INTERPRETATION OF PROCTOR TEST RESULTS
Knowledge of the optimum water content and the maximum dry unit weight of soils is very important for construction specifi cations of soil improvement by compaction. Specifi cations for earth structures (embankments, footings, etc.) usually call for a minimum of 95% of Proctor maximum dry unit weight. This level of compaction can be attained at two water contents—one before the attainment of the maximum dry unit weight, or dry of optimum, the other after attainment of the maximum dry unit weight, or wet of optimum (Figure).
Normal practice is to compact the soil dry of optimum. Compact the soil wet of optimum for swelling (expansive) soils, soil liners for solid waste landfi lls, and projects where soil volume changes from changes in moisture conditions are intolerable.
Illustration of compaction specifi cation of
soils in the fi eld.
When a heavily compacted soil mass (near to maximum dry unit weight) is sheared, it tends to expand (dilate) and gets looser. Usually this expansion is not uniform; some parts of the soil mass are looser than other parts. The fl ow rate of water in the soil will increase as water can easily (compared to the intact one) fl ow through the looser parts, possibly leading to catastrophic failure.
Heavily compacted soils tend to show sudden decrease in strength when sheared. In engineering, if failure is to occur we prefer that it occurs gradually rather than suddenly so that mitigation measures can be implemented. In some earth structures (for example, earth dams) you should try to achieve a level of compaction that would cause the soil to behave ductile (ability to deform without rupture). This may require compaction wet of optimum at levels less than 95% of the maximum dry unit weight (approximately 80% to 90% of maximum dry unit weight).
BENEFITS OF SOIL COMPACTION:
Compaction is the most popular technique for improving soils. The soil fabric is forced into a dense confi guration by the expulsion of air using mechanical effort with or without the assistance of water. The benefi ts of compaction are:
1. Increased soil strength.
2. Increased load-bearing capacity.
3. Reduction in settlement (lower compressibility).
4. Reduction in the fl ow of water (water seepage).
5. Reduction in soil swelling (expansion) and collapse (soil contraction).
6. Increased soil stability.
7. Reduction in frost damage.
Improper compaction can lead to:
1. Structural distress from excessive total and differential settlements.
2. Cracking of pavements, fl oors, and basements.
3. Structural damage to buried structures, water and sewer pipes, and utility conduits.
4. Soil erosion.