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

S. Datta and D. Roy

ABSTRACT: Microbe-induced reduction of soil erodibility, since natural, is expected not to disrupt the natural environmental system. Although the role of bacteriogenic EPS in aggregating soil particles is widely recognized but the impact of various environmental parameters e.g., groundwater velocity and nutrient availability on bacteriogenic EPS in reducing the erodibility of soil is not very clear. In this study, a species of EPS producing soil bacteria Bacillus megaterium RB-05 isolated from a naturally cemented intertidal silt site was used to investigate the influence of flow velocity and nutrient availability on microbially mediated reduction of erodibility of sand. Durations of nutrient availability and media circulation velocity were observed to influence the bacterial population, amounts and composition of EPS found within sand specimens. Drained shear strength of loose sand samplers was found to increase due to EPS-related interparticle aggregation. EPS produced under fluvial activities seems to be more capable in aggregating sand grains as well as reducing erodibility of sand. Results of model sand erosion test further strengthen this conclusion.

KEYWORDS: Extracellular polymeric substance (EPS), Bacillus megaterium RB-05, Erodibility, Flow velocity, Nutrient availability.

Effective Treatment Technique for Producing a Highly Pozzolanic Clay in a Construction Work

A. Poowancum, P. Kotkhangphlu, and J. Ayawanna

ABSTRACT: A high reactivity pozzolanic clay was developed in this work as a partially replacing cement material for eco-friendly construction. Kaolinite clay was treated at a transformation temperature range of meta-kaolin formation for 6 h in a simple electric furnace. The heated kaolinite clay was separately cool down with 2-different techniques, a normal cool down using a free-cooling rate and a quenching cool down, to make an active phase in a pozzolanic activity. The pozzolanic activity of the treated kaolinite clay was evaluated via the Frattini test, and the compressive strength of the treated clay-cement mixed samples was reported. In comparison to the normal cool down technique, the quenching cool down technique in the heat treatment process generated a higher reactivity clay with a higher degree of reactive metakaolin phase. The Frattini test revealed a decrease in Ca2+ and OH- ions in the Ca(OH)2 solution, which confirmed the pozzolanic reaction between the Ca(OH)2 and the high reactive kaolinite clay from the quenching technique. The replacement of cement with a highly reactive pozzolanic kaolinite clay effectively enhanced the compressive strength above the standard requirements within 2-days curing. Also, the compressive strength of the treated clay-cement mixed samples after 2-days curing was comparable to the strength after 7- days curing of the cement samples. This study thus revealed a new effective technique to produce a high potential clay as a cement replacement material in the construction works.

KEYWORDS: Pozzolanic materials, Kaolinite clay; Cooling treatment technique, Compressive strength, Construction.

Estimating Hydraulic Conductivity Assisted with Numerical Analysis for Unsaturated Soil – A Case Study

Md Rajibul Karim, David Hughes, and Md Mizanur Rahman

ABSTRACT: Meteorologically induced pore water pressure changes and associated changes in effective stress often affect the behaviour of geotechnical structures such as slopes. Seasonal fluctuations in pore water pressure can lead to stiffness degradation which is also known to have caused a number of failures across the world. These effects are likely to become more severe in the future as dryer summers and wetter winters are expected to become more frequent climate scenario in many parts of the world. To analyse the behaviour of a slope subjected to atmospheric boundary interactions, a number of parameter may be used including the soil water characteristics curve, saturated and/or unsaturated hydraulic conductivity of soil, and strength parameters. Some of them (e.g., hydraulic conductivity) are very difficult to deduce with high degree of certainty because of natural variability of soils and limitations in testing procedure. This paper outlines how numerical techniques combined with conventional field or laboratory investigation can serve as a useful technique to overcome some of these limitations specially in deducing hydraulic conductivity. The effectiveness of these techniques will be tested using a well-documented case study form the United Kingdom.

KEYWORDS: Pore water pressure, Atmospheric boundary, Climate change, Effective stress, Shear strength, Slopes, Soil water characteristics curve.

Development of a Stress-Strain Path Controlled Triaxial Apparatus to Understand the Behaviour of Silty Sand

A. T. M. Z. Rabbi, M. M. Rahman, K. Mills, and D. A. Cameron

ABSTRACT: Triaxial tests are widely used to determine the shear strength, material properties and instability behaviour of soil. The conventional isotropically consolidated drained and undrained triaxial compression tests under constant confining stresses, fail to simulate many field stress conditions such as K0-consolidation for zero radial strain or the reduction of lateral confinement at constant shear stress and associated instability behaviour of slopes. Such strain or stress path controlled tests need special arrangements and control systems. In this paper, a newly developed triaxial apparatus, capable of stress-strain path controlled test, is described. The main feature of this apparatus is the precise measurement and control system, which permits individual control of the cell pressure, pore water pressure, vertical stress and axial strain. The apparatus was used to study the stress-strain behaviour of a South Australian silty sand under different stress-path testing such as isotropic and K0-consolidated undrained and drained shear, constant shear drained (CSD) and constant mean stress (CMS) tests. Critical state conditions were achieved with uniform soil deformation at large axial strains, except in the case of the constant shear drained (CSD) tests where a gradual reduction of lateral confinement accelerated sample failure.

KEYWORDS: Stress-path, Triaxial apparatus, Critical state, Constant shear drained, Constant mean stress.

Laboratory Investigations on the Shear Behaviour of Sand-Tyre Derived Aggregate Mixtures

J.S. Vinod, M. Neaz Sheikh, Soledad Mashiri, and Dean Mastello

ABSTRACT: A significant amount of research has been carried out in recent years to investigate possible options on the reuse of scrap tyres in civil engineering applications. One of the sustainable options is to utilise scrap tyre as tyre shreds/tyre chips (generally called as Tyre Derived Aggregate, TDA) and sand mixture as a lightweight fill material in the construction of infrastructure. Utilising TDA in infrastructure projects has multiple benefits including environmentally sustainable recycling and reuse of the scrap tyre thereby easing the consumption of natural fills, reduced material costs and enhanced geotechnical properties of the soil. Understanding the shear and volume change behaviours of TDA and sand mixture is critical before recommending the mixture as a suitable lightweight-reinforcing structural fill. In this study, the effect of the addition of TDA on the shear behaviour of sand was investigated using large scale direct shear and triaxial apparatus. It has been observed that TDA has significant influence on the shear and volume change behaviours of sand. Also, overall improvements in the soil characteristics, such as enhanced shear strength, can be achieved by the addition of TDA in sand.

KEYWORDS: Tyre derived aggregates, Sand, Triaxial testing, Direct shear testing

Stress-Strain and Deformation Characteristics of an Unsaturated Soil-Cement Interface under Different Overburden Stresses and Grouting Pressures

M. A. Hossain and J. H. Yin

ABSTRACT: The most important parameters, by which the shear strength of any interface may be affected, are overburden stress and degree of saturation. Nowadays, grouting pressure is considered as another important parameter, which affects the interface behavior. In addition to gravity grouting, pressure grouting has been widely used to grout insitu soil-cement grout interfaces, like interfaces of soil-nail, soil-pile, and soil-anchor. In the present study, a series of interface direct shear tests were performed between a compacted completely decomposed granite (CDG) soil and cement grout under different overburden stresses, matric suctions, and grouting pressures. The stress-strain and deformation characteristics of the pressure grouted interface are similar to that of the CDG soil. However, the dilation values of soil-cement interface under different grouting pressures are smaller compared to CDG soil. The interface shear strength envelopes are approximately linear, and the apparent interface friction angle and adhesion intercept increase with matric suction for particular grouting pressures. On the contrary, the apparent interface friction angle decreases with pressure grouting for different matric suctions except saturated condition at which it remains constant.

KEYWORDS: CDG soil, Direct shear, Matric suction, Grouting pressure, Overburden stress, Cement grout, Interface.

Influence of the Sample Preparation on the Mechanical Characteristics of Hostun Sand

Q. Gu, D. König, and M. Goudarzy

ABSTRACT: The paper assesses the effect of sample preparation on the small and large strain characteristics of Hostun sand that has been widely used for many years in model tests and researches. For approaching to this goal, experimental efforts were conducted on fully saturated samples prepared using the dry air pluviation (AP) and moist tamping (with various initial water contents) (MT) methods. The experiments were conducted using bender elements for small strain level (e.g. the maximum shear modulus Gmax) and triaxial device for large strain level. The results show that the initial fabric of specimens can be affected by sample preparation. For an example, loose specimens prepared by the AP method show larger maximum shear modulus in comparison with those prepared by the MT method. On the other hand, dense specimens compacted using the AP method show less maximum shear modulus in comparison with those compacted using the MT method. However, the experiments showed the negligible effect of sample preparation on the large strain characteristics of the adopted sand under drained condition. The experiments revealed that the dry deposition method provides samples with the more isotropic fabric in comparison with those prepared by the moist tamping. This effect can be obviously seen in dense samples.

KEYWORDS: Stiffness, Critical state line, Hostun sand, Triaxial device, Bender elements, Wave velocity.

Piled Raft on Sandy Soil – An Observational Study

V. Balakumar, Min. J. Huang, Erwin Oh, Nilan S. Jayasiri, Richard Hwang, and A. S. Balasubramaniam

ABSTRACT: The increasing recognition of the combined piled raft foundation system is mainly due to the economics and the savings that can be achieved in the foundation design without compromising the safety and serviceability requirements. While detailed investigations through field monitoring of the piled raft supporting several tall and heavily loaded structures have been reported (Hooper, 1974; Cooke et al., 1981; Poulos, 2008; Yamashita, 2012), it appears, not so many case histories exist on the applicability of piled raft in the case of moderately loaded structures. The present study is related to the monitoring of the piled raft supporting a 12 storied apartment building and the results have been subjected to validation through numerical analyses. The results have been compared with the published results for a similar structure. As a practical problem, the study also discusses the effect of a compressible layer sandwiched between two competent layers obtained from an analytical study.

KEYWORDS: Piled raft, ANSYS, Compressible layer.

Lateritic Soil Stabilization by Addition of Steel Slags

S. Chaiyaput and J. Ayawanna

ABSTRACT: The addition of electric arc furnace slag and ladle furnace slag on strength improvement of lateritic soil was studied in this work. Liquid limit, plasticity index, and the California bearing ratio of lateritic soil mixed with the slags were determined in comparison to those of ordinary lateritic soil. A scanning electron microscope was used to confirm the effect of those two slags on microstructures related to chemical components of raw materials and the strength of lateritic soil. The deterioration of California bearing ratio, liquid limit, and plastic index values were obtained when the electric arc furnace slag was mixed in lateritic soil. Meanwhile, the California bearing ratio of lateritic soil was highly improved with the addition of ladle furnace slag, owing to the hydration reaction between water and excess lime in ladle furnace slag with free silica in lateritic soil. The values of the plasticity index were also comparable to the ordinary lateritic soil. Microstructures confirmed a highly compacted surface of mixed lateritic soil with ladle furnace slag. The ladle furnace slag is therefore one of the promising alternative low-cost materials for the soil-stabilizing application.

KEYWORDS: Steel slags, Soil stabilization, Construction, Mechanical properties, Improvement.

M.Yamin, M. F. Attom, S. Atabay, and R. Vandanapu

ABSTRACT: This paper investigates the effect of compaction effort on shear strength parameters of clay. Four clayey soils, two with high plasticity and two with low plasticity were used in this study. The initial physical properties of the clay such as gradation, optimum moisture content, maximum dry density, and Atterberg limits were determined in accordance with American Standard for Testing and Materials (ASTM) standard procedures. All soil specimens were remolded at five different compaction levels at three different water contents: dry, optimum and wet conditions. Empirical formulae were suggested to obtain optimum moisture content and maximum dry unit weight at an energy level utilizing the results obtained from the standard Proctor test. The shear strength parameters of the prepared specimens were determined using direct shear test. Another set of empirical formulae were also suggested to obtain the cohesion and friction angle at an energy level utilizing the cohesion and friction angle obtained from direct shear test at maximum dry unit weight and optimum moisture content from standard Proctor test.

KEYWORDS: Compaction energy, Cohesion, Angle of internal friction, Shear strength.

Using a Stochastic Model to Study the Ground Motion of Comprehensive Subsurface Structure

C. H. Hsu, S. J. Chao, S. G. Chern, and H. Hwang

ABSTRACT: The seismic response analysis of buildings and bridges use earthquake acceleration time histories as input data. But there is always a lack of earthquake time histories in regions where buildings are constructed, which makes ground motion simulation is one of the most important problems in civil engineering domain. In this study, the strong motion stations of boring log data collected within Lanyang plain have been combined with the subsurface layers of Lanyang plain obtained by using geophysical method to construct a comprehensive subsurface structure of Lanyang plain. Available papers and reports on nonlinear dynamic soil characteristics have been reviewed to determine appropriate soil parameters for representing soils in Lanynag plain. In this research, the stochastic seismic model developed by Boore (1983, 2005) will be used to simulate a set of rock-outcrop synthetic acceleration time histories. The acceleration time histories at the rock-outcrop from this study will be compared with the real earthquake record for all the soil sites, the acceleration time histories at the ground surface will be generated from nonlinear site response analyses using the computer program SHAKE91. For the strong motion station, we compare the earthquake spectrum and real earthquake spectrum to investigate the appropriateness in the Ilan area. These ground motions may be used in the field of disaster reduction, such as the calculation of the ground response subject to earthquakes using the SHAKE computer program or the evaluation of the safety of buildings and bridges.

KEYWORDS: Synthetic ground motion, Response spectra, Comprehensive subsurface structure.

The Effect of the Inclined Core of the Earth Fill Dam on the Settlement Rate

A. M. Ghaemi, B. K. Ahangari, C. A. Noorzad, and D. K. Goshtasbi

ABSTRACT: Since the constituent material of the earth fill dams are granular, they have little resistance to tensile stresses and are undergoing deformations as settlement. A large number of parameters affect the performance of dams. The aim of this paper is to evaluate the effect of the inclined core of earth fill dam as a parameter that is less regarded for the dam settlement rate. Therefore, considering the different angles for the core (60°, 75°, and 90°), the dam settlement (Case Study: Sardasht dam) was investigated. In this study, numerical and statistical methods (fuzzy set) were used to analyze the results, first analyses were conducted by numerical modeling using PLAXIS software, and then a settlement criterion was obtained using existing techniques in statistics. Then, to obtain the considered relationship, the fuzzy linear regression model was used. Finally, after dynamic and static analysis with the mentioned methods, it was observed that with increasing core slope, the dam settlement decreases. The outcomes of fuzzy regression equations also confirm this issue.

KEYWORDS: Settlement rate, Inclined core, Body slope, Numerical modelling, PLAXIS, Fuzzy set regression.

The Boundary Condition of a 3D Continuum Model for a Quasi 2D Plane Strain Condition

S. Chaiyaput and M. Sugimoto

ABSTRACT: The half-model has been used in the finite element analysis, based on the symmetric condition against the tunnel’s longitudinal, vertical plane, but this design concept cannot apply to a staggered lining. Therefore, the authors have proposed a multi-ring model to present the segmental lining behavior of a 3D continuum ground model in the case of staggered building, but the proposed 12-ring model shows the different axial force distributions in circumferential directions between the center of the model and the end of the model, especially in cases of soft soil. Accordingly, to make clear, the mechanism of the above difference and to establish the analysis condition of the proposed model for quasi 2D plane strain condition, this paper evaluates the influence of boundary condition on the transverse cross section at the end of the segmental lining on the segmental lining behavior, based on the analyzed axial force and bending moment in soft and stiff ground conditions using both the 2-ring model and a 12-ring model. As a result, it was confirmed that the proposed 2-ring model could simulate the segmental lining behavior under quasi 2D plane strain condition.

KEYWORDS: Segmental lining, Staggered arrangement, Continuum ground model, 3D FEM, Boundary condition.

3D Evaluation of PWP Development Due to Tunnel Excavation in an Over Consolidated Clay

D. L. Vettorello, F. A. M. Marinho, and P. D. G. Orlando

ABSTRACT: The stress relief promoted by the excavation process may induce the reduction of the pore water pressure within the soil when an undrained condition occurs. According to the magnitude of the stress relief and the initial hydrostatic pressure, the pore water pressure reduction may generate suction in the soil, thus increasing the soil shear strength. Effects of the stress relief promoted by a circular tunnel excavation process in stiff clay was investigated by means of a series of 3D numerical analyses. These analyses considered a NATM unsupported excavation advance of a shallow and small diameter tunnel in a saturated soil mass. The results highlight the development of a suction zone in front of the tunnel face where the soil shear strength is increased. The extension of this suction zone and its dissipation rate through time, however, are influenced by the soil massif coefficient of earth pressure at rest, and hydraulic conductivity, respectively.

KEYWORDS: Tunnels, 3D Numerical model, Pore water pressure development, Stress relief, Overconsolidated clay.

Seismic Behaviour of Complex H-Shaped Buried Structures

B. Salehi, A. Bahmanpour, and M. Derakhshandi

ABSTRACT: Seismic loading parameters are always characterized by high complexity and uncertainty. The uncertainty increases in complex structures, which makes the process more complicated, particularly for structures that are partly buried deep and partly at the surface. In this research, the seismic behavior and stress changes of complex H-shaped structures have been studied using 2D FE analysis. Five points in the geometry of the model were selected at which the normal and shear stresses were extracted. Granular soil with variable strength parameters was investigated in the sensitivity analysis. The structures had height-to-width ratios of 1, 1.5 and 2. The results show that the stress increased in structures under near-field earthquake records. A decrease in shear stress was observed for all structural ratios with an increase in the internal friction angle. A rise in the internal friction angle at the midpoint of the wall also increased the shear stress. This growth was observed for all far-field earthquake records up to an internal friction angle of 35° and then descend. At a constant friction angle, climbing structure ratio H/L from 1 to 1.5 improved the shear stress at least 20%. However, as H/L changed from 1.5 to 2, the minimum stress increase was 85%. For far-field earthquakes, the stress values depended on the horizontal acceleration and H/L because they are influenced by the frequency content and internal friction angle.

KEYWORDS: Time history, Finite element method, Complex structure, Shear stress.

Settlement’s Prediction of Piles in Tropical Soil

P. H. L. Bezerra, A. S. Moura, F. C. da Silva Filho, F. V. C. G. Filho, and E. Cerqueira Junior

ABSTRACT : The prediction of deep foundation settlements remains challenging due to the scarcityofstudies about it. This paper aimed to evaluate the accuracy of the usage of load transfer methods in predicting the settlement of bored piles in granular soil in Brazil’s Northeast. For this, two small piles were installed and submitted to load tests, a small amount of expansive polystyrene was placed under one of the pile’s tips to evaluate the load distribuition of the elements.. For the settlement’s prediction, methods based on load transfer functions, such as analytical and numerical (using RSPile and UniPile ), were employed. The comparinson of the predicted values with the experimental measurements showed agreement in the elastic zone of soil’s behavior for all the methods. For higher loads, discrepancies occurred. The method proposed by Massad (1992) was the most effective among the used methodologies. Using the mentioned programs, results were close to the experimental values.

KEYWORDS: Settlement prediction, Load transfer, Bored piles.

Cyclic Resistance of High Plasticity North Java Clays and Silt

E. Rismantojo, E. Ginting, and M. Ochoa

ABSTRACT: In an attempt to develop a database of the cyclic resistance of fine-grained soils in Indonesia a series of cyclic triaxial test was performed on fine-grained soils collected from two locations near shore of north Java Island, Indonesia. These samples were classified as fine-grained soils with clay-like behavior and, according to various liquefaction criteria, they are non-liquefiable. Our study used a cyclic triaxial test to apply multistage cyclic axial stresses in a form of sinusoidal stress with a frequency of 1 Hz. All specimens are isotropically consolidated passing their preconsolidation stress in order to achieve a normally consolidated state with OCR of about 1. Our findings suggest that cyclic resistance of North Java clay-like fine-grained soils follows the SHANSEP concept and could be normalized at OCR of 1. At fifteen cycles of uniform sinusoidal loading the tested clay and silt samples have a normalized cyclic strength, , or a cyclic resistance ratio (CRR) of approximately 0.31 and a cyclic strength ratio, , of 0.70.

KEYWORDS: Clay-like behavior, Cyclic resistance ratio, Cyclic strength ratio, Plasticity index, Water content to liquid limit ratio.

Durability of Erosion Control Geomesh of Paddy by ‘Accelerated Biodegradation Test’

S. Halkude and R. Katdare

ABSTRACT: Paddy straw is abundantly available as crop waste and has a sacrificial quality to biodegrade early, which can be advantageously utilized to develop a low-cost Paddy Straw Geomesh (PSG) to control rain-induced surface erosion of a newly constructed embankment for the critical period of few months of the rainy season. The present study reports employing, ‘Accelerated Biodegradation Test’ to quickly estimate the test time of PSG specimens in the laboratory to reach its durability in comparison with the field test time which is generally longer due to slow disintegration of organic material in climatic conditions. This acceleration is achieved by increasing fungi spore concentration with respect to field fungi concentration, using various concentrations ranging from 3.29 x 105 through 3.29 x 109 spores per gram of soil while taking ‘Accelerated Biodegradation Test’. The findings of ‘Accelerated Biodegradation Test’ report 40% decrease in test time of PSG in a laboratory, using higher fungi spore concentration in soil embodiments, as compared to those in the field.

KEYWORDS: Accelerated biodegradation test, Laboratory embedment, Paddy straw geomesh, Fungi spore concentration, Tensile strength.

Stability Analysis of an Overall Failure Excavation Case in Hang Zhou

T. N. Do

ABSTRACT: In this paper, stability of an overall failure excavation case in Hang Zhou, China was analyzed using the finite element method (FEM). The retaining system of the excavation was fully modelled, including walls, horizontal struts, and vertical center posts. For comparison, the structural elements were simulated using plates with both elastic and elastoplastic behaviors. The soil response nesar failure was assumed to follow the Morh-Coulomb model. Results showed that the FEM using the elastoplastic retaining system gave a more reasonable estimate of stability of the excavation than that using the elastic support system. With the elastoplastic retaining system, yielding firstly occurred on the wall and then on the struts, which caused large movement of surrounding soil toward the excavation. On the other hand, with the elastic retaining system, failure of the excavation was only due to the great plastic heave of soil at the excavation bottom. The predicted movement of soil and wall was nearly one meter as using the ealstoplatic support system but several meters as using the elastic one.

KEYWORDS: Deep excavations, Stability analysis, Finite element method, Failure mechanism.

Vol. 52 No. 2 June 2021

Stability Analysis of an Overall Failure Excavation Case in Hang Zhou

T. N. Do

ABSTRACT: In this paper, stability of an overall failure excavation case in Hang Zhou, China was analyzed using the finite element method (FEM). The retaining system of the excavation was fully modelled, including walls, horizontal struts, and vertical center posts. For comparison, the structural elements were simulated using plates with both elastic and elastoplastic behaviors. The soil response nesar failure was assumed to follow the Morh-Coulomb model. Results showed that the FEM using the elastoplastic retaining system gave a more reasonable estimate of stability of the excavation than that using the elastic support system. With the elastoplastic retaining system, yielding firstly occurred on the wall and then on the struts, which caused large movement of surrounding soil toward the excavation. On the other hand, with the elastic retaining system, failure of the excavation was only due to the great plastic heave of soil at the excavation bottom. The predicted movement of soil and wall was nearly one meter as using the ealstoplatic support system but several meters as using the elastic one.

KEYWORDS: Deep excavations, Stability analysis, Finite element method, Failure mechanism

DOI: 10.14456/seagj.2021.41

Vol. 52 No. 2 June 2021

Durability of Erosion Control Geomesh of Paddy by ‘Accelerated Biodegradation Test’ 

By S. Halkude and R. Katdare

ABSTRACT: Paddy straw is abundantly available as crop waste and has a sacrificial quality to biodegrade early, which can be advantageously utilized to develop a low-cost Paddy Straw Geomesh (PSG) to control rain-induced surface erosion of a newly constructed embankment for the critical period of few months of the rainy season. The present study reports employing, ‘Accelerated Biodegradation Test’ to quickly estimate the test time of PSG specimens in the laboratory to reach its durability in comparison with the field test time which is generally longer due to slow disintegration of organic material in climatic conditions. This acceleration is achieved by increasing fungi spore concentration with respect to field fungi concentration, using various concentrations ranging from 3.29 x 105 through 3.29 x 109 spores per gram of soil while taking ‘Accelerated Biodegradation Test’. The findings of ‘Accelerated Biodegradation Test’ report 40% decrease in test time of PSG in a laboratory, using higher fungi spore concentration in soil embodiments, as compared to those in the field.

KEYWORDS: Accelerated biodegradation test, Laboratory embedment, Paddy straw geomesh, Fungi spore concentration, Tensile strength.

DOI: 10.14456/seagj.2021.40

Vol. 52 No. 2 June 2021

Cyclic Resistance of High Plasticity North Java Clays and Silt

E. Rismantojo, E. Ginting, and M. Ochoa

ABSTRACT: In an attempt to develop a database of the cyclic resistance of fine-grained soils in Indonesia a series of cyclic triaxial test was performed on fine-grained soils collected from two locations near shore of north Java Island, Indonesia. These samples were classified as fine-grained soils with clay-like behavior and, according to various liquefaction criteria, they are non-liquefiable. Our study used a cyclic triaxial test to apply multistage cyclic axial stresses in a form of sinusoidal stress with a frequency of 1 Hz. All specimens are isotropically consolidated passing their preconsolidation stress in order to achieve a normally consolidated state with OCR of about 1. Our findings suggest that cyclic resistance of North Java clay-like fine-grained soils follows the SHANSEP concept and could be normalized at OCR of 1. At fifteen cycles of uniform sinusoidal loading the tested clay and silt samples have a normalized cyclic strength, ((q_cyc⁄2))⁄(σ_c^’ ), or a cyclic resistance ratio (CRR) of approximately 0.31 and a cyclic strength ratio, ((q_cyc⁄2))⁄s_u , of 0.70.

KEYWORDS: Clay-like behavior, Cyclic resistance ratio, Cyclic strength ratio, Plasticity index, Water content to liquid limit ratio.

DOI: 10.14456/seagj.2021.39

Vol. 52 No. 2 June 2021

Settlement’s Prediction of Piles in Tropical Soil

P. H. L. Bezerra, A. S. Moura, F. C. da Silva Filho, F. V. C. G. Filho, and E. Cerqueira Junior

ABSTRACT : The prediction of deep foundation settlements remains challenging due to the scarcityofstudies about it. This paper aimed to evaluate the accuracy of the usage of load transfer methods in predicting the settlement of bored piles in granular soil in Brazil’s Northeast. For this, two small piles were installed and submitted to load tests, a small amount of expansive polystyrene was placed under one of the pile’s tips to evaluate the load distribuition of the elements.. For the settlement’s prediction, methods based on load transfer functions, such as analytical and numerical (using RSPile and UniPile ), were employed. The comparinson of the predicted values with the experimental measurements showed agreement in the elastic zone of soil’s behavior for all the methods. For higher loads, discrepancies occurred. The method proposed by Massad (1992) was the most effective among the used methodologies. Using the mentioned programs, results were close to the experimental values.

KEYWORDS: Settlement prediction, Load transfer, Bored piles

DOI: 10.14456/seagj.2021.38

Vol. 52 No. 2 June 2021

Seismic Behaviour of Complex H-Shaped Buried Structures

B. Salehi, A. Bahmanpour, and M. Derakhshandi

ABSTRACT: Seismic loading parameters are always characterized by high complexity and uncertainty. The uncertainty increases in complex structures, which makes the process more complicated, particularly for structures that are partly buried deep and partly at the surface. In this research, the seismic behavior and stress changes of complex H-shaped structures have been studied using 2D FE analysis. Five points in the geometry of the model were selected at which the normal and shear stresses were extracted. Granular soil with variable strength parameters was investigated in the sensitivity analysis. The structures had height-to-width ratios of 1, 1.5 and 2. The results show that the stress increased in structures under near-field earthquake records. A decrease in shear stress was observed for all structural ratios with an increase in the internal friction angle. A rise in the internal friction angle at the midpoint of the wall also increased the shear stress. This growth was observed for all far-field earthquake records up to an internal friction angle of 35° and then descend. At a constant friction angle, climbing structure ratio H/L from 1 to 1.5 improved the shear stress at least 20%. However, as H/L changed from 1.5 to 2, the minimum stress increase was 85%. For far-field earthquakes, the stress values depended on the horizontal acceleration and H/L because they are influenced by the frequency content and internal friction angle.

KEYWORDS: Time history, Finite element method, Complex structure, Shear stress.

DOI: 10.14456/seagj.2021.37

Vol. 52 No. 2 June 2021

3D Evaluation of PWP Development Due to Tunnel Excavation in an Over Consolidated Clay

D. L. Vettorello, F. A. M. Marinho, and P. D. G. Orlando

ABSTRACT: The stress relief promoted by the excavation process may induce the reduction of the pore water pressure within the soil when an undrained condition occurs. According to the magnitude of the stress relief and the initial hydrostatic pressure, the pore water pressure reduction may generate suction in the soil, thus increasing the soil shear strength. Effects of the stress relief promoted by a circular tunnel excavation process in stiff clay was investigated by means of a series of 3D numerical analyses. These analyses considered a NATM unsupported excavation advance of a shallow and small diameter tunnel in a saturated soil mass. The results highlight the development of a suction zone in front of the tunnel face where the soil shear strength is increased. The extension of this suction zone and its dissipation rate through time, however, are influenced by the soil massif coefficient of earth pressure at rest, and hydraulic conductivity, respectively.

KEYWORDS: Tunnels, 3D Numerical model, Pore water pressure development, Stress relief, Overconsolidated clay

DOI: 10.14456/seagj.2021.36

Vol. 52 No. 2 June 2021

The Boundary Condition of a 3D Continuum Model for a Quasi 2D Plane Strain Condition

S. Chaiyaput and M. Sugimoto

ABSTRACT: The half-model has been used in the finite element analysis, based on the symmetric condition against the tunnel’s longitudinal, vertical plane, but this design concept cannot apply to a staggered lining. Therefore, the authors have proposed a multi-ring model to present the segmental lining behavior of a 3D continuum ground model in the case of staggered building, but the proposed 12-ring model shows the different axial force distributions in circumferential directions between the center of the model and the end of the model, especially in cases of soft soil. Accordingly, to make clear, the mechanism of the above difference and to establish the analysis condition of the proposed model for quasi 2D plane strain condition, this paper evaluates the influence of boundary condition on the transverse cross section at the end of the segmental lining on the segmental lining behavior, based on the analyzed axial force and bending moment in soft and stiff ground conditions using both the 2-ring model and a 12-ring model. As a result, it was confirmed that the proposed 2-ring model could simulate the segmental lining behavior under quasi 2D plane strain condition.

KEYWORDS: Segmental lining, Staggered arrangement, Continuum ground model, 3D FEM, Boundary condition.

DOI: 10.14456/seagj.2021.35