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# LRFD

SLOPE STABILITY ANALYSIS ACCORIDING TO:

AASHTO – LRFD BRIDGE DESIGN SPECIFICATIONS – 9TH EDITION

The LRFD approach consists in checking the following equation:

Where Q and R can represent forces, as well as stresses, deformations, displacements, etc. In this equation, Q represents an amplified value of actions, while R is a reduced value of resistances. The fundamental concept of LRFD design is expressed by the following relationship:

In which

Qi : force effect

Rr : factored resistance

Rn : nominal resistance (i.e., ultimate capacity)

φ : resistance factor

According to the Specifications, it is necessary to perform the slope stability analysis using the Strength I approach. However, also the other two verifications (Service I e Extreme Event I) have to be performed.

However, referring to the Strength I approach, it is prescribed to employ a unitary load factor γi for all the loads included in this combination. Therefore, the verification can be rewritten as follows:

The resistance factor, φ, has to be evaluated as specified in the following.

Where the geotechnical parameters are well defined, and the slope does not support or contain a structural element: φ = 0.75

Where the geotechnical parameters are based on limited information, or the slope contains or supports a structural element: φ = 0.65

In SLOPE, the LRFD approach is equivalently applied using the following equation:

where

Therefore, the coefficient to be input in SLOPE takes the following values:

Where the geotechnical parameters are well defined, and the slope does not support or contain a structural element: = 1/φ = 1/0.75 1.30

Where the geotechnical parameters are based on limited information, or the slope contains or supports a structural element: = 1/φ = 1/0.65 1.50

This coefficient has to be given as input in SLOPE as shown in the following window, concerning the main parameters.