Nghiên cứu đặc điểm cấu trúc nền đất yếu tuyến đường giao thông ven biển đoạn từ hải phòng đến nam định và đề xuất giải pháp xử lý nền bằng cọc cát biển xi măng TT TIENG ANH

MINISTRY OF EDUCATION AND TRAINNING UNIVERSITY OF TRANSPORT AND COMMUNICATIONS - - NGUYỄN THỊ DỊU STUDY ON THE STRUCTURE OF SOFT GROUND OF COASTAL HIGHWAY HAI PHONG - NAM DINH AND GROUND IMPROVEMENT PROPOSED BY SEA SAND – CEMENT COLUMNS Field of study : Transport Construction Engineering Specialization Code : Geotechnical engineering : 9580205 SUMMARY OF DOCTORAL DISSERTATION HANOI - 2021 The research work was completed at: University of Transport and Communications Superviors: Assoc.Prof TA Duc Thinh, University of Mining and Geology Assoc.Prof NGUYEN Duc Manh, University of Transport and Communications Reviewer 1: Reviewer 2: Reviewer 3: The dissertation was defended in front of the Doctoral Council at University of Transport and Communications at h ./ ./ /2021 Thesis can be found in the library: - National library of Vietnam - Library of University of Transport and Communications INTRODUCTION Research background The coastal Hai Phong - Nam Dinh highway is 64.1 km long.It is a part of the project of Vietnam's coastal highway construction, starting from Nui Do port (Quang Ninh) to Ha Tien (Kien Giang) The highway is designed according to standard grade III with pavement grade A1, and the speed of 80km/h The structure of the ground in the coastal highway Hai Phong - Nam Dinh is rather complicated Quaternary sediment appears in several layers of soft soil with very low load-bearing capacity, high deformation, clay, sand clay, and sand in soft and plastic state of Thai Binh and Hai Hung formations Therefore, it is necessary to improve or reinforce the soft ground before constructing the highway to ensure the settlement and stability requirements according to current standards Currently, in Vietnam, there are many soft soil improvement methods, including methods of shallow or deep improvement; mechanical or chemical treatment; drainage consolidation, or preloading Each improvement method has its advantages, disadvantages, and application range The effectiveness of soft soil improvement depends on the technical factors, the technology, the nature of soft soil behavior, and the soft ground structure The ground structure of the coastal highway Hai Phong - Nam Dinh consists of a large thickness of soft soil, distributed near the ground surface Thus, deep treatment methods such as PVDs, vacuum suction, sand columns, soil-cement columns, and rigid piles will be a reasonable choice However, these methods also have their limitations For example with the PVD method, there is often a disturbance of the soil around the vertical artificial drain during construction (disturb effect), a broken vertical artificial drain when there is a lot of settlement, or interruption of the drainage path due to poor quality construction, reducing the consolidation capacity of the soft soil, so in many cases settlement still occurs during operation The method of sand columns, sand wells, or compacted sand columns, in addition to the important requirements of qualified river sand materials, is easy to cause a loud noise, affecting the environment and surrounding works and is costly Moreover, the exploitation of river sand is very difficult, increasingly scarce, adversely affecting the ecological environment With soil-cement columns, the ability to bring good effects on reducing settlement and increasing stability with an embankment on soft soil, but the high cost and high requirements on engineering as well as construction equipment are also very important The vacuum method is less suitable in treating soft soil for long constructions with narrow cross-sections such as this coastal highway Due to the above urgent reasons, the dissertation has been conducted with the purpose of developing the method of sea sand-cement columns This work has studied theoretical and experimental basis for the small-scale laboratory physical model, design and construction procedure, and especially using sea sand near the construction site as column material, to reduce the environmental effect caused by river sand exploitation Research objective - Clarification of soft ground structure characteristics and division of soft ground structure along the coastal highway Hai Phong - Nam Dinh - Proposition of ground improvement technique by sea sand – cement columns - Assessment of effectiveness of ground improvement by sea sand – cement columns Study object and scope - Study object: Soft ground structure and soft ground treatment methods by sea sand – cement columns - Scope of the study: The soft ground structure along the Hai coastal highway PhongNam Dinh Research contents - An overview study on soft soil, soft soil structure, and soft soil treatment methods in the world and in Vietnam - Study on the characteristics and division of soft soil structure of the coastal highway Hai Phong - Nam Dinh - Study and propose a new method to treat soft soil with sea sand - cement columns - Study to build a miniature physical model in the labs, analyze the numerical model to evaluate the initial effectiveness of the method of treating soft soil with sea sand cement columns, applied to the coastal highway of Hai Phong - Nam Dinh Research methodology Research approach - Systemic approach: the coastal highway from Hai Phong - Nam Dinh has been invested and built; coastal areas of Hai Phong - Nam Dinh widely distribute soft soils that need to be treated when building roads; soft soil treatment method suitable to Vietnamese conditions, using sea sand near the source as treatment material - Inheritance approach: Inheriting research results published in the world and in Vietnam - Theoretical approach: Research and apply relevant theories to solve the problems posed in the research content of the topic - Experimental approach: Deploy experimental work - Modern approach: Using experimental models, numerical models to solve problems in the research content of the topic Research Methods - Methods of synthesis and systematization of documents: Collect, analyze and systematize documents related to soft soil and methods of treating soft soil - Expert method: Organizing seminars and scientific seminars to consult experts and scientists on issues related to the research content - Theoretical research methods: Researching and applying the theory of changing geotechnical properties of soil and rock, soil mechanics theory and related theories to divide the soft soil structure, build the base the theory of sea sand - cement pile method to treat soft soil - Experimental research methods: Conduct field investigations and surveys, take soil samples; Experiment in the room - Modeling method: Building a physical model, analyzing the numerical model of the working process of sea sand - cement piles Creativeness and innovativeness - Clarifying the characteristics of the foundation structure and distinguishing the types of soft soil structure along the coastal route of Hai Phong - Nam Dinh section - Proposing a new method of treating soft soil with sea sand - cement piles, the process of calculating the design and construction of piles - Building an experimental research model, a numerical model to simulate the working of the foundation system - sea sand column-cement and initially evaluate the effectiveness of the method of treating soft soil with sea sand column – cement Scientific and practical contribution Results of the dissertation have contributed to the theoretical and experimental on the soft ground treatment methods by sea sand – cement columns Provide the new solution for the soft ground treatment of the Hai Phong-Nam Dinh coastal highway Document for teaching and scientific research of students, teachers, and civil engineers CHAPTER REVIEW OF STRUCTURE OF SOFT GROUND AND GROUND IMPROVEMENT TECHNIQUES 1.1 Overview of research activities on soft ground structures in the world The concept of soft ground structure is used to indicate the spatial arrangement of the ground layers (including soft soil) with the characteristics of origin, age, composition, state, physical and mechanical properties Recent studies of soft ground associated with the structure of the ground, Kamon and Bergado (1991) [56] stated that the ground is soft when there are soft clay soils distributed in areas with high groundwater levels Omar and Jaafar (2000) [68] studied the soft soil characteristics of the coastal area in the town of Cyberjaya (Malaysia), they found that the soft soil layers distributed along the coastal area mainly consist of organic silty clay with an average thickness of 2.0m from surface, even the thickness of very soft clay layers in some location reaches values of 10.m Underneath the organic silty clay layer is usually soft to firm clay Skempton (1953) [71] proposed the Activity Index, AI According to Skempton, soft soils with AI index > 1,25 usually represent soils with high mineral content of montmorillonite, which directly affects the shrinkage behavior of soil); Soils with AI index value less than 1,25 are those containing high content of kaolinite minerals The studies of C Ma et al (2010) [42], Zeng et al (2011) [60], Yadu (2013) [61], Uddin (2018) [47], Ye et al (2015) [62] showed that the soil in the coastal areas is mainly composed of soft to extremely soft mud The mineral composition is mainly kaolinite, illite, montmorillonite, and chlorite Soil layers in the soft ground are often highly sensitive to changes in moisture content, in which the volume of these soil layers often increases as soil moisture increases 1.2 Overview of research activities on soft ground structures in Vietnam Nguyen Thanh (1984) [23] conceived that “the ground structure is the soil layer used as the foundation for construction works, which is characterized by the laws of distribution of rock-soil formations with depth, in which each layer has a difference in structure, origin, age, composition, thickness, state, and geological properties” Le Trong Thang (1998) [24] stated that "the ground structure is an interactive part between the structure and the geological environment where the structure built upon it (ground), determined by the law of distribution in space, the ability to change over time of rock-soil formations This interaction is taken place within the area of influence of construction activities At the spatial limit, as the ground structure has a distribution of soft soil layers, it can be considered as a "soft ground structure" Pham Van Ty (1999) [29] claimed that "ground structure is understood as the spatial arrangement relationship of geological elements (soil layers), soil structure, quantity, shape, and size characteristics, composition, state, and properties of these constituent elements” Ta Duc Thinh (1990) [25] studied the spatial variation laws of some physico-mechanical parameters of the soft soil of the Hai Hung formation of soft soil ground in the northern delta by a mathematical model Nguyen Huy Phuong (2004) [16] introduced a concept that “the ground structure is the spatial relationship of the soil and rock layers, their compositional, architectural, and structural characteristics, as well as the engineering geological characteristics of soil and rock layers located in the compression zone of the construction work activities” In 2016, Nguyen Van Phong et al [17] divided the soft ground structure in the northern coastal area into types, say (I, II, III, IV); forms, namely (a, b, c, d) 1.3 Overview of recent research on soft ground treatment methods in the world 1.3.1 Shallow ground improvement methods The most common methods are the group of ground improvement using binders such as cement, lime, blast furnace slag, and the group of using soil reinforcements including geotextile, geobag, geogrid Recent studies on the use of binders such as cement, lime, have focused mainly on optimization of soil to binder materials ratio for different ground structures (Nieminen, 1977, Vitanen, 1977); on evaluating the behavior of lime-treated expansive soils (Holinn et al., 1983); on adding more cementitious materials into the treated soil such as cement, lime, the gypsum powder (Kujala Lahtinen, 1992); on using lime and cement to improve silty clayey soils in Malaysia (Bujiang B.K Huat, Shukri Maail Thamer Ahmed Mohamed, 2005) [39]; or on improving very soft soils using cement Ho Chan (2008) [64] The ground technique group using soil reinforcements such as geotextile, geobag, geogrid are widely used to prevent slope failure and embankment failure This technique group has been applied in many countries in the world such as in the USA, England, France, Holland, Japan, China, and Vietnam as well 1.3.2 Deep ground improvement techniques The deep ground improvement technique, namely Prefabricated Vertical Drains, also known as Wick Drains or band drains were initially introduced by Kjellman [74] in 1948 The PVDs act as drainage paths to take pore water out of soft compressible soils that consolidate faster under a constant surcharge load Since the 1970s, PVDs have been commonly used to replace sand wells The use of vacuum technique to replace backfill soil for loading when treating soft soils with PVDs was first mentioned by Kjellman in 1952 (Griffin and Kelly, 2014) [48] In 1958, Murayama [65] introduced a new deep ground technique, the key of this technique is to use compacted sand Afterward, this technique was employed in studies of Ezoe et al (2019) [37] for soft clayey, and sandy clayey soils In 1995, a new technology of nonvibration compaction by static compaction equipment was applied for ground treatment (Harada et al., 2004) [58] In 2008, pressure grouting technology using sand materials was developed and applied in practical projects of ground improvement in Japan Deshpande and Vyas (1996) used geotextiles to wrap around piles of coarse materials [45] to treat deep soft grounds Recently, several experimental and numerical models have been built to figure out primary factors affecting the performance of crushed stone piles covered with geotextiles (Dutta et al., 2016) [72]; the finds of those works show that the optimum spacing between the piles wrapped with geotextile is 0,25D, where D is pile diameter Deep ground improvement techniques using soil-cement, soil-lime columns were developed and widely used in Europe since the 1960s such as in Sweden and Finland, England, Germany [75], [53] In Asia, the soil-cement method has been widely used in Japan since 1970, then in Thailand, China Soil-cement columns are not only widely used in stabilizing soft ground but also as retaining walls for deep excavations, stabilizing slopes, and preventing liquefaction [50] Many factors affect the effectiveness of soft soil treatment by soil-cement columns, in which the ratio of water/cement is an important factor that affects the strength development of soil-cement columns (Horpibulsuk et al., 2012) [70] 1.4 Overview of recent research on ground improvement techniques in Vietnam To date, there have been several techniques applied to stabilize soft ground in Vietnam, including shallow soil treatment with cement, lime, preloading; and deep improvement methods like PVDs, sand wells, soil-cement columns, soil-lime columns, vacuum 1.4.1 Shallow ground improvement methods The use of cement and lime materials to improve shallow ground was firstly used in 1967, the study was conducted in the Hanoi University of Science and Technology, accordingly, the compressive strength of soil-cement samples was found to be increased as the cement content increased In the case of using lime, the compressive strength reaches the maximum value at 9% of lime; however, as the lime content increases up to a value of 12% the compressive strength reduces significantly 1.4.2 Deep ground improvement techniques Ground improvement using deep treatment techniques has been applied in Vietnam since 1980x To date, several techniques have become popular and been widely used such as soil-cement columns, soil-lime columns, PVDs, sand columns, compacted sand columns, vacuum[30], [36], [27], [28], [12], [13], [34], Ta Duc Thinh (2002) [27] proposed a method of treating soft soil with sand-cement-lime columns, in which cement contents were of 5; 7,5; 10; 12,5 and 15%, and lime content were 5, 7, 9, and 11% compared to dry sand mass In 2002, the Institute of Irrigation Science performed research using Jet-grouting technology to repair and waterproof irrigation and hydroelectric projects in Vietnam [9]; to date, this technique has been widely applied to soft soil treatment or foundation reinforcement for many different types of construction works Methods of sand columns, compacted sand columns, preloading with PVDs or vacuum are also widely used in road construction engineering Expressways such as Hanoi - Cau Gie, Hanoi - Lao Cai, Hanoi - Hai Phong, National Highway 1A, Hanoi - Hai Phong Expressway, Trung Luong - My Thuan have applied those techniques to improve the soft ground Moreover, several national standards on soft ground treatment have been issued such as TCVN 9403:2012, TCVN 9842:2013, TCVN 11713:2017 , serving as an important legal basis in the treatment of soft soil in Vietnam 1.5 Conclusion remark of Chapter - Soft soils are defined as soils that have special composition, state, and properties They have high water content, high compressibility, low strength, deformation is large The soft ground structure is the concept that refers to the spatial arrangement of the ground layers (including soft soils) with the characteristics of origin, age, composition, state, physical and mechanical properties of them Recently, there have been many methods of soft ground improvement, each method has its advantages, disadvantages, and effective application range Selecting the appropriate soft ground treatment method depends on many factors, in which notably on the type of construction work, soft soil properties, and especially soft ground structure characteristics - The use of deep improvement methods for soft ground such as PVDs, vacuum, sand columns, sand wells, compacted sand columns, soil-cement columns to construct coastal roads in the Hai Phong - South section Dinh should pay attention to the economy, the ability to a source of local materials, in which the natural environment needs to be protected CHAPTER CHARACTERISTICS OF SOFT GROUND STRUCTURE IN THE HAI PHONG – NAM DINH COASTAL HIGHWAY 2.1 Soft soil characteristics in the coastal area of Hai Phong – Nam Dinh 2.1.1 Age and Origin The soft soils in the coastal area of Hai Phong - Nam Dinh has Holocene age, the Thai Binh and Hai Hung formations belong to Quaternary sediments Origin includes river (a), sea (m), river-sea (am), river-marsh (ab), river-sea-marsh (amb), sea-wind (mv) [11] 2.1.2 Material composition 2.1.2.1 Soil grain size composition: According to TCVN 9355:2012, soft soils in the study area mainly consists of clayey soil, sandy clay, and clayey sand; state of soft soil is included liquid and plastic states 2.2.2.2 Organic ccontent: in the study area, the soft ground was characterized by low values of organic content soils, organic content was found in a range of 2,17-4,3%, common value is of 3-4% 2.2.2.3 Salt content: Soil in the coastal area of Hai Phong - Nam Dinh belongs to the type of low salt contamination, with a salt content of (0,24 - )1%, commonly (0,3 0,6)% 2.1.3 Construction characteristics of soft soils in the study area According to [19], [28], [11], construction characteristics of soft soils in coastal area in Hai Phong – Nam Dinh is very minimal, they are mainly in the states of liquid and high plastics, are low in bearing capacity (Ro ≤ 50kPa), deformation modulus is trivial (Eo≤ 3000kPa) Soft sandy clay, very soft to soft, has a very low bearing capacity (50 ≤ Ro 5000kPa), have river-sea (em) and sea (m) origin Silty sand, clayey sand have Ro ≥ 100kPa, Eo > 5000kPa; these types of sandy soils are not classified as soft soils, but they are sensitive to dynamic load, inappropriate for construction activities 2.1.4 Physico-mechanical properties 2.1.4.1 Mechanical properties Preconsolidation pressure of soft soils in study area is found to be low, with its value of (47Pa  84Pa), low permeability (k = (0,3  3,9).10-7cm/s), high compressibility (0,2  0,4), For the soft to very soft clay soils, the cuu = 8kPa  21,6kPa, uu ≈ 1o; clayey sand: cuu = 2kPa, uu = 7o26’ Typical SPT values were found of N30 =  5; the u = 10Pa  31Pa (from vane shear test); CPTu qt = 0,36MPa  MPa, fs = 0,005MPa  0,01MPa, u2 = 0,02MPa  0,8MPa The ratios of horizontal consolidation coefficient to that of vertical one Ch/Cv = 1,04 7,76 (silty soft clay), Ch/Cv = 2,636,35 (very soft clay); Ch/Cv = 1,7  6,1 (silty sandy clayey soils) [11] 2.1.4.2 Physical properties: According to [11], soft swamp soils in the study area consists of muddy clay soils, muddy sandy clay have high viscosity value (1,08 ÷ 2,66); high moisture content (40%  60%); low unit weight (1,63 g/cm31,74g/cm3) Soft river, sea, river-sea origin soils are often found to be in liquid to plastic state (IL = 0,79 ÷ 1); high moisture content (32%  50%); unit weight is pretty low (1,70 g/cm31,83 g/cm3) 2.2 Division of soft ground structure along the Hai Phong - Nam Dinh coastal highway The soft ground structure of the entire Hai Phong - Nam Dinh coastal highway is divided into Classs (I, II), and types (Ia, IIa, Ib, IIb, Ic, and IIc) with discontinuous distribution, according to which: Class I, the soft soil layer is distributed right on the ground (under the surface level); below this soft soil is a medium to dense sand, sometimes loose sand Class II, the soft soil layer is distributed right on the ground, but below are soft, firm, and stiff clay soils The thickness of typess Ia, and IIa is less than 5m, typess Ib and IIb is of (5m  15m), and types Ic, IIc is larger than 15m Based on the thickness of different ground structures, the recommended improvement techniques are as follows: - For the ground structures typess Ia, IIa which has a thickness of soft soil layer less than 4m: shallow improvement methods such as cement mixing method, lime mixing method, or replace the soft soil layer by a better soil (in terms of bearing capacity) or placing geocomposite materials like geotextile, geogrid - For the types Ib, IIb (thickness is in range of 5m 15m) and types Ic, IIc (thickness is larger than 15m): deep improvement methods is highly recommended 2.3 Conclusion remark of Chapter Based on the age, origin, soft soil types in the coastal area as well as in the study scope are of Holocene age, Thai Binh and Hai Hung formations, which belong to Quaternary sediments in the Northern Delta (Q2tb, Q2 1-2hh) with mainly marine (m) or river-sea (am) origin The soft ground structure along the Hai Phong-Nam Dinh coastal highway is divided into classes (I, II), types, namely Ia, IIa, Ib, IIb, Ic, and IIc, in which, Class I has a soft soil layer distributed right on the ground, below are sandy soil layers with better construction performance Class II has a soft soil layer distributed right on the ground (same as in Class I), but, below are clay layers with better construction performance Types “a” has a thickness of soft soil less than 5m, the thickness of type “b” from 5m to 15m, and type c is larger than 15m The division of soft ground structure types along the Hai Phong-Nam Dinh coastal highway is crucial in the selection of ground treatment methods, ensuring technical and economic efficiency Selection of treatment methods suitable to the characteristics of soft soil, weak ground structure, and natural conditions of the area where the highway was built on the ground CHAPTER RESEARCH AND PROPOSING SOFT SOIL IMPROVEMENT METHOD USING SEA SAND – CEMENT COLUMNS 3.1 Basis for proposing method of treating soft soil improvement with sea sand cement columns The types of foundation structures Ib, Ic, IIb, IIc with soft soil thickness of 5-15m and greater than 15m are classified in Chapter 2, the selection of soft soil treatment methods for the building is reasonable The river sand source is increasingly depleted, its exploitation has been facing many difficulties, adversely affecting the ecological environment The technique of sea sand-cement column allows the use of sand materials near the construction site, with a complete theoretical and experimental basis and design, construction, and inspection procedures It will reduce the costs and the usage of river sand sources It will be suitable to treat the soft soil of the Hai Phong-Nam Dinh coastal highway 3.2 Theoretical principles of soft soil treatment method by sea sand - cement column 3.2.1 Mechanical compaction principle Treatment of soft soil with the sea-cement column is to use a specialized equipment to put dry sea sand - cement mixture into the soil foundation in the form of circular crosssectional columns and the soil does not take out Sea sand - cement material will take the place of the soil, the water and air in the soil will escape, the pore volume is reduced, the soil foundation is compacted The change in volume of soil mass before and after treatment with sea sand-cement columns is due to the change in pore volume in the soil mass The variation of soil volume is linearly proportional to void coefficient variation: 𝑉𝑜 : ∆𝑉 = ∆𝜀 (3.1) where 0,  is the initial void ratio and improved void ratio The (1+𝜀0 ) effectiveness of soil compaction depends on the volume of sea sand-cement put into the soil foundation, the porosity of the soil as well as the amount of water and air escaping from the soil 3.2.2 Increase principle of sea sand – cement column strength and surround soft soil strength * The process of increasing the strength of sea-cement sand columns: The strength of sea-cement sand columns are formed by a complex physicochemical process, with two 14 3.5.2.1 Calculation of settlement and load capacity in the case of sea sand - cement column improved soft soil According to Swedish design guidelines, when the strength of the column is not greater than 150kPa, the calculation of settlement and load-bearing capacity of the column-soft soil is converted to a homogeneous ground (equivalent ground) The results show that, when the cement content is less than 5%, the post-treated ground can be considered as a homogeneous soil foundation by converting the characteristics of shear strength, deformation of columns, and ground soil for replacement ratio The settlement of the soil-column ground after treatment can be calculated according to the theoretical methods of linear deformation environment, the most popular being the "equivalence layer" method and the layer-by-layer settlement method [26] The load capacity of the treated ground can be calculated by methods based on the theory of linear deformation such as: Puzưrevxkiy: P0 = γh π cotgφ+(φ+ ) π cotgφ+(φ− ) + π.cotgφ π cotgφ+(φ− ) (3.4) c Maxlov: Pgh = πy(b.tgφ+h+γ.tgφ π cotgφ+(φ− ) + γh (3.5) π φ Jaropolxkiy: Pgh = ( − ) c b.cotg 2 +h+γ.tgφ π cotgφ+(φ− ) + γh (3.6) Where:  - average internal friction angle of treated ground; c - average cohesion of the treated ground;  - average unit weight of the treated ground; h - considered foundation depth; b - the width of the foundation The above settlement and load-bearing calculation methods apply to calculating of natural ground When calculating the design of sea-cement sand columns, it is necessary to distinguish in two cases: slow construction and fast construction Slow construction is the case that after completing the soil treatment, it is necessary to wait a certain time before starting construction In this case, the volume of sea sand cement put into the soil foundation to treat soft soil is considered as an external load acting on the soil foundation Under the effect of this load, an additional stress z will appear in the ground, causing deformation of the ground (in the longitudinal and transverse directions), the value is determined: z =  + u (3.7), in where  is the effective stress acting on the soil particles; u - neutral stress absorbed by water By time, the effective stress increases, the neutral stress decreases, but at any time in the ground, the above correlation still exists In the case of slow construction, the processes of mechanical compaction, consolidation, and physico-chemical reaction between cement and the environment have ended, the entire external load (mass of sea sand cement) due to soil particles collector (z = ), neutral stress is suppressed (u = 0), the ground deformation reaches a stable value, the soil foundation is completely compacted, becoming a new ground like the natural ground The suitable method of calculating settlement and load-bearing capacity of the treated soil is the linear deformation environment 15 Fast construction is right after completing the treatment of soft soil, construction is built immediately At this time, the processes of mechanical compaction, consolidation of the ground soil, and the physicochemical reactions of cement with soft soil have not ended The physico-mechanical parameters of the ground soil used to calculate the settlement and load-bearing capacity of the foundation are still in the process of change, have not reached the stable value (constant), so obviously, the calculation results are not accurate The calculated settlement of the soft soil will be larger than the reality and the calculated foundation soil's bearing capacity will be smaller than the reality Therefore, in the case of fast construction, the treated ground cannot be considered as natural ground However, if it is in favor of safety, it is still possible to use the results of calculating the settlement and load-bearing capacity of the ground as for the natural ground to serve the design of the foundation of the building 3.5.2.2 Calculation of settlement and load capacity in the case of sea sand - cement reinforced soft soil When the cement content in the sea sand-cement mixture is over 10%, in essence, the sea sand-cement column method is similar to the soil-cement column In this case, it is possible to calculate the settlement and load capacity of the treated foundation according to the calculation methods for soil-cement columns Some authors consider the reinforced column foundation as a conventional block foundation without deformation and only calculate the settlement of the soil at the bottom of the conventional block foundation Others calculate the equal strain method with the assumption that the column and the soil surrounding the column are a conventional mass and that the axial deformation of the reinforcement column corresponds to the settlement of the soil around the column As long as the axial stress is less than the ultimate creep strength of the column, the axial stress of the column depends on the compressive modulus of the column material and of the soil around the column and is calculated by the below equation: σc = Pc Ac = σ M ac + d (1−ac ) (3.8) Mc where, c - axial stress of the column; Pc - total load acting on the columns; Ac - crosssectional area of the reinforced column;  - average stress at the bottom of the foundation; ac - replacement area ratio; Md - the compressive modulus of the soil around the column, it is usually taken as 150cu with cu - the shear resistance of the soil around the column, as determined by a Field Vane Shear Test or Cone Penetration Test; Mc compressive modulus of column, it is taken as (50-100)Cc with Cc - unit cohesion of column material The settlement of the soil foundation is determined by the sum of the settlement of the reinforced soil block with depth H and the settlement of the foundation under the reinforced block The settlement of the reinforced soil mass is determined by the equation: S= 𝜎 𝑀 𝐻= 𝜎𝐻 𝑎𝑐 𝑀𝑐 +(1−𝑎𝑐 )𝑀𝑑 (3.9) The settlement of the soil under the reinforced block is determined by conventional 16 methods, but the settlement reduction factor taken into account, it is the ratio between the settlement of the reinforced soil mass and the settlement of the unreinforced soil For predicting load capacity, sea sand-cement columns are used to reinforce the soil, essentially similar to soil-cement columns Therefore, it is possible to use the methods of calculating the bearing capacity of soil-cement columns proposed by Broms (Sweden), Bergado, and many others (AIT), TCVN9403:2012 to calculate the bearing capacity of sea sand - cement columns 3.6 Constitution of design, construction, and inspection procedures of sea sand cement column reinforced soft soil In order to be able to apply the sea sand-cement column method in practice to treat soft soils, besides the theoretical and experimental basis, it is necessary to develop a procedure of designing, constructing, and inspecting columns, ensure reliability, and feasibility 3.6.1 Constitution of design procedure 3.6.1.1 Design of sea sand-cement improved soft soil Designing sea sand - cement columns to improve the soil foundation, that is, to act as sand columns, sea sand - cement materials occupy the pore volume in the soil, reducing the total pore volume, the soil particles are rearranged, the ground is compacted and the bearing capacity of the soil foundation is increased At this time, the strength of the column is insignificant, the role of cement in the column material mixture is only a adhesive of sea sand grains so that the column is not cut, broken and the sand particles not move into the ground Therefore, the cement content in the basic column material mixture does not need to be too high The procedure of sea sand - cement column design is carried out in the following sequence of steps: (1) Geotechnical survey and division of the soil foundation structure; (2) Determination of cement content in the column material mixture (desired column strength) and related specifications and requirements; (3) Determining the void ratio and required parameters; (4) Determination of column length, column diameter, number of columns and distance between columns 3.6.1.2 Design of sea sand-cement reinforced soft soil Using sea sand - cement columns to reinforce the soil foundation is using the high strength of the sea sand - cement columns themselves, increasing the stress concentration ratio thanks to their stiffness to support the construction load In this case, the bearing capacity of the column must be large enough The sequence of design of sea sand - cement columns to reinforce the basic foundation includes steps such as (1) Geotechnical survey and division of the foundation structure of the construction area; (2) Determination of cement content in the column material mixture (desired column strength) and related specifications and requirements; (3) Calculation of column length, column diameter, column load capacity, number of columns and distance between columns 3.6.2 Constitution of the construction procedure On the basis of working drawings, the construction process of sea sand - cement column is carried out in the sequence of the following main steps: (1) Selection of construction equipment; (2) Preparation of construction site; (3) Construction of trail columns; (4) 17 popular construction; (5) Quality evaluation of the effectiveness of the method 3.6.2.1 Selection of construction equipment For small loading projects, the construction site is small, column length less than 15m, UGB-50M equipment can be used (Figure 3.9, 3.10) This is a multifunction drilling machine, with a capacity of 150 horsepower, which can be drilled with a two-way twisting reel that rotates forward and backward with great torque This equipment has the same construction mechanism and sequence of the sand-cement-lime column that was successfully tested in Quang Ninh, Thai Binh [27] Figure 3.3 Twisted auger drill for sea sand – cement column construction [27] Figure 3.4 Twisted auger drill machine UGB-50M [27] Figure 3.5 Sea sand – cement column produced by UGB-50M [27] When the column length is greater than 15m, the construction site is spacious, without affecting the surrounding works, equipment from Sweden, Japan, China can be used according to the principle of column driving machine or vibrating hammer, the mixture is poured inside the tube The equipment can be used such as Hitachi PD 100, Cobelco 100P, Nippon Sharyo DH 408, DH 608, with tonnage from 40T to 65T 3.6.2.2 Preparation of site Plan The construction site is prepared according to the design and environmental requirements, as well as in compliance with the relevant current technical standards Sea sand and cement materials imported into the construction site must have a certificate of inspection of technical characteristics specified in the design and current standards 3.6.2.3 Trial column construction The number of trial columns to be constructed is in accordance with the regulations of the design agency and the current technical standards for quality control The purpose of test column construction is to confirm design requirements and create critical control values for equipment, materials, technical processes of the same type when popular construction The positions of columns on the ground must be located by specialized equipment If equipment or twist drills are used, the construction control parameters include drilling speed down and up; the rotational speed of the drill rod; compressed air pressure, and the amount of sea sand and cement used 3.6.2.4 Popular construction As a result of the quality evaluation of test columns meeting the design requirements, popular construction is carried out in the same sequence as when trial columns If the column quality is not satisfactory, it is necessary to calculate and adjust the design parameters 3.6.2.4 Quality evaluation and effectiveness of soil improvement method 18 Depending on the requirements for settlement, stability, and load capacity in the short term as well as in the long term, the soft soil treatment works will be inspected and evaluated by various relevant field tests 3.6.3 Inspection Sequence After completing the column construction, it is necessary to evaluate the quality and effectiveness of the soft soil treatment and conduct inspection While the inspection standards of sea sand-cement columns have not yet been issued, TCVN 9403:2012 or other published relevant documents can be used The sequence of steps to take over columns includes: (1) evaluating the results of soil treatment (evaluating the quality of the columns, assessing the quality of the soil, determining the settlement and the loadcarrying capacity of the treated soil); (2) and making inspection records 3.7 Conclusion of chapter - The scientific basis of the method of sea sand-cement column treated soft soil is the processes of increasing the load capacity and reducing the settlement of the soil foundation, including mechanical compaction, increase in the strength of the sea sand cement and surrounding ground soil; permeability consolidation and physico-chemical effects between cement and ground - The compressive strength of the sea sand-cement mixture depends on the cement content With the cement content of 5%, 7%, 10%, 13%, 15%, the compressive strength at the age of 28 days is 0.65MPa, 1.05MPa, 1.30MPa, 1.78MPa, 2.45MPa, respectively - Experimental study on the small-scale physical model of sea sand - cement columns has shown that the load- settlement relationship in the case of single column and group of columns compression consists of stages: linear phase (phase) compaction phase), nonlinear phase (under the bottom edge of the foundation, plastic deformation area begins to appear, the soil begins to be damaged), the displacement phase increases rapidly before failure, consistent with previous studies Here, the dependence between the load and the settlement of the soil is similar to that of the high-strength cement-soil column - Numerical modeling of the sea sand - cement column improved soft soil shows that the behavior of the reinforced sea sand - cement column seems to be linear when the applied load is less than 6.5kN, then is the plastic behavior when the applied load is from 6.5kN to 12.4kN, followed by the full yield behavior when the applied load is greater than 12.4KN, where the load hardly increases but the deformation continues increase The 3D numerical results are quite similar to the experimental results, serving as the basis for calculation and prediction of column group behavior for the cross section and actual size of the column as in the case of high-strength cement-soil columns Settlement and bearing capacity of the sea sand - cement columns treated soft soil are calculated according to the theory of linear deformation environment in the case of column design to improve the soil (cement content less than 5%); The calculation is similar to that for soil-cement column concept in the case of designing columns to reinforce the soil (cement content greater than10%) - The parameters for sea sand - cement column design include the diameter, length; distance columns; bearing capacity, and settlement of the composite soil Column construction equipment can use existing equipment or fabricate suitable equipment for 19 Vietnamese conditions CHAPTER 3D NUMERICAL MODELING FOR EFFECTIVENESS ANALYSIS OF SEA SAND – CEMENT COLUMN IMPROVED SOFT SOIL 4.1 Technical parameters of Hai Phong-Nam Dinh highway and proposed methods of soft soil improvement 4.1.2 Technical parameters The Hai Phong-Nam Dinh coastal highway is designed according to the standard of grade III, pavement class A1, speed of 80km/h embankment width of 12m, two-lane 2x3.5m = 7m, the wayside width of 2x2.5m (= 5m) The embankment height is to 8m depending on the natural elevation The slope of the embankment is 1: 1.5 4.1.3 Soil foundation structure Along the coastal Hai Phong-Nam Dinh highway, Soil ground structures are divided into two classes (I, II) and three types (a, b, c), as presented in Chapter 4.1.4 Proposed methods of soft soil improvement Based on the technical parameters, the selection of method and design of soft soil treatment depends on the soft soil structure and the existing construction equipment capacity 4.1.4.1 Soft soil structure of type Ia, IIa There is soft soil distributed on the soil surface, the thickness is less than 5m, it is recommended to choose shallow treatment methods such as lime or cement mixing method, soil replacement, compaction, and geotextile 4.1.4.2 Soft soil structure of type Ib, IIb There is soft soil distributed on the surface of the soil, the thickness is from 5m to 15m, it is recommended to choose a deep treatment method with sea sand - cement columns The process of calculating, designing, constructing, and inspection columns is presented in Chapter Construction equipment for sea sand-cement columns in this case is recommended for the UGB-50M spiral drill (Figure 3.10) 4.1.4.3 Soft soil structure of type Ic, IIc There is soft soil distributed on the soil surface, the thickness is greater than 15m, it is recommended to choose a deep treatment method with sea sand - cement columns If the bottom of compressive zone is a good soil layer, it is recommended to design columns reinforced soft soil, and if below compressive zone is a soft soil layer, it is recommended to design columns improved soft soil Recommended construction equipment is Hitachi PD 100, Cobelco 100P or Nippon Sharyo DH 408 or suitable equipment for Vietnamese conditions 4.2 3D numerical modeling for effectiveness analysis of sea sand – cement column improved soft soil 4.2.1 Selection of technical parameters of 3D model The typical design location has a soft soil structure of class Ib, including the following soil layers: Layer 1- backfill soil of 1.0m thick; layer 2- soft clay of 6.5m; layer - soft clay of 8.0m; layer - compacted mixed sand of 5.0 m Some mechanical parameters of the soil layers are listed in Table 4.1 20 Table 4.1 Mechanical parameters of the soil structure No Layer Layer Layer Layer (4a) Layer Soil type backfill, 1,0m Clay, 6,5m Clay, 8m Compactes, mixed sand, 5m  c (g/cm3) (g/cm3) Ip 1,73 1,73 1,68 1,88 IL e a1-2 c  (o ) -1 (kPa ) (kPa) 6o11’ 1,19 20,58 0,87 1,261 9,10 6o11’ 1,1 27,08 0,85 1,145 11,1 6027’ 1,47 4,96 - 0,823 0,033 13o58’ 6,2 6,2 6,7 5,4 4.2.2 Sea sand column parameters Sea sand - cement columns are designed with a diameter of 0.5m, a length of 16.5m, square arrangement, column distance of 2.0m (Figure 4.1) 4.2.3 3D numerical modeling 3D numerical modeling was built using FLAC3D software The model is a slide of half of the roadbed with columns, that allows to determine the influence of the column group and the arch effect on the top of the columns in the embankment (Figure 4.2) In the analysis, embankment and soft soil layers, underlying sandy soil layer, sea sandcement columns were selected for the model of linear elasticity, perfectly plasticity according to the Mohr-Coulomb failure model (a) Model elevation (b) 3D mesh of model Figure 4.1 cross section of Figure 4.2 numerical model embankment Table 4.2 The parameters of the suggested constitutive models Vật liệu / Materials Model Parameters Backfill MC E = 2,48 MPa,  = 0,3,  = 6o11’, c = 6,2 kPa, = 17,3 kN/m3 Clay MC E = 2,48 MPa,  = 0,3,  = 6o11’, c = 6,2 kPa, = 17,3 kN/m3 Clay MC E = 1,93 MPa,  = 0,3,  = 6o27’, c = 6,7 kPa, = 16,8 kN/m3 Compacted sand MC E = 6,15 MPa,  = 0,3,  = 13o58’, c = 12,4 kPa,  = 18,8 kN/m3 Embankment Interface MC clay-column E = 30 MPa,  = 0,2,  = 19 kN/m3 ks = kn = 1108 kN/m/m,  = o8’, c = 6,2 kPa 21 Table 4.3 Mechanical parameters of sea sand – cement columns (according to D.Wang et al) [46] E  qu c’ (kPa) Ko  ’ (o)  (MPa) (kN/m3) 0,5 670 0,24 22 161 43 0,32 13 1,0 947.6 0,24 22 273 43 0,32 13 1,5 1160,6 0,24 23 386 43 0,32 13 2,0 1340,2 0,24 23 498 43 0,32 13 2,5 1498,4 0,24 24 611 43 0,32 13 4.3 Analysis of Effectiveness of soft soil improvement using sea sand – cement columns 4.3.1 Effect of sea sand - cement columns on soft soil settlement The model embankment over unreinforced soft soil was built and assigned with its own load and some parameters of the model (Figure 4.3.a) The settlement of soft soil is very large (117cm), far exceeding the allowable value (30cm) according to 22TCN262-2000 The model of embankment over sea-cement column reinforced soft soil is shown in Figure 4.3.b The settlement is significantly reduced, it is only 16cm Thanks to the reinforcement with sea-cement (b) sea sand – cement (a) unreinforced soil columns, the total settlement due to reinforced soil, qu = 1,5 the embankment load are only MPa about 1/7 of the settlement of the Figure 4.3 3D numerical results in terms of roadbed on unreinforced soft soil settlement under body embankment load 4.3.2 Effect of load on soft soil settlement Figure 4.4 shows the settlement of embankment on the unreinforced and reinforced soft soil subjected to uniform load p = 15kPa The settlement of embankment on unreinforced soft soil is predicted to be 39 cm The settlement of the embankment on the reinforced soft soil is greatly reduced, it is only cm it is clearly shown that the reinforcement is clearly observed, the settlement of the embankment on the reinforced soft soil is only about 1/10 compared to the unreinforced case As a result, using sea sand cement columns to reinforce the soft soil not only significantly reduces the settlement of the embankment, but also effectively prevents the (a) Unreinforced soft soil (b) Sea sand cementdestruction of soft soil, reinforced soft soil qu = increasing the load capacity, 1.5 MPa 22 such as increasing the elastic Figure 4.4 settlement of embankment when the uniform modulus, and widen the load of 15kPa elastic range of the road embankment 4.3.3 Effect of sea sand - cement column on horizontal displacement of embankment Horizontal displacement of embankment in cases of unreinforced and sea-cement column reinforced soft soil subjected to the body weight of embankment (Figure 4.5) The horizontal displacement at the end of the embankment talus in the case of the embankment over unreinforced soft soil is estimated at 49.4cm This value is only 4.8cm for the case of sea-cement column reinforced soft soil, the (a) Unreinforced soft soil (b) Sea sand cementreinforced soft soil qu = phenomenon of pushing up on the 1.5 MPa ground next to the embankment talus is not observed The Hình 4.4 horizontal displacement at the end of compression zone is observed at a embankment talus subjected to the body weight of embankment certain depth in soft soil 4.3.4 Effect of sea sand-cement column stiffness on settlement, horizontal displacement, and stress on soft soil and column head There is a significant difference between the stress applied to the soft soil in the case of unreinforced and reinforced soil (Figure 4.6) The stress value applied to the soft soil in the reinforced case is about half that of the unreinforced case (45kPa and 90kPa) The stress acting on the soft soil reduces which leads to a decrease in the settlement of the soft soil Figure 4.6 Effect of uniform load on soft soil stress on unreinforced and reinforced case 4.3.4.1 Effect of sea sand-cement column strength on embankment settlement Figure 4.7 shows the relationship between the total settlement of the embankment and the strength of sea-cement column corresponding to different uniform load Obviously, as the applied load increases, the settlement of the embankment increases 23 It can be seen that, when the uniform load is insignificant, the embankment settlement in the cases of the strength qu = 1.5MPa and 2.5MPa is almost the same When the Figure 4.7 Effect of sea Figure 4.8 Effect of sea load is significant, the sand-cement column sand-cement column strength embankment settlement on column displacement depends significantly on the strength on embankment settlement strength of the column (Figure 4.7) In addition, Figure 4.7 also shows that the increased strength of the column will significantly reduce the settlement of the embankment The settlement corresponding to the strength of the column of 1.5MPa is only a half of the settlement corresponding to the strength of the column of 0.5MPa Figure 4.8 shows the vertical displacement of the columns corresponding to the load level of 20kPa When the column strength increases from 1.5MPa to 2.5Mpa, the displacement of the column significantly reduces compared to the column strength of 0.5MPa Figure 4.9 Effect of sea sand-cement column strength on horizontal displacement of embankment 4.3.4.2 Effect of sea sand-cement column strength on horizontal displacement of embankment When the compressive strength of the column increases, the horizontal displacement at the end of embankment talus is significantly reduced (Figure 4.9) At the pressure level of 50kPa, the observed horizontal displacement is 78cm, 30cm, and 14cm, corresponding to the compressive strength of the column of 0.5 MPa, 1.5 MPa, and 2.5 MPa 4.3.4.3 Effect of sea sand column strength on the stress applied to soft soil and column head When the compressive strength of sea-cement columns increased from 0.5MPa to 1.5MPa, the applied stress on soft soil was significantly reduced (Figure 4.10), the result is similar to previous studies of J.Han and M.Gabr [55], of HLLiu et al [51] When the strength increases from 1.5MPa to 2.0Mpa, the stress acting on the soft soil does not change significantly It is concluded that the optimal design value of sea sand - cement column is achieved when qu = 1.5MPa Figure 4.11 also shows the difference of stress applied to the top of column with column strength values 24 Figure 4.10 Effect of sea sand column strength on the stress applied to soft soil Hình 4.11 Effect of sea sand column strength on the stress applied to column head (p = 20kPa) 4.3.5 Effect of sea sand-cement column length on embankment settlement and soft soil stress 3D numerical model of the length of column changes respectively L = 8.5m (located in at bottom of first weak layer); L = 13.5m (located in the middle of the second weak clay layer); L = 16.5m (columns through out two weak layers to the load-bearing sand layer) The compressive strength of the column is 2.5MPa Figure 4.12 Effect of sea sand-cement Figure 4.13 Effect of sea sand-cement column length on embankment settlement column length on column displacement at p = 20kPa 4.3.5.1 Effect of column length on settlement and horizontal displacement of embankment When the column length is 8.5m, the settlement is 1.4 times greater than that of the column 13.5m, and approximately times when the column is installed on good ground (16.5m) (Figure 4.12) it means that the highest effectiveness is reached as the maximum length of the reinforced sand column is greater than the depth of the soft soil When the column is placed in soft soil, the settlement of the soft soil and the displacement of the column head are approximately the same, while the displacement of the column (2.5cm) will be smaller than the settlement of soft soil (5cm) when the column is anchored into the bearing layer (Figure 4.13) The horizontal displacement of the embankment talus is similar to the settlement result As the column length increases, the embankment bed 25 will be more stable in the horizontal direction (Figure 4.14) 4.3.5.2 Effect of column length on the stress applied to soft soil When the length of the column is 8.5m and 13.5m, the stress transmitted to the soft soil is almost the same When the length of the column is 16.5m, the stress applied to the soft soil is reduced relative to the two cases above (Figure 4.15) Figure 4.14 Effect of column length on Figure 4.55 Effect of column length on the horizontal displacement at the toe of column head stress slope of embankment 4.3.5.3 Effect of column length on the stress applied to the column head The stress acting on the columns is not the same within the embankment (Figure 4.16), the difference is not too large when the column length changes For example, for the first column, the stresses acting on the column head are 1.70MPa, 1.83MPa, and 2.08MPa, respectively, with column lengths of 8.5m, 13.5m, and 16.5m, respectively Figure 4.16 Effect of column length on column head stress at p=20 kPa 4.4 Conclusion of chapter - Reinforcement of soft soil by sea sand-cement columns significantly reduces settlement, horizontal displacement of soft soil under the effect of embankment load, and uniform surcharge load - The results of numerical model analysis also show that the arch effect inside the embankment, the stress transmitted to the sea-cement sand column is many times greater than that to the soft soil, thus soft soil stress is reduced - When the strength of the column increases, the settlement of the embankment and the horizontal displacement of the toe of slope decreases, which means that the stability of the embankment increases Numerical results also figure out that there exists an optimal 26 column strength that gives the best effect in terms of technical and economy - the value is around qu = 1.5MPa - The depth of reinforcement greatly affects the settlement of the embankment under embankment load and uniform surcharge load When the length of the column penetrates the soft soil, reaching to substratum, it will bring the best bearing effect, but stress transmitted to the soft soil and the stress transmitted to the column head has little influence CONCLUSIONS The research results of the dissertation allow to draw the following conclusions: Soft soil including clay, mixed clay, mixed sand in plasticity is widely distributed along the coastal Hai Phong - Nam Dinh highway that will be built, with Holocene age, Thai Binh, and Hai Hung Quaternary of the Northern Delta sediments (Q23tb, Q21-2hh), mainly originating from river-sea (am) or sea (m) Soft soil structure of the highway scope is divided into two classes (I, II) and types (a, b, c),: Class I has soft soil distributed near the ground surface on sandy soil; Class II has soft soil distributed near the ground surface, the next is clay type soil Type a has less than 5m soft soil, Type b has 5m to 15m soft soil, Type c has greater 15m soft soil The scientific basis of the technique of sea sand-cement column improved soft soil includes the below processes, including mechanical compaction; the increase in strength of the column and the ground around the column thanks to the cement binder; the permeable consolidation and the physico-chemical effects between the cement and the surrounding soil Experimental study shows that, when the cement content changes by 5%, 7%, 10%, 13%, and 15%, the compressive strength of sea sand-cement samples at 28 days of age is 0.65 MPa, 1.05 MPa, 1.30 MPa, 1.78 MPa, and 2.45 MPa, respectively The sea sand-cement column method is calculated and designed depending on the soft soil structure and the cement content in the column material mixture If the foundation structure consists of soft soil layers of large thickness, and the soft soil below the compressive action zone, then sea sand - cement column design for the purpose of improving the soft soil with cement content of less than 5% will be suitable If the soil below is soft and the affected area of the work has good soil layers, the design of seacement-sand columns for the purpose of reinforcing the soft soil with a cement content of more than 10% will be appropriate In the case of specific soft ground structure, the numerical results show that: - The settlement of the embankment is reduced from 117cm (unreinforced case) to 16cm (reinforced case); - The horizontal displacement of the toe of the embankment slope reduces from 49.4cm (unreinforced case) to 4.8cm (reinforced case) - When the sea sand - cement column strength increases, the settlement of the embankment and the horizontal displacement of the toe of embankment slope decreases With the same load level, the settlement corresponding to the column strength of 1.5MPa is only a half of the settlement corresponding to the column strength of 0.5MPa; The observed horizontal displacement is 78cm, 30cm, and 14cm, corresponding to the column compressive strength of 0.5MPa, 1.5MPa, and 2.5MPa When the compressive 27 strength of sea sand – cement column increases from 1.5MPa to 2.0MPa, the stress acting on the soft soil does not change significantly, which shows that the optimal value in the design of sea-cement columns should be selected, the optimal compressive strength is about 1.5MPa; - The length of sea sand - cement column affects the settlement of the embankment The embankment settlement in the case of 8.5m length is 1.4 times greater than that of 13.5m length, and is approximately times greater than that of 16.5m length However, the length of the column has little effect on the value of stress transmitted to the column head and the soft ground at a specific load value RECOMMENDATIONS Full scale experimental study at site of the sea sand - cement column enables us to evaluate the economic and technical efficiency of the method Study on the equipment investigation for construction of the sea sand - cement column is suitable to Vietnam's conditions Building technical standards allows the application of the sea sand-cement column reinforced soft soil in Vietnam Consideration of the environmental change factors is influent on the stability of sea sand - cement columns PHD CANDIDATE PUBLICATION Evaluation of sand-cement column solution for soil improvement in the North Coastal Highway, Vietnam, Proceeding of the 4th International conference VIETGEO, 294 - 302 Nghiên cứu công nghệ gia cố đất yếu cọc cát biển - xi măng phục vụ xây dựng cơng trình hạ tầng vùng ven biển, Kỷ yếu Hội nghị khoa học toàn quốc Địa kỹ thuật xây dựng phục vụ phát triển bền vững VIETGEO, ngày 2526/10/2019 Vĩnh Long, Nxb Khoa học Kỹ thuật, 251-255 Cấu trúc đất yếu tuyến đường giao thơng ven biển Hải Phịng-Nam Định đề xuất công nghệ gia cố phù hợp, Kỷ yếu Hội nghị khoa học toàn quốc Khoa học trái đất Tài nguyên với phát triển bền vững (ERSD), ngày 12.11.2020 Trường Đại học Mỏ-Địa chất, Hà Nội, 19-25 Nghiên cứu khả sử dụng cát biển xử lý đất yếu phương pháp cọc gia cố xi măng, Tạp chí Khoa học Kỹ thuật Mỏ-Địa chất, Tập 61, Kỳ 6, tháng 12, 102-108 Nghiên cứu đề xuất phương pháp tính độ lún sức chịu tải đất yếu gia cố cọc cát biển – xi măng, Kỷ yếu Hội nghị khoa học toàn quốc Khoa học trái đất Tài nguyên với phát triển bền vững (ERSD), ngày 12.11.2020 Trường Đại học Mỏ-Địa chất, Hà Nội, 97-104 Nghiên cứu xây dựng quy trình cơng nghệ xử lý đất yếu cọc vật liệu hỗn hợp cát biển - xi măng - tro bay, Tạp chí Khoa học Kỹ thuật Mỏ-Địa chất, Tập 61, Kỳ 6, tháng 12, 1-9 Swelling Potential of Clayey Soil Modified with Rice Husk Ash Activated by Calcination for Pavement Underlay by Plasticity Index Method (PIM), Advances in Materials Science and Engineering, Volume 2021, https://doi.org/10.1155/2021/6688519 (SCIE/Q2) Study on application of sea sand-cement column in soft soil improvement for Hai Phong-Nam Dinh coastal highway, Proceedings of CIGOS 2021 - Part of the Lecture Notes in Civil Engineering book series of Springer (Accepted) ... Nội, 19-25 Nghiên cứu khả sử dụng cát biển xử lý đất yếu phương pháp cọc gia cố xi măng, Tạp chí Khoa học Kỹ thuật Mỏ-Địa chất, Tập 61, Kỳ 6, tháng 12, 102-108 Nghiên cứu đề xuất phương pháp tính... học Kỹ thuật, 251-255 Cấu trúc đất yếu tuyến đường giao thơng ven biển Hải Phịng -Nam Định đề xuất công nghệ gia cố phù hợp, Kỷ yếu Hội nghị khoa học toàn quốc Khoa học trái đất Tài nguyên với phát... Vietnam, Proceeding of the 4th International conference VIETGEO, 294 - 302 Nghiên cứu công nghệ gia cố đất yếu cọc cát biển - xi măng phục vụ xây dựng cơng trình hạ tầng vùng ven biển, Kỷ yếu