How is the moisture/density relationship of soil obtained? The two standard methods available for determining the moisture/density relationship differ mainly in the amount of energy used to compact the soil sample. The standard Proctor Test is defined in American Society for Testing and Materials (ASTM) D 698, "Laboratory Compaction Characteristics of Soil Using Standard Effort." Proctor Tests are conducted by compacting a soil sample into a mold of know volume. The compactive energy for a standard Proctor Test is provided by a 2.5-kg (5.5-lb) hammer falling 305-mm (12-in). For the modified Proctor Test, a 4.5-kg (10-lb) hammer falling 457-mm (18-in) is used. Compaction of the sample occurs in three lifts and 25 blows of the hammer are applied to each lift. The volume of the mold is known. In each test procedure the mold is 102-mm or 152-mm (4-in or 6-in) in diameter. The initial soil sample is dried and separated into four parts. Water is added to each part to create soil-water mixtures at different moisture contents. Soil from one mixture is added to a mold and compacted. The dry unit-weight of the soil in the mold is determined and plotted on a graph at the value corresponding to the moisture content. The process is repeated for each mixture. The four points so determined provide a parabolic curve with its peak at the value of the maximum dry density and optimum moisture content of the soil.
The specified values for percentage of maximum dry density and moisture generate a range of acceptable values for compaction. A specific dry density less than maximum can have two values of moisture content. For that reason, a range of moisture values is specified. The value of the moisture content is as important an evaluation factor as the percentage density attained. Acceptance under the established specification requirements is based on both percentage of maximum dry density and the range of moisture content. If both criteria are not met, the specified conditions have not been attained.
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How do you measure the compaction of soils? The prevailing method for measuring compaction during placement ot the soil is with a nuclear moisture density gauge. The methods for using a nuclear gauge for soil moisture/density testing are defined in ASTM D 2922, "Density of Soils and Soil-Aggregate in Place by Nuclear Methods (Shallow Depths)," ASTM D 5195, "Density of Soil and Rock In-Place at Depth Below Surface by Nuclear Methods" and ASTM D 3017, "Water Content of Soil and Rock In-Place by Nuclear Methods." A technician with a nuclear gauge can quickly and effectively check both the moisture and density of in-place soil. An onboard computer calculates the percentages of maximum density and moisture content. Special training and a license are required for safe handling of the instrument because of the nuclear material enclosed in the device.
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