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 = 0.30SDSW.
AASHTO LRFD Bridge Design Specification, Section 11.6.5.1 considers two sources of seismic force: soil lateral pressure (PAE), and wall inertial forces (PIR), 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 PAE + 0.5PIR and 0.5PAE + PIR (with 0.5PAE at least equal to the static active soil pressure). AASHTO further calculates PIR as kh(Ww + Ws), where kh is the seismic horizontal acceleration coefficient, Ww is the weight of the wall, and Ws is the weight of soil above the heel.
For typical Seattle sites, the range of kh 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 As (ground acceleration) as FPGAPGA, where PGA (peak ground acceleration) is about 0.45G for Seattle (PGA is tabulated for soft rock – Site Class B/C). FPGA 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 As = 0.52 for most Seattle sites. kh is taken as 0.5As where 1 to 2 inches of sliding during an earthquake is acceptable, otherwise kh = As. For slope stability, this gives a typical kh = 0.26 (I commonly see kh = 0.3 used for slope stability analysis). Note: AASHTO uses FPGA = 1.2. Both AASHTO and WSDOT assume kv = 0.
Conclusion: use 0.5kh 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 1.0kh 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
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