Geotechnical Centrifuge Center

The properties of soil are well known to be dependent on the effective confining stresses, owing to its self-weight. When modeling a geotechnical system at a scale of 1:N, the confining stresses are N times smaller than the prototype. In order to achieve correct stress levels in the physical model, the geotechnical centrifuge artificially increases the self-weight of the physical model by applying an increased “gravitational” acceleration. In this way, a 1:N scale model tested at N times the gravitational acceleration (Ng) in a centrifuge will have the same effective stresses as the full-scale prototype, properly maintaining scale similarity and avoiding scale effects.

Under such enhanced gravity conditions, the time is scaled down by N2 for diffusion problems and N for dynamic events. This leads to an additional major advantage in terms of simulating long-duration processes in a very short time. For example, consolidation settlements, which may take 1 year in nature, can be simulated in less than 1 hour in the centrifuge by accelerating the model at 100g (the time is scaled by N2 for consolidation problems). In this way, the centrifuge offers the possibility to perform physical model tests of geotechnical systems in a much faster and more economical way than full-scale or field tests. In particular, for the seismic response of soil-foundation-structure systems, centrifuge modelling may be the only realistic solution, since large scale shaking table testing is prohibitive in terms of cost.
 

The drum centrifuge consists of a main channel (1), in which the model is built or placed, and a tool platform (2), where actuators and sensors can be mounted together with the data acquisition systems. With a diameter of 2.2 m, 0.7 m height and 0.3 m depth, the channel can be accelerated up to 440g carrying a payload of up to 2 tons. A key advantage of the drum centrifuge is that the entire channel can be filled with soil, creating a model deposit of 5 m length, which corresponds to a prototype of 2 km length, over 100 m depth and 300 m width. Smaller models can be tested using strongboxes, prepared outside and placed on the channel.

The rotation of the channel and the tool platform is provided by an external (4) and an internal (5) shaft, respectively. The tool platform can spin together with the channel, or independently. This allows stopping the tool platform during a test, allowing changes or adjustments of actuators and sensors mounted on the platform without stopping the test. In such a case, a shield is lowered to protect the stationary platform from the spinning channel. Communication between the on-board computer and the control room is provided by sets of electrical slip rings (6, 10, and 13). An additional slip ring is mounted on the tool platform over the internal shaft (5), allowing supply of water to the spinning model from an external source.