Soft Soil Retrofitting Study with Bamboo Concave Mattress Against Stability of Jombang - Kertosono Railway Double Track Slope Due to Earthquake Load - Suprayitno, Suprayitno; Darjanto, Helmy (IJIEEB International Journal of Integrated Education, Engineering and Business eISSN 2615-1596 pISSN 2615-2312, 2018)