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

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

1 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

2 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

3 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

4 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

5 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

6 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

7 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

8 Contents

This dissertation starts with an introduction, following by four main chapters and finally ends with conclusions and recommendations

Chapter 1 Overview of compacted earth dam construction

Chapter 2 Fundamental principles of compacted earth dam construction

Chapter 3 Experimental study on compacted earth dam construction under high humidity conditions

Chapter 4 Selection of reasonable compacting factor for earth dam construction

in North Central region under high humidity conditions

CHAPTER 1 OVERVIEW OF COMPACTED EARTH DAM

CONSTRUCTION 1.1 Overview of the earth dam construction

1.1.1 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 1

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 2 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 Soil for construction

2.2.1 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=1

2.2.3 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<0,1mm, the capillary height decreases from mica to quartz angular, to rounded quartz Capillary height increases rapidly following the reducing of particle diameter [39]

Figure 2.13 Height of desiccant (capillary) hc of soil types [39]

Type of soil Particle diameter, d (mm) hc (cm)

Mixed sand <2m occupy 3 ÷ 10% 100 ÷300

Mixed clay <2m occupy 10÷ 30% 300 ÷400

Capillary height values hc in Table 2.13 show that for raw soil especial gravel, hc

is very small and smaller than 30cm For general sand is hc<50cm [39]

2.2.4 Consolidation characteristic of the soil

Natural soil exists as 3 types (solid, liquid, gas) Therefore, when the load effects

on soil to evaporate air and water called consolidation characteristic of soil Volume decreased gradually until the pore water pressure inside reaches equilibrium, reducing the load can cause swelling which makes saturated soil To

the process of consolidation must be started is preloading and can take several years to reach final settlement

2.3 Compaction and soil humidity selection

2.3.1 The theory of compaction

As we already know:Material can be divided into 3 types, solid of soil, liquid of water and gas of air Normally solid and liquid cannot be compressed, so compacting soil makes the soil particles surrounded by water pressed full into the interstices of the soil particles and pushes air out of the blocks The soil porosity decreases small while the hardness (density) increases Obviously, the compacting process rearranges the three soil types under the influence of external forces Study purposes is to utilize the minimal compaction performance to obtain the most effective compaction leads to the development of theory and application techniques for compacting soil

2.3.2 Selection of compacting factor and humidity for cohesive soil

• Compacting factor requires to avoid subsidence when the water is stored:

- Dry soil W≤Wopt : γk≥0,95γk.max

- Wet soil W>Wopt: γk≥0,90γk.max

• Choosing γk depends on waterproofness: Wet soil W>Wopt

• Selecting the factors based on the special mechanics characteristics:

K≥0,95 and W≤Wopt 3%

• According to the standard: For construction work at level II and higher K≥0,97, construction work at level III and lower K≥0,95, and W=Wotp3%

2.4 Conclusion of chapter 2

Chapter 2 focuses on the physics essence analysis, mineral components, mechanics characteristics of clay and of cohesive soil The study on the scientific basis of physical essence and chemical relate to application of cohesive soil in construction of soil works These studies are classical studies of the scientists in

the world and Vietnam It shows the natural elements, geography, climate and history of the soil due to the characteristics study of them

Standard system among Russia, Vietnam and Western countries are different on soil classification but the essence is similar In this thesis, the author does not change and it is cited with original notes This issue will be undertaken in the ISO system

The most obvious point of the application of study is the dependence of the shear strength on the compacting factor of the soil and the soil type In design and construction as well as technological applications, we should consider applying

to the specific, natural conditions and materials extracted from the current the region

Chapter 2 also fully updates research results for the soil in South Central, Central Highlands and Southeast region of Vietnam from other scientists to apply in the practical studies

The previous study shows that soil with high compactibility and certain mechanics characteristics for serving the earth dam construction depend on the compacting factor; cohesive soil is high hygroscopic and high capillarity; drainage and consolidation following loading time

When we chose the mechanics characteristics of the damming material: For soil with special mechanics properties (swelling, shrinkage, and wet disintegration)

we should comply with current regulations; For the normal soil, we should optimize the internal mechanics properties consolidation following loading time The standards system of Vietnam is currently approaching towards using compacting factor in the design and construction, and it is suitable with damming material However, when we combine a compacting factor value for all soil types,

it will be a huge waste and difficult in the construction process

For the soil in study area which do not have special mechanics properties, we should approach optimization of the internal mechanics properties consolidation following loading time

CHAPTER 3 EXPERIMENTAL STUDY OF COMPACTED EARTH DAM CONSTRUCTION UNDER HIGH HUMIDITY CONDITIONS 3.1 Introduction

In this chapter, the author will focus on experimental studies to determine the curve between humidity (W) and the compaction factor (K) of soil: W~K and curve between mechanics characteristics: Friction angle (φ), cohesion (c) and hydraulic conductivity (k) with compacting factor: (φ, c, k) ~ K After that, we find the value of compacting factor that damming material is suitable with the design At the same time, the author also studies about solutions to reduce soil humidity for damming

3.2 Experimental content

The author spent a lot of time to perform the experiments in the laboratory and

in the execute construction work for two soil types: alluvial sediments river sandstone and clay-stone rock sediments ruins Laboratory experiments determine the soil humidity at the material storage at several periods of the year,

17 mechanics characteristics, swelling, shrinkage, and wet disintegration Construction work experiments determine the particle components, hydraulic conductivity k, cohesion c, and friction angle φ after the soil covered in the dam

to reach the requirements of the design Construction work experimental results will be compared and be verified with laboratory experimental results

Table 3.3 Statistical table of laboratory test number

1 Ta Trach 17 mechanics

characteristics experiment

Sample group 427 Curve experiment K, k, φ,

3 Thuy Yen 17 mechanics

characteristics experiment

Sample group 11 Curve experiment K, k, φ,

c, W

Sample group 110

The experiments were carried out from August 2010 to February 2016 for soil sediment and ruins soil types in the works Ta Trach dam, Thuy Yen dam and Ngan Truoi dam The number of experimental sample group is summarized in Table 3.3

3.3 Experimental results in the laboratory

3.3.1 Sediment

Experimental results in Table 3.4 show that soil sediment in the study region is the soil type of weak swelling or no swelling and weak disintegration or no disintegration

Table 3.4 Mechanics characteristics about swelling and disintegration of sediment soil

4 Disintegrating disintegrate weak or no disintegrate weak or no

Through laboratory studies, the author built a curve between [compacting factor (K) and humidity of soil (W)] with [friction angle (φ), cohesion (c), hydraulic conductivity (k) and dry density] as Figure 3.9, Figure 3.10 and Figure 3.11

Figure 3.9 Hydraulic conductivity and compacting factor curve (k ~ K) of sediment soil