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In this paper, the influence of the foundation configuration (raft or separate footings) on tunnel–soil–framed building interaction is investigated using geotechnical centrifuge testing. Tunnelling-induced soil movements and deformation fields, framed building displacements and structure shear distortions (with associated modification factors) are illustrated. Framed building stiffness and footing bearing capacity are also evaluated experimentally. Results show that the foundation configuration plays an important role in determining the ground response to tunnelling, affecting soil displacement fields, as well as the distribution of soil shear and volumetric strains. In particular, foundation settlements and differential horizontal displacements are larger for separate footings compared to raft foundations. The effects of building width, weight and eccentricity (with respect to the tunnel) on foundation settlements and structural distortions are quantified for separate footings and contrasted against results for raft foundations. The modification factor of the maximum building shear distortion is linked to the relative soil–building shear stiffness; interestingly, for buildings with similar values of relative stiffness, the level of shear distortion within framed buildings is lower for separate footings than rafts. [ABSTRACT FROM AUTHOR]

On 12 October 2013, the 24-storey high 'Space' building collapsed in Medellín, Colombia. The building had a reinforced concrete structure founded on large-diameter piles with enlarged bases. During and after construction the building exhibited structural problems and excessive settlements of some piles. Because of the evidence of structural damage, the building was evacuated on 11 October 2013, 1 day before the collapse. Isolated large-diameter piles excavated in a firm, high-plasticity saprolitic residual clay, supported building columns. Building collapse was triggered by a significant increase of the vertical load transmitted by some columns, which, in turn, is explained by widely different settlements experienced by neighbouring piles and the resulting loading transfer mechanisms. The first part of the paper presents the results of the field exploration and laboratory tests, as well as the structural and architectural layout, including the construction process and other forensic aspects that could influence the collapse of the structure. Afterwards, the joint structure–foundation–soil numerical model developed and its predictions are described. Matching the records of pile settlements, measured during construction and afterwards until building collapse, with model predictions allowed the validation of the model. The causes that triggered the collapse were identified and some other lessons were learned. [ABSTRACT FROM AUTHOR]

Bearing strength surfaces, the locus of all combinations of vertical load, horizontal shear and moment that can induce shallow foundation bearing failure, can assist in envisaging the response of shallow foundations to general loading. Additionally, however, there is the question of the effect of earthquake loading on foundation bearing strength. Several researchers have investigated this from the perspective of the vertical capacity of the foundation. Not surprisingly, all show that the bearing strength under vertical load is reduced by inertia loads in the soil beneath the foundation. Somewhat surprisingly, these results are not directly relevant to the earthquake bearing strength calculations for shallow foundations, as the important role of the static actions sustained by the foundation prior to the earthquake may be overlooked. This technical note shows, using the bearing strength surfaces given in Part 5 of Eurocode 8, how it is that shallow foundation bearing strength for cohesive soils is not sensitive to the earthquake horizontal acceleration, but dry cohesionless soil may be more susceptible. [ABSTRACT FROM AUTHOR]

Skirted foundations have been widely applied as offshore foundations for several decades. In design, the use of failure envelopes is convenient for assessing stability under combined loading. A large number of studies on failure envelopes exist in the literature based on experiments and numerical analyses. Most of these studies focus on ultimate capacity and static loading. This paper presents a numerical study focusing on cyclic degradation and failure envelopes for skirted foundations subjected to combined cyclic and static loading. It was found that the shapes of the failure envelopes are little affected by degradation expressed by the number of equivalent cycles. In addition to failure envelopes, contours of displacements were computed in the three-dimensional load space (vertical, horizontal and moment load) for a more complete description of the response. As an example, the well-defined cyclic contour diagrams of Drammen clay were utilised to demonstrate how foundation response diagrams can be established. The database is accompanied by a simplified procedure to account for cyclic degradation through equivalent number of cycles, different normalised load-displacement response and variation in foundation geometry. The framework of procedures can be used to estimate foundation stiffness and capacity and the results can serve as a basis for the development of foundation macro-element models. [ABSTRACT FROM AUTHOR]

Circular foundations are widely employed in offshore engineering to support facilities and are generally subjected to fully three-dimensional loading due to the harsh offshore environmental load and complex operational loads. The undrained capacity of surface circular foundations on soil with varying strength profiles and under fully three-dimensional loading is investigated and presented in the form of failure envelopes that obtained from finite element analyses. The combined ultimate limit state of circular foundations is defined as the two-dimensional failure envelopes in resultant H-M loading space accounting for the vertical load and torsion mobilisation. The effects of vertical load and torsion mobilisation, soil shear strength heterogeneity and loading angle from moment to horizontal load on the shape of normalised H-M failure envelopes are explored. A series of expressions are proposed to describe the shape of failure envelopes obtained numerically, enabling essentially instantaneous generation of failure envelopes and optimisation of a circular foundation design based on constraint of any input variable through implementation in an automated calculation tool. An example application is ultimately provided to illustrate how the proposed expressions may be used in practice. [ABSTRACT FROM AUTHOR]

Jack-up installation is typically discontinuous with seawater ballast being taken on and shed again periodically to elevate the hull out of the ocean as the spudcan footings penetrate the soil. Any pauses in the footing penetration provide the opportunity for consolidation to occur in sufficiently permeable cohesive soils. The undrained shear strength underneath the spudcan is increased during consolidation, which enhances the penetration resistance within a limited extent immediately after the consolidation. This phenomenon has been explored in a limited number of centrifuge tests, but cannot be reproduced using conventional finite-element methods. An effective stress large-deformation finite-element approach based on periodic mesh regeneration is employed to capture the entire process of 'penetration-consolidation-penetration', with the modified Cam-clay model being incorporated to represent the behaviour of normally consolidated kaolin clays. The numerical model established is verified by comparison with two series of centrifuge tests before exploring the influence of key parameters, including consolidation depth, consolidation duration, coefficient of consolidations, anisotropy of permeability and loads held during consolidation. The normalised consolidation duration is identified as the dominant factor that affects the post-consolidation peak in penetration resistance. A simple method is proposed to estimate the post-consolidation penetration resistance. [ABSTRACT FROM AUTHOR]

Dynamic tests on model soil-pile systems under 1 g (natural gravity) conditions are carried out to assess the effects of soil non-linearity on the horizontal pile-to-pile interaction factors of inclined piles. This paper examines 2 × 2 symmetric free-head floating inclined pile groups embedded in cohesionless soil and subjected to a low-to-high amplitude of lateral harmonic pile head loadings for a wide range of frequencies. Piles with head level centre-to-centre spacing of five times the pile diameter with three practical pile inclinations of 0° (vertical), 5° and 10° aligned parallel to the direction of loading are reported. Results from the experiments indicate a significant impact of soil non-linearity on the horizontal interaction factors. Moreover, utilising such experimentally measured interaction factors in conjunction with available experimental solutions for horizontal impedance functions of inclined single piles, the principle of superposition is assessed in obtaining the horizontal impedance functions of inclined pile groups. Comparison between the results based on the principle of superposition and experimentally available solutions shows a good agreement for the low-to-intermediate range of loading frequency. At higher frequencies, however, loading-direction-dependent soil-pile interface non-linearities developed for inclined piles cannot be captured in a straightforward way. To this end, a multiplication factor is proposed for the superposition method. [ABSTRACT FROM AUTHOR]

The design of shallow foundations has traditionally used a mixture of plasticity-based solutions to find the ultimate limit state and either a factor of safety on the plasticity solution or a linear elastic solution to attempt to design for the serviceability limit state. The serviceability limit state is intrinsically linked to the deformation mechanism that occurs beneath the shallow foundation in service. A better understanding of these soil movements can pave the way for more rational design approaches. In this paper, small-scale experimental work is used to show that the deformation mechanism beneath strip and circular foundations continuously changes as the footing is displaced. The mechanisms observed at intermediate settlements, noted to be typical design points, are best described by a mixture of solutions. Linear mixes of idealised fields were analysed using an upper bound approach to determine the load-displacement behaviour of each mix. The envelope of lowest upper bounds indicated that the optimal mix of fields changes depending on the footing settlement. At typical design points for shallow foundations mixtures dominated by ellipsoidal cavity expansion mechanisms were found to be optimal for both axisymmetric and plane strain cases. Comparison of theoretical and experimentally measured predictions indicated that using linear mixes of fields gives a good approximation to the true behaviour and may be used for settlement-based design approaches in the future. [ABSTRACT FROM AUTHOR]

This paper reports a set of centrifuge tests to investigate the increase in bearing capacity of a skirted circular foundation that results from preloading with consolidation. Results are compared with coupled numerical analyses using the modified Cam Clay model. Fundamental mechanisms are revealed from the stress paths followed during preloading and consolidation at the element level, accounting for both the change in mean normal effective stress and in the hardening parameter. A formulation is developed to predict the gain in bearing capacity as a function of the level of preloading applied on the foundation and the degree of consolidation achieved. The formulation provides good agreement with the centrifuge results. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of the performance of two instrumented surface footings at the Bothkennar Clay research site in the UK. Footing A was loaded to failure over 4 days, reaching a net bearing capacity of = 138 kPa; footing B was loaded to 89 kPa, at an identical loading rate, and left to consolidate under maintained load for about 11 years. The preloaded footing was then loaded to failure over 3 days, reaching = 204 kPa. The increase in bearing capacity was significantly larger than that expected due to consolidation effects alone, and it is anticipated that the occurrence of creep and other ageing processes may have played a major role in the observed response. The complete loading history of the two footings is simulated by means of coupled axi-symmetric finite-element analyses in which the foundation soil is described using an elastic-viscoplastic model that mimics isotach viscosity. The ground profile and the model parameters are derived based on the extensive laboratory and field test data available in the literature. The numerical analyses are able to describe accurately the footings behaviour during first loading, the development of delayed settlement under maintained load and the increase in bearing capacity due to preloading. The paper emphasises various issues regarding the application of elastic-viscoplastic models to model boundary value problems in conditions close to failure. [ABSTRACT FROM AUTHOR]

Shallow foundations supporting bridge piers, building frames, shear walls and monuments are often subjected to extreme lateral loading such as wind in offshore environments, or strong seismic shaking. Under such loading conditions, foundations may experience a host of non-linear phenomena: sliding on and uplifting from the supporting soil or even soil failure in the form of development of ultimate bearing capacity mechanisms. This type of response is accompanied by residual settlement and rotation of the supported structural system. Nevertheless, inelastic foundation performance can provide potential benefits to the overall seismic integrity of the structure. Thanks to such non-linearities, energy dissipation at or below the foundation level may eventually limit the seismic demand on structural elements. Several theoretical and experimental studies have provided encouraging evidence to this effect. This paper has a dual objective: first, to study the behaviour of shallow foundations under vertical and lateral monotonic loading and under lateral slow cyclic loading of progressively increasing amplitude; second, to explore the differences in foundation response between reduced-scale 1 and centrifuge 50 model testing. Emphasis is placed on interpreting their discrepancies by unveiling the role of scale effects. The role of soil densification due to multiple loading cycles with uplifting is also highlighted. [ABSTRACT FROM AUTHOR]

A fully instrumented experiment was conducted to investigate the soil-structure interaction of single short, stiff laterally loaded piles. A hollow steel pipe pile with a diameter of 102 mm, a thickness of 6.4 mm, and a length of 1.524 m was installed in well-graded sand and subjected to increasing lateral load. The pile and surrounding soil were fully instrumented using advanced sensors, including flexible shape acceleration arrays, thin tactile pressure sheets, and in-soil null pressure sensors. The sensors attached to the pile were used to develop the compressive soil-pile interaction pressures and the lateral displacement along the pile length. The tactile pressure sheet sensors provided the soil-pile interaction compressive pressures on the circumference of the pile at a specific depth and along the length of the pile. The measured soil-pile interaction compressive pressures combined with the measured lateral displacement along the pile length were used to develop the soil-pile interaction force-displacement relationships ( curves) using direct measurements. In addition, the in-soil null pressure sensor measurements were used to develop the distribution of horizontal stress changes around the pile as the lateral pile displacement increased. When appropriate, the measured results were compared with data and methods available in the literature. [ABSTRACT FROM AUTHOR]