When constructing retaining walls in California that exceed 4 feet in height, which CBC requirement must be met regarding drainage?
Correct Answer
B) Install drainage behind the wall with a subdrain system
CBC Section 1807.2.3 requires drainage systems behind retaining walls to prevent hydrostatic pressure buildup. This includes subdrain systems, drainage material, and proper outlet systems. California's seismic activity makes proper drainage critical to prevent wall failure from combined seismic and hydrostatic forces during wet seasons.
Why This Is the Correct Answer
CBC Section 1807.2.3 mandates comprehensive drainage systems behind retaining walls exceeding 4 feet in height. This requirement includes subdrain systems with proper drainage material and outlet systems to prevent hydrostatic pressure buildup. California's seismic conditions make this critical, as combined hydrostatic and seismic forces during wet seasons can cause catastrophic wall failure without adequate drainage relief.
Why the Other Options Are Wrong
Option A: No specific drainage requirements for walls under 6 feet
This is incorrect because CBC specifically requires drainage systems for retaining walls over 4 feet in height. There are no exemptions for walls under 6 feet - the 4-foot threshold triggers mandatory drainage requirements under Section 1807.2.3.
Option C: Install a French drain at the base only
A French drain at the base only is insufficient per CBC requirements. The code mandates a complete drainage system behind the wall including subdrain systems, drainage material, and proper outlets - not just base drainage which cannot adequately relieve hydrostatic pressure along the wall height.
Option D: Provide waterproofing on the back face only
Waterproofing alone on the back face is inadequate. While waterproofing may be beneficial, CBC Section 1807.2.3 specifically requires active drainage systems including subdrains to relieve hydrostatic pressure. Waterproofing without drainage can actually trap water and increase pressure buildup behind the wall.
Memory Technique
Remember '4-DRAIN': walls over 4 feet need complete DRAINAGE systems, not just waterproofing or base drains.
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