Cone penetration testing (CPT) has become one of the most commonly adopted in situ tests for site investigations, due to its reliability and repeatability of the measurements and the potential for direct use in design. For CPTs in clays, previous studies have paid considerable attention to the selection of penetration resistance factor N_kt considering the effects of soil stiffness, stress and strength anisotropy, soil layering and soil sensitivity, and so on. In contrast, this paper focuses on the effects of spatially variable soil properties on the interpretation and use of CPT data. This has been achieved using large-deformation finite-element modelling with random fields. It is shown that when using CPT data to obtain the true point-to-point statistics of the ground, scale effects could lead to the (unconservative) overestimation of the low estimate and underestimate of the high estimate for variable ground with small scales of fluctuation. Suggestions are made for correcting CPT-measured data to allow for this effect. In contrast, if using CPT data to provide design inputs for (larger) foundations, the same scale effects may be considered to increase beneficially the characteristic values of soil strength for sizing and reduce characteristic soil strengths for installation assessment.

In this study an effective stress model for cement-treated soil is employed to carry out a systematic study on cement-treated soil slab used to control the wall displacement in excavations with thick, soft clay layers. Deterministic analyses were first conducted to assess the drainage state of the treated soil slab. These indicate that, under timescales which are representative of those in actual construction, cement-treated soil slab conditions may range from nearly undrained to drained. Random finite-element analyses also show that the global undrained and drained behaviour of the treated soil slab differ significantly. Undrained loading leads to a rapid rise in stress with strain to a peak strength with subsequent softening. Drained behaviour, on the other hand, leads to a more compliant response, which suggests that the main issue in the drained condition is likely to be large displacement, rather than complete collapse. A rational design framework for cement-treated soil slab is proposed, which explicitly considers the distribution in global behaviours of the treated soil slab. This differs from current limit state approaches which are largely based on statistical properties of the input data, such as the unconfined compressive strength of sampled cores.