Luận văn thạc sĩ: Analyzing the causes of strong seepage on Xahuong dam and Proposing solutions for handling

However, after waterproof handling, the permeable phenomena still appears atdownstream slope from elevation +740 m to +85.0 m including the water intakeculvert This safety report also re

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de Liége

THUY LOI UNIVERSITY & UNIVERSITY OF LIEGE

FACULTY OF CIVIL ENGINEERING

Presented by

MAI THI NGAT

ANALYZING THE CAUSES OF STRONG SEEPAGE

ON XAHUONG DAM AND PROPOSING SOLUTIONS

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SUSTAINABLE HYDRAULIC STRUTURES

REASSURANCES

NAME: MAI THI NGAT

Major: Sustainable Hydraulic structure

Student Number: 148ULG09

I hereby declare that I am the person who conducted this master thesis under the guidance of Dr Ho Sy Tam and Prof.Radu Sarghiuta with the research topic in the thesis “Analyzing the causes of strong seepage on XaHuong dam and proposing the

solution for handling”.

This is a new research topic which does not overlap with any dissertation before, so there is no copy of any public dissertation The contents of the thesis are presented in accordance with regulations; the data resources and materials used in research are quoted sources.

If there is any problem with the contents of this thesis, I would like to take full responsibility as prescribed.

SIGN

MAI THI NGAT

MaiThiNgat Supervisor: Dr Ho Sy Tam

Co-Supervisor: Prof Radu Sarghiuta

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MASTER THESIS

ACKNOWLEDGEMENTS

Master Thesis in major of sustainable hydraulic structure “Analyzing the causes

of strong seepage on XaHuong dam and proposing the solution for handling” was completed in August, 2016

In the process of implementation of the thesis, I always get the encouragement and devoted directions from my instructors _ Dr Ho Sy Tam and Prof Radu Sarghiuta I am really grateful for their invaluable help.

I also would like to express our sincere thanks to all of my teachers in Sustainable Hydraulic structure Master course at Thuy Loi University, along with professors from University of Liege had imparted valuable specialized knowledge for

me so that i can get this result.

Finally, I sincerely thank my family, my friends, and especially my classmates who had exchanged enthusiastically, contributed and encouraged me to complete this thesis.

Sincerely

SIGN

MAI THI NGAT

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CATEGORY

1.2 RESEARCH OBJECTTIVIES Án HT HT HH HH nHưệt 2

CHAPTER 1 GENERAL INTRODUCTION 4

1.1.1 Location of Project 4T©a - - G c1 HH KH ng 4

1.1.4 XaHuong T€S€TVOIT - - Q1 HH TH HH net 6 1.1.5 XaHuong Dam - - - c1 1n TH TH HH HH 10

1.1.5.3 Downstream dam Slope ccscecescessessneessceceseeceseeesaeceseeceeeceeeeeaeeesaeeees 12 1.2 SEEPAGE PROBLEM TO XAHUONG DAM key 13 1.3 STUDIES ON SEEPAGE INSTABILITY THROUGH EARTH DAM 16

1.3.1 o,ï.io 16

1.3.1.3 Hydraulic gradient 00.0 ce 19 1.3.1.4 Darcy ÏAW HH HT HH HH HH 19

1.3.1.6 Basic principle of seepage line 00 eee eeeceeeeceneeeeteceeseeeseeeaeteeeaeeeeeeseees 22

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MASTER THESIS

CHAPTER 2: STUDY ABOUT CAUSES MAKING

SEEPAGE INSTABILITY THROUGH THE BODY OF

2.1 INTRODUCTION ABOUT CALCULATION SOEFTWARE 33

2.1.1 DeSCTIPtiONn n6 33

2.2 CALCULATION - HT ng HH ni ghh 40

CHAPTER 3: SEEPAGE TREATMENT SOLUTIONS 60

3.1 PROPOSED SOLUTION 1.00 eccecccesceeeceseeeseseeeceeeseceaeceeecaeceseceaeseeeeneeeaeeeeeeseees 60

3.1.2 Solution for case 4: effect of anisotropic 1nferÌayT ««++-«<++ss++ 64

CHAPTER 4: CONCLUSION & RECOMMENDATION 68

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LIST OF FIGURE

Figure I- 1: Location of XaHuong reservoir ensembles ccccccccccccccsccseccceeseceseeeseeessseeaes 4 Figure 1- 2: XAHUONG T€S€TVOÌI Ăn TH TT HH Hệ 7

Figure 1- 4: Dam crest from the right ADUtMEN ccccccccescceesseceeeseeeeseeeesneeeessesenee 10

Figure 1-5: Dam crest from the left ADULMENE «key 10

Figure 1- 6: Crest of parapet WdlÏ SG E981 11v vn tren Il Figure 1- 7: Foot of parapet Wl l ccccccscccssccessccesscessecessecesseeeseceseeceeeceecesseeeseeseeeseaeeess Il Figure 1- 8: Dam Slope in the left ADUtMENE c.cccccscccessceetccesseeeseeseseeseseseseesesenseennees 12

Figure 1- 10: Overall downstream dam SLOPE eccccssccescccessceesecesseeenseceneeesaeeeaeeseaeeseaeenss 13 Figure I- 11: Dam slope m = 2.5, from elevation of +83.0m to dam cresf 13 Figure I- 12: The first dam berm at elevation Of +83.0 11 cccccscccscccesscessecestetenseeeneeees 13 Figure I- 13: Dam slope m = 3.0 from elevation Of +71.5 tO +63 13 Figure 1- 14: Handling the seepage of dam slope from elevation +71.5 to +63.0 I5

Figure 1- 15: Concentrated rocks for seepage drainage ON sÏOD€ -s-«<<<5 T5

Figure 1- 17: Cross-Section Of (ÍŒEH G3181 899111 1k1 1 tk ngư 26

Figure I- 19: Effect of Anisotropy on Seepage through an Earth Dam 29

Figure 2- 3: Importing region from AutoCA DFOBTGIH sccSccssssssessseeeeee 36

Figure 2- 5:Defining hydraulic boundary COHHÏOPS à à.ccSẶSSSSS+ssekisseeerxses 37

Figure 2- 8: SOWiNg dAta SG n0 Hư 38 Figure 2- 9: Displaying T€SHÏHS Ă TH TH HH Hư 39

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- Seepage calculation results fOr CASC 1 ccescccccscccccssccceesseetestseetstesetsasees

- Calculated result of slope slide stability (Normal load combination)

2 ~.-S Seepage calculation results fOr CASC 2.i eccecccscceseesetseeseeeseeenetseeeseeenees

Ÿ «« Calculated diagram of slope stability (Normal load combination)

««<«« Calculated result of slope slide stability (Normal load combination)

- Seepage calculation results for case3(rafio= Ï4Í) «««-«<«<«+

- Seepage calculation results for CaS€3(TATIO=2O) «-«-«-<<<«+

- Contributed material layers cccccccccccescccesscteessceeeneeeeesseesesseeeeeuseeensaaees

- Seepage calculation results for Z= 70M cccccccccccessecesseeeseeesseeeseeeenseeeas

- Calculated diagram for ZÔÍI ch he

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Contributed material layers

Calculated diagram of seepage stabilitySeepage calculation result

Calculated diagram of slople stability (Normat load combination),Figure 3- 10: Calculated result of slope slide stability (Normal load combination)

MaiThiNgat ‘Supervisor: Dr Ho Sy Tam

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LIST OF TABLE

Table 2 1: Mechanical and physical indicators of fill-soil for dam body andfoundation

Table 2 2: Ouput data of case 3

Table 2 3: Output data of case 4

Table 3 1: Ouput data

4315867

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1.1 THE URGENCY OF THE PROJECT

Earth dam is a type of dam built by the existing soils in the building region such

as clay, clayed , sandy loam, sand, gravel, cobbles Earth dam has simple and stablestructure, capable of highly mechanized during the construction and in most cases.Barth dam Widely applied in most countries This type of dam has advantage of

using local materials which are available at construction area, so it has cheaperconstruction costs comparing to other types of the same scale dams However, earthdam also contains many risks, easy to occur unsafe incident to dams if the designingWork and construction does not guarantee the requirements such as foundationtreatment, dam structure selection, appropriate material planning for fill soil of an

‘embankment dam as well as densification ensure uniformity and tightness of each filllayer, According to statistic, permeability occupies high rate in the cause of makingreservoir built with local materials unsafe

In our country, most of the earth dams are made of homogeneous soil Whenwater level rise and lowered erratically, it will destabilize the slope of dam, leading tosliding, subsidence, local erosion

Therefore, the calculation of stability mode for the earth dam is veryimportant Usually we only calculate permeability in homogeneous environments

to significant errors comparing to fact If the dam body or the dam's waterproofingparts are constructed with materials relatively homogeneous with small value ofheterogenous coefficient then we can solve the seepage problem with homogencous

‘environment.

Moreover, beside case of normal calculation (isotropic environment), we mustpay attention to the heterogeneity of the material (anisotropy of permeability) Theinhomogeneous - anisotropic usually occur because of earth dam constructionMaiThiNgat T ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

technology with horizontal soil layers, having the difference in permeability

coefficient between horizontal and vertical layers (kŠk”); whereas: kề, kỳ are

permeability coefficients of horizontal x and vertical y

In fact, we often see the type of land with permeable foundation, soilfoundation and fill soil of dam includes many different layers The problems of thistype are complicated, because we have to mention the environment with multiplelayers as well as complex boundary conditions The seepage problem solutions that we

ad learned only approximate and simple

When calculating permeability, we must analyze the viability of the materialwith anisotropic permeability coefficient with different values (© take measures (©

‘overcome the adver: consequences of distortion repellent

Recently, there are 2 methods to calculate permeability: permeabilitycalculation by analytical method (straight-line rate method of Lence - American

‘engineer, published in 1934) and by numerical model method (using software SEEP /

`W version in 2007 by GEO-SLOPE International, Ltd, Development Canada)

‘Today, beside the the significant progress in using numerical methods inparticular and the strong development of modem technology in general, we can solvethe permeability problem more quickly and easily In my thesis which is “Analyzingthe causes of strong seepage on XaHuong dam and proposing the solution forhandling”, the auther will apply calculation software namely GEOSLOPE to calculatethe anisotropic permeability of XaHuong dam,

12 RESEARCH OBJECTIVES

‘The main purposes of this research are to study the causes of the permeabilityphenomenon occuring inside XaHuong dam body and based on basjc theoretical tocalculate and propose solutions to handle this problem

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13 METHODOLOGY TO STUDY THE SUBJECT

Using Geoslope software (Seep/W and Slope/W) to calculate seepage and slope

stability of dam in different eases, especially in wo cases: Anisotropic seepage and

anisotropic interlayer, based on calculations results, comparing and asse: ing effect ofseepage instability to XaHuong dam; then analyzing and giving the best waterproofingsolutions for seepage problem of XaHuong dam

1.4 RESEARCH SCOPE OF THE STUDY

In this research, I just focus on study application of GEO-SLOPE to calculatestability for XaHuong earth dam in isotropic and anisotropic cases

MaiThiNgat 3 ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

CHAPTER 1 GENERAL INTRODUCTION

11 INTRODUCTION OF THE PROJECT

L.L.1 Location of Project area

Is managed by Limited Company MTV Thuy Loi Tam Dao, Xa Huongreservoir is located in Buffalo Valley at the foot of Tam Dao mout n of Xa Huongvillage (Minh Quang, Tam Dao, Vinh Phuc), away about 2 km from the NationalHighway 2B at Km13 and about 15 km North East to Vinh Yen city [1]

Figure I- I: Location of XaHuong reservoir ensembles

‘Topographical and geomorphological conditions

Headwork area of XaHuong dam cut across a narrow valley - in the foot of TamDao mountain (Vinh Phuc) With a slope towards the northeast — southwest, the mainriver originates from the Tam Dao mountain (Tam Dao has mountain side with steep

slopes and high level of coverage) [1]

MaithiNgat 4 ‘Supervisor Dr Ho Sy Tam

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1.1.3 Geological features

In the hydrological report of the Company Consulting and Technology Transfer(in 2009) , the characteristics of natural features, climate as well as meteorology inarea of work was described as following:

‘There are 3 pits (symbol KMI - KM3) had dilled at the dam route area,wheareas:

~ ‘The borehole KMI with a depth of 30:5 m was laid out on the dam berm atelevation +83.0 m

= ‘The boreholes Km2 and KM3 with a depth of 20 m was laidout on the berm at

elevation $71.5 m

Besides, drill holes are arranged to form the vertical and horizontal geologic

‘ross sections , Stratigraphic layers are described from top to bottom [1]

= Dam fill - soil (symbolized A): reddish brown clay, gray, hard plastic to the

semi-rigid state, tight texture, stones mingled with small debris with thicknessđeoreasing towards the foot ofthe dam, the bottom layer at elevation +59.0 m

‘At layer A conducted the experiment pouring water into borehole KMI, KM2and KM3, with hydraulic conductivity K = 10-4 to 10-5 envs

= Stone weathered completely -Tropical IAI (symbol 2): clay mingled stonedebris which have still not weathered all, has brown red, white and semi-rigidstate Layer 2 is present in three drilled holes with relatively uniform thickness,approximately 3-4 m At group of completely weathered rock, conducted

pouring water testing at Kml and KM3, hydraulic conductivity was 2 x 10°

mức

= Stone weathering light, fresh - Tropical IIB (symbol 3): underlying layer 2,slightly discolored rocks, closed cracks, unbroken peeled drilling; very rigid,

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MASTER THESIS

hard (o break by hammer pounding At moderate weathered group, conductedpouring water at KM2, hydraulic conductivity is K = 210-5 mức

Based on the design documents of the Corporation Construction Consulting'VN-2013 about urgent waterproofing handling for Xa Huong dam body; we can drawnthe following as ‘sments about geological conditions:

= From the results ofthe survey, the dam fill - soil had uneven compaction factor

ddue to distribution area and the height of the dam From elevation +848 m

-+80.5 m to the dam surface, filed Soil have better compacted factor, but thereare some places sil have unsatisfactory compacted factor Or we ean say thatfilled soil quality isnot satisfactory if comparing with the standard design of theđâm and earth dam construction in past as well as present (with the requireddensity Ke 20.95)

+ According the the previous design, the dam was remolded homogenate, but theactual check shown that filled soil of the dam body is not homogenate, reflected

by the results of experiments undisturbed soil samples

= Results geophysical survey by electrical symmetry depth also showed dam

body locally have voids (in the dry part has high resistivity p = 2000 +

low res

Qm and more hydrated wet section has electricity ivity p

operation in 1984 with a capacity of over 12 million m’ of water, enough irrigation

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water for 1,850 hectares farmland of the three districts Tam Dao, Binh Xuyen and TamDuong (Wikipedia, the free encyclopedia)

Figure 1- 2: XaHuong reservoir

‘The crest elevation of XaHuong dam is +94m, dam toe elevation is +50m andthe largest dam height is 41m, This reservoir is also put on the list of importantprojects of the Mi try of Agriculture and Rural Development

Being one of the reservoirs with a large dam height and volume in VinhPhueprovince, XaHuong reservoir plays an important role in the development of the

economy, especially the development of the province After being put into operation,

so far, XaHuong reservoir has undergone some major repairs in order to improve the

1009 and 2013, 2015level of safety of the construction in the years 1991

MAJOR SPECIFICATIONS:

# Reservoir

= Construction grade: I

~ Design flood frequeney: Pạy = 2%

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MASTER THESIS

75%

«Irrigation ensure percentage:

= Flood control regulation: Year

+ Normal Water level (NWL): 91.5m

+ Hitising water level (CFL): 93.5m

~ Dead water level (DWL): 66 m

= Reservoir's surface area corresponding to NWL: 0.853km?

“ Earth dam

Xa Huong dam is homogeneous dam with a height of about 41m, 252m length

and Sm crest width ; the crest elevation of earth dam is + 94m, the crest elevation

‘MaiThiNgat 8 ‘Supervisor: Dr Ho Sy Tam

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of breakwater wall is + 95m Upstream slope has slope coefficient are 4.5 ; 3.5and 2.75, which are separated by berms at the elevations +64m; +70m, +84m,494m; The upstream slope with slope coefficient of 25, 30 and 3.5, areseparated by berm at elevations +83m; +71.5m; and +60m.

Offtake culvert is arranged at the dam right abutment, with reinforced concretestructure size b x h x 14 m The elevation of the sewer inlet is +64.0 m,Length of culvert L = 200 m, Form of culvert is non-pressure box (RC boxculvert), using reinforced concrete material; steel valve gate operated by V30)and is regulated by flat valve placed in the culvert tower in upstream Design

flow Q=2.1 m'/s and culvert slope i = 5%.

dissipation, whereas the design discharge

1546m

4 Management house

MaiThiNgat a ‘Supervisor: Dr Ho Sy Tam

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Figure 1-6: Crest of parapet wall Figure 1-7: Foot of parapet wall

(Images taken from the safety report of XaHuong Dam [I])With the height of about 41m, the homogeneous dam XaHuong has dam crestwhich is 252m length, Sm width, The earth dam crest has elevation of + 94m and

‘parapet wall has elevation of + 95m,

‘Structured by crushed gravel, due to the impact of natural conditions, dam crest

is no longer being flat as the original state (Figure 1- 4), In the top of the dam crest, astone parapet wall was built with the height of Im In some locations at the crest andfoot ofthis wall, the external concrete mortar layer

7)

peeled (Figure 1- 6 and Figure I~

“The drainage ditch behind the parapet wall was strongly filled so many sections,

no longer have ability of drainage (Figure 1.6)

Besides, there is no lighting equipment in the dam crest

1.1.5.2 Upstream dam slope

Slope coefficient of upstream slopes are m = 4.5; 3.5 and 2.75, which are

separated by the dam berms at elevations of 464.0; +70.0; +84.0 and 94.0 (Figure 1- 8and Figure 1- 9) The slope is protected by stone riprap on ballast and gravel layer toprevent wave action, The dam slope is flat and has no phenomenon of disproportion,peeling stone or uneven,

‘MaiThiNgat TT ‘Supervisor: Dr Ho Sy Tam.

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Figure 1-8: Dam slope in the left Figure 1-9: Dam slope in the right

abutment abutment

(Images taken from the safety report of XaHuong Dam [1))1.1.8.3 Downstream dam slope

Downstream slope has slope coefficient m = 2.5; 3.0 and 3.5, are separated by

the dam berms at elevations of +83m; +71.5m; and +60m (Figure 1-12 & Figure 1-13)

"

Due to the relatively flat of dam slope, the grass grows evenly; leading to the

downstream slope is protected by grass

At the present time, there are some problems to the surface drainage ditchwhich is executed by stone, lied on the dam berm, at the dam foot and along the slope

‘Some positions is been peeling and some are covered by grass (Figure 1- 10 to Figure

113)

Besides, drainage ditch at the foot of prismatic drainage is made of soil.Monitoring the seepage flow becomes difficult because a part of water from the intakeculvert flow back into this ditch, however we can’t distinct whether water in this ditch

is permeable water or water poured into this ditch from the downstream side ofculvert,

‘MaiThiNgat 15 ‘Supervisor: Dr Ho Sy Tam.

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Because of working time and especially by seepage water through the dambody, some locations of dam berm are deformed,

AC the prismatic drainage in the downstream, a flow appears from this prismatic

to drainage ditch

Figure 1- 10; Overall downstream dam Figure I- 11: Dam slope m = 2.5, from

slope elevation of +83.0m to dam erest

Figure 1-12: The first dam berm at Figure 1- 13: Dam slope m = 3.0 from

elevation of +83.0m elevation of $71.5 to +83.0

(Images taken from the safety report of XaHuong Dam [1)1.2 SEEPAGE PROBLEM TO XAHUONG DAM

Was the 2" highest earth dam in Vietnam, XaHuong reservoir was constructed

in 1977 and in 1984 it was put into usage After nearly 40 years of exploitation, someworks and items have been repaired and uperaded several times

MaiThiNgat 1ã ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

In Safety report of XaHuong dam, the monitoring phenomenon in reality aredescribed as follow: in September of 1990, when the reservoir capacity reached atclevation +860 m, the permeable water appeared and leaked more toward thedownstream slope ftom elevation +770 m and +74.0 m Therefore, in 1991 The

Ministry of Water Resources (former) had handled waterproof by drilling grouting of

‘cement and clay into the dam body and simultaneously, upstream slope was paved bystone quarry from elevation +84.0 m to +94.0 m and cobblestone chit from elevation484.0 mo +64.0 m

However, after waterproof handling, the permeable phenomena still appears atdownstream slope from elevation +740 m to +85.0 m (including the water intakeculvert)

This safety report also records after the storm No, 5 in 2012 when the waterlevel raised to the designed elevation +91.50m, strong seepage phenomenon occurred

at downstream slope of earth dam, leading to drenched dam slope and creating flow ondrainage trench, then the dam body had signs of cracks, hollow inside, This is notallowed in the dam safety regulations, particularly for earth dam as the dam XaHuong,Bevides, the dam has not built the safety system yet when exceeding the flood designlevel or historical flood,

In early 2015, the next time of drilling jet grouting was conducted The range

was around the culvert with a length of about 75 m, 0.5 m from the bottom of drain tothe elevation +71.25 mm

AU the time of the survey, the October of 2015, the water level in the reservoir

\as +84.55 m, the downstream slope completely dried, in stark contrast to time beforethe waterproofing handling, As reported by the management unit, before the

‘waterproofing handling, when the level of water in the reservoir rose up to the same

elevation, the permeability flow appeared at downstream at different positions,

forming the seepage dumps Management unit had to us stone quarry as the drainageditch of seepage and accumulated stone quarry on berm to handle permeabilityfrequently

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Because of termites nesting phenomenon so in 2008, the dam was processed bydrilling termite spraying.

Figure 1- 14: Handling the seepage of dam slope from elevation +71.5 to +83.0

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‘MASTER THESIS

With the complicated movements of weather situations and the risk of damfailure, the need of finding out the causes of strong seepage on the dam body andfinding out solutions to that problem is extremely urgent to ensure the safety of

‘XaHluong dam,

(Source: Data and images taken from Safety report of XaHuong dam [1])

1.3 STUDIES ON SEEPAGE INSTABILITY THROUGH EARTH DAM

1.3.1 Seepage flow

Research purposes about seepage of the earth dam are to address the following

issues:

= Identifying the discharge of seepage flow through the dam, foundation and

shore fo assess water loss in calculations and balance the reservoir,

Simultaneously, based on the basis of this calculation, we will determine the

form of waterproofing for foundation and dam body.

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~ Identifying the location of the seepage line to arrange construction materialsand evaluate the stability of the downstream slope The determination of thelocation of the saturated line also is to choose the appropriate drainage formalong its size in order to improve the downstream slope stability.

~ Calculating permeable gradient to assess the level of underground erosion in

‘general and the local underground erosion aims to specify the reasonable size ofthe dam body , waterproofing structures, drainage and components of reversefiltering layer

1.3.1.1 Causes of permeability

Soils subjected of multi-phase mixture (Wikipedia, the free eneyelopedi)

= Solid phase is reinforced soil particles.

~ Liquid phase is water

~ Gas phase is arin the gap between the reinforced soil particles

Water in the soil can be in different states: water in vapor form, water inclinging form, water in thin films form, capillary water and gravity water The air inthe pores of the soil not only interacts with water in vapor form, it also dissolves inwater, about 2% of the volume of water

According to the saturated nature of water, the environment of permeable soil isdivided into two categories: saturated soil and unsaturated soil

~ Saturated soil is the environment which only comprise of two main phases: soiland water filled in the pore Cause of permeability in the saturated soil is due tothe movement of the seepage flow or gradient of hydraulic water column

= Unsaturated Soils is multiphase mixtures In addition to the three phases: soil,

water and air, the boundary of water air where occurs the surface tension isconsidered as the fourth independent phase, Beside the main cause creatingMaiThiNgat 7 ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

permeability in unsaturated soil which is the gradient of hydraulic water column(including high-pressure gradient and the elevation gradient), it also due to themoisture gradient and sticky suck gradient (degrees of stick suck is U, - Ủy,with U, is the pressure of air voids; Uw is the pressure of pore water)

1.3.1.2 Basie principle of seepage flow

As well as any phenomenon that occurs in nature, the movement of water isalways associated with the principle of conservation of energy, water flows from highenergy place to low energy place Hence, to understand about seepage flow, we need4o know about water energy at different positions

Energy of cubic water at different positions includes 3 parts

= Potentiometric: depend on elevation of water at a moment of time in a standard

plane

~ Pressure : depend on pressure of water

~ Kinetic energy: depend on the movement of water

‘These above energy components are shown as form of water head, total of |water head is shown’

ay

Whereas:

H Total water head

Pˆ Void pressure

‘ye Unit weight of water

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Z Water head (from calculated elevation to chosen compared datum)(potential head)

‘The equation (1.1) is called Bernoulli equation, shows that energy of watermoves and is used to compare energy at different positions To seepage flow in soil,

velocity head is much smaller than potential head and pressure head, so it can be

ignored

Applied force to water move from point A to point B in the soi Js generated bythe difference of total water head between the two points Water moves from place

‘which has the higher total water head to lower total water bead When using the same

‘compared datum, the difference of water head Ah between 2 points is not change anddon’t depend on selection of compared datum [6]

1.3.1.3 Hydraulic gradient

Hydraulic gradient is dimensionless quantity is used to express the loss of water

head between two points Its represented by the ratio between the difference of water

level between two considerable points Ab and the length of the seepage flow betweenthese two points:

(12)

Ah: the difference of total water head

AL: displacement distance

1.3.14 Darey law

From 1856, Darey had conducted experimental studies and observed thechanges of seepage flow through corresponding samples with changed water head,Darcy had explored that seepage flow petmeable flow is proportional to hydraulicslope j and today this relation is called the law of permeability Darcy

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With ris coefficient to determine the status of the seepage flow and the range is

in 1< r <2, depending on the size of the soil particles (or voids) and the hydraulic

slope

~ When r =1 seepage flow is in laminar state and formula (1.5) becomes Darcyformula, This is applied when seepage flow in environment of fine particles

such as sand, ch:

~ When r= 2 seepage is in turbulent state, Hydraulic resistance is proportional to

the square of velocity, usually occurs in the environment of Coarse (rock,

gravel.) larger hydraulic lope G>!)

= Seepage flow in the transition zone: Ì<r <2

MaiThiNga 30 ‘Supervisor Dr Ho Sy Tam

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The governing partial differential equation for seepage through aheterogeneous, anisotropic, saturated, unsaturated soil can be derived by satisfyingconservation of mass for a representative elemental volume If the assumption is madethat the total stress remains constant during a transient process, the differential

‘equation can be written as follows for the three dimensional transient

ân aye

cares

ax an ay" oh, ôn êh ah,aXS" —. q6)

I ky and kz co-efficient of permeability of soil in x, y and z direction,

spectively

‘m= water storage (the slope of the soil-water characteristic curve)

For steady state seepage, only the co-efficient of permeability is required

‘because the time dependent term disappears and the water storage term drops out But

to solve transient seepage problem associated with a saturated - unsaturated soil

system, the two soil properties (i.e co-efficient of permeability and water storage) arerequired (Thieu, Fredlund, al, 2001)

1.3.1.5 Hydraulic conductivity

Hydraulic condu ty (nws) basically depend on the average size the voids in

structure of the soil Generally speaking, small particle size will have small void sizeand small hydraulic conductivity and soil material will be less permeable

K permeability coefficient is to measure water conductivity of soil, whichaccording to the linear permeability law of Darcy, it defines the relationship betweenhydraulic reductions and speed the underground water permeability, It also depends onthe nature of the soil (pore size, particle size, shape and composition the graingrinding, ete ), is well as the nature of the ground water (vise sity, mineralchemi temperature etc ), permeability coefficient of the speed measurements.MaiThiNgat BỊ ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

Regarding the number, it equals permeable velocity when hydraulic permeability

reduction

1.3.1.6 Basie principle of seepage line

Potential enerey and kinetic energy of water masses

AS any phenomenon occurring in the nature, the movement of water always.

attached to the principle of conservation of energy, water down from higher potential

to lower potential place Hence, to understand seepage line inside the soil, it’s essential

to know water potential at different positions,

Basic rules on the movement of seepage flow are represented by Darcy's law:

ved aynWhereas:

vvis the permeability velocity (em /s)

I permeability gradient (hydraulic gradient)

kis the permeability coefficient (cm/s)

V values in the formula is the average flow rate of seepage flowsymbolize” when viewing the entire seepage line filled with liquid

‘The average flow rate in the void of soil or rock fissures calculated using the formula:

vin as)Whereas

v's the average permeable velocity of pores of the seepage environmentv: the average permeable velocity v of symbolic line, caleulated by theformula

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1: porosity of the environment (soil or fractured rock)

mW q9)Whereas

W" Volume of the hollow portion in the total volume of the environmentSeepage flow determined by the formula:

q=vA (110)

Whereas:

q: Seepage flow; (m3 /s)

'V: Seepage velocity; (em / s)

‘A: Cross-section area of seepage flow (em’)

1.3.1.7 Permeable basic equation

For stable permeability case, it means that both the flow rate and seepagepressure does not depend on time; the seepage velocity components are formed:

aay

Whereas h; Seepage water level

On the other hand, water infiltrate through soil suitable with continuousconditions of motion of incompressible fluid so it satisfy the continuity equation,

q13)MaiThiNgat 3 ‘Supervisor: Dr Ho Sy Tam

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MASTER THESIS

from Darcy formula and continuity we have

(113)

It we call permeable velocity is 9, then

n-th (14)From (1.11) and (1.14) we have

(15)

“Take the derivative (1.15) and put into (1.12) we have:

ae ay ae ao

(1.12) and (1.16) shows that function of the water column h and that velocity @

is the harmonic function Solve this Lapolaxo equation with specific boundary

conditions, we can determine the water column h and velocity @ at any point in the

permeable environment and from there determine the contours of water column h =

const and contours that @ = const On that basis, we can calculate the permeable

velocity and pressure

1.3.1.8 Planar permeable equation

In case of seepage flow is flat movement (not depend on direction of axial 62),differential equation (1.11) becomes:

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oy

&

1.20)

„ (1.20)oy

‘The connection between potential function and line function (is shown as Caushy Reyman formula

-d3n

(1.22)Equation (1.20) can determine the flow path has a constant value y = const

‘And from that calculate discharge by the formula:

4.23)

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MASTER THESIS

dun: Seepage discharge between 2 line function n and m

Vor Vạ¿ Values of 2 line function n and m,

Line function y and potential velocity @ has relation

naturally occurring strata as well as in earth dams the horizontal and vertical

permeability are often not equal The higher conductivitytends to occur parallel to thestratification, Thís makes the hydraulic conductivity anisotropic; that is, the

‘conductivity is not the same in all directions

ero

[

Figure I- 17: Cross-section of dam

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‘The Laplace equation as expressed by equation (1.27) and for which the flownet is a graphical solution applies only to isotropic material Hence, a simpletransformation may be made to the natural flow region in order to obtain an equivalenttropie flow region within which the flow net may be sketched This may be

demonstrated by starting with the Laplace equation for anisotropic material

oh, oh+ -0 125)

tay hay os)

If anew dimension X is defined by

nik, 1K" 426)

q2?)

Which is the Laplace equation for isotropic material

‘This indicates that if the x dimensions are transformed to X according to

‘equation (1.26) an isotropic region is obtained within which the flow net may be

<rawn In this transformation the x dimensions are varied and the y dimensions arekept constant, Altematively the transformation could be canied out in the y direction

in which ease the x dimensions would be maintained constant

‘This process of transformation is illustrated in Figure 1.18 In this example thepermeability k, is greater than the permeability k, This means that the transformedsection is smaller than the natural section in the x direction, For one dimensional flowfrom left to right the flow net has been sketched in the isotropic transformed section inthe Figure, In this flow net the flow lines and equipotential lines have been drawn toform a square pattern If this flow net is transferred back into the natural section, itisseen that the square pattern is not maintained and the shapes now become rectangular

In order to calculate the rate of seepage flow Q per unit width, equation (1.28)

(Geomechanics 1) may be used § nce there are now two permeability (ky and ky) it

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MASTER THESIS

may not be immediately apparent which value of the permeability should be used for k

in equation (1.28) The value of the permeability to be used in conjunction with thetransformed section to calculate the rate of s ;page flow may be developed as follows:

MaiThiNgat By ‘Supervisor: Dr Ho Sy Tam

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(2) by s9,

Figure 1- 19; Effect of Anisotropy on Seepage through an Earth Dam

From the natural section (anisotropic)

Q vay per unit width

(1.28)

per unit with

Where k is the coefficient of permeability to be used with the transformed section

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anisotropy, that is, the greater the magnitude of the ratio of the horizontal to vertical

permeability the more distorted the flow net becomes when itis redrawn on the naturalđam section The figure also indicates that the greater the degree of anisotropy thecloser the line of seepage moves towards the downstream slop of the dam [8]

SEEP/W has the capability of considering an ani

is specified as

Ky=ratio*K, (1.32)

K, is always specified and K, is always computed from K, and the specified

ratio,

A ratio of 2, for example, means K, is two times greater than K,, and a ratio of 0.1

In SEEPAW, using the anisotropy ratio physically means that the material isperfectly stratified: that is, II layering extends from the left side to the right side of themodel domain and that the layering is the same throughout the embankment, It isimportant to understand the physical significance of this ratio [10]

1.3.2.2 Analysis about cause of permeability

1.3.2.2.1 The characteristic of dam construction process

‘Compacted earth dam was constructed from stacked layers When constructed,people sprayed each soil layer thickness 25:30 em, using the device until it reachsdesigned tightness, then continue to spread another layer During the constructionprocess, if humidity of exploited soil smaller than controlled humidity when dam up(usually selected by the moistrure WTN), then it need to be irrigated water to replenishMaiThiNgat 30 ‘Supervisor: Dr Ho Sy Tam

Tiêu đề	Analyzing The Causes Of Strong Seepage On Xahuong Dam And Proposing Solutions For Handling
Tác giả	Mai Thi Ngat
Người hướng dẫn	Dr. Ho Sy Tam, Prof. Radu Sarghiuta
Trường học	Thuy Loi University
Chuyên ngành	Sustainable Hydraulic Structures
Thể loại	master thesis
Năm xuất bản	2016
Thành phố	Ha Noi

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Số trang	79
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