Operational Soil Stiffness from Back-Analysis of Pile Load Tests within Elastic Continuum Framework
Fawad S. Niazi and Paul W. Mayne
ABSTRACT: New sets of shear stiffness reduction curves are developed from the back-analyses of 299 static axial pile load tests from 61 sites towards the implementation of a non-linear load-displacement (Q-w) response method for pile foundations. The initial shear modulus (Gmax) is derived from the measured shear wave velocity (Vs) profiles at the pile sites, usually obtained from seismic cone penetration tests (SCPT). A Randolph-type closed-form elastic continuum solution for axial compression loading is used, modified for additional applications involving axial tension loading cases. The back-analysis of shear moduli at various load levels results in derivations of new shear modulus reduction curves, specifically normalized shear stiffness (G/Gmax) vs. logarithm of pseudo-strain: γp = wt/d, where wt = pile top displacement and d = pile diameter. These curves incorporate the effects of pile type and installation method, and also show the influence of soil plasticity. A complete step-by-step methodology is presented for use and application of these new stiffness reduction curves within the extended system of closed-form elastic continuum solutions. In a companion paper, these solutions are further extended towards their application to a stacked pile model, representing certain practical field situations of non-homogeneous and non-Gibson type soil profiles, and accounting for the concept of progressive failure with depth.
KEYWORDS: Pile foundations, Displacements, Load tests, Shear modulus, Shear wave velocity, Soil stiffness, Plasticity index
Elastic Continuum Solution of Stacked Pile Model for Axial Load-Displacement Analysis
Fawad S. Niazi and Paul W. Mayne
ABSTRACT: In a companion paper, new sets of shear stiffness reduction curves developed from the back-analyses of 299 static axial pile load tests were presented towards the implementation of a non-linear load-displacement (Q-w) response analysis within the framework of Randolph-type closed-form elastic continuum solution. These curves were developed with the following underlying assumptions: (1) soil stiffness is linear with depth (although certain situations may portray a different trend), and (2) the back-analyzed field stiffnesses can be obtained keeping the operative modulus profile constant throughout the loading (even though shaft resistance is expected to be mobilized prior to the end bearing). In an effort to make some improvements with respect to these conditions, certain provisions of the elastic continuum solution are exploited to present a methodology for drawing the stiffness reduction curves as functions of depth. These curves are further utilized in modeling the pile as a stack of smaller shaft segments embedded in multi-layered soils, where each layer is assigned its own distinctive averaged stiffness. The load-displacement analysis of all pile segments, associated with their adjacent soil layers, can thus be performed using the stiffness reduction curves applicable to their respective depths. The overall load-displacement response is obtained through integration of the analysis result of all layers. Flow charts are presented detailing steps for plotting the depth-dependent stiffness reduction curves. Similarly illustrative figures are included showing the procedures for implementing the stacked pile model for compressible as well as rigid piles.
KEYWORDS: Pile foundations, Displacements, Shear modulus, Shear wave velocity, Soil stiffness, Layered soil profile
Lateral Loading Tests on Piled Rafts and Simplified Method to Evaluate Sectional Forces of Piles
J. Hamada, T. Tsuchiya, T. Tanikawa and K. Yamashita
ABSTRACT: Both analytical and experimental studies are necessary when developing a seismic design concept for piled raft foundations, especially in highly active seismic areas such as Japan. This paper presents static cyclic lateral loading tests on large-scale piled raft foundations carried out to investigate the influence of vertical load and pile spacing ratios during earthquakes. The test models were pile groups and piled rafts with a concrete footing supported by 16 piles in a 1g field. The results showed that most of the lateral force was carried by raft friction when there was large contact earth pressure beneath the raft, and that piles experienced pulling forces from the raft, behaving like anchors at large deformations. The tested foundations were simulated using a simplified method based on Mindlin’s solution, with theoretical equations derived making some approximations and assumptions. The simulated results agreed well with the test results.
KEYWORDS: Piled raft, Lateral loading test, Simplified modeling, Simplified theoretical method
Applicability of Simple Method to Piled Raft Analysis in Comparison With Field Measurements
K. Yamashita, T. Tanikawa, and J. Hamada
ABSTRACT: An applicability of the simple method of the combined pile group and raft method proposed by Clancy and Randolph (1996) to piled raft analysis was examined through comparisons with the field monitoring results from four case histories in Japan. To deal with multi-layered soils with finite depth, the simple method was modified using the Steinbrenner’s solution. The shear modulus of soil used in the analysis was determined by degrading the shear modulus at small strains using a reduction factor, where a set of reduction factors were employed in Case 2 while a single reduction factor was used in Case 1. Consequently, it was found that the presented approach based on the method of Clancy and Randolph gave an approximate average settlement and load sharing between the pile group and the raft, when the reduction factor of shear modulus was 0.4 in Case 1 and 0.3 in Case 2.
KEYWORDS: Piled raft foundation, Design method, Analysis, Settlement, Load sharing
Engineering Assessment of Ground Vibrations Caused by Impact Pile Driving
K. Rainer Massarsch and Bengt H. Fellenius
ABSTRACT: Ground vibrations are an important design consideration for piles driven by impact hammers. The first task is to determine allowable vibration levels; the second task is to predict the intensity of ground vibrations during driving and the attenuation of ground vibrations with increasing distance. The paper describes the Swedish vibration standard which is applicable for pile driving. Commonly used vibration parameters, associated with the evaluation of vibration measurements, are discussed. The importance of pile impedance for ground vibrations is highlighted. A simplified calculation method is proposed which can be used to estimate vertical ground vibration velocity as a function of distance from the driven pile. Two case histories have been evaluated and compared with theoretical predictions, using the proposed method of analysis.
KEYWORDS: Ground vibrations, Pile driving, Vibration limits, Vibration parameters, Prediction, Frequency, Damage
Analysis of Results of an Instrumented Bidirectional-Cell Test
Bengt H. Fellenius
ABSTRACT: The bidirectional-cell test has been around since the early 1970s. The first commercial development came about in the early 1980s in Brazil and about a decade later in the USA. The use of strain-gage instrumented tests was pioneered by the US supplier of the test method. The results of a bidirectional-cell test on a strain-gage instrumented 1.2 m diameter, 40 m long, bored pile were analyzed to establish the load distribution in the test pile, the distributions of beta-coefficient along the test pile, and the unit shaft shear resistance versus movement relative to the soil. The response to the applied load was modeled in an effective stress analysis to determine the t-z and q-z functions, as fitted to the measured upward and downward curves. The equivalent pile head-down load-distribution was modeled from the functions, including the separate modeling of the pile response for the pile head, pile shaft, and pile toe. The calculated pile head loadmovement curve was compared to a load-movement curve manually calculated directly from the test data.
KEYWORDS: Bidirectional-cell loading test, Effective stress analysis, t-z and q-z functions, Equivalent head-down loadmovement; Equivalent head-down load-distribution
Deep Barrette Pile Capacity with Aging Effect
W. Teparaksa
ABSTRACT: Two instrumented barrette piles of the highest high-rise building in Bangkok city were tested in order to verify the ultimate pile capacity. Their sizes were approximately 1.2×3.0x66m depth and their pile tips seated in the second very dense silty sand layer. The first tested barrette pile (TP1) was tested at 43 days after pile completion while 72 days curing time was allowed for the second tested barrette pile (TP2). Aging Effect of barrette pile was observed. Barrette pile number TP1 showed low loading capacity with high settlement while higher loading with lower settlement was recorded in the tested barrette pile number TP2. The value of adhesion factor (α) and friction factor (β) of the first tested pile (TP1) were studied and it shows well agreement with typical values of Bangkok soil while the second showed that its values were at higher bound of the typical.
KEYWORDS: Barrette pile, Aging effect, Friction factor, Adhesion factor, Bentonite slurry, Polymer slurry
Case Study of Dynamic Responses of a Single Pile Foundation Installed in Coal Ash Landfills using Effective Stress Analysis and EQWEAP
C. W. Lu and D. W. Chang
ABSTRACT: Coal ash continues to be a major production of energy generated material and its use are predicted to increase. The disposal of coal ash has become an important issue when not only chemical but also physical and engineering characteristic of the coal ash is necessary to be researched. The large quantity of disposal of coal ash is even more complicated to handle. This paper is focused on researching dynamic behaviors of coal ash soils obtained in a landfilled field in north Taiwan and also the dynamic interaction of a single pile foundation sitting in the landfills. An effective stress analysis, firstly, was adopted for examining seismic response of 4 m height single pile with 10 m depth into the ground, of which soil parameters in the constitutive model was confirmed basis of stress path and stress- strain relationship of the coal ash soils. Then, the pile response was researched again by EQWEAP. This paper found out that 1) when the pore water pressure reached 40 kpa at around 5sec, the coal ash landfill liquefied, 2) the movement of pile foundation became larger after 5sec, 3) the nonlinearity of the soils resulted in reducing responded horizontal acceleration of the soils at different depth after 5sec. It also showed that both numerical codes were applicable of reproducing pile responses in this an extreme weak coal ash in this study.
KEYWORDS: Dynamic responses of single pile foundation, Soil liquefaction, Coal ash, Three dimensional effective stress analysis, EQWEAP
The Response of A “Plug” in An Open-Toe Pipe Pile
Bengt H. Fellenius
ABSTRACT: Two published case histories are taken as reference to a discussion on how to consider the effect of soil core inside a pipe pile driven open-toe, as opposed to the response of a pile driven closed-toe. The analysis of the measurements shows that the comparison has to be made in terms of deformation, not capacity. Both piles have similar shaft resistance along the outside of the pipe. For the closed-toe pile, toe resistance acts along the full cross section. For the open-toe pile with a soft core, some toe resistance is mobilized by the force against the steel annulus. The soil force that acts at the bottom of the core, pushes the core upward much like the upper portion of a pile tested in a bidirectional test and the upward movement is resisted by shaft resistance along the inside of the pile. However, the core is very soft compared to a pile and the movement of the pile toe is quickly spent, resulting in a limited magnitude of inside shaft resistance, moreover one acting only along a limit length of the pile up from the pile toe. Recommendations for how to analyze the response of an open-toe pile are presented and a comparison is provided between the results of a simulated static loading test on a closed-toe and an open-toe pipe pile are presented, showing that the comparison—and piled foundation design—have to recognize and consider the pile toe movement during service conditions.
KEYWORDS: Closed-toe and open toe pipe piles, Soil core response, Effective stress analysis, t-z and q-z functions, Load-deformation considerations, Influence of residual load.
Effects of Toe Grouting on Axial Performance of Drilled Shafts Socket in Intermediate Geomaterial
S.S. Lin, Y.L. Yin, K.C. Fu, Y.K. Lin, C.J. Kuo, and Y.H. Chang
ABSTRACT: In this paper, the axial performance of two heavily instrumented drilled shafts, with and without toe grouting, socket in intermediate geomaterials in Taipei are evaluated based on the results of pile load tests. The load versus settlement at pile head and the t-z curves along shaft, especially for the part socket into intermediate geomaterials, are main concerns. The t-z curves interpreted from the measured data along shaft are also simulated by the hyperbolic model. The value of friction factor (β) of the shaft with or without grouting is also compared in the paper. It’s found toe grouting improved not only the end bearing capacity but also the frictional resistance of the tested shaft.
KEYWORDS: Pile load test, Intermediate geomaterial, Drilled shaft, Hyperbolic model
Reliability-Based Design of Proof Load Test Programs for Foundations
Y. Abdallah, S.S. Najjar and G. Saad
ABSTRACT: There is currently an inconsistency in the recommendations that are available in pile design codes and practices regarding the required number of proof-load tests and the level of the proof loads. This paper presents the results of a comprehensive investigation that is conducted to study the effect of choosing different proof-load test programs on the reliability of piles. This is achieved by utilizing a Bayesian approach to update the capacity distribution of piles at a site given the results of the proof-load test program. In the updating exercise, an effort is made to update both the mean and the lower-bound capacity to maximize the benefit of the collected proof load data. The significance of the results presented lies in the fact that these results constitute necessary input to any practical decision framework for choosing the number and the magnitude of the proof load test that would maximize the value of information of the test program.
KEYWORDS: Proof loads, Reliability, Piles, Lower bound capacity, Bayesian updating
Probabilistic Approaches for Ultimate Resistance of Drilled Shafts in Sands Considering Spatial Variability
Z. Luo, L. Wang, W. Gong, and C. Hsein Juang
ABSTRACT: In this paper, existing probabilistic approaches for determining the ultimate resistance of drilled shafts in sands considering the spatial variability of soil properties are evaluated and compared. The first approach is realized through random field modeling implemented with Monte Carlo simulation (MCS). The second approach is a simplified method based on the spatial averaging technique. The third approach is yet another simplified method based on the spatial correlation between spatial averages. The last two (simplified) approaches can be efficiently implemented without MCS so that much less computational effort is required. The comparison study shows that the three probabilistic approaches yield practically identical results (i.e., probability of failure in the designed drilled shafts). This study also highlights the importance of considering the spatial variability of soil properties and the model bias in the design of drilled shafts. Results indicate that: (1) neglecting spatial variability often leads to an overestimation of the probability of failure; (2) ignoring the model bias results in either overestimation or underestimation of the probability of failure, depending on the compression load applied to the drilled shafts.
KEYWORDS: Drilled shafts, Standard penetration test, Probability, Random field modeling, Spatial variability, Spatial average, Spatial correlation.
SPECIAL FEATURE STORY ON “Liquefaction Problems in the 21st Century”
I. Towhata
ABSTRACT: This paper concerns liquefaction problems that were not solved in the 20th Century. Most of the problems lie in inexpensive structures and existing structures that were out of scope in the past. Efforts are going on to overcome the problems in spite of the imbalance of the required cost and the financial capacity. Seismic performance design is one of the future directions but there are again problems to tackle. It is stressed that qualification of the properties and lands are important because people can recognize the lying hazards and can prepare for the future safety.
KEYWORDS: Liquefaction problems, Seismic performance design, Lying hazards, Future safety
