To prevent failure during compaction of the overlying RAP layer, what should the minimum LBR value of the soil be?

• A. 40
• B. 20
• C. 30
• D. 50

Answer: (A)

A stable subgrade under the weight and dynamic loads of compaction is essential for the RAP layer to compact properly. LBR reflects how well the soil can bear load without deforming; a higher LBR means a stiffer, stronger foundation. If the underlying soil doesn’t meet a sufficient LBR, it will deform or fail when the rollers compact the RAP above it, leading to rutting, pumping of fines, and poor contact in the RAP layer. So, the minimum LBR value must be high enough to resist those pressures and keep the subgrade from yielding during compaction. Among the options, the value that meets this requirement without being excessively high is the correct choice, because lower values would not provide enough stiffness, while a higher value would also satisfy the need but is not the minimum.