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Losing that sinking feeling: stone column ground installation at the Port of Valencia

The Xita Dock in the Port of Valencia is one of the most important shipping terminals in Spain. It has consistently ranked as Spain’s number one port for real registered foreign trade volumes (imports and exports) and channels traffic from all sectors of the economy and practically any type of commodity.

One of the main imports is coal clinker, which is currently stored on a platform next to the Xita dock. The issue with this is that in high winds, coal clinker dust clouds are created, resulting in pollution and dust particle fallout within the dock and surrounding areas.

The Spanish authorities decided that the best way to tackle the problem was to build a giant warehouse on the platform that could house the clinker and shelter it from the elements. To ensure the warehouse could support the weight of coal and didn’t subside into the ground, major ground engineering work was required.

It was at this point that ground improvement specialist, Pennine, part of Balfour Beatty Ground Engineering, was brought in as part of a joint initiative with Spanish contractor Geocisa. Work in the port started in January 2008 and is due for completion in August. Valued at €5 million, the project involves the installation of some
8,500 stone columns into the ground to enhance the bearing capacity and control settlements.

“This is one of our larger projects,” says Jon Chevin, Overseas Contracts Manager at Pennine. “Our guys are working 24 hours a day, six days a week. We’re training the Spanish rig drivers too, which has enabled us to improve productivity in recent months.”

Pennine which has a long-established relationship with Geocisa was introduced to this project because of its track record on other large-scale projects around the world. The company has worked on the Valencia F1 track, Palm Islands in Dubai, UAE and is currently carrying out major marine vibro work in Vancouver, Canada.

Good vibrations

To produce the 8,500 stone columns required to support the warehouses, Pennine is using a ‘wet top-feed vibroflot’ technique. This involves suspending a heavy, torpedo-shaped vibrating poker (vibroflot) – from a 150 tonne crane and plunging it into the ground. As the vibroflot creates a hole in the ground, stone is fed from the top into the hole, to create a column of stone which is then compacted by the vibroflot. This is then raised a short distance from the base of the bore and more stone is
introduced into the hole and compacted to the required pressures. This operation is repeated in stages until a compacted stone column is formed to the surface.
Separating the tip of the vibroflot from the crane is a 30 m collar section. Throughout this section, there is a series of isolators that are used to prevent vibration travelling to the crane’s cab.

“Top-feed is quicker than bottom feed, but requires the hole to stay open while you pour in the stone. This is determined by the soil type as the vibroflot will just not get through ground conditions with a Standard Penetration Test value of 20 or so. Fortunately the soil in Valencia – which is mainly sand and silt – lends itself well to this technique,” says Chevin.

The vibroflot itself is 500 mm wide, but creates holes of up to 800 mm in diameter due to the consolidating effects of the vibration. These columns go as deep as 30 m into the underlying sand and silt and are positioned on a triangular grid at distances between 1.6 to 2 m apart.

“The total surface area of the site is approximately 1,000 m long and 60 m wide,” says Chevin. “As we move into the central part of the site, the columns are much closer together. This is done deliberately because the coal clinker will be piled up in a giant mound, with the greatest weight in the centre.”

To produce the correct level of consolidation, pressure is calculated against depth as the holes are being bored using Pennine’s Datalogger monitor ing system. This feeds the information from the vibro piling rig cabin into a central computer to measure the progress of each individual column.

Operators are asked to take the pressure levels during each stage of column construction to values of up to approximately 300 bars, dependent upon ground  conditions and ground response. “An experienced driver knows when the column has reached the optimum level.” explains Chevin.

“If we over compact the column, you end up with wider columns and use more stone, which is a waste of time and money. It’s up to us to ensure that we get it right every time, so that we can deliver on time and on budget.”

“We have four crane mounted vibroflots on the site and each column takes about 40 minutes to produce. We set a target to produce 300 linear metres of stone column per shift per rig, but regularly exceed this, so we’re well on course to meet our targets,” adds Chevin. The equipment used on site is designed and custom built by Pennine at the company’s manufacturing plant in Bacup, Lancashire, UK.

Pennine, Bacup, Lancashire, UK
Edition: Edition 40

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