Chapter 7: Slope Stability

Section 1: Overview

Overview

Evaluate all slopes, whether a cut or a fill and whether in soil or in rock, for global (overall) stability. Slopes steeper than 3:1 must have a documented evaluation. When warranted, evaluate for both short-term (undrained) and long-term (drained) conditions under Strength I limit state condition per current edition of AASHTO LRFD Bridge Design Specifications. However, the load factors are not compatible with limit equilibrium analysis and resistance factor is yet to be calibrated & implemented in commercially available software, overall stability analysis still be performed under Allowable Stress Design (ASD) methods.

Conditions

Perform slope stability analyses under all applicable conditions using Limit Equilibrium software such as GSTABL, Slide2, etc. At a minimum, evaluate the following conditions:

1) Short-term (undrained) condition,

2) Long-term (drained) condition, and

3) Rapid drawdown (flood) condition.

For embankments and cut slopes consisting of high plastic clay soil, shear strength may degrade due to exposure to weathering action (shrinkage-swelling). Perform long term stability check based on residual shear strength of the soil.

Specific site conditions may require evaluation for additional types of failure, such as bearing capacity, settlement, and undercutting (for rock cuts).

Perform embankment settlement analysis under service limit state to evaluate the performance of the embankment or structure on top of embankment. If deformation will adversely affect the facility, develop ground improvement or mitigation measures. Identify the most appropriate ground improvement method based on project needs and approval from the department. Include ground improvement limits, details, instrumentation and performance requirement in the plan and specification.