Methodology

The analysis of a retaining wall requires a variety of simplifying assumptions relating to the state of stress in the soil and the distribution of forces to reinforcements.

In the following categories, various methodologies are available depending on the selected design standard:

• Active Pressure Method
• Tension in Reinforcement
• Passive Pressure Method

Not all of the options will be displayed. Only the ones that are supported by the selected design standard are shown. In some cases, certain design standards prohibit the use of some methods for reasons of oversimplifying the analysis. In such cases, the prohibited methods are not displayed.

Active Pressure Method

To analyze a retaining wall, the soil force acting behind the wall is assumed to be in the active state, and relates to the active pressure coefficient (commonly referred to as "Ka"). There are various methodologies available for calculating this force, including the following:

• Coulomb Method - considers a planar active boundary with accounting for interface friction, wall batter, slope angle (see Das 2010; 2012)
• Rankine (simplified) - considers a planar active boundary without accounting for interface friction, wall batter or slope angle (see Das 2010; 2012)
• Rankine (generalized)- considers a planar active boundary without accounting for interface friction, but accounts for wall batter and slope angle (see Das 2010; 2012)
• Rankine-Bell Method - the same as the Rankine generalized method, but with provisions for passive pressures (AS 4678:2002, Annex E)
• Generalized Wedge Method - also known as Culmann method or Active Trial Wedge methods (Caltrans 2011); conducts a rigorous wedge analysis based on the actual weight of soil and reaction forces at the wedge boundaries, including the cohesion and adhesion of soil and soil-structure, respectively. The value of Ka is not calculated; rather, the entire active force is computed via a force balance.
• Kerisel & Absi (EN1997 Annex C1) - assumes a log-spiral active boundary. Tabulated values of Ka are provided in Kerisel & Absi (1990).
• Brinch Hansen (EN1997 Annex C2) - assumes a log-spiral active boundary, with accounting for many soil parameters.

The following table summarizes the assumptions of each method. It is highly recommended to select a method that most suits your problem.

Tension in Reinforcement

For segmental walls only. There are a number of methods that can be used to calculate the tensile force in a reinforcement layer. Some design standards allow you to choose which method to use. The following options are available:

• Simplified method - a simplified method that estimates the vertical stress at each layer based on the soil directly above it, and then multiplying by the internal value of Ka and the layer's tributary height to estimate the tensile force (AASHTO 2020).
• Coherent Gravity method - a method that generally relies on calculating the external active force, eccentricity, and resultant vertical stress on every reinforcement layer, and then multiplying by the internal value of Ka and the layer's tributary height to estimate the tensile force (BS 8006-1).
• Tieback wedge method - similar to the coherent gravity method but with a few minor differences in assumptions. Under specific conditions it simplifies to a closed-form equation (BS 8006-1).
• Stiffness method - a method that considers the stiffness of the entire reinforced structure holistically and distributes the tensile forces according to the stiffness of each reinforcement layer (Allen & Bathurst 2001; 2015; 2018).

Depending on the selected design standard, the actual steps for each method may vary, even if the method shares the same name across different regions. Consult the design standard manual of the applicable region for more details.

Passive Pressure Method

Some design standards allow for passive pressures to be computed at the front face of the wall. These forces increase the resistance of the wall against certain failure modes and relate to the passive pressure coefficient (commonly referred to as "Kp"). The following methodologies are available for calculating this force:

• Caquot & Kerisel (1949) - assumes a log-spiral active boundary. Tabulated values of Kp are provided in Caquot & Kerisel (1949), and estimation charts are provided in AASHTO (2020).
• Kerisel & Absi (EN1997 Annex C1) - assumes a log-spiral active boundary. Tabulated values of Kp are provided in Kerisel & Absi (1990).
• Brinch Hansen (EN1997 Annex C2) - assumes a log-spiral active boundary, with accounting for many soil parameters.
• Use at-rest pressure - conservatively assumes that Kp = Ko (the at-rest lateral earth pressure coefficient), which is often reasonable because many jurisdictions prohibit assuming the full passive pressure.