As far as nondestructive testing methods are concerned, acoustic emission techniques are relatively recent additions to the rock monitoring area (dating from the late 1930's) and to the metal testing area (dating from the 1950's). Its application to soils is an even more recent event with little activity prior to the 1970's. However, over the past five to ten years, interest has been generated in the soils area to the point where at least five equipment manufacturers are currently marketing acoustic emission systems specifically for geotechnical engineering applications. This activity is seemingly well founded, for acoustic emissions are indeed generated by deforming soil masses and technical feasibility is now firmly established. This state-of-the-art paper on acoustic emission activity in soils presents these findings on the basis of fundamentals, small-scale laboratory tests, and large-scale laboratory tests. Furthermore, the technique has been applied to field situations in a number of cases. These include slope stability monitoring of dams and embankments, soil movements arising from horizontal and vertical deformations, seepage monitoring, and grout/hydrofracture monitoring. Specific case histories in each group are presented. Collectively taken, the information available seems encouraging enough for many investigators to use the technique for a wide variety of applications. With a multi-faceted attack, the current qualitative status of the technique (that is, no acoustic emission indicates stability; low acoustic emission indicates small movement; moderate acoustic emission indicates larger movement; high acoustic emission indicates instability) should move into a better defined quantitative status. In this latter case, acoustic emission signatures of different soils could lead to instant assessment of actual stress levels in any given situation.

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An investigation of ten rock types in unconfined compression has led to four distinct types of stress versus acoustic emission response curves, that is, signatures. These are the originally proposed Mogi type, and three variations of this generalized behavior, that is, unstable, dense, and dense unstable types. They have been correlated to conceptual ideas on brittle rock behavior. Frequency analysis on all rock types at each stage of the various stress ranges has also been performed. Trends in peak frequency shifting are presented with implications as to the use of this information.

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