Đề tài Research on the vacuum consolidation method for soft soil improvement in works construction

Nội dung text: Đề tài Research on the vacuum consolidation method for soft soil improvement in works construction

1. MINISTRY OF MINISTRY OF AGRICULTURE EDUCATION AND AND RURAL DEVELOPMENT TRAINING THUYLOI UNIVERSITY  PHAM QUANG DONG RESEARCH ON THE VACUUM CONSOLIDATION METHOD FOR SOFTSOIL IMPROVEMENT IN WORKS CONSTRUCTION Major : GEOTECHNICAL ENGINEERING Code : 62-58-60-01 SUMMARY OF DOCTORAL DISSERTATION HANOI – 2015
2. The thesis is completed at: THUYLOI UNIVERSITY  Promotors: 1. PROF. DR. TRINH MINH THU 2. PROF. DR. NGUYEN CHIEN Reviewer 1: Reviewer 2: Reviewer 3: The thesis was defended successfully at the Doctoral graduation committee at ThuyLoi University – 175 Tay Son – Dong Da – Hanoi at hour date moth year The thesis could be found at: National Library Or Water Resources University Library - 175 Tay Son – Dong Da – Hanoi
3. 1 PREFACE 1. Background of the topic The vacuum consolidation method has been applied successfully all over the world, and now it is chosen to improve the softsoil in Vietnam because this method has advantages as: shorten construction time, save loading materials, easily remove loading material after improvement, no environment pollution in construction, especially appropriate for ground improvement in a large and/or long area. Vietnam has applied this method to improve the ground for some works, with technology and equipment managed by foreign partners. However, the knowledge and mastering of technology and equipment, and building of relationship among parameters of soft soil layer during vacuum consolidation process for soft soil in Vietnam are topical and essential. Therefore, the topic “Research on the vacuum consolidation method for soft soil improvement in works construction” has large scientific and practical significance. 2. Purposes of the topic - To define the variation pore water pressure and deformation (settlement) of soft soil layer during vacuum consolidation. - To establish the relationship between Plasticity Index (PI), degree of consolidation (U), thickness of improved soft soil layer (H), and consolidation time (t) when improving the soft soil by vacuum consolidation method. 3. Object and Scope of research - The softsoil in different projects at Dinh Vu – Hai Phong, Duyen Hai – Tra Vinh, Nhon Trach – Dong Nai, Thai Binh Thermal Power Plant – Thai Binh. - The softsoil in some another areas which have similar physical and mechanical behaviors. 4. Contents of research (1) Literature reviews the vacuum consolidation solutions for improving soft soil foundation all over the world and in Vietnam. Evaluate the existing problems in technics and point out the matters that the thesis is focusing on solving. (2) Apply the background theory to figure out parameters of vacuum consolidation process and compare with physical model test in laboratory and with the field treatment. (3) Study the installation and the operation procedures to be well controlled the vacuum consolidation technology. (4) Studying the variation pore water pressure and the deformation of soft soil layer in vacuum consolidation process by physical model. The study results are compared with calculated results from numerical model to see the conformity of digital
4. 2 calculation model. (5) Determining the rationality of the numerical model to calculate and formulate the relationship between plastic index, consolidation, depth of softsoil improvement and consolidation time when apply the vacuum consolidation method. 5. Research Method (1) Theory calculation and analysis method: research the vacuum consolidation problem, study the contents related to solving the vacuum consolidation problem. (2) Practical method: physical model experiment to determine the variation process of pore water pressure and deformation at interested locations and depths during the application of vacuum consolidation. (3) Statistics method: Processing the experimental and statistic data to establish and build the relationship between the parameters of vacuum consolidation process. (5) Expert method: organize workshops, scientific reports to collect comments from experts and scientists in the researched field. 6. Scientific and practical meanings a) Scientific meaning There are few studies on factors that affect the vacuum consolidation process in Vietnam, therefore the results of this thesis on the variation pore water pressure and settlement in the softsoil of Vietnam can be used to predict the behaviors of softsoil when improving by vacuum consolidation. Currently, there is no software applied for the vacuum consolidation problems, therefore, the selection of the appropriate finite element software has great scientific meaning. To have basis for giving initial forecast on the consolidation process when improving the soft soil layer using vacuum consolidation method, the ability of building a relationship between consolidation parameters, consolidation time, plasticity index and the depth of soft soil layer for improvement is essential. b) Practical meaning The research results will determine the variation law of soft soil layer, and establish its relationship when improving the softsoil by vacuum consolidation, give a tool for geo-technician to give initial forecast on consolidation process when improving the softsoil by this method. 7. Contributions of the thesis (1) Establish a large scale physical model which is the first model which applies the vacuum consolidation method to improve the soft coastal underlying soil has been performed at the Geotechnical Laboratory, ThuyLoi University to study the variation process of pore water pressure and the deformation of softsoil at different interested locations and depths.
5. 3 (2) Select appropriate software to calculate the vacuum consolidation together with loading for the problem in the laboratory and at site. (3) Compute the relationship between plasticity index, degree of consolidation, thickness of soil layer for improvement and consolidation time when improving the soft soil by the vacuum consolidation method. 8. Layout of the Thesis Preface Chapter 1: Overview on the vacuum consolidation method for improving the softsoil and literature review Chapter 2: Experimental study on the vacuum consolidation method for softsoil improvement in the laboratory Chapter 3: Calculation model for vacuum consolidation problems Chapter 4: Establishment of a relationship between parameters of vacuum consolidation problems Conclusion and recommends List of publications References Chapter 1 OVERVIEW OF VACUUM CONSOLIDATION METHOD FOR SOFTSOIL IMPROVEMENT AND THEORY OF THE METHOD 1.1. Softsoil There have been many different definitions about softsoil, but it could be summarized that softsoil is the soil not suitable for works construction. The building in softsoil requires very careful improvement to ensure the structrures will be safe during the construction and operation time. 1.2. Overview of research and application of vacuum consolidation method 1.2.1. The application of vacuum consolidation method for improving softsoil in the world The vacuum consolidation method for improving softsoil was introduced for the first time in 1952 by W. Kjellman, in 1980, the vacuum consolidation was improved by a combination of loading and prefabricated vertical drain. In 1989, Menard Construction Company (from France) applied this improvement, since then, the method has been widely applied in many countries. Since 1997, Cofra Construction Company of Netherland had an improvement is to remove the protective membrane which made the construction difficult and vulnerable, however, a loading layer must be used to compensate the removed atmosphere pressure difference. Basically, the
6. 4 vacuum construction method could be divided into 2 main types which are sealed air membrane and no sealed air membrane construction. 1.2.2. The situation of study on vacuum consolidation method There are many factors that will create effect on the vacuum consolidation method process, these factors have been researched by many authors from practical results in the lab and at site of actual works. The results of these researches also point out that depending on the type of soil, pressure level, type of prefabricated vertical drain, distance of prefabricated vertical drain affects the consolidation level of the ground. However, in addition to the above conditions which create the differences in research results; the horizontal consolidation coefficient and the disturbance level around the prefabricated vertical drain are major factors that affect the consolidation process and resulting in this difference. 1.2.3. The research and application of vacuum consolidation method for softsoil improvement in Vietnam The application of vacuum consolidation for improving softsoil has been applied in some works in Vietnam. Based on the application result, this method proved itself as a new and an effective one. However, up to now, the design and construction using this method hold by foreign partners only. Therefore, it is essential to have research on the nature of consolidation process and understand the appropriate construction technology in the condition and actual situation to widely apply in the softsoil improvement in Vietnam. There is not many research on vacuum consolidation method in Vietnam, and the researches are often based on the results of works improving at site. There is no practical model in the laboratory to study the variation parameter of pore water pressure and deformation variation of soil, as well as proper numerical model to check and compare. 1.3. Theory of vacuum consolidation method 1.3.1. Consolidation of softsoil The nature of consolidation is the decreasing of void ratio of soil due to the release of pore water by permeability process, as such, the soil particles transfer stress directly on each other, thus improve the connection of soil structure. If the principles of mechanical compaction uses the loading to increase total stress, thus increase the effective stress. On the other hand, the vacuum consolidation will decrease the residual stress in pore, thus increase the effective stress without changing the total stress (fig 1.12).
7. 5 áp suất khí quyển áp suất khí quyển 0 ~100 kPa áp lực 0 ~100 kPa áp lực áp suất khí ứ Mực ng suất hiệu quả Mực n•ớc ngầm n•ớc ngầm ứng suất hiệu quả ứng suất hiệu quả khi không bơm hút ứng suất tổng ứng suất tổng Độ sâu (m) Độ sâu (m) ứng suất hiệu quả khi bơm hút ứng suất d• ứng suất d• khi bơm hút ứng suất d• tr•ớc khi bơm hút Khi không kết hợp hút chân không Khi kết hợp hút chân không Fig 1.12. Vacuum consolidation principles 1.3.2. Basic differential equation Terzaghi suggestted the basic differential equation of consolidation phenomenon as follows: u 2u C (1-1) t v z2 According to N. Carrillo, the 3 dimensions consolidation problem will be written in this form: u 2u 2u 1 u C C (1-2) v 2 r 2 t z r r r in which: Cv – vertical consolidation coefficient; Cr – radius-direction consolidation coefficient; u – residual pore water pressure. 1.3.3. Theory of consolidation To solve the consolidation problem with similar compression consolidation method and Barron – Terzaghi method, the scientists re-solved the problem of Barron – Terzaghi applying for the vacuum consolidation problem, in which there is solution of Wollongong University. Average value of pore water pressure at time t is determined by the formula 1-33: 2 ut pva pva 8 2m 1 2 1 8 (1-33) 1 exp T u u u  2 2 2 c L2  h 0 0 0 m 1 2m 1 vh The average consolidation and time can be determined by formula: 2 8 2m 1 1 8 U 1 exp 2 T (1-35) t  2 2 2 h m 1 2m 1 2 c L  vh In which: Cvh= Ch/Cv=kh/kv; L=l/de; l: Length of prefabricated drain; 2 Th=Cht/de ;  - coefficient of lateral expansion; pva – vacuum pressure.
8. 6 1.4. Settlement prediction method 1.4.1. Asaoka method According to this method, the final settlement value will be predicted based on actual monitoring data. This data could modeled approximately as a straight line (Fig 1.20). St+Δ = βSi+A (1-36) In which: β – slope of the most accurately stimulated regression line; A – extended intersection of line stimulating the vertical axis; S t+ Đ•ờng 3 After determining the stimulated S100% đ•ờng 3 S8 S regression line, the final settlement 7 St = St+ S6 value in Asaoka is measured as: Đ•ờng 2 -1 S100% đ•ờng 2 tan  A S5 S S (1-37) 4 100% S3 1  Đ•ờng 1 S100% đ•ờng 1 S2 S1 0 S S S S 0 1 100% đ•ờng 1 t Fig 1.20. Asaoka line 1.4.2. Inflection point method The inflection coefficients in theory and field are determined as the following equations: dU (1-40) M theory d log Tr max dSt (1-41) M field d log t max The final settlement value will be measured based on the ration: M S field (1-42) c M theory Conclusion of Chapter 1 The vacuum consolidation method has been studied and applied successfully worldwide and it has been partly applied in Vietnam with technology and equipment mainly from foreign partners. There are not many studies on factors affecting the consolidation process, and the application in works is at small number. Therefore, to proactively manage the technology and apply appropriately in the condition of softsoil of Vietnam, it is essential to have studies on variation law of parameters of underlying soil using
9. 7 physical models. In this thesis, the author suggests using large physical model to investigate the variation pore water pressure and the deformation of soil at interested points and depth. Chapter 2 EXPERIMENTAL STUDY ON VACUUM CONSOLIDATION METHOD FOR SOFTSOIL IMPROVEMENT IN LABORATORY 2.1. Purposes of research The purposes of practical research in laboratory are to determine the variation of settlement and pore water pressure for softsoil at different points and depth during vacuum consolidation process. 2.2. Experimental model 2.2.1. Introduction of model The experiments model is a physical model which was built in the laboratory of Geotechnical Engineering, ThuyLoi University. The model includes the rectangular cube, size (2.0x1.0x1.2) m and monitoring equipment of pore water pressure, settlement deformation, vacuum pump system The layout of the experimental model is in figure 2.1 9 1 8 2 4 10 3 Lớp cát thoát n•ớc 200 4 1200 400 7 5 Mẫu đất thí nghiệm 1000 2000 450 1200 1000 600 6 1 Dây truyền tín hiệu từ đầu đo áp lực n•ớc lỗ rỗng 6 Thùng chứa máy bơm chân không 2 Đầu đọc số liệu datalogger 7 Máy bơm chân không 3 ống nhựa truyền áp lực chân không mm 8 Đồng hồ đo lún 4 Đồng hồ đo áp lực chân không 9 Giá đỡ thiết bị 5 Van điều áp 10 Màng kín khí Fig 2.1. Simple scheme of the model The physical model stimulates a block of soft soil for research. In which, the prefabricated drain type CT-D910 has size 100x4m is located at effective distance 1.0x1.0m with the length is through the depth of soil block. Piezometer (PIE) monitoring pore water pressure is located at interested depth next to the prefabricated drain and between two prefabricated drains, the Tenxomets (TEN) monitoring are located right on the surface near two prefabricated drains. The
10. 8 sand layer 0.2m on the surface of soil surface has both drainage and loading functions. The physical models are built for the case of softsoil improvement with prefabricated vertical drain (Physical model 1, physical model 2) and without vertical drain (physical model 3). The layout of monitoring equipment for settlement and pore water pressure at interested points of physical models are shown in Fig 2.2, 2.3 and 2.4. Thiết bị đo áp lực n•ớc lỗ rỗng (Piezometer) Đồng hồ đo áp lực chân không Màng kín khí Màng kín khí Vải địa Vải địa Lớp cát thoát n•ớc ống thu n•ớc Thiết bị đo lún ống thu n•ớc 200 200 TEN 1-2 TEN 1-1 PIE 1-1 Bấc thấm 1200 PIE 1-2 1200 Bấc thấm 1000 1000 Bấc thấm PIE 1-3 Bấc thấm 500 250 500 500 500 500 500 500 500 500 2000 2000 Fig 2.2. Layout of locating physical model 1 Thiết bị đo áp lực n•ớc lỗ rỗng (Piezometer) Đồng hồ đo áp lực chân không Màng kín khí Màng kín khí Vải địa Vải địa Lớp cát thoát n•ớc ống thu n•ớc Thiết bị đo lún ống thu n•ớc 200 200 TEN 2-2 TEN 2-1 PIE 2-3 Bấc thấm 1200 1200 Bấc thấm 1000 1000 Bấc thấm PIE 2-1 Bấc thấm PIE 2-2 500 250 500 500 500 500 500 500 500 500 2000 2000 Fig 2.3. Layout of locating physical model 2 Thiết bị đo áp lực n•ớc lỗ rỗng (Piezometer) Đồng hồ đo áp lực chân không Màng kín khí Màng kín khí Vải địa Vải địa Lớp cát thoát n•ớc ống thu n•ớc Thiết bị đo lún ống thu n•ớc 200 200 TEN 3-2 TEN 3-1 PIE 3-3 1200 1200 1000 1000 PIE 3-1 PIE 3-2 500 250 500 500 500 500 500 500 500 500 2000 2000 Fig 2.4. Layout of locating physical model 3 2.2.2. Experimental soil The experimental soil is taken in the coastal area of Pvtex Dinh Vu – Hai Phong. The experimental soil sample (in the physical models) is prepared from this type of soil which has the similar physical and mechanical properties of the original soil (very soft sandy clay). To investigate the effectiveness of
11. 9 improvement method, it is necessary to determine the physical and mechanical properties of the original softsoil. The undrained shear resistance in depth before experiment is shown in fig 2.7. 2.2.3. Experiment equipment 0.0 The model experiment equipment (Fig 0.1 Su tr•ớc thí nghiệm 2.1) includes: pump system to create 0.2 vacuum pressure, pore water pressure 0.3 probe (Piezometer), data logger, 0.4 deformation measurement equipment, 0.5 Độ sâu (m) sâu Độ membrane for air sealing, prefabricated 0.6 vertical drain, water collector 0.7 connecting to the prefabricated vertical 0.8 drain and pump. This equipment is 0.9 1.0 1.2 1.4 1.6 1.8 2.0 almost used for works at site. Sức kháng cắt Su (kPa) Fig 2.7. Su of soil in depth before experiment Fig 2.8. Pore water pressure probe - Fig 2.10. Datalogger - Geokon Geokon Fig 2.13. Vacuum pump system Fig 2.20. Physical model
12. 10 2.3. Experiment procedures (1) Prepare the box experiment flume and prepare the sample. (2) Determine the physical and mechanical properties before experiment. (3) Plug in the prefabricated vertical drain. (4) Install monitoring equipment for pore water pressure. (5) Create water drainage surface and install water collection system. (6) Seal the experiment model. (7) Install the settlement and vacuum pressure measurement gauges. (8) Connect and activate the pore water pressure probes. (9) Connect the pump system and operate the model. 2.4. Experimental results 2.4.1. Experimental results of model 1 The relation between the settlement and time of physical model 1 in the vacuum consolidation process is shown in Fig 2.21. Due to the equipment conditions, the physical model 1 can only create the vacuum at maximum 36 kPa. 12 50 10 45 8 40 6 35 4 30 2 Lún mặt thí nghiệm cạnh bấc thấm (TEN 1-1) 0 Lún mặt thí nghiệm giữa 2 bấc thấm (TEN 1-2) 25 -2 áp lực chân không 20 Độlún (cm) -4 15 -6 lựcchân khôngáp (kPa) 10 -8 -10 5 -12 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.21. Relationship between the practical settlement and time of physical model 1 The results from Fig 2.12 show that the surface settlement develops relatively fast in the first day of loading. After that, the settlement reduces gradually and almost stable from the 16th day after loading. The settlement between 2 prefabricated vertical drains is lower than next to the prefabricated vertical drain, however, this difference is very small. To predict the final settlement based on the practical figures of physical model 1, it is used Asaoka forecast method. The predicted results show the agreement of this method with minor difference between the predicted and experiment settlement next to the prefabricated vertical drain and between 2 prefabricated vertical drain are 0.9% and 0.7% accordingly.
13. 11 The relationship between pore water pressure and time of physical model 1 during the vacuum consolidation process is shown in Fig 2.24 40 40 35 30 35 25 20 30 15 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 50 cm (PIE 1-1) 10 25 5 ALNLR thí nghiệm giữa 2 bấc thấm ở độ sâu 50 cm (PIE 1-2) 0 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 75 cm (PIE 1-3) 20 -5 áp lực chân không -10 15 -15 -20 10 -25 áp lựcchân khôngáp (kPa) áp lựcn•ớc áp lỗrỗng (kPa) -30 5 -35 -40 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.24. The relationship between pore water pressure and time of physical model 1 Fig 2.24 shows that the pore water pressure in the interested depth of soft soil before loading is static pore water pressure. The residual pore water pressure reduces relatively fast in the first loading day, then gradually reduces and is stable after 16th day of loading. The residual pore water pressure next to the prefabricated vertical drain is larger than between 2 prefabricated vertical drains in the first day of loading, after that, this difference becomes smaller and tends to be asymptotical. The nearer of the soil surface, the more decrease of the residual pore water pressure. 2.4.2. Experimental results of model 2 The relation between the settlement and time of physical model 2 in the vacuum consolidation process is shown in Fig 2.25. Due to the equipment conditions, the physical model 2 can only create the vacuum at maximum 41kPa. 20 50 18 16 45 14 12 40 10 8 35 6 Lún mặt thí nghiệm cạnh bấc thấm (TEN 2-1) 4 30 2 Lún mặt thí nghiệm giữa 2 bấc thấm (TEN 2-2) 0 25 -2 áp lực chân không Độlún (cm) -4 20 -6 -8 15 -10 -12 10 lựcchân khôngáp (kPa) -14 -16 5 -18 -20 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.25. Relationship between the practical settlement and time of physical model 2
14. 12 The results from Fig 2.25 show that the settlement develops relatively fast in the first day of loading. After that, the settlement reduces gradually and almost stable from the 20th day after loading. The settlement next to the prefabricated vertical drain is greater between 2 prefabricated vertical drains, however, this difference is very small. The result of using Asaoka method to predict the settlement has proved its compliance with a difference of 10.3% and 11.6% accordingly to the position next to fabricated vertical drain and between 2 fabricated vertical drains. The relationship between pore water pressure and time of physical model 2 during the vacuum consolidation process is shown in Fig 2.28 50 50 45 40 45 35 30 40 25 20 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 75 cm (PIE 2-1) 35 15 10 ALNLR thí nghiệm giữa 2 bấc thấm ở độ sâu 75 cm (PIE 2-2) 30 5 0 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 50 cm (PIE 2-3) 25 -5 áp lực chân không -10 20 -15 -20 15 -25 -30 10 áp lựcchân khôngáp (kPa) áp lựcn•ớc áp lỗrỗng (kPa) -35 -40 5 -45 -50 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.28. The relationship between pore water pressure and time of physical model 2 Figure 2.28 shows that the pore water pressure in the interested depth of soil before loading is static pore water pressure. The residual pore water pressure reduces relatively fast in the first loading day, then gradually reduces and is stable after 20th day of loading. The residual pore water pressure next to the prefabricated vertical drain is larger than between 2 prefabricated vertical drains, however, this difference is gradually smaller and in the last stage of loading, it is extremely small. 2.4.3. Experimental results of model 3 The relation between the settlement and time of physical model 3 in the vacuum consolidation process is shown in Fig 2.29. Due to the equipment conditions, the physical model 3 can only create the vacuum at maximum 40kPa.
15. 13 12 50 10 45 8 40 6 35 4 Lún mặt thí nghiệm cách biên 50 cm (TEN 3-1) 2 Lún mặt thí nghiệm cách biên 100 cm (TEN 3-2) 30 0 áp lực chân không 25 -2 20 Độlún (cm) -4 15 -6 -8 10 -10 5 lựcchân khôngáp (kPa) -12 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.29. Relationship between the practical settlement and time of physical model 3 Fig 2.29 shows that the surface settlement develops relatively slow in the vacuum consolidation process, the settlement values at points are insignificant difference. At the time of loading end (26th day), the settlement curve is still slope and unstable. The final forecasted settlement values according to Asaoka at point where is 0.5m and 1.0m from the margin of soil block are 7.53cm and 7.72cm. the practical settlement when the loading ends (26th day) at the above points are 5.37 and 5.57 cm accordingly, at this time, the settlement curve is still slope, unstable, as such, the forecasted settlement value according to Asaoka is appropriate. The relationship between pore water pressure and time of physical model 3 during the vacuum consolidation process is shown in fig 2.32 60 50 50 45 40 40 30 35 20 30 10 25 0 20 -10 ALNLR thí nghiệm cách biên 50 cm ở độ sâu 75 cm (PIE 3-1) 15 -20 ALNLR thí nghiệm cách biên 100 cm ở độ sâu 75 cm (PIE 3-2) -30 ALNLR thí nghiệm cách biên 50 cm ở độ sâu 50 cm (PIE 3-3) 10 áp lựcchân khôngáp (kPa) áp lựcn•ớc áp lỗrỗng (kPa) -40 áp lực chân không 5 -50 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Fig 2.32. The relationship between pore water pressure and time of physical model 3 The result in fig 2.32 shows that the pore water pressure in interested depth before loading is static pore water pressure. The residual pore water pressure starts decreasing after about 1 day of loading. Due to the shortage of prefabricated vertical drain, the disposal of residual pore water pressure occurs relatively slowly, in the end of loading, the curve of residual pore water
16. 14 pressure disposition is still slope and unstable. The residual pore water pressure values at points with the same depth in underlying soil are insignificant difference during the vacuum consolidation process. 2.5. Technical effect of vacuum consolidation To evaluate the effect of vacuum consolidation method, after finishing the experiment, soil samples are taken to determine properties in the laboratory. After that, the results will be compared with the test results before the experiments. The undrained shear resistance before and after experiment of physical models are shown on Figs 2.36, 2.38 and 2.40. 0,0 0,0 0,0 Su sau thí nghiệm cách biên dọc 0,5 m Su sau thí nghiệm giữa 2 bấc thấm Su sau thí nghiệm giữa 2 bấc thấm 0,1 Su sau thí nghiệm cách biên dọc 1,0 m 0,1 Su sau thí nghiệm cạnh bấc thấm 0,1 Su sau thí nghiệm cạnh bấc thấm Su tr•ớc thí nghiệm Su tr•ớc thí nghiệm Su tr•ớc thí nghiệm 0,2 0,2 0,2 0,3 0,3 0,3 0,4 0,4 0,4 0,5 0,5 0,5 Độ sâu (m) Độ sâu (m) Độ sâu (m) sâu Độ 0,6 0,6 0,6 0,7 0,7 0,7 0,8 0,8 0,8 0,9 0,9 0,9 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 Sức kháng cắt Su (kPa) Sức kháng cắt Su (kPa) Sức kháng cắt Su (kPa) Fig 2.36. Su before Fig 2.38. Su before Fig 2.40. Su before and and after experiment and after experiment after experiment of of physical model 1 of physical model 2 physical model 3 The physical and mechanical criteria and the undrained shear resistance of soil before and after improvement of physical models show that: the internal friction angle ( ) increases (1.84-2.87) times, the cohesion (C) increases (3.17- 4.53) times, the permeability coefficient (k) decreases (6.28-14.51) times, average undrained resistances next to prefabricated vertical drain and between two prefabricated vertical drains increase (7.71-9.07) and (4.71-6.53) times accordingly. Conclusion of Chapter 2 (1) Proactively manage the usage procedures of equipment, installation and operation of vacuum consolidation process. (2) Determine the effect of prefabricated vertical drain when improving the underlying soil using vacuum consolidation method through 2 experiments with and without prefabricated vertical drain when consolidating with similar level of vacuum pressure for the coastal softsoil in Viet Nam. (3) The experiment determines the variation of
17. 15 settlement value, pore water pressure at points next to prefabricated vertical drain and between 2 prefabricated vertical drain in interested depth for light clay mud, the practical results from physical models show the compliance with theory. (4) Asaoka method for forecasting the settlement when having practical result is appropriate. Chapter 3 CALCULATION MODEL FOR VACUUM CONSOLIDATION PROBLEMS 3.1. Calculation model In the thesis, the author used two modules SEEP/W and SIGMA/W of GeoStudio 2004 software to calculate the pore water pressure and settlement at the interested points and depth to investigate the differences between experimental results in the laboratory and field experimental results of the practical projects. 3.2. Simulation of vacuum consolidation problem The procedures of calculating vacuum consolidation problem by integrating two modules SEEP/W and SIGMA/W of Geostudio software are shown in figure 3.1. B•ớc ban đầu Xác định phạm vi làm việc Xác định phạm vi làm việc (khổ giấy, tỷ lệ, đơn vị ) (khổ giấy, tỷ lệ, đơn vị ) L•u giữ bài toán L•u giữ bài toán Khai báo vật liệu Khai báo hàm thấm (E, F , C, à ) Vẽ l•ới phần tử hữu hạn & Vẽ l•ới phần tử hữu hạn & Mô đun SIGMA/W Mô đun SEEP/W gán điều kiện biên gán điều kiện biên Lựa chọn kiểu phân tích Lựa chọn kiểu phân tích (b•ớc thời gian, b•ớc lặp ) (b•ớc thời gian, b•ớc lặp ) Gia tải theo số cấp áp lực thực tế B•ớc tính toán Lựa chọn b•ớc thời gian Lựa chọn b•ớc thời gian t•ơng ứng với từng cấp gia t•ơng ứng cho từng cấp tải - ứng suất đ•ợc lấy từ gia tải - điều kiện áp lực pha ban đầu t•ơng ứng n•ớc lỗ rỗng ban đầu từ Mô đun SIGMA/W tr•ớc đó pha ban đầu tr•ớc đó Mô đun SEEP/W Tích hợp SIGMA/W và Gia tải áp lực chân không SEEP/W bằng lựa chọn cho từng cấp gia tải (bằng Couple áp lực ng•ợc) Xuất kết quả (Biến dạng lún và áp lực n•ớc lỗ rỗng) Figure 3.1. Calculation flowchat solving vacuum consolidation problem
18. 16 3.3. Calculation of application for physical model Simulation and computation for soil mass with the same size of soil mass was experimented under the diagram of plane problem. Preloading load including 0.2 m layer of sand with natural volumetric weight of 16 kN/m3, pressure of vacuum load is collected on average during vacuum loading, so the pressure selected in the calculation is 32 kPa for physical model 1 and 38 kPa for physical model 2, physical model 3, loading time is 26 days respectively. 3.4. Comparison of experimental results and calculation of physical model Settlement, pore water pressure calculation and experiment of physical model 1 with the duration as shown in figures 3.11 and 3.12. 16 50 14 12 45 10 40 8 6 35 Lún mặt thí nghiệm cạnh bấc thấm (TEN 1-1) 4 30 2 Lún mặt thí nghiệm giữa 2 bấc thấm (TEN 1-2) 0 Lún mặt tính toán cạnh bấc thấm 25 Độ lún (cm) -2 Lún mặt tính toán giữa 2 bấc thấm 20 -4 áp lực chân không -6 15 -8 -10 10 lựcáp chân không (kPa) -12 5 -14 -16 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.11. Relationship of settlement, calculation and experiment with duration of physical model 1 50 40 45 40 35 35 30 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 0,5 m (PIE 1-1) 25 ALNLR thí nghiệm giữa 2 bấc thấm ở độ sâu 0,5 m (PIE 1-2) 30 20 15 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 0,75 m (PIE 1-3) 10 ALNLR tính toán cạnh bấc thấm ở độ sâu 0,5 m 25 5 ALNLR tính toán giữa 2 bấc thấm ở độ sâu 0,5 m 0 20 -5 ALNLR tính toán cạnh bấc thấm ở độ sâu 0,75 m -10 áp lực chân không -15 15 -20 áp lựcáp n•ớc lỗ-25 rỗng (kPa) 10 -30 lựcáp chân không (kPa) -35 -40 5 -45 -50 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.12. Relationship of pore water pressure calculation and experiment with duration of physical model 1 Settlement, pore water pressure calculation and experiment of physical model 2 as shown in figures 3.13 and 3.14.
19. 17 20 50 18 16 45 14 12 40 10 8 Lún mặt thí nghiệm cạnh bấc thấm (TEN 2-1) 35 6 Lún mặt thí nghiệm giữa 2 bấc thấm (TEN 2-2) 4 30 2 Lún mặt tính toán cạnh bấc thấm 0 Lún mặt tính toán giữa 2 bấc thấm 25 Độlún (cm) -2 -4 áp lực chân không 20 -6 -8 15 -10 -12 10 lựcchân khôngáp (kPa) -14 -16 5 -18 -20 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.13. Relationship of settlement, calculation and experiment with duration of physical model 2 60 50 50 45 40 40 30 ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 0,75 m (PIE 2-1) 35 20 ALNLR thí nghiệm giữa 2 bấc thấm ở độ sâu 0,75 m (PIE 2-2) ALNLR thí nghiệm cạnh bấc thấm ở độ sâu 0,5 m (PIE 2-3) 30 10 ALNLR tính toán cạnh bấc thấm ở độ sâu 0,5 m 0 ALNLR tính toán giữa 2 bấc thấm ở độ sâu 0,75 m 25 ALNLR tính toán cạnh bấc thấm ở độ sâu 0,75 m -10 áp lực chân không 20 -20 15 áp lựcáp -30 n•ớc lỗ rỗng (kPa) 10 lựcáp chân không (kPa) -40 -50 5 -60 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.14. Relationship of ALNLR, calculation and experiment with duration of physical model 2 Settlement, pore water pressure calculation and experiment with duration of physical model 3 as shown in figures 3.15 and 3.16. 12 50 10 45 8 40 6 Lún mặt thí nghiệm cách biên 0,5 m (TEN 3-1) 35 4 Lún mặt thí nghiệm cách biên 1,0 m (TEN 3-2) 2 Lún mặt tính toán cách biên 0,5 m 30 0 Lún mặt tính toán cách biên 1,0 m 25 Độlún (cm) -2 áp lực chân không 20 -4 15 -6 10 lựcchân khôngáp (kPa) -8 -10 5 -12 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.15. Relationship of settlement, calculation and experiment with duration of physical model 3
20. 18 70 50 60 45 50 40 40 30 35 20 30 10 0 25 -10 ALNLR thí nghiệm cách biên 0,5m ở độ sâu 0,75 m (PIE 3-1) 20 -20 ALNLR thí nghiệm cách biên 1,0cm ở độ sâu 0,75 m (PIE 3-2) -30 ALNLR thí nghiệm cách biên 0,5m ở độ sâu 0,5 m (PIE 3-3) 15 -40 ALNLR tính toán cách biên 0,5m ở độ sâu 0,75 m lựcáp chân không (kPa) áp lựcáp n•ớc lỗ rỗng (kPa) ALNLR tính toán cách biên 0,5m ở độ sâu 0,5 m 10 -50 ALNLR tính toán cách biên 1,0m ở độ sâu 0,75 m 5 -60 áp lực chân không -70 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Thời gian (ngày) Figure 3.16. Relationship of pore water pressure calculation and experiment with duration of physical model 3 Figure 3.11 to Figure 3.16 show the agreement between theory and experiment as settlement, pore water pressure calculation and experiment at positions next to prefabricated vertical drain greater than positions between two prefabricated vertical drain. Value of settlement, pore water pressure calculation and experiment has no significant differences during the process of vacuum consolidation. This result shows the conformity of numerical model using calculation. 3.5. Calculation and testing for practical projects 3.5.1. Pvtex Dinh Vu – Hai Phong Construction Project Settlement, pore water pressure calculation and experiment with duration of Pvtex Dinh Vu – Hai Phong construction project are shown in figures 3.25 and 3.26. 100 150 80 140 60 130 40 120 110 20 100 0 Độ lún tính toán của công trình Pvtex Đình Vũ - Hải Phòng 90 -20 Độ lún thực nghiệm của công trình Pvtex Đình Vũ - Hải Phòng 80 70 Độlún (cm) -40 áp lực gia tải -60 60 -80 50 40 -100 30 (kPa) lực giatải áp -120 20 -140 10 -160 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Thời gian (ngày) Figure 3.25. Relationship of settlement, calculation and experiment with duration of Pvtex Dinh Vu – Hai Phong Construction Work
21. 19 600 150 550 140 130 500 120 450 110 400 100 ALNLR tính toán ở độ sâu 15 m 350 90 ALNLR tính toán ở độ sâu 10 m 80 300 ALNLR thực nghiệm ở độ sâu 15 m 70 250 ALNLR thực nghiệm ở độ sâu 10 m 60 200 áp lực gia tải 50 150 40 30 (kPa) lực giatải áp 100 áp lựcn•ớc áp lỗrỗng (kPa) 20 50 10 0 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Thời gian (ngày) Figure 3.26. Relationship of pore water pressure calculation and experiment with duration of Pvtex Dinh Vu – Hai Phong Construction Project 3.5.2. Duyen Hai 3 – Tra Vinh Electrothermal Project Settlement, pore water pressure calculation and experiment with duration of Duyen Hai 3 – Tra Vinh Electrothermal Plant Project are shown in figures 3.35 and 3.36. 200 100 160 90 120 80 80 70 40 Độ lún thực nghiệm của công trình nhà máy nhiệt điện Duyên Hải 3 - Trà Vinh 60 0 Độ lún tính toán của công trình nhà máy nhiệt điện Duyên Hải 3 - Trà Vinh 50 áp lực gia tải -40 40 Độlún (cm) -80 30 -120 20 (kPa) lực giatải áp -160 10 -200 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Thời gian (ngày) Figure 3.35. Relationship of settlement, calculation and experiment with duration of Duyen Hai 3 – Tra Vinh Electrothermal Plant Project 300 100 250 90 ALNLR thực nghiệm ở độ sâu 5 m 200 80 ALNLR thực nghiệm ở độ sâu 10 m 70 150 ALNLR tính toán ở độ sâu 10 m 60 100 ALNLR tính toán ở độ sâu 5 m áp lực gia tải 50 50 40 áp lực gia tải (kPa) lực giatải áp 0 30 -50 20 áp lựcn•ớc áp lỗrỗng (kPa) -100 10 -150 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Thời gian (ngày) Figure 3.36. Relationship of pore water pressure calculation and experiment with duration of Duyen Hai 3 – Tra Vinh Electrothermal Plant Project
22. 20 3.5.3. Nhon Trach 2 – Dong Nai Electrothermal Project Results of settlement, calculation and experiment with duration of Nhon Trach 2- Dong Nai Electrothermal Plant Project are shown in figure 3.43. 200 200 160 180 120 160 80 140 40 Độ lún tính toán của công trình nhà máy nhiệt điện Nhơn Trạch 2 - Đồng Nai 120 0 Độ lún thực nghiệm của công trình nhà máy nhiệt điện Nhơn Trạch 2 - Đồng Nai 100 Độlún (cm) -40 áp lực gia tải 80 -80 60 -120 40 (kPa) lực giatải áp -160 20 -200 0 0 10 20 30 40 50 60 70 80 90 100 110 Thời gian (ngày) Figure 3.43. Relationship of settlement, calculation and experiment with duration of Nhon Trach 2 – Dong Nai Electrothermal Plant Project Figures 3.25, 3.26, 3.35, 3.36 and 3.43 show that settlement and pore water pressure calculation have close agreement to experimental results with no significant differences between calculation results and experimental results during the process of vacuum consolidation. This result shows that the numerical model used in calculation is appropriate. Conclusion of chapter 3 (1) Given the procedures of solving the vacuum consolidation problem by combining two modules SEEP/W and SIGMA/W of Geostudio software. (2) Applied calculation and testing for 3 experimental models in the laboratory, the calculated results show the consistency using these two modules. (3) Confirmed suitability of numerical model through calculation, compared with results of practical projects. Chapter 4 DEVELOPMENT OF RELATIONSHIP BETWEEN PARAMETERS OF THE VACUUM CONSOLIDATION PROBLEM 4.1. Introduction The thesis uses the selected numerical model and calculates the application for types of poor soil of practical works as stated in chapter 3 and poor soil of Thai Binh electrothermal plant project in case of improving underlying soil with height of 10m to 30m. Thereby the relationships between consolidation time (t) with plasticity index (PI), consolidation (U) and thickness of improvement softsoil (H) are established in order to serve the rapid identification of
23. 21 unnecessary time for vacuum preloading to achieve required degree of consolidation under the softsoil treatment. 4.2. Mechanical properties of types of poor soil Mechanical properties of types of soft soil are collected from tables 3.3, 3.4, 3.5 and 4.1. 4.3. Calculation results The relationships between consolidation level and time of softsoil types corresponding to the thickness of improvement soft soil are shown in figure 4.1 to figure 4.5. 160 160 140 140 120 120 100 100 80 80 60 60 40 Đất yếu Đình Vũ - Hải Phòng 40 Đất yếu Đình Vũ - Hải Phòng 20 Đất yếu nhiệt điện Thái Bình 20 Đất yếu nhiệt điện Thái Bình Độ Độ cố kết (%) 0 Đất yếu Duyên Hải - Trà Vinh Độ cố kết (%) 0 Đất yếu Duyên Hải - Trà Vinh -20 Đất yếu Nhơn Trạch - Đồng Nai -20 Đất yếu Nhơn Trạch - Đồng Nai -40 -40 -60 -60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Thời gian (ngày) Thời gian (ngày) Figure 4.1. Relationship between degree of Figure 4.2. Relationship between degree of consolidation and consolidation time with consolidation and consolidation time with 10m thickness of soft soil for treatment 15m thickness of soft soil for treatment 140 140 120 120 100 100 80 80 60 Đất yếu Đình Vũ - Hải Phòng 60 40 Đất yếu nhiệt điện Thái Bình 40 Đất yếu Đình Vũ - Hải Phòng 20 Đất yếu Duyên Hải - Trà Vinh Đất yếu nhiệt điện Thái Bình Độ cố kết Độ (%) 0 Đất yếu Nhơn Trạch - Đồng Nai cố kết Độ (%) 20 Đất yếu Duyên Hải - Trà Vinh -20 0 Đất yếu Nhơn Trạch - Đồng Nai -40 -20 -60 -40 0 10 20 30 40 50 60 70 80 90 100 110 120 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Thời gian (ngày) Thời gian (ngày) Figure 4.3. Relationship between degree of Figure 4.4. Relationship between degree of consolidation and consolidation time with consolidation and consolidation time with 20m thickness of soft soil for treatment 25m thickness of soft soil for treatment 140 120 100 80 60 Đất yếu Đình Vũ - Hải Phòng 40 Đất yếu nhiệt điện Thái Bình 20 Đất yếu Duyên Hải - Trà Vinh Độ cố kết Độ (%) 0 Đất yếu Nhơn Trạch - Đồng Nai -20 -40 0 10 20 30 40 50 60 70 80 90 100110120130140150160 Thời gian (ngày) Figure 4.5. Relationship between degree of consolidation and consolidation time with 30m thickness of soft soil for treatment The results of figure 4.1 to figure 4.5 show that the consolidation level and consolidation time of poor soil types depend on plasticity index and thickness of softsoil layer. The greater plasticity index of poor soil, the longer time of consolidation.
24. 22 4.4. Development of relationship between consolidation time (t), plasticity index (PI), consolidation level (U) and thickness of improvement softsoil (H) Case 1: Development of above relationship upon determined thickness of improvement softsoil. Case 2: Development of above relationship upon determination of consolidation level. 4.4.1. Relationship between consolidation time and plasticity index and consolidation level upon determined thickness of softsoil Relationship between consolidation time and plasticity index and consolidation level of poor soil types corresponding to thickness of improvement softsoil is shown in figure 4.6 to figure 4.10. Figure 4.6. Relationship between consolidation time Figure 4.7. Relationship between consolidation time and plasticity index and degree of consolidation and plasticity index and degree of consolidation with with 10m thickness of soft soil for treatment 15m thickness of soft soil for treatment Figure 4.8. Relationship between consolidation time Figure 4.9. Relationship between consolidation time and plasticity index and degree of consolidation with and plasticity index and degree of consolidation with 20m thickness of soft soil for treatment 25m thickness of soft soil for treatment Figure 4.10. Relationship between consolidation time and plasticity index and degree of consolidation with 30m thickness of soft soil for treatment
25. 23 4.4.2. Relationship between consolidation time, plasticity index and thickness softsoil upon determination of consolidation level The relationship between consolidation time, plasticity index and thickness softsoil upon determination of consolidation level is shown in figure 4.11 to figure 4.14. Figure 4.11. Relationship between the consolidation Figure 4.12. Relationship between the consolidation time with plasticity index and the thickness of soft time with plasticity index and the thickness of soft soil with the degree of consolidation of 80% soil with the degree of consolidation of 85% Figure 4.13. Relationship between the consolidation Figure 4.14. Relationship between the consolidation time with plasticity index and the thickness of soft soil time with plasticity index and the thickness of soft with the degree of consolidation of 90% soil with the degree of consolidation of 95% Notes: t10, t15, t20, t25, t30, t80, t85, t90, t95 is consolidation time (t) when the thickness softsoil varies from 10m to 30 m and the consolidation level from 80% to 95%. Results from figure 4.6 to figure 4.14 show the relationship between consolidation time, plasticity index, height of softsoil and consolidation level of poor soil types upon applying the vacuum consolidation method. This relationship is represented through equations from t80 to t95 and from t10 to t30 corresponding to the thickness softsoil from 10m to 30m and the consolidation level from 80% to 95%. Conclusion of Chapter 4 (1) Establish equations t80, t85, t90, t95 and t10, t15, t20, t25, t30 about the relationship between consolidation time, plasticity index, consolidation level and thickness of softsoil, corresponding with each consolidation level and thickness softsoil as identified. (2) Based on the equations t80, t85, t90, t95 and t10,
26. 24 t15, t20, t25, t30 it is estimated about time of consolidation, degree of consolidation for soft clay with plasticity index of 18.4% to 33.8% when foundation is treated by vacuum consolidation method, with the different thickness of soft soil foundation for treatment ranging from 10m to 30m, corresponding to type of prefabricated, distance of prefabricated and level of pre-determined load. CONCLUSIONS AND RECOMMENDATIONS I. The conclutions (1) The first large-scale experimental model of Vietnam has been conducted in the Geotechnical laboratory – ThuyLoi University in order to investgate the variation of pore water pressureand settlement at different points and depth upon the vacuum consolidation. (2) The researcher has taken an active role in the process of using equipment, installation and operation of vacuum consolidation technology for softsoil improvement to deploy broader for various construction projects. (3) From research on the physical model and numerical model, the effectiveness of vacuum consolidation was found in combination with prefabricated vertical drain for accelerated consolidation process (32.3-36.4)% compared with no prefabricated vertical drain. (4) By the experimental results in the laboratory, technical efficiency of vacuum consolidation method has been evaluated, the internal friction angle of soil has increased (1.84-2.87) times, cohesion has increased (3.17-4.53) times, and undrained shear strength has increased (4.71-9.07) times. (5) The integration of SEEP/W and SIGMA/W has been done for calculation, analysis, comparison with experimental results in the laboratory and the field has confirmed the suitability of this model. (6) Building a relationship between the time of consolidation with degree of consolidation, plasticity index and thickness of soft soil for treatment by vacuum consolidation method. II. Limitations and future study (1) Physical model is presented by soil mass of same working conditions with the field, however the effects of neighborhood factors in 3 dimensions have not been considered so it is necessary to investigate such effects through follow-up experimentals. (2) Due to limited experimental equipment, the experiments with higher vacuum pressure levels have been unfulfilled. (3) In order to complete the technology of softsoil improvement with loading prefabricated vertical drain combined with vacuum consolidation, it is necessary to add further research on horizontal deformation, the variation of vacuum pressure depend on depth, permiability characteristic of softsoil during treatment
27. LIST OF PUBLICATIONS 1. Nguyen Chien, Pham Quang Dong (2009), "Initial results of study on appropriate prefabricated vertical when dealing with softsoil improvement by the vacuum consolidation method", Journal of Science and Technology Water Resources and Environment, (24), 72-79. 2. Nguyen Chien, To Huu Duc, Pham Quang Dong (2011), "Initial results of field vacuum consolidation experiment for ground improvement in Long Thanh – Dau Giay highway project", Journal of Science and Technology Water Resources and Environment, (32), 77-83. 3. Nguyen Chien, Pham Quang Dong (2012), "The application vacuum consolidation method for improvement softsoil for construction hydraulic works in coastal zone", Journal of Geotechnical Engineering, (2), 3-9. 4. Pham Quang Dong, Bui Van Truong, Trinh Minh Thu (2013), "Research on process change pore water pressure and deformation of soft soil by vacuum consolidation physical model", Journal of Geotechnical Engineering, (2), 12-21. 5. Pham Quang Dong, Trinh Minh Thu, Nguyen Chien (2013), ”Initial results of study on improving the soft soil by vacuum consolidation method”, Vietnam transport infrastructure with sustainable development, 17-8-2013, Da Nang, Viet Nam, Construction Publishing House, Hanoi. 6. Bui Van Truong, Pham Quang Dong (2013), ”The experimental research in the laboratory on consolidation method by prefabricated vertical drain for improvement of poor underlying soil”, 1st annual scientific conference, December 6, 2013, Water Resources University, Hanoi.