1g Model Tests Research Articles

New indices of the recovery effect of the large deformation shield tunnel based on a light-disturbance grouting model test

The sudden surface surcharge is a key disturbance that causes excess transverse deformation of the existing shield tunnel underneath. The light-disturbance grouting technique, which is applied on two sides of a tunnel spring-line, is an effective way to control or even recover the large tunnel structural deformation. Based on the 1g model test of grouting for the shield tunnel in the sand layer using the Shanghai Metro shield tunnel as a prototype, a numerical model is established based on the FLAC3D software. Meanwhile, the numerical simulation method of simulating grouting involves the application of the virtual expansion stress. The grouting transfer efficiency and recovery bias are proposed in this paper and used as the evaluation indexes of the recovery effect of light-disturbance grouting on the shield tunnel. The findings indicate that irrespective of various overloading modes and symmetrical single-row grouting, the transfer efficiency remains approximately 12%, which may potentially serve as a optimization index for determining the number of grouting rows. Moreover, the observed disparity in recovery bias within tunnels experiencing asymmetric deformation underscores the significance of joint behavior, emphasizing the necessity for closer monitoring during grouting procedures.

Experimental Study on Cumulative Deformation of Pile Group in Saturated Clay under Horizontal Cyclic Loading

In order to investigate the cumulative deformation of the pile group in saturated clay under horizontal cyclic loading, a series of 1g model tests were conducted using the self-made loading equipment in this paper. Firstly, the loading equipment and testing procedure are introduced. Then, the cumulative deformation of the pile group, the dynamic response of the soil, and the bending moment of the pile shaft under horizontal cyclic loading are studied. Finally, the horizontal cyclic stiffness of the pile group is analyzed based on the experimental results. It can be found that the cumulative displacement, the rotation angle of the bearing platform, the pile shaft bending moment, and the pore water pressure can attain 90% of the peak values within the first 1000 cycles, and the growth rate slows down in subsequent loading cycles. Moreover, the bending moment of each pile increases with the burial depth and gradually decreases after the peak values. Notably, the horizontal cyclic stiffness of the pile group grows with the cycle loading times and decreases with the loading amplitude.

The surface bearing capacity of a strong granular layer on weaker sand

Stronger granular layers are often placed as working platforms over weaker sand subgrade. The design of a working platform involves the calculation of a two-layer bearing capacity under rectangular loading. Existing design methods are either overly simplified, based on infinitely long strip loads and validated by a small number of small-scale 1g model tests, or rely on numerous or empirically derived charts that are difficult to use or implement into design software. In this paper a new and highly practical design method is proposed in which the bearing capacity is determined simply from the shear strengths and unit weights of the two soil layers. The method was derived from extensive finite-element analysis and finite-element limit analysis (FELA) parametric studies in both plane strain and axisymmetric geometries. It was validated against published physical model tests, other FELA analyses and existing design methods. It can be applied to all rectangular shape ratios with dry and saturated layers.

Dynamic response of single-bucket foundation in clay under vertical variable amplitude cyclic loadings

The application of multi-bucket foundation has an increasing trend in deeper water. The horizontal dynamic response of multi-bucket foundation mainly depends on the vertical cyclic behavior of single bucket in this multi-bucket foundation. In order to investigate the vertical dynamic response of single bucket foundation, a series of 1g model tests were carried out under vertical cyclic loadings in over-consolidated clay. Test results show that the vertical displacement of model bucket increases and the stiffness of bucket-soil decreases with the increase of cyclic loading level parameter ζb, which is defined as the ratio of maximum force in loading cycle and static ultimate capacity. The vertical capacity increased by about 23% and 19.5% after constant amplitude unidirectional loading and variable amplitude loading, respectively. And the vertical capacity remains unchanged after constant amplitude bi-directional loading. The vertical displacement and stiffness of bucket-soil increase with the increase of cyclic times N. Superposition theory was applied to express the vertical displacement and stiffness of bucket-soil under vertical variable amplitude cyclic tests. The stiffness law was also analyzed using the spring model. The study on the vertical behavior of single-bucket foundation can provide some reference for the design of multi-bucket foundation.

The failure induced by embedment loss of gravity installed anchors in clay

The reliability of mooring systems is a critical issue for deepwater floating structures. Gravity installed anchors (GIAs) have been effectively employed in a significant number of mooring systems, whose embedment loss and kinematic behaviors under the cable loading conditions also attract attention to the reliability of the anchors during service. Particularly, the embedment loss of the anchor may result in the failure of the anchor. The present work focuses on systematically investigating the failure state of anchors in clay induced by embedment loss under various complex conditions. A theoretical model for analyzing the comprehensive behaviors of the anchor is constructed, which is validated by previous centrifuge tests and 1g model tests. A rational criterion for determining the state of failure induced by embedment loss is proposed and applied to the analytical results of the theoretical model. The factors that have a substantial impact on embedment loss of the anchors, including the anchor embedment depth, anchor orientation, cable loading angle at the seafloor, soil adhesion factor, soil strength, and cable properties are investigated in the analytical cases. OMNI-Max anchors, being one of the most remarkable GIAs and having complex kinematic behaviors, are selected to reveal the influences of these factors on the failure state of the anchors. Based on the results of numerous analytical cases, the failure surfaces in the three-dimensional spatial data field are presented, which reveal the critical state of the anchor under different conditions. Furthermore, these failure surfaces are stated using a surface function. The function can be used as a simple judgment condition for the state of failure owing to anchor embedment loss. The findings of earlier investigations support the applicability of the surface function. In addition, the promising application of the surface function in practice is highlighted.

Responses of model monopile to cyclic lateral loadings in clay

Large diameter monopiles are usually used as the foundation of offshore wind turbines. Monopile foundations are subjected to laterally cyclic marine environmental loadings which tends to accumulate displacements and rotation of the monopile. To preliminarily evaluate the cyclic responses of monopile in marine clay, 1g model tests have been conducted to combine with reported centrifuge tests to efficiently illustrate the behavior. This study investigates the laterally monotonic and cyclic responses of the model pile embedded in kaolin clay. As expected, obvious differences are observed between results of the 1g model tests and the centrifuge tests owing to the effect of stress levels, but important quantitative relations between results of these two types are revealed: two equations closely related to cyclic loading magnitude ratio ζb with same cyclic loading symmetry ratio ζc, could be qualitatively obtained to predict the pile responses in centrifuge tests by using the result of 1g model tests. The normalized unloading stiffness of model pile in 1g test is less than that of obtained from centrifuge tests with the same ζb values. This result not only highlights that it is necessary to account for the stress level of soil to understand cyclic lateral responses of monopile foundations, but also reveals that it is possible to qualitatively insight responses of monopile through 1g model tests.

1g model test of piled-raft foundation subjected to vibration load and its simulation considering small confining stress

The numerical simulations of 1 g model tests are presented to evaluate the feasibility of the proposed elasto-plastic constitutive model, which can precisely describe the less compressibility of clean sand under small confining stress. The dynamic responses of piled-raft foundations subjected to cyclic train loads were investigated by comparing the results of model tests and simulations. Different frequencies of dynamic loading and different grounds including dry loose sand, dry/saturated medium dense sand, were taken into consideration. It is confirmed that for all types of grounds and input vibration, the numerical results showed a good match with model tests data, especially the settlement and acceleration of piled-raft foundation. Both numerical simulations and experiments showed that the higher the frequency of the dynamic load is, the greater the peak acceleration will be. The proposed constitutive model has greatly improved the accuracy of the numerical simulations on the small-scale 1 g model tests because this model can accurately describe the mechanical characteristics of sand from very small confining stress to normal confining stress, especially the less compressibility and more dilative behavior of sand under small confining stress. • The proposed constitutive model can precisely describe the less compressibility of sand under small confining stress (CS). • The model tests were well reproduced by the numerical analysis with proposed constitutive model in the different grounds. • The proposed model has greatly improved the accuracy of the numerical simulation on the small-scale 1 g model test. • Excess pore water pressure was small both in the tests and simulations, because of more dilative behavior of sand at small CS.

Optimisation of vibroflotation by amplitude in 1g tests and coupled Eulerian–Lagrangian simulations

Vibroflotation includes densification of loose sands by means of shear deformation processes imparted by horizontal vibrations of vibrator probes. 1g model tests replicating the vibroflotation process were conducted using a model vibrator to identify online compaction control parameters. The effectiveness of the compaction was evaluated by means of a cone penetration test. An accelerometer and a trigger were installed in the model vibrator to measure the amplitude of the vibrator and the phase angle of the eccentric mass during compaction. The variation of amplitude and phase angle during compaction was studied and it was observed that amplitude and phase angle reduced with the increasing density of sand. Two tests were conducted with and without considering amplitude as a compaction control parameter. One test included a pre-determined fixed compaction time and the other used amplitude as a compaction completion indicator. It was observed that tests conducted considering amplitude as a compaction control parameter led to reduced compaction time. Coupled Eulerian–Lagrangian-based numerical simulations were carried out to study the feasibility of amplitude-controlled compaction under realistic stress state conditions.

Pullout behaviour of inclined shallow plate anchors in sand

This paper presents an experimental study on the pullout behaviour of inclined shallow plate anchors subjected to axial pull in sand. The 1g model tests were performed to examine the effects of anchor inclination and sand–anchor interface conditions on the load–displacement response and the associated failure and deformation mechanisms of plate anchors at various embedment ratios and sand densities. The anchor pullout capacity was found to increase continuously with the load inclination angle to the vertical (α), and the increase was more significant for α from 45° to 90°. The effect of sand–anchor interface conditions was negligible for horizontal plate anchors (α = 0°), but it became increasingly significant at larger inclination angles. The effects of these two factors both decreased with an increasing embedment ratio. Their influences on the failure and deformation mechanisms were measured and analysed using a digital image correlation (DIC) technique. Based on the test data and results available in the literature, a simple empirical method for the prediction of pullout resistance of inclined plate anchors in sand is calibrated and recommended.

Large scale model testing to investigate the influence of granular cushion layer on the performance of disconnected piled raft system

This paper presents the results of 1g model tests performed on instrumented model foundations, i.e., unpiled raft, single piled raft, single disconnected piled raft (DPR) and 3 × 3 DPR in sand under vertical load by using different granular materials and thickness of the granular cushion layer to investigate the effects of granular cushion on the performance of DPR system. The Settlement Efficiency (η) of the DPR which is nonlinear in nature is found to increase as the cushion thickness decreases and the mean particle size (d50) of the cushion material increases. A cushion thickness of two times the pile diameter and cushion material having d50 more than or, equals to 2 mm might serve as the suitable cushion material for the optimum performance of 3 × 3 DPR. The neutral axis and the zone of occurrence of maximum axial load of the pile in DPR are found to move downward with increasing cushion thickness. The piles in DPR are found to carry 35% of the total superstructure load within the settlement range considered in the study.

Experimental Investigation of Skirted Foundation in Sand Subjected to Rapid Uplift

Abstract This paper reports results from 1g model tests carried out under single gravity on a skirted foundation installed in sand and subjected to a rapid uplifting force. The effects of displacement rates ranging from 5 mm/s to 450 mm/s on the ultimate capacity, suction pressure inside the skirt compartment, and time of extraction were investigated. Test results indicate that the displacement rate significantly affected the magnitude of uplift resistance, as well as the magnitude of suction under the foundation lid, but had little effect on the relationship between stress and the displacement of the foundation. The shapes of the uplift capacity-displacement curve and the suction-displacement curve were similar for all experimental displacement rates.

1g model tests of piled-raft foundation subjected to high-frequency vertical vibration loads

The piled-raft foundation (PRF) has been used in high-speed railway in China. If the design and construction of the foundation is done without taking proper precautions, then the vibration loads induced by the high speed trains may cause excessive settlement of the foundation, significantly affecting the stability and safety of trains. To carefully examine the settlement mechanisms of PRFs constructed in different grounds subjected train loads, systematic 1 g model tests were conducted. The dynamic responses of PRFs in dry loose sand, medium dense sand, and saturated medium dense sand ground, subjected to different frequencies of sinusoidal vibration loads, were investigated. Additionally, reinforcement effect of cement-treated partial ground improvement (PGI) for dry loose sand ground, a commonly used countermeasure against settlement of PRF, was also investigated via model tests. It is found that the dynamic loading frequency affected the settlement of PRF, and the pattern of PGI reinforced at different depths of pile also significantly affected the settlement. The model test also showed that load-sharing ratio between pile and raft also changed significantly during vibration loading, implying that extensive precaution should be taken in design and construction of PRFs for high-speed railway constructed in soft ground.

Experimental studies on a circular open caisson

The resistance offered by the soil to the cutting edge of a caisson and soil flow around the cutting edge will vary continuously during sinking. In this study, a series of 1g model tests have been carried out to investigate the load–penetration response and the soil flow mechanism in sand around the cutting edge of a circular open caisson. Full- and half-open caisson tests have been conducted considering different tapered angles of the cutting edge. The load–penetration response is investigated using the full-open caisson tests with different (a) tapered angles of the cutting edge, (b) types of penetration and (c) depths of sinking. The soil flow mechanism is investigated using the half-open caisson tests with different (a) tapered angles of the cutting edge, (b) magnitudes of penetration and (c) depths of sinking. The soil flow mechanism is evaluated using image-based deformation measurement techniques. The results of the study presented in the form of load–penetration response and soil flow mechanism are useful in addressing the interaction between the cutting edge of the caisson and soil during the sinking operation.

Response amplification of back-rotated piles

Piles are largely back-rotated in sliding slope or subjected to lateral spreading. This paper reveals for the first time that response of these piles (e.g., displacement, rotation, bending moment, and shear force) is amplified against forward rotating piles. In particular, magnification is detrimental, once normalized rotational stiffness (NRS) of the piles is around a singularity value (i.e., normalized singularity stiffness, NSS). New expressions are developed to gain the NSS value, the magnification degree, and the sliding depth to incur the singularity. The NRS is assessed using 1g model tests. The solutions are adopted to capture the response of the model piles, to detect new failure mechanism of Showa Bridge, and to check the safety of Christchurch bridges. The main conclusions are as follows: (i) piles are prone to response amplification, when subjected to lateral spreading or in sliding slopes. (ii) The NRS is only slightly affected by soil movement profiles and sliding depths. (iii) Showa Bridge collapsed from displacement amplification of back-rotated piles. Finally, (iv) the roller connections between girder and piers, and an integral abutment and piers are proved to be effective to curb the amplification. The amplified response needs to be assessed in practice to lessen failure of back-rotated piles.

Coupled Eulerian Lagrangian based numerical modelling of vibro-compaction with model vibrator

Abstract The use of field experience and empirical relations for the design of a vibro-compaction encourages the development of the understanding of vibratory compaction for granular material. Field measurements are difficult to conduct in homogeneous loose sands, and there is a lack of adequate control of the test conditions in the field. Hence, a model vibrator was fabricated to perform 1g vibro-compaction model tests. The effectiveness was evaluated in terms of cone penetration tests before and after compaction. The 1g model test results suggested that the degree and radial extent of compaction increased with frequency for the considered sand for a fixed vibrator mass and eccentricity. A coupled Eulerian Lagrangian-based numerical model simulating the 1g model tests was created, and the results from the physical model tests and numerical simulations were found to be in good agreement. Following this validation, the numerical framework was used to study the influence of a range of frequencies and the nature of the sand on the degree and extent of compaction. The numerical simulations were found to not only provide spatial and temporal compaction analysis freedom but also to serve as a substitute for laborious and time-consuming physical model tests.

Model tests on the bearing capacity of precast open-ended micro pipe piles in soft soil

Laboratory 1g model tests were carried out to study the bearing capacity and load transfer mechanisms of precast open-ended micro pipe piles in soft soil. The single pile, 2 × 2 and 3 × 3 pile groups were designed and tested under vertical compression, uplift and horizontal loads, respectively. A steep fall pattern occurred in the load–displacement curves of piles under vertical compression and uplift loads. The measured soil plug length and axial force indicated that the piles could be considered as fully plugged friction piles under vertical compression load. The pile groups behaved in a non-integral failure mode under vertical uplift load and a conical wedge failure mode under horizontal load. At the early loading stages under vertical compression and uplift loads, the pile shaft resistance of the upper part tended to be fully mobilised earlier than that of the lower part. At the later loading stages, the pile shaft resistance of the upper part decreased, whereas the shaft resistance of the lower part still showed a little increase. The maximum bending moment of the pile occurred at a depth of about 10Dp under horizontal load.

A hypoplastic macroelement model for a caisson foundation in sand under monotonic and cyclic loadings

Nowadays, the numerical analysis of caisson foundations of offshore structures is a big challenge in engineering design. A simple, fast and accurate numerical tool is proposed in this article based on the macroelement concept. The novel macroelement is within the framework of hypoplasticity and can consider static monotonic and cyclic combined (multidirectional) loads for a caisson foundation in sand. The incremental nonlinear constitutive formulas are defined in terms of generalised forces and displacements and an enhanced function of failure surface is introduced. A series of well-documented laboratorial reduced-scale 1g model tests are adopted to validate the novel numerical tool. Results demonstrate a satisfied predictive performance of the proposed macroelement that can be used in caisson foundation design and constitutes an alternative to the traditional finite element analysis.

Experimental evaluation of ultimate bearing capacity of the cutting edge of an open caisson

Open caissons are sunk into the ground by their own weight. A cutting edge of the caisson having a tapered inner face on loading – that is, raising of the steining – results in bearing failure by displacing the soil which is in contact with the cutting edge. The bearing capacity of the cutting edge and the soil flow mechanism depend on the configuration of the cutting edge, sinking depth and soil type. This paper presents the results of a series of 1g model tests, which investigate the effect of varying tapered angles of the cutting edge on the penetration resistance of the open caisson. The vertical failure load and corresponding vertical bearing capacity factor, N′γ, and the soil flow mechanism around the cutting edge are investigated. The soil flow mechanism and the influence of surcharge formed at the top level of the cutting edge due to advancement of the caisson in the ground are examined using the image-based deformation measurement technique. The results highlight that the cutting angle of the cutting edge and sinking depth play important roles in the load–penetration response and soil flow mechanism.

An innovative booster for dynamic installation of OMNI-Max anchors in clay: Physical modelling

An innovative booster concept is proposed to increase both the kinetic and potential energies and hence to increase the penetration depth of the OMNI-Max anchor within the clayey soil with high strength gradient. This paper carried out 1g model tests to investigate the working efficiency of the booster on the anchor penetration depth both in normally consolidated (NC) and lightly over-consolidated (LOC) soils. The similarity relationships of the anchor dynamic penetration within the soil were first deduced, and then a series of dynamic penetration tests were performed to investigate the parameters that influence the anchor penetration depth. The parameters include the soil strength characterisation, impact velocity, booster weight, inclined impacting angle, and water drag and entrainment effects. The anchor dynamic penetration process within the soil was measured by a micro-electromechanical systems (MEMS) accelerometer, through which the soil strain-rate effect and the hydrodynamic aspects including the water drag force and potential water entrainment along the anchor-soil interface were analysed. Finally, energy-based and force-based prediction models were put forward to estimate the penetration depths of OMNI-Max anchors and hybrid anchors (i.e. an OMNI-Max anchor with a booster).

Deformational behaviour of steel sheet piles during jacking

In geotechnical engineering the simulation of penetration processes is an important tool for the prediction of the bearing capacity of structures and of the soil behaviour in the near field. Generally the numerical analysis of penetration processes is limited to penetrating bodies, which experience no deformation during simulation. The reason for this limitation is determined by the enormous computational costs. This paper presents a novel method to simulate the penetration process considering the deformation behaviour of slender profiles with relatively low bending and torsional stiffness such as sheet piles. The numerical analyses are carried out using the Coupled Eulerian-Lagrangian (CEL) approach. The modelling method is validated by 1g-model tests. Applying this method to the jacking simulation of sheet piles, in particular U- and Z-sections, it can be shown, that the stress state between the surfaces of web and flange increases due to soil plugging. Hence, higher shaft resistance in the corner regions forces the profile to deform. The influence of the profile’s deformation on the reaction forces is significant and must be considered.