Saptono, Singgih, et al. “The Influences of Normal Load Variation in Direct Shear Strength Test Parameters Tuff from Candi Ijo Yogyakarta”. International Journal of Engineering & Technology, vol. 8, no. 1.1, Jan. 2019, pp. 184-9, https://doi.org/10.14419/ijet.v8i1.1.24658.