D.F.T. Nash

Frequency Domain Determination of G~0 Using Bender Elements

Methods for determination of shear wave velocity (vs) with bender elements using frequency domain techniques are reviewed. An enhanced method is proposed, which is quicker and provides more information than existing techniques. The method can be implemented using a spectrum analyzer—a common piece of hardware. The validity of the method is demonstrated by means of a series of bench top tests on a reconstituted clay sample. The results are compared both with existing frequency domain practice and with established time-domain techniques. The results reveal that the system comprising benders elements and soil sample is dispersive. The relationship between velocity and frequency is established in detail for several propagation paths through the sample, allowing some insight to be gained into the factors influencing the dispersion characteristics.

Horizontally Mounted Bender Elements for Measuring Anisotropic Shear Moduli in Triaxial Clay Specimens

Novel horizontally mounted bender element devices capable of high-quality transmission and reception of horizontally propagated shear waves polarized in orthogonal planes across the mid-height of a triaxial clay specimen are described. Mounting of these mini benders, in a T-shaped configuration, is in the pads of a radial strain belt on 100 mm diameter triaxial samples, or alternatively as bender probes (similar in concept to mid-height pore pressure probes) suitable for use on triaxial samples down to 38 mm diameter. This latter type may also be used in bench-top tests or on site. The effective fabrication procedures that have been developed are described. The instrumentation systems used to drive and receive signals are outlined, and estimates of the magnitude of the shear strains developed by the bender elements and the accuracy with which shear wave velocities can be determined are discussed. These new bender elements enable both anisotropic shear moduli to be measured over the same path length on a single triaxial specimen that can be taken to a wide variety of anisotropic stress states. Comparison with measurements using conventional platen-mounted bender elements suggests these may underestimate the shear modulus by up to 20%.

Influence of Destructuration of Soft Clay on Time-Dependent Settlements: Comparison of Some Elastic Viscoplastic Models

AbstractThe time-dependent settlement of soft soils following the application of surface loading may be modeled using elastic viscoplastic constitutive models to describe the soil behavior. For applied loadings that increase the stresses toward and beyond the in situ yield stress, the predicted behavior is strongly influenced by the associated breakdown of the clay structure and the way in which this is modeled. Four elastic viscoplastic models are compared and it is shown that, despite presentational differences, all calculate creep rate in fundamentally the same manner. This paper describes some predictions for a hypothetical case prediction exercise organized recently by the Norwegian Geotechnical Institute that compared different calculation methods used in settlement analyses of soft soil. The results presented here are from a one-dimensional coupled consolidation analysis implemented in a spreadsheet-based software and using a finite-element program. Parameters were obtained from an oedometer test, ...

The Effects of Controlled Destructuring on the Small Strain Shear STiffness G0 of Bothkennar Clay

The important differences between natural and reconstituted soils are well recognised. In a natural structured clay, the microstructure enables it to exist at states outside the state boundary surface for the reconstituted soil, resulting in greater peak undrained strength and yield stress at a given void ratio. The structure of soft clays is gradually destroyed by strain; understanding the process of destructuring is important both in developing constitutive models and in understanding the differences between field and laboratory behaviour.

A Comparison of Four Elastic Visco-Plastic Models for Soft Clay

The time-dependent settlement of soft clays following application of surface loading may be modelled using elastic visco-plastic constitutive models to describe the soil behaviour. For applied loadings that increase the stresses to around the in-situ yield stress, the predicted behaviour is strongly influenced by the associated breakdown of clay structure and the way in which this is modelled. Four elastic visco-plastic models have been compared and all calculate the creep rate in fundamentally the same manner by comparison to the state at the same effective stress on a reference isotache.