ABSTRACT

The relative repeatability of the fall cone, motorized laboratory vane and torvane, when testing the undrained shear strength of extremely weak, saturated, cohesive sediments, has been established.

The twenty-six sediment samples tested were collected from an artificial settling pond, tested for shear strength and analyzed for their chemical, mineralogical and physical properties. Similarity among the samples was verified by x-radiography and a statistical analysis of their physical parameters. The repeatability of shear strength measurements within each instrument and the significance of differences among the instruments were then determined.

The fall cone showed the best repeatability with the motorized laboratory vane next and the torvane last. The individual repeatability of all three devices when testing extremely weak sediments is poor. Differences among the instruments were not significant at the 5 percent level. Instrument characteristics and testing methods rather than sediment characteristics, are most likely responsible for differences among the instruments and for the low repeatability.

None of the instruments are especially well suited for testing extremely weak sediments, however, because of its greater repeatability the fall cone is preferred to the other two devices.