Text: Jos Wassink
© Stephan Timmers
Gravitation is the driving force behind all kinds of soil processes such as subsidence, settling, setting and uplifting. The greater the gravitational force, the smaller the models can be that simulate these soil processes. That is the logic behind the GeoCentrifuge commissioned at knowledge institute Deltares – neighbours and research partners of TU Delft.
A human being does not feel the force of gravity, 1 g. A fighter pilot can withstand something like 9 g when wearing a pressure suit, after which he loses consciousness. The GeoCentrifuge goes much further, up to 150 g. At 100 g, a 2-centimetre layer of clay exerts as much weight as a 2-metre layer does in reality. This means that a cross-section of a dyke can be tested on a 1-in-100 scale model as a test sample behind a thick glass plate in the ‘strong box’.
Such a test sample…
…is placed in the hinged container with multiple cameras and connected to water, hydraulic pressure, sensors and whatever else is needed.
The door is sealed airtight all around using an inflatable draught excluder to prevent pressure loss under the centrifuge container flying past, and thus imbalance of the centrifuge.
In the control room, operator Rob Zwaan enters the mass of the test setup in the centrifuge container on the control panel
The centrifuge then automatically moves the counterweight with two rotating spindles to the correct position to regain balance.
Two electric motors of 135 kilowatts each drive the centrifuge via a gearbox. At a low speed, the machine automatically trims away the last imbalance by moving another small mass internally. Then the way is clear to rev up the 27-tonne machine. At most, the centrifuge makes three revolutions per second. For the container, that means a speed of 347 kilometres per hour flat along the wall.
Hence, the centrifuge is surrounded by a 40-centimetre-thick concrete wall with five layers of steel reinforcement. The entire structure rests on a circular concrete floor of 2.5 metres thick anchored 18 metres deep to the Pleistocene sand layer using 77 piles.
The first floor houses peripheral equipment such as the hydraulic pressure generator (170 kilowatts)…
…and the refrigeration unit to dissipate the frictional heat of the air.
A chilled continuous circulating air flow of up to 10 cubic metres per second flows down from the grille on the first floor, past the centrifuge and under the ground back to the cooling system.
Ten double air conditioning units on the roof dissipate heat.