Spent TBM Slurry at 12 Percent Solids: Solidifying Without Doubling the Tonnage

A slurry shield driving a 2.8 km metro tunnel generates 1,100 cubic metres of spent bentonite slurry a day, and the separation plant returns about 340 tonnes of filter cake at 30 percent solids to the muck bay. The contractor had been trucking that cake as a wet slurry instead, at 12 percent solids, because the press was undersized. Each truck was carrying 2.6 tonnes of solids and 19 tonnes of water, and the gate fee was charged per tonne.

Slurry management is where tunneling budgets quietly break, and SAP is usually proposed as the fix without anyone doing the tonnage arithmetic first. This article sets out where the water actually is, why dosing on wet weight misleads, and the sequence that keeps disposal costs from doubling.

Where the Tonnage Comes From: 12 Percent Solids, 88 Percent Water

At 12 percent solids, a cubic metre of slurry weighs about 1.08 tonnes and carries 130 kg of solids. The other 950 kg is water. To make that material pass a landfill stability test, you must either remove water or bind it, and binding it does not reduce mass by a single kilo. This is the first thing contractors get wrong: they expect solidification to shrink the load.

What solidification actually buys is acceptance. A stackable, non-bleeding material can go to a non-hazardous cell, can be tipped without a liner, and can sit in a trailer overnight. A 12 percent slurry often cannot do any of those things, so it either travels further or costs more per tonne at the gate. Those two effects, not volume reduction, are where the money is.

The Trap: Dosing on Wet Weight Instead of Dry Solids

Quotes for this work are often written as a percentage of wet weight, which hides a factor of eight. At 1.5 percent of wet weight on a 12 percent slurry, you are adding 1.5 kg per 12 kg of solids, or 12.5 percent on a dry basis. Move to a 30 percent filter cake and 1.5 percent of wet weight becomes 5 percent on dry solids, three times less polymer for the same solids throughput.

Always convert before comparing suppliers or before agreeing a rate. A simple rule: dose percentage on dry solids equals the wet-weight percentage multiplied by 100 and divided by the percent solids. At 12 percent solids, 1.5 percent wet becomes 12.5 percent dry. Any supplier who quotes without asking for your percent solids is quoting a number that cannot be checked.

Separate First, Solidify Second

Mechanical separation is almost always cheaper than chemical binding. A decanter centrifuge will take a 12 percent slurry to 28 to 32 percent cake for roughly 3 to 5 dollars per cubic metre of feed in power, flocculant and wear. Granules to bind the same water cost 15 to 25 dollars per cubic metre. Run the centrifuge first and the granule bill drops by more than half, even after the cake is harder to pump.

Where space rules out a decanter, a geotextile dewatering bag in a lined bay is the low-capex alternative: 12 percent slurry goes in, 25 to 30 percent cake comes out after 3 to 6 weeks depending on the fines. It is slow, but on a 24-month drive there is usually enough sequencing to make it work, and the bags can be run in parallel with the shield advance.

Truck-Out Maths at a 22 Tonne Payload

Take 340 tonnes a day of 12 percent slurry, which is 41 tonnes of solids and 300 tonnes of water, about 315 cubic metres. At a 22-tonne payload that is 15 loads a day, and at 38 dollars a tonne gate fee the daily bill is 12,900 dollars. After centrifugation to 30 percent solids the same solids leave as 137 tonnes of cake, 7 loads, 5,200 dollars at the gate, plus 1,400 dollars of centrifuge cost.

Granules do not change those numbers much on their own. Their role is to make the cake legal to tip and stable enough to store, at 0.4 to 0.8 percent of cake weight, which on 137 tonnes is 550 to 1,100 kg a day. That is a few hundred dollars against a 7,000 dollar daily saving from separation. Put the money into the press first and the polymer second.

For slurry work, choose a coarse cut with high absorption under load rather than a fine powder, because cake is stacked and the gel must hold water under 2 m of its own weight. The polyacrylate SAP granule range is a sensible place to start the shortlist.