of glaciolucustrine silty clay over silt till on top of limestone bedrock. Consolidation of this thick clay layer takes a long time unless additional drainage is added.
The Geotechnical engineer on this project, Faris Khalil, P.Eng, of UMA/AECOM, indicated that on this project that a substantial percentage of subsoil consolidation needed to be completed within the available two year construction time. The design objective for this project was to have 50 mm of post construction settlement. Usually the options in this part of Manitoba were to use staged construction, vertical sand drains, or PVDs. The choice of PVDs was based on cost, short installation time, and less interference with the construction process.
The concept behind Prefabricated Vertical Drains (PVDs) is to install a matrix of drainage channels into the subsoil under the embankment to reduce drainage paths. Typically PVDs are installed in a grid pattern with a spacing range between 1.5 to 2 meters center to center depending on the compressible layer thickness, the waiting period available, and the loading condition. The PVDs are installed into the ground for the full or partial depth of the compressible layer depending on the design objective. In this case the depth was 20 meters which was the depth of the clay layer under the site. Over 130,000 lineal meters of wick drain were punched into the subsoil beneath the proposed embankment.
A granular drainage mat was provided to provide a drainage path for the water that comes up from the PVDs allowing the water to drain to the sides of the embankment area into the side ditch. The embankment is then built on top
of the drainage mat. As the embankment grows the pressure on the matrix of soil and PVDs increases and the water begins to flow up the PVDs, into the drainage layer, and out to the side ditch. The drains are designed so that consolidation will occur within a suitable construction time frame. On this project the embankment will be as high as about 6 m and the maximum associated settlement is estimated to be in the order of 450 mm.
The prefabricated vertical drains (also known as wick drains) are a composite material supplied in rolls 152 m long. The drain consists of an inner core made out of corrugated polypropylene which is wrapped with a heat-bonded non-woven geotextile. A standard wick drain is 93 mm wide and approximately 4 mm thick. The wick drains supplied on this site each had a maximum drainage capacity of 6.6 litres/min.
One interesting aspect of this project was the use of the newest style of hydraulic prefabricated vertical drain (PVD) installation equipment. This patented hydraulic equipment mounts to the mast of an excavator and uses hydraulic motors to press the drains into the ground with over 130 kN of static force. The PVD is carried into the ground inside a steel mandrel that is driven into the ground and then extracted. The PVD is wrapped around a small metal plate at the bottom of the mandrel to hold it in place and to keep the soil out of the mandrel as it is driven down. When the mandrel is extracted the metal plate anchors the PVD to the bottom of the hole. In the typically saturated soils where PVDs are used the mandrel presses easily into the ground without predrilling or other preparation.
An advantage of the new hydraulic equipment over older cable operated designs is that vibratory assist is integral to the drive unit and is efficiently transmitted along the mandrel if a difficult section of soil is encountered. Using the vibration assist adds an additional 350 kN of dynamic force for a total static plus dynamic force of 480 kN. On this project the clay became progressively stiffer as the depth increased and at approximately the 14 m depth mark vibration assist needed to be turned on. The requirement for vibro assist had been indicated in the study of the borehole logs. The logs showed that the SPT count increased with the depth of the clay. Typically the vibro assist is required with an SPT blow count value of 13 N or greater. With the vibro assist the wick drain unit had no trouble pressing the PVDs into this soil.
The prefabricated vertical drain portion of this project took about a month and a half including weather delays. A good production day would include 230 wicks of 20 m each or about 4000 lineal meters per day. In total over 6,500 wicks were inserted.
This was the first PVD job that Craig Erb from Hugh Munro construction was aware of in Manitoba. Compared to the sand column drains they installed last year on a similar job the PVD process was like “night and day.” Last year he had three drilling rigs drilling 450 mm holes 20 m deep at 3 m spacings to achieve the same type of drainage. The sand columns also needed a full time excavator for the tailings, a sand truck, and a water truck. The PVDs went much more quickly and are expected to provide similar drainage at a significantly lower price.
Hugh Munro Construction is now completing the embankments over the PVD treated area. Bridge construction is planned to take place in the summer of 2008 and the rail alignment will be switched over in 2009.
Layfield Environmental Systems Ltd is a full-service geosynthetics installation contractor specializing in geomembranes, floating covers, and soil reinforcement geosynthetics. Layfield operates across Canada, the Western US, and internationally providing 30 years of geosynthetic solutions.