Geotechnical Centrifuge Modelling Research Articles

Effect of antecedent groundwater conditions on the triggering of static liquefaction landslides

Real-time early warning systems for shallow landslides are typically built upon real-time measurements and forecasts of rainfall and empirical correlations between past patterns of rainfall and landslide occurrence. Whereas these relationships describe whether certain combinations of rainfall and preexisting groundwater levels are of elevated risk of landslide triggering, not all combinations leading to landslide events necessarily have the same consequences in terms of landslide mobility (velocity and distal reach of the landslide). In this paper, the technique of geotechnical centrifuge modeling is used to quantitatively evaluate the hypothesis that the mobility of a landslide triggered under elevated antecedent groundwater conditions is higher than scenarios under drier antecedent conditions. Five identical slope models with a shallow depth to bedrock were subjected to different antecedent conditions ranging from zero groundwater flux to nearly saturated conditions prior to rainfall. The results from these scenarios show that higher antecedent groundwater conditions can result in landslides with velocities about three times higher and travel distances about eight times higher than low antecedent conditions due to static liquefaction of the soil at the base of the slope.

Base liquefaction: a mechanism for shear-induced failure of loose granular slopes

For the deviatoric strain-softening associated with static liquefaction to occur in a landslide, the soil must be contractile, be subjected to a monotonic loading trigger, and be sufficiently saturated to permit the generation of excess pore-water pressures upon loading. It is hypothesized in this paper that static liquefaction might preferentially occur in the saturated granular soil located at the base of the landslide in certain circumstances rather than the well-drained inclined portion of the slope. This hypothesis was tested using the technique of geotechnical centrifuge modelling with a loose granular slope, which was brought to failure under a step-wise increase in groundwater flux. The rising pore-water pressures eventually led to a small localized failure at the toe of the slope. This toe failure acted as the monotonic loading trigger to shear the loose contractile saturated sand at the base of the slope and cause liquefaction to occur in the base region. A back-analysis of the landslide indicates that these types of analyses should be viewed with great caution as the progressive nature of the event following triggering will be likely to lead to erroneous back-calculations of mobilized strength.

Closure to “performance of a transparent Flexible Shear Beam container for geotechnical centrifuge modelling of dynamic problems” by Ghayoomi M., Dashti S., and McCartney J.S.

In response to the discussion, this closure presents a new set of analyses to confirm the satisfactory performance of the recently-developed transparent Flexible Shear Beam (FSB) container and its limitations. The lateral deformations of the box estimated using Finite Element analyses and measured during centrifuge experiments were compared. The maximum deformation to height ratio was sufficiently small to retain at-rest lateral earth pressures for loose to medium-dense cohesionless soils and all cohesive materials. In addition, higher frequency modes of vibration were estimated for the container, and were found to occur where the earthquake energy is less significant. Further, the box is expected to approximately replicate free field conditions under 1-D horizontal shaking for the range of soil properties under investigation. Overall, the deformation and vibration analyses indicate that the selection of appropriate rubber material properties and boundary conditions are critical when analyzing the performance of the container.

Discussion of “Performance of a transparent flexible shear beam container for geotechnical centrifuge modelling of dynamic problems by Ghayoomi, Dashti and McCartney”

Owing to the increasing use of PIV (1) for optical displacement measurements in geotechnical centrifuge modelling, the use of transparent walls on model containers has been on the increase. In dynamic centrifuge modelling this process has been hampered by the trade-off between the ability to optically measure displacements in rigid transparent containers (2) and the more appropriate dynamic boundary conditions provided by flexible containers (3). The transparent flexible shear beam container described by Ghayoomi et al. (4) thus appears to provide a solution to this trade-off, enabling experiments visualising mechanisms of failure such as those carried out at 1g by Knappett et al. (5) to be replicated at more appropriate stress levels in the centrifuge.

The state-of-the-art centrifuge modelling of geotechnical problems at HKUST

Geotechnical centrifuge modelling is an advanced physical modelling technique for simulating and studying geotechnical problems. It provides physical data for investigating mechanisms of deformation and failure and for validating analytical and numerical methods. Due to its reliability, time and cost effectiveness, centrifuge modelling has often been the preferred experimental method for addressing complex geotechnical problems. In this ZENG Guo-xi Lecture, the kinematics, fundamental principles and principal applications of geotechnical centrifuge modelling are introduced. The use of the state-of-the-art geotechnical centrifuge at the Hong Kong University of Science and Technology (HKUST), China to investigate four types of complex geotechnical problems is reported. The four geotechnical problems include correction of building tilt, effect of tunnel collapse on an existing tunnel, excavation effect on pile capacity and liquefied flow and non-liquefied slide of loose fill slopes. By reporting major findings and new insights from these four types of centrifuge tests, it is hoped to illustrate the role of state-of-the-art geotechnical centrifuge modelling in advancing the scientific knowledge of geotechnical problems.

Geotechnical Centrifuge Modeling of Super-High Embankment Filled by Red Soft Soil in Western Yunnan Province

High and even super-high embankment filled by red soft soil are often used in road construction in the mountainous areas of Western Yunnan province, China. Due to the poor engineering properties of the red soil, a proper analysis of the stability of the embankment is necessary. This paper aims to analyze the deformation and stability of a typical super-high embankment by comparing the geotechnical centrifuge modeling and FLAC-2D results. The paper finally concludes that: 1) The red soft soil in Yunnan province can be used as fill material; 2) Flat slope and geo-grid can effectively restrain the deformation of the slope; 3) Geotechnical centrifuge modeling well reflect the real engineering performance of the embankment.

Deformation and Failure Characteristics of Embankment upon the Slope in Permafrost Area under Different Foundation Slope

To investigate the effect of foundation slope on stability of embankment upon the slope in permafrost area, 3 groups of model tests with different foundation slope are designed using the mechanical similarity based on geotechnical centrifuge modeling, when the freezing-thawing depth of the embankment reaches the greatest. The results show that: (1) The foundation slope has effect on the stability of the embankment. The deformation mainly concentrates on the soil layers above the freezing-thawing interface, and the deformation mutation point takes place at the freezing-thawing interface. (2) According to fracture characteristics and failure severity of the embankment, failure modes can be divided into the cracking failure in shallow layer and in deep layer. (3) The cause of unstable failure is the deficiency of shear resistance strength of the weak belt, the soil layers above the freezing-thawing interface slips along the freezing-thawing interface under gravity load. (4) Under the experimental conditions, the critical value of the foundation slope influencing on the stability of the embankment is about 1:6 when the height of the slope embankment is 5.0 m.

Numerical Simulation and Analysis of Centrifuge Model Tests with Nonhomogeneous Materials in Geotechnical Engineering

The primary methods are antetype observation and model tests which to check the actual engineering status in geotechnical engineering field. The antetype observation is the best direct and convictive method, but approach miscellaneous and spend hugely. The general model tests can not fulfil the same stress between model and antetype. Geotechnical centrifuge model test can not only minish the measure of model and fulfil the comparability condition, but also can found all kinds of non-symmetrical models and simulation all kinds of complicated engineering. So the geotechnical centrifuge model test is applied widely in the geotechnical engineering. This paper used the RFPA-Centrifuge and recured to the principle of geotechnical centrifuge model test, evaluated the safety of model only by increase the physical strength. Though the numerical calculating in nonhomogeneous models with different scales showed that stress, displacement and failure mode were accord with conform ratio of centrifuge model tests. Showed the advantage that the results of RFPA can be validated each other with results of physical tests. For some specifical complicated items in geotechnical engineering, make a good test model is not only very hard and have to spend much time, but also need expensive test equipment and much money for test materials. It is very good if we can use a method to conquer these shortages. So it is advisable that using the mechod which geotechnical centrifuge tests combine RFPA-Centrifuge numerical simulation analysis method.

Geotechnical Centrifuge Shaking Table Tests and Numerical Simulation of Soil-Structure

Two geotechnical centrifuge model tests of a soil-structure system with different burial depths are performed to investigate the interaction between soil and structure. The tests are performed at 50 gravitational centrifuge accelerations and the input motion is Kobe wave. This paper focuses on the accelerations and displacements in the soil-structures system. The peak accelerations and displacements along the axis of the structure and along the vertical line 17cm away from the axis are presented. The acceleration and displacement response due to the interaction between soil and structure are studied.

Centrifuge and numerical modelling of ground-borne vibration from surface sources

For the study of ground-borne vibration from surface sources, it is commonly assumed that soil properties remain constant with depth. However, various studies in the field of geotechnical engineering have shown that soil properties (stiffness and damping) change with depth due to the effect of increasing confining stress. Ground-borne vibration is hence unrealistically simulated if constant soil properties are assumed throughout a half space or a soil layer. This paper presents a study of the effect of the variation of soil properties with depth, referred to as soil non-homogeneity, on ground-borne vibrations induced by surface sources. The study involved physical modelling using a geotechnical centrifuge and numerical modelling using EDT and FLAC to gain further understanding of experimental results. In the centrifuge model, a dynamic excitation was applied to a rigid foundation resting on the surface of sand within a cylindrical container. Dynamic soil response was measured using accelerometers on the surface and within the soil. Comparison of the experimental and numerical results demonstrated that it is important to account for the variation of soil properties with depth when studying ground-borne vibration from surface sources.

Geotechnical Centrifuge Modeling of Chloride Diffusion through Soils

AbstractGeotechnical centrifuge modeling provides a means of accelerating the transport of contaminants through a soil matrix. Contaminant transport mechanisms occurring in the geotechnical centrifuge model can be used to verify various mathematical and analytical models. This technical note presents modeling of chloride (Cl) diffusion through soils in a geotechnical centrifuge. Acceleration level, N, of 100 g was used for centrifuge modeling of Cl migration through 7.5-m-deep soil strata for a period of 150 and 600 days. From the centrifuge test results, the Reynolds number, R, and Peclet number, P, were found to be several orders less than unity. This indicates that the dominant Cl transport mechanism in the soil is diffusion. Finite-element modeling was performed, using SEEP/W and CTRAN/W, to validate the experimental results, and excellent matching has been obtained. The study also highlights that geotechnical centrifuge modeling can be used as a viable alternative to field scale experimentation.

Geotechnical centrifuge model tests for explosion cratering and propagation laws of blast wave in sand

This paper presents the explosion cratering effects and their propagation laws of blast waves in dry standard sands using a 450 g-t geotechnical centrifuge apparatus. Ten centrifuge model tests were completed with various ranges of explosive mass, burial depth and centrifuge accelerations. Eleven accelerometers were installed to record the acceleration response in sand. The dimensions of the explosion craters were measured after the tests. The results demonstrated that the relationship between the dimensionless parameters of cratering efficiency and gravity scaled yield is a power regression function. Three specific function equations were obtained. The results are in general agreement with those obtained by other studies. A scaling law based on the combination of the π terms was used to fit the results of the ten model tests with a correlation coefficient of 0.931. The relationship can be conveniently used to predict the cratering effects in sand. The results also showed that the peak acceleration is a power increasing function of the acceleration level. An empirical exponent relation between the proportional peak acceleration and distance is proposed. The propagation velocity of blast waves is found to be ranged between 200 and 714 m/s.

Experimental Study on Stresses Distribution under Embankment

For high-speed railway subgrade settlement, there was usually a big difference between calculated and measured value. Studying the stresses distribution under embankment, giving more accurate simplified method，was a approach to improve the accuracy of calculated settlement. The stresses distribution under embankment were studied through tests on experimental embankment segment, geotechnical centrifuge model tests, and numerical analysis and calculation. Studies showed that the measured stresses distribution was curve, the stresses under the embankment centre were highest, under the vicinity of shoulder the stresses changed smoothly, the stresses was above zero at toe of the side slope.

Blast Wave Effect on Apparatus and Propagation Laws in Dry Sand in Geotechnical Centrifuge Model Tests

7 geotechnical centrifuge model tests for buried explosion in dry sand were investigated by using 450 g-t geotechnical centrifuge apparatus. Blast wave effect on apparatus and propagation laws in dry sand were studied under the conditions of different explosive charges and different centrifuge acceleration levels. 11 accelerometers were buried around the explosives for recording the acceleration response in sand. Other 1 accelerometer was installed on the centrifuge arm to monitor blast wave effect on centrifuge apparatus. The results demonstrate that: The effect of blast wave on centrifuge apparatus can be ignored. The peak acceleration is a power increasing function of the acceleration level. An empirical relation of exponent can be found between the proportional peak acceleration and the proportional distance.

Shear band systems in plane strain extension: analytical solution and comparison with experimental results

Shear banding represents a local failure mechanism of a soil structure as a response to shear loading. In soil structures of different spatial scales systems of regularly spaced shear bands can be observed as a consequence of extensional loading. The phenomenon of single shear bands, defined as thin zones of localized deformation with a discontinuity of the strain field at its boundaries, is well understood. Inside the shear band the material undergoes inelastic strain softening accompanied by shearing and dilation, whereas the material outside the shear band unloads accompanied by elastic contraction in extension tests. Despite numerous experimental and numerical investigations, the physical mechanisms and parameters determining the spacing of parallel shear bands remained unknown. The paper in hand presents an analytical solution for the spacing of the shear bands and a comparison with a large base of experimental data gained from 1g and ng (geotechnical centrifuge) model experiments. The analytical solution is based on the assumption that the elastic energy rate in the unloaded zone between the shear bands tends to a minimum value. The spacing was calculated as the energetically preferred solution for a broad range of cohesive-frictional granular materials. The dependency of the calculated spacing on initial and boundary conditions as well as on material parameters was found to be in good agreement with the experimental results.

Physical modelling of wetting-induced collapse in embankment base

The relevance of the oedometer tests used for the prediction of wetting-induced deformations in embankments is examined. Single and double oedometer tests are carried out. A comparison is made between laboratory tests and geotechnical centrifuge modelling at 100g conducted to examine an inundated embankment made of a sand–clay mixture. A 20 cm high embankment model is built and instrumented. The material is compacted on the ‘dry side' of the optimum Proctor curve at a low compaction rate in order to emphasise settlement phenomena. The inundation simulation is conducted in two successive sequences during centrifuge flight up to a water table of 5 cm. The results prove that the prediction of the dry density after settlement due to inundation is good.

Recent contributions of geotechnical centrifuge modelling to the understanding of jack-up spudcan behaviour

The paper presents an overview of the recent contributions of centrifuge modelling to the understanding of soil–structure interaction and the development of design and predictive methods in the field of mobile jack-up drilling rig foundations. Both advantages and limitations of the centrifuge methods are detailed and key examples are presented. The benefits provided by centrifuge modelling to the development of analysis methods that are now being used within the jack-up industry are highlighted. To conclude, industry trends and research opportunities are discussed, as is the possible role of the geotechnical centrifuge in finding solutions to these new needs.

Liquefaction Study of Heterogeneous Sand: Centrifuge

From past numerical research and small scale laboratory tests, it has been observed that more excess pore water pressure (EPWP) is generated during earthquakes in a heterogeneous sand deposit than in the corresponding homogeneous sand with relative density equal to the average relative density of the heterogeneous sand. This interesting phenomenon is investigated here in large scale experiments using geotechnical centrifuge modeling techniques. A series of liquefaction tests have been conducted at C-CORE’s geotechnical centrifuge facility: Two on variable sand and one on uniform sand deposits. A one level frame structure resting on two strip footings was also placed on that sand deposit to study the effect of soil variability on building foundations. Experimental results such as accelerations, EPWPs and settlements were monitored and measured throughout the tests. Recorded results support the conclusion of previous research that more EPWP is generated in heterogeneous sand deposits than in the corresponding homogeneous sand. The liquefaction mechanism of heterogeneous sands leading to this phenomenon is discussed in this paper.

Small-Scale Modeling of Reinforced Concrete Structural Elements for Use in a Geotechnical Centrifuge

This paper discusses the modeling of reinforced concrete structural elements for use in geotechnical centrifuge modeling of soil-structure interaction problems. Centrifuges are employed in geotechnical modeling so that the nonlinear constitutive behavior of soil in small-scale models can be correctly modeled at prototype scale. Such models typically necessitate large scale factors of between 1∶20 and 1∶100, which is significantly larger than most conventional small-scale structural modeling. A new model concrete has been developed consisting of plaster, water, and fine sand as a geometrically scaled coarse aggregate that can produce a range of model concretes with cube compressive strengths between 25–80 MPa. Reinforcement is modeled using roughened steel wire (beams) or wire mesh (slabs). To illustrate the validity of the modeling technique, a series of three- and four-point bending tests were conducted on model beams designed to represent a 0.5×0.5 m square section prototype beam at 1∶40 scale, and mod...

Centrifuge modelling of soil slopes reinforced with vegetation

This paper reports a series of geotechnical centrifuge model tests conducted to investigate the mechanical reinforcement of slopes by vegetation. Some of the model slopes contained young willow trees, which were grown in controlled conditions to provide different root distributions and mechanical properties. Slopes were brought to failure in the centrifuge by increasing water pressures. The failure mechanisms were investigated photographically and using post-test excavation. By measuring the soil properties and pore pressures in each test when failure occurred, slope stability calculations could be performed for each slope failure. These back-calculations of stability suggest that only a small amount of reinforcement was provided by the root system even when it was grown for 290 days before testing. In contrast, the use of the measured root properties and a commonly used root reinforcement model suggests that significant reinforcement should have been provided by the roots. This disparity is probably due to either inappropriate assumptions made in the root reinforcement model or soil alteration produced by root growth. Such disparities may exist in the application of root reinforcement models to full-scale slopes and therefore require additional study. The modelling technique outlined in this paper is suitable for further investigation of root mechanical interactions with slopes.