Vibro stone columns

How does Vibro stone columns work, step by step?

1. 1

   Step 1: Read the ground and fix the design

   Ground improvement is designed off the ground investigation, not off a rate. Particle size, fines content, undrained strength, organic content, water level and the depth to a competent stratum all narrow the shortlist, and the settlement the structure above can tolerate narrows it further. The geotechnical designer and the specialist contractor then agree the treatment: which areas get columns, how deep they go, how they are laid out, and what the treated ground has to achieve. Typical schemes put columns on a square, rectangular or triangular grid at 1.5-3 m centres, replacing something like 5% to 15% of the plan area with stone - but every dimension on the design for a particular site comes from their calculations and their experience of the local ground, checked against the same investigation the foundation designer used. An investigation dense enough to design from usually means probes or boreholes at something like 20-40 m centres, closed right up wherever the fill turns variable.

2. 2

   Step 2: Build and certify the working platform

   A vibro rig is tall, heavy and top-loaded, and it stands on the softest ground on the project - which is the whole reason it is there. The platform is a designed structure: typically 600 mm to 1.2 m of well-graded granular material, sometimes over a geogrid or geotextile, laid on a proven formation, with ramps at a gradient the rig can climb loaded and an engineered stand-off from any open excavation. A vibro rig weighs 30-60 tonnes with the poker hung, and nearly all of that arrives through a small track footprint. It is signed off in writing before the rig tracks on, inspected while the works run, and reworked and re-certified when rig traffic ruts it. The fatal accidents in this trade are rig overturns on platforms that were adequate on day one and were never looked at again.

3. 3

   Step 3: Prove the design in a trial

   Before production starts, trial columns are installed at the design layout in a representative area and tested. The trial answers questions no calculation can: whether the poker reaches design depth without refusing on buried obstructions, how much stone the ground actually takes - in the order of a tonne for every metre of column is a common starting expectation - how long a column takes, which at production depths is usually a matter of minutes rather than an hour, and whether the treated ground performs as assumed. Production parameters - the criteria for depth, for stone consumption and for compaction - are set from the trial and written down. A trial that fails is cheap. Discovering the same failure under a finished floor slab is not.

4. 4

   Step 4: Penetrate to depth, top-feed or bottom-feed

   The poker is vibrated down under its own weight, with air or water flush assisting, displacing and densifying as it goes. Where the bore stands open in stable ground, stone - clean, single-sized crushed rock or crushed concrete, typically around 40-75 mm - is tipped in from the surface around the poker. That is the top-feed method: quick and cheap, but reliant on the hole staying open. Where the ground is soft or wet and the bore would collapse, the bottom-feed method delivers smaller stone, typically 20-40 mm so that it will pass down the delivery tube, straight to the tip of the poker, which never leaves the hole. The choice belongs to the specialist contractor and follows the ground and the water level, not the price list.

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   Step 5: Build the column in compacted stages

   The column is not backfilled in one go. The poker is raised a short distance - commonly 0.3-1 m - a charge of stone is placed, and the poker is driven back down through it to compact the stone and force it outwards into the surrounding soil. That re-plunging is the whole technique - it is what makes the column dense and what locks it into the ground. The rig records what it does: depth reached, the current the vibrator draws as it compacts, and the stone taken at each stage. Those records are the first quality document on the job, and a column that took far more or far less stone than its neighbours is where the investigation starts. A rig in steady production commonly places 150-400 linear metres of column in a shift, so a 5,000 m² warehouse slab treated at 2 m centres - something over 1,200 columns - is three to six weeks of work once the platform is signed off.

6. 6

   Step 6: Verify the treated ground and cap it

   Verification is agreed before mobilisation and executed as the work proceeds: penetration testing before and after treatment to show the change in the soil between the columns, typically one profile for every few hundred to a couple of thousand square metres treated; plate load tests on individual column heads, commonly at something like one column in fifty to one in a few hundred; and one or more zone tests loading a group of columns together under kentledge - often tens of tonnes of it - to prove the composite behaviour the design assumed. The rates for a particular job belong to the specification and are agreed before the rig arrives, not negotiated after a disappointing result. Results are assessed against the acceptance criteria with the designer, and shortfalls are re-treated rather than argued about. The treated area is then blinded or capped and protected, because treated ground left open to weather and site traffic is treated ground being spoiled.