抄録
Determination of undrained shear strength Su and sensitivity St of soft soils and sediments are important in geotechnical design as well as in sediment management. Usually these properties are deduced from laboratory or field tests. In lab, vane shear test (VST) is commonly used in clayey soft soils. Other testing is available such as the full flow penetrometers mainly used for in situ offshore engineering and for centrifuge modeling applications. Among them, the T-bar test (TBT) is not really used in geotechnical lab at 1 g level in small layers of soft soils put in containers. This paper deals with measurement of undrained shear strength (Su) of soft clays (kaolin clay) and dam sediment under conditions of natural drying, consolidation and drainage, using two different methodologies, namely T-bar and vane shear tests. Vane shear test is a standard test for Su measurement but not applicable for obtaining a continuous profiling of Su versus depth and it provides rather discrete measurements. Hence T-bar test has been used to observe a continuous phenomenon during shearing. Many studies have been reported comparing standard vane shear and T-bar tests to ensure applicability of the latter. Two different types of soils was tested to check the repetitiveness of testing and to validate previous works performed on Su measurement of kaolin clay using T-bar tests.Besides, in order to measure remolded shear strength, the vane is rapidly rotated through several revolutions until the clay becomes remolded. Again the same procedure is followed to measure the remolded shear strength. The ratio of two shear strengths measured (i.e. peak strength and remolded strength respectively) gives the sensitivity St. In addition to this, for comparative studies of TBT and VST, the methodology involves the rotation of vane blade 90 degrees anticlockwise and then 90 degrees clockwise. Although, this method is not usually used for VST testing, it enables to compare Su measured in anticlockwise rotation with the downward movement of T-bar. Likewise, Su is measured during clockwise rotation with the upward movement of T-Bar. In order to obtain compatibility between measurements made with T-bar and vane shear tests, it is interesting to study the ratio of measurements made by both tests. So, in order to do that, the peak value of each curve from vane shear test is compared with the average value of Su measured by T-bar test along the depth of blade insertion. This practice is appropriate for comparison because Su has been assumed as uniformly distributed over the entire height of the vane. So, in order to test the suitability of the comparison, the above practice is followed for all tests in kaolin clay and dam sediments. But still, it was interesting to observe the average of the ratios measured by different numbers of tests in kaolin and dam sediments and to compare them in order to see the applicability of the tests for different types of soils. So, average values of ratio (r), r = Su (VST)/Su (TBT) are proposed. All test results are reported and discussed in the paper and conclusions are given.
