(Luận văn thạc sĩ) an evaluation of the effectiveness of head enlarged soil cement columns (HCC) in ground improvement

VIETNAM NATIONAL UNIVESITY HANOI VIETNAM JAPAN UNIVERSITY NGUYEN DUC TRUNG AN EVALUATION OF THE EFFECTIVENESS OF HEAD-ENLARGED SOIL CEMENT COLUMN (HCC) IN GROUND IMPROVEMENT MASTER’S THESIS Hanoi, 2019 VIETNAM NATIONAL UNIVESITY HANOI VIETNAM JAPAN UNIVERSITY NGUYEN DUC TRUNG AN EVALUATION OF THE EFFECTIVENESS OF HEAD-ENLARGED SOIL CEMENT COLUMN (HCC) IN GROUND IMPROVEMENT MASTER’S THESIS MAJOR: INFRASTRUCTURE ENGINEERING CODE: ………… RESEARCH SUPERVISOR: Dr NGUYEN TIEN DUNG Hanoi, 2019 TABLE OF CONTENTS ABSTRACT iii ACKNOWLEDGEMENTS iv LIST OF ABBREVIATIONS v LIST OF TABLES vi LIST OF FIGURES vii CHAPTER - INTRODUCTION 1.1 General introduction of deep mixing method 1.2 Necessity of research 1.3 Objective and scope 1.3.1 Objectives: 1.3.2 Scope of research: CHAPTER - LITERATURE REVIEW 2.1 Overview of cement deep mixing method 2.1.1 Brief view of deep mixing method 2.1.2 Application of CDM 2.1.3 Classification 2.1.4 Equipment and machine 2.1.5 Construction procedure 10 2.1.6 Fixed type and floating type improvement 11 2.2 Innovation of conventional CDM method 11 2.2.1 T-shaped soil-cement column 11 2.2.2 The PF method 13 2.3 Theory of settlement evaluation 14 2.3.1 Brief overview of settlement 14 2.3.2 The use of 1D, 2D and 3D model in evaluating settlement 17 2.4 The equivalent elastic modulus and 1D settlement of composite ground 31 2.4.1 Equivalent elastic modulus 31 2.4.2 One-dimensional settlement 32 i 2.5 A suggested method for evaluating the settlement of spread footing on improved ground 33 2.6 Verification from comparison of predict and measured load-settlement curves 33 CHAPTER - METHODOLOGY 35 3.1 The performance of research 35 3.2 The analyze of research proposal 37 3.2.1 The percentage of settlement reduction 37 3.2.1 The percentage of material reduction 38 3.3 Verification by load-settlement curve 38 3.3.1 Initial elastic modulus of composite ground 38 3.3.2 Bearing capacity of composite ground (qu,comp) 39 CHAPTER - SETTLEMENT ANALYSIS 40 4.1 Theoretical analysis 40 4.1.1 Ideal case and assumed parameters 40 4.1.2 Results and discussion 42 4.2 A case study 44 4.2.1 General information of project 44 4.2.2 Soil profile and footing parameters 44 4.2.3 Settlement analysis 46 4.2.4 Economic analysis 49 4.2.5 Load-settlement curve from calculation and measurement 50 CHAPTER - CONCLUSIONS 61 5.1 Conclusion 61 5.2 Limitation and suggestion 61 REFERENCES 63 ii ABSTRACT Cement deep mixing methods (CDM) are seen to be used widely for improving ground, especially for infrastructure project such as embankment, dam, quay, etc The principle of deep mixing method is mixing admixture with in-situ soil to create constant diameter mixed soil columns and therefore the soft ground is improved consequently It can be seen that, the load of these construction mentioned above is considered as large load area which mentioned in most CDM books and design manuals Whereby, the used of one-dimensional settlement (1D) is applied In recent years, the application of CDM for civil construction such as buildings, parking lots which use pad foundations has increased quickly It requires a research on estimating settlement of improved ground overlain by limited load area which cannot be seen widely Indeed, for limited applied load area such as spread footing, under 3D condition, load distributed significantly at upper layers and reduces along the depth of ground so that the conventional CDM showed its limitations when the constant diameter mixed soil columns improve soil equally In other words, both soft and stiff layers are improved equally instead of focusing on soft layers (which normally are upper layers) Consequently, the settlement could be increased by applying conventional CDM or soil cement column (SCC) for spread footing To overcome this circumscription, an optimal shaped of mixed soil column called head enlarged soil cement column (HCC) is studied for answering two questions: - What is the optimal shape of mixed soil columns that performs the minimum settlement of spread footing resting on improved ground? How to determine it? - What percentage of material can be saved by applying that optimal shape of mixed soil columns? Both theoretical and case studies were considered The results showed that, by applying HCC, settlement of spread footing could be reduced 10%, and material can be saved up to 12% iii ACKNOWLEDGEMENTS First and foremost, I gratefully appreciate Dr Nguyen Tien Dung, who has an incredible passion for geotechnical engineering He spent hours trying to explain to me anytime even when being very busy I cannot count how many times we discussed geotechnical engineering comfortably as friends Geotechnical engineering is attractive but complicated and I can understand how hard he had to undergo for interpreting for me He shows perseverance when supervising and inspiring me the passion for geotechnical engineering gradually I believe my progress has a huge contribution from him I am hugely indebted to him I would like to acknowledge the sincere inspiration from Prof Nguyen Dinh Duc and Prof Hironori Kato Their lectures covered not only specialist knowledge but also the responsibility and mission of a new generation of Vietnam I am grateful to Dr Phan Le Binh for his support in the last two years since I have studied at Vietnam Japan University Thanks to him, I have learned the professional courtesy of Japanese people as well as Japanese culture I would also like to thank Prof Junichi Koseki, Assoc Prof Kenji Watanabe, Assist Prof Hiroyuki Kyokawa as well as other members of Koseki lab, where I had 80 meaningful days internship at The University of Tokyo It was very helpful to me I would also like to acknowledge the staff of Vietnam Japan University, especially Mr Nguyen Ngoc Dung and Mr Bui Hoang Tan for their help and support Special thanks to my best undergraduate friend, Nguyen Trung Thanh, a geotechnical researcher at University of Wollongong His explanations in geotechnical engineering helped me a lot in this study His successful way in research encouraged me more than anything else Thanks are due to my family, especially to my beloved wife, Thuy Le for her deep understanding and encouraging me to take part in this master’s course from the beginning to the end iv LIST OF ABBREVIATIONS as improvement area ratio ascc CDM D Dh improvement area ratio of conventional CDM column cement deep mixing method diameter of conventional soil cement column (m) diameter of cap of HCC (m) Df Dt E50 Eu Ec Ecomp Es HCC L Lc Lh Lt embedded depth (m) diameter of tail of HCC (m) scant elastic modulus of soil at 50 percent (kPa) undrained elastic modulus of soil (kPa) elastic modulus of soil cement column (kPa) elastic modulus of improved ground (kPa) elastic modulus of soil (kPa) head-enlarged soil cement column length of conventional soil cement column (m) length of cone of HCC (m) length of head of HCC (m) Length tail of HCC (m) Ms NC OCR PF PI qu SCC su TDM constrained modulus of soil (kPa) normal consolidation over consolidation ratio point foundation plasticity index (%) unconfined compressive strength (kPa) conventional soil cement column undrained shear strength of soil (kPa) T-shaped deep mixed column ratio of length of cap and the total length of HCC  ratio of diameter of cap of HCC and diameter of tail of HCC  xy horizontal stress increment (kPa) vertical stress increment (kPa) z ratio of distance between two columns and the diameter of column  poisson ratio of soil in drained condition  poisson ratio of soil in undrained condition u v LIST OF TABLES Table 2.1 Relative importance of immediate, consolidation, and secondary compression for different soil types (Holtz, 1991) 15 Table 2.2 Method for estimating equivalent elastic modulus 24 Table 2.3 Theoretical values of Binc at fully saturation 29 Table 4.1 Info of footing categories and parameters 45 Table 4.2 Unconfined compression test results 56 vi LIST OF FIGURES Figure 2.1 The effectiveness of using CDM for clayey soil Figure 2.2 The effectiveness of using CDM for sandy soil Figure 2.3 The application of CDM for on land construction Figure 2.4 The application of CDM for on land construction Figure 2.5 Type of column installation Figure 2.6 Classification of deep mixing method Figure 2.7 Mixing shaft Figure 2.8 Typical equipment of on-land CDM construction Figure 2.9 Equipment of CDM method Figure 2.10 Procedure construction of CDM 10 Figure 2.11 Type of ground improvement 11 Figure 2.12 The T-shaped soil cement column overlain by embankment 12 Figure 2.13 Displacement of soil under TDM and SCC 13 Figure 2.14 PF method 13 Figure 2.15 Plain strain condition 16 Figure 2.16 Intensity of pressure based on Boussinesq approach 19 Figure 2.17 Pressure at point of Depth z bellow the center of the circular area acted on by pressure qo 20 Figure 2.18 Flexible rectangular loaded area 21 Figure 2.19 Shallow foundation under unit load 22 Figure 2.20 Variation of IG and  23 Figure 2.21 Variation of strain influence factor with depth and L/B 26 Figure 2.22 The field e-log’ curves 28 Figure 2.23 Settlement calculation from e- curve 28 Figure 2.24 Stress increment of a soil element at the center below circular load 29 Figure 2.25 Settlement ratio for circular and continuous foundation 30 Figure 2.26 Stress distribution of soil and columns under rigid foundation 31 vii Figure 2.27 Comparison of predict and measured settlement values 34 Figure 3.1 Shape of SCC and HCC 36 Figure 3.2 Initial and secant elastic modulus 39 Figure 4.1 Square footing and HCC parameters 40 Figure 4.2 Idealized soil profile 41 Figure 4.3 Settlement of HCC column 42 Figure 4.4 Volume reduction of HCC 43 Figure 4.5 Plan view of the JINCHEON factory and views of a typical footing 44 Figure 4.6 Soil profile of JINCHEON project 46 Figure 4.7 Comparison of settlement induced by HCC and SCC 48 Figure 4.8 Settlement by 3D elastic method (up to z = 10.4 m) 48 Figure 4.9 Variation of Scorr,HCC,min/Scorr,SCC 48 Figure 4.10 Variation of volume reduction 49 Figure 4.11 Plan view of SAMSE factory project 50 Figure 4.12 Soil profile of SAMSE factory project 51 Figure 4.13 Settlement and volume reduction analysis of SAMSE project 52 Figure 4.14 Static loading test on instrumented PF group 52 Figure 4.15 Test installation 53 Figure 4.16 Strain gauge installation 54 Figure 4.17 PVC sampling 55 Figure 4.18 Attached sampler 55 Figure 4.19 A typical unconfined compression test result 56 Figure 4.20 The estimation of secant elastic modulus at 50 percent (E50) 58 Figure 4.21 Strain influence from elastic theory and empirical study 59 Figure 4.22 Load-settlement curves estimated from measurement, linear and nonlinear analytical calculation 60 viii 4.3.2 Soil profile and footing parameters s'v (kPa) f' (degree) SPT N-value Soil profile 100 200 300 10 15 20 10 20 30 Es (MPa) 40 Fill layer Clayey sand Depth, z (m) Clay loam 10 Water table Loam Silty loam 15 20 25 Silty clay loam c Silty clay Figure 4.12 Soil profile of SAMSE factory project Figure 4.12 shows a soil profile of SAMSE factory project collected from drilled hole HK1 including layers which are: fill materials (0 to 1.6 m), clayey sand (1.6 to 6.2 m), clay loam (6.2 to 10.5 m), loam (10.5 to 12.6 m), silty loam (12.6 to 15 m), silty clay loam (15 to 24.8 m) and silty clay (24.8 m downward) Five upper layers have small SPT value (i.e N-values are smaller than 10) The N-value increased up to 18 in the deepest layers of the test (silty layer) The equivalent modulus of soil (Es) was evaluated similarly to the Jincheon factory project as mentioned above The effective internal friction angle was estimated from the SPT N-value followed a reliable method proposed by Hatanaka and Uchida (1996) 4.3.3 Settlement and volume reduction analysis The procedures of estimating settlement and volume reduction are analogous to the JEF project discussed previously The comparison of settlement induced by HCC and SCC, the ratio of minimal settlement of HCC and that of SCC are shown in Figure 4.13(a, b & c) and Figure 4.13(d), respectively The result showed that by using PF, the settlement reduction is 5% compared to SCC, at the values of  and  are 0.34 and 1.6, respectively, corresponding to  = 2.5 (i.e ascc = 0.2) 51 0.0108 0.0099 0.0107  value 1.10 1.40 1.80 0.0098 Corrected Settlement, Scorr (m) Corrected Settlement, Scorr (m) 0.0098 1.20 1.60 2.00 0.0097 Scorr,PF,min 0.0097 0.0096 3D Elastic  = 1.5 (DSCC=0.9 m) 0.0096 0.2 0.3 0.4 0.5 0.6  value 0.7 0.8  value 0.0107 1.10 1.40 1.80 0.0106 0.0106 1.20 1.60 2.00 0.0105 0.0105 Scorr,PF,min 0.0104 3D Elastic  = 2.0 (DSCC=0.67 m) 0.0104 0.0103 0.2 0.9 0.3 0.4 1.00 0.0118 0.99  value 0.0117 1.10 1.40 1.80 0.0116 1.20 1.60 2.00 0.0115 Scorr,PF,min 0.0114 0.0113 3D Elastic  = 2.5 (DSCC=0.53 m) 0.0112 0.0111 0.2 0.3 0.4 0.5 0.6  value 0.7 0.8 0.9 (b) 0.0119 Settlement ratio, Scorr,HCC,min/Scorr,scc Corrected Settlement, Scorr (m) (a) 0.5 0.6  value 0.7 0.8 value 1.50 1.5 (DSCC=0.89 m) 2.0 (DSCC=0.67 m) 0.99 2.50 2.5 (DSCC=0.53 m) 0.98 0.98 0.97 0.97 0.96 0.96 0.95 3D Elastic L=8.5 m 0.95 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1  value (c) (d) Figure 4.13 Settlement and volume reduction analysis of SAMSE project 4.3.4 A static load testing program on PF group For verifying the method as well as the results of calculation mentioned above, an in-situ static loading test was carried out by EXT Co., Ltd as shown in Figure 4.14 An increment applied load pressing on a steel plate underlain by 03 PF columns performed reliable data for the verification analytical settlement evaluation of this study Figure 4.14 Static loading test on instrumented PF group 52 4.3.4.1 The geometry and installation of PF column (b) (a) (c) Figure 4.15 Test installation: (a) the geometry of PF columns, (b) increment load applies on steel plate, (c) displacement sensors on steel plate and ground For conducting the static load test, a group of 03 PF columns was constructed The diameter of head and tail of column are 1.2 and 0.8 m, respectively The level of top of column is considered as surface ground A 0.15 m thickness of steel plated placed on 03 PF columns for transferring applied load to treated ground Note that, the dimension of the plate is not fully cover all of 03 PF columns as shown in figure 4.15 (a) A circular hydraulic jack was placed on the plate, under the loading system for increasing the applied load to the plate as shown in Figure 4.15 (b) A system of 06 sensors for measuring the displacement of steel plate Two of them are electric sensors which placed at the center edge of plate and connected to the instruments (shown in figure 4.16 (c)), the rest ones at the corner of plate are mechanic sensors 53 4.3.4.2 Installing strain gauges 0.5m 1.25m 2.0m 3.0m 4.0m 5.0m (b) 6.0m 7.0m 8.5m (d) (a) (c) Figure 4.16 Strain gauge installation: (a) installation of sensors along the depth of PF, (b) setting up sensors into PF, (c) strain gauge instruments, (d) sensor in PF For the static loading test, each PF column was installed strain gauges immediately after finishing mixing soil column when the binder is still soft The sensors inside the PF are connected to the test instruments for collecting data of load-settlement test The installation of strain gauges is shown in Figure 4.16 4.3.4.3 Sampling The samples of PF were taken after each period of time corresponding to 14 and 28 days for the laboratory tests There are two types of sampling which are PVC and attached samplers The PVC showed in Figure 4.17 was simply pushed on the surface of PF for collecting samples at upper layers whereas the attached samplers shown in Figure 4.18 were attached to the mixing blade of PF machine for taking the sample at the deeper layers 54 Figure 4.17 PVC sampling Figure 4.18 Attached sampler 4.3.4.4 Unconfined compression test Five group of PVC and a group of attached samplers were prepared for the test Each group has three samples so that the total samples for axial compression test are 18 The diameter and length of samples taken from attached samplers are 50 and 100 mm, respectively while that of PVC are 85 and 187 mm The detail info and results of unconfined compressive strength of PF are shown in Table 4.2 It could be seen that the unconfined compressive strength of PF are ranged from 1563 to 3679 (kPa) This could be explained that the samples were taken from PF columns at construction site so that the quality is not as homogeneous as ones produced in laboratory 55 Table 4.2 Unconfined compression test results No 10 11 12 13 14 15 16 17 18 Sampling PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC Attached sampler Attached sampler Attached sampler Sample's name C147-04 C147-05 C147-06 C156-04 C156-05 C156-06 C211-04 C211-05 C211-06 C239-04 C239-05 C239-06 C244-04 C244-05 C244-06 SAMPLE-01 SAMPLE-02 SAMPLE-03 qu (kPa) 1,779 2,972 2,600 3,792 3,176 2,493 3,015 1,919 1,564 3,037 3,426 3,030 2,766 2,528 3,679 2,146 1,563 2,993 E50 (kPa) 307,000 654,000 1,609,000 1,244,000 1,148,182 417,777.8 761,875 1,184,000 296,250 789,375 1,436,000 1,020,000 1,287,273 55,4375 1,630,909 352,500 200,666.7 586,000 Figure 4.19 showed the stress-strain curve when increasing load on PF sample The concave curve at the origin demonstrates an unusual initial elastic modulus of insitu PF compared to convex curve as normally seen from sample produced in laboratory It could be explained that at the initial stage of test, PF sample has not fully mobilized its stiffness due to the appearance of impurities surrounding sample 3000 Stress (kPa) 2500 2000 1500 1000 500 0 0.01 0.02 0.03 0.04 Strain Figure 4.19 A typical unconfined compression test result 56 4.3.4.5 A set of secant modulus Similar to initial elastic modulus as discussed in subsection 3.3.1, the secant value of elastic modulus at 50 percent, E50, of the unconfined compressive strength related to the strength qu Due to the unusual initial elastic modulus as discussed above, a corrected value of Emax is needed for estimating E50 From the trend of stress-strain curve in figure 4.19, it can be inferred that the slope represents the trend of elastic modulus, which should be the red line as shown in figure 4.20 The slope of Emax could be more inclined at the origin which implies Emax > E50 It also can be seen that the slope of E50 is nearly equal to the tangent modulus (Etan) at stress of half of compressive strength (i.e q = qu/2) From this point of view, E50 could be estimated as follows: E50 ≈ Etan = (q2 - q1) / (2 – 1) (4.4) where q1 and q2 are stress increment value corresponding to the strain value at 1 and 2, respectively subjected to q1 < qu/2 < q2 and 1 < (qu/2) < 2 (e.g q1 = 1000 kPa, q2 = 1500 kPa as shown in Figure 4.20) It is usual that elastic modulus at 50 percent of mixed soil column is seen to be used as the modulus of mixed soil column which is applied for determining the equivalent elastic modulus of treated soil (Kitazume & Terashi, 2013) 18 samples give 18 values of E50 The average E50 is estimated based on the geometric mean as following equation: E50,avg  n     E50,i   i 1  n (4.5) The average value of E50 of 18 samples is 722,439 kPa Note from figure 4.45 (a) that the steel plate does not fully cover 03 PF columns It means that the stiffness of mixed soil columns has not mobilized fully Hence, a corrected factor should be considered for determining the appropriated value of elastic modulus A corrected value k = 1.1 is assumed for the settlement calculation that produces E50 = 800,000 kPa 57 Figure 4.20 The estimation of secant elastic modulus at 50 percent (E50) 4.3.4.5 Load-settlement analysis The stress distribution under spread footing decrease along the depth of ground, and hence, the strain influence zone is also distribute significantly at upper layers As elastic theory, the value of strain is nearly nil at the depth of 4Be (Note: here, Be is the circle diameter of foundation) In fact, the strain is down to zero (not nearly) at depth of 4Be is accepted A wellknown method for estimating settlement of shallow foundation is the Schmertmann (1978) method as mention in Figure 2.21 It is the modified of Schmertmann (1970), where the triangular distribution was replaced by polygonal one As discussed previously, that empirical study indicated that the strain decrease to zero at depth from 2Be to 4Be corresponding to the ratio of width and length of foundation (i.e B/L) is from to 10 58 Therefore, the depth of 4B is considered for estimating settlement in this research For the layers below the depth of 4B, strain of subsoil can be omitted Figure 4.21 Strain influence from elastic theory and empirical study (Mayne & Poulos, 1999) In order to evaluating the settlement of spread footing, both linear and nonlinear settlement estimation as mentioned in Eq (2.14) and Eq (2.36), respectively are considered It is obvious that the load-settlement curves from in-situ test are nonlinear Strain increase quickly when the stress increase over the strength value (qu) of treated ground But for the values of stresses smaller than half of strength (qu/2), the measured stress-strain curve is almost linear, which implies that treated soil works as elastic material and hence, the linear settlement estimation mentioned in Eq (2.14) can be applied and give reliable results The practical method mentioned in Eq (2.36) was also applied for evaluating the nonlinear load-settlement curve, and for the comparison As discussed previously, the stiffness of treated column is not fully mobilized because the steel plate was not cover all of 03 PF columns, so that a corrected factor is needed for estimating the stiffness of soil column The corrected factor kcrr = 1.1 is assumed for the calculation (i.e E50,crr = 1.1×E50) This corrected factor considered the volume of soil columns placed outside of area of steel plate 59 Load (kN) 1000 2000 3000 4000 0.002 0.004 Settlement (m) 0.006 Eq (2.14) 0.008 0.01 measurement Eq (2.36) 0.012 0.014 0.016 0.018 Figure 4.22 Load-settlement curves estimated from measurement, linear and nonlinear analytical calculation Figure 4.22 shows the comparison of load-settlement curves resulted from measurement, linear and nonlinear calculation It is clear that all of three curves are linear and overlapped at small load (Q < 1100 kN) The theoretical linear and measured curve are still similar when increasing load up to 2000 kN, then the measured curve fall down quickly, especially when load increase over the strength value (i.e Qu = 2500 kN) which implies material does not work elastically Whereas the practical nonlinear approach is close to the measurement at load larger than Qu In conclusion, the use of linear and nonlinear method can be accepted for estimating the settlement spread footing resting on PF columns for loads smaller than half of strength of treated column (Qu/2) Both of two methods showed reasonable results for the load smaller than Qu/2 It can be explained that for loads smaller than Qu/2, the moduli of treated soil is approximately equal to scant elastic modulus at 50 percent (i.e E ≈ E50) as mentioned above, which cause soil behavior as elastic material 60 CHAPTER CONCLUSION 5.1 Conclusion This research first introduces an improved CDM method named Head Enlarged Soil Cement Column (HCC) Instead of making CDM columns of constant diameters, HCC method produces head-enlarged CDM columns that can improve the stiffness of natural soil profile more effectively, especially for ground under shallow footings The research presented a comparative settlement study to evaluate the effectiveness of the HCC method A shallow footing and practice-like soil profile are assumed to perform the settlement analysis The following key conclusions can be drawn from the analysis For the examined footings, within the optimal range of = 1.4 to 1.6 settlement from the footing on optimal HCC is about 90 to 95 % of that from the footing on SCC columns, under the conditions of equal loading condition and improved volume Under the same loading conditions and settlement amounts, volume of each HCC column can be reduced from to 12.5% compared with the volume of the SCC column Therefore, for large-scale ground improvement projects, the savings would be very significant The settlement in this study is a parametric study with much-simplified conditions.The results of settlement estimation from two case studies proved the feasibilty of applying HCC for improving the soft soil 5.2 Limitation and suggestion It is obvious that every technology has limitations The HCC shown its disadvantage when applying for one-dimensional condition It means that for large loaded area such as embankment where the used of 1D settlement is seen to be used for estimating settlement, SCC is more effective than HCC In term of settlement, 61 compared to SCC, HCC performs a larger value under the condition of using same material Under 3D compressibility, from the relationship between stress and strain (i.e zz = [zz - (xx+yy)]/E), it can be inferred that the magnitude of strain developed with that of stress Hence, the larger value of elastic modulus reach, the smaller strain can be enhanced As explained above, upper layers have to be imposed large load distribution Small value of E-value and large value of stress double the large value of strain at upper layers That increase E-value of upper layers by head enlarged columns reduces strain value remarkably The use of elastic theory (for both linear and nonlinear method) for estimating settlement as mentioned above is reasonable for the load smaller than a half of strength of treated ground For the large loads (Q > Qu), the soft layers beneath the improved soil deform significantly due to their small moduli which contribute mostly to the settlement of foundation Thus, the application of elastic theory is not suitable However, under the 1D condition, applied load is distributed equally along the depth of ground cause the equal stress at every layers (z,1 = z,2 = z,n) This cause the less effectiveness of using HCC 62 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UNIVESITY HANOI VIETNAM JAPAN UNIVERSITY NGUYEN DUC TRUNG AN EVALUATION OF THE EFFECTIVENESS OF HEAD- ENLARGED SOIL CEMENT COLUMN (HCC) IN GROUND IMPROVEMENT MASTER’S THESIS MAJOR: INFRASTRUCTURE ENGINEERING... only binder, binder and air or the mixing of binder, air and water - The combination of mechanical mixing and high pressure injection mixing Figure 2.7 Mixing shaft 2.1.4 Equipment and machine... compressibility of soil Basically, the compressibility of soil caused by (1) the deformation of soil elements, (2) the rearrangement of soil elements, and (3) the reduction volume of water in the ground The