Kpaudel, the simplest and maybe more coherent references are in Bowles, Foundation Analysis and design, chapters 9 (Kh vertical) and 16 (Kh horizontal). They are pretty good as an input in FEM methods.
Other references are Gazetas 1991, Foundations vibrations, in Feng, foundation engineering handbook. More references: Salgado, the engineering of foundations.
Kh is also the initial, linear part of a p-y curve.