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  3 Compaction 3.1 DENSITY MEASUREMENTS The relative density of a granular deposit is a major factor in determining if stabilization is required or desirable. It is defined in terms of void ratio, e(see equation 2.3). In the laboratory, the minimum value of e is approachedby pouring the soil sample into a container with minimum drop height. Thestate of maximum density is approached by tapping, tamping, or vibratingthe cylinder until the volume no longer decreases. ASTM standards describethe procedures in detail (ASTM D4253 and D4254). The in-situ void ratio,which must be known in order to compute relative density, cannot bedetermined in the laboratory since all granular samples are disturbed, andcannot be reconstituted in the laboratory with any reasonable precision. (Bysampling granular deposits which have been grouted, it is possible to obtainin-situ properties. Procedures are discussed in the chapter dealing withacrylic grouts. Frozen samples can also be used, but may induce error due tothe volume change when water freezes.)In-situ density of a granular deposit at or near the ground surface isgenerally determined by field tests. These include sand cone, rubber balloon,and nuclear methods. These tests are described in ASTM Standards D-1556,D-2167, and D-5195. Density of deep deposits may be estimated from theresults of probe tests such as the Standard Penetration Test (SPT), and the Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  Cone Penetration Test, CPT. See ASTM Standards D-1586 and D-3441.Field density tests are used extensively to determine if modification is neededand to monitor the progress and results of modification (compaction)efforts.When field compaction is to be done it is necessary to establish thegoals to be reached. These goals must not only be adequate for the proposedconstruction, but must also be economically attainable. They are generallyestablished by laboratory tests, which are empirical in nature. Nonetheless,they are backed by a wealth of field data, and are universally used.The earliest procedure to come into widespread use was the ProctorTest, intended for use in the design of roads and air field runways. It is stillin use today and described in ASTM Standard D-698. However, as vehiclesand planes grew larger and heavier, the Proctor was modified to account forthe greater supporting capacity needed (ASTM Standard D-1551). All of these compaction tests consist of applying compactive effort to a containedsoil sample by dropping a weight a number of times on layers of the sample.Details are given in Table 3.1. Each test requires compacting several samplesof the same soil with the same compactive effort, but at different watercontents. The resulting data are plotted as shown in Figure 3.1From the test data, the maximum dry density and the optimum watercontent to achieve that density can be determined. It is, of course, extremelydifficult in the field to compact to precisely the maximum density at preciselythe optimum water content. Specifications, therefore, are always written tocall for a percentage such as 90 or 95 %  of maximum density and a watercontent somewhat below optimum (at water contents higher than optimum,the supporting ability of the compacted soil degrades rapidly).Figure 3.1 shows typically that for any soil as the compactive effortincreases, the maximum density increases and the optimum water contentdecreases. The approximate relative moisture density relationships for thesoil designations of the Unified Classification System are shown inFigure 3.2. T ABLE  3.1  Laboratory Compaction Test Data TestNo. DesignationNumberof layersWeight of hammer, lbsHeight of drop, in.Number of blows/layerCompactiveeffort, inft-lbs/ft 3 A Stand. Proctor 3 5.5 12 25 12,400B Mod. AASHO 5 10.0 18 25 56,200C — 5 5.5 12 25 20,600D — 3 10.0 18 25 33,750E Mod. AASHO(CBR Mold)5 10.0 18 55 56,200 + Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  3.2 SHALLOW COMPACTION Shallow compaction in the field is accomplished by rolling or vibrating.Rolling is done with ‘‘sheepsfoot’’ drums, round steel drums and rubbertired vehicles, as shown in Figure 3.3. Rollers come in various sizes andweights, and rolling is continued until field tests verify that the desireddensity has been attained. Each roller traverse over a specific area is called apass. Each successive pass produces less compaction, so the number of passes is economically limited by the diminishing returns. This is illustratedin Figures 3.4. and 3.5. Vibratory machines range in size from small hand-propelled units tolarge motor-driven machines. They also show diminishing returns as thenumber of passes increases. The applicability of equipment to various soilsis shown in Table 3.2a. Compaction characteristics of various soils areshown in Table 2.9.The applicability of various types of compactors to different soils isshown in Table 3.2b. F IGURE  3.1  Moisture density relations. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  T ABLE  3.2A  Applicability of Compaction Equipment Equipment Most-suitable soils Typical applications Least-suitable soilsSmooth wheel rollers, static orvibratingWell-graded sand-gravelmixtures, crushed rock,asphaltRunning surface, base courses,subgrades for roads andrunwaysUniform sandsRubber-tired rollers Coarse-grained soils withsome finesRoad and airfield subgradeand base course proof-rollingCoarse uniform cohesionlesssoils, and rockGrid rollers Weathered rock, well-gradedcoarse soilsSubgrade, subbase Clays, silty clays, uniformlygraded materialsSheepsfoot rollers:Static Fine-grained soils with morethan 20 %  finesDams, embankments,subgrades for airfields,highwaysClean coarse-grained soils,soils with cobbles, stonesVibrating As above, but also sand-gravelmixturesSubgrade layersVibrating plate (light) Coarse-grained soils, 4 to 8 % finesSmall patches Cohesive soilsTampers, rammers All types Difficult-access areasImpact rollers Wide range of moist andsaturated soilsSubgrade earthworks (exceptsurface)Dry, cohesionless soilsHausmann, M. R., ‘‘Engineering Principles of Ground Modification’’, p 27, McGraw-Hill Co., 1990. Reproduced by permission of McGraw-Hill Companies. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.


Jul 23, 2017
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