TÓM tắt TIẾNG ANH một mô hình tính lún của móng cọc có xét đến phân bố của ma sát dọc thân cọc

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF CIVIL ENGINEERING DUONG DIEP THUY A METHOD FOR ESTIMATING PILE GROUP SETTLEMENT CONSIDERING DISTRIBUTION OF PILE SHAFT FRICTION MAJOR: GEOTECHNICAL CODE: 62580211 SUMMARY OF DOCTORAL DISSERTATION HA NOI, 2018 The doctoral dissertation has finished at Hanoi University of Civil Engineering Science Instructor: AProf Dr Pham Quang Hung – Ministry Of Education And Training Dr Le Thiet Trung – National University of Civil Engineering Reviewer 1: Prof Dr Do Nhu Trang Reviewer 2: Prof Dr Trinh Minh Thu Reviewer 3: AProf Dr Doan The Tuong The doctoral dissertation will be defended at the level of the State Council of Dissertation Assessment's meeting at the National University of Civil Engineering At…… hour……., day………month ……….year 2018 The dissertation is available for reference at the libraries as follows: - National Library of Vietnam; - Library of National University of Civil Engineering; HA NOI, 2018 INTRODUCTORY THE PURPOSE OF THE DISSERTATION - Author studied the τ-z curves (the relationships between the unit shaft friction τ versus the pile movement z) and q-z curves (presents the pile end unit bearing q versus the pile toe movement z) based on pile load test results which used strain gauges along pile in numerous major projects in Vietnam, and recommended the τ-z an qz curves be applied for some soil types in Vietnam - Suggested a method for estimating pile group settlement considering distribution of pile shaft friction - Develop a calculation algorithm for the proposed method and write a calculation program STUDY SUBJECTS AND RESEARCH SCOPE 2.1 Study subjects - Model of settlement for pile group 2.2 Research Scope - The piles are constructed in homogeneous and layered layers; - The stiffness and section of pile is constan along pile; - The ground is considered elastic half space; - The piles bear the entire work load; - No consideration of negative friction when pile and pile group working in weak clay STUDY METHODOLOGY - Research theories: + Overview of the working of pile group; + Recommended the τ-z and q-z curves be applied for some soil types in Vietnam; + Distribute the friction components along pile and bearing pile tip + Write a calculation program - Theoretical and analytical calculations: + Based on the proposed settlement method, try to calculate with three ground cases and compare these cases with the calculation methods being used The hypothetical ground case is intended to clearly see the advantages of the proposed method; + Compare results from the proposed model with data from real experiments and monitoring results of some projects in Vietnam RESEARCH CONTENT - Overview of the settlement methods are used in Vietnam and the world From that orientate for the PhD thesis; - Recommended the τ-z and q-z curves be applied for some soil types in Vietnam based on pile load test and O’cell test results which used strain gauges along pile in numerous major projects in Vietnam; - Describe the method of settlement calculation considering the distribution of the friction component along the pile; - Evaluation of advantages and disadvantages of the method base on the calculation results in some hypothesis cases; - Verify proposed method by comparison with the results of measurement of single piles and pile group of some authors have been published in Geotechnical journals These measurements are made with different pile sizes and geologic conditions; - Comparison of calculation results with monitoring settlement data of some projects in Vietnam NEW CONTRIBUTIONS OF THE DISSERTATION - Study the method of settlement calculation considering the distribution of the friction component along the pile base on τ-z and q-z curves - Write a calculation program base on proposed method THE STRUCTURE OF THE DISSERTATION In addition to the introduction, table of contents, list of references, scientific published, drawings, tables, appendices, thesis consists of 148 pages is organized in five chapters: Preamble; Chapter 1: Overview of settlement methods and pile-soil interaction model; Chapter 2: Analyze and compare the (t-z) and (q-z) curves of some soil types; Chapter 3: A Settlement Considering Distribution of Pile Shaft Friction (SDF); Chapter 4: Analysis and evaluation of settlement results of SDF method; Conclusions and recommendations List of Published works References Appendix calculations CHAPTER 1: OVERVIEW OF SETTLEMENT METHODS AND PILE-SOIL INTERACTION MODEL 1.1 OVERVIEW OF SETTLEMENT METHODS The results show that, due to the interaction between piles in pile group and the interaction between the piles and the ground, the settlement of a pile group differs significantly from that of a single pile at the same average load level The effects generated in the pile group should be considered when studying the behavior and design of pile foundation In addition, studies indicate that the depth and area affected by the soil under the pile tip depend on the size of the group and the magnitude of the load At present, there are many models to forecast the settlement of the pile group from simple to complex In particular, common methods used to predict settlement of pile group can be as follows: 1.1.1 Emprical and semi emprical method Most empirical methods are based on real experiments or experimental models The semi-empirical method combines the theoretical calculation with the experimental results The methods can be mentioned as: settlement ratio, method based on results from CPT, SPT experiments 1.1.2 Equivalent raft method The method focused to determine size of equivalent raft at tip pile level The additional stresses in soil due to the building load can then be determined by the theory of Bousinesq, or by the “2:1 method” 1.1.3 Interaction factor method According to this method, the authors propose how to determine the settlement of single piles and the factors of interaction between the piles from which the settlement of piles in the group is determined Studies have been made taking into account various interactions Some studies may be as follows: Poulos & Davis, 1980 [73]; Zhang & Lee, 2010 [90]); Hain & Lee (1978) [56]; Randolph & Wroth (1979) [78]; O’Neill & Ha (1982) [69]; Mylonakis & Gazetas (1998) [66] 1.1.4 Numerical methods Numerical methods used in Geotechnical problems include finite element method (FEM) and finite difference method (FDM) FEM and FDM methods are numerical methods for solving problems described by partial differential equations along with specific boundary conditions 1.2 PILE – SOIL INTERACTION MODEL 1.2.1 Some assumptions of pile-soil interaction models by τ-z and q-z curves The pile is assumed to be in contact with the surrounding soil over the entire length of the pile The pile depth and total pile length are equal The pile has a vertical axis (the z axis has a positive direction downward) and is loaded at the center of the pile P0 Parallel displacement axes of the pile are denoted z and positive in positive direction Z The pile material is assumed to be linear elastic at all loads The soil around the pile is considered to consist of homogeneous layers In most cases, the heterogeneous influence of horizontal layers is measured by estimation 1.2.2 τ-z curve in the sand soil The τ-z curve by Coyle & Sulaiman (1967) [46] is one of the first suggestions for the τ-z curve The authors conducted experiments on small piles in sand and correlated the results of laboratory experiments with the field results of piles in sand There are many τ-z curves that have been studied and developed, such as Vijayvergiya (1977) [88]; Kraft, Ray & Kagawa (1981) [57]; Mosher (1984) [63]; Briaud & Tucker (1984) [40]; API (1993) [36]; Zhang and He (2010) [90] 1.2.3 τ-z curve in the clay soil From the results of the static load test of the pile in the clay and the results of the experiments, Coyle & Reese [45] proposed three load transmission curves Vijayvergiya [88], Kraft, Ray & Kagawa [57] indicate that the τ-z curve for piles working in sand can be used for piles in clay Aschenbrener & Olson [37] based on the static field test results of the piles lowered in clay to find the relationship between load and pile displacement with different soil properties Finite element analysis and finite difference were performed by Heydinger & O'Neill [54] to construct a τ-z curve for piles in clay 1.2.4 Resistance of pile tip Mosher (1984) [63], [64] extends the study of Coyle & Castello (1981) [47] to determine the q-z relationship for piles in sand Briaud & Tucker [41] gives the calculation of the residual stress at the pile tip during the application Vijayvergiya [88] suggested that the q-z curve for piles worked in sand and could be used for piles in clay The curves of Zhang and He [90] are also recommended for sandy soil and clay 1.3 CONCLUSION CHAPTER Some research above shows that: The empirical formulas have the advantage of being simple and easy to use as they depend only on the size geometry should only be applied when calculating preliminary; Equivalent raft method is suitable with small group pile foundation; Numerical methods allow for the overall problem model and are considered the best method today However, the calculation using software based on the base model and the input parameters for the base model requires the engineer to have a lot of experience; Interaction factor method, from determine the settlement of single piles and the factors of interaction between the piles from which the settlement of piles in the group is determined The addition of displacement requires the ground beneath the pile to be homogeneous The tasks set out for the thesis are as follows: - Study the method of settlement calculation considering the distribution of the friction component along the pile; Recommends using the τ-z and q-z curves based on O'cell's experiments; Write a calculation program base on proposed method; Verify method proposed CHAPTER 2: ANALYZE AND COMPARE THE τ-z AND q-z CURVES OF SOME SOIL TYPES 2.1 SOME LOCATIONS OF TEST Based on data from static load test and O'cell, which has attached strain gauge along pile at some project in Hanoi such as Vietinbank Tower - North Thang Long [16], [21], [22]; Petroleum Hotel - Tu Liem [14]; IndoChina - Xuan Thuy Cau Giay [14]; U Silk city - Van Khe - Ha Dong [19], [20]; Pearl Southern Tower - Truong Chinh - Thanh Xuan [17], [18]; Pacific plaza project - Ly Thuong Kiet - Hoan Kiem [14]; "Determining the unit friction resistance for bored piles and barette piles for some typical types of land in Hanoi" with the results of different projects (Nguyen Bao Viet, Pham Quang Hung - 2014) [23] Based on the experiment data of some above mentioned projects, the author has compared the models of τ-z and q-z to choose suitable model for soil in Hanoi area 2.2 τ-z AND q-z CURVES FOR SOME KIND OF SOIL 2.2.1 τ-z curves - With clay soil, most of the survey models give the largest unit friction resistance equal to the value of the undrained shear strength For design calculations, it is possible to use curves in accordance with Vijayvergiya [88] or Heydinger & O'Neill [54] with the addition of the ratio of displacement diameter piles to safety 0.6% according to Hoang Thanh Hai [14] - For most sandy soils, the proposed the unit friction values ranging from 47 kPa to 115 kPa are quite suitable for the ground in Hanoi In these cases we not have experiment the recommended τu value can be used It is possible to use curves in accordance with API [36] and Vijayvergiya [88], which are suitable for sandy soils in Hanoi The zu values are based on small-diameter pile experiments, so with large pile diameter have to adjuste Can be used displacement / pile diameter ratio be about 0,65% with sandy soil having a friction angle ϕ = (15o-20o), equal to 0,55% with friction angle ϕ = (20o-30o) and 0,4% with sandy soil with friction angle ϕ = (30o-35o) - With sand and gravels from the experimental sites in Hanoi, the highest unit friction value ranged from 80 kPa to 250 kPa Can be used API [36] and Vijayvergiya [88] with a zu displacement ranged from to mm - With gravels in some of the experimental sites in Hanoi, the highest unit friction value ranged from 200 kPa to 400 kPa Can be used API [36] and Vijayvergiya [88] with a zu displacement ranged from to mm 2.2.2 q-z curves - From experimental data of gravels, gravels and sand, it was found that the ratio of zu/D to gravels, gravels and sand in Hanoi was between 3% and 7% - Vijayvergiya curve [88] can be used with an exponent of 1/1,5 - Due to the limited number of experimental piles, the results may not yet be representative of gravel, gravel and sand in Hanoi CHAPTER 3: A SETTLEMENT CONSIDERING DISTRIBUTION OF PILE SHAFT FRICTION (SDF) 3.1 SETTLEMENT METHOD SDF To determine the settlement of pile foundation, SDF method distributes the load acting on the pile, based on the τ-z and q-z curves, into forces located at the center of the piles and pile tip All forces place in the soil The coordinates of each pile to build 3D model Using the Mindlin's equation to determine the stress at any point caused in the ground The distribution of the friction component along the pile and the stress calculation is based on the interaction between the pile and the soil Figure 3.1 decribed the vertical force distributions in single piles and 3D models 3.1.1 Assumptions of the method - The ground is considered elastic half space; - The piles bear the entire work load 3.1.2 Input parameters + Geological conditions include number of soil layers, soil name, soil layer thickness, level of groundwater, τ-z and q-z curves, poisson coefficient, E of soil layers; + Pile parameters include type of pile, pile size, pile length, pile foundation size and pile layout in pile cap; + Load acting on the pile cap; Figure 3-1 Distribute the force along pile and 3D of SDF method + Parameters input, such as pile size segment, hypothetical displacement, number of repetitions 3.1.3 Calculation steps: Step 1: Determine the force on the pile head Step 2: Determine the τ-z and q-z curve for layers Step 3: Divide a pile equally into n segments Length of each segment is dh = L/n, in which L is the length of the pile Each segment must be located within one soil layer Step 4: Distribute the force in the pile To distribute the force P0,j in the pile into friction components along the pile Fi,j (the friction force corresponds to the ith pile of the pile j) located at the center of the pile segments and resistance of pile tip Pt,j The selection of the τ-z and q-z cuvers influenced greatly to the stresses under the pile tip and settlement of the piles If the friction force of the ith is Fi,j and end bearing is Pt,j, the n force balancing for the piles: P = ∑ F + P 0, j i, j t, j i =1 end of each iteration, we have friction and tip resistance at step i and i-1 By interpolating from two steps, we can determine friction forces and pile tip resistance corresponding to P0 P0 P0 P0 f0 f0 τ –z P1 Fi P1 f1 f1 τ– z Top Bottom … P2 Pn q– z fn fn Pt Toe of pile b) a) Figure 3-6 Distribute force in the pile Pt c) (a) τ-z and q-z curves; (b) pile segments; (c) Result If P1,0 > P0, settlement z1 can be assumed again If z1 is insignificant (less than 10-8 mm for example) but P1,0 still larger than P0 then we can ignore the tip resistance and proceed the calculation with only friction at pile segments Pile segments near pile tip can be omitted until the P1,0 ≤ P0 condition at the first step is found) This condition occurs when the force P0 at the pile head is insignificant Friction components of upper soil layers have mobilized enough strength and friction and/or tip resistance of the soil layer below are not mobilized Step 5: 3D Model From step 1, the force is determined on the pile In the case of load acting on the piles and geological conditions within the piles is the same, from the concentrated forces Fi,j and Pt,j were calculated in a single pile applied to all piles in the group From the x and y 10 coordinates of each pile to build 3D model (figure 3.6c) Thus, in this case, it is only necessary to distribute the friction force for one pile and apply to all piles In the case of load on piles, pile length and/or geology within pile cap is different, the load distribution process must be performed m times (corresponding to the number of piles) from step to step Step 6: Calculate stress Used the Mindlin [62] equation as below: - After determining friction components Fi,j at the center of the pile segments and pile tip resistance Pt,j at the pile tip, based on the coordinates of the piles to build 3D model The point of stress calculation can be center or other points - Using the Mindlin's equation to determine the stress at any point caused in the ground as 3.5 equation m n σ z = ∑∑ i =1 j =1 P (Fi, j ,1 − Fi, j ,2 + Fi, j ,3 + Fi, j, + Fi, j,5 ) 8π (1 −υ) (3.5) In where: From Fi,j,1 to Fi,j,5 values are determined by Mindlin [62] corresponding to the ith pile and jth pile segment Step 7: Calculate settlement of soil layers below the pile tip plane After having a diagram of the stress distribution in the soil (below the pile tip), the settlement is calculated according to the current methods The settlement of the soil under the pile tip is S2 3.1.4 Some comments about SDF method The SDF method distributes the load acting on the pile, based on the τ-z and q-z curves, into forces located at the center of the piles and pile tip Using the Mindlin's equation to determine the stress at any point caused in the ground The method considers the interaction between the pile and the soil at any stress state – deformation In the majority of cases pile length and force at the pile head is the same The SDF method in this case is simple and the quick calculation time In some cases, when large pile groups can use software such as Sap2000 to distribute loads to the piles, the length of the piles and 11 the geology within the piles are also different At this case, we have to distribute the friction component and the pile tip resistance m times (m is the number of piles in the group) The calculation time and the declared volume will be very large As such, the SDF method will not be limited by the length of the piles or geology within the range of piles The applicability of the SDF method is high because it does not require a lot of parameters The τ-z and q-z curves can be used theoretically, but if there are experimental results, the results will be optimized The SDF method still has some disadvantages such as soil model and the results depend on the choice of the τ-z and q-z curves However, without the τ-z and q-z curve from the experiment, the selection of the τ-z and q-z curve according to the recommendations of the available studies was still good 3.2 PROGRAM SEDIF 1.0 3.2.1 Scientific basis, purpose and programming language choice SeDiF 1.0 is designed to automate computational steps that have been made, to build a graphical interface, and to include in the program the results of the thesis such as the τ-z and q-z curves, calculate stress and settlement of pile group Program helps to calculate design quickly and efficiently The programming language used to write program is Visual Basic.net This language with many utilities such as using the net environment can import data directly from the interface or import the calculated data from the Exel file 3.2.2 Functions and interface of SeDiF 1.0 program 3.2.2.1 Functions of SeDiF 1.0 program File: Allows to set new, open existing calculation files, save the computed file or save to new file, exit the program Data: Enter the data to calculate the include: Soil; q-z; τ-z; Pile parameters: Enter the information such as pile shape (pile, square, ring) and pile material (concrete, steel or reinforced concrete), pile section, material and pile layout (if 12 reinforced concrete piles), pile length; Parameter number of data to calculate include Stress: Parameters to run the program; Distribution of friction on each pile; Drawing friction graph, vertical force graph; Stress and stress graph Settlement Caculation: Calculated by Egorov; Steinbrenner; one-way compacting; settlement consolidation methods Help: How to use the software and explain the parameters 3.2.4.2 Some interfaces of the SeDiF 1.0 program Figure 4-20 Input soil data Figure 4-25 Stress calculation CHAPTER ANALYSIS AND EVALUATION OF SETTLEMENT RESULTS OF SDF METHOD 4.1 CALCUTATION WITH ASSUMPTION SOIL Consider two models of calculation: Group of 16 piles; and group of piles All piles in the model are 400x400mm, the distance between the piles is 1.2m (3D) The load acting on the pile is 100 tons Proplem (BT1): One layer (2) Proplem (BT2): Two layers (2) – (1); Proplem (BT3): Two layers (1) – (2) All three problems will be calculated with cases of foundation with the number are 16 piles and piles 13 a) b) (2) Sandy Medium dense c) (2) Sandy Medium dense (1) Soft clay (a) Proplem - (b) Proplem (1) Soft clay (2) Sandy Medium dense (c) Proplem Figure 4-3 Three cases calculation Settlement calculation result Table 4-2 Calculation result for model (16 piles) Soil BT BT BT SP (mm) 30,2 167,8 32,0 Mosher (mm) 5,4 5,4 5,3 Vijayvergiya (mm) - Table 4-3 Calculation result for model (6 piles) Soil SP (mm) Mosher (mm) Vijayvergiya (mm) BT 11,3 1,9 BT 29,5 1,9 BT 11,3 1,94 + The distribution of friction in piles greatly affects the stress and settlement Specifically, with different τ-z curves, the maximum friction values τu and the maximum displacement zu between the Mosher and Vijayvergiya curves are different + Stresses according to the proposed method in all cases are much smaller than the equivalent raft method This difference increases as the number of piles increases This can be explained by the expansion an angle of ϕ/4 would significantly reduce the stress with the small pile But there is no significant reduction in stress with the large pile group 14 + Problem (BT1) and Problem (BT2) will have the same result on the distribution of the friction component along piles Selfstress in two cases is a bit different + The settlement of BT1 and BT2 is very large difference due to weak soil layer (1) + Even if the above ground is weak (Problem 3), the friction of mobilization also significantly reduces the settlement of the pile group 4.2 EVALUATION BETWEEN SDF METHOD AND REAL TEST RESULT 4.2.1 Yasunori Koizumi test result Yasunori Koizumi (1967) 3d 3d [56] analyzed the influence of the surrounding soil when 3d constructed the piles and 3d compared the settlement of single pile and pile group 10 Single piles were tested 4,2 m from pile group Experimental τ (kPa) 1,3m piles are steel piles with a 20 kPa diameter of 300mm and 1,7m Silty sand 24 kPa thickness of 1,6mm and length of 5,55m The spacing of the 24 kPa piles is 900mm from center to Mud clay 40 kPa center (3D) The pile is 1,3m 3,85m 30 kPa from the ground Single pile is 25 kPa numbered and the remaining piles in the group are numbered from to 10 Figure 4.11 shows Figure 4-11 The pile layout the pile layout max The τ-z curves are shown in Figure 4.15 with the largest displacement zu of mm The q-z curve is shown in Figure 4.16 with the largest displacement of 3% of the pile diameter 15 25 20 20 15 15 τ (kPa) τ (kPa) 25 10 10 t-z at 0,5m - 1,5m depth 5 t-z at 0m - 0,5m depth 0 10 15 Displacement z (mm) 20 10 Displacement z (mm) 15 20 a) The τ-z curve at 0m – 0,5m depth b) The τ-z curve at 0,5m – 1,5m depth 30 35 25 30 25 τ (kPa) 20 τ (kPa) 15 20 15 10 t-z at 1,5m - 2,5m depth 10 t-z at 2,5m - 3,5m depth 0 10 Displacement z (mm) 15 20 c) The τ-z curve at 1,5m – 2,5m depth 10 Displacement z (mm) 15 20 d) The τ-z curve at 2,5m – 3,5m depth 40 30 35 25 30 20 τ (kPa) τ (kPa) 25 20 15 15 10 t-z at 3,5m - 4,5m depth 10 t-z at 4,5m - 5,5m depth 5 0 10 Displacement z (mm) 15 20 0 10 15 Displacement z (mm) 20 e) The τ-z curve at 3,5m – 4,5m f) The τ-z curve at 4,5m – 5,5m depth depth Figure 4-15 The τ-z according to Vijayvergiya (1977) [88] 16 250 q (kPa) 200 150 100 q-z at 5,5m depth 50 0 10 15 Displacement z (mm) 20 Figure 4-16 The q-z curve according to Vijayvergiya (1977) 4.2.2 Comparison and evaluation After calculating the stress at the center of the foundation, settlement of pile group is calculated by the Berardi & Lancellotta (1991) [39] method The Es elastic modulus is 520 times the maximum shear resistance of the soil (according to Roberto Cairo (2006) [78]) Figures 4.17 and 4.18 show the mobilized friction of pile and the axial force in the pile Unit friction in pile (kN/m2) 10 20 30 40 Axial force in pile (kN) 50 100 150 200 0 P=154 kN P=145 kN 1 P=133,3 kN P=100 kN Depth (m) P=33,33 kN Depth (m) P=66,67 kN 2 P=154 kN P=145 kN 4 P=133,3 kN P=100 kN P=66,67 kN 5 6 Figure 4-17 Friction force in pile P=33,33 kN Figure 4-18 Axial force in the pile 17 Calculation results are shown in figure 4.19 The result from the SDF method was compared with the data from the experiment of Yasunori Koizumi [56] and the results of Roberto Cairo [78] Roberto Cairo [78] used a centrifuge to determine and test Koizumi's measurement results [56] Load (kN) 200 400 600 800 1000 1200 1400 1600 Displacement (mm) 10 15 Koizumi (1967) Robert Cairo (2006) 20 SDF method 25 Figure 4-19 Compare Koizumis, Robert Cairo and SDF results Use the proposed method SDF to calculate settlement of pile group (with the curve τ-z and q-z according to Vijayvergiga [88]) The results show that: At the small load stage, the results from the proposed method seem to be consistent with the actual measurement When the high load stage, there is a difference between the results calculated by the proposed method and the measurement results of the pile group The results obtained from the SDF method for settlement are smaller than Koizumi test The τ-z curve with the small displacement it has all the friction resistance and the q-z curve is difficult to determine In the case high load stage, the tip resistance component is principal In the plastic deformation phase, the pile is slipped in the soil at which the full load is due to the nose and the SDF method does not describe the work at this stage 4.3 PROJECT APARTMENT IN NEW URBAN AREA VAN GIANG, HUNG YEN 4.3.1 Introduction and geological conditions 18 The 1A-01 apartment building is located in the north of Van Giang new urban area, with the main function is apartment and commercial center on the 1st floor The 1A-01 tower consists of towers, including Tower and Tower 3, each of which consists of three blocks: 25 storeys, 22 storeys, 22 storeys, each with a size of 43,57m x 83,39m x 84,3m Tower consists of two blocks: 19 - 19 storeys with total dimensions of 36,80m x 55,91m x 66,30m Within the scope of the study, the authors only calculated with towers 1, and The height of blocks in the 1st floor, basement car and typical floor is 6m; 4,5m, and 4,2m Tháp Tower Tower Tháp 33 Tháp 55 Tower Tower Tháp 11 Tháp Tower Figure 4-68 The layout of the towers According to the report of the geological survey of the project in October 2009 [3], the geological characteristics at bored holes B4, B5, B6 can be shown in Table 4.15 Underground water level is from +2,82 to +3,35m Based on geological and hydro-geological conditions, the project is built on a complex stratum, with underground water level close to the natural ground The project is located near the Red River, in the ground may appear underground flow Therefore, the solution proposed by the design unit is to use a 45x45cm reinforced concrete pile foundation with a 32 m long pile (consisting of piles of 16,0 m) These piles are bonded and work together under a reinforced concrete foundation of 2,0 m The mobilizate friction and axial force in the pile are shown in figures 4.74 and figure 4.75 19 Unit Friction (kN/m2) Unit Friction (kN/m2) 50 100 150 0 5 10 10 Depth (m) Depth (m) 0 15 40 60 80 100 15 20 20 25 25 30 30 35 35 a) B1 bored hole 20 b) B3 bored hole Figure 4-74 Friction of mobilization along pile - tower Axial Force (kN) Axial Force (kN) 400 800 1200 1600 5 10 10 Depth (m) Dephth (m) 0 15 800 1200 1600 15 20 20 25 25 30 30 35 35 a) B1 bored hole 400 b) B3 bored hole Figure 4-75 Axial force in the pile - tower 20 The stress at the center of the pile (bored hole B1 and B3) is shown in figure 4.76 50 Stress (kPa) 100 150 200 32 50 Stress (kPa) 100 150 200 32 34 37 36 38 47 Depth (m) Depth (m) 42 40 42 44 52 46 57 48 50 62 a) B1 bored hole 52 b) B3 bored hole Figure 4-76 Axial force and stress - tower Chapter focuses on the comparative of SDF methodology through cases: 1) Calculation with assumption soil; 2) evaluation between SDF method and real test result; 3) Comparison of results by SDF method with monitoring data of some projects Some conclusions are given as follows: + The distribution of friction in the pile greatly influences the stress and settlement + At the small load stage, the results from the proposed method seem to be consistent with the actual measurement When the high load stage, there is a difference between the results calculated by the proposed method and the measurement results of the pile group However, the high load stage is almost non-existent in the actual working of the pile foundation so SDF method can be applied to calculate the design; + Comparison of results calculated by SDF method and monitoring data is different The cause of the SDF method has not 21 been completely described the real problem Despite the differences, the results of the calculation using the SDF method are acceptable CONCLUSIONS AND RECOMMENDATIONS CONCLUSION Studying the new settlement method for pile foundations has practical significance in predicting the state of deformation limitation: + To design can use τ-z curve according to Vijayvergiya [88] or Heydinger & O'Neill [54] for clay soil The largest friction resistance is equivalent to the undrained shear strength value and additionally the condition of the ratio between the maximum displacement / pile diameter is about 0,6% (according to Hoang Thanh Hai [14]) For sandy soil it is possible to use the τ-z curve according to API [36] and Vijayvergiya [88] The maximum of unit friction value τu ranged from 80 kPa to 250 kPa and additionally the condition of the ratio between the maximum displacement / pile diameter is about 0,65% for sandy soil with ϕ = (15o-20o), 0,55% for sandy soil with ϕ = (20o30o) and 0,4% for sandy soil with ϕ = (30o-35o); + Can be used q-z curve according to Vijayvergiya curve [88] with an exponent of 1/1,5 From experimental data of gravels, gravels and sand, it was found that the ratio of zu/D to gravels, gravels and sand in Hanoi was between 3% and 7%; + The proposed SDF method is based on the τ-z and q-z curve to distribute the friction component along the pile length and pile tip resistance It then calculates the stresses at any location in the ground The method takes into account the interaction between the pile – soil at the stress – deformation states; + Develop a calculation algorithm for the proposed method and write a calculation program SeDiF 1.0 has an intuitive interface, easy to use and reliable results; + At the small load stage, the results from the proposed method seem to be consistent with the actual measurement When the high load stage, there is a difference between the results calculated by the proposed method and the measurement results of the pile group However, the plastic deformation stage is almost non-existent in the 22 actual work of pile foundation Therefore, the SDF method is suitable for calculating the settlement design for pile foundation; + The applicability of the SDF method is good because it does not require many parameters TO PROPOSE THE SCOPE OF APPLICATION SUBJECT The reasonable scope of the method for the conditions is as follows: - The piles are constructed in homogeneous and layered layers; - The stiffness and section of pile is constan along pile; - The ground is considered elastic half space; - The piles bear the entire work load; - No consideration of negative friction when pile and pile group working in weak clay FURTHE RESEARCH DIRECTIONS Further study of the dissertation can be developed as followed: + Research on the influence of soil at the bottom of the pile cap; + The study offers better suggestions on τ-z and q-z curves for some type of soils in Vietnam; + Research to consider the interaction of piles - piles change the settlement of the pile itself from which the distribution of friction, pile tip will also change and return to impact on the settlement + Research applied to other pile materials LIST OF PUBLISHED WORKS Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2014) A new model for pile group settlement considering distribution of friction along pile Vietnam Geotechnical Journal, Vietnam Geotechnical Institute (VGI) No 1/2014 pp 42-49 ISSN 0868-279X Duong Diep Thuy, Pham Quang Hung (2014) Evaluation of some settlement calculation methods for pile group are being used in Viet Nam Review of Ministry of construction No 8/2014 pp 86-89 ISSN 0866-0762 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2015) Comparision of some models descript the relationship between the 23 unit friction mobilization and displacement of pile segment f-w for some kind of clay in Ha Noi Review of Ministry of construction No 7/2015 pp 76-79 ISSN 0866-0762 Duong Diep Thuy, Pham Quang Hung (2015) Verify neutral plane method in calculating settlement of pile group Journal of Science and Technology in civil Engineering No 23 -3/2015 pp: 6268 ISSN: 1859-2996 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung, Hoang Thanh Hai (2015) Comparision of some models descript the relationship between the unit friction mobilization and displacement of pile segment f-w for some kind of sand in Ha Noi National Mechanics Conference 2015 in Danang Danang Publishing House 8/2015, pp 541-550 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2016) Compare a model for pile group settlement considering distribution of friction along pile (SDF) and full – scale pile groups test Review of Ministry of construction No T1/2016 pp 132-135 ISSN 08660762 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2016) A Method for Estimating Pile Group Settlement Considering Distribution of Pile Shaft Friction (SDF) – Application for Pile Groups in Vietnam Geotechnical Engineering Journal of the SEAGS & AGSSEA Vol 47 No March 16 pp 69-78 ISSN 0046-5826 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2016) Determination of the unit shaft friction along pile from static load test Review of Ministry of Construction 11-2016 Pp 160-163 ISSN 0866-0762 Duong Diep Thuy, Pham Quang Hung, Le Thiet Trung (2016) Some problems of pile group settlement in Vietnam The 2nd National Cconference on transport infrastructure of vietnam with sustainable development Construction Publishing House T8/2016 pp 309-316 10 Duong Diep Thuy, Le Thiet Trung (2018) Study the relationship between the settlement of single pile and pile group Vietnam Journal of Construction – Copyright Vietnam Ministry of Construction 012018 Pp 83-87 ISSN 0866-8762 24 ... measured by estimation 1.2.2 τ-z curve in the sand soil The τ-z curve by Coyle & Sulaiman (1967) [46] is one of the first suggestions for the τ-z curve The authors conducted experiments on small piles... τ-z; Pile parameters: Enter the information such as pile shape (pile, square, ring) and pile material (concrete, steel or reinforced concrete), pile section, material and pile layout (if 12 reinforced... the maximum friction values τu and the maximum displacement zu between the Mosher and Vijayvergiya curves are different + Stresses according to the proposed method in all cases are much smaller