Proctor compaction test is made to determine the moisture content at which soil will be compacted to obtain ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 10/10/2023 (Evening)
Detailed Explanation
In the Proctor test, the same soil is compacted in a standard mould at several different water contents with the same compactive effort. The dry density of each specimen is calculated and plotted against water content. The peak of this curve gives the maximum dry density (MDD), and the water content at the peak is the optimum moisture content (OMC).
Why “maximum dry density” Is Correct
The whole purpose of the test is to tell site engineers how much water to add so that rolling gives the densest, strongest soil. That target is the maximum dry density, reached at the OMC.
Why the Other Options Are Wrong
- A. specific gravity: Specific gravity is a property of the soil solids, found with a pycnometer or density bottle, not by compaction.
- B. minimum dry density: Compaction aims to increase density; a ‘minimum dry density’ is not a design target.
- D. porosity: Porosity does fall with compaction, but the test is defined in terms of dry density, not porosity.
Key Point
Proctor test output = OMC + MDD. Field compaction is then specified as a percentage of this MDD (relative compaction).
Key Concepts for Students
- Light (standard) compaction: 2.6 kg rammer, 310 mm drop, 3 layers × 25 blows (IS 2720 Part 7).
- Heavy (modified) compaction: 4.9 kg rammer, 450 mm drop, 5 layers × 25 blows (IS 2720 Part 8).
- Dry density is used, not bulk density, because added water would otherwise hide the real packing of solids.
Practice more: this is question 4 of Compaction of Soil MCQs – Part 1. Try the whole set and check every answer instantly.