Geotextile Tube Dewatering: Dose the Slurry Before the Tube, Not In It

A dredging contractor filled an 8 metre tube with slurry at 12 percent solids and waited three weeks for a cake. At the end of it, the tube was still bulging, the filtrate was cloudy, and the cycle time had doubled against the plan. The polymer had been dosed into the fill port with a hose, which is the cheapest way to do it and the reason the result was inconsistent along the length of the tube.

Geotextile tube dewatering works because the fabric retains solids while letting water out. Anything that improves the retention of fines or the release of water shortens the cycle, and the place to do it is in the slurry line, upstream of the tube.

What the polymer does to the slurry

A 12 percent dredged slurry is a suspension with a yield stress close to zero. It flows, it carries fines into the fabric pores, and the fines blind the fabric from the inside, which is what turns a three-day fill into a three-week one. Adding 0.5 to 1.5 kg of polyacrylate per cubic metre raises the yield stress, binds the fines into larger structures, and lets the fabric pass water instead of trapping particles in its pores.

The result is a faster fill, a higher cake solids at the end of the cycle, and a filtrate that is clearer, which matters if the discharge has a turbidity limit. Cake solids typically move from 20-25 percent to 35-45 percent, which is the difference between paying for water transport and not.

Order of addition, and why it matters

If the slurry carries a high fines fraction, a flocculant goes in first and the polymer second, with a static mixer between them. The flocculant builds the flocc; the polymer holds water inside it and stiffens the mass. Reverse the order and the polymer coats the particles before the flocculant can reach them, which is the single most common reason a jar test works and a full-scale fill does not.

Jar tests are still the right starting point, but run them in the order you will dose, at the solids concentration you will see, and with the same mixing energy. A jar test at 200 rpm for two minutes tells you nothing about a pipeline at 3 m/s.

Tube diameter is the other variable worth modelling. A 12 m circumference tube holds roughly 11 cubic metres per metre of length, and the fill time scales with the square of the diameter for the same fabric and slurry. That is why a large tube tolerates a slow-draining slurry and a small one does not: the small tube has less fabric area per cubic metre, so the filtrate path is longer relative to the volume. Size the tube for the slurry, not for the pump.

The three ways it fails in the field

Dosing into the tube, as above, gives an uneven cake: stiff at the inlet, soup at the far end. The tube then has to be filled in stages, and the cycle time advantage disappears.

Overdosing gives a gel that plugs the fabric. The tube stops weeping, the internal pressure rises, and on a seam-limited tube that is a burst risk rather than a schedule problem. Keep the dose under 2 kg/m³ and watch the filtrate rate; when it drops sharply, stop adding.

Ignoring the salinity of the dredge spoil is the third. Brackish or marine sediment at 20,000 mg/L chloride will cut the swelling of a sodium polyacrylate by more than half, so the dose that worked in a freshwater jar test under-delivers by the time the barge reaches the sea. Test in the actual pore water.

For the next project, put three numbers on the dosing sheet: slurry solids, pore-water salinity, and target cake solids. The water absorbing granule range includes grades selected for saline conditions. A tube that fills in three days instead of three weeks pays for the polymer in the first cycle.