Summary of doctoral dissertation: Selection study of the reasonable compacting factor for compacted earth dam construction under high humidity condition in the north central

This thesis studied the soil and the soil properties in the North Central region and proposed reasonable and safe solutions for the design and the construction of compacted earth dams. It meets the requirements for development of economic, social security and defense of the country.

MINISTRY OF EDUCATION & TRAINING MINISTRY OF AGRICULTURE & RURAL DEVELOPMENT THUY LOI UNIVERSITY TRAN VAN HIEN SELECTION STUDY OF THE REASONABLE COMPACTING FACTOR FOR COMPACTED EARTH DAM CONSTRUCTION UNDER HIGH HUMIDITY CONDITION IN THE NORTH CENTRAL REGION OF VIETNAM Specialization: Hydraulic Engineering Code No: 62 58 40 01 SUMMARY OF DOCTORAL DISSERTATION HANOI, 2016 This dissertation has been accomplished at Thuy loi University Promotor: Assoc Prof Ph.D Le Van Hung Co-promotor: Prof Ph.D Le Kim Truyen Reviewer No 1: Assoc Prof Ph.D Vu Huu Hai Reviewer No 2: Assoc Prof Ph.D Nguyen Huy Phuong Reviewer No 3: Assoc Prof Ph.D Nguyen Binh Thin The Doctoral thesis defense will take place at the meeting of the University Doctoral Committee at Thuy loi University, 175 Tay Son street, Dong Da, Hanoi, Vietnam On 16th December, 2016 The dissertation is available at: - The National Library - Library of Thuy loi University INTRODUCTION Problems statement Earth dams are important structures, which are built across rivers or streams to let water rise up and form the reservoirs Earth dams are still widely used in our country due to the advantageous characteristic of using the local material They are cost-efficient through usage of simple construction techniques and simple foundation treatment The earth dams construction are always affected by the flow After diverting the river the engineers have to solve several issues such as water drainage in the foundation pit and foundation reinforcement At the same time, the construction must be quick to avoid the flood For that reason, the construction usually takes place in one dry season In recent years, the constructions of earth dams in the North Central region usually encounter problems, which are cohesive soil in heavy long rain and humid climate conditions The key constructions in provinces from Thanh Hoa to Thua Thien Hue such as: Truoi, Ta Trach, Ngan Truoi, Cam Thuy, Thuy Yen – Thuy Cam, Da Han, Ban Mong, are in the areas that always experience high humidity in the air and long rainy times in rainy season The air humidity in many months is greater than 80% Therefore, to ensure the design quality such as cohesive soil compaction achieved K≥0,97 corresponding to the soil moisture W=Wopt  3% is very difficult As a result, the construction time lasts longer, and it does not meet the construction progress or economic and social development requirements This reduces the effectiveness of the investment capital and leads to a long time of capital recovery Vietnam has many experiences and achievements on earth dam construction technologies but also faced many catastrophic failures At present and near future, according to the plan and direction of economic development to 2030 and vision of 2050, North Central region will have to build many earth dams under high humidity conditions In order to achieve the goal of completing these constructions, ensuring the construction quality, following the construction schedule with a reasonable cost, while minimizing the damages as well as negative impacts on the society, we need to have proactive solutions during the design and construction of compacted earth dams under high-humidity conditions It urges the authors to find out the in-time solution for the theoretical and practical issues Objectives This thesis studied the soil and the soil properties in the North Central region and proposed reasonable and safe solutions for the design and the construction of compacted earth dams It meets the requirements for development of economic, social security and defense of the country Objects and study areas This thesis studied the soil and the soil properties for earth dams construction, solutions for the design and the construction of compacted earth dams in the North Central region (from Thanh Hoa province to Thua Thien Hue province) In detail, the study firstly focused on the theories and practicing of earth dam construction in North Central region, and then the results were applied to select the reasonable compacting factor and the humidity for compacted earth dam construction under high humidity condition Study contents In order to achieve the main objectives, the author focused on the following issues: a) Studying the materials for earth dam construction and natural conditions of the North Central region (from Thanh Hoa province to Thua Thien Hue province), assessing the soil characteristics of this area to build the compacted earth dams; b) Proposing solutions to reduce soil humidity for the earth dam construction in the region; c) Conducting experimental study to select appropriate compacting factor for the earth dam construction under the high humidity condition of the study areas; d) Developing a process to determine appropriate speed for the earth dam construction with the selected compacting factor and humidity of the soil; e) Applying the study results to assess the construction safety of Da Han dam and Ta Trach dam Methodology - Literature review; - Studying on the basic theory, inherited the study results on the earth dam construction using compaction method, which was published in Vietnam and internationally; - This study is a combination of experimental testing and theoretical analysis using some supporting softwares; - This study also based on the observation, summarizing and analysis the results from practical experience; - The study results were reported and discussed in many conferences Scientific and reality applications The study results enhance knowledge in studying and practicing of the compacted earth dam construction In theory: This thesis summarized the basic contents of the process to evaluate and select the materials as well as to identify compacted earth dam construction solutions, based on summing up, inheriting and developing previous studies on the earth dams, which were applied in many regions having different or similar conditions This process changes with the compacting factor and the humidity of the soil in the North Central region In practice: This thesis built up a basic database, solutions as well as specific applications for some projects within the study areas from Thanh Hoa province to Thua Thien Hue province The author proposes not only a process to choose reasonable humidity and compacting factor but also an effective solution to reduce the humidity of the soil for the earth dam construction in the study areas New contributions (1) The soils used for the earth dam in this area are mostly sediment on sedimentary bedrock In this thesis, the characteristics of cohesive soils (sediment) in North Central region was evaluated thoroughly, including non-swelling or weakly swelling soil and non-breakup or weakly breakup soils; (2) Compacting factor (K) was selected for the earth dam construction: when impervious core was sedimentary soil, we selected K≥0,94 correspondence to the humidity of W=Wopt+(4÷6)%; For the dam body, which was typically the ruins soil or a mixture of soil and rocks from foundation excavation of other component structures, we selected K≥0,97; (3) This thesis proposed the solutions to reduce effective moisture for the earth dam construction under high humidity conditions by using a sand filter layer with thickness of 50cm right underneath the cohesive soil stockpile This is suitable for practicing of the earth dam construction in the North Central region, Vietnam; (4) This thesis proposed a process to identify the compacted earth dam construction speed This process was applied with high humidity soil and achieved high efficiency It helped to speed up the earth dam construction in the study areas Contents This dissertation starts with an introduction, following by four main chapters and finally ends with conclusions and recommendations Chapter Overview of compacted earth dam construction Chapter Fundamental principles of compacted earth dam construction Chapter Experimental study on compacted earth dam construction under high humidity conditions Chapter Selection of reasonable compacting factor for earth dam construction in North Central region under high humidity conditions CHAPTER OVERVIEW OF COMPACTED EARTH DAM CONSTRUCTION 1.1 1.1.1 Overview of the earth dam construction Evolution of the compacted earth dam construction in the world Earth dam has a long history of development in Egypt, China, India and some other countries because of the excessive advantages such as the use of local materials Nowadays, it is more widely used than before According to the ICOLD statistics, local materials dam increased from 75% to in 1977 to 82.9% in 1998 in total built dam types 1.1.2 The development of the compacted earth dams construction in Vietnam In Vietnam, before 1964, the construction of reservoirs was slow There are a few reservoirs built during this period After 1964, especially since the country's unification, the construction of reservoirs developed rapidly According to statistics of management by the Thuy loi directorate until 2015: There are 63 earth dams with the height of 30m and above; 492 earth dams of from 15m to below 30m high; including 44 soil dams, 19 rocks and soils mixture dams 1.2 The study about the compacted earth dam construction in Vietnam After 1975, thanks to the research of scientists has successfully processed about special mechanical properties of soil It is necessary to build a series of reservoirs in the Central South region and the Central Highlands However, when constructing dams in the Central North region as Ta Trach, Ngan Truoi, Truoi reservoir has faced unsolvable problem with very high soil and air humidity It is impossible to use the normal solution to reduce soil humidity to can satisfy technical requirements for earth dam construction by compaction method; resulting in delay in the construction schedule works causing great wastage for society Therefore, researching solutions to select the reasonable soil humidity and compacting factor for earth dam construction in this region are the topical issues 1.3 Conclusion of chapter Through the overview study on this review, the author found that the study for compacted earth dam construction in high humidity conditions for the North Central region is necessary The earth dam construction in high humidity conditions will change the damming speed as well as mechanical soil properties of compacted earth dams To quantify the changes, it is necessary to study in detail: - Studying about material of the earth dam and the natural conditions of the North Central region, from Thanh Hoa to Thua Thien Hue, evaluating soil mechanic characteristics of the region for the earth dam construction; - Proposing the solutions to reduce soil humidity for the earth dam construction in the region; - Selecting the reasonable compacting factor for the earth dam construction in high humidity conditions; - Determining the appropriate damming speed with the selected compacting factor and humidity; - Application of new study results on the safety assessment of the Da Han and Ha Trach earth dam constructions CHAPTER FUNDAMENTAL PRINCIPLES OF COMPACTED EARTH DAM CONSTRUCTION 2.1 Introduction From the overview study results, the thesis finds out that the scientific basis and practices of the Central North region have a persistent problem of "constructing earth dam in high natural humidity condition”, which needs further studies In this chapter, the author studies the scientific basis of soil for the earth dam construction in high natural humidity condition of the natural environment of the Central North region 2.2 2.2.1 Soil for construction Source of formulating Soil is the layer of material bulks, formed by weathering rocks and small feces, not consolidation and distribution from the ground to hard rock 2.2.2 Shear strength of soil - Mor-Coulomb theory According to Terzaghi, 1936 [41] shear strength of saturated soils identified through vandalism standards Mohr - Coulomb and effective stress as follows: Vandalism envelope equation (often called Coulomb expressions): 𝜏𝑓′ = 𝑐 ′ + 𝜎𝑛′ 𝑡𝑔𝜑′ (2.11) In this: φ’ – friction angle or shear strength; c’ – cohesion unit According to the relationship between Mohr circle and Vandalism covering, we have the corner of vandalism face: 𝛼𝑓 = (900 + 𝜑′ ) 2.2.3 (2.12) Hygroscopic characteristic of the soil Capillary height depends on the mineral composition and shape of holes; it decreases from mica to quartz, to feldspar For grains d