Sample Preparation and Compaction PROCEDURES

Figure 13-1.—Apparatus for soil compaction testing. fitted with a detachable base plate and a removable extension collar that is 2 1/2 inches high. The larger cylinder is the CBR mold. It is 6 inches in diameter, 7 inches high, and is fitted with a base plate and a 2-inch-high extension collar. When you are compacting a soil sample, a 2 1/2-inch-thick spacer disk is placed inside the CBR mold to control the thickness of the compacted sample. With the spacer disk in place, the volume of the mold is about 0.0735 cubic feet. The CBR mold is used for samples containing material retained on the No. 4 sieve. The compaction tamper consists of a drop tamper in a cylindrical guide. The tamper has a drop weight that weighs 10 pounds and has a striking face that is 2 inches in diameter. The guide sleeve regulates the height of drop to 18 inches. To use the compaction tamper, you place the guide on top of the specimen and then draw the tamper to the top of the guide and allow it to drop. Other items that you need to perform compaction testing are a balance or scale for weighing the material 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 for striking excess material from the top of the mold after compaction. Sample Preparation and Compaction Procedures About five specimens, containing successively increasing moisture contents, are needed to determine the OMC at which the maximum density for a given compactive effort will occur. For the Proctor 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, or about 60 to 70 pounds total. Before the compacting begins, the sample is air- dried and a moisture content of the air-dried material is determined. Airdrying is done by spreading out the material in the sun or in front of an electric fan. The water content of the air-dried material is determined as a basis for estimating the amount of water you need to add to each trial specimen. The driest specimen should contain just enough water to produce a damp mixture that crumbles readily. For each succeeding specimen, increase the water content by about 2 percent until the wettest specimen is quite wet and plastic. The compaction procedures for nongravelly and gravelly 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, 25 tamper 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 two mold sizes and soil volumes. To compact the soil, you first attach the base plate and collar to the mold. Then you fill the mold to the top of the collar with the material placed in five equal layers, compacting each layer with the appropriate 25 or 55 equally distributed blows. After compacting the 13-2