A series of isotropic compression tests and drained and undrained triaxial compression tests have been performed on glacial sediment from Iceland. Langjökull sediment, which is well graded, is thought to have reached a critical grading during deposition and transportation. Multiple parallel normal compression lines (NCLs) were found, but a unique critical state line (CSL) could be identified. This is unlike other so-called “transitional” soils, whose grading varies between reasonably well graded to gap graded, which tend to have distinct NCLs and critical state lines depending on the specimen density. It is thought that in the case of the Langjökull sediment studied, its particular strain history that involved incessant shearing during deposition accounts for the difference in behaviour. This provides the interesting case of a soil that has been crushed to a critical grading in situ, which depends on the mineralogy of the grains, which was then sampled and tested. Despite the unique grading, samples with a range of different void ratios can be prepared and the combination of grading and density seems to set a fabric that cannot be changed by compression, resulting in multiple parallel NCLs. At the critical state, however, the fabric has been destroyed and the CSL is unique.

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Adding discrete fibers to soils can improve their strength, however fiber reinforcement remains scarce in practice. Previous studies on the performance of soils reinforced with discrete fibers consist mainly of laboratory studies with either clay or, most often, uniform sand as the host soil, so that there is a lack of data on other types of soils such as weathered soils, which tend to be well graded. Unlike uniform soils, which are generally dilative, well graded soils usually show a contractive behavior. This study examines the effect of adding fibers to a completely decomposed granite (CDG) typical of many residual soils which has the characteristics to be sensitive to material and sample preparation and also to be compressive during shearing. It is found that adding discrete fibers to the CDG homogenizes it as the reinforced soil is not sensitive to the method of material or sample preparation. It is also found that despite its compressive nature, fibers mobilize extra strength compared to the unreinforced soil, and this effect does not reduce at large confining stresses.

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