Liquefaction Induced Downdrag for the Juan Pablo II Bridge at the 2010 Maule Earthquake in Chile
Bengt H. Fellenius, Babak Abbasi, and Balasingam Muhunthan
ABSTRACT: Sandy soil layers reduce volume during and following liquefaction, which results in settlement of the overlying soil layers. In case of pile constructed in the liquefying soil, the liquefaction-induced settlement induces downward directed shear stress (negative skin friction) along the pile causing the pile to settle—be dragged down. Depending on the site conditions, the change in the axial response resulting from liquefaction-induced settlement, and downdrag can have a significant impact on piled foundation performance in seismic regions. This study presents an analytical method to quantify the effects of liquefaction-induced downdrag on drilled shafts. The method relies on combining two diagrams. One diagram shows the distributions of force along the pile in negative and positive direction displaying a force equilibrium. The other diagram shows the distribution of soil and pile movement, displaying a settlement equilibrium. The analysis method consists of combining the force movement of the pile and the soil so that the two equilibriums occur at the same depth, called the “neutral plane”. The method, called the “unified analysis method”, is applied to an observed case of downdrag during the February 7, 2010, Maule Magnitude 8.8 earthquake in Chile showing that the calculated settlements are close to those observed at the site. The results of the unified analysis indicate that the major effect on the pile settlement was from liquefaction-induced settlement below the pile toe level, as opposed to downdrag. The case study shows the importance of combining forces and movements in the analysis of piled foundation settlement.
KEYWORDS: Piled foundations, Liquefaction, Downdrag, Negative skin friction, Settlement analysis
Deep Compaction of Sand Causing Horizontal Stress Change
K.R. Massarsch and B.H. Fellenius
ABSTRACT: Deep soil compaction is usually required for the control of total and differential settlement, and mitigation of liquefaction. An important, often neglected aspect, is the increase in horizontal stress which occurs due to deep compaction. The increase in horizontal stress means that also the preconsolidation stress and thus the overconsolidation ratio have increased. Re-analysis of calibration chamber tests employing CPT and DMT soundings show that both the CPT and DMT can measure changes in horizontal stress and, thus, be used to show a simple relationship between the increase in horizontal stress index from DMT and the overconsolidation ratio. The application of the tangent modulus method is illustrated using information by CPT and DMT. A hypothesis is proposed that explains aging effects in compacted soil by the redistribution of horizontal stresses after treatment. The significance of the overconsolidation ratio for the liquefaction resistance of loose, water-saturated soils is illustrated. The increase in effective stress due to compaction is of significance for analysis of compacted fill, and in particular for the assessment of settlement. Stress changes due to compaction are also important for other types of advanced geotechnical analyses.
KEYWORDS: Sand, Compaction, Settlement, Liquefaction, CPT, DMT, OCR
Some Factors That Influence the Prediction of the Behaviour of Piled Rafts via Simplified (Numerical) Analyses
Renato P. Cunha, Harry G. Poulos and John C. Small
ABSTRACT: This article evaluates two distinct external effects on the settlement results from standard analyses of piled raft foundation systems. The influence of the excavation level and the influence of the number of piles underneath the pile will be separately assessed by two independent analyses for two published case histories, respectively a house located in Gothenburg and another in Uppsala, Sweden. They have been initially presented by Hansbo (1993) and Hansbo and Källström (1983). Both structures were founded over a soft, highly plastic marine clay of varying thickness, where the foundation was designed by using the concept of “creep piling”, i.e., piles in a state of full load mobilization. The analyses were carried out with the numerical tools DEFPIG and GARP, by considering a series of simplified assumptions for the load pattern, raft and pile characteristics and subsoil profile. The soil, pile and load characteristics have been considered, with analyses that allowed (and not) the effect of the excavation level (1st case history), and with variation (optimization) of the number of piles (2nd. case history). The exercise emphasizes the importance of the consideration of the excavation level for the proper assessment of the settlement pattern underneath the raft, and the beneficial aspect on the optimization of the number of piles in the piled raft design.
KEYWORDS: Piled Raft, Soil Excavation, Numerical Analysis, Foundation Design, Soft Clay, Optimization
Evaluation of Analytical and Numerical Techniques to Simulate Curtain Pile Walls in a Tropical Soil of the Federal District of Brazil
S. Jacazz, Y. M. P. de Matos, F. F. Monteiro, R. P. Cunha, J. C. Ruge & G. Gassler
ABSTRACT: This paper is fruit of an active interaction between several universities/academics and the University of Brasília. It has focus on the design of retaining walls. This design is increasingly present in engineering projects for urban areas, given their continuous development. In the Federal District of Brazil, many retaining walls are built in order to optimize space. These excavations need retaining works in order to maintain the terrain in place and to avoid any collapse. The aim of this paper is to tentatively assess the behavior of retaining structures made of “pile curtain” by using existing commercial finite element (F.E.) and “winkler spring” type softwares. The case study is characterized by a 13m deep excavation founded in the well-known porous clay of Brasilia. The excavation´s instrumentation provided displacements along depth, used to evaluate the structure´s behavior. In terms of the approach by using a winkler type software, the best technique to evaluate the subgrade reaction coefficient (modulus) was an empirical equation presented by Bowles (1988). For the numerical predictions with finite element method, the Hardening Soil model tended to show a slight better result when compared to the traditional Mohr Coulomb model. It is concluded that simple techniques, and experience of the engineer, are sometimes fundamental to better simulate such types of structure founded on complex unsaturated tropical soils.
KEYWORDS: Retaining Structure, Numerical Simulation, Tropical Soil, Winkler Model, Subgrade Reaction Coefficient
Biodegradable Prefabricated Vertical Drains: from Laboratory to Field Studies
Thanh Trung Nguyen, Buddhima Indraratna, and Pankaj Baral
ABSTRACT: Biodegradable prefabricated vertical drains (BPVDs) made from natural fibres have been in use for several decades to improve soft soil, especially in East and Southeast Asia despite the fact that this type of drain has still not been fully addressed and evaluated. This study presents a series of laboratory tests where a drain made from coconut cores wrapped in Indian jute sheath filters is compared to conventional synthetic prefabricated vertical drains (SPVDs). Discharge volume tests are carried out with and without soil clogging to understand how jute drains can resist soil clogging under increasing confining pressure. Along with these macro-hydraulic tests, the influence that the microcharacteristics of natural fibre drains can have on their hydraulic conductivity is also examined using micro-CT scanning and an optical microscopic to capture the micro-details of these drains. This study shows that the porous structure of BPVDs is much more complex than SPVDs, which causes them to have a lower discharge capacity. Unlike SPVDs, micro-properties also play an important role in the hydraulic properties of BPVDs. A pilot project in soft soil at Ballina, Australia, where BPVDs were installed in parallel to SPVDs, was used to evaluate their performance in assisting soil consolidation considering the biodegradation of natural fibres. The identical performance of these two types of PVDs added further evidence to prove how well BPVDs can facilitate soil consolidation.
KEYWORDS: Biodegradable fibre drains, Soil improvement, Discharge capacity, Micro-features, Jute fibres
P-Cone: A Novel Cone Penetration Test Device for Deep Foundation Design
H.M. Nguyen and A.J. Puppala
ABSTRACT: A novel cone penetration test device, the P-cone, has been developed to assist in deep foundation design and this P-cone device combines features of CPTU cone sounding technologies with capabilities to perform bidirectional loading at a given soil depth condition. Using two independent systems, the P cone measures shear stress versus movements of cone shaft and stress versus penetration of the cone tip at desired depths. P-cone tests were performed on large compacted clayey silt specimens in a laboratory fabricated chamber. Tests performed showed that the movements to fully mobilize the shaft shear resistance and tip resistance were close to 0.2 mm and 2.0 mm, respectively. The soil failure shapes around the cone tip investigated was found without the horizontal stress build-up around the cone tip and shaft.
KEYWORDS: P-cone, Cone penetration, Head-down test, Bidirectional test, End bearing test
Analysis of Stiffened Granular Piled Raft
Madhav Madhira, Jitendra Kumar Sharma and Raksha Rani Sanadhya
ABSTRACT: Piled rafts are usually provided to reduce total and differential settlements of foundation and structures built on deep deposits of soft or problematic soils. Granular piles may be used in place of conventional concrete or steel piles because of their several additional advantages. The load carrying capacity of these granular piles (GP) can be increased for improved performance by stiffening of granular piles their top portion of length by relatively strong material having better strength and stiffness, i.e. higher deformation modulus in comparison to the material of granular pile in the lower portion. Geogrid encased columns, SDCM (stiffened deep cement mixing), fibre reinforced granular columns, etc. are common forms of stiffening GP. The present study deals with the analysis of partially stiffened granular pile with rigid raft based on the continuum approach. The overall response of the top stiffened GP-raft foundation is evaluated in terms of settlement influence factor, settlement reduction factor in comparison to un-stiffened granular piled raft, normalized GP-soil interface shear stresses, percentage load shared by GP, normalized contact pressure distribution beneath the raft and percentage load transfer to the base of GP with relative stiffness factor and relative length of stiffening.
KEYWORDS: Granular piled raft, Stiffness factor, Relative stiffness of granular pile, Relative length of stiffening, Settlement influence factor
Can a Pile Load Tested to ‘Failure’ be Used as a Working Pile?
Madhav Madhira, Jitendra Kumar Sharma and Raksha Rani Sanadhya
ABSTRACT: There are a few available loading test methods to obtain a load-settlement curve of a pile. Likewise, there are many definitions to determine the ‘ultimate’ pile capacity from a load-settlement curve. Although pile load tests have been widely used over the past decades, there are still many questions regarding its practice and interpretation. Frequently asked questions include: when does a pile test considered to have failed? From an economic point of view, a failure in pile loading test can cost quite a lot of money. To what load can the pile be loaded till it is considered to have failed? Can a pile loaded to failure still be used as a working pile? What is a bidirectional pile load test (BD-test)? When should a BD-test be used? Can a pile tested with a BD be used as a working pile? What are the differences between kentledge or reaction piles static loading test with the bidirectional test? Do the different pile tests produce the same results? This paper aims to shed light on these questions, one case history where the pile tested to ‘failure’ and later used as working piles is presented.
KEYWORDS: Pile static load test, Bidirectional test, Ultimate pile capacity, Fail Pile
Displacement of Piles from Pressuremeter Test Results – A Summary of French Research and Practice
R. Frank
ABSTRACT: This paper presents the ‘load-transfer functions’ t-z and p-y methods for determining the axial and lateral displacements of single piles. They are based on the results of pressuremeter tests. The methods from the results of the Ménard pre-borehole pressuremeter (MPM) and the self-boring pressuremeter tests (PAF, for ‘Pressiomètre AutoForeur’ in French) are described. Especially, the t-z and p-y methods derived from the MPM test results are commonly used in French practice. For both t-z and p-y curves, some theoretical background (usually FEM calculations in linear elasticity) is given. The results of the t-z and p-y analyses are compared to the measurements from loading tests on full scale piles. A proposal concerning “barrettes” is also presented.
KEYWORDS: Pile, Displacement, Settlement, Lateral behaviour, Ménard pre-borehole pressuremeter, Self-boring pressuremeter, Barrette
Effect of Seismic Action on Settlement and Load Sharing of Piled Rafts Based on Field Monitoring
K. Yamashita, J. Hamada and T. Tanikawa
ABSTRACT: This paper offers three case histories of piled rafts combined with deep mixing wall grids, in which long-term monitoring on settlement and load sharing between the piles and the raft was performed. The buildings are located on soft ground consisting of liquefiable sand and soft clayey soil. The buildings experienced the 2011 Tohoku Earthquake (M=9.0, about 380 km distance from the epicentre) and the seismic response during the earthquake was successfully recorded for the two buildings. Based on the static and dynamic monitoring results, the effect of seismic action on the settlement and load sharing between the piles and the raft were investigated. It was found that no significant change in foundation settlement was observed after the earthquake. However, some change in load sharing between the piles and the raft caused by the seismic action was noted for the relatively short shaft-bearing piles, while almost no change in the load sharing was observed for the toe-bearing piles or long shaft-bearing piles.
KEYWORDS: Piled raft foundation, Deep mixing wall grid, Soft ground, Settlement, Load sharing, Field monitoring, The 2011 Tohoku Earthquake
Model Vibration Tests on Piled Raft and Pile Group Foundations in Dry Sand
Anh-Tuan Vu, Tatsunori Matsumoto and Kohei Kenda
ABSTRACT: In this research, the authors carried out vibration load tests on piled raft and pile group models to investigate dynamic behaviours of the foundations. Foundation models consisting of 6 piles, with or without batter piles, were used in the experiments. They were pile rafts (6PR and 6BPR) if the raft was in contact with ground surface, while they were pile groups (6PG and 6BPG) if the raft was not in contact with ground surface. To create dynamic load acting on the foundation, a vibro-hammer, placed on the raft, was used. The vibro-hammer can provide vibration load (active shaking) mainly in the vertical direction (called vertical loading) or in the vertical and horizontal directions simultaneously (called combination loading) by rotating two discs of eccentric mass synchronously in opposite directions or the same direction, respectively. Active shaking tests were conducted on 4 types of pile foundation models (6PR, 6BPR, 6PG and 6BPG) in a consistent dry sand ground. The experimental results indicate that the piled rafts are more effective foundation type to decrease settlement and inclination under dynamic loading than the pile groups.
KEYWORDS: Piled raft, Pile group, Dry sand, Model test, Dynamic load
BEM and FEM Approaches to the Analysis of Negative Skin Friction on Piles
G. Russo, L. Di Girolamo and G. Marone
ABSTRACT: Negative skin friction (NSF) may be a relevant problem in the design of piles in soft soils, when significant areas around the piles are loaded at the ground surface. The downdrag on piles is traditionally evaluated by reversing a part of the shaft resistance in an applied load and considering it in the evaluation of a safety factor against a bearing capacity failure. Such an approach is totally inadequate, because NSF is actually a problem of soil-pile interaction. Two methods of analysis of a pile subjected to both an external load and NSF are presented: Boundary Element Method (BEM) and Finite Element Method (FEM). The former method is based on a BEM approach and concentrates non-linearity effects at the pile-soil interface while the latter is a FEM approach using the package PLAXIS 2D. The accuracy of the methods is checked back analysing a well-documented case history of bored piles in soft soils.
KEYWORDS: Negative Skin Friction (NSF), Dragload, Downdrag, Boundary Element Method (BEM), Finite Element Method (FEM)
Effective Stress Friction Angle of Normally Consolidated and Overconsolidated Intact Clays from Piezocone Tests
Z. Ouyang and P.W. Mayne
ABSTRACT: The effective stress friction angle (Φ′) is an important fundamental property for all soil types. A modified effective stress limit plasticity solution is presented in this paper for the evaluation of Φ′ of normally-consolidated to overconsolidated clays from piezocone penetration tests (CPTu). The solution takes account of stress history effect by introducing the equivalent stress concept from critical state soil mechanics (CSSM). Values of Φ′ obtained from laboratory triaxial compression tests on high quality samples are taken as the benchmark reference. The method is applicable to clays that are intact, insensitive, and inorganic. Example results of piezocones performed in normally consolidated kaolin in the laboratory and field tests on overconsolidated clay from Alaska are presented to elaborate application of the solution. A compiled database from 132 piezocone soundings in intact clays at field sites, 1-g chamber tests, and centrifuge series is compared with triaxial tests to show the full range of 18° < Φ′< 45° of natural and artificial clays.
KEYWORDS: Clay, Friction angle, Piezocone, Effective stress friction angle, Overconsolidation ratio
Numerical and Simplified Methods for Soil-pile Interaction Analysis
F. Dezi, S.Carbonari, M. Morici, G. Leoni
ABSTRACT: The paper presents a review of the analytical and numerical procedures developed by the authors for the dynamic analysis of soil-pile foundation systems subjected to the propagation of seismic waves in the soil. Inclined and vertical single piles and groups constituted by piles with a generic inclination are addressed. For the former, an analytical approach based on the beam on dynamic Winkler foundation approach is adopted; the pile is modelled as a Euler-Bernoulli beam and the soil-pile interaction is captured by defining soil impedances relevant to the harmonic vibrations of rigid disks available in the literature. The coupled flexural and axial behaviour of the pile is solved analytically exploiting exponential matrices. The pile group dynamic problem is similarly formulated but the solution is achieved exploiting the finite element approach. Besides numerical models, simplified approaches based on static equivalent methods and simplified formulas are also addressed to estimate the maximum kinematic stress resultants on vertical piles subjected to lateral seismic excitations. The reliability of the presented tools in capturing the dynamic stiffness and the overall kinematic response of pile foundations is shown by comparing results with those available in the literature or achieved through refined finite element models. From an engineering point of view, the proposed approaches assure a sufficient accuracy and may substitute refined computational demanding numerical models.
KEYWORDS: Soil-pile interaction, Pile foundations, Kinematic interaction, Numerical models
The Behaviour of Pile Group and Combined Piled-Raft Foundation in Liquefiable Soil under Seismic Conditions
Aniruddha Bhaduri, Vansittee Dilli Rao and Deepankar Choudhury
ABSTRACT: This paper highlights the beneficial usage of Combined Pile-Raft Foundation (CPRF) over conventional pile group foundation subjected to seismic loading in liquefiable soil. Firstly, a single pile resting on a liquefiable soil is numerically modelled and subsequently validated with available dynamic centrifuge test result by using finite difference based computer programme, FLAC3D. Thereafter, the model is extended for simulating CPRF and pile group.Further parametric studies are performed to understand the effect of pile spacing (s), pile length (l) and different seismic motions on the behaviour of CPRF and pile group. Results are presented in terms of normalised bending moment (M/Mmax), shear forces and pore water pressure (PWP) ratio. Increase in shear resistances in the range of (35 – 60)% and (40 – 70)% are observed for the piles in CPRF over the conventional pile group foundation, having a pile spacing of 2 to 5 times of its diameter (d) and the (l/d) of 14 to 20, respectively. These outcomes portray the advantages of employing CPRF over pile group founded in liquefiable area under seismic loading.
KEYWORDS: Liquefaction, CPRF, Pile Group, Centrifuge, FLAC3D
Foundation Investigation and Analysis for Tall Tower Developments
C. M. Haberfield, J.E. Finlayson, and A. L .E. Lochaden
ABSTRACT: Many tall buildings are supported on piled rafts and / or deep bored cast in situ piles. Good engineering design requires soil-structure interaction analysis and a clear understanding of the factors controlling the performance of the footing system. These rely on a sound understanding of the ground characteristics and individual and group pile performance, including adequate collection of data and testing, which can only be achieved through detailed and targeted ground investigation and in situ testing. This paper focuses on the ground investigation methods available and how the results are used to achieve a reliable estimate of footing system performance using soil-structure interaction analysis. It highlights the importance of accurate inputs into the analyses, especially in respect to the stiffness characteristics of the ground and the load displacement performance of individual piles. This is illustrated through a number of case studies of tall tower projects that the authors have been involved in.
KEYWORDS: Geotechnical investigation, Soil-structure interaction, Tall towers, Analysis, Design, Case study
Effects of Cyclic Behaviour during Pile Penetration on Pile Performance in Model Load Tests
S. Moriyasu, T. Matsumoto, M. Aizawa, S. Kobayashi, and S. Shimono
ABSTRACT: This study focuses on the effect of ‘cyclic’ behaviour of pile installation methods on the penetration resistance and bearing capacity of a model pile. A series of laboratory model tests were conducted to investigate this cyclic effect by comparing three kinds of piling methods: monotonic jack-in, pseudo-dynamic push-in and pull-out, i.e. ‘surging’, and vibratory driving in dry or saturated sand. Surging or vibratory pile driving decrease the pile penetration resistance due to negative soil dilation caused by the cyclic shearing of the soil surrounding the pile. Static load tests show that surging and vibratory pile driving provide the same or larger pile head load as the jack-in method does. Furthermore, the fluctuation of the pore water pressure strongly indicates a change in soil dilation. Both surging and vibratory pile driving prevent positive dilation more than the jack-in method due to differences in cyclic shearing and monotonic loading.
KEYWORDS: Pile installation method, Jack-in, Surging, Vibratory pile driving, Pore water pressure
FD Analysis on Piled Raft Foundation Settlements under Vertical Loads
D.W. Chang, H.W. Lien and M.H. Hung
ABSTRACT: A three-dimensional finite difference analysis has been developed to estimate the foundation settlements for vertically loaded piled raft foundations. Thin-plate theory was adopted to model the finite raft with boundary effects. Alternate spring models were used to model soil resistances under the raft while the resistances of pile were model by calculating the pile stiffness from wave equation analysis. The newly proposed analysis was examined with finite element solutions. It was found that variations of soil resistance underneath the raft and the pile-soil-pile interactions are the keys to the applicability of such analysis.
KEYWORDS: Geocell reinforcement, Composite model, Flexible pavements, Parametric study
Analysis for Laterally Loaded Offshore Piles in Marine Clay
S. Jeong, and Y. Kim
ABSTRACT: The load distribution and deformation of offshore drilled shafts under lateral loading in Incheon grand bridge are investigated by experimental field loading tests and lateral load-transfer approach through p-y curve analysis. The main focus is on improved wedge failure model developed by considering three-dimensional combination forces and new hyperbolic p-y criterion. Through comparisons with field case studies, it is found that the rigidity of the drilled shaft is a critical factor in drilled shafts in marine clay, and the methodology proposed in this study yields more accurate and realistic results considering pile-soil interaction for laterally loaded drilled shafts in marine clay.
KEYWORDS: Drilled shaft, Lateral loading, Marine clay, p-y curve, Wedge failure model, Pile rigidity
Pile Design in Seismic Areas: Small or Large Diameter?
R. Di Laora
ABSTRACT: This work investigates the role of pile diameter in resisting seismic actions, with reference to two example subsoils, namely a dry sand and a fully saturated NC clay. After a ground response analysis in free-field conditions for different values of peak rock acceleration, mobilized soil stiffness and surface acceleration are used as ingredients for assessing the kinematic and inertial moment in a concrete pile. An optimum pile diameter is identified as the one that, while guaranteeing safety, corresponds to the minimum cost. It is also proven that, with a constant value of reinforcement area and length, increasing pile diameter (i.e. increasing safety factor and cost) leads rapidly to failure. Likewise, if pile reinforcement is designed only for inertial action, increasing pile diameter is severely detrimental.
KEYWORDS: Pile design, Seismic action, Kinematic interaction, Earthquake-induced bending, Pile diameter
