Centrifuge modeling on seismic response for soil-foundation-building systems supported by shallow foundations and deep basements

Structures with shallow foundations are susceptible to excessive settlement and rotation in the event of earthquake-induced soil liquefaction. Numerical modelling of the problem remains challenging, due to persisting uncertainties regarding dynamic soil response and soil–structure interaction. In this paper, numerical simulations are employed to study the seismic response of a structure on a shallow mat foundation, resting on a liquefiable sand layer. A coupled hydromechanical analysis is performed employing the finite-differences code FLAC2D, modelling non-linear soil response with the constitutive model PM4Sand. A broad set of element tests on Hostun sand (widely used in centrifuge modelling) are performed and used for extensive model calibration. The calibrated model is then validated against centrifuge test results. The validation is not restricted to the recorded pore pressure, acceleration and settlement time histories, but extends to the deformation mechanisms extracted from centrifuge testing through image analysis, allowing for an in-depth assessment of the numerical simulation. Overall, the analysis is in good agreement with the centrifuge model test. The pore pressure build-up and the final foundation settlement and displacement fields are predicted with adequate accuracy. Although the accumulated displacements are well reproduced, the failure mechanism is not fully captured. This discrepancy is attributed to dissipation-related phenomena, which are not accurately reproduced in the numerical analysis.

Uneven settlement often induces the slope instability problem and the consequent inclination in upper structure. The response of the slopes with shallow foundation, vertical pile foundation, and inclined pile foundation subjected to uneven settlement are investigated using centrifuge model tests. The uneven settlement induces a significant progressive failure in the slopes with shallow and vertical pile foundation. By contrast, the slope with inclined foundation maintains good stability. Full displacement field of the slope-foundation system is obtained by employing an image-based measurement method. The inclined pile foundation exhibits much smaller top center displacement and inclination angle than the shallow foundation and vertical pile foundation. The slope failure mechanism is revealed through an integrated analysis of deformation localization and local failure. On that basis, the reinforcement mechanism of the inclined pile foundation is illustrated by the weakening of the deformation localization in the slope and hence preventing the local failure. The corresponding reinforcement effect on the slope is further presented in terms of the reduction of deformation and the restriction to uneven settlement influenced area. Therefore, the inclined pile foundation with an angle of 60° is confirmed to be effective in resisting the uneven settlement and reducing the slope deformation within acceptable foundation inclination.

Reliability assessment of the physical modeling of liquefaction-induced effects on shallow foundations considering nonuniformity in the centrifuge model

Liquefaction-induced effects are a major threat to shallow foundations built on saturated sand deposits, in seismically active regions. This type of structure is frequently used to support different structures, namely bridges, buildings, gravity walls, etc. Currently, there are few procedures in practice with limited scientific basis for estimation of the liquefaction-effects on foundations built on liquefiable soil. Therefore it is very important to develop more reliable predictions of the performance of shallow foundations built on liquefiable ground and, even more importantly, to develop more efficient techniques to improve that performance. This paper aims at describing a preliminary investigation carried out as part of a research project- SERIES- involving dynamic centrifuge modelling of seismic liquefaction effects and mitigation in shallow foundations. The observations from a centrifuge model test performed at Cambridge University Engineering Department’s Schofield Centre are used to evaluate the settlements of two shallow foundations applying different bearing pressures to the soil. Also, the results presented aim at evaluating the development of excess pore pressures during the seismic simulations, post-earthquake response to the high transient hydraulic gradients, and the propagation of the accelerations under the footings and free-field during the seismic motion. The results obtained enhance current understanding on liquefaction effects and provide valuable information that can be used to examine current design procedures. This will hopefully contribute to design safer and cheaper structures built on shallow foundations in regions prone to earthquake-induced liquefaction.

Using laser displacement transducer scanning technique in centrifuge modeling of reverse fault–foundation interaction

Liquefaction remains a major threat to shallow foundations built on saturated deposits of sand, existing semi-empirical rules being inadequate for the modern requirements of performance-based design. In order to clarify the effects of liquefaction on shallow foundations and to enhance the use of mitigation methods to minimize its consequences, a broad research programme based on centrifuge modelling is being carried out at Cambridge University. This paper summarizes and critically assesses the data produced by 8 centrifuge tests, intended to describe the behaviour of bridges built on shallow foundations and to identify the different phenomena occurring in the soil during and after the earthquake loading. The deformation mechanisms of shallow foundations and the unforeseen relevance of some phenomena are discussed. The experimental results highlight some of the limitations of the information collected from case histories and have practical implications for the use of densification as a liquefaction resistance measure.

Dynamic centrifuge tests of structures with shallow foundations on soft clay reinforced by soil-cement grids

Underground space is commonly exploited both to maximise the utility of costly land in urban development and to reduce the vertical load acting on the ground. Deep excavations are carried out to construct various types of underground infrastructure such as deep basements, subways and service tunnels. Although the soil response to excavation is known in principle, designers lack practical calculation methods for predicting both short- and long-term ground movements. As the understanding of how soil behaves around an excavation in both the short and long term is insufficient and usually empirical, the judgements used in design are also empirical and serious accidents are common. To gain a better understanding of the mechanisms involved in soil excavation, a new apparatus for the centrifuge model testing of deep excavations in soft clay has been developed. This apparatus simulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to simulate excavation in a centrifuge model. The new system has the benefit of giving the correct initial ground conditions before excavation and the proper earth pressure distribution on the retaining structures during excavation, whereas heavy fluid only gives an earth pressure coefficient of unity and is unable to capture any changes in the earth pressure coefficient of soil inside the zone of excavation, for example owing to wall movements. Settlements of the ground surface, changes in pore water pressure, variations in earth pressure, prop forces and bending moments in the retaining wall are all monitored during excavation. Furthermore, digital images taken of a cross-section during the test are analysed using particle image velocimetry to illustrate ground deformation and soil–structure interaction mechanisms. The significance of these observations is discussed.

Shallow foundation suffers from considerable settlement, displacement and tilting under earthquakes. This is particularly due to the shaking associated with earthquakes that lead to the generation of horizontal seismic load transferred through the soil to the foundation. Also, liquefaction could take place during the earthquake in the saturated loose sand. To alleviate the detrimental effect of earthquakes, ground improvement is required. This study examines the response of the shallow square foundation rested on loose sand soil reinforced with geogrid reinforcement when subjected to 2023 Turkey earthquake by using a shaking table system. Different number of geogrid layers are installed; (one, two, three and four), also various geogrid configurations were examined which are (straight, trapezoidal and reverse trapezoidal). The acceleration response, settlement, horizontal displacement, rotation and pore water pressure developed in the sand soil and the shallow foundation during 2023 Turkey earthquake has been examined. The settlement and the horizontal displacement, foundation rotation, acceleration and pore water pressure were measured using rope displacement transducers, tilt sensors, accelerometers and pore water transducers respectively. The results showed that the acceleration amplifies when passing through loose sand. The results also indicated that the shallow foundation experienced noticeable settlement, horizontal displacement and rotation when subjected to the seismic loads. On the other hand, the installation of geogrid proved to be effective in controlling the problems associated with earthquakes. The optimum geogrid reinforcement is occurred when three layers of geogrid placed in reverse trapezoidal configuration (3RT) since it gave the best reduction in the acceleration amplification and the highest decrease in the foundation settlement, displacement and tilting which is about (60-66) %. Nevertheless, the effectiveness of geogrid minimizes when the sand soil becomes saturated. In addition, liquefaction occurs during earthquakes especially at the shallower depths because of the decrease in the shear strength of saturated soil.

The propagation of reverse faults through soil to the ground surface has been observed to cause damage to surface infrastructure. However, the interaction between a fault propagating through a sand layer and a shallow foundation can be beneficial for heavily loaded foundations by causing deviation of the fault away from the foundation. This was studied in a series of centrifuge model tests in which reverse faults of dip angle 60° (at bedrock level) were initiated through a sand layer, close to shallow foundations. The tests revealed subtle interaction between the fault and the shallow foundation so that the foundation and soil response depend on the foundation loading, position, breadth and flexibility. Heavily loaded rigid foundations appeared best able to deviate fault rupture away from the foundation but this deviation could be associated with significant foundation rotations. However, a lightly loaded foundation was unable to deviate a reverse fault and the fault emerged beneath the foundation. This led to gapping beneath the foundation as well as significant rotations and may cause severe structural distress. As well as providing insight into the mechanisms of behaviour, the data from the tests is used to validate finite element analyses in a separate article.

Centrifuge modelling using sands excels for large deformation ultimate limit state (ULS) problems. Stiffness remains more challenging due to the inherent particle size effects that lead to displacements being incorrectly simulated. A series of modelling of models tests has been conducted to investigate the influence of the relative size between a vertically loaded shallow foundation and the mean sand particle size on the measured stiffness. The results highlight that the number of particles within a failure mechanism directly affects the observed stiffness. The number of particles can be estimated through taking the footing width to mean particle size ratio and a linear relationship relating this ratio and the foundation stiffness.

Small-strain foundation response has mostly been studied analytically, with limited experimental verification against 1g physical model tests. This paper revisits the problem of small-strain foundation response, conducting a series of centrifuge model tests, aiming to eliminate the limitations of 1g testing. A centrifuge modelling technique is developed, combining static pushover and dynamic impulse testing for similar systems. To allow for derivation of meaningful insights, a novel procedure for in-flight measurement of the distribution of shear modulus with depth is also developed. The latter combines spectral analysis of surface waves (SASW) measurement of the shear modulus G0 at the surface, and estimation of the distribution of the shear modulus G with depth using acceleration measurements in shaking tests. A novel centrifuge tube–actuator is developed and employed to discharge spherical projectiles against single-degree-of-freedom models lying on shallow foundations on sand. This allows generating dynamic impulse excitation, which is used to measure the small-strain dynamic rocking stiffness. The developed actuator is versatile, and was also used for in-flight SASW testing. The centrifuge model tests are shown to confirm the widely used and well-known formulas. This good agreement can also be seen as a confirmation of the validity of the developed experimental techniques.

Currently, there is no reliable design procedure which considers all aspects of liquefaction effects on shallow foundations. There are many light and heavy structures resting on saturated sand with high liquefaction potential in seismic areas. The aim of this experimental and numerical study is to evaluate the performance of two shallow foundations with different contact pressures in liquefaction. The results of the centrifuge experiment of shallow foundations with surcharges of three-story and nine-story buildings on liquefiable sand are presented in detail. Although entire soil profile is liquefied, no liquefaction is observed under the foundations. There is a clear difference in settlement mechanisms observed beneath the shallow foundation and in the free-field. The heavy foundation fluctuated more strongly compared with the lighter one. The effect of soil permeability and contact pressure on foundation response is investigated during numerical study. Subsequently, the experiment is simulated two dimensionally using a fully coupled nonlinear constitutive model (UBCSAND) implemented in a finite-difference program, FLAC-2D. The results show that settlement of foundations increased with the increase of soil permeability. Trends of excess pore water pressure are captured reasonably by the soil model, but the settlement mechanisms are different. The soil model underestimates total liquefaction-induced settlement of shallow foundation, especially for light foundation.

An investigation into the lateral loading response of shallow bucket foundations for offshore wind turbines through centrifuge modeling in sand

Centrifuge modeling of single pile-shallow foundation interaction in reverse fault