Brief summary of engineering doctoral thesis: Analysis of influencing parameters and basics of determining resistance factors of drilled shafts used in bridge substructures in Ho Chi Minh city

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Brief summary of engineering doctoral thesis: Analysis of influencing parameters and basics of determining resistance factors of drilled shafts used in bridge substructures in Ho Chi Minh city

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Nội dung Text: Brief summary of engineering doctoral thesis: Analysis of influencing parameters and basics of determining resistance factors of drilled shafts used in bridge substructures in Ho Chi Minh city 1 MINISTER OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS NGO CHAU PHUONG ANALYSIS OF INFLUENCING PARAMETERS AND BASICS OF DETERMINING RESISTANCE FACTORS OF DRILLED SHAFTS USED IN BRIDGE SUBSTRUCTURES IN HOCHI MINH CITY MAJOR: BRIDGE AND TUNNEL ENGINEERING CODE: 62.58.02.05.03 BRIEF SUMMARY OF ENGINEERING DOCTORAL THESIS Hanoi20142 This thesis completed at: Faculty of Civil Engineering University of Transport and Communications SUPERVISORS: 1. Assoc.Prof. Dr. Tran Duc Nhiem 2. Assoc.Prof.Dr. Nguyen Ngoc Long Reviewer 1: Prof. Dr. Nguyen Nhu Khai, National University of Civil Engineering Vietnam. Reviewer 2: Prof. Dr. Nguyen Dong Anh, Institute of Mechanics Vietnam. Reviewer 3: Dr. Do Huu Thang, Institute of Transport Science and Technology Vietnam. Thesis is defended in front of the UniversityGraded Committee of thesis evaluation according to Decision 1359QĐĐHGTVT, on date 17th June 2014 signed by the Rector of University of Transport and Communications on date………………….. 2014. Readers can find this thesis at: Vietnam National Library Library of the University of Transport and Communications

1 MINISTER OF EDUCATION AND TRAINING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS NGO CHAU PHUONG ANALYSIS OF INFLUENCING PARAMETERS AND BASICS OF DETERMINING RESISTANCE FACTORS OF DRILLED SHAFTS USED IN BRIDGE SUBSTRUCTURES IN HO-CHIMINH CITY MAJOR: BRIDGE AND TUNNEL ENGINEERING CODE: 62.58.02.05.03 BRIEF SUMMARY OF ENGINEERING DOCTORAL THESIS Hanoi-2014 This thesis completed at: Faculty of Civil Engineering University of Transport and Communications SUPERVISORS: Assoc.Prof Dr Tran Duc Nhiem Assoc.Prof.Dr Nguyen Ngoc Long Reviewer 1: Prof Dr Nguyen Nhu Khai, National University of Civil Engineering Vietnam Reviewer 2: Prof Dr Nguyen Dong Anh, Institute of Mechanics Vietnam Reviewer 3: Dr Do Huu Thang, Institute of Transport Science and Technology Vietnam Thesis is defended in front of the University-Graded Committee of thesis evaluation according to Decision #1359/QĐ-ĐHGTVT, on date 17th June 2014 signed by the Rector of University of Transport and Communications on date………………… 2014 Readers can find this thesis at: - Vietnam National Library - Library of the University of Transport and Communications INTRODUCTION By applying statistics and probability and reliability theory to engineering foundations, the thesis proposes a pattern to determine resistance factors of drilled shafts used in bridge substructures based on statistics characteristics of the ratio between real measured values and estimated values for Resistance (R) and load effect (Q) Then, by analyzing statistics characteristics of the capacity based on 24 results of static axial compressive load test of drilled shafts that were constructed by the wet method (bentonite) in cohesive and non-cohesive composite soils in HoChi-Minh city, the thesis captures the determination of resistance factors for four different calculation methods of pile resistance design based on soil base strength condition Literature Review: Drilled shafts construction technology was first used in America (1890), in over the word (1950) and in Vietnam (1990) However, the calculation theory has been developed more slowly One of the trends in the world is to research new problems of applying statistics and probability theories and reliability theory to correct resistance factors based on statistics characteristics of the ratio between real measured values and estimated values for Resistance (R) and load effect (Q) extracted from a reasonable number of construction projects Research results are gradually applied to update and to implement for some clauses in standards or specifications and design instructions of developed countries such as in Europe, Japan…and America Over two more recent decades in Vietnam, accompanied with the development of infrastructure in great scale built in soft soil bases or in urban areas, drilled shafts foundations becomes one of the best solutions The foundations are also widely applied in Ho-Chi-Minh city However, there is not any backgrounds to determine resistance factors of drilled shafts based on analyzing statistics characteristics and reliability analysis using current advanced theories Therefore, studying of the backgrounds to determine the resistance factors based on reliability analysis is new and attracts international and domestic researchers It is the reason why I selected this topic to research Thesis title: “Analysis of influencing parameters and bases to determine resistance factors of drilled shafts used in bridge substructures in Ho-ChiMinh City” Objectives: Determine resistance factors according to soil base strength which is equivalent to methods presented in specifications being applied Structure: Drilled shafts used in bridge substructures Research Scope: Predictive resistance and real resistance obtained from the results of static axial compressive load tests for drilled shafts in HoChi-Minh City casted in cohesive and non-conhesive composite soils (sand, sandy, clay, clay mud, ) by wet method; to determine general resistance factors according to soil base strength condition for four different methods of pile resistance design: 1) Russian Method specified in TCXDVN 20598; 2) Japanese Method (JRA 2002 SHB -Part IV); 3) Reese&O'Neill (1988) and 4) O'Neill&Reese (1999) Problems relating to load statistics characteristics, general resistance factors for various types of soil, local areas, and type of structure as well as pile shalf and tip resistance factors are not performed in this thesis and they are recommented for future studies Scientific and practical meaning of the topic: - Apply advanced theories of statistics analysis and reliability to propose a pattern to determine resistance factors of drilled shafts based on statistics data of the ratio between real measured values and estimated values for Resistance (R) and load effect (Q) - The thesis has analyzed and determined statistics characteristics of the ratio between real measured values and estimated values; to determine resistance coefficients for the four methods from 24 static load test results of drilled shafts constructed in cohesive and discrete composite soils in HoChi-Minh city subjected to static axial compression and other applicable data - Research results of the thesis can be used as reference documents in design bearing capacity of drilled shafts used for bridge substructures constructed in Ho-Chi-Minh city or similarly geological areas Chapter GENERAL 1.1 Drilled shafts and its application in infrastructure construction 1.1.1 Definitions, structural characteristics and technology Drilled shafts of bridge substructures (Drilled Shafts) : are a part of piers and abutments; they are constructed by raw concrete casting in pre-bored holes with or without steel case inside The piles are subjected to loads transferred from foundation foots and then transfers the loads into surrounding soil base Wet drilled shafts construction method (wet method): to drill holes and to cast the piles in water or in bore mud and a temporary tube wall segment is put in the boring hole top Applicable for cohesive, discrete and high groundwater level areas Drilled shafts cross-section maybe cylindrically constant throughout the pile length, this pile type is called simple one; or cylindrical-shaped but widened at bore hole tip area 1.1.2 The utilization of the pile in Vietnam and in the world Through analysis, the author recognizes the need to use the drilled shafts is growing both in Vietnam and in the world Almost foundation solutions for traffic, civil and industries from medium to large scale in Vietnam are using drilled shafts foundations 1.1.3 Current status and characteristics of drilled shafts used in HCM City Through analysis, the drilled shafts foundation for constructions here is also applied a lot in recent years Most of the piles are constructed by the wet method (in bentonite) through the mixture soil layers with combined cohesive and discrete soils, these layers can be weak, average or good in load bearing 1.1.4 Some structural characteristics, drilled shafts technology in Vietnam Due to the characteristics of the technology, the complexity of geology; experience level of the participants in the management, design and construction limits and especially the system of processes, standards are still in the process of integration and are not complete and existing many problems Therefore, the quality of the drilled shafts or pile resistance depends very much on the aforementioned elements 1.2 Design drilled shafts based on reliability according to Load and Resistance Factor Design method (LRFD) The design method LRFD is based on reliability,as the load effects with their particular factors (Qtk) shall not exceed the resistance with their particular factors (Rtk) Through analysis of the historical development of the design philosophies and design standards such as Allowable Stress Design (ASD), Limited States Design or Load Factor Design (LSD; LFD), Reliability- Based Design (RBD) and the method with reliability factors separately or Load and Resistance Factor Design (LRFD), the author found that design calculations of drilled shafts foundation according LRFD method is an advanced method and have being trusted and applied by many countries in the world 1.3 Analysis of literature to determine drilled shafts resistance factors based on reliability used for bridge substructures in the world 1.4 Analysis of literature showing the LRFD application and determining resistance factors for bridge design in Vietnam 1.5 Current challenging problems Some current problems in bridge design standards 22TCN272-05 and AASHTO LRFD 2012 (2007) are shown in Table 1.1 Table 1.1 List of current problems in the standards 22TCN272-05 and AASHTO LRFD 2012 (2007) Problem 22TCN272-05 AASHTO LRFD 2012 (2007) Method for determination 05 methods existed 01 method of resistance in cohesive and from before 1988 O'Neill&Reese (1999) discrete soil Resistance factors are not Sandy soil, cohesive Cohesive and discrete soil specified for: and discrete soil Officially Applied Year 2005 2007 Many method based Determination of ultimate 5% pile diameter or merged on TCXDVN269resistance under static load settlement pile 2002 The resistance factors are not the standard values for Recomendations when all states of America and of course not accurate for resistance factors used other countries, including Vietnam Some shortcomings of the related scientific studies: - The study of resistance factors correction of deep foundation of Paikowsky et al (2004): Didn’t mention the resistance factors of method O'Neill & Reese (1999), just mention method Reese & O'Neill (1988) for sandy and clay mixture soil conditions on the basis of 44 load test results of drilled shafts in Florida - Liang (2009): Proposed resistance factors for method O'Neill & Reese (1999), but only suggested for sandy and clay conditions in the U.S - Murad et al (2013): Proposed resistance factors for method O'Neill & Reese (1999) for mixed cohesive and discrete soil condition in Louisiana & Mississippi on the basis of 34 pile load test results, but there were 26 values extrapolated to static load test results due to not try to break the pile - There is no study regarding the research objectives of this thesis in Vietnam From the above-mentioned problems, the author proposes the targets, content and research methodology of the thesis as decribed in items 1.6 and 1.7 1.6 Targets of the topic Quantitative study of factors affecting the estimated resistance results of the four methods compared with actual field resistance of drilled shafts under the ground conditions in the area of HCMC This means that the author has determined the statistics characteristics of the ratio of the real measured resistance and the expected one (resistance bias factor, λR); To research the basis of determining the resistance factors and to propose the resistance factors for drilled shafts foundations of bridge substructures in HCMC area for the four methods 1.7 Content and Research Methodology To research the basis of determining the resistance factors for drilled shafts using probability and statistics theory and advanced reliability theory Specifically, the survey collected from 24 results of static pile load tests in HCM City, the author conducted a study to identify typical statistics of the ratio of the measured and estimated resistances (Resistance bias factor, λR); From that way, the authod determined the resistance factors for the four methods on the basis of reliability analysis Chapter DETERMINATION OF RESISTANCE FACTORS OF DRILLED SHAFTS BASED ON RELIABILITY THEORY According to AASHTO LRFD, drilled shalfs axial resistance factors according to soil base strength condition are factors determined based on the statistical characteristics of the nominal resistance, mainly calculated from the variability of characteristic parameters of the ground around the pile, the pile size, level of expertise (professional) of human - device participating in the implementation phase of the project and the uncertainty of prediction method for nominal resistance; but also related to the statistical characteristics of load effects through the identification process 2.1 Method to analyze the statistical characteristics 2.1.1 Determination of minimum size of samples Sample size is zα /2 + zθ )2 estimated by: n (= = (ε /σ ) C (2.1) (ES)2 In which: σ and zα/2, zθ: common standard deviation and standard deviation with error probabilities α, θ from the normal distribution; ɛ: allowable error; C: is a constant related to error probability Type I and Type II For example, to determine the sample size for the thesis: With some prediction methods of drilled shafts resistance that accept averaged estimated error of about 50% (=1/FS, FS=2: safety factor) with reliable interval of 0,95 (i.e., α=0,05) and θ = 0,2 Previous studies indicate standard deviations of the resistance bias factor from 0,27 to 0,74 Thus, the effect factor is: ES = 0,5/0,74 = 0,456 and C=7,85 By applying the formula (2.1) to estimate the required sample size for the study: 7,85 = n = 17, > 17( samples ) ( 0,5 / 0, 74 )2 To compare with recommendation of Murad (2013), the number of test piles for the study area at least is ≥ 20 piles Thus, with 24 results of static axial compressive load tests for drilled shafts in Ho Chi Minh City area can be considered reliable enough for analysis in order to meet the research objectives of the thesis 2.1.2 Testing method of suitable probability distribution for the random bias factor Through analysis, the Shapiro-Wilk method or the Pearson chi-square (when the sample size is less than 50) is recommended with the following principles: the empirical distribution consists with assumed theoretical distribution (standard or logarithmic, ) when the match probability (P) is greater than 0.05 2.1.3 Correction method for statistical characteristics of random bias factor For foundation structures, the laws of probability distributions of random bias factor often match or nearly match the normal standard distribution or standard logarithm Through research, the authod proposes two correction methods of statistical characteristics for logarithmic distribution form according to the the principle (Allen, 2005): Based on the graph of the cumulative probability function to examine the conformity with one of the two cases, 1) consistent with the entire collection data (FTAD method -fit to All data) or 2) only consistent with the area of small values at Figure 2.1 Cumulative probability distribution tail (BFTT-Best method density function of resistance bias fit to tail) (Figure 2.1) factor 2.2 Reliability Analysis Method When analyzing the reliability, the incident probability is the condition that the limited state has been reached The adjustment factors are selected to ensure that incident probability of each limited state is very small and acceptable The probability density functions of load effects (Q) and resistance (R) with the assumption of two independent normally distributed variables (Figure 2.2) Safety range or the safety factor is the difference between R and Q, the quantitative quantity for the safety is reliability or safety probability, Ps: (2.2) Ps = R > Q ) = G =- Q > 0) =( β ) P( P( R Φ Incident probability: Pf is calculated as: (2.3) Pf = P ( G < ) = 1- Ps = − Φ ( β ) In which: Φ(.): normalized distribution functions; β: index of reliability Index of reliability is determined based on averaged number and standard deviation as follows: µ R - µQ (2.4) µG = β = σG 2 σ R + σQ Figure 2.2 Normalized distribution probability density functions Figure 2.3 Normal Logarithm distribution probability density function If R and Q follows the normal logarithm distribution, safety range, G, is determined as follows: (Figure 2.3): G=ln(R)-ln(Q)=ln(R/Q) (2.5) Here, β is determined as the ratio of logarithm averaged number G and logarithm standard deviation, ξG G (2.6) β= ξG 2.3 Methods to determine pile body resistances The thesis has researched four methods to determine the pile body resistance: Method in accordance with the safety factor of the design philosophy of allowable stress (ASD); first-order secondary moment method (FOSM); First-order reliability method (FOSM); Monte Carlo method (MCS) After analyzing the advantages and disadvantages of these four method, the author proposes to select Monte Carlo analysis method to determine the resistance factorss Safety range, G, is applied to determine resistance factors as R and Q follow the normal logarithm distribution: QD + γ L) QL f(R, Q= G ln ) = Q ϕ ( λD D + λL ) QL λR (γ D (2.7) 2.4 Propose a procedure and pattern to determine the resistance factors The procedure and pattern to determine the pile resistance factors comply with the ensurement basis of target reliability as follows: To determine limited state according to soil base strength conditions for drilled shafts (22TCN272-05, AASHTO LRFD), strength state function: g(R,Q)=ϕR – (γDQD+γLQL)= λR(γDk+γL)/ϕ - (λDk+ λL); To select statistical parameters of design load effect (Q) and load factors: the representive is static load bias factor (λD) and live load effect bias factor (λL) complied with the standard AASHTO LRFD To analyze the statistical characteristics of resistance (R): the representive is resistance bias factor, λR, which is the ratio of measured ultimate resistance (Rtd) and predicted nominal resistance (Rdt): a To determine the measured ultimate resistance Rtd from results of pile static load tests according to soil base condition, this is the trial load value at a settlement of 5% of pile diameter or merged settlement pile (AASHTO LRFD 2012, TCVN 9393-2012); b To predict the nominal resistance (Rdt) based on calculation theory; c To determine the resistance bias factor, λR=Rtd/Rdt; d To analize, calculate the statistical parameters (μ, σ) and to verify the form of distribution density function (standard, logarithm, ) suitable for λR; To analyze and to determine the resistance factors of drilled shafts (ϕ) on the basis of analyzing reliability follwing Monte Carlo method with the target reliability index satisfied, βt; To recommend to correct the resistance factors for calculation method The above procedure is shown in Figure 2.4 11 The formula to determine the unit resistance at the pile tip and pile shaft according to the two standards are briefly introduced in Table 3.1 and Table 3.2 Table 3.1 Summary of formula to determine nominal unit resistance of drilled shafts according to 22TCN 272-05 and AASHTO LRFD 2012 22TCN 272-05 (brief RO88-272) AASHTO LRFD 2012 ( brief OR99-AL12) Unit shaft Unit tip resistance, qp Unit shaft resistance, qs Unit tip resistance, qp resistance, qs Cohesive soil (clay, soil with clay dust content higher 50%) qp=Nc Su ≤4 (MPa), qp=Nc Su ≤4 (MPa), qs= α Su (MPa) qs= α Su (MPa), where: where: where: Su(MPa) α α =0,55, với Su / pa ≤ 1,5 N c = Z / D )] ≤ 6[1 + 0,2( N c = Z / D )] ≤ , 6[1 + 0,2(

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Mục lục

  • INTRODUCTION

  • Chapter 1. GENERAL

    • Drilled shafts and its application in infrastructure construction

      • Definitions, structural characteristics and technology

      • The utilization of the pile in Vietnam and in the world

      • Current status and characteristics of drilled shafts used in HCM City

      • Some structural characteristics, drilled shafts technology in Vietnam

    • Design drilled shafts based on reliability according to Load and Resistance Factor Design method (LRFD)

    • Analysis of literature to determine drilled shafts resistance factors based on reliability used for bridge substructures in the world

    • Analysis of literature showing the LRFD application and determining resistance factors for bridge design in Vietnam

    • Current challenging problems

    • Targets of the topic

    • Content and Research Methodology

  • Chapter 2. DETERMINATION OF RESISTANCE FACTORS OF DRILLED SHAFTS BASED ON RELIABILITY THEORY

    • Method to analyze the statistical characteristics

    • Determination of minimum size of samples

    • Testing method of suitable probability distribution for the random bias factor

    • Correction method for statistical characteristics of random bias factor

    • Reliability Analysis Method

    • Methods to determine pile body resistances

    • Propose a procedure and pattern to determine the resistance factors

    • Uncertainty factors and statistical characteristics of load effect

    • Uncertainties affecting to drilled shafts resistance

    • Analyzing selection of methods to predict drilled shafts resistance

    • Selection of method to determine actual measured ultimate resistance of drilled shafts

    • Analyzing the statistical characteristics for resistance bias factor of drilled shafts based on soil base strength in Ho Chi Minh City

      • Survey to collect data base of static axial compressive load tests to serve for current research.

      • Table 3.4. Synthetic table of survey data of experimental results of drilled shafts under static load test in HCMC area and comparision with a number of research works of foreign authors

      • Analysis of data statistic characteristics

      • Table 3.5. Actual measured and predicted nominal resistances, statistical characteristics of resistance bias factor (λR) of drilled shafts according to 4 methods for 24 piles under static load tests

    • Determining statistical characteristics of parameters that affect to determination of resistance factors of drilled shafts

    • Results obtained from Chapter 3

  • Chapter 4. DETERMINATION AND PROPOSAL OF RESISTANCE FACTORS OF DRILLED SHAFTS ACCORDING TO SOIL BASE STRENGTH IN HO CHI MINH CITY

    • Selection and proposal of target reliability index for drilled shafts design

    • Determination of axial resistance factors of drilled shafts according to soil base strength

    • Evaluation and comparison of resistance factors in current applying standards and results of present thesis

    • Results obtained from Chapter 4

  • GENERAL CONCLUSION

  • PUBLICATIONS

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