The use of passive piles can be an effective method for stabilising unstable slopes. Unfortunately, no rigorous analytical solution has so far been proposed for assessing the ultimate, lateral pile–soil pressure distribution under drained conditions for the design of passive piles in a slope. The present work focuses on the reliability of a finite-element model (FEM) used to assess the ultimate limit state conditions of passive piles in frictional soils. The paper also provides an estimate of the ultimate load of a single pile and a row of piles in a slope of frictional soil. The results are obtained with a series of two-dimensional analyses (evaluating the role of boundary conditions) and three-dimensional analyses on an infinite slope (to evaluate the role of the embedment ratio, the influence of slope inclination and the arching effects in pile rows). The analyses were performed using the Abaqus finite-element code associated with a couple of user-defined subroutines for defining the initial and boundary stress conditions. The computed ultimate loads are compared with theoretical findings obtained from a simple extension to drained conditions of Viggiani's approach to undrained conditions. Depending on pile embedment and soil layer thickness and strength, three rupture mechanisms are discussed from a theoretical standpoint. The FEM converges very efficiently and reliably in one rupture mechanism and for deep pile embedments, whereas convergence is slow and difficult in the other cases and requires a very high elastic soil stiffness.

Direct observation of gas in peat layers, generated by slow degradation in anoxic conditions, raised concern in the Netherlands about its potential impact on the geotechnical response of dykes founded on peat. To address this issue, an experimental investigation was initiated aimed at quantifying the main consequences of the presence of gas on the mechanical response of peats. The results of a series of triaxial tests on natural peat samples flushed with carbonated water are presented and discussed. Controlled amounts of gas were exsolved by undrained isotropic unloading, and the samples were sheared under undrained conditions. During gas exsolution, the samples suffered volumetric expansion, at a rate which is ruled by the relative compressibility of the fluid and the soil skeleton. The gas in the pore fluid dominates the stress-strain response upon undrained shearing, causing lower excess pore pressure compared to fully saturated samples. The experimental results suggest that local fabric changes occur during gas exsolution. However, for the amounts of gas investigated, these fabric changes seem to be almost reversible upon compression. Although the ultimate shear strength is hardly affected by gas, the reduction in the mobilised shear strength at given axial strain thresholds is dramatic, compared to fully saturated samples. The study suggests that the presence of gas must be cautiously accounted for at low stresses, when a reference stiffness is chosen for serviceability limit states, and when operative shear strength definitions, based on mobilised strength for given strain thresholds, are chosen in the assessment of geotechnical structures on peats.

Conventional triaxial tests on peats are strongly criticised due to the very high shear strength parameters obtained from standard data elaboration, leading to unrealistic factors of safety when used in geotechnical design and assessment. Various operational approaches have been proposed in the literature to overcome this difficulty; however, they seem to lack consistent mechanical background. Some of the issues related to the shear strength evaluation of peats from triaxial tests come from the non-uniform stress and strain states developing in the samples well before failure is attained, due to end restraint effects. Undrained triaxial compression tests were performed on reconstituted peat to examine the influence of end restraint on the deviatoric stress, excess pore pressure and deviatoric strain response. Samples were tested with standard rough end platens and with modified platens to reduce the friction between the sample and bottom and top caps. Four different initial height-to-diameter ratios were examined, to reduce the consequences of rough end platens on the sample response. The results indicate that end restraint contributes dramatically to overestimating the shear strength of peat, due to the increase in both the calculated deviatoric stress and the measured excess pore pressure at the bottom of the sample. Suggestions are given to quantify the influence of end restraint in the interpretation of standard data, in an attempt to suggest viable procedures to determine more reliable effective and undrained shear strength parameters from standard triaxial tests.

Constitutive models for soils are developed and validated against laboratory tests assuming these give representative information on the true material behaviour. However, data from standard laboratory tests reflect the sample response rather than the true material behaviour, due to nonuniformities in stresses and strains generated over the experimental test. The work examines the implications of end restraint on the definition of the stress–dilatancy rule of highly compressible soils with a finite element numerical approach. The numerical model replicates a reconstituted peat, typically characterized by a combination of high compressibility and high friction angle, which increases the severity of end restraint effects. Simulated results show that the global measurements from standard triaxial tests with rough end platens would not give the proper stress–dilatancy rule, if they were interpreted as the response of a single soil element at the constitutive level. Both overestimation and underestimation of the true dilatancy compared to the material response can be observed, depending on the deformation mode. To support the validity of the numerical results, experimental findings from drained triaxial tests on reconstituted peat are presented. Practical indications are given on how the standard interpretation of drained triaxial tests data on peats can be improved.

Soil-retaining structures, such as anchored, gravity and diaphragm walls can be used effectively to stabilise unstable, shallow slopes. The present work focuses on assessing the passive soil pressure that can be mobilised by passive and active retaining structures used for slope stabilisation purposes; passive structures are taken to be those left free to move and find their own equilibrium against the soil pressure, and active structures those equipped with pre-stressed ground anchors that lead to an upward movement against the unstable sloping soil. The numerical analyses performed with the Abaqus finite-element code and the comparison drawn with available theoretical solutions show that, in the case of passive retaining structures, the maximum horizontal soil pressure that can be exerted by the unstable soil layer on the retaining structure is independent of soil–wall friction and coincides with Rankine's passive value. However, in the case of active retaining structures, the passive soil pressure may be much greater than Rankine's value and should be evaluated as suggested by Eurocode 7 (geotechnical design). Such a high soil pressure is only meaningful for the purpose of sizing the retaining structure, however, because (just uphill from the stress region perturbed by the soil–wall friction and ground anchor) the stress state at rupture coincides with Rankine's theory in the case of active retaining structures too. Thus, even in the case of anchored retaining structures, the maximum soil pressure that can be exploited for slope stabilisation coincides with Rankine's value. These conclusions have important consequences for the dimensioning of soil-retaining structures for slope stabilisation purposes. A practical application for slope stabilisation is discussed in the final part of the paper.