Figure 13-1.—Apparatus for soil compaction testing.fitted with a detachable base plate and a removableextension collar that is 2 1/2 inches high.The larger cylinder is the CBR mold. It is 6inches in diameter, 7 inches high, and is fitted with abase plate and a 2-inch-high extension collar. Whenyou are compacting a soil sample, a 2 1/2-inch-thickspacer disk is placed inside the CBR mold to controlthe thickness of the compacted sample. With thespacer disk in place, the volume of the mold is about0.0735 cubic feet. The CBR mold is used for samplescontaining material retained on the No. 4 sieve.The compaction tamper consists of a drop tamperin a cylindrical guide. The tamper has a drop weightthat weighs 10 pounds and has a striking face that is2 inches in diameter. The guide sleeve regulates theheight of drop to 18 inches. To use the compactiontamper, you place the guide on top of the specimenand then draw the tamper to the top of the guide andallow it to drop.Other items that you need to perform compactiontesting are a balance or scale for weighing thematerial in grams, a 3/4-inch and a No. 4 sieve,moisture canisters, and tools, such as a mixing pan,spoon, trowel, spatula, and a steel straightedge forstriking excess material from the top of the mold aftercompaction.Sample Preparation and CompactionProceduresAbout five specimens, containing successivelyincreasing moisture contents, are needed todetermine the OMC at which the maximum densityfor a given compactive effort will occur. For theProctor mold, about 6 pounds for each specimen(about 30 pounds total) is needed. For the CBR mold,you will need about 12 to 14 pounds per specimen, orabout 60 to 70 pounds total.Before the compacting begins, the sample is air-dried and a moisture content of the air-dried materialis determined. Airdrying is done by spreading out thematerial in the sun or in front of an electric fan. Thewater content of the air-dried material is determinedas a basis for estimating the amount of water youneed to add to each trial specimen. The driestspecimen should contain just enough water toproduce a damp mixture that crumbles readily. Foreach succeeding specimen, increase the water contentby about 2 percent until the wettest specimen is quitewet and plastic.The compaction procedures for nongravelly andgravelly soils are the same with two exceptions. First,the 4-inch Proctor mold is used for fine-grained soil,and the CBR mold is used for gravelly soil. Second, 25tamper blows per layer are used for the Proctor mold,and 55 blows per layer are used in the CBR mold.That results in equal compactive efforts for the twomold sizes and soil volumes.To compact the soil, you first attach the baseplate and collar to the mold. Then you fill the mold tothe top of the collar with the material placed in fiveequal layers, compacting each layer with theappropriate 25 or 55 equally distributed blows. Aftercompacting the13-2
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