Nghiên cứu hiệu quả hệ thống nêm cát thu lọc nước thấm trong nâng cấp sửa chữa đập đất vừa và nhỏ tt tiếng anh

Placement of adverse filters in rehabilitation and upgrade of small and medium dams/ reservoir storing water would be difficult for the following reasons: i technical calculations for fi

VIET NAM ACADEMY FOR WATER RESOURCES

PHAM DINH VAN

RESEARCH ON EFFICIENCY OF THE SAND WEDGE SYSTEM FOR COLLECTING AND FILTERING PERMEABLE WATER IN REHABILITATION AND UPGRADE OF SMALL AND MEDIUM

Scientific supervisors:

1. Prof.Dr Nguyen Quoc Dung - Vietnam Academy for Water Resources

2. Dr Phan Truong Giang - Vietnam Academy for Water Resources

Reviewer No 1: Assoc Prof Dr Nguyen Huy Phuong - Ha Noi

University of Mining and GeologyReviewer No 2: Assoc Prof Dr Le Xuan Kham - Thuy loi UniversityReviewer No 3: Prof Dr Truong Dinh Du - Vietnam Water Resources

Development Association

The thesis is defended against State level thesis assessment council in Vietnam Academy for Water Resources No 171 Tay Son Street, Dong Da Dist., Hanoi

9

The thesis can be further referred to at:

- National Library of Vietnam

- Library of Vietnam Academy for Water Resources

In Vietnam, earth dams account for 90% of all types of dams In which, small and medium dams were built quite a long time, with local materials so vulnerable Currently, the seepage through of earth dam is very common and it accounts for 45% cause of the instability of earth dams

Earth dams in our country account for 90% of dams, in which small and medium sized dams were built quite a long time ago with local materials, thus being vulnerable Permeation on dams is very common, with many different variations, and accounts for 45% of instability causes

It’s impossible for earth dam to be completely free of permeation What matters is that permeation must not cause piping, overly water loss, and instability

in earth dam slope Piping often occurs on contact surfaces, where there is shifting of fine-grain materials into the layer of coarse-grain materials, or onto exposed surfaces To prevent piping, an adverse filter is one solution

Typical structure of adverse filters presented in books today include many variations, mostly for new dams The primary function of an adverse filter is to prevent hazardous piping in fine-grain soil that needs protection Placement of adverse filters in rehabilitation and upgrade of small and medium dams/ reservoir storing water would be difficult for the following reasons: (i) technical calculations for filters are complicated, thus engineers often design them in accordance with structural composition; (ii) materials to make filters are not available or not having required aggregate grain composition; (iii) digging at toe

of dam slope to install drains may compromise dam safety; (iv) in practice, it’s difficult to execute designs, therefore, it often requires reduction of water level in reservoir, affecting downstream irrigation

The structure “sand wedge to collect and filter seepage water at downstream

of earth dam” is a solution proposed by the author and the research team in the Hydraulic Construction Institute, accepted and granted a Copyright of Invention

in the gazette no CBA 351- dated 6/2017 This invention is a solution to collect and filter seepage water at the toe of earth dam slope, to lower phreatic line and to improve stability and safety of earth dams However, this invention is currently at the conceptual stage and not yet tested in practice to determine its efficiency, placement, calculation and design

Efficiency of “sand wedges” (also wedge-shaped sand blocks or) lies in their size and scale, distance between sand wedges, reduction of phreatic line, safe construction (while the dam is storing water), durability of sand wedges etc was

studied by the author and reflected in this thesis

2 OBJECTIVES OF THE STUDY

The study is to propose “sand wedges to collect and filter seepage water at downstream of earth dam” as replacement to conventional adverse filter made of sands-stones-gravels It also makes recommendations for placement of sand wedges so as to ensure functionality (collect and filter seepage water) and safety

of construction (while the dam is storing water)

3 SUBJECTS AND SCOPE OF THE STUDY

- Subjects of the study: Small and medium homogeneous earth dams (as defined in Decree no 114/2018/NĐ-CP dated 14/9/2018 by the Government on safety management of dams and reservoirs) storing water and suffering overly permeation, which require treatment to ensure dam safety even during construction

- Scope of the study: Exit point of the phreatic line when sand wedges are installed; Stability coefficients in construction (overall and local sliding safety factors; max permeation gradients during construction); Quality of water collection and filtering is evaluated by analyzing water samples in a 1:1 model

4 STUDY METHODOLOGIES

The thesis applies the following study methodologies:

Document study: Study existing books, articles, documents, technical standards etc related to permeation that affect safety of earth dams, structure of adverse filters, materials of filters etc

Inheritance study: Inherit dam safety criteria by Pham Ngoc Quy, edge with

a groove to filter water by Nguyen Quoc Dung etc

Mathematic modelling study: Use commercial software (in particular Midas GST, with copyrights) to calculate and identify permeation phreatic lines (in 2D and 3D), overall and local safety factors of existing dams, to determine probability of piping during construction and after sand wedges are installed Experimental study: Install 3 sand wedges and a system to monitor elevation

of phreatic lines (4 pits to gauge water level); measure seepage flow using meters and graduated thimbles; measure filter quality (through water turbidity, NTU) etc

in earth dam of Dong Be reservoir (Thanh Hoa province) Measurements are observed over the 2 years (2016 and 2017)

5 SIGNIFICANCE IN SCIENCE AND IN PRACTICE

Significance in science: Propose and conduct field experiments of sand

wedge system to collect and filter seepage water at downstream of small and medium earth dams; show efficiency in lowering phreatic lines, improve stability and mitigate permeation at downstream of earth dams The sand wedge system has simplified structure which is suitable for fixing and stopping permeation when conventional water-draining parts are not working

Significance in practice: Provide an additional simple solution to collect

and filter seepage water in downstream, improve safety and stability of small and medium earth dams

6 NEW CONTRIBUTION OF THE THESIS

Propose and establish theoretical basis for sand wedges to collect and filter seepage water in downstream slope of small and medium earth dams

Through field experiments and mathematical modelling, the study proposes potential placements of sand wedges as anti-permeation solution for small and medium earth dams storing water

7 CONTENTS AND STRUCTURE OF THE THESIS

The thesis in structured with 3 Chapters, plus Prologue, Conclusions, Recommendations, a list of published scientific studies and reference materials (18 tables, 67 figures and charts, appendices of experiment results, 5 published related studies and 39 reference documents) Contents and structure of the thesis are as follow:

PROLOGUE

CHAPTER 1: OVERVIEW OF PERMEATION AND CONTROL OF PERMEATION IN EARTH DAM

CHAPTER 2: THEORETICAL BASIS OF SAND WEDGES

CHAPTER 3: EFFICIENCY OF SAND WEDGES IN FIELD EXPERIMENTS

CONCLUSIONS AND RECOMMENDATIONS

LIST OF PUBLISHED SCIENTIFIC STUDIES

LIST OF REFERENCE DOCUMENTS

8 DOCUMENTARY BASIS OF THE THESIS

The thesis is composed on basis of the author’s years of study and research, analysis of documents regarding permeation that compromise safety of earth dams, structure of adverse filters, materials of filters etc and use of geotechnical software Midas GST to calculate and analyze permeation and how to stabilize small and medium earth dams (2D and 3D models) In particular, the study

conducted field experiments (Install 3 sand wedges and a system to monitor elevation of phreatic lines, measure seepage flow using meters and graduated thimbles; measure filter quality etc in the earth dam of Dong Be reservoir (Thanh Hoa province) The author and the research team in Hydraulic Construction Institute, accepted and granted a Copyright of Invention in the gazette no CBA 351- dated 6/2017

CHAPTER 1: OVERVIEW OF PERMEATION AND CONTROL OF

PERMEATION IN EARTH DAM

1.1 Permeation is among the main causes of dam safety failure

According to [29] (Todd Hill, P.E.; Department of Land Rehabilitation; Earth dams – typical potential failures; presentation in workshop dated 12/7/2016

in Hanoi), permeations caused about 45-50% of dam failures in the U.S

According to report of American Large Dam Association, ASCE/USCOLD (1975), there are 4 main causes of dam failures: (1) Overtopping (38%) – when gate is open, spillway capacity is not sufficient; (2) permeation, piping (33%) – permeation through foundation, body and abutment of dam, and permeation through contact surface of soil embankment and the structure; (3) instability (23%) – loss of stability in foundation of gravity dam, slope sliding in earth dams; (4) other causes (6%) – malfunctions of gates, rubbish blockage, sabotage [1] In short, permeation is among main causes of dam failures (second to overtopping)

In Vietnam, there are about 6.648 irrigation reservoirs and most of those have earth dams [1], [5] About 6.000 small and medium irrigation reservoirs were built during 1960 – 1980 with limited survey technology, lacking design standards and codes, actual construction not meeting technical specifications, inadequacy or absence of operation protocols, regular maintenance, and forecasting capacity [29] Investment report of the project “Dam rehabilitation and safety improvement” [29] concludes that: Permeation is one of the main causes of earth dam failures, therefore anti-permeation treatment is a solution to minimize risks

of dam failures

1.2 Anti-permeation treatments for reservoir storing water

1.2.1 Objectives of anti-permeation treatments

Prevent occurrence and spread of seepages, piping and bubbles by placing adverse filters in positions with high gradient, add incremental load if necessary; Negate and reduce pressure of pore water, uplift pressure and seepage pressure by releasing seepage water out of dam body; Prevent blistering and erosion on dam

surface; Reduce water loss from a reservoir by build additional sloping walls, walls/ ditches at the core

1.2.2 Structural composition of earth dam with components to prevent and control permeation

Figure 1.1 is the cross-section of an earth dam with structural components to better prevent and control permeation through dam body [4], [7], [16], [22], [34]

Downstream shell Chimney drain

Chimney filter Impervious core

Cutoff trench

Figure 1.1 Structure of earth dam with different filter and drain components [34] 1.2.3 Measures to enhance anti-permeation for dam body while reservoir is storing water

In Vietnam, there are various measures with different cons and pros: drill and inject mortar to create anti-permeation membrane; anti-permeation upstream sloping wall; anti-permeation wall/ditch

1.2.4 Control permeation for dam while reservoir is storing water

Structures to collect and drain seepage water (also known as filter) for earth dams may include: Adverse filter to prevent piping; system to collect and drain seepage; relief well on dam toe to reduce uplift pressure

These structures can be standalone or combined to address permeation for a particular earth dam In principles, designs and placements of filters and drains for seepage water in dam body must perform the following functions: drain water permeated through dam body and foundation toward downstream; prevent seepage flow to come out on dam slope and downstream dam abutment; reduce phreatic lines to improve stability of downstream dam slope; prevent deformation due to permeation

1.3 Shortcomings in designs of filters, drains for seepage water in earth dam

Components to collect, filter and rain water toward downstream of dam is indispensable in most earth dams (except for dams with height less than 5m, dams not prone to water pressure etc.) However, in fact, current designs of anti-permeation components still have many shortcomings, even unwanted mistakes

For example, Washakie dam in the U.S [39], Quy Lo dam in Nghe An provice, Trieu Thuong dam in Quang Tri provice, Ham Lon dam in Ha Noi city [29]

1.4 Mechanism of dam failure due to permeation and piping

Occurrence and spread of piping in earth dams can be divided into 2

categories (1) Phreatic line in dam body rises to the top of downstream slop, gradient is high at exit point and some soil specks is blistered, gradually spread to

a larger area and to the dam crest, causing erosion on dam crest, which leads to overtopping and then dam failure (2) Piping occurs at toe of downstream dam, spreads to upstream, and at certain extent, causes dam failure

Treatment of phreatic line in dam body rising up in downstream slope When

analyzing for anti-permeation treatment, we must identify highly safe zones, moderately safe zones and risky zones [18] In order to identify these zones, we have to locate upper phreatic line and lower phreatic line in dam body as corresponding to varying coefficients of permeation in dam body and dam foundation

Water level

Phreatic line K minmin =[K]

Phreatic line K minmin =1.2[K]

Phreatic line J max =[J]

Sa fet

y z one

Phreatic line J max =[J]/1.2

Figure 1.2 Safety factor considering exit point of phreatic line [18]

1.2), there must be measures to lower phreatic line by building additional permeation walls or renovate seepage water collection system

anti-1.5 Standards guiding design of filters

In rehabilitation and upgrade of small and medium dams, use of adverse filters with design standards for filters in new earthworks (Vietnamese standard TCVN 8422: 2010 [23] or American standard FEMA-1970 [36]) would be difficult to execute, due to the following reasons: (i) technical calculations for filters are complicated, thus engineers often design them in accordance with structural composition; (ii) materials to make filters are not available or not having required aggregate grain composition; (iii) digging at toe of dam slope to install drains may compromise dam safety; (iv) in practice, it’s difficult to

execute designs, therefore, it often requires reduction of water level in reservoir, affecting downstream irrigation

The thesis aims at improving design of filters to simplify calculations so that

it can be used in small and medium dams storing water

1.6 Introduce the invention “sand wedges to collect and filter seepage water at

downstream of earth dam”

The aim of this invention is to renew, replace existing drains with sand wedges to collect and filter seepage water, with limited digging required at dam toe, so as to ensure safety even when the dam is storing water; installed sand wedges (filter components) can be covered in soil, one at a time, so that dam safety won’t be compromised

In Figure 1.3, in permeated area of dam slope foundation (7), dig wedge ditches at distances of L (Figure 1.3a) or place new slope embankments on old earth dam body (Figure 1.3b) Width of ditch (W) is often selected as multiple of digger bucket’s width, at least 1m; two side walls are made orthotropic while back wall has a slope proportional to stability of dam toe, oftentimes 1:0,5

The filter made of yellow coarse-grain sand (4) is placed on the back of

specific conditions when constructing sand filter

Figure 1.3 Cross-section along seepage flow through sand wedges [14]

The special feature in structure of sand wedges is: Instead of designing filters with incoherent grains (sand, stones, gravels) having suitable aggregate to prevent piping between layers, sand wedges use a layer of coarse grains with its core being a grooved ditch (that collects water) This new design creates a structure to

collect and filter water at downstream foundation of earth dam in order to lower phreatic line, collect seepage water and release it without letting soil specks to be washed off Sand wedges in this invention will work similarly as chimney filter and drain

In description of the invention, the author has not specified suitable distance between sand wedges Efficiency in lowering phreatic line and quality of filters are also not yet proven in practice

1.7 Conclusion of Chapter 1

In rehabilitation and upgrade of small and medium earth dams in Vietnam today, engineers often pay attention to anti-permeation measures and less to measures that control permeation Components to collect and drain seepage water are often based on structure, suitable for new dams but not so much for operational dams storing water

Design standards of adverse filter require complicated calculations and procedures For rehabilitation and upgrade of earth dams storing water, especially small and medium ones, how to address water drainage and anti-permeation in a way that ensure such functions and dam safety is a critical matter worthy of attention and study

“Sand wedges to collect and filter seepage water” is an invention proposed

by the author and his research team as a new measure to collect seepage water behind earth dams, using a new method which is grooved ditch, following capillary principle

CHAPTER 2: THEORETICAL BASIS OF SAND WEDGES

2.1 The matter

Sand wedges to collect and filter seepage water is a new measure to collect seepage water behind earth dams It is applicable for small and medium dams but must be studied to verify its efficiency in theory and in practice before it can be widely adopted In theory, permeation must be calculated and permeation safety must be verified, particularly: during construction, after construction and in boundary conditions of calculation

2.2 Methodologies

2.2.1 Methodologies in studying permeation in earth dams

Analytic method: This method uses basic rules of permeation and related theories to determine characteristics of seepage flow, including mechanics of fluids and hydraulics This method can only solve calculations with simple

boundary conditions (homogeneous dam, impervious ground) [8], [16]

2.2.2 Digital modelling to analyze earth dams having sand wedges

In a study [17], the author used analytic method (hydraulic) and digital modelling with 2 commercial software (Seep/W by Geoslope, Canada, and Midas GST by Korea) to identify phreatic line in the cross-section of a 2-block dam having boundary conditions similar to those studied under this thesis Calculations suggest that analytic method and digital modelling to identify phreatic line yield similar results in case of homogeneous dam body and different results in case of inhomogeneous dam body

2.3 Select software to calculate permeation and verify permeation safety

Currently, software such as Seep/W – Geoslope, Midas GTS, Plaxis, Flax, Abaqus etc can all be used to identify characteristics of seepage flow in earth dams Considering its advantages in terms of graphics and convenience for simulating filter strips, pipes, sand wedges, ditches, the author selected Midas GTS to analyze permeation and stability on 2D and 3D models Figure 2.1 shows block diagram to calculate permeation phreatic line and stability when sand wedges are used

2.4 Analyze permeation in earth dams using Midas software

2.4.1 Identify safe zones and risky zones prone in term of permeation

Take the example of a 15 m-high earth dam, with changeable permeabilities

level is 1.5 m from the top of the dam Permeabilities of dam body and foundation are summarized by the author from dams with height less than 15 m included in WB8 project Figure 2.3 and Figure 2.4

Identify top limiting line by fixing dam foundation permeability as corresponding to the highest phreatic line, then change permeability of dam body

fixing dam foundation permeability as corresponding to the lowest phreatic line,

1.2 x [K] cp (Figure 2.5)

susceptible to permeation because exit point of phreatic line is way too high on downstream slope Treatment is needed by installing sand wedges

Figure 2.1 Diagram to calculate phreatic line and stability

a) Model of filter strip b) Cells and components c) Water catcher in filter strip

Figure 2.2 Simulation of water-catch capacity of filter strip

Figure 2.4 Statistical diagram of foundation permeabilities of dams  15 m

TH1 TH5

Figure 2.5 Phreatic lines in dam body in various calculation cases

2.4.2 Stability when digging grooves to install sand wedges

Take the example of a dam with 15 m height storing water, body

Stability when digging singular grooves: Width of hypothetical dig hole varies

from 20 m, 10 m to 5 m and 1 m Compared to condition before digging to find

an appropriate distance Dig scenarios are as shown in Figure 2.6

Figure 2.6 Diagram of groove with different width of dig hole (scenario A)

Width of groove (W) should not exceed 5 m to ensure safety (W<5 m)

However, based on practical experience, the author recommends making sand