Introduction
Embedment requirements play a pivotal role in the construction and longevity of Segmental Retaining Walls (SRWs). Contractors and engineers working on these projects need a thorough understanding of the precise embedment criteria to ensure the walls’ stability and durability. This article provides a comprehensive overview of these criteria, considering factors like frost protection, erosion control, and global stability. We’ll delve into the guidelines provided by authoritative sources such as the NCMA Design Manual, FHWA-NHI-10-024/25 (Federal Highway Administration), and AASHTO* (American Association of State Highway and Transportation Officials).
Understanding Embedment and Its Importance
Embedment refers to the depth to which a retaining wall is buried into the ground. It is a critical factor in ensuring that the wall remains stable and can resist external forces such as soil pressure and ground movement. Proper embedment prevents issues like wall scour, sliding, or even collapsing due to erosion. The depth of embedment depends on various factors, including wall height, front slope, soil type, and specific project requirements.
Considerations for Frost and Global Stability
- Frost Considerations: While frost heave can impact some regions, it is not the primary concern for flexible modular retaining walls. Hence, placing the bottom of wall below the local frost line is not a requirement for these systems like it is for rigid structures. The focus lies more on factors like erosion protection, bearing capacity, and overall global stability. Where frost susceptible soils are a concern, increasing the leveling pad (base material) thickness can limit movement in the facing from frost heave.
- Global Stability: Global stability is a critical consideration for retaining walls, encompassing the wall’s ability to resist sliding, overturning, and full slope collapse. Proper embedment contributes significantly to global stability by anchoring the wall into stable soil layers. This prevents excessive movement and ensures that the wall can withstand lateral pressures caused by external forces or slope conditions. While general guidelines on embedment should be considered the minimum, global stability can be improved by increasing embedment as an alternative to excessive reinforcement (geogrid) lengths.
| Toe Slope Condition | Minimum Total Embedment (NCMA) | Minimum Bench Embedment (AASHTO/FHWA) | DIY Retaining Wall Recommends |
|---|---|---|---|
| Horizontal (walls) | H’/20 | H/20 | 0′ + H’/20 |
| Horizontal (abutments) | H’/10 | H/10 | 0′ + H’/10 |
| 3H:1V | H’/10 | H/10 | 1.3′ + H’/10 |
| 2H:1V | H’/7 | H/7 | 2′ + H’/10 |
| 1.5H:1V | – | H/5 | 2.7′ + H’/10 |
| Minimum | 0.5’ | *2.0’ | per code |
| *Provision for 1′ minimum embedment exists when there is no erosion potential, no future excavations planned, and the soils are not frost susceptible | |||
Note, NCMA bases minimum embedment on exposed height, H’, and does not acknowledge bench considerations. AASHTO bases minimum embedment on total height and requires a 4′ bench for sloping toe conditions in addition to the table requirements. We here at DIY Retaining Wall recommend that at a minimum, that the 4′ horizonatl bench embedment meet the minimum level slope requirements for that wall height.
Increased embedment should be evaluated by an engineer for the following cases:
- Walls constructed along rivers and streams to prevent scour
- With soils that have shrinking and swelling potential (fat clays)
- Seismic regions
- Steep slopes beyond the toe of the wall
- Large settlement potential or weak bearing capacity of underlying soils
Any wall over 4′ in total height, near slope, or supporting a surcharge requires an engineer. If you need an engineer for your project, or you are an engineer wanting futher clarification, check out our services.
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