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e e e 3-31 Compaction (moisture-density relationship). - - The moisture-density relationship of soils (for a given camp active effort) should be known before shear testing of disturbed (borrow) materials can be instigated or before field control of moisture and density can be maintained. Compaction and field control may be necessary either (I) to assure adequate density and shear strength in levee or dike fills or (2) to increase the erosional resistance of low density, weakly coherent soils which form the bottoms and sides of channels. Compaction tests should be performed on disturbed samples of selected representative soils on which shear tests are required for design and/ or field control of moisture and density are required during construc- tion operations. Compaction tests are also used to evaluate collapse potential of soils. Most highly collapsible soils are also generally highly erosive. Permeability. - - In those cases where seepage from channel banks may be a problem, the permeability of natural materials may be required to construct flow nets and to analyze seepage forces. Permeability tests should be performed on undisturbed materials which are represent- ative of soils in the problem area. Field permeability tests will usually be the most effective means of determining permeability rates (a) in non-coherent soils which are difficult to sample representatively, handle and transport to the laboratory and (b) in lenticular and highly stratified soils. Methods of making field permeability. tests are described in NEH, Section a2!, Chapter 2; Section 16~, Chapter 2; Special Publ~cation SP-SW-0262 of the American Society of Agricultural EngineersI/; and the Earth Manual, First Edition, U. S. Bureau of Reclamatio~/. A permeability test may also be performed as a secondary and companion test to the unconfined compression test when the purpose of the latter is to provide values for use in an erosional analyses of coherent or cemented soils.2/ (see Chapter 6). The permeability test described hereafter is not necessary when the unconfined compression test is made for the purpose of providing values for a bank stability analysis. A permeability test, as a companion test to the unconfined compression test, should be performed in a falling head permeameter on undisturbed specimens having a ratio of height to diameter equal to approximately two. As soon as consistent head readings are obtained, the test should be discontinued; the specimen should be removed from the permeameter immediately and subjected to an unconfined compression test to determine the coherence of the saturated specimen.