Soils close to optimum water content require what type of compactive effort to achieve the required relative compaction?

Soils near optimum water content generally require less compactive effort to achieve the desired level of relative compaction. This is based on the relationship between moisture content and soil density. When soil is at or close to its optimum moisture content, the particles are able to achieve maximum density with less energy input because the moisture lubricates the particles, allowing them to rearrange more efficiently during compaction.

At optimum moisture content, the soil's cohesion is maximized, and the voids of the soil are minimized, which enhances the soil's ability to compact efficiently. This characteristic allows for effective packing of soil particles with less energy, as opposed to soils that are too dry or too wet, which require more effort to reach the same compaction levels.

Higher moisture content can lead to a reduction in required compaction energy as well, but within the specific range around optimum, it is the surface tension of water that allows soil particles to settle more snugly together, making additional compactive effort less necessary. Therefore, achieving the required relative compaction close to optimum water content can be accomplished with less overall compaction effort.