If soil is unstable, a granular bedding material is specified before laying new pipe. How thick should this material be?

• A. 1-2 in
• B. 4-6 in
• C. 6-8 in
• D. 2-4 in

Answer: (C)

When soil is unstable, you need a thicker cushion under the pipe so the load is spread more evenly and the pipe is protected from settlement and deformation. Six to eight inches of granular bedding provides a solid, uniform base that fills small voids and smooths irregularities in the trench bottom. This thickness is standard for unstable soils, offering enough bearing and leveling room without being excessive. Thinner options (1–2, 2–4, or 4–6 inches) don’t provide sufficient cushioning or load distribution in soft or uneven ground, increasing the risk of settlement or misalignment.