Cyclic Settlement Research Articles

Geocell-reinforced bed anchored with additional vertical elements under repeated loading

This paper investigates the behavior of geocell-reinforced beds combined with extra vertical elements embedded into the geocell pockets at their joints. During repeated loading, the soil mass inside geocell pockets can loosen, leading to decrease in geocell layer extension. Vertical elements can help to reduce the soil loosening, alleviate deformation and distortion of the geocell wall directly under loading zone and provide anchorage at the geocell outer boundaries. Results show that, with the use of vertical elements, geocell performance is significantly enhanced due to better load distribution and extra anchorage. An average 40% reduction in cyclic settlement was observed using combined geocell and vertical elements compared with a bed reinforced with geocell alone. Also, with an optimum height of the vertical elements, a 42% reduction in geocell consumption even at higher performance can be achieved using a less extensive geocell mattress. Provision of vertical elements caused the geocell length on each side of the footing to be decreased from 1.1D to 0.5D for the same performance in some cases (D: footing diameter). Prototype experiments showed that the geocell reinforcement anchored with vertical elements experiences less deformation, distributing load over a wider area. Thus, geocell and vertical elements combine to act like a geocell with greater thickness and larger width, helping project economy by lower use of geocell material even if the additional cost of vertical elements may counteract. Calibrated numerical modelling compared favorably with experimental observations, providing insight into stress and strain distribution in the soil and geocell.

Mesoscale Mechanism of Asphalt Track Bed in Reducing Cyclic Settlement of Ballast Layer under High-Speed Train Traffic Loads

Asphalt track bed, in which an asphalt mixture is inserted underneath the ballast in a conventional granular track bed, has been experimentally proven to have better safety, stability, and economic efficiency. However, most existing theoretical studies were based on continuous mechanics theory, mainly focusing on the impact of the asphalt layer on the macroscale response of the track bed or subgrade, and rarely on the particle-scale mechanical behavior of ballast due to the limitations of the method. Hence, aiming at the improved performance of asphalt track bed and its mesoscale mechanisms, the discrete element method (DEM) model of asphalt track bed is established in this study and then validated with the measurements obtained from on-site tests. Subsequently, the validated model is used to investigate the effect of asphalt layer on the dynamic response and cyclic settlement of ballasted track bed under high-speed train moving loading. Results reveal that the asphalt layer has an ability to reduce vibration of the track bed, which contributes to less active ballast and more stable interparticle structure, thereby decreasing the accumulative settlement of the ballast layer. Besides, the asphalt layer can improve the uniformity of the force chain network inside the ballast layer. The well distributed and less intense force chain network in the ballast results in a slower degradation rate. A series of parameter analyses is performed in the end, concluding that the required thickness of the ballast layer in the asphalt track bed could be reduced by approximately one-third without changing the bearing capacity of the subgrade, and the asphalt track bed structure is a more economical and effective track form for high-speed or heavy-haul railway lines. This study discusses the improvement of ballasted railway track by insertion of asphalt layer and its mesoscale working mechanisms, and the results provide guidance for the design of asphalt track bed in high-speed railways or heavy-haul railways.

Settlement analysis of shallow strip footing under combined static and cyclic loadings

ABSTRACT Shallow foundations in the vicinity of industrial zones are often subjected to machine induced cyclic loads. One of the key problems associated with these kinds of foundation is the determination of amount of settlement under cyclic load. A close observation of the existing studies reveals lack of studies explaining the influence of depth of embedment and frequency of loading on the cyclic load induced settlement of the foundation. An attempt has been made to fill these voids in this study. In the present study, the soil structure interface is modeled using nonlinear elasto-plastic springs and settlement is observed by varying different parameters like the relative density of soil, embedment ratio, factor of safety, intensity and frequency of cyclic loading. It is found that the settlement becomes negligible after certain number of load cycles, i.e. the critical number of load cycles. An empirical expression is developed to estimate the non-dimensional settlement of the foundation using nonlinear regression. It is found that the static load on the foundation is the most important factor influencing the combined static and cyclic settlement response of the foundation. Also, the ultimate settlement is attained earlier for higher values of frequency of loading.

Cyclic settlement of ballast layer due to train passages at high speed and its reduction by asphalt trackbed

Ballasted track has been widely adopted in railway lines, even in high-speed railways. As the train speed increases, ballast layer has to undertake excessive settlement or severe degradation. To preserve rail network safety and ride quality more optimally, it is vital to get further insight into the cyclic accumulative deformation of ballast layer. This study concentrates on investigating cyclic settlement of railway ballast with an emphasis on the inside particle-scale interactions in conventional ballasted trackbed, and further in asphalt trackbed, a promising structural form of the granular track. The entire transversal cross-section of granular trackbed is modeled based on discrete element method with realistic polygon elements. The numerical model is validated with the measurements from full-scale model tests and then used to study the deformation behavior of railway ballast. Subsequently, the trackbed model with insertion of asphalt layer is established to investigate the reduction of permanent deformation in asphalt trackbed. The granular trackbeds under different train loading are analyzed in the end. Results indicate that the inter-particle structure in the ballast is stabilizing with an increasing number of loading cycles, contributing to less particle migration, so the growth rate of permanent deformation appears a slowing trend. Besides, the asphalt layer optimizes the distribution of contact chains in the ballast and reduces the intensities of contact forces, leading to a remarkable decrease in permanent deformation. Moreover, the higher train speed results in more active ballast particles so that the trackbed becomes hard to stabilize especially when the shakedown threshold is reached. The adverse impact derived from the train speed can be weakened effectively by the asphalt layer. This paper discusses the cyclic settlement of railway ballast and its reduction induced by the asphalt layer. These conclusions will guide the potential methods to alleviate the permanent deformation in high-speed railways, such as employing the asphalt trackbed or controlling the train speed.

Experimental assessment of the performance of a bridge pier subjected to flood-induced foundation scour

Scouring around piers is recognised as one of the main causes of bridge failure. Local scour refers to the localised erosion of soil at piers and abutments caused by flow-induced vortices around their base. This paper studies experimentally the response of a bridge pier, supported on a cylindrical embedded foundation, subjected to flood-induced scour. A hybrid two-step methodology is developed to study the hydraulic and the mechanical part of the problem. In the first step, the hydraulic problem of local scour around a pier is experimentally modelled in 1g using a recently developed miniaturised tidal generator. The experimentally generated scour hole is then three-dimensionally scanned to produce a three-dimensional (3D)-printed mould. The latter is used in the second step to reproduce the scour hole in an Ng model, subsequently tested in a drum centrifuge to study the mechanical part of the problem under proper stress scaling. Foundation performance before and after local scour is studied through vertical, lateral monotonic and slow cyclic pushover tests. The effects of general (uniform) scour are also investigated by removing a soil layer of constant thickness. Local scour is shown to have a minor effect on vertical bearing capacity. In stark contrast, the lateral performance is significantly affected, with the foundation moment capacity Mult being reduced by up to 38%. The effect of general scour is even more pronounced, leading to 48% reduction of Mult. The rate of cyclic settlement accumulation is also much more severely affected by general scour as compared to local scour. Overall, the effects of local scour on foundation performance differ substantially to those of general scour, bringing into question the common simplification of ignoring the geometry of the scour hole, making no distinction between local and general scour.

Experimental study on cyclic settlement of piles in silt soil and its application in high-speed railway design

Pile-supported embankment is popularly used for constructing high-speed railways on soft soils. With train running at high speed, the dynamic load on the pile top generated by the train passages increases noticeably. To keep the long-term settlement of the railway track within a reasonable range, it is necessary to investigate the settlement of piles in soils simultaneously subject to the monotonic load caused by the weight of the track structure and embankment as well as the cyclic load caused by the train passages. In this paper, model tests on piles in silt soil under combined monotonic and cyclic loading were conducted. The behavior of a pile in saturated silt was investigated by measuring pile settlement, soil pressure on the pile surface, and pore water pressure in the soil adjacent to the pile. Based on the model testing results, the ratio of the mean value of cyclic load to the ultimate bearing capacity of the pile (Pm/Pu) and the ratio of the half amplitude of the cyclic load to the ultimate bearing capacity of the pile (Pcyc/Pu) were identified as the main factors dominating the development of pile settlement. A higher Pcyc/Pu leads to faster accumulation in the pile settlement, and a higher Pm/Pu facilitates this process. A chart with three zones determined by the combination of Pcyc/Pu and Pm/Pu has been proposed to distinguish the settlement trends of the piles: stable zone, metastable zone and unstable zone. The combination of Pcyc/Pu and Pm/Pu located in the stable zone generates very small settlement. Moreover, for the loading combination in the metastable zone, an analytical model was proposed to predict the cyclic pile settlement subject to cyclic load with large cycles. The advantage of the model is that the coefficient of friction at the pile surface and the cyclic strain degradation parameter are incorporated. Finally, a case study is presented to demonstrate the application of the proposed chart and the prediction method on cyclic pile settlement in the design of piled embankment of high-speed railways.

Cyclic Settlement of Footings of Different Shapes Resting on Clayey Soil

An experimental investigation is carried out to investigate the impact of the footing shape, when rested on clayey soil under cyclic loading condition. The model footings used in this study are circular, square and the area of footings is fixed. Cyclic load test is carried out on the cohesive soil with three undrained shear strengths (20 kPa, 40 kPa and 70 kPa). Two depths of foundation embedment (at surface and 5 cm) to know the effect of the depths of the foundations on the change of settlement and total vertical stress and two rates of loading (3 mm/sec and 6 mm/sec) are used. It has been observed that the bearing capacity varies in increasing order as Solid, Circular and Square. It is found that the cyclic settlement in the square foundation is less than the circular foundation. The results reveal that the shape of the footing has a significant effect on its bearing capacity and the settlement characteristics. The vertical stress reaches a constant value which is greater below circular footing and it is about (70.9 - 92.7) % greater than below square footing.

Effects of freeze-thaw on settlement of fine grained soil subjected to cyclic loading

The dominant soil deposits in the seismically active Arctic region are of fine-grained characteristics. Behavior of such soil deposits under dynamic loading is typically influenced by the freeze-thaw phenomenon as well as the variation of the ground temperature. In this extensive experimental study, settlement of fine-grained soils due to seismic loading is investigated. Reconstituted soil samples were tested under laboratory conditions to study the effect of freeze-thaw and lowered ground temperature. A strong correlation between the settlement potential and developed excess pore pressure during cyclic loading was found to exist for all freeze-thaw conditions. The settlement increased with increasing excess pore pressure. Subjecting the soil to freeze-thaw cycles decreased the settlement potential. A substantial decrease in volumetric strain of the soil occurred within the first two cycles of freeze-thaw. The number of dynamic loading cycles and the level of induced shear strain were also found to influence the settlement; the larger the number of loading cycles and induced shear strain the greater the settlement, for all freeze-thaw conditions. Another factor affecting the cyclic settlement was the variation in ground temperature. The soil experienced the smallest settlement at near-freezing temperature of 0.5 °C.

Cyclic response of coir fibre-reinforced sand

The reinforcement technique is a better choice to the conventional ground improvement technique under appropriate conditions to enhance the bearing capacity of soil. The behaviour of reinforced earth was studied by considering generally static loads. In some situations, structures are also subjected to cyclic loads; such as those occurred by the machine foundations, traffic loads, earthquake, etc. Review of earlier studies has indicated that very few literatures are available with regard to performance of coir reinforcement under cyclic loading. The objective of the present study is to assess the behaviour in terms of load-carrying capacity and cyclic settlement using coir fibres as reinforcement and by conducting cyclic load settlement tests using model footings resting on sand reinforced with coir fibres. The present experimental study indicated that, coir reinforcement shows decrease in cyclic resistance ratio as well as cyclic stiffness with increase in u/B ratio. In the present study, better cyclic resistance of sand was observed in coir fiber-reinforced sand compared to unreinforced sand under all excitation parameters tested.

An experimental study on the cyclic settlement of sand and cemented sand under different inclinations of the bedding angle and loading amplitudes

Although the directional dependence of soil behaviour is a subject of great interest, yet studies of its effects on cyclic settlement are lacking. Therefore, this paper investigated the macro-scale cyclic settlement under various inclinations of bedding plane (α = 0° to 90°) and different loading amplitudes (qc/qu), and to assess the degree of improvement provided by cement additions which reduce directional dependence. There was high variability in the cyclic settlement of uncemented sand according to bedding plane inclination. The difference between settlement ratios at 0° and 90° were 5.1, 29 and 55% after 100 load cycles with amplitudes of 10, 20 and 30%, respectively. Most of this difference occurred when α ≥ 60°. However, settlement variation became marginal after a large number of load cycles for all amplitudes. Sand treated with cement was improved and it showed a decrease in settlement ratio and elimination of directional dependency. With 7% cement, settlement variation with α did not exceed 12% for all loading amplitudes. However, these effects were still pronounced at lower cement contents, especially when α = 60–90°.

Cyclic settlement behavior of strip footings resting on reinforced layered sand slope

The paper presents a study of the behavior of model strip footings supported on a loose sandy slope and subjected to both monotonic and cyclic loads. The effects of the partial replacement of a compacted sand layer and the inclusion of geosynthetic reinforcement were investigated. Different combinations of the initial monotonic loads and the amplitude of cyclic loads were chosen to simulate structures in which loads change cyclically such as machine foundations. The affecting factors including the location of footing relative to the slope crest, the frequency of the cyclic load and the number of load cycles were studied. The cumulative cyclic settlement of the model footing supported on a loose sandy slope, un-reinforced and reinforced replaced sand deposits overlying the loose slope were obtained and compared. Test results indicate that the inclusion of soil reinforcement in the replaced sand not only significantly increases the stability of the sandy slope itself but also decreases much both the monotonic and cumulative cyclic settlements leading to an economic design of the footings. However, the efficiency of the sand–geogrid systems depends on the properties of the cyclic load and the location of the footing relative to the slope crest. Based on the test results, the variation of cumulative settlements with different parameters is presented and discussed.

Volumetric Threshold Shear Strain for Cyclic Settlement

The volumetric cyclic threshold shear strain for cyclic settlement, γtv, is evaluated for seven different sands and clays at different degrees of saturation, S, from the results of 11 Norwegian Geo...

Cyclic settlement behaviour of spudcan foundations

This paper examines the cyclic settlement behaviour of spudcan foundations in sand, using centrifuge model data. By means of a servo-electrohydraulic system, loading episodes comprising several thousand cycles of sinusoidal loading were applied to model spudcan foundations. The findings showed that, in dry sand, non-preloaded spudcans settle by penetration into the soil, resulting in a large increase in cyclic stiffness. In contrast, preloaded spudcans appear to settle by densification of the sand, leading to a smaller increase in cyclic stiffness. In saturated sand, where excess pore pressure can accumulate, the effects of preloading are less evident. A non-preloaded spudcan tends to generate a bulb of negative pore pressure around itself, conferring some apparent preloading on the near-field soil, and thereby resisting further penetration. In a preloaded model, the negative pore pressure bulb is smaller and surrounded by a zone of positive pore pressure. As the positive pore pressure propagates into the negative pore pressure bulb, the effective stress of the sand around the spudcan decreases, allowing further penetration. This is manifested as an apparent loss of preload. Thus non-preloaded and preloaded spudcans may settle by densification and penetration.

Cyclic settlement behaviour of spudcan foundations

This paper examines the cyclic settlement behaviour of spudcan foundations in sand, using centrifuge model data. By means of a servo-electrohydraulic system, loading episodes comprising several thousand cycles of sinusoidal loading were applied to model spudcan foundations. The findings showed that, in dry sand, non-preloaded spudcans settle by penetration into the soil, resulting in a large increase in cyclic stiffness. In contrast, preloaded spudcans appear to settle by densification of the sand, leading to a smaller increase in cyclic stiffness. In saturated sand, where excess pore pressure can accumulate, the effects of preloading are less evident. A non-preloaded spudcan tends to generate a bulb of negative pore pressure around itself, conferring some apparent preloading on the near-field soil, and thereby resisting further penetration. In a preloaded model, the negative pore pressure bulb is smaller and surrounded by a zone of positive pore pressure. As the positive pore pressure propagates into the negative pore pressure bulb, the effective stress of the sand around the spudcan decreases, allowing further penetration. This is manifested as an apparent loss of preload. Thus non-preloaded and preloaded spudcans may settle by densification and penetration.