The ACI 318-08 code provides guidelines for designing reinforced concrete retaining walls, including counterfort retaining walls, which are used for tall walls or heavy earth loads where a conventional cantilever wall would be inefficient or require excessive thickness. Below is a comprehensive guide to designing a counterfort retaining wall step-by-step according to ACI 318-08.
Understanding Counterfort Retaining Walls
A counterfort retaining wall is similar to a cantilever retaining wall but includes triangular counterforts (thin vertical webs) on the back face of the stem.
- The counterforts act as tension ties, connecting the stem (vertical wall) to the heel slab and base slab.
- This reduces the bending moment in the stem and base slab by making them work like continuous slabs rather than cantilevers.
Applications:
- Wall heights > 6 m (20 ft).
- High earth pressures with heavy surcharge loads.
- Economical for long walls with close spacing of counterforts (8–12 ft apart).
Detailing Requirements (ACI 318-08)
- Minimum reinforcement ratio for wall stem: 0.0018 × gross area (Ag).
- Spacing of main reinforcement: ≤ 18 in. or 3× wall thickness.
- Provide temperature and shrinkage steel in both directions (min. 0.0018Ag).
- Counterfort anchorage: Reinforcement must extend into both the stem and the heel slab.
- Waterproofing and drainage behind the wall to prevent hydrostatic pressure buildup.
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