What is the target compaction for the slab subgrade?

• A. 90%
• B. 95%
• C. 100%
• D. 85%

Answer: (B)

The main idea tested is what density the soil under a slab needs to reach before the concrete goes on. For slab-on-grade work, the target is 95% relative compaction of the Standard Proctor maximum dry density. This level gives a dense, uniform bearing surface that reduces differential settlement and helps the slab distribute loads evenly.

Why that level fits best: 95% is tight enough to minimize settlement and rutting under the weight of the slab and foot traffic, yet not so tight that it overworks the soil or impedes drainage. If the subgrade is too loose (like 85%), it can consolidate under load, causing cracks or uneven support. If it’s at 100%, some soils can become overly stiff and lose drainage, which isn’t typically necessary or desirable for a stable slab base. While some projects tolerate slightly lower targets, 95% is the common specification that balances performance and practicality. Field tests (nuclear gauge or similar) confirm the percent of maximum dry density achieved, using the appropriate Proctor test.