Application of two novel CPTu-based stratification models

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Øysand research site: Geotechnical characterisation of deltaic sandy-silty soils

This paper describes the geology and geotechnical engineering properties of the fluvial and 18 deltaic gravelly-sandy-silty sediments at Øysand, Norway. Geophysical and geotechnical site 19 investigations carried out between 2016 and 2018 at the site are presented. Field testing included state-20 of-the-practice and state-of-the-art soil characterisation techniques such as total sounding, seismic 21 cone penetration testing, seismic flat dilatometer, multichannel analysis of surface waves, electrical 22 resistivity tomography, ground penetrating radar, piezometers, thermistors strings, slug tests, and 23 permeability tests using a newly developed CPT permeability probe from NGI. Several sampling 24 techniques were used at the site to assess sample quality. Laboratory testing consisted of index tests 25 and advanced triaxial tests with bender elements to estimate shear strength and stiffness. Data 26 interpretation, engineering soil properties and state variables derived from this analysis are presented, 27 along with comments on data quality. Engineering problems investigated at Øysand so far, are related 28 to: the impact of using different CPTU types, sample quality assessment by obtaining soils with state-29 of-the-practice and state-of-the-art techniques (such as gel push sampler and ground freezing), and 30 frost heave susceptibility

Elastic-plastic solutions for expanding cavities embedded in two different cohesive-frictional materials

An analytical solution of cavity expansion in two different concentric regions of soil is developed and investigated in this paper. The cavity is embedded within a soil with finite radial dimension and surrounded by a second soil, which extends to infinity. Large-strain quasi-static expansion of both spherical and cylindrical cavities in elastic-plastic soils is considered. A non-associated Mohr–Coulomb yield criterion is used for both soils. Closed-form solutions are derived, which provide the stress and strain fields during the expansion of the cavity from an initial to a final radius. The analytical solution is validated against finite element simulations, and the effect of varying geometric and material parameters is studied. The influence of the two different soils during cavity expansion is discussed by using pressure–expansion curves and by studying the development of plastic regions within the soils. The analytical method may be applied to various geotechnical problems, which involve aspects of soil layering, such as cone penetration test interpretation, ground-freezing around shafts, tunnelling, and mining

Calibration of cone penetrometers according to International Organization for Standardization requirements

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Size Effects in Cone Penetration Tests in Sand

Size effects in miniature cone penetration tests (CPTs) are examined by performing a series of 1 g laboratory tests using three penetrometers of 3, 6, and 12 mm in diameter (D) in two grades of dry Leighton Buzzard sand respectively. It is found that the size effects primarily depend on three non-dimensional geometrical parameters, including relative penetration depth (H/D), normalised surface roughness of the cone (Ra/d50), and normalized cone size (D/d50). Test results showed that: (1) H/D is a major size factor influencing the cone resistance at relatively shallow depths, and its influence may disappear while the localized failure mechanism dominates. (2) the cone resistance may increase with a decreasing value of D/d50 in some circumstances, and this effect attenuates in loose sand; (3) the cone resistance is positively related to Ra/d50, especially for cones with an intermediate rough interface. These size dependent behaviour is attributed to the dependency of the failure pattern and sand properties on the stress level, strain level, and non-local interactions of underlying microstructures and the dependency of the shearing resistance of sand-cone interface on Ra/d50

Engineering behaviour and mechanical - empirical relationships for a problematic New Zealand tropical residual soil

Unlike sedimentary clays, many residual soils do not exhibit clear mechanical-empirical relationships to assist in their engineering characterisation. In contrast, this paper discusses one residual clay in which such relationships may be determined, and examines whether the effects of structure in this clay may be assessed using a framework previously developed for sedimentary clays. The Northland Allochthon residual clay of New Zealand is a problematic soil of the fersiallitic type, prone to slope instability. Atterberg limit tests on soils from five field sites in the same geological unit show considerable variation, but that they are mechanically related. Triaxial tests were performed on reconstituted and intact soil specimens from one field site. Normalization of the strength envelope using the equivalent stress on the intrinsic compression line suggests that soil structure, destroyed in reconstituted specimens, plays a role in the shear strength of this soil in its intact state. Overconsolidated behaviour, in the absence of geological preloading, suggests the existence of a pseudo-preconsolidation pressure associated with weathering processes. The results show that the saturated mechanical behaviour of this residual soil is in line with that of sedimentary clays and that mechanical-empirical relationships developed for such clays may be applied in this case

A cavity expansion–based solution for interpretation of CPTu data in soils under partially drained conditions

A cavity expansion–based solution is proposed in this paper for the interpretation of CPTu data under a partially drained condition. Variations of the normalized cone tip resistance, cone factor, and undrained‐drained resistance ratio are examined with different initial specific volume and overconsolidation ratio, based on the exact solutions of both undrained and drained cavity expansion in CASM, which is a unified state parameter model for clay and sand. A drainage index is proposed to represent the partially drained condition, and the critical state after expansion and stress paths of cavity expansion are therefore predicted by estimating a virtual plastic region and assuming a drainage‐index–based mapping technique. The stress paths and distributions of stresses and specific volume are investigated for different values of drainage index, which are also related to the penetration velocity with comparisons of experimental data and numerical results. The subsequent consolidation after penetration is thus predicted with the assumption of constant deviatoric stress during dissipation of the excess pore pressure. Both spherical and cylindrical consolidations are compared for dissipation around the cone tip and the probe shaft, respectively. The effects of overconsolidation ratio on the stress paths and the distributions of excess pore pressure and specific volume are then thoroughly investigated. The proposed solution and the findings would contribute to the interpretation of CPTu tests under a random drained condition, as well as the analysis of pile installation and the subsequent consolidation