Introduction

One-dimensional unloading of soil generally produces a more rapid drop of vertical effective stress than horizontal stress. Under certain circumstances, the horizontal effective stresses can actually exceed the vertical effective stresses – the horizontal stresses remain ‘locked in’ during unloading. Such stress conditions are described mathematically by an earth pressure coefficient at rest (𝐾𝑜 = 𝜎'_ℎ/𝜎'_𝑣) that exceeds 1.0.

In general, the first step in a stress-deformation simulation using SIGMA/W is to establish the initial stress conditions. The initial stress conditions are particularly important if a more advanced constitutive model (e.g. modified Cam clay) is used in a subsequent analysis such as staged construction of an embankment.

Initial stresses can be established in SIGMA/W via an In Situ analysis using either the gravity activation procedure or the 𝐾0 procedure. The horizontal stresses generated by the gravity activation method are limited to the case where the horizontal effective stresses are less than or equal to the vertical effective stresses (that is, 𝐾𝑜 <= 1.0). The 𝐾0 procedure can be used to establish horizontal effective stresses that exceed the vertical effective stress.