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| Question | Answer |
| Compaction | Densification of an unsaturated soil by a reduction in the volume of voids filled with air, while the volume of solid and water remain essentially the same. |
| Cosolidation | The process in which water is squeezed out under the action of a continuous static load. |
| Objectives of Compaction | 1) Increase shear strength 2) Decrease compressibility 3) Decrease permeability 4) Control swelling and shrinkage 5) Reduce liquefaction potential 6) Prolong durability |
| Four factors affecting the extent of compaction: | 1. Compaction effort 2. Soil type and gradation 3. Moisture content 4. Dry unit weight (dry density). |
| Compaction –General Principles | see pg 6 |
| purpose of a laboratory compaction test | is to determine the proper amount of mixing water to use when compacting the soil in the field and the resulting degree of denseness which can be expected from compaction. |
| Structure of Compacted Clay Soil | ‐ For a given compactive effort and dry density, the soil tends to be more flocculated (random) for compaction on the dry side as compared on the wet side. ‐ Increase in water content and compactive effort tends to disperse (parallel oriented clay particle) the soil. |
| Effect of Compaction on Cohesive Soil Properties | Increasing the water content results in a decrease in permeability on the dry side of the optimum moisture content and a slight increase in permeability on the wet side of optimum. |
| Effect of Compaction on Cohesive Soil Properties | Increasing the compactive effort reduces the permeability since it increases the dry density, thereby reducing the voids available for flow. |
| Effect of Compaction on Cohesive Soil Properties | ‐ Under lower pressure soil compacted on the wet side of the optimum is more compressible than the sample compacted on the dry side of the optimum. ‐ Under high pressure the trend is opposite. |
| Effect of Compaction on Cohesive Soil Properties | Strength of compacted clayey soil generally decreasing with moisture content. |
| Conventional Earthwork | 1. Clearing & Grubbing 2. Excavation 3. Transportation and Placement 4. Moisture conditioning – if soil is too dry spray water from a water truck and mix thoroughly the soil using bulldozers or other equipment so that water is uniformly distributed. 5. Compaction |
| Four common types of rollers: | 1. Smooth‐wheel rollers ‐ subgrade and finishing operation with sandy and clayey soil. 2. Pneumatic rubber‐tired rollers: ‐ Heavily loaded with several rows (4‐6) of tires ‐ Suitable for sandy and clayey soil ‐ Compaction is the combined effect of pressure & kneading 3. Sheepsfoot rollers: ‐drums with a large number of projections ‐ compact by pressure and manipulation ‐Can be used for variety of soils, but work best in silts and clays 4. Vibratory rollers: ‐ suitable for granular soils (sand & gravel) ‐ could be attached to Smooth‐wheel, Pneumatic rubber‐tired, and Sheepsfoot rollers ‐ compact by pressure, manipulation and vibration |
| A roadway section will include the following four courses in ascending order: | • Subgrade • Sub‐base course • Base course • Surface course |
| Three Methods to determine the Field Unit Weight of Compaction | 1. Sand cone method 2. Rubber balloon method 3. Nuclear method (often used) |
| Essential Points: | 1. Compaction is the densification of a soil by the expulsion of air and the rearrangement of soil particles. 2. The Proctor test is used to determine the maximum dry unit weight and the optimum water content and serves as the reference for field specifications of compaction. 3. Higher compactive effort increases the maximum dry unit weight and reduces the optimum water content. 4. Compaction increases strength, lowers compressibility, and reduces the permeability of soils. 5. A variety of field equipment is used to check the dry unit weights achieved in the field. Popular field equipment includes the sand cone apparatus, the rubber balloon apparatus, and the nuclear density meter. |