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Shear modulus and damping properties of sands from cyclic self-boring pressuremeter tests Murthy, R. Thandava


The thesis considers the problem of assessment of shear modulus and damping properties of cohesionless soils using the self-boring pressuremeter. The self-boring pressuremeter test, which is widely acknowledged as the closest approach to undisturbed in-situ testing, is very frequently used to obtain values of shear modulus in deformation calculations. However, damping calculations from the SBPMT have been rarely attempted. It is understood that both the shear modulus and damping appear to be complex functions of many variables, as a result of which there has been a wide range of values reported in literature making it difficult to choose appropriate values for a particular problem. This thesis presents a study of the influence of various factors such as stress and strain levels, creep, equipment effects and testing procedure on the shear modulus and damping of sands using the SBPMT. The importance of the standardisation of testing procedure and interpretation methods is discussed. The mathematical models that could be used to calculate damping, assuming a visco-elastic model, from the unload-reload loops are presented. The values of shear modulus were found to depend highly on the testing procedure. Any amount of unloading seemed to yield reasonable unload-reload moduli provided the Wroth (1982) criterion for elastic unloading is adhered to. However, installation disturbance affected the obtained moduli in the initial region of the pressure-expansion curve. Comparison of the shear modulus profiles with reference profiles of the modulus from seismic cone penetration tests showed reasonable agreement provided the dominant equipment effects are acknowledged. It was found that reasonable modulus reduction curves can be obtained irrespective of the stress level. Creep seems to have a major effect on the shear modulus. It appears that cavity strain/minute during the holding phase before unloading has to be kept extremely low if reasonable results are to be obtained. Interpretation methods are semi-empirical involving elaborate correction procedures. The use of an average stress seems to provide good results when loops are carried out at the similar strains. Equipment effects and calibration of the equipment were the most important factors affecting the results at this stage of the SBPM research. Damping was found to be affected highly by strain level and creep. No significant effect of stress level on damping was observed. The intermediate range of rates of inflation or frequency of loading used in this investigation did not seem to affect damping results. It was observed that average damping curves could be obtained over a strain range for any depth. The equipment effect was the most dominant factor affecting damping as in the case of the shear modulus. Understanding the limitations of the visco-elastic model, it was observed that the model could provide a good calculation technique for damping. Both the shear modulus and damping seem to be fairly consistent with the reference results from the SCPT and other published results suggesting that the SBPMT could become a popular test to obtain these properties.

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