Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.Nghiên cứu ảnh hưởng của hệ cọc bê tông cốt thép đến các đăng trưng của dòng thấm trong nền cát dưới cống qua đê.
MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUYLOI UNIVERSITY DINH XUAN TRONG RESEARCH ON THE EFFECTS OF THE REINFORCED CONCRETE PILE SYSTEM ON THE SEEPAGE CHARACTERISTICS IN THE SANDY FOUNDATIONS OF THE UNDER-DIKE CULVERTS Major: Major code: Hydraulic engineering 58 02 02 SUMMARY OF TECHNICAL PHD THESIS HANOI, 2023 This scientific work has been accomplished at ThuyLoi University Scientific supervisor 1: Prof.Dr Nguyen Quoc Dung Scientific supervisor 2: Prof.Dr Pham Ngoc Quy Reviewer No.1: Assoc.Prof.Dr Hoang Viet Hung, ThuyLoi University Reviewer No.2: Assoc.Prof.Dr Nguyen Chau Lan, University of Transport and Communications Reviewer No.3: Assoc.Prof.Dr Do Minh Duc, VNU University of Science The thesis will be defended at the meeting of the University Doctoral Committee at Room - K1, ThuyLoi University, 175 Tay Son, Dong Da, Ha Noi at 08:30AM on July 05th, 2023 Thesis can be found at the library: - The National Library; - The Library of Thuy loi University INTRODUCTIONS Rationale of the study Vietnam has about 13.200km of dikes with nearly 2.600km of sea dikes and 10.600km of river dikes Particularly in the Red River delta (RRD), there are about 3.000 km of river dikes have been decentralized, and 1.855 under-dike culverts of various types Under-dike culverts are important items, as are the weaknesses of the dike systems The majority of break dikes are located at the culvert site, and about 25% of culvert incidents are due to seepage instability Some initial comments on the incidents: (i) All culverts are constructed on sand foundations; (ii) The solution for treating the foundations is to use reinforced concrete (RC) piles; (iii) The seepage treatment solution is to driving sheet pile at upstream or both upstream and downstream; and (iv) The culvert foundation is empty, the culvert is almost standing on the pile head The question arises: (1) Why seepage-related incidents still happen in practice, even though the design ensured the safety of the seepage in accordance with current regulations? (2) Pile driving into the sandy foundation has changed the natural state of the soil, and how does it affect the seepage characteristics in the foundation? (3) If the piles has adverse effects, what are the solutions to prevent and ensure the safety of the culverts while they are operating? To answer these questions, the thesis will quantitatively study the impacts of the RC pile system on the seepage characteristics in the sand foundation under the culverts The results of the study respond to the above-mentioned scientific and practical urgency Research objectives - The study clarifies the effects of the RC pile system on the seepage characteristics (seepage head, seepage pressure and seepage gradient) in the sand foundation of the under-dike culverts; - Propose a method for determine the seepage characteristics in the sandy foundation of under-dike culverts when considering the impact of RC piles; contribute to supplementing the scientific base, completing the design process, help to build safer under-dike culverts Subject and scope of the study - Subjects of the study: The characteristics of seepage in the homogeneous sand foundation of under-dike culverts with prefabricated RC piles - Scope of the study: The research for two characteristics related to the safety of the culverts are the seepage head and seepage gradient in the foundation of the under-dike culverts (open culverts) in the RRD region Research methodology Theoretical methods; Experimental method; Mathematical modelling method; Methods of synthesis, statistics, analytics and processing; Expert method Scientific and practical implications Scientific implications: Determine the law of variation of seepage pressure, the distribution of seepage gradient in the foundation of culvert with RC piles compared to the foundation without piles Based on this, it is proposed and recommended to adjust the traditional calculation method to increase work safety Practical implications: Clarify the cause of damage to some under-dike culverts that have occurred in practice, thereby providing appropriate treatment solutions Organization of the thesis Besides to the introduction and conclusion, the thesis is presented in chapters: - Chapter Overview of under-dike culverts and seepage in the sand foundation under the culverts; - Chapter Theoretical basis and research methods for the effects of the RC pile system on the seepage characteristics; - Chapter Experimental study on the effects of the RC pile system on the seepage characteristics in the culvert foundation; - Chapter Calculation of seepage characteristics in the sand foundation with RC piles at the Tac Giang culvert - Ha Nam province CHAPTER OVERVIEW OF UNDER-DIKE CULVERTS AND SEEPAGE IN THE SAND FOUNDATION UNDER THE CULVERTS 1.1 River dike systems and under-dike culverts in the Red River delta The river dike system has been decentralized in the RRD with a total length of 2.976,722 km; in which the dikes grade III or higher is 1.930,594 km The entire region has 2.098 culverts under dike, in which grade III dikes to a special grade have 1.049 culverts The characteristics of the under-dike culverts: (1) Construction time has been long and many culverts are degraded; (2) Various missions; (3) Complex flow and bearing modes; (4) Complex geology, mostly young sediments; (5) Inspection and supervision have not been carried out regularly and effectively The forms of the under-dike culverts: Culverts are built as open or underground culverts using concrete or RC, masonry bricks, stone masonry materials; culverts can have one or more bays 1.2 Characteristics, classification of the geological structure of the underdike culverts foundation in the RRD region 1.2.1 Geological characteristics of the dikes, under-dike culverts foundation The stratigraphy of the foundation of dikes and under-dike culverts is completely variable, with many layers of soft soil The working model for the dike foundation is divided into three horizons: (1) The cover horizon by cohesive soils has small permeability; (2) The aquifer is clay sand, silty to coarse sand with medium to large permeability; and (3) Less permeable clay horizon or bedrock When the aquifer has silty sand and fine sand layers of the Thai Binh formation under the covered horizon have different thicknesses; the level of seepage sensitivity is rated by many authors from low to very high 1.2.2 Classification of the under-dike culverts foundation structure in RRD Based on the survey, design documents of 110 under-dike culverts in 25 dike routes in the provinces of the RRD and inheriting the approach of previous authors on the classification of seepage sensitivity, the thesis has classified the stratigraphic structure of the under-dike culverts foundation in the region into three types of foundation structures with six subtypes: - Type No.1: The entire culvert foundation, which is clay (or sandy clay) has small permeability and large thickness, so there is no possibility of seepage deformation - Type No (culvert foundation with clay sand layer): Sub-type 2a (cover layer is clay or sandy clay) is less sensitive to seepage; Sub-type 2b (culvert placed directly on the clay sand layer) is more sensitive to seepage deformation; - Type No.3 (culvert foundation with sand layer): Sub-type 3a (cover layer is clay or sandy clay) is less sensitive to seepage; Sub-type 3b (cover layer is clay sand) is sensitive; Sub-type 3c (culvert placed on sand layer) is very seepage sensitive 1.3 Seepage, seepage deformation in the under-dike culverts foundation 1.3.1 Seepage in the soil at the foundation of the under-dike culverts When the culvert works, the difference between the upstream and downstream level will cause water to move through the voids of the soil, forming a seepage flow Permeability at the bottom of the culvert is pressure seepage, while at the sides or on the top of the culvert it is non-pressurized The seepage characteristics at the bottom of the culvert include the seepage head (related to the seepage pressure), the seepage velocity, and the seepage gradient 1.3.2 The seepage deformation in the foundation of the under-dike culverts - Types of seepage deformation: Internal erosion, contact erosion, heaving of soil, flat soil, extrusion circuit, and sand boil - Harm: Possibility of making the structure unstable and cause uneven settlement - Solutions to ensure seepage stability: Upstream apron, cutoffs, sheet-pile, slurry wall cutoffs, and adverse filter In practice, many solutions are often combined 1.3.3 The seepage incidents occurred in the under-dike culverts in the RRD According to the survey results in 2006, up to 22,73% of under-dike culverts were related to seepage Many incidents of instability seeping in under-dike culverts have occurred, but there is no general rule A notable problem is that most of the culverts that have been damaged were built on sand and have piles 1.4 Solutions to reinforce the under-dike culverts foundation in the RRD The design of the treatment of the under-dike culvert foundation on sand- clay sand to ensure that the settlement conditions are within the allowable limits and the difference does not exceed the elasticity of the joints Commonly used solutions include increasing the softness of the culvert, increasing the depth of the foundation, changing the size or type and hardness of the foundation, and treating the foundation with grouting drilling Under-dike culverts in the RRD are often used RC piles to reinforce the foundation in the form of friction pile foundation - low cap The pile cross section is square, mostly using a side size ap = 30cm (93,0%), pile spacing dp mainly ranging from 4ap to, and in most culverts, piles arranged according to a rectangular or square grid 1.5 Overview of research situation in Vietnam and the world 1.5.1 Studies on seepage in the sand foundation of the under-dike culverts - In the world, there have been many studies on seepage in the sand foundations with relatively complete results: + Calculation of seepage characteristics by Bligh (1910), Pavolovsky (1922, 1931), Lane (1934), Khosla et al (1936), Tsugaep (1956), etc + Seepage deformation: Many studies have been carried out based on the view that geometric factors affect the instability of soil structure due to seepage flow Meanwhile, many authors approach hydraulic criteria based on the flow velocity in the soil pore and the seepage gradient + Designing the creep at the bottom of the works has been done by Bligh (1910), Griffith (1913), Lane (1934), Terzaghi (1939), Sellmeijer et al (2011), etc - Researches in the country focus on seepage deformation of dikes foundation has been conducted by Nguyen Tran (1983 – 1985), Pham Van Quoc (2001), To Xuan Vu (2002), Bui Van Truong (2009), Nguyen Quoc Dat (2013), etc 1.5.2 Studies on seepage in sand foundation with RC piles Many studies on sand foundation treatment with precast RC piles have been carried out in Vietnam and around the world However, the researches has mainly focused on topics such as pile bearing capacity, densification, settlement, and stress of the soil, there have been no study on the effect of piles on the seepage characteristics in the foundation 1.6 Problems and research direction Research problems: 1) Does the RC pile system change the seepage head and the seepage gradient in the foundation, thereby increasing the seepage deformation and leading to the instability of the structure? (2) How to assess the impacts of RC piles on the seepage flow to ensure safe works? Research direction: Research on the effects of RC pile system on the seepage characteristics in homogeneous sand foundation of open under-dike culverts in the RRD; and thereby propose a method to determine the these characteristics 1.7 Concluction chapter - Under-dike culverts are an important item but also weakness of the dike system To ensure safety, many solutions for seepage and foundation treatment have been implemented, with the main solution being sheet-pile and RC piles - Based on the collected documents, the thesis classifies the stratigraphic structure of the under-dike culverts foundations in the study area into three stratigraphic types with six subtypes Type No.3 is considered to have a very sensitive level of permeability - Most of the seepage incidents occurred in the under-dike culverts built on the sub-type No.3c foundation structure and treated with RC piles This raises the question of why permeability instability and reinforced concrete piles affect the seepage characteristics of sluices across dikes built on sandy foundations - Previous studies have mainly focused on two directions: For foundation without piles, there have been quite comprehensive studies on seepage and seepage deformation; for pile foundations, research has only gone as far as assessing the pile bearing capacity, densification, settlement, and stress of the soil Therefore, it is necessary to conduct studies on the scientific basis as well as to choose research methods to obtain quantitative results on the change of these characteristics when the culvert foundation is driven with RC piles CHAPTER THEORETICAL BASIS AND RESEARCH METHODS FOR THE EFFECTS OF RC PILE SYSTEM ON SEEPAGE CHARACTERISTICS 2.1 The mechanisms of formation and the effects of seepage flow on the stable working of the under-dike culverts 2.1.1 Theory of permeability - Bernoulli's equation: The seepage flow dissipates energy when moving from a high-energy location to a low-energy location The energy dissipated by the seepage flow is transferred to the soil texture and is equal to the difference in the total head between the two locations: ZA + uA u = Z B + B + ∆h ⇔ H A= H B + ∆h ⇔ ∆h= H A − H B γn γn - Basic law of seepage flow (Darcy's law): vt = kth J (2-3) (2-4) - The basic equation of seepage flow: + Homogeneous, isotropic soil: ∂2 h ∂2 h + = ∂x ∂y ∂2 h ∂2 h + Homogeneous, anisotropic soil: kthx ∂x + kthy ∂y = (2-8) (2-9) 2.1.2 The effects of seepage flow on stress in soil Seepage in the soil increases pore water pressure and reduces effectively stress; thereby, reducing the compression resistance and shear resistance of the soil 2.1.3 The effects of seepage flow on the working of the under-dike culverts The seepage pressure acting on the structure reduces the frictional force between the bottom and the foundation, reduces the stability coefficient and can generate or increase tensile stress The seepage gradient is an important parameter for the safe works of the culverts When the gradient exceeds the allowable value, seepage deformation can occur γ γ ' or J gh = J gh = k + (nđ − 1) (2-22, 23) γn γn The seepage forces move soil particles in the direction of the seepage flow When the seepage force is large enough, the soil mass can become unbalanced and lead to instability 2.2 The effects of RC piles on the permeability of sand foundation Permeability corresponds to the ability to allow water to pass through the pores of the soil; when the void ratio increases, the permeability coefficient increases, and vice versa When driving the piles into the soil, they will compact the soil in an area with radius (3 ÷ 5,5)ap and depth (3 ÷ 5)ap below the pile tip The degree of reduction of the void ratio in the affected area (∆e): np ∆e = e0 − ∑ ∆e1 (2-25) e0 is the initial void ratio, np is the number of piles, ∆e1 is the reduction in void ratio due to 01 pile: 1+ e ∆e1 = 72 (2-26) Thus, driving piles into the soil reduces porosity and the permeability coefficient of the soil 2.3 The factors affecting the seepage characteristics in the sand foundation of the under-dike culverts with the precast RC piles - Group of hydraulic factors: properties of water, the difference between upstream and downstream head (∆H) - Group of soil factors: size and distribution of particle, particle shape, fine particle content, void ratio (e), saturated (G), permeability coefficient (kth), unit weight (ρđ), stratigraphic structure and thickness of permeable layer (Tc) - Group of works factors: shape and length of creep (Lth); quantity, location and depth of sheet-piles, works load, shape and size of piles, distance of piles, method of piles installation 2.4 The theoretical basis of experimental research and physical modeling 2.4.1 Similar theories and model scale The experimental model for pressure seepage is built base on similar conditions to ensure the uniformity of the motion equations and the continuity of the flow This means that the following criteria should be met: λQ = λk λl2 (2-36) CHAPTER EXPERIMENTAL STUDY ON THE EFFECTS OF THE RC PILE SYSTEM ON THE SEEPAGE CHARACTERISTICS IN THE CULVERT FOUNDATION 3.1 Statement of problems Culvert foundations with RC piles may have positive or negative impacts on seepage characteristics This needs to be evaluated quantitatively by empirical research to ensure appropriate and timely behaviour 3.2 Research scenario Table 3.1 Research scenario ap 0,00 0,06 0,12 0,24 0,00 0,06 0,00 0,50 1,00 2,00 0,00 0,50 0,000 0,833 1,666 3,332 0,000 0,833 MNS 4,5 KB2.2 4,5 KB2.3 4,5 KB3.0 4,5 10 KB3.1 4,5 11 KB3.2 4,5 12 KB3.3 MNS 4,50 4,50 16,15 16,15 16,15 16,15 Lp Lp dp hct hch ap 0,12 0,24 0,00 0,06 0,12 0,24 1,00 2,00 0,00 0,50 1,00 2,00 1,666 3,332 0,000 0,833 1,666 3,332 4,5 4,5 4,5 4,5 4,5 4,5 MN§ III I II MN§ III I II MN§ III Lp II MNS H H I hct III M2 M2 M2 M3 M3 M3 Lp Lth MNS MN§ hct II hch H hct I No Scenario Sample Cu hch 2,54 2,54 2,54 2,54 4,50 4,50 dp H M1 M1 M1 M1 M2 M2 Lp hch hct KB1.0 KB1.1 KB1.2 KB1.3 KB2.0 KB2.1 Lp hct Lp hch Lp Lth hch Lp No Scenario Sample Cu Lp Lp Lp = 0,0; = 0,0 = 0,0; hch hct Lth Lp Lp Lp = 0,5; = 0,833; = 0,06 hct hch Lth Lp Lp Lp = 1,0; = 1,666; = 0,12 hch hct Lth Lp Lp Lp = 2,0; = 3,332; = 0,24 hct hch Lth Figure 3.1 Illustration of research scenarios 3.3 Test procedures (1) Preparation of soil samples for testing; (2) Installation of works simulation equipment and measuring equipment; (3) Saturate the sandy soil sample; and (4) Conduct experiments 3.4 Results of calculations, experiments and analysis To consider the effect of the RC piles on ht, J at the bottom of the culverts, assuming the seepage flow follows the diagram as shown in Figure 3.2 and splits the culvert foundation into 03 regions (Figure 3.1) including: Zone I – Inlet area 11 (pipe No.9); Zone II – Sluice bottom slab area (pipe No.13 to 32); Zone III – Outlet area (pipe No.35) 10x50=500 TL 01 TL 03 02 05 04 18 23 26 29 32 35 38 41 43 45 19 20 24 25 27 28 30 31 33 36 34 37 42 44 46 10 07 11 16 21 08 12 17 22 39 40 HL Seepage pipes 30 Assumption stream line 13 14 15 06 Surface of soil sample HL 09 560 30 Surface of soil sample 48x50 = 2400 Figure 3.2 Layout diagram of seepage pipe The degree of variation in the seepage head (ht), seepage gradient (J), and total seepage pressure (Wth) is expressed by the rht (%), rJ (%), rW (%) index When these indices are greater than 0; ht, J, Wth of the scenario with piles increases compared to the case without piles and decreases when (rht, rJ, rW) < 3.4.1 Seepage head a) Inlet zone (Zone I – seepage pipe No.9): The scenarios Lp = 250mm have the largest increase in seepage head; with Lp = 62,5mm and Lp = 125mm, the seepage head increases little (less than 0,5%) 2,4 2,4 Cu = 2,54 Cu = 4,50 Cu = 16,15 1,6 Cu = 2,54 Cu = 4,50 Cu = 16,15 2,0 1,6 rht (%) rht (%) 2,0 1,2 1,2 0,8 0,8 0,4 0,4 0,0 0,00 0,0 0,05 0,10 0,15 0,20 0,0 0,25 a) Effect of rate (Lp/Lth) 2,8 Cu = 2,54 Cu = 4,50 Cu = 16,15 1,5 2,0 Lp/hct=0,5; Lp/hch=0,833; Lp/Lth=0,06 Lp/hct=1,0; Lp/hch=1,666; Lp/Lth=0,12 Lp/hct=2,0; Lp/hch=3,332; Lp/Lth=0,24 2,4 2,0 rht (%) rht (%) 1,6 1,0 b) Effect of rate (Lp/hct) 2,4 2,0 0,5 Lp/hct Lp/Lth 1,2 0,8 1,6 1,2 0,8 0,4 0,4 0,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,0 3,5 Lp/hch c) Effect of rate (Lp/hch) Cu 12 16 d) Effect of coefficient Cu Figure 3.11 Effects of (Lp, hct, hch, Lth, Cu) on rht in zone I 12 20 Thus, the RC pile system increases the seepage head in the inlet area compared with the case of no pile; the degree of increase depends on the ratios (Lp/hct, Lp/hch, Lp/Lth) and Cu In the same soil, with (Lth, hct, hch) is constant, the rht increases as Lp increases b) Zone II - Bottom slab zone - At the top of the bottom slab (seepage pipe No.13), the seepage head in the foundation with piles increases compared to the case without piles The largest increase was 12,05% (KB3.3); the lowest was 4,4% (KB1.1) 15,0 5,0 0,0 0,00 5,0 0,0 0,05 0,10 0,15 0,20 0,25 0,0 Lp/Lth a) Effect of rate (Lp/Lth) 14 12 10 0,0 1,0 1,5 1,5 2,0 rht (%) rht (%) 5,0 0,5 1,0 b) Effect of rate (Lp/hct) Cu = 2,54 Cu = 4,50 Cu = 16,15 10,0 0,5 Lp/hct 15,0 0,0 Cu = 2,54 Cu = 4,50 Cu = 16,15 10,0 rht (%) 10,0 rht (%) 15,0 Cu = 2,54 Cu = 4,50 Cu = 16,15 2,0 2,5 3,0 3,5 Lp/hct=0,5; Lp/hch=0,833; Lp/Lth=0,06 Lp/hct=1,0; Lp/hch=1,666; Lp/Lth=0,12 Lp/hct=2,0; Lp/hch=3,332; Lp/Lth=0,24 Lp/hch c) Effect of rate (Lp/hch) Cu 12 16 20 d) Effect of coefficient Cu Figure 3.12 Effect of (Lp, hct, hch, Lth, Cu) on rht in zone II – top of bottom slab - At the end of the bottom slab (seepage pipe No.32), the seepage head in the case with RC piles increases and decreases significantly compared to the case without piles In the scenarios Lp = 250mm, the seepage head is reduced compared to the case of no pile; the largest reduction is 15,02% (Cu = 16,15) and the smallest is 5,56% (Cu = 2,54); in the scenarios Lp = (62,5 and 125) mm, the seepage head tends to increase The boundaries of the increase - decrease rht are the limit values [Lp/Lth]gh, [Lp/hct]gh, [Lp/hch]gh In the same soil type, when the ratios (Lp/Lth), (Lp/hct), (Lp/hch) are less than the limit value, rht > 0, the seepage head in the case of foundation with piles increases compared to the case of no pile; when these ratios are larger than the limit value, rht < 0, seepage head decreases compared to the foundation without RC piles The largest increase was when (Lp/Lth = 0,076; 13 Lp/hct = 0,63; Lp/hch = 1,05) 30 30 Cu = 2,54 Cu = 4,50 Cu = 16,15 0,05 0,10 0,15 0,20 0,63 -20 0,0 0,25 a) Effect of rate (Lp/Lth) 2,70 2,80 1,05 2,54 rht (%) rht (%) Cu = 2,54 Cu = 4,50 Cu = 16,15 -10 1,5 2,0 b) Effect of rate (Lp/hct) 25 20 15 10 -5 -10 -15 -20 30 10 1,0 Lp/hct Lp/Lth 20 0,5 1,69 -10 1,52 rht (%) 10 0,203 -10 -20 0,00 0,183 0,076 rht (%) 10 Cu = 2,54 Cu = 4,50 Cu = 16,15 20 1,62 0,194 20 -20 0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 Lp/hct=0,5; Lp/hch=0,833; Lp/Lth=0,06 Lp/hct=1,0; Lp/hch=1,666; Lp/Lth=0,12 Lp/hct=2,0; Lp/hch=3,332; Lp/Lth=0,24 Lp/hch c) Effect of rate (Lp/hch) C u 12 16 20 d) Effect of coefficient Cu Figure 3.13 Effect of (Lp, hct, hch, Lth, Cu) on rht in zone II – end of bottom slab c) Zone III – Outlet zone (seepage pipe No.35) 0,05 0,10 0,15 0,20 -10 -20 -30 0,0 0,25 Lp/Lth 30 20 Lp/hct=0,5; Lp/hch=0,833; Lp/Lth=0,06 Lp/hct=1,0; Lp/hch=1,666; Lp/Lth=0,12 Lp/hct=2,0; Lp/hch=3,332; Lp/Lth=0,24 -10 0,50 1,00 1,50 2,00 -20 2,70 -20 2,51 1,05 -10 -30 0,00 10 rht (%) 10 2,50 2,0 b) Effect of rate (Lp/hct) 2,62 rht (%) 20 1,5 40 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 1,0 Lp/hct a) Effect of rate (Lp/Lth) 40 0,5 1,63 0,194 0,180 0,076 -20 1,57 10 -10 20 1,51 10 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 rht (%) 0,188 rht (%) 20 -30 0,00 40 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 0,63 40 3,00 -30 -40 3,50 Lp/hch c) Effect of rate (Lp/hch) Cu 12 16 d) Effect of coefficient Cu Figure 3.15 Effect of (Lp, hct, hch, Lth, Cu) on rht in zone III – Outlet region 14 20 The increase and decrease the seepage head was greatest in sample M3 (increased 31,06% - KB3.1; decreased 27,66% - KB3.3) and vice versa, sample M1 had the smallest increase and decrease (increase by 25,7% when Lp = 62,5mm and 11,98% decrease when Lp = 250mm) When Cu is constant, (Lp/Lth) > [Lp/Lth]gh, (Lp/hct) < [Lp/hct]gh, < (Lp/hch) < [Lp/hch]gh, rht > 0; the opposite case rht < The index rht increases the largest when (Lp/Lth = 0,076; Lp/hct = 0,63; Lp/hch = 1,05) 3.4.2 Seepage gradient The seepage gradient at the outlet (No.35 - HL) has a great change due to the effects of the RC piles Scenario (KB1.3, KB2.3, KB3.3), J decreased compared to the no pile scenario and vice versa in the remaining scenarios Sample M3 (Cu = 16,15) has the largest gradient variation; increased by 31,06% (KB3.1) and decreased by 27,66% (KB3.3); sample M1 has the smallest gradient amplitude, increased by 25,67% (KB1.1) and decreased by 11,98% (KB1.3) 0,05 0,10 0,15 0,20 -10 -20 -30 0,25 0,0 Lp/Lth 30 20 rJ (%) 10 2,0 b) Effect of rate (Lp/hct) 2,62 rJ (%) 20 1,5 40 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 1,0 Lp/hct a) Effect of rate (Lp/Lth) 40 0,5 1,63 0,194 -30 0,00 0,180 0,076 -20 1,57 10 -10 20 1,51 10 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 rJ (%) 0,188 20 rJ (%) 40 Cu = 2,54 Cu = 4,50 Cu = 16,15 30 0,63 40 -10 Lp/hct=0,5; Lp/hch=0,833; Lp/Lth=0,06 10 Lp/hct=1,0; Lp/hch=1,666; Lp/Lth=0,12 Lp/hct=2,0; Lp/hch=3,332; Lp/Lth=0,24 -30 0,00 0,50 1,00 1,50 2,00 2,70 -20 2,51 1,05 -10 2,50 3,00 -20 -30 3,50 Lp/hch c) Effect of rate (Lp/hch) Cu 12 16 20 d) Effect of coefficient Cu Figure 3.1 Effect of piles on seepage gradient at outlet area (35 – HL) In the same soil sample (Cu constant), when (Lp/Lth, Lp/hct, Lp/hch) is less than the limit values, the seepage gradient increases compared to the case without RC piles (rJ > 0) and decrease (rJ < 0) in the opposite cases The degree of increase in seepage gradient is greatest when (Lp/Lth = 0,076; Lp/hct = 0,63; Lp/hch = 1,05) 15 3.4.3 The seepage pressure acting on the bottom slab of the culvert Because of the effects of the piles, the seepage head increases and decreases compared to the case without piles, which causes changes in the value, the distribution diagram and the total seepage pressure 12 10,3 Cu = 2,54 rw (%) Cu = 4,50 8,1 Cu = 16,15 100 150 200 250 279 50 257 79 234 -3 300 Lp (mm) Figure 3.19 Effect RC piles on total seepage pressure When (Lp/hct < 0,63; Lp/hch < 1,05; Lp/Lth < 0,076), rW>0 and proportional to Lp, reached the maximum value in the case of Lp ≈ hch; when (Lp/hct > 0,63; Lp/hch > 1,05; Lp/Lth > 0,076), rW > and gradually decreases to as Lp increases When (Lp/hct > 2,23; Lp/hch >3,72; Lp/Lth > 0,27) for sample M1; (Lp/hct > 2,06; Lp/hch > 3,43; Lp/Lth > 0,25) with sample M2 and (Lp/hct > 1,87; Lp/hch > 3,12; Lp/Lth > 0,23) in sample M3; total seepage pressure in the foundation with RC piles is reduced compared to that without piles 3.4.4 Explain physical phenomena Assume the pile foundation as a “base of the equivalent permeability foundation” with a width of unit, depth hm = Lp, and length Lm (Figure 3.20) H Lp hm Tc Lm Lbd MN§ hch t hct hct Lp hch hm Lm Lbd MN§ hch Lp H MN§ Tc Tc hct hch Lm Lbd MNS t H MN§ t H t hct Tc MNS MNS hm MNS Lm Lbd a) Lp = c) hch < Lp < hct b) < Lp < hch d) Lp > hct Figure 3.20 Illustration of “base of the equivalent permeability foundation” + When ≤ Lp ≤ hch: The Lth value decreases as Lp increases and reaches the minimum value when Lp = hch; seepage head at the outlet increases progressively and reaches the maximum value when Lp = hch; 16 + When hch < Lp ≤ hct: Because of the influence of horizontal creep length (Ltn) increases, leading to Lth increase, the seepage head decreases with Lp increase; + When Lp > hct: The length of the vertical creep (Ltd) increases with hm, the influence of Ltn also increases because Tc decreases (due to the increase of Lp), leading to ht decrease with Lp Moreover, due to the effects of RC piles, the soil permeability coefficient decreases The seepage flow through “base of the equivalent permeability foundation” is impeded by the RC piles This changes the seepage head and seepage gradient compared to the foundation without RC piles 3.5 Propose a method to determine the seepage characteristics in the sandy foundation of the under-dike culverts with RC piles 3.5.1 Contents of the method (1) Assume that the culvert foundation does not have RC piles; calculate the seepage head (ℎ𝑡𝑡0𝑐𝑐 ) at the bottom slab locations, the largest exit gradient (Jrmax) according to conventional methods (2) Calculation of the seepage head (ℎ𝑡𝑡𝑐𝑐 ) at the locations of the bottom slab and 𝑐𝑐 ) when taking into account the influence of RC the largest exit gradient (𝐽𝐽𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 piles according to the formula: - For seepage head: At the inlet: At the top of the bottom slab: At the end of the bottom slab: At the outlet: - For the seepage gradient at the outlet: htc = kh ht0c htvc = khv htv0 c htcđ = khđ htđ0c htcc = khc htc0c htcr = khr htr0c J rcmax = k J J r max (3-8) (3-9) (3-10) (3-11) (3-12) (3-13) where kh, kJ are the adjustment coefficients for seepage head, seepage gradient (3) Calculate the seepage pressure and check for possible seepage deformation according to current regulations 3.5.2 Set up empirical formulas to determine the adjustment coefficients kh, kJ The adjustment coefficient of the seepage head (kh) and seepage gradient (kJ) reflects the degree of change of the seepage head (rht), and seepage gradient (rJ) between the case with piles and without piles Use the least squares sum method with a linear model based on test results findings to develop an empirical formula 17 to determine these adjustment coefficients The results are: 3 Lp Lp Lp khv = 1, 00 − 0, 61 − 3,89.10−8 Cu3 + At the inlet: Lth hct hch 2 Lp Lp Lp +0,12 + 1, 20.10−5 Cu2 ; R = 0,9987 L h h th ch ct 3 L L L At the top of the −3 p p −8 p k hđ = 1, 04 − 9, 29.10 − 8,59.10 Cu3 + bottom slab: Lth hct hch L Lp Lp −3 p ; R2 = 0,9966 +0, 04 + 1,90.10 Cu L h h th ch ct 3 (3-54) (3-55) Lp Lp Lp At the end of the khc = 1, 20 + 0, 06 + 2,97.10−6 Cu3 − bottom slab: Lth hct hch Lp Lp Lp Lp −0, 03 Cu − 0,11 Lth hct hch hch Cu At the outlet: ; R2 = 0,9988 (3-56) 3 3 3 Lp Lp Lp khr = 1,37 − 1, 03 + 3, 27.10−6 Cu3 − Lth hct hch 0,5 0,5 Lp Lp Lp Lp (3-57) −0,04 Cu − 0, 20 ; R = 0,9995 L h h h C th ct ch ch u Lp Lp Lp kJ = 1,37 − 0, 22 + 1,51.10−5 Cu3 − Lth hch hct 0,5 0,5 Lp Lp Lp Lp (3-58) −0,04 Cu − 0, 20 ; R = 0,9995 L h h h C th ct ch ch u Conditions of application of the formulas: (1) Pressure seepage flow in homogeneous sand; (2) The culvert has sheet-pile on both the river side and the field side; (3) Precast RC piles; ap = 30 cm; arrange piles according to the square grid with dp = 4,5ap; Lp = (0 ÷ 2)hct 3.6 Conclusion of Chapter (1) RC piles alter the seepage head, seepage pressure, and seepage gradient in the culvert foundation The level of change is dependent on Lp, hct, hch, Lth, Cu (2) To ensure the safety of seepage, the thesis has proposed a method for calculating the seepage head, and seepage gradient base by adjusting the calculation results of the case to ignore the effect of piles (adjustment coefficients kh and kJ determined by empirical formulas established based on test data) 18 CHAPTER CALCULATION OF SEEPAGE CHARACTERISTICS IN THE SAND FOUNDATION WITH RC PILES AT THE TAC GIANG CULVERT - HA NAM PROVINCE 4.1 Statement of problems Experimental research results have confirmed and quantitatively evaluated the effects of the pile system on the seepage characteristics in the homogeneous sand foundation of under-dike culverts in the RRD The question is whether these results are studied on a miniaturized physical model and whether they are reliable when applied in practice The thesis selects the Tac Giang culvert to apply and evaluate the suitability and reliability of experimental research results 4.2 Introduction about Tac Giang culvert - Ha Nam province Tac Giang culvert was built on Hong right dike, Ha Nam province The culvert foundation is located on small-fine sand layer The solution for seepage treatment is to driven the upstream and downstream steel sheet-pile of the culvert The solution for foundation treatment by precast RC piles, size 30x30cm, length 5m, arranged according to the square grid with pile spacing is 1,3m For details see Figure 4.2 22,3 HONG RIVER CHAU GIANG RIVER +9.40 Seepage wells +5.30 +3.20 -1.50 -2.00 T12 B G I -2.00 -2.50 T6 T11 N P T5 S Downstream sheet pile Upstream sheet pile -6.70 Concrete piles -13.70 15,5 24,3 10 -3 Layer No.4: Sandy - k th= 7.10 cm/s Figure 4.2 Longitudinal section of Tac Giang culvert and seepage calculation diagram 4.3 The incident of Tac Giang sewer in 2012 and treatment solution - Incident progress: In August 2012, the culvert had a problem due to seepage, after stilling basintank there was strong sand boil, cloudy water; the operation house is sinking; there are many places of sinkholes on the dike body 19 - Cause of the incident: Internal erosion in the foundation and at the embankment between the flank wall and the dike body - Treatment solution: Using soil-cement wall to treat internal erosion and grounting drilling cement-sand to fill voids in the foundation 4.4 Determine the seepage characteristics under foundation of the Tac Giang culvert 4.4.1 Cases, scenarios and calculation methods - Case study: (1) Not considering the effects of RC piles (the problem used in the original design) and (2) Taking into account the effects of RC piles - Calculation scenario: Table 4.4 Summary of research scenarios No Scenarios TG1 TG2 TG3 TG4 MNTL (m) 4,23 4,40 5,18 7,10 MNHL (m) 2,14 2,00 2,05 2,18 ΔH (m) 2,09 2,40 3,13 4,92 Note August 5, 2013 August 28, 2017 July 22, 2018 Design water level complex - Calculation method: Using SEEP/W software to calculate for case (1) and "adjustment coefficient method" to calculate for case (2) 4.4.2 Calculation results Table 4.7 Calculation results of seepage head for Tac Giang culvert Scenarios TG1 TG2 Location B G I N P S B G I N P S ht0c (m) 1,961 1,559 0,960 0,660 0,422 0,085 2,252 1,791 1,103 0,758 0,519 0,098 kh htc 1,0000 1,0020 1,0461 1,1449 1,2258 1,2258 1,0000 1,0020 1,0461 1,1449 1,2258 1,2258 1,961 1,562 1,004 0,756 0,517 0,104 2,252 1,795 1,154 0,868 0,636 0,120 Scenarios 20 TG3 TG4 Location B G I N P S B G I N P S ht0c (m) 2,937 2,335 1,438 0,989 0,627 0,127 4,616 3,671 2,260 1,554 1,065 0,200 kh htc 1,0000 1,0020 1,0461 1,1449 1,2258 1,2258 1,0000 1,0020 1,0461 1,1449 1,2258 1,2258 2,937 2,340 1,504 1,132 0,769 0,156 4,616 3,678 2,364 1,779 1,305 0,245 Table 4.8 Maximum seepage gradient at the outlet of Tac Giang culvert Scenarios TG1 TG2 TG3 TG4 ΔH (m) 2,09 2,40 3,13 4,92 Jrmax 0,106 0,122 0,159 0,250 kJ 1,2256 1,2256 1,2256 1,2256 J rcmax 0,130 0,150 0,195 0,307 4.4.3 Analyze and evaluate research results 4.4.3.1 Analyze and evaluate the suitability and reliability of research results a) Analyze and evaluate through observation data during the operation Using the calculation results of the case of no piles, the case with piles and observe data during the operation at wells T12 and T11 (before and after the upstream sheet-pile), T6 and T5 (before and after the downstream sheet-pile) to evaluate the suitability and reliability of the experimental research results and the proposed method in Chapter Table 4.11 Calculation and observe results of seepage head at T12, T11, T6, T5 Calculation results when Scena ignoring the effect of piles (m) -rios T12 T11 T6 T5 TG1 1,569 0,960 0,647 0,419 TG2 1,801 1,102 0,742 0,516 TG3 2,349 1,438 0,968 0,622 TG4 3,693 2,260 1,522 1,058 Calculation results when considering the effect of piles (m) T12 T11 T6 T5 1,572 1,805 2,354 3,700 Observation data T12 T11 T6 T5 1,60 1,01 0,76 0,51 1,004 0,740 0,514 1,153 0,850 0,633 1,85 1,20 0,90 0,65 1,504 1,109 0,762 2,40 1,54 1,15 0,80 No data 2,364 1,742 1,297 Table 4.12 Evaluation of the change of seepage head due to the effect of piles Scenar -ios ΔH (m) TG1 2,09 TG2 2,40 TG3 3,13 TG4 4,92 Average Degree of change in seepage head (%) between adjustment results and numerical model T12 T11 T6 T5 0,20 4,61 14,49 22,58 0,20 4,61 14,49 22,58 0,20 4,61 14,49 22,58 0,20 4,61 14,49 22,58 0,20 4,61 14,49 22,58 Degree of change in seepage head (%) between observation data and numerical model results T12 T11 T6 T5 1,99 5,22 17,55 21,72 2,69 8,87 21,23 25,89 2,15 7,13 18,77 28,62 There are no data to compare 2,28 7,07 19,18 25,41 Correlation between seepage head and the difference between upstream and downstream level at observe wells T12, T11, T6, T5 in the cases of no piles taking into account the effect of piles, actual observe data is shown in Figure 4.9 21 3,0 Observe data ht = 0,7521∆H - 6.10-8 R² = Adjusted calculation data Calculating data from numerical model 3,5 3,0 2,5 ht = 0,7436∆H - 8.10-8 R² = 2,0 1,5 1,0 ht = 0,7763∆H - 0,0323 R² = 0,9661 0,5 Seepage head, well T11, m Seepage head, well T12, m 4,0 1,0 1,5 2,0 2,5 ΔH, m 3,0 3,5 4,0 4,5 ht = 0,5343∆H - 0,07 R² = 0,9455 1,0 ht = 0,4539∆H - 2.10-8 R² = 0,5 ht = 0,2984∆H + R² = 0,0 1,4 Observe data ht = 0,3542∆H - 7.10-8 R² = Adjusted calculation data Calculating data from numerical model ht = 0,3237∆H + 0,0727 R² = 0,9419 1,5 5,0 4.10-8 Seepage head, well T5, m Seepage head, well T6, m 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0,5 2,0 0,0 0,0 0,0 Observe data ht = 0,4783∆H - 7.10-8 R² = Adjusted calculation data Calculating data from numerical model 2,5 1,0 1,5 2,0 2,5 ΔH, m 3,0 3,5 4,0 4,5 5,0 Observe data ht = 0,2719∆H - 0,0508 R² = 0,993 Adjusted calculation data Calculating data from numerical model 1,2 1,0 0,8 0,5 ht = 0,2361∆H + 0,0404 R² = 0,8615 0,6 0,4 ht = 0,2218∆H - 0,0415 R² = 0,993 0,2 0,0 0,0 0,5 1,0 1,5 2,0 2,5 ΔH, m 3,0 3,5 4,0 4,5 0,0 5,0 0,5 1,0 1,5 2,0 2,5 ΔH, m 3,0 3,5 4,0 4,5 5,0 Figure 4.9 Correlation between calculated and observation results at wells The average error between the calculation results of the seepage head by the adjustment coefficient method and the observation data at the well T12 is 2,07%, T11 is 2,35%, T6 is 4,10% and The mean T5 was 2,31% In terms of practical significance, the above error value is negligible b) Evaluation of research results by salt-marking method combined with electric imaging measurement at the field Brine is poured into the T11 well; wells T3, T6, T8 are used for monitoring; arrange an electric image measurement line at the end of the stilling basin Based on the deterioration of resistivity over time, the distribution spectrum of the resistivity on the electrical imaging route before and after pouring saline to evaluate the movement direction of the seepage flow The results show that, six years after the internal erosion failure was treated, the seepage flow at the bottom of Tac Giang culvert still complicated The seepage flow crosses diagonally from the culvert center with a large seepage velocity at the face adjacent to the bottom slab This may be due to the concentration of seepage in the gaps between the culvert and the foundation c) Conclusion about the reliability of research results Based on the results applied to Tac Giang culvert, it can be confirmed that the RC piles has changed the seepage head, thereby leading to the change in seepage 22 gradient and seepage pressure This should be kept in mind when designing works to ensure safety The experimental research results and the method of calculating proposed in the thesis are reliable 4.4.3.2 Analyze and evaluate research results and initial design results - Seepage pressure: When considering the effects of the pile system, the value of the seepage head within the bottom slab of the culvert tends to increase and thereby, increases the uplift pressure on the bottom slab of works - Seepage gradient: In the design case, ∆H = 4,92m, the result of the design phase was Jrmax = 0,242; of this study when ignoring the effect of piles is Jrmax = 0,25 and when considering the effect of piles J rcmax = 0,307 > [J] = 0,3 – not meeting the requirements as prescribed 4.5 Conclusion of Chapter The results applied to Tac Giang culvert - Ha Nam shows that: - The results calculated by the method proposed by the thesis are quite consistent with the observation data; accurately reflect the increasing trend of the seepage head due to the effects of the RC pile system - The average increase of the seepage gradient at the outlet of the culvert when considering the effects of the RC pile system is 22,56% The exit seepage gradient calculated by the adjustment coefficient method is 0,307 > [J] = 0,3 in the design case Thus, along with other causes, the impacts of the incident occurred in 2012, and this increase can lead to unsafe seepage for the structure - Research results by the method of salt-marking combined with electrical images show that the seepage flow is oblique in relation to the centre of the culvert; the seepage velocity is quite large in the area adjacent to the bottom of the culvert The cause may be due to contact seepage between the culvert and the foundation Thus, it can be asserted that the experimental research results and the methods for determining the seepage characteristics based on the adjustment coefficients of the thesis are reasonable and reliable, can be used in research and design work 23 CONCLUSION The results achieved by the thesis - The thesis has classified the stratigraphic structure of the foundation of underdike culverts in the RRD region; - The thesis has established the empirical research equation and built a physical model to assess changes in seepage characteristics - The thesis has quantified the effects of RC piles on the seepage characteristics in the sandy foundation - The thesis proposed a methodology for calculating seepage characteristics on the basis of empirical formulas These formulas are established from test results - The thesis has applied research results for Tac Giang sluice (Ha Nam Province) to assess the reliability Achievements of the thesis (1) Clarify the effects of the precast RC pile system on the seepage characteristics in the sand foundation of the under-dike culverts in the RRD (2) Establish formulas from (3-54) to (3-58) that determine the adjustment coefficient for the seepage head (kh) and seepage gradient (kJ) to serve the calculation of seepage characteristics when considering the effect of RC piles Limitations and recommendations for further research - Limitations: The thesis has not been studied for heterogeneous foundations; only considers the disadvantage that the joints between the bottom slab and the connecting part are damaged; only studied for driving piles, the ratio dp/ap = 4,5 and arranged in a square grid In addition, the stiffness of the lateral walls of the model influence in part the soil properties - Recommendations for further research: Continue to study the case of multilayer foundations; for culverts with both front and back yards; for cases of different pile sizes, layout forms, and installation methods Experimental study with a spatial model to evaluate the effect of piles and sheet-pile on the seepage under the floor, and the embankment between the flank wall and the dike body 24 LIST OF PUBLICATIONS OF THE AUTHOR Dinh Xuan Trong, Nguyen Quoc Dung, Pham Ngoc Quy, Pham Thi Huong, “Methods for determining the seepage characteristics in the sandy foundations of under-dike culverts with reinforced concrete piles,” Journal of Water Resources Science and Technology, Vietnam Academy for Water Resources, ISSN 1859-4255, No 75, pp 58-65, 12/2022 Dinh Xuan Trong, Nguyen Quoc Dung, Pham Ngoc Quy, Pham Thi Huong, “Research on the effects of reinforced concrete piles on the seepage characteristics in sand foundation of under-dike culverts by physical model,” Journal of Water Resources and Environment, Thuyloi University, ISSN 1859-3941, No.81, 99 114-122, 12/2022 Dinh Xuan Trong, “Research on the effects of the uniformity coefficient on the suffusion sensibility of the sand foundations of the under-dike culverts in the Red River delta,” Vietnam Journal of Agriculture and Rural Development, ISSN 1859-4581, No 23, pp 101-107, 12/2022 Nguyen Nhu Trung, Nguyen Quoc Dung, Dinh Xuan Trong, Bui Van Nam, Nguyen Canh Thai, Nguyen Van Diep, Nguyen Thi Thu Huong, “Research on application of salt tracer method and resistance method for determining water flow through the dyke body of Tac Giang sluice, Ha Nam province,” Vietnam Journal of Marine Science and Technology, ISSN 1859-3097, No 20, vol (4B), pp 215-226, 2020 Dinh Xuan Trong, Do Thi Thuy Dung, Nguyen Canh Thai, “Solutions for each type of incident risk for river levees and under-dike culverts in the Red River Delta and Central Vietnam”, in Proceedings of the Science and Technology 2014 – 2019, Vietnam Academy for Water Resources, Ha Noi: Science and Technics Publishing House, ISBN 978-604-67-1399-9, pp 601 – 615, 11 – 2019 Dinh Xuan Trong, “Establish an experimental model to study the internal erosion phenomenon in sandy foundations of under-dike culverts and to take into account the influence of reinforced concrete piles,” Journal of Water Resources Science and Technology, Vietnam Academy for Water Resources, ISSN 1859-4255, No 45, pp 80-89, 2018