Geotechnical Centrifuge Tests Research Articles

지반 동적거동모델에 따른 부지응답해석 영향연구

전단파괴 이전 지반의 동적비선형거동특성은 일반적으로 함수형 피팅모델과 Masing 법칙을 이용하여 수치해석프로그램에 사용된다. 그러나 대부분의 함수형 피팅모델은 특정 전단변형률 영역에서 실험결과 대비 전단탄성계수와 감쇠비의 오차를 유발하는 것이 일반적인 현상이다. 이러한 오차의 원인은 현재 피팅모델로 표현하기 어려운 지반재료의 고유 특성에 기인할 수 있다. 지금까지 상기 문제를 해결하기 위하여 몇몇 피팅모델이 제안되었으나, 오차의 영향이 지진 시 부지응답해석에 미치는 영향은 아직까지 구체적으로 검토된 바는 없다. 본 논문에서는 상기 영향 검토를 응답이력해석을 통하여 실시하였다. 세 개의 서로 다른 함수형 피팅모델을 이용하여 부지응답해석을 시행하였으며, 그 결과는 동적원심모형시험 결과의 원형 계측치를 기준으로 검증을 실시하였다. 실험과 해석 간의 오차는 입력지진 크기가 증가함에 따라 커짐을 알 수 있었다. 저-중간 강도의 입력지진 범위에서 함수형 피팅모델에 따른 해석의 정확도 차이는 실용적인 측면에 있어서 큰 차이가 나지 않음을 알 수 있었다. Nonlinear soil behavior before failure under dynamic loading is often implemented in a numerical analysis code by a mathematical fitting function model with Masing's rule. However, the model may show different behavior with an experimental results obtained from laboratory test in damping ratio corresponding secant shear modulus for a certain shear strain rage. The difference may come from an unique soil characteristics which is unable to implement by using the existing mathematical fitting model. As of now, several fitting models have been suggested to overcome the difference between model and real soil behavior but consequence of the difference in dynamic analysis is not reviewed yet. In this paper, the effect of the difference on site response was examined through nonlinear response history analysis. The analysis was verified and calibrated with well defined dynamic geotechnical centrifuge test. Site response analyses were performed with three mathematical fitting function models and compared with the centrifuge test results in prototype scale. The errors on peak ground acceleration between analysis and experiment getting increased as increasing the intensity of the input motion. In practical point of view, the analysis results of accuracy with the fitting model is not significant in low to mid input motion intensity.

Centrifuge testing to simulate buried reinforced concrete pipe joints subjected to traffic loading

Pipeline water leakage has become a serious problem in many countries. It has been widely noted that most of the damage to the pipelines occurs in the joints where two pipes are connected to each other. This paper presents the results of a geotechnical centrifuge testing program in which the response of a 12 m long (in prototype scale) large-diameter reinforced concrete pipeline with gasketed bell-and-spigot joints subjected to three standard American Association of State Highway and Transportation Officials design load configurations has been investigated. The results show that most vertical pipe movements occurred during the first 10 cycles of traffic loading. Under design tandem loading, the pipe joint displacements were significantly higher than those under the other two traffic load configurations. An increase of soil cover depth resulted in a reduced influence of surface loading, the effect of which was the most significant for two single pairs of wheels of design trucks in passing mode. Furthermore, two pipes on the left side and two pipes on the right side from the tested joint were influenced significantly by the surface loading, while the pipeline movements were not symmetrical. Although the joint directly under the load experienced the largest rotation, the possibility of leakage in the second joint in the spigot-to-bell direction was also high, due to large differential displacement between the pipes.

A comparison of full profile prediction methods for a spudcan penetrating sand overlying clay

Spudcans are the traditional footings used for offshore mobile jack-up rigs. However, the installation of spudcans in sand overlying clay may lead to punch-through failure, which can cause serious damage to the jack-up rig and endanger personnel. This article compares three new methods proposed in the literature and an interpretation of the International Organization for Standardization (ISO) guideline for predicting the full penetration resistance profile. The penetration resistance profile for each of the methods is characterised by two key calculations: the peak resistance in the sand and the bearing capacity within the underlying clay. The punch-through distance – an indicator of the potential for and severity of punch-through failure – is estimated from these calculations. In comparison with 71 geotechnical centrifuge tests, the ISO guideline provides poor predictions, consistently underestimating the peak resistance in the sand and the underlying bearing capacity in the clay. Although all three of the new methods provide a superior response, by assessing the accuracy, scatter and geometric skew of the predictions, two of the methods are shown to be biased in at least one of the key calculations used to define the penetration resistance profile, thus producing bias in the prediction of the punch-through distance. However, one method yields largely unbiased predictions.

Physical modelling of rainfall-induced flow failures in loose granular soils

The tragic consequences of the March 2014 Oso landslide in Washington, USA were particularly high due to the mobility of the landslide debris. Confusingly, a landslide occurred at that exact same location a number of years earlier, but simply slumped into the river at the toe of the slope. Why did these two events differ so drastically in their mobility? Considerable questions remain regarding the conditions required to generate flow failures in loose soils. Geotechnical centrifuge testing, in combination with high-speed cameras and advanced image analysis has now provided the landslides research community with a powerful new tool to experimentally investigate the complex mechanics leading to high mobility landslides. This paper highlights recent advances in our understanding of the process of static liquefaction in loose granular soil slopes achieved through observations of highly-instrumented physical models. In particular, the paper summarises experimental results aimed to identify the point of initiation of the chain-reaction required to trigger liquefaction flow failures, to assess the effect of slope inclination on the likelihood of a flowslide being triggered, and to quantify the effect of antecedent groundwater levels on the distal reach of landslide debris with the objective of beginning to explain why neighbouring slopes can exhibit such a wide variation in landslide travel distance upon rainfall-triggering.

Tuned mass damper effects on the response of multi-storied structures observed in geotechnical centrifuge tests

Tuned mass dampers (TMDs) are widely used to reduce vibrations in structures. However, very little research is available on the experimental investigation of TMDs and their performance in soil-structure systems. In this paper, a series of geotechnical centrifuge tests was conducted to investigate the effects of TMDs on the response of a multiple-storey sway frame structure undergoing dynamic soil-structure interaction (SSI). Structural responses were recorded for a wide range of input motion characteristics, damper configurations and soil profiles. The practicality associated with the use of TMDs in the damping of resonant structures in light of unexpected earthquake characteristics different from design earthquakes was experimentally demonstrated. Tuning a TMD to soil-structure system properties rather than fixed-base structural properties was found to double the improvement in damping and reduce the original peak response by nearly half. The potential effectiveness of a de-tuned mass damper in light of significant SSI was also demonstrated.

Influence of slope inclination on the triggering and distal reach of hydraulically-induced flow slides

In the absence of water, a dry granular landslide mass released on an incline will experience greater acceleration than an otherwise identical mass released on a plane of lower inclination. However, when water is introduced to the system, the outcome is more complicated. While it requires less water pressure to trigger a landslide on a steep slope, the additional saturated soil in a gentle slope can make it more susceptible to flow liquefaction, which in turn can make it the slope of greater risk in terms of the landslide velocity and distal reach. Results from seven geotechnical centrifuge tests were evaluated to compare the behavior of flow slides triggered in a loose granular soil layer inclined at 20° and 30°. The results indicate that the influence of slope inclination on mobility is highly dependent on antecedent groundwater conditions. In low antecedent groundwater scenarios, the mobility of the failures triggered in the 20° and 30° were similar. However, in a high antecedent groundwater scenario, a flow slide triggered on the 20° slope was observed to travel twice the distance, and reaching nearly twice the maximum velocity than a flow slide triggered on the steeper 30° slope. These results provide experimental data documenting the complex interaction between slope inclination and antecedent groundwater conditions to control the triggering and distal reach of hydraulically-induced flow slides.

Performance of micropiled raft in sand subjected to vertical concentrated load: centrifuge modeling

Initial applications of micropiles have involved retrofitting foundations of existing buildings. In these applications, the overall performance of the micropiles–raft (MPR) foundation system is similar to a piled raft foundation where the load is transmitted through both the raft and micropiles. However, there is no guidance available regarding the performance of MPR foundations. In this study, geotechnical centrifuge testing was conducted to investigate the behavior of MPR foundations in sand and evaluate their performance characteristics. The study investigated the effect of raft flexibility on a number of important design parameters, including raft total and differential settlements, raft contact pressure, raft bending moment, and load sharing between the raft and micropiles. In addition, the use of micropiles as settlement reducers was investigated. The results showed that the micropiles carried 42%–59% of the applied load for the MPR configuration considered, which resulted in redistribution of the raft contact pressure. It was found that the Poulos–Davis–Randolph (PDR) method can be used to evaluate the performance of MPR systems with relatively stiff rafts; however, it is not applicable for MPR with flexible raft. A correction factor was proposed to account for the raft flexibility in the PDR method.

Nonlinear Soil–Foundation–Structure and Structure–Soil–Structure Interaction: Engineering Demands

Two geotechnical centrifuge tests were designed to elicit the influence of soil–foundation–structure interaction (SFSI) and structure–soil–structure interaction (SSSI) effects on the seismic demands of an inelastic frame structure founded on individual spread footings. Four experimental cases were considered: (1) SFSI of an individual inelastic frame structure (baseline case); (2) in-plane SSSI between the frame structure and a large elastic wall structure designed to respond predominately in a rocking mode; (3) anti-plane SSSI between the frame and wall structures; and (4) combined in-plane and anti-plane SSSI between the frame structure and two wall structures. Results from Cases 1, 2, and 4 are analyzed considering 13 demand parameters. The peak seismic demands from the baseline case are compared to the peak demands from the SSSI cases to elicit the impacts of building adjacency. The number of earthquake motions that resulted in increases or decreases in the seismic demands in the frame structure are tracked for the SSSI cases. For these tests, SSSI consistently results in increased peak foundation moment and foundation base shear demands, regardless of the geometric layout of the structures. These increases propagate throughout the superstructure, resulting in increases in moment, shear and curvature ductility demands in the structural components. SSSI effects are observed to be most significant for low–to–moderate intensity earthquake motions, suggesting that particular consideration should be given to the likelihood of SSSI for highly probable earthquakes.

Predicting damping ratio of fine-grained soils using soft computing methodology

Accurate prediction of dynamic soil properties is very important to basic understanding of soil behavior and also practical soil modeling. Shear modulus and damping ratio play a vital role in the design of geotechnical structures subjected to dynamic loads. In this study, artificial neural network (ANN), and adaptive neuro-fuzzy inference system (ANFIS) were employed for prediction of damping ratio of fine-grained soils. Most effective factors that affect this parameter include shear strain, plasticity index, and effective confining pressure. A wide-ranging database of soil element tests was used to develop an advanced model, capable of predicting soil damping ratio accurately. Results of geotechnical centrifuge tests were also involved during the training process for adequate generalization of the algorithm for future predictions. Contributions of the effective variables were evaluated through a parametric study. It was found that the ANN model developed with feed-forward back propagation (FFBF) algorithm exhibits higher performance in prediction of soil damping ratio than those developed by radial basis function (RBF) and ANFIS. The results indicate that the soft computing-based model could provide accurate and reasonable predictions, compared with the available practical charts.

MEMS accelerometers for measuring dynamic penetration events in geotechnical centrifuge tests

Micro-electro mechanical system (MEMS) accelerometers are small, inexpensive sensors that have only recently been used in geotechnical centrifuge tests. This is unlike piezoelectric accelerometers, which are by comparison large and expensive but have been used extensively in geotechnical centrifuge tests over the past couple of decades. This paper examines the response of a single-axis ±500g MEMS accelerometer under both static and dynamic conditions in a centrifuge environment. The potential for MEMS accelerometers to be used to measure the depth of objects buried in soil is examined and the achievable resolution is discussed. Unlike piezoelectric accelerometers, which only measure changes in acceleration, MEMS accelerometers can measure both constant and changing accelerations. The merit of this feature is demonstrated through tests in which MEMS and piezoelectric accelerometers are embedded within a dynamically installed model anchor. The MEMS accelerometer is capable of measuring accelerations during both the free-fall phase and the soil embedment phase, whereas the piezoelectric accelerometer is only able to measure the changing accelerations that dominate during the soil embedment phase. Velocity profiles derived from numerical integration of the MEMS accelerations give mudline anchor velocities that agree with independent measurements and anchor embedment depths that agree with direct measurements.

Centrifuge modeling of large-diameter underground pipes subjected to heavy traffic loads

Large-diameter pipes, as well as heavy vehicles, have become increasingly prevalent, which imposes uncertainties on pipe design. This paper describes the procedure and results of a series of geotechnical centrifuge tests performed on a large 1400 mm diameter reinforced concrete pipe subjected to heavy traffic loading up to 850 kN. The influence of soil cover depth, as well as the positions and magnitude of traffic loads, on the bending moments of the pipe were investigated. The centrifuge test results were found to be in reasonable agreement with those from full-scale tests. The pipe would experience the most unfavorable conditions when the heaviest axle of the traffic vehicle was located directly above the pipe crown. A deeper soil cover would lead to higher initial stresses in the pipe, as well as reduced influence of traffic load. However, even for a soil cover depth of 4 m, there is significant bending moment induced by the heavy truck loading, which cannot be ignored during pipeline design. A comparison was made between the centrifuge test results and several widely adopted design methods, and unconservative calculation results were noticed for large-diameter rigid pipes lying at a shallow soil cover depth subjected to heavy traffic loading.

Bearing capacity on sand overlying clay soils: a simplified conceptual model

The derivation of a new conceptual model for predicting the peak penetration resistance of circular footings installed vertically into sand overlying soft clay is presented in this paper. Based on visualisation experiments and finite-element analyses, a failure mechanism of a frustum of sand being forced into the underlying clay, with the outer angle reflecting the dilation in the sand, is assumed. The analytical basis of the new conceptual model follows the approach for silo analysis, and takes into account the stress level and dilatant response of the sand. A comprehensive analysis of the performance of the new model, and other existing analytical methods, in retrospectively predicting the peak resistance of a database of 49 geotechnical centrifuge tests is provided. Significant improvements over existing punching shear and load spread models are shown, as a result of incorporating the strength properties of the sand in a consistent manner. The new model has application to the offshore mobile ‘jack-up' industry, where inaccurate predictions of peak capacity in layered soils continue to cause damaging punch-through failures.

동적원심모형 시험을 이용한 부지응답해석 검증시 입력 지진의 결정

In order to verify the reliability of numerical site response analysis program, both soil free-field and base rock input motions should be provided. Beside the field earthquake motion records, the most effective testing method for obtaining the above motions is the dynamic geotechnical centrifuge test. However, need is to verify if the motion recorded at the base of the soil model container in the centrifuge facility is the true base rock input motion or not. In this paper, the appropriate input motion measurement method for the verification of seismic response analysis is examined by dynamic geotechnical centrifuge test and using three-dimensional finite difference analysis results. From the results, it appears that the ESB (equivalent shear beam) model container distorts downward the propagating wave with larger magnitude of centrifugal acceleration and base rock input motion. Thus, the distortion makes the measurement of the base rock outcrop motion difficult which is essential for extracting the base rock incident motion. However, the base rock outcrop motion generated by using deconvolution method is free from the distortion effect of centrifugal acceleration.

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.

Seismic soil–foundation–structure interaction observed in geotechnical centrifuge experiments

This paper reports the experimental results from two geotechnical centrifuge tests examining seismic soil–foundation–structure interaction of frame structures. In the first test, two three-dimensional frame structures were placed apart, and in the second test, the same frame structures were located adjacent to each other. One of the frame structures was founded on shallowly embedded spread footings, and the other was founded on a deeper basement. During the second test, seismic structure–soil–structure interaction was also examined. In this paper, the experimental set-up is described in detail, some kinematic interaction observations are made, and seismic footing response results are reported. The results of the tests indicate that the seismic response of the shallow footings of a flexible frame structure is complex. This result is important because these complexities are not well incorporated in current soil–foundation–structure interaction analysis procedures. The adjacency of two frame structures, and thus structure–soil–structure interaction, further complicates analysis procedures. In the second test, the spread footings rock, slide, and settle less when they are directly adjacent to a deep basement. These results imply that SSSI can be beneficial or detrimental, depending on the earthquake motion and the structural system. In a companion paper, the seismic response of the frame superstructures and the footing force demands are also examined.

Dynamic response of liquefiable sand improved by microbial-induced calcite precipitation

Microbial-induced calcite precipitation (MICP), a novel bio-mediated ground improvement method, was explored to mitigate liquefaction-prone soils. Geotechnical centrifuge tests were used to evaluate cementation integrity and the response of MICP cemented sands to dynamic loading. The cementation integrity testing reveals a change in behaviour from ‘soil like' to ‘rock like', with an increase in treatment level. Results from dynamic testing demonstrate a clear increase in resistance to liquefaction of MICP-treated sands compared to untreated loose sand. The MICP sands were treated to varying levels of cementation (light, moderate and heavy cementation levels) and assessed using non-destructive shear wave velocity measurements. The centrifuge models were all subjected to ground motions consisting of sine waves with increasing amplitudes. Accelerations, pore pressures and settlements were measured in the soil during shaking, and the changes in soil behaviour and post-shaking shear wave velocity for soils prepared to different cementation levels are discussed. Increased resistance to liquefaction was demonstrated with a decrease in excess pore pressure ratios in the MICP-treated models, as well as in reduced post-shaking settlements; however, surface accelerations were amplified at heavy levels of cementation. A tradeoff between improving liquefaction resistance and minimising undesirable higher surface accelerations needs to be considered when designing the soil improvement level.

Investigation of bearing capacity factor of T-bar penetrometer at shallow depths in clayey soils

The T-bar penetrometer is normally used in both in-situ and on the centrifuge to establish profiles of the undrained shear strength of fine-grain soils. However, adequate measurement of the undrained shear strength at shallow depths requires understanding the failure mechanism at these depths. For onshore and offshore structures such as pipelines, the assessment of soil parameters, in particular the undrained shear strength at shallow depth can be very critical to achieve both efficient and economic design. However, this requires the evaluation of the value of T-bar bearing factor (Nb) corresponding to both shallow and very shallow depths. This paper describes a series of geotechnical centrifuge tests undertaken using three different sizes of the T-bar penetrometer on two types of clayery soil. The results of the undrained strength obtained from each T-bar were compared with the results obtained from the bigger T-bars. The curves that describes the variation of the T-bar factor (Nb) versus the penetration depth were established from the comparison of all results obtained from the three different sizes of the T-bar. The results of (Nb) obtained in this study were compared with results obtained from both numerical and analytic solutions proposed in the literature. Generally, the experimental data obtained in this study presented Nb values higher than those predicted by others authors for all H/D ratios. However, the comparison indicates that the Nb values obtained in this study showed relatively reasonable agreement with those values obtained from the literature by adopting a non-homogenous strength profile.

Performance of an equivalent shear beam (ESB) model container for dynamic geotechnical centrifuge tests

In dynamic centrifuge tests, appropriate boundary conditions are required in order to simulate the seismic semi-infinite soil layer responses within the confines of a finite size model container. An ESB (equivalent shear beam) model container first designed at the University of Cambridge was built with a stack of light-weight aluminum frames separated by rubber to experimentally achieve this goal. In this paper, a significant number of dynamic centrifuge tests and the corresponding seismic response analyses were performed to evaluate the dynamic performance of a newly constructed ESB model container and to shed light on the range of testable soil conditions. In the set of conducted tests, it appears that the end walls of the ESB model container behave in accordance with the dynamic response of the soil deposit, despite a difference in the natural period depending on the relative density of the sand deposit. This is attributed to the differences in mass and stiffness of the end walls compared to those of the contained soil model. For partially filled model container, significant differences in seismic responses are observed in the end walls and in the soil deposit due to seismic interaction caused by the upper unfilled frames of the container. These findings suggest that dynamic model tests using this ESB model container should be conducted with the container completely filled. In addition, on the basis of a comparison with the seismic soil behavior inside a rigid-walled model container, it is clear that the ESB model container can provide a more representative lateral boundary configuration for dynamic site response studies.

Study on land subsidence under different plot ratios through centrifuge model test in soft-soil territory

Since the mid-1990s, multi-storied buildings, high-rise buildings and skyscrapers have been emerging due to the new round of great-scale development on the soft-soil littoral. Land subsidence in soft-soil areas caused by engineering environmental effect, especially by construction of high-dense building groups, attracts more and more attention. The plot ratio may be one of the most important factors on engineering-induced land subsidence. Thus, three geotechnical centrifugal model tests were conducted under three conditions of different plot ratios to study the ground settlement mechanism of high-dense building groups in soft-soil territory. It is concluded that the subsidence superimposition effect is obvious in the central area of high-dense building groups; the settlement increases with the growth of plot ratio. The urban planning department should take this subsidence situation into account when determining the construction plot ratio to utilize the earth more aptly and safely.

Centrifuge modelling of pile-soil interaction in liquefiable slopes

Piles passing through sloping liquefiable deposits are prone to lateral loading if these deposits liquefy and flow during earthquakes. These lateral loads caused by the relative soil-pile movement will induce bending in the piles and may result in failure of the piles or excessive pile-head displacement. Whilst the weak nature of the flowing liquefied soil would suggest that only small loads would be exerted on the piles, it is known from case histories that piles do fail owing to the influence of laterally spreading soils. It will be shown, based on dynamic centrifuge test data, that dilatant behaviour of soil close to the pile is the major cause of these considerable transient lateral loads which are transferred to the pile. This paper reports the results of geotechnical centrifuge tests in which models of gently sloping liquefiable sand with pile foundations passing through them were subjected to earthquake excitation. The soil close to the pile was instrumented with pore-pressure transducers and contact stress cells in order to monitor the interaction between soil and pile and to track the soil stress state both upslope and downslope of the pile. The presence of instrumentation measuring pore-pressure and lateral stress close to the pile in the research described in this paper gives the opportunity to better study the soil stress state close to the pile and to compare the loads measured as being applied to the piles by the laterally spreading soils with those suggested by the JRA design code. This test data shows that lateral stresses much greater than one might expect from calculations based on the residual strength of liquefied soil may be applied to piles in flowing liquefied slopes owing to the dilative behaviour of the liquefied soil. It is shown at least for the particular geometry studied that the current JRA design code can be un-conservative by a factor of three for these dilation-affected transient lateral loads.