Analyzing the causes of strong seepage on xahuong dam and proposing solutions for handling

42 Figure 2- 15: Calculated result of slope slide stability Normal load combination ..... 44 Figure 2- 18: Calculated result of slope slide stability Normal load combination ..... 47 Fig

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

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

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

CATEGORY

PREMISE 1

THE URGENCY OF THE PROJECT 1

1.1 RESEARCH OBJECTIVES 2

1.2 METHODOLOGY TO STUDY THE SUBJECT 3

1.3 RESEARCH SCOPE OF THE STUDY 3

1.4

CHAPTER 1 GENERAL INTRODUCTION 4

INTRODUCTION OF THE PROJECT 4

1.1 1.1.1 Location of Project area 4

1.1.2 Topographical and geomorphological conditions 4

1.1.3 Geological features 5

1.1.4 XaHuong reservoir 6

1.1.5 XaHuong Dam 10

1.1.5.1 Dam crest 10

1.1.5.2 Upstream dam slope 11

1.1.5.3 Downstream dam slope 12

SEEPAGE PROBLEM TO XAHUONG DAM 13

1.2 STUDIES ON SEEPAGE INSTABILITY THROUGH EARTH DAM 16

1.3 1.3.1 Seepage flow 16

1.3.1.1 Causes of permeability 17

1.3.1.2 Basic principle of seepage flow 18

1.3.1.3 Hydraulic gradient 19

1.3.1.4 Darcy law 19

1.3.1.5 Hydraulic conductivity 21

1.3.1.6 Basic principle of seepage line 22

1.3.1.7 Permeable basic equation 23

1.3.1.8 Planar permeable equation 24

1.3.2 Calculation of perfect anisotropy 26

1.3.2.1 Definition 26

1.3.2.2 Analysis about cause of permeability 30

CHAPTER 2: STUDY ABOUT CAUSES MAKING SEEPAGE INSTABILITY THROUGH THE BODY OF XA HUONG DAM 33

INTRODUCTION ABOUT CALCULATION SOFTWARE 33

2.1 2.1.1 Description 33

2.1.2 Steps to calculate 35

CALCULATION 40

2.2 2.2.1 Case 1: Normal working filter layer 42

2.2.2 Case 2: Clogged filter layer 44

2.2.3 Case 3: Effects of anisotropic permeability 46

2.2.4 Case 4: Effect of Anisotropy interlayer 52

CHAPTER 3: SEEPAGE TREATMENT SOLUTIONS 60

PROPOSED SOLUTION 60

3.1 3.1.1 Solution for case 3: Effect of anisotropic permeability 60

3.1.2 Solution for case 4: effect of anisotropic interlayer 64

ASSESSMENTS ABOUT RESULTS 67

3.2

CHAPTER 4: CONCLUSION & RECOMMENDATION 68 REFFERENCES 70

LIST OF FIGURE

Figure 1- 1: Location of XaHuong reservoir ensembles 4

Figure 1- 2: XaHuong reservoir 7

Figure 1- 3: Upstream view of XaHuong Dam 10

Figure 1- 4: Dam crest from the right abutment 10

Figure 1- 5: Dam crest from the left abutment 10

Figure 1- 6: Crest of parapet wall 11

Figure 1- 7: Foot of parapet wall 11

Figure 1- 8: Dam slope in the left abutment 12

Figure 1- 9: Dam slope in the right abutment 12

Figure 1- 10: Overall downstream dam slope 13

Figure 1- 11: Dam slope m = 2.5, from elevation of +83.0m to dam crest 13

Figure 1- 12: The first dam berm at elevation of +83.0 m 13

Figure 1- 13: Dam slope m = 3.0 from elevation of +71.5 to +83.0 13

Figure 1- 14: Handling the seepage of dam slope from elevation +71.5 to +83.0 15

Figure 1- 15: Concentrated rocks for seepage drainage on slope 15

Figure 1- 16: Seepage drainage on berm at elevation +71.5 16

Figure 1- 17: Cross-section of dam 26

Figure 1- 18: Transformation for Anisotropic Conditions 28

Figure 1- 19: Effect of Anisotropy on Seepage through an Earth Dam 29

Figure 2- 1: Geoslope software interface 33

Figure 2- 2: Creating analysis properties 35

Figure 2- 3: Importing region from AutoCAD program 36

Figure 2- 4: Defining material layers 36

Figure 2- 5:Defining hydraulic boundary conditions 37

Figure 2- 6: Drawing material layers 37

Figure 2- 7: Drawing boundary conditions 38

Figure 2- 8: Solving data 38

Figure 2- 9: Displaying results 39

Figure 2- 10: Viewing report 39

Figure 2- 11: Drawing permeable grid and sliding center 40

Figure 2- 12: Contributed material layers 40

Figure 2- 13: Calculated diagram of seepage stability for case 1 42

Figure 2- 14: Seepage calculation results for case 1 42

Figure 2- 15: Calculated result of slope slide stability (Normal load combination) 43

Figure 2- 16: Calculated diagram of seepage stability for case 2 44

Figure 2- 17: Seepage calculation results for case 2 44

Figure 2- 18: Calculated result of slope slide stability (Normal load combination) 45

Figure 2- 19: Calculated diagram of seepage stability for case 3 46

Figure 2- 20: Calculated diagram of slope stability (Normal load combination) 46

Figure 2- 21: Seepage calculation results for case 3 (ratio=7) 47

Figure 2- 22: Calculated result of slope slide stability (Normal load combination) 47

Figure 2- 23: Seepage calculation results for case 3(ratio=10) 48

Figure 2- 24: Calculated result of slope slide stability (Normal load combination) 48

Figure 2- 25: Seepage calculation results for case3(ratio=14) 49

Figure 2- 26: Calculated result of slope slide stability (Normal load combination) 49

Figure 2- 27: Seepage calculation results for case3(ratio=20) 50

Figure 2- 28: Calculated result of slope slide stability (Normal load combination) 50

Figure 2- 29: Contributed material layers 52

Figure 2- 30: Calculated diagram of seepage stability for Z=70m 53

Figure 2- 31: Seepage calculation results for Z=70m 53

Figure 2- 32: Calculated result of slope slide stability (Normal load combination) 54

Figure 2- 33: Calculated diagram for Z=80m 55

Figure 2- 34: Seepage calculation results for Z=80m 55

Figure 2- 35: Calculated result of slope slide stability (Normal load combination) 56

Figure 2- 36: Calculated diagram for Z=84m 57

Figure 2- 37: Seepage calculation results for Z=84m 57

Figure 2- 38: Calculated result of slope slide stability (Normal load combination) 58

Figure 3- 2: Calculated diagram of seepage stability 61

Figure 3- 3: Seepage calculation result 62

Figure 3- 4: Calculated diagram of slople stability (Normal load combination) 62

Figure 3- 5: Calculated result of slope slide stability (Normal load combination) 63

Figure 3- 6: Contributed material layers 64

Figure 3- 7: Calculated diagram of seepage stability 65

Figure 3- 8: Seepage calculation result 65

Figure 3- 9: Calculated diagram of slople stability (Normal load combination) 66

Figure 3- 10: Calculated result of slope slide stability (Normal load combination) 66

LIST OF TABLE

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

foundation 41

Table 2 2: Output data of case 3 51

Table 2 3: Output data of case 4 58

Table 3 1: Output data 67

PREMISE

THE URGENCY OF THE PROJECT

1.1

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 stable structure, capable of highly mechanized during the construction and in most cases Earth dam is widely applied in most countries This type of dam has advantage of using local materials which are available at construction area, so it has cheaper construction costs comparing to other types of the same scale dams However, earth dam also contains many risks, easy to occur unsafe incident to dams if the designing work and construction does not guarantee the requirements such as foundation treatment, dam structure selection, appropriate material planning for fill soil of an embankment dam as well as densification ensure uniformity and tightness of each fill layer According to statistic, permeability occupies high rate in the cause of making

reservoir built with local materials unsafe

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

Therefore, the calculation of stability mode for the earth dam is very important… Usually we only calculate permeability in homogeneous environments Concept of permeability of earth dams in case of homogeneous soil often do not lead

to significant errors comparing to fact If the dam body or the dam's waterproofing parts are constructed with materials relatively homogeneous with small value of heterogeneous coefficient then we can solve the seepage problem with homogeneous environment

Moreover, beside case of normal calculation (isotropic environment), we must pay attention to the heterogeneity of the material (anisotropy of permeability) The inhomogeneous - anisotropic usually occur because of earth dam construction

technology with horizontal soil layers, having the difference in permeability coefficient between horizontal and vertical layers (ktx,kty); whereas: ktx, kty are permeability coefficients of horizontal x and vertical y

In fact, we often see the type of land with permeable foundation, soil foundation and fill soil of dam includes many different layers The problems of this type are complicated, because we have to mention the environment with multiple layers as well as complex boundary conditions The seepage problem solutions that we had learned only approximate and simple

When calculating permeability, we must analyze the viability of the material with anisotropic permeability coefficient with different values to take measures to overcome the adverse consequences of distortion repellent

Recently, there are 2 methods to calculate permeability: permeability calculation 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 in particular and the strong development of modern technology in general, we can solve the permeability problem more quickly and easily In my thesis which is “Analyzing the causes of strong seepage on XaHuong dam and proposing the solution for handling”, the auther will apply calculation software namely GEOSLOPE to calculate the anisotropic permeability of XaHuong dam

RESEARCH OBJECTIVES

1.2

The main purposes of this research are to study the causes of the permeability phenomenon occuring inside XaHuong dam body and based on basic theoretical to calculate and propose solutions to handle this problem

METHODOLOGY TO STUDY THE SUBJECT

1.3

Using Geoslope software (Seep/W and Slope/W) to calculate seepage and slope stability of dam in different cases, especially in two cases: Anisotropic seepage and anisotropic interlayer, based on calculations results, comparing and assessing effect of seepage instability to XaHuong dam; then analyzing and giving the best waterproofing

solutions for seepage problem of XaHuong dam

RESEARCH SCOPE OF THE STUDY

1.4

In this research, I just focus on study application of GEO-SLOPE to calculate

stability for XaHuong earth dam in isotropic and anisotropic cases

CHAPTER 1 GENERAL INTRODUCTION

INTRODUCTION OF THE PROJECT

1.1

1.1.1 Location of Project area

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

Figure 1- 1: Location of XaHuong reservoir ensembles

1.1.2 Topographical and geomorphological conditions

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

Management house

Management road

Water intake culvert

Earth dam

Flood discharge spillway

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 in

area of work was described as following:

There are 3 pits (symbol KM1 - KM3) had dilled at the dam route area, wheareas:

- The borehole KM1 with a depth of 30.5 m was laid out on the dam berm at

At layer A conducted the experiment pouring water into borehole KM1, KM2 and KM3, with hydraulic conductivity K = 10-4 to 10-5 cm/s

- Stone weathered completely -Tropical IA1 (symbol 2): clay mingled stone debris which have still not weathered all, has brown red, white and semi-rigid state 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 Km1 and KM3, hydraulic conductivity was 2 × 10-5m/s

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

hard to break by hammer pounding At moderate weathered group, conducted pouring water at KM2, hydraulic conductivity is K = 2×10-5 m/s

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

- From the results of the survey, the dam fill - soil had uneven compaction factor due to distribution area and the height of the dam From elevation +84.8 m +80.5 m to the dam surface, filled Soil have better compacted factor, but there are some places still have unsatisfactory compacted factor Or we can say that, filled soil quality is not satisfactory if comparing with the standard design of the dam and earth dam construction in past as well as present (with the required density Kc  0.95)

- According the the previous design, the dam was remolded homogenate, but the actual 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  = 2000  2500

m and more hydrated wet section has electricity low resistivity  = 50  100

m These positions can be the termite nest or fill soil has not been compacted

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

Figure 1- 2: XaHuong reservoir

The crest elevation of XaHuong dam is +94m, dam toe elevation is +50m and the largest dam height is 41m This reservoir is also put on the list of important projects of the Ministry of Agriculture and Rural Development

Being one of the reservoirs with a large dam height and volume in VinhPhuc province, 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 level of safety of the construction in the years 1991, 2009 and 2013, 2015

- Irrigation ensure percentage: P = 75%

- Flood control regulation: Year

- Normal Water level ( NWL): 91.5m

- Hi-rising water level (CFL): 93.5m

- Dead water level (DWL): 66 m

- Basin area : FLV = 24km2

- Reservoir’s surface area corresponding to NWL: 0.853km2

- Reservoir’s surface area corresponding to DWL : 0.15km2

- Reservoir’s storage corresponding to CFL: 15.8×106m3

 Earth dam

of breakwater wall is + 95m Upstream slope has slope coefficient are 4.5 ; 3.5 and 2.75, which are separated by berms at the elevations +64m; +70m, +84m, +94m; The upstream slope with slope coefficient of 2.5, 3.0 and 3.5, are separated by berm at elevations +83m; +71.5m; and +60m

 Culvert

Offtake culvert is arranged at the dam right abutment, with reinforced concrete structure size b × h = 1 × 1.4 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 box culvert), 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 m3/s and culvert slope i = 5%

 Main spillway

Spillway of XaHuong reservoir is form of Broad-crested weir , is regulated by Arch-shaped valve-gate and arranged in left abutment of dam Size of valves is B× h = 10 × 4 m, the elevation of spillway threshold is +87.5 m The width of water slope is Bd = 10m The entrance of spillway is form of gradually narrowing, serial form following spillway are water slope - chute and energy dissipation, whereas the design discharge = 259m3/s and the length of Chute L = 154.6m

 Management house

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

(Images taken from the safety report of XaHuong Dam [1])

With the height of about 41m, the homogeneous dam XaHuong has dam crest which is 252m length, 5m 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, a stone parapet wall was built with the height of 1m In some locations at the crest and foot of this wall, the external concrete mortar layer is peeled (Figure 1- 6 and Figure 1- 7)

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 +64.0; +70.0; +84.0 and 94.0 (Figure 1- 8 and Figure 1- 9) The slope is protected by stone riprap on ballast and gravel layer to prevent wave action The dam slope is flat and has no phenomenon of disproportion, peeling stone or uneven

Figure 1- 8: Dam slope in the left

abutment

Figure 1- 9: Dam slope in the right

abutment (Images taken from the safety report of XaHuong Dam [1])

1.1.5.3 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) [1]

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 ditch which 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 1.13)

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 intake culvert 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 of

culvert

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

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

SEEPAGE PROBLEM TO XAHUONG DAM

1.2

Was the 2nd 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, some

works and items have been repaired and upgraded several times

In Safety report of XaHuong dam, the monitoring phenomenon in reality are described as follow: in September of 1990, when the reservoir capacity reached at elevation +86.0 m, the permeable water appeared and leaked more toward the downstream slope from elevation +77.0 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 by stone quarry from elevation +84.0 m to +94.0 m and cobblestone chit from elevation +84.0 m to +64.0 m

However, after waterproof handling, the permeable phenomena still appears at downstream slope from elevation +74.0 m to +85.0 m (including the water intake culvert)

This safety report also records after the storm No 5 in 2012 when the water level 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 on drainage trench, then the dam body had signs of cracks, hollow inside This is not allowed in the dam safety regulations, particularly for earth dam as the dam XaHuong Besides, the dam has not built the safety system yet when exceeding the flood design level 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 to the elevation +71.25 m

At the time of the survey, the October of 2015, the water level in the reservoir was +84.55 m, the downstream slope completely dried, in stark contrast to time before the 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 use stone quarry as the drainage

Because of termites nesting phenomenon so in 2008, the dam was processed by drilling termite spraying

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

Figure 1- 15: Concentrated rocks for seepage drainage on slope

Figure 1- 16: Seepage drainage on berm at elevation +71.5

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

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

STUDIES ON SEEPAGE INSTABILITY THROUGH EARTH DAM

- Identifying the location of the seepage line to arrange construction materials and evaluate the stability of the downstream slope The determination of the location of the saturated line also is to choose the appropriate drainage form along 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 of the dam body , waterproofing structures, drainage and components of reverse filtering layer

1.3.1.1 Causes of permeability

Soil is subjected of multi-phase mixture (Wikipedia, the free encyclopedia):

- Solid phase is reinforced soil particles

- Liquid phase is water

- Gas phase is air in the gap between the reinforced soil particles

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

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

- Saturated soil is the environment which only comprise of two main phases: soil and water filled in the pore Cause of permeability in the saturated soil is due to the 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 is considered as the fourth independent phase Beside the main cause creating

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

1.3.1.2 Basic principle of seepage flow

As well as any phenomenon that occurs in nature, the movement of water is always associated with the principle of conservation of energy, water flows from high energy place to low energy place Hence, to understand about seepage flow, we need

to 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:

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

The equation (1.1) is called Bernoulli equation, shows that energy of water moves 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 soil is generated by the difference of total water head between the two points Water moves from place which has the higher total water head to lower total water head When using the same compared datum, the difference of water head ∆h between 2 points is not change and don’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 It’s represented by the ratio between the difference of water level between two considerable points ∆h and the length of the seepage flow between these two points:

With r is 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 Darcy formula This is applied when seepage flow in environment of fine particles such as sand, clay…

- 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 slope (j>1)

- Seepage flow in the transition zone: 1< r <2

The governing partial differential equation for seepage through a heterogeneous, anisotropic, saturated, unsaturated soil can be derived by satisfying conservation of mass for a representative elemental volume If the assumption is made that the total stress remains constant during a transient process, the differential equation can be written as follows for the three dimensional transient case:

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) are

required (Thieu, Fredlund, et al., 2001)

1.3.1.5 Hydraulic conductivity

Hydraulic conductivity (m/s) basically depend on the average size the voids in the soil, on the other hand depends on the arrangement of soil particles, the shape and structure of the soil Generally speaking, small particle size will have small void size and small hydraulic conductivity and soil material will be less permeable

K permeability coefficient is to measure water conductivity of soil, which according to the linear permeability law of Darcy, it defines the relationship between hydraulic reductions and speed the underground water permeability It also depends on the nature of the soil (pore size, particle size, shape and composition the grain grinding, etc .), as well as the nature of the ground water (viscosity, mineral chemical, temperature etc .), permeability coefficient of the speed measurements

Regarding the number, it equals permeable velocity when hydraulic permeability

reduction = 1

1.3.1.6 Basic principle of seepage line

Potential energy 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:

v=kJ (1.7)

Whereas:

v is the permeability velocity (cm / s)

J permeability gradient (hydraulic gradient)

k is the permeability coefficient (cm / s)

V values in the formula is the average flow rate of seepage flow

"symbolize" 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:

v' v n/ (1.8)

Whereas:

v ' : the average permeable velocity of pores of the seepage environment

v: the average permeable velocity v of symbolic line, calculated by the

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

q: Seepage flow; (cm3 / s)

V: Seepage velocity; (cm / s)

A: Cross-section area of seepage flow (cm2)

1.3.1.7 Permeable basic equation

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

h

x h

y h

Whereas h: Seepage water level

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

from Darcy formula and continuity we have:

h vx x h vy y h vz z

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

is the harmonic function Solve this Lapolaxo equation with specific boundary conditions, we can determine the water column h and velocity φ at any point in the permeable environment and from there determine the contours of water column h = const and contours that φ = 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 oz), the basic differential equation (1.11) becomes:

h

vx k

x h

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

And from that calculate discharge by the formula:

q n m_  n m (1.23)

Whereas:

qn_m: seepage discharge between 2 line function n and m

ψn, ψm: values of 2 line function n and m

Line function ψ and potential velocity φ has relation

The Laplace equation as expressed by equation (1.27) and for which the flow

net is a graphical solution applies only to isotropic material Hence, a simple

transformation may be made to the natural flow region in order to obtain an equivalent

isotropic flow region within which the flow net may be sketched This may be

demonstrated by starting with the Laplace equation for anisotropic material

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

drawn In this transformation the x dimensions are varied and the y dimensions are

kept constant Alternatively the transformation could be carried out in the y direction

in which case the x dimensions would be maintained constant

This process of transformation is illustrated in Figure 1.18 In this example the

permeability kx is greater than the permeability ky This means that the transformed

section is smaller than the natural section in the x direction For one dimensional flow

from left to right the flow net has been sketched in the isotropic transformed section in

the Figure In this flow net the flow lines and equipotential lines have been drawn to

form a square pattern If this flow net is transferred back into the natural section, it is

seen 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 Since there are now two permeability (kx and ky) it

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 the transformed section to calculate the rate of seepage flow may be developed as follows:

Figure 1- 18: Transformation for Anisotropic Conditions

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

From the natural section (anisotropic)

Q = vxy per unit width

 per unit with

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

SEEP/W has the capability of considering an anisotropic coefficient The effect

is specified as

KY=ratio*Kx (1.32)

Kx is always specified and Ky is always computed from Kx and the specified ratio

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

In SEEP/W, using the anisotropy ratio physically means that the material is perfectly stratified; that is, all layering extends from the left side to the right side of the model domain and that the layering is the same throughout the embankment It is important 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 cm, using the device until it reachs designed tightness, then continue to spread another layer During the construction