Chapter 10: Global Stability - Terraced

Figure 10-1: Terraced Wall Applications

10.1   Whenever walls are constructed in a terraced arrangement, or any of the other conditions listed, the designer must consider the overall global stability of the structure. With today’s software we now have the ability to precisely model and evaluate terraced walls using an approach which refines the use of a Bishop’s analysis in a methodology developed by Prof. Dov Leshchinsky called the Limit Equilibrium Method (LEM). The approach builds off the long used geotechnical principals of global stability, combined with an SRW facing and utilizes a two-step approach. During step one, an analysis on the internal stability of the defined configuration is done where the calculations quantify the position and amount of load along the length of each geogrid layer. During this first step, the pullout loads versus capacity are reviewed at the wall facing and at the back of the embedded geogrid layers. The wall designer now has a method and a design tool to fully analyze complex wall geometries including applied live and dead loads as well as pseudo-static loading conditions. Step two, which is in keeping with the current standard of practice, requires a full global analysis. See the AB Engineering Manual for more details and contact the AB Engineering Department for the AB Walls design software.

10.2   When determining the influence of the upper wall onto the lower wall, it is common to consider the walls as independent if they are spaced apart a minimum of twice the height of the lower wall. If they are spaced closer, the lower wall must be designed to carry the surcharge of the upper wall(s), Figure 10-1.

10.3   Terrace height and grading considerations (Figure 10-2):

1. The top of the lower terrace should be the same elevation as the bottom of the next terrace.
2. Grade the soil between the terraces starting at the back of the lower wall and sloped to bury at least one full block of the upper terrace. Proper embedment should always be considered.
3. Terrace heights should be adjusted to even course numbers to facilitate 2-course maximum spacing.

Figure 10-2: Grid Considerations for Terraced Retaining Walls

10.4   Grid Considerations (Figure 10-2):

1. It is common to have the design grid lengths for each terrace equal to at least 60% of the total terraced structure height.
2. Maintain 2-course spacing by placing the top layer of grid in the last two courses of the lower terraced and starting on top of the first course of the upper terrace.
3. Have the top layer of grid extend under the base of the upper terrace.

10.5   Greater attention to compaction and compaction testing should be placed on the foundation soils below the upper terraces and in transition areas where the wall splits from one wall into two. If the soils are not properly compacted in these areas, settlement can occur over time that could cause aesthetic concerns.

10.6   Toe and heel drains shall be routed as to not exit on the lower terraces. Drainpipes shall be extended to provide a path for water to be channeled away from the wall structure. Pipes at exit locations shall be marked to facilitate identification of where water is draining from.

10.7   These recommendations do not eliminate the need to consider a global stability analysis.

1. Follow global stability recommendations in Chapter 9 Global Stability - General.

10.8   For terraced structures that are classified as tall walls, they should take into account Chapter 8 - Tall Wall Considerations.