T. W. Barraclough, J. R. Blackford, S. Liebenstein, S. Sandfeld, T. J. Stratford, G. Weinl¨ ander, and M. Zaiser July 8, 2018 1 Experimental method Ice chips were cooled to − 80 ◦ C and crushed into a dry powdery material using a stan- dardised process [1]. This material was then placed in a covered container in the cold lab for four hours to allow it to reach the temperature of − 10 ◦ C at which the deformation experiments were carried out. This resulted in a cohesive aggregate of ice granules with density ρ = 370 kg / $_{m}$− 3 and grain sizes ranging from 0 . 1 to about 0 . 5 mm (mean grain size ξ ≈ 0 . 2 mm). As is typical for laboratory- or machine-made snow, the density of our samples is at the upper end of the range observed for natural snow. The artificial snow thus produced was sifted through a 1.68 mm aperture sieve into a flat rectangular box with aluminium alloy side walls and a glass bottom. Excess snow was removed with a straight edge and the box was closed with a second sheet of glass screwed on top of the specimen. The container was then rotated upright to provide a specimen w = 250 mm wide, L = 195 mm high and d = 20 mm thick, enclosed in a container with transparent front and back walls within an apparatus as shown in Fig. 1. The total time from sieving to deformation was approximately 5 minutes. The specimen was compacted by an anvil moving downwards at fixed rate. During deformation, the driving force was recorded by a load cell located above the anvil. The recorded force data were filtered to remove high frequency oscillations associated with the motor drive. Typical compressive forces at the end of a deformation experiment (compressive strain ε$_{xx}$ ≈ 0 . 3) were of the order of 250 N. As the specimen is compacted, it exerts lateral forces on the side walls of the container. These forces were evaluated by determining, at the end of a deformation experiment, the sideways deflection of the container side walls using a pair of dial test indicators, and comparing with calculations made under the assumption that the response of the side walls is that of an elastic thin plate, with elastic properties determined by a calibration experiment. This yielded a total lateral force of about 45 N at compressive strain ε$_{xx}$ = 0 . 3. An 18 megapixel camera was located in front of the specimen with illumination pro- vided by a flashgun and diffuser located behind the specimen. Images of the transmitted light were recorded at 0.25 s intervals. To analyze spatio-temporal deformation patterns, each image series was analysed using the digital image correlation software GeoPIV [2]. arXiv:1501.02184v1 [cond-mat.soft] 16 Dec 2014 Supplementary Information (SI): Propagating compaction bands in confined compression of snow: Experiment and Modelling 1