Homework Sourse1 What limitations might there be in using Bjerrums correcti
Solution
1.
The undrained shear strength (su) of fine-grained soils that can be measured in situ and in laboratory is one of the key geotechnical parameters. The unconfined compression test (UCT) is widely used in laboratory to measure this parameter due to its simplicity; however, it is severely affected by sample disturbance. The vane shear test (VST) technique that is less sensitive to sample disturbance involves a correction factor against the soil plasticity, commonly known as the Bjerrum\'s correction factor, . This study aims to reevaluate the Bjerrum\'s correction factor in consideration of a different approach and a relatively new method of testing. Atterberg limits test, miniature VST, and reverse extrusion test (RET) were conducted on 120 remolded samples. The effect of soil plasticity on undrained shear strength was examined using the liquidity index instead of Bjerrum\'s correction factor. In comparison with the result obatined using the Bjerrum\'s correction factor, the undrained shear strength was better represented when su values were correlated with the liquidity index. The results were validated by the RET, which was proven to take into account soil plasticity with a reliable degree of accuracy. This study also shows that the RET has strong promise as a new tool for testing undrained shear strength of fine-grained soils.
History of the correction factor The history of the Swedish correction factor is a summary of SGI Information 3 by Larsson, et al. (2007). The initial correction factor was introduced for fall cone tests. The correction factor was based on comparisons with pile loading tests done in 1900 and back-calculations from failures with circular slip-surfaces. As geotechnical test methods were further developed, the fall cone tests were calibrated again during the 1930’s. That included data from full-scale loading tests, registered failures and landslides. Measurements showed that the fall cone test had to be corrected with respect to the liquid limit. The vane test that is used today was introduced by Cadling and Odenstad in 1950. The test was calibrated based on back-calculated landslides and on one full-scale loading test. The fall cone test was then re-calibrated based on the vane test. It was known that the measured strength values needed to be corrected, the correction from fall cone test was then implemented. The correction was depending on the liquid limit, but no standard had been chosen for determining the corrected shear strength. To set a Swedish recommendation for the correction, SGI had a technical meeting in 1969. It was decided that measured values from both vane test and fall cone test must be adjusted with a correction factor according to the equation; fu f (2.1) where fu = corrected undrained shear strength = correction factor f = uncorrected undrained shear strength The correction factor is depending on the liquid limit of the soil and the recommendation from SGI was to use Figure 2.1 below. This reduction was by some considered to be undersized and thereby a risk of overestimating the strength of the soil. To cope with this, the recommendation was therefore to use carefully and somewhat conservative chosen values of strength. Figure 2.1 Correction factor from 1969 Research has been made to refine this correction. In Andréasson study in 1974, Bjerrum’s theories for the plasticity index were converted to the use of liquid limit. This study was found to give similar design values of shear strength, as when using 4 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2012:61 the SGI method from 1969 with carefully chosen values. This method was therefore not chosen as a new recommendation from SGI. Another study was performed by Helenelund in 1977, which corresponded well with Andréasson’s method. The two methods diverges when the liquid limit is high, where Helenelund’s method has a larger reduction of shear strength. This method was not implemented either but these studies formed the basis for the new SGI recommendation in 1984. The present standard for correction of shear strength was developed in 1984 and is based on both prior experience and newly gathered information. Empirical relations from preconsolidation, loading and Atterberg limits were taken under consideration when developing the new standard. The factor of correction is presented in equation 2.2 below and is shown in Figure 2.2. 0,5 0.43 0.45 wL (2.2) where wL = corrected undrained shear strength Figure 2.2 Overview of correction methods The correction factor was based on results from tests in clays with an OCR of approximately 1.3, the method of correction is therefore applicable for normally and lightly overconsolidated clays. Clays with OCR1.5 are defined as overconsolidated and are also corrected with regard to OCR. The correction factor for overconsolidated clays is calculated by results in lower values on µ. The correction for overconsolidated clays is based on an investigation made by Larsson and Åhnberg where overconsolidated Swedish clays were studied.
2.A post -failure investigation of a highway embankment constructed over organic and sensitive subsoils was made with a series of field vane tests. The results indicated that the embankment was quite stable, and, therefore, the reliabilities of both the it. = 0 slip - circle analysis and the field vane results seemed to be highly questionable. Undisturbed samples taken from an area unaffected by the embankment failure were used to determine the undrained shear strength of these subsoils. The results of both the laboratory vane and unconfined compression tests were appreciably below the results of the field vane tests, but they were not low enough to completely explain the instability of the embankment. It was concluded that a progressive failure of the highly sensitive organic silty clay was partly responsible for the inaccuracy of the = 0 slip -circle stability analysis. However, the major error resulted from the inaccuracies of the field vane results. The effect of the rate of 67 , 4 rotation of the laboratory vane upon the undrained shear strength was found to be insignificant. Since the vane basically determines the shear strength developed on the vertical plane, the difference between the shear strength developed on the horizontal plane and vertical plane was studied. Finally, the method of evaluating rod - friction was investigated. It was found that the use of the \"dummy\" rod for the evaluation of the frictional resistance developed on the field vane\'s torque rod was inaccurate. This fact was demonstrated by the comparison of original field vane test results with results obtained from a modified slip -joint field vane, capable of directly measuring the rod -friction. It was concluded that a modified slip - joint field vane accurately measured the in situ undrained shear strength. The fact that silty clays found at the highway embankment site were sensitive was not indicated by either the original field vane or the modified slip -joint field vane. The actual sensitivity of these subsoils was determined by both the laboratory vane and the liquidity index of the soils. It was concluded that the in situ sensitivity could not be obtained by the modified slip -joint field vane. The one point that the entire investigation indicated was that before the field vane test results can be intelligently used, additional information about the subsoils is required.
