Laboratory investigation of the mechanical properties of Cowden till under static and cyclic conditions
Creators: name: Ushev, Emil
Publisher: name: Imperial College London
Year: 2017

description: This Thesis describes a comprehensive laboratory investigation into the mechanical properties of Bolders Bank glacial till sampled at the PISA project Cowden test site, near Hull in Humberside. The PISA project aimed to modernise the foundation design procedures for offshore windturbines, many of which are being installed in strata such as the stiff low pasticity till encountered at Cowden. Advanced laboratory testing should be of value to a far greater range of onshore and offshore applications, covering both static and cyclic loading cases. High quality rotary and block samples of Cowden till were employed in advanced triaxial and other tests, making use of local pore pressure probes, high resolution local strain sensors and dual axial bender elements. Static experiments established profiles of compression and extension shear strengths and stiffness with depth, as well as tests that investigated behaviour under higher effective stress conditions imposed by pile installation, the influence of strain rate and sample size. Ring shear experiments investigated residual shear strength and soil-steel interface behaviour. Parallel triaxial programmes were run on both natural and reconstituted specimens. Small strain drained probing tests investigated the till's cross – anisotropic elastic stiffness. A comprehensive programme that imposed thousands of undrained cycles investigated the influence of cyclic loading amplitude and deviatoric stress offset on the potential for cyclic failure, as well as its impact on stiffness, damping, pore pressure development and permanent strain accumulation. The Cowden till's response to static and cyclic loading is interpreted within the critical state framework which recognises the existence of Kinematic Yield Surfaces (KYS) within the till's large scale yield surface. The experiments reveal that the till's large strain mechanical behaviour fits well with key features of classical, ductile critical state models with almost no difference between ultimate and residual angles of shearing resistance and ultimate compression shear strengths mapping onto critical state v – ln(p') lines that were broadly parallel to those found under K0 or isotropic normal compression paths. However, corrections had to be made for the effects of the till's variable gravel contents when comparing reconstituted and natural specimens. Extension tests also showed quite different behaviour at large strains to those seen in compression. Other significant deviations from the classical models include the till's anisotropic elastic stiffness response which was confined to a small kinematic (Y1) yield surface and the highly non-linear behaviour seen once the surface was engaged. The non-linear secant stiffnesses were found to vary with mean effective stress (p') raised to a partial power (around 0.5) over the stress range of interest. The site's glacial genesis led to profiles of undrained shear strength and Yield Stress Ratio (YSR) that were unlike those expected for sedimentary clays. The till responded as if mechanically overconsolidated and manifested significantly higher horizontal stiffness than vertical. The programme of cyclic experiments also revealed many interesting features. The till could respond to undrained cycling in either fully stable, unstable or metastable fashion, depending on the applied mean deviatoric stresses and the imposed cyclic amplitudes. Behaviour was fully stable within a second Y2 kinematic yield surface which was far larger than the Y1 elastic surface, and within this region exhibited almost constant cyclic stiffness and damping ratio without developing significant permanent strains or drifts in p' over thousands of cycles. A notable feature was the till's tendency to contract under cyclic loading once the cyclic large scale yielding (Y3) surface was engaged. Although high level cyclic loading could lead to abrupt or creep failure, tests that did not fail under cycling demonstrated no loss of post-cyclic undrained shear strength.