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compact2 - 53:030 Class Notes C.C Swan University of Iowa...

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Period #26: Compaction of Soils (II) A. Review 1) Compaction is the process by which loose soils are applied to a construction site and densified. The objective of compaction is to remove air voids from the soil. 2) The degree of compaction of a soil is expressed in terms of its dry density ρ d or dry unit weight γ d . (These measure grain packing.) 3) Moisture density tests are typically performed on soils before they are applied to construction sites. Moisture density tests involve compacting a soil at a number of different water contents w with a specific amount of compactive energy . Results are plotted as dry unit weight versus water content. 4) The maximum degree of compaction that can be possibly be achieved for a given soil at a given water content w is the so-called zero-air voids dry unit weight ( γ d ) zav : ( γ d ) zav = G s γ w /(1+wG s ) In practice, this density can never be achieved. 1 53:030 Class Notes; C.C. Swan, University of Iowa
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2 B. Percent Compaction or Relative Compaction In the field the questions that have to be answered are: a) to what dry density must the soil be compacted? and b) how can this be achieved efficiently? For many construction applications involving roadway subgrades and trench backfills, etc., there are typically codes provided by federal, state, and local agencies specifying the minimum percent compaction or relative compaction that must be achieved. The guidelines typically specify that the field compaction must meet or exceed a certain level as follows: ( γ d ) field R ( γ d ) max-lab where: R is a relative compaction, and ( γ d ) max-lab is the maximum dry density achieved for a given level of compactive effort (Standard Proctor, for example). For many roads owned by local municipalities, codes exist stating that soils in roadway subgrades must be compacted so that the minimum dry density achieved exceeds 95% of the Standard Proctor Maximum dry density.
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