Behaviour of Single and Group Helical Piles in Sands from Model Experiments Jongho Bak, Byung-hyun Choi, Junwon Lee, Jonghwan Bae, Kicheol Lee and Dongwook Kim MATEC Web Conf., 278 (2019) 03007 Published online: 08 April 2019 DOI: 10.1051/matecconf/201927803007