Piles In Soft Clay Research Articles

Nonlinear numerical simulation of physical shaking table test, using three different soil constitutive models

Dynamic response records of pile performance during earthquakes are limited mainly due to the challenges of recording the seismic soil–pile response. This limitation has led to an inadequacy in providing a standardized basis for the calibration and validation of the available analytical and numerical methods developed for seismic soil–pile superstructure interaction problems. To bridge this gap, a series of numerical simulations of scaled, shaking table tests of model piles in soft clay has been developed in the current study. This paper aims to accurately identify all aspects and critical parameters in the numerical simulation and propose the most suitable soil constitutive model. Three soil constitutive models are selected as advanced models for soft soil, namely, the modified Mohr–Coulomb, Drucker–Prager/cap plasticity, and Cam–clay models. Similar to the physical test case study, this numerical analysis uses dimensional analysis techniques to identify scale modelling criteria and develop a scaled soil and pile-supported structure model correctly. The 3D nonlinear numerical models are developed using the Abaqus software.

Compressive Capacity of Conventional and Under Reamed Piles in Soft Clay

Under reamed pile is a well-known solution for carrying roads, light structures, walk sides, communication and electrical tower and rail. In this laboratory study, the compression conduct of conventional pile and undreamed pile with single and double bulbs models with shaft diameter of 10 mm embedded in soft clay soil layer overlaying sand soil was investigated. The sand layer with a depth of 300mm was compacted into six sub layers to relative density 70% in a steel container of a diameter 300mm in size. The depth of soft clay was 250mm and undrained shear strength (Cu) 35 kPa which compacted into five sub layers on sandy soil layer. A series of tests of model are performed on conventional and underreamed pile. Different lengths of piles are manufactured like 250mm, 290mm, 330mm, 370mm and 410mm and with bulb of a diameter 20mm. Moreover, single bulb and dual bulbs were utilized for these piles, a comparison in behaviour between underreamed pile and conventional pile (without bulb) is fulfilled. This study appeared finalize the ultimate compressive capacity of underreamed pile of single bulb is (1.9, 2.6, 3.3, 3.4 and 3.2) times greater than conventional pile while the underreamed of double bulbs is (4.1, 3.7, 4.3, 4.1 and 3.7) times greater than conventional pile for ratios of L/ D (25, 29, 33, 37 and 41) respectively. A proposed relationship used to estimate ultimate compressive capacity for conventional pile and underreamed pile with single and double bulbs when dimensions of pile are known or used to estimate dimensions of pile for specified load capacity.

Effect of Soil Improvement Techniques on Increasing the Lateral Resistance of Single Piles in Soft Clay (Numerical Investigation)

Soft clay soil extends in many areas all over the world. The construction on soft clay is one of the most important problems in engineering practice. When the design engineers face this problem, they always use deep foundations. In many cases, the vertical loads are satisfied but the lateral capacity of the piles is not satisfied. In this paper, a series of 3D finite element analysis was performed to investigate the effect of soil improvement techniques on increasing the lateral pile capacity. The 3D finite element results indicate that the improvement in the lateral pile capacity is strongly dependent on the combined effect of the lateral extent and the thickness of the improved zone around the pile head. The analysis indicates that the lateral pile capacity increased to about 240% greater than the capacity in native soft clay when a portion of soft clay is excavated and replaced with dense sand. In addition, the lateral pile capacity increased to a value greater than 600% when the soft clay is mixed with cement.

Drained-Undrained Shaft Resistance of Piles in Soft Clays

The determination of the stress distribution around driven piles in soft clays constitutes a complex problem. The stress distribution is affected by a range of factors such as soil permeability, soil strength, sensitivity, remoulding, distance from adjacent piles and number of piles. The mechanisms of pile installation and subsequent consolidation are investigated by considering that pile installation may be represented by the expansion of a long vertical cylindrical cavity. The stress paths followed by typical soil elements at the soil–pile interface are analyzed by means of the theoretical relationships obtained during the undrained expansion of a cylindrical cavity in soft cohesive soils and modified to take into account the severe remoulding caused by pile installation. It is shown that the shaft resistance of driven piles may be calculated by considering either a drained stress path or an effective stress path during undrained loading. It is also shown that soft clay remoulding caused by pile driving is a major factor that must be taken into account for a reasonable estimation of the limiting skin friction.

Vertical static and dynamic pile-to-pile interaction in non-linear soil

A vertically loaded floating pile in clay affects a neighbouring pile by increasing the latter's displacement due to its own load. As a result, a group of rigidly capped piles exhibits a force/settlement ratio (‘vertical stiffness’) that is smaller than the sum of the individual stiffnesses of each pile – ‘efficiency’ in static stiffness less than 1. However, under dynamic steady-state loading the response of the pile group is an oscillatory function of frequency, and at certain frequencies a complete reversal of the static trend occurs, with the elastic dynamic group ‘efficiency’ exceeding not only the static ‘efficiency’, but also unity. To assess the realism of such behaviour, finite-element inelastic soil models were utilised to explore the influence of soil non-linearity on pile-to-pile interaction factors, under both static and dynamic loading. It is found that, with realistically inelastic undrained clay behaviour, the influence of a loaded pile on its neighbour diminishes radically with increasing amplitude of imposed displacement. The presence of a number of in-between piles, as well as the neighbouring pile's own rigidity, has no substantial effect on the interaction. The observed trends are explained by recourse to simple physical arguments. The diagrams provided for the pile-to-pile interaction factor are utilised to obtain the vertical dynamic impedance (i.e. stiffness and damping) of a 2 × 2 and a 3 × 3 rigidly capped pile group. It is found that these impedances are in accord with those resulting from three-dimensional analysis of the complete pile group. The difference between elastic and inelastic efficiency factors is shown to be substantial. The validity of the numerical results is strictly limited to piles in soft clays, whose resisting stress on the pile shaft equals their undrained shear strength.

A p–y curve model for laterally loaded XCC pile in soft clay

This paper presents a p–y curve model for laterally loaded X-sectional cast-in-place concrete (XCC) piles, which are a type of non-circular cross-sectional pile, in soft clay. The recently developed empirical solutions for the elastic stiffness and the ultimate lateral resistance of the two-dimensional laterally loaded X-shaped cross-sectional pile segment–soil system by the authors using the complex variable elasticity theory and finite element limit analysis are used as the two key parameters in constructing the p–y curve. An elastic–perfectly plastic p–y curve is immediately obtained with the linear elastic state described by the elastic stiffness of the two-dimensional system, and the perfectly plastic state is controlled by the ultimate lateral resistance. Furthermore, to consider the nonlinear behaviour, a more robust model using the hyperbolic function is selected for producing the p–y curve with the elastic stiffness and the ultimate lateral resistance incorporated. Subsequently, the p–y model is introduced to the Winkler model to generate solutions for the response of laterally loaded XCC piles. The proposed model considers the non-circular cross-sectional effect of the XCC pile and can also be regarded as a simple method for constructing the p–y curve for other non-circular cross-sectional piles.

Study on Bearing Capacity of Belled Uplift Piles in Soft Clay Area

The study on the bearing capacity characteristics of the belled uplift piles in soft clay foundation is of guiding significance for the type selection and bearing performance of the piles. A full-scale test on the uplift piles in soft clay was carried out in Qianhai District of Shenzhen. The test results show that the belled uplift piles are applicable to the soft clay. Compared with the equal section uplift piles, the Q–S curves are smoother, the ultimate bearing capacity can be increased by more than 50%, and the piles perform more stability in the later stage of the test. The displacement of pile foundation, the elongation of pile body and the rebound ratio of pile foundation are studied. The results show that the foundation with the belled uplift piles are applicable in the soft clay areas, and the small-angle belled uplift pile head suitable for the soft clay area is proposed, which is of great significance for the popularization and use of the belled uplift piles.

Development of dynamic p–y curves for piles in soft clay by using 1-g shaking table test

ABSTRACTIn this study, a series of 1-g shaking-table tests was performed for a single pile in soft clay deposit with various conditions. Based on the test results, the dynamic p–y curve was determined by connecting the peak points of dynamic p–y loops. To construct the p–y curve for practical forms, Matlock’s p–y for clay deposit was utilized to represent the initial stiffness () and the ultimate capacity () of the clay. These two terms are required to formulate the p–y curve as a hyperbolic function. The suggested p–y curve was compared with p–y curves in the literature, and verified with centrifuge test results and numerical analysis results.

Development of Design Charts for Installation of Granular Piles in Soft Clays

The vertical loads on the soil reinforced with Granular Pile results in axial compression of both the soil and the Granular Pile material. The imposed stresses result in radial bulge of the pile material against the surrounding soil though it is restricted to the upper portion of pile length. Considering this nature of the Granular Pile, analytically solution for the strength of the composite ground reinforced with Granular Piles is discussed herein. The radial bulge extends up to the depth equal to the four times the diameter of the Granular Pile. Various factors such as undrained shear strength of the soil and the internal friction angle of the pile material are used to derive the capacity of the Granular Pile. The proposed solution is used to calculate the capacity of Granular Pile varying the properties of surrounding soil and Granular Pile material. Values of undrained shear strength (cu) were varied from 10 to 50 k Pa. Bulk density (γ) of the clay soil is assumed as 20 k N/m3. Angle of internal friction for Granular Pile material (ϕc) is varied from 380 to 420. The design charts between safe bearing pressure and the angle of internal friction for Granular Pile material are drawn. The diameter of Granular Pile is kept from 0.3 m to 1.05 m and the spacing between them is varied from 2D to 3.5D. The triangular pattern of Granular Pile arrangement is considered. For different spacing of the Granular Pile and undrained shear strength, the design charts explained and illustrated through an example.

Degradation of lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading

In this article, the degradation of the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading is studied numerically. A modified kinematic hardening constitutive model is employed to simulate the degradation of soft clay after cyclic loading. The modified model is verified by comparing the numerical simulation results with the results of centrifuge model tests. Furthermore, the modified model is applied to numerical simulations for evaluating the lateral bearing capacity of piles in soft clay subjected to cyclic lateral loading. The degradation of the lateral bearing capacity of piles in soft clay after different cyclic displacement levels and different numbers of cycles is investigated. The study reveals that the modified kinematic hardening constitutive model can effectively estimate the cyclic degradation behavior of piles in soft clay subjected to cyclic lateral loading. The degradation of the ultimate lateral bearing capacity progresses slowly with increasing cyclic displacement level for fewer cycles, and the degradation develops significantly at higher levels of cyclic displacement after applying a larger number of cycles.

Prediction of Response of Existing Building Piles to Adjacent Deep Excavation in Soft Clay

This study investigates building settlements near excavations in soft clay. A simplified theoretical method is proposed to predict the additional settlements and axial forces of excavation‐adjacent existing building floating piles in soft clay. The soil displacement is simplified as a line or broken line along the depth direction, depending on the distance from the excavation. A hyperbolic model is applied to calculate the skin friction and tip resistance induced by the vertical soil displacement. The parameters of the hyperbolic model are corrected to fit data from in‐service piles. Based on the load‐transfer curve method, the additional settlements and axial forces are determined. The measured data of 17 floating piles from two excavation cases in Hangzhou, China, show good agreement with the calculated values. The results show that the position of the neutral point of the loaded pile varies with the soil settlement. Because of the upper structure, the theoretical settlements for piles near the excavation are larger than those obtained from the measured values; for distant piles, this relationship is reversed. The proposed prediction methodology is expected to guide the design of practical excavations.

Seismic behaviour of soft clay and its influence on the response of friction pile foundations

In recent years, much of the research in geotechnical earthquake engineering has focused on liquefaction of loose, saturated sands and silts. However, the dynamic behaviour of soft, clayey soils and their interaction with pile foundations during the earthquakes have received relatively little attention. In this study, an attempt is made to investigate the dynamic behaviour of soft clay and its interaction with pile foundations during earthquakes using high gravity centrifuge testing. A model single pile and two sets of 3 × 1 row model pile groups with different pile spacing were embedded in soft kaolin clay and tested under the action of model earthquakes at 50 times the earth’s gravity. The strength and stiffness of clay were evaluated using a T-bar test and an air hammer device respectively. The focus of this research is to investigate the dynamic response of friction piles in soft clay. However, this depends on the dynamic response of the soft clay layer around the pile. To this end, one-dimensional ground response analysis was performed using DEEPSOIL software to emphasise the importance of non-linear analysis in characterising the seismic behaviour of soft clays. It will be shown that clay response depends both on the earthquake intensity and the shear strength and stiffness of the clay layer. This has a direct bearing on the response of single piles and pile groups, with larger amplification occurring for small intensity earthquakes and attenuation occurring for stronger earthquakes.

Prediction of Quantitative Response of Under-Reamed Anchor Piles in Soft Clay Using Laboratory Model Study

Abstract The foundation of many civil engineering structures is required to be designed as an anchor foundation subjected to upward load action. The shallow as well as deep anchor in soft clay is an important but complex soil-foundation system that must be analyzed theoretically. As direct determination of the quantitative response by the field test is a very difficult and expensive exercise, the physical modelling technique is considered as an easy alternative for prediction of (i) ultimate pulling capacity and (ii) pull-rise curve. This paper highlighted the potential use of two parameters viz. (i) factor of geometrical relationship (K) and (ii) cohesion factor (N), obtained by conducting laboratory tests on two models of different scale factors for the intended purpose. The K and N parameters were constant for both the prototype foundation and small size model tested in laboratory. The under-reamed concrete pile in soft clay was considered as a soil-foundation system and shallow as well as deep under-reamed anchor piles were investigated in this study. Besides, straight shafted concrete piles were also considered in both categories of shallow and deep anchors. It was found that the physical modelling technique using two characteristic system parameters and by incorporating the model scale factor effect enables the prediction of the earlier stated quantitative response of prototype under-reamed anchor pile foundation in soft clay with a reasonable degree of accuracy. A methodology involving curve fitting procedure was also developed to predict the “pull-rise” curve for a prototype soil-foundation system under consideration.

Effects of pile shape and pile end condition on the lateral response of displacement piles in soft clay

A series of lateral tests conducted in a centrifuge on displacement piles in kaolin is used to examine the effects of pile shape, pile end-condition and clay overconsolidation on the lateral load transfer (p–y) curves in soft clay. These experimental tests are supported by finite-element analyses, which examine the responses of the circular, square and H pile sections used in the centrifuge tests. The experimental and numerical results reveal an important effect of pile shape on p–y curves, but no discernible effect of the pile end condition. These results also indicate comparable dependencies of net ultimate pressures on normalised depth and undrained shear strength. The findings enable development of a new formulation for p–y curves in soft clay, which incorporates the effects of pile shape and uses the triaxial compression consolidated undrained shear strength as the reference undrained strength.

Effect of Slope on p-y Curves for Laterally Loaded Piles in Soft Clay

Pile foundations are often subject to lateral loading due to various forces on a variety of structures like high rise buildings, transmission towers, power stations, offshore structures and highway and railway structures. The present investigation is to study the effect of slopes on p-y curves (where p is the static soil reaction and y is the pile deflection) due to static lateral loading in soft clay (Consistency index Ic = 0.42). A series of laboratory model tests were carried out on the instrumented model pile on sloping ground (slopes of 1V:1H, 1V:1.5H, 1V:2H, 1V:3H and 1V:5H) and with varying embedment length to diameter ratio (L/D) of 20, 25 and 30. From the experimental results, the bending moment curves along the pile shaft are double differentiated to obtain the soil resistance (p) and double integrated to obtain the deflection (y) using curve fitting method. New p-y curves for piles located on crest of soft clay with different sloping ground surface under static lateral loading are developed. Moreover, the effect of sloping angles on proposed p-y curves was studied.

Cyclic lateral response and failure mechanisms of semi-rigid pile in soft clay: centrifuge tests and numerical modelling

Previous studies on laterally loaded piles in clay have mainly focused on flexible and rigid piles. Little attention has been paid to semi-rigid piles (whose pile–soil stiffness lies somewhere between those of rigid and flexible piles), which may behave as either flexible piles or rigid piles, depending on the change in soil stiffness during cycling. This study aims to understand the cyclic lateral response of a repeatedly loaded semi-rigid pile in soft clay and the failure mechanisms of the soil around the pile, through a series of centrifuge model tests and three-dimensional finite element analyses using an advanced hypoplastic clay model. Numerical parametric studies were also performed to investigate the evolution of soil flow mechanisms with increasing pile rigidity. It is revealed that the semi-rigid pile behaved as if it were a flexible pile (i.e., flexural deformation dominated) during the first few cycles, but tended to behave like a rigid pile (i.e., rotational movement prevailed) during subsequent cycles, which progressively softened the surrounding soil. As a result, the mechanisms of soil flow around the semi-rigid pile exhibited an intermediate behaviour combining the mechanisms of both flexible and rigid piles. Three distinctive mechanisms were identified: a wedge-type mechanism near the surface, a full-flow mechanism (within the transverse sections) near the middle of the pile, and a rotational soil flow mechanism (in the vertical symmetrical plane of the pile) near the lower half of the pile. By ignoring the rotational soil flow mechanism, which has a much lower resistance than the full-flow mechanism, the American Petroleum Institute code (published in 2007) underestimated the cyclic bending moment and the lateral pile displacement by 10% and 69%, respectively. Application of jet grouting around the semi-rigid pile at shallow depth significantly altered the soil flow mechanism (i.e., it was a solely wedge-type mechanism around the grouted zone).

Performance of geosynthetic-reinforced granular piles in soft clays: Model tests and numerical analysis

The laboratory model tests and numerical analyses have been performed on reinforced granular piles installed in very soft clay. The granular piles were reinforced with geosynthetic in the form of vertical encasement, horizontal strips and combined vertical-horizontal reinforcement. The short term-displacement control model tests were carried out either only a granular pile loaded or with an entire area loaded. The laboratory results in the form of vertical load intensity-settlement behaviour were compared with that obtained from FEM software, PLAXIS 3D. The results indicated significant improvement in ultimate load intensity and stiffness of treated ground due to inclusion of geosynthetic.

Field testing of one-way and two-way cyclic lateral responses of single and jet-grouting reinforced piles in soft clay

Piles supporting transmission towers, offshore structures (such as wind turbines), or infrastructures in seismic areas are frequently subjected to either one-way or two-way cyclic lateral loadings. Relatively little attention, however, has been paid to compare and understand the effects of different loading regimes (one-way or two-way cycling) on lateral responses of piles in soft clay. For this reason, a series of field tests in soft clay are carried out to compare one-way and two-way cyclic responses of single piles and of jet-grouting reinforced piles. The field tests reveal that the single pile subjected to two-way cycling experiences much more rapid degradation in lateral stiffness and capacity, but accumulates much smaller residual pile deflection (δ p), than the single pile under one-way cycling. This is because the reverse part of the two-way cycling also generates plastic strain, causing additional softening and strength reduction in the soil surrounding the pile. After each cycling, non-zero bending moment (i.e. locked in moment, or M L) is retained in the single piles, and the M L increases with the δ p. The one-way cycling leads to two times larger M L than the two-way cycling, as it causes greater δ p. The maximum M L in the pile after one-way cycling can be up to 40% of the maximum bending moment induced during the previous cyclic loading stage. After application of jet-grouting surrounding the upper part of the single pile, it greatly reduces degradation of lateral pile stiffness, accumulation of δ p and therefore development of M L. Compared to the field measurements, the API (API RP 2A-WSD, recommended practice for planning, designing, and constructing fixed offshore platform-working stress design, 21st edn. API, Washington, 2000) code underestimates the lateral stiffness of the pile under one-way cycling, while overestimates that of the pile under two-way cycling, leading to a non-conservative prediction of bending moment in the latter pile.

Effects of scour-hole dimensions and soil stress history on the behavior of laterally loaded piles in soft clay under scour conditions

Bridge pile foundations in the midst of water current are often subjected to scour, which induces the loss of soil support around the piles and thus results in a significant decrease of foundation capacities or even the failure of bridges. In the current practice, when the scour-affected behavior of the pile foundations is analyzed, either the whole scour-hole geometry or the possible changes in the stress history of the remaining soils is often ignored. In reality, however, scouring creates scour holes with certain dimensions around the pile foundations and the remaining soils that are not scoured away undergo an unloading process at the same time, which will increase the over-consolidation ratios of the remaining soils and accordingly changes of their mechanical properties. This paper examines the behavior of laterally loaded piles in soft clay under scour conditions by using the p-y method. The conventional p-y curves have been modified appropriately to reasonably reflect the effects of three-dimensional scour-hole geometry as well as the stress history of the soils, with the aid of integration of Mindlin’s fundamental solutions. A field test is used to serve as a reference case and to investigate the effects of stress history, scour depth, scour width, and scour-hole slope angle on the responses of laterally loaded piles in soft clay. The results indicate that neglecting the stress history effect can be unconservative for scour-affected pile foundations in soft clay, whereas neglect of the scour-hole dimensions and geometry would lead to over-conservative predictions/design of the laterally loaded piles under scour condition.

Cyclic and seismic response of single piles in improved and unimproved soft clays

Presented in this paper are results of two centrifuge tests on single piles installed in unimproved and improved soft clay (a total of 14 piles), with the relative pile–soil stiffness values varying nearly two orders of magnitude, and subjected to cyclic lateral loading and seismic loading. This research was motivated by the need for better understanding of lateral load behavior of piles in soft clays that are improved using cement deep soil mixing (CDSM). Cyclic test results showed that improving the ground around a pile foundation using CDSM is an effective way to improve the lateral load behavior of that foundation. Depending on the extent of ground improvement, elastic lateral stiffness and ultimate resistance of a pile foundation in improved soil increased by 2–8 times and 4–5 times, respectively, from those of a pile in the unimproved soil. While maximum bending moments and shear forces within piles in unimproved soil occurred at larger depths, those in improved soil occurred at much shallower depths and within the improved zone. The seismic tests revealed that, in general, ground improvement around a pile is an effective method to reduce accelerations and dynamic lateral displacements during earthquakes, provided that the ground is improved at least to a size of 13D × 13D × 9D (length × width × depth), where D is the outside diameter of the pile, for the pile–soil systems tested in this study. The smallest ground improvement used in these tests (9D × 9D × 6D), however, proved ineffective in improving the seismic behavior of the piles. The ground improvement around a pile reduces the fundamental period of the pile–soil system, and therefore, the improved system may produce larger pile top accelerations and/or displacements than the unimproved system depending on the frequency content of the earthquake motion.