Research and Graduate Studies in Civil Engineering

Seismic analysis and design of reinforced soil retaining walls

By Kianoosh Hatami, PhD, PEng

Reinforced soil retaining wall structures have become increasingly popular and widespread around the world in the past two decades due to their distinctive advantages over conventional retaining walls. On the other hand, their mechanical response during construction and under service loads is complicated and therefore, a subject of intensive research. Their response to ground motion in seismically active areas poses an even more complicated problem to engineers for a safe, serviceable and yet economical design approach. At present, the available codes for seismic design of reinforced soil wall systems (e.g., FHWA 1996, AASHTO 1998, NCMA 1998) follow a pseudo-static approach to determine the total lateral earth force behind the wall. In addition, the assumed distribution of dynamic earth pressure behind the wall is based on limited studies on metallic reinforced soil walls (e.g., Segrestin and Bastick 1988). The results of current studies (e.g. Bathurst and Hatami 1998) have indicated important shortcomings of the current design approaches which include (Figures 1 and 2):

Figure 1: Numerical model of a reinforced soil retaining wall system: Terminology (click image below to enlarge):

Numerical Model.

Figure 2(a): Fixed-toe condition (click image below to enlarge):

Fixed-toe condition.

Distribution of reinforcement load in the wall at end of construction and end of base acceleration (shaded). Predicted distributions from Rankine (1) and Coulomb (2) theories are shown for comparison.

Figure 2(b): Sliding-toe condition (click image below to enlarge):

Sliding-toe condition.

1) The distribution of reinforcement load along the wall height is different for geosynthetic and metallic reinforcement materials. The stiffness values of geosynthetic reinforcement materials are at least one order of magnitude lower than the stiffness of typical metallic reinforcement products. The reinforcement stiffness shows little effect on the reinforcement load response under static loading (e.g., a constructed wall under service loading conditions). However, the reinforcement stiffness becomes an important factor in the magnitude and distribution of reinforcement load behind the wall when subjected to ground motion. The distribution of dynamic reinforcement load in a wall with geosynthetic reinforcement and restricted toe condition is essentially uniform whereas the load distribution in metallic reinforcement increases linearly with depth. Accordingly, the elevation of lateral earth pressure resultant in geosynthetic reinforced soil walls would be underestimated by adopting the design methodologies that are based on metallic reinforced soil walls (e.g., AASHTO 1998).

2) The peak ground acceleration (PGA) parameter, which is used in the current pseudo-static design approaches is a poor indication of the dynamic effect of ground motion on the wall structure. The selection of seismic coefficients based on PGA only, does not include the influence of different ground motion characteristics (namely, frequency content, intensity and duration) on the seismic response of the wall. The results of current study (Hatami and Bathurst 2001) have demonstrated that the characteristics and magnitude of wall seismic response can be substantially different when the wall is subjected to different recorded ground motions that are scaled to the same PGA value (Figure 3).

Figure 3: History of facing lateral displacement at the top of a 3.6m-high segmental reatining wall subjected to recorded (1 to 6) and harmonic (7 and 8) ground accelerations (8 in total) scaled to PGA=0.15g (click image below to enlarge):

History of facing lateral displacement.

Therefore, a seismic design methodology should properly include the dynamic characteristics of ground motion and retaining wall structure for a reliable design approach.

3) The internal stability analysis of reinforced soil retaining walls using Mononabe-Okabe theory is based on the assumption that a shear failure wedge develops inside the reinforced zone. Results of present studies (e.g. Bathurst and Hatami 1998) indicate that the shear failure plane in reinforced soil walls with typical reinforcement configurations subjected to ground motion forms a two-part wedge failure mechanism which contains (and is practically outside) the reinforced zone (Figure 4).

Figure 4: Shear failure wedge behind a propped-panel wall subjected to base acceleration (click image below to enlarge):

Shear failure wedge.

The objective of the present research is to enhance the current understanding of seismic response of geosynthetic reinforced soil walls and therefore, to develop improved methodologies and simplified guidelines for their design against seismic loading.

References

  1. AASHTO 1998. Interims: Standard specifications for highway bridges. American Association of State Highway and Transportation Officials, Washington, DC, USA.
  2. National Concrete Masonry Association (NCMA) 1998. Segmental Retaining Walls - Seismic Design manual (Supplement to Design Manual for Segmental Retaining Walls authored by RJ Bathurst), Herdon, VA, USA, 118p.
  3. Bathurst RJ and Hatami K, 1998. "Seismic Response Analysis of A Reinforced Soil Retaining Wall", Geosynthetics International (special issue on Earthquake Engineering), Industrial Fabrics Association International (IFAI), USA, 5(1-2), pp. 127-166.
  4. FHWA 1996. Mechanically stabilized earth walls and reinforced soil slopes design and construction guidelines. In V. Elias and B.R. Christopher (eds). Federal Highway Administration (FHWA) Demonstration Project 82, Washington, DC, USA.
  5. Hatami K and Bathurst RJ. "Investigation of Seismic Response of Reinforced-Soil Retaining Walls", Proceedings of the 4th International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, March 2001, Rolla, MO, USA.
  6. Itasca Consulting Group 1998. "FLAC - Fast Lagrangian Analysis of Continua", Version 3.40, Itasca Consulting Group, Inc., Minneapolis, Minnesota, USA.
  7. Segrestin P and Bastick M, 1988. Seismic design of reinforced earth retaining walls - the Contribution of Finite Element Analysis. In T. Yamanouchi, N. Miura & H. Ochiai (eds), Theory and Practice of Earth Reinforcement - Proceedings of the International Geotechnical Symposium on Theory and Practice of Earth Reinforcement, Fukuoka, Kyushu, Japan, October 1988: 577-582. Balkema, Rotterdam.