Retaining Wall Design

Last updated November 18, 2024
By Ian Story

Retaining wall design is a highly inexact science that relies on many assumptions and specifications. Some parts of the process are codified, but many of the decisions to be made are based on engineering judgment, informed by past studies, geotechnical references, and sources outside of the building codes (for example, transportation departments and associations).

Discussion on Forces

Seismic Self Weight

Basics of Retaining Wall Design suggests using ASCE 7-16, equation 15.4-5 (Section 15.4.2) to calculate seismic forces due to self weight. For an importance factor of 1, this gives V = 0.30S_DSW.

AASHTO LRFD Bridge Design Specification, Section 11.6.5.1 considers two sources of seismic force: soil lateral pressure (P_AE), and wall inertial forces (P_IR), where the weight of any soil directly above the heel is included in the weight of the wall for purposes of calculating inertial forces. Research indicates that these two forces tend to act out of phase (i.e., when the seismic component of earth pressure is at a maximum, the inertial force is near a minimum, and vice versa). To account for potential concurrence of phases, the AASHTO specification uses the larger of P_AE + 0.5P_IR and 0.5P_AE + P_IR (with 0.5P_AE at least equal to the static active soil pressure). AASHTO further calculates P_IR as k_h(W_w + W_s), where k_h is the seismic horizontal acceleration coefficient, W_w is the weight of the wall, and W_s is the weight of soil above the heel.

For typical Seattle sites, the range of k_h values appears to be between 0.3 and 0.55, depending on how much sliding is acceptable. Calculating sliding is a challenging task, but the Simplified Newmark Sliding Block Analysis gives a rough ballpark estimate. According to this paper (Jibson), a slope/retaining wall designed for 0.30g is expected to slide a mean distance of around 1 inch in an earthquake with a PGA of 0.45g and a duration of 10 seconds. For reference the equivalent sliding for a wall designed for 0.35g is 0.4 inch and the equivalent sliding for a wall designed for 0.40g is 0.2 inch (all approximate means, with variation).

The WSDOT Geotechnical Design Manual calculates A_s (ground acceleration) as F_PGAPGA, where PGA (peak ground acceleration) is about 0.45G for Seattle (PGA is tabulated for soft rock – Site Class B/C). F_PGA is a conversion factor to other site classes, and works out to 1.15 for PGA=0.45 and Site Class D. This gives us a typical A_s = 0.52 for most Seattle sites. k_h is taken as 0.5A_s where 1 to 2 inches of sliding during an earthquake is acceptable, otherwise k_h = A_s. For slope stability, this gives a typical k_h = 0.26 (I commonly see k_h = 0.3 used for slope stability analysis). Note: AASHTO uses F_PGA = 1.2. Both AASHTO and WSDOT assume k_v = 0.

Conclusion: use 0.5k_h as the seismic coefficient for self-weight (0.2 is a good value for reduced sliding), but include weight of soil over the toe in self weight. Alternatively, for walls with very large heels, ignore the seismic lateral force from the soil and use 1.0k_h as the self-weight seismic coefficient. Use whichever method gives the lower factor of safety.

Cases to Check:

Construction Phase – Overturning

Some sources assert that seismic forces may be neglected for temporary retaining walls. For example:, the WSDOT Bridge Design Manual states that “Any retaining wall that is expected to be in service for more than three years shall be designed for seismic loading.”

Apply all forces that would be present right at the moment of tipping. This includes earth pressures (use active pressure, because overturning requires movement of the wall)

Resources

• Basics of Retaining Wall Design, Hugh Brooks
• AASHTO LRFD Bridge Design Specification
• WSDOT Bridge Design Manual, Chapter 8
• WSDOT Geotechnical Design Manual
• Predicting Earthquake-Induced Landslide Displacements Using Newmark’s Sliding Block Analysis, Randall W. Jibson