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9 As can be seen, the pogo stick and wacker are much smaller and lighter than the sheepsfoot and self-propelled roller. Soils compacted with the lighter equipment must be placed in thin layers not exceeding 4 inches, while soils compacted by the heavier equipment can be placed in layers up to 12 inches. FIGURE 2.5-tt - VIBRATORY ROLLER. Drums vibrate as roller moves. 2.6 MOISTURE CONTROL The amount-of moisture (water) contained in a soil prior to placement determines several important characteristics of the material during and after com- paction: How easy the material is to work with (e.g., not mud or dust). How much effort will be required to obtain the greatest practical density. How dense a soil can be made by a given amount of compactive effort. The moisture content of the soil under consideration falls into three categories: Optimum (just right) - When the moisture content is at the Optimum Level, there is just enough moisture to allow most grains or par- ticles of soil to move or slide into a more compact structure while the air filling a sig- nificant portion of voids in the soil is forced out. A TOPTlMUM MOISTURE CONTENT, A GIVEN COMPACTIVE EFFORT WILL PRODUCE THE GREATEST DENSITY. Too Dry - When not enough moisture is present, the grains or particles resist change or movement between surfaces and the mate~ rial does not reach the desired density after application of a specified compactive effort. Too Wet - When too much water is present, it allows water to fill a large portion of the voids in the sample. When compactive effort is applied, the pressure is transferred to the water and air, resulting in little change in the soil structure. 2.7 COMPARISON OF PATHWAYS AVAILABLE FOR THE FLOW OF WATER THROUGH A GIVEN MATERIAL We have seen how the structure of sand, silt, and clay provide very different patterns or pathways for the flow of water through the soil. In order to give a comparison of the area available for water to flow through various materials, the following illustration is presented. These commonly used materials will be shown: Sand - properly compacted Silt - properly compacted Clay ~ properly compacted Concrete - properly placed Asphalt - properly compacted One-fourth inch steel plate - in new condi- tion To establish a basis for quick comparison, a shaded square will be used to represent the portion of solid particles and the portion of voids (normally filled by water or air) of a given area of a material we are viewing. The area representing the solid portion is shaded and the void area is clear. LOOKS LIKE Sand e Silt ~ Clay (/j @ Conaete Asphalt - JA" Steel Plate AREA RESISTINU~ THE FLOW OF WATER AREA AVAILABLE FOR WATER TO FLOW THROUGH FIGURE 2.7-) - COMPARISON OF AREA AVAILABLE FOR WATER TO FLOW THROUGH. (NOT TO SCALE) REPRESENTED BY SHADED SQUARE AREA RESISTINGlI THE FLOW OF WATER AREA A V AILABLE FOR WATER TO FLOW THROUGH AREA RESISTING=- THE FLOW OF WATER AREA A V AILABLE FOR WATER TO FLOW THROUGH AREA RESISTING~ THE FLOW OF WATER AREA AVAILABLE FOR WATER TO FLOW THROUGH AREA RESISTING--....=- THE FLOW OF WATER AREA A V AILABLE FOR WATER TO FLOW THROUGH AREA RESISTING THE FLOW OF WATER AREA A V AILABLE FOR WATER TO FLOW THROUGH