Design and Performance of an In-Flight Rainfall Simulator in a Geotechnical Centrifuge

The objective of this paper is to present the design details and performance of an actuator for simulating in-flight rainfall at enhanced gravity levels in centrifuge. The developed simulator is capable of inducing in-flight rainfall of various intensities (varying from 10 mm/h to 80 mm/h in prototype dimensions) and durations (over a year continuously in prototype dimensions) on geotechnical structures using specially designed pneumatic nozzles. The intensity and duration of rainfall can be regulated at any point of time in the in-flight condition to replicate prototype natural hazards, ranging from long-term medium intensity rainfall to a short spell of very high intensity rainfall. The various components of the developed simulator are discussed, with special emphasis on measures adopted to nullify Coriolis effects on droplet trajectory. Furthermore, the simulator produces rainfall in the form of fine mist at high gravities, which neutralizes chances of erosion due to impact of raindrops. In the present paper, a total of six calibration tests at high gravities and four centrifuge model tests on a typical silty sand slope were carried out using a 4.5-m radius large beam geotechnical centrifuge facility available at IIT Bombay, India. The analysis and interpretation of calibration results indicated that uniform rainfall intensity could be achieved over entire model surface area, and they were further applied to implement the scaling laws involved in modelling of rainfall in a geotechnical centrifuge. The model tests on silty sand slope indicated a rise in phreatic surface with ingress of rainwater, accompanied by face deformations and surface settlements, which increased in magnitude with higher rainfall intensities. In addition, the phreatic surfaces induced during rainfall using the developed simulator and time corresponding to failure observed in centrifuge were compared with numerical results using Geostudio 2012 (Geo-Slope, Calgary, Canada) and were found to corroborate well.