under ground dams design
TRANSCRIPT
Underground Dams’ Design
Underground dam is...A facility that dams up groundwater flow, stores in the pores of the stratum and uses groundwater in a sustainable way.
Underground dam have no huge “Tank” under the ground, generally have a lot of porosity in the aquifer (underground). In other words, underground dam reserves the groundwater in “hard” porous sponges.
Underground Dams
Types of Underground Dam• Classification by dam purpose.
• Classification by method of construction.
• Classification by reservoir type.
• Classification by material of construction
1) Dam up type (Storage type, Run-off control type)This dam type is planned to store groundwater. The reservoir, which dams up
groundwater and regulates its discharge, accordingly increases the groundwater level and allows stable intake of groundwater.
2) Saltwater intrusion prevention typeThis dam type is planned to prevent intrusion of saltwater into the groundwater and to
protect available water resources. The reservoir unconditionally allows groundwater pumping and the resultant adjustment of the groundwater level.
Classification by purpose of underground dam
Classification by Purpose
Dam up type (storage type)
Saltwater intrusion prevention type
1) Ground improvement method (grouting method)In general, is applied by foundation improvement of surface
dam, using injection of cement milk under the ground and hardening the milk, and so constructing the impermeable barrier (grout curtain). This method is applied to underground dam in small scale.
2) Impermeable body driving methodThis method is to construct a dam body by driving steel sheet
pile (or concrete sheet pile). This method is used for shallow unconsolidated layer.
3) Diaphragm wall methodDiaphragm wall method is applied to underground dam in large
scale. Among of the several types of diaphragm method used for underground dam construction, (Soil Mixing Wall method).
Classification by Construction Method
Classification by Construction Method (cont.)
1) Fully subsurface storage typeThis dam type is ordinary case of underground dam and
reservoir is not visible directly.
2) Partially surface storage typeThis dam type has functions not only to reserve groundwater
but also to store surface water on the ground in the reservoir area.
3) Surface dam hybrid typeAt the surface dam, reservoir water is stored in the ground in
addition to on the ground by the effectiveness of the watertightness barrier, which is created by the foundation treatment such as grouting works.
Classification by Reservoir Type
Classification by reservoir type (Cont.)
Example of fully Sub-surface storage type underground dam
Example of partially surface storage type underground dam(Kanjin underground dam)
Surface reservoir ofKanjin underground dam
Classification by Material of Construction (Nilsson, 1988)
Dam Classification by Material of Construction (Nilsson, 1988)
World Underground Dams
Average Dam Heights (Nilsson, 1988)
Dam type Average height (m)
Injection screen 10 Brick wall 6 Concrete dam 6 Stone masonry dam 5 Reinforced concrete dam 4 Clay dike 3 Plastic sheets 2
Data on water storage in sand
Empty spaces between sand (voids) are filled when a dry river bed is flooded.
Tiny voids get saturated slower and less water can be extracted compared to coarse material.
The courser the sand the higher the volume of water that can be extracted.
Silt FineSand
MediumSand
CoarseSand
Size (mm) < 0.5 0.5 to 1.0 1.0 to 1.5 1.5 to 5.0
Saturation 38% 40% 41% 45%Waterextraction 5% 19% 25% 35%
Water extraction rate: (350 liters of water can be extracted from 1 m3)
Examples of Subsurface Dams in Japan
Examples of Subsurface Dams in Japan (cont.)
Map of Subsurface Dams in Japan
1. Excellent storage aquiferAn aquifer with large effective porosity and hydraulic conductivity must be available in the planned area.
2. Impermeable basement The basement stratum forming the reservoir floor and side boundaries must be relatively watertight so as to form efficient groundwater reservoir.
3. Sufficient recharge to the reservoir areaSufficient and appropriate groundwater recharge must be available in the reservoir area of underground dam. Normally, high precipitation and infiltration will be required corresponding to the planned amount of storing water.
Requested Natural Conditions for Underground Dams
Underground dam
Dam body: The construction of underground in order to shut out the groundwater flow. Drainage facility: The facility to drain the surplus water from the underground dam. Intake facility: The facility to take the reserved water from the underground dam.Operation and maintenance facilities: The facility to operate and maintain the underground dam. Recharge facility: The facility in order to increase the storage volume of underground dam by infiltration.
Technical Terms of Underground Dams
Dam crest : The crest of the cut-off wall.
Dam length : Length of the dam body.
Dam height : Height of the dam body (including penetration part).
Penetration part : Part penetrated into the basement layer.
Technical Terms of Underground Dam
Cut-off area : The sectional area constructed by cut-off wall (including Penetration part area.
Uncut-off area : The sectional area between working floor and dam crest.
Penetration part area : The sectional area constructed in the basement by cut-off wall.
Working floor : Flat plain for construction of cut-off wall.
Technical Terms of Underground Dam (cont.)
Critical water level : Permitted maximum water level in the reservoir.
Full water level: Water level when an overflow starts from the dam crest.
Low water level (Dead water level) :Minimum water table that is reached as a result of groundwater use in the reservoir area.
Effective water capacity: Storage capacity obtained by subtracting the dead water capacity from the gross water capacity.
Dead water capacity: Water volume below low water level.
Gross reservoir capacity: The sum of effective water capacity and dead water capacity.
Technical Terms of Underground Dam (cont.)
Porosity• The porosity or pore space is the amount of air space or void
space between soil particles.
Specific Yield• Specific yield is the ratio of volume of water that drains from a saturated rock due to gravity to the total volume of rock.
• A sample with smaller grain sizes will have a lower specific yield because of the Surface Tension.
• The specific storage of an aquifer can be defined as the volume of water that a unit volume of the aquifer under a unit decline in the average head releases from storage due to expansion of water and compression of the aquifer.
tatal
drainedy V
VS
Specific Storage (Ss)
n
nSS 1
Storage coefficient (Sc)
• It's the volume of water that a permeable unit will absorb or loss from storage per unit surface area per unit change in head.
• Sc= B Ss (Confined aquifer: 0.001-0.00001)• Sc= Sy +h Ss (Unconfined aquifer: 0.2-0.3)
(h Ss)is neglected because it is <<Sy
Figures taken from Hornberger et al. (1998)
Unconfined aquiferSpecific yield
Confined aquiferStorativity
S = V / A hS = Ss b
b
hh
Hydraulic conductivity (K)•It’s a function of properties of both porous media and the water passing through it which represent the specific rate (L / T ) of water passing through the porous media. K= k (ρ g/µ)
• k (permeability) a function of porous media only which present the actual permeability of that media, ρ density of fluid, g the acceleration of gravity and µ dynamic viscosity of fluid
TransmissivityIts amount of water that can be transmitted horizontally through a unit width by the full saturated thickness.
T = K B
Criteria for Choosing Dam Site• sandy soil (wadi images and surface information).• Less salt content.• Large average annual flow rates (annual rainfall data is the
measure).• Land gradient <5% (Topography and DEM).• Less drainage density (WMS calculation).• Less evaporation rate (meteorological data).• Bed rock not too deep (experience 20-70 m, geological maps,
geophysics, etc.).• Do not built on fractured rocks or large boulders to prevent seepage.• Build on solid bedrocks instead or 1 meter in solid and impermeable
soil.•
Case study in JapanAssumed subsurface dam
Site Springs(Bangle)
Model Boundary and Grid Design
Construction of an Underground Dam(a) a trench perpendicular to flow
in an alluvial valley is dug down to bedrock;
(b) the bottom and wall of the trench is lined with plastic;
(c) an extraction well is emplaced in the trench and then the trench is backfilled;
(d) salt extracting plants are grown in the alluvium immediately upstream of the dam to reduce salinity build-up. these are harvested and fed to cattle offsite
Underground Dams . a practical solution for the water needs of small communities in semi-arid regions by Telmer and Best.
Hydrological Study• Data required:- Meteorological data: (Rainfall depth, Temperatures, etc.)
- Geological data: (maps, logs, sounding, geophysics, rock types, stratigraphy, boreholes, faults, fractures, etc.).
- Hydrogelogical data: (aquifer types, hydraulic conductivity, porosity, etc.) may be obtained from soil types and grain size tables in text books or reports from previous projects, water levels, etc.
Potential and Available Storage Volumes
Wadi Area (Alluvium)Effective PorosityPermeable Thickness% of potential storage=0.20% of available storage=0.80Increased height due to dam
. .. . . .
a
a a
PSV A n HASV A n H A n HASV PSV
aAnH
H
Simple Groundwater Recharge Estimation Procedure
Design Output• Basin area.• Max dam height.• Crest elevation.• Free board.• Dam length.• Total capacity of the dam reservoir.• Monthly availability of the supply rate of water.• Material of construction (Hydraulic conductivity of the
cutoff wall).• Method of construction.• Cost estimation.
Cutoff Wall• Common values of the cutoff wall
hydraulic conductivity:
10-4 cm/sec.=0.001 m/day10-5 cm/sec.=0.0001 m/day10-6 cm/sec.=0.00001 m/day
Conceptual Model of Underground Dam
2 2
2
( , )( , )
( , )2 ( , ) 2
( , )2 ( , ) 2
xx yyw
xx yyw
xx yyw
h h Q x yK h K h R x yx x y y A
h h Q x yK K R x yx x y y A
u h
u u Q x yK K R x yx x y y A
h u
Analysis of Underground Dam: Groundwater Model
2D steady state, unconfined flow with recharge and Pumping:
-Analytical solutions only for simple conditions
-Numerical solutions: Finite Element, Finite Difference
Tim e level (k)
i,ji,j+1
i+1,ji,j-1
i-1,j
i+1,j,ki,j,ki-1 ,j,k
i,j-1 ,k
i,j+1,k
i,j,k-1
t+dt
t
(a)
(b)
الفراغي (b(التقسيم الزمني) في) aو الجوفي للخزاناألفقي هي kالمسقط بالشكل المعادالت n+1الموضحة في
المعادالت nهي k-1و Elfeki (2003)في
1 1 1, ,
1,
1 1 1 11, , , 1,
1 11 , ,2 2, ,
1,
Fully Implicit Finite Difference Method
2 ( , ) 2 ( , )n n n ni j i j
ni j
n n n ni j i j i j i j
n n i j i ji j i j
ni j
u us u uK K R x y W x yt x x y yu
u u u uK K
x xu ustu
1 1 1 1, 1 , , , 1
1 1, ,2 2 2 ( , ) 2 ( , )
n n n ni j i j i j i j
i j i j
x
u u u uK K
y yR x y W x y
y
1 1 1 1 1, , 1, , , 1,
1 1 1 12 2 2 21 , , , ,2 2 2 2,
1 1 1 1, 1 , , , 1
1 1 1 12 2 2 2, , , ,2 2 2 2
Simplifications leads ton n n n n ni j i j i j i j i j i j
n i j i j i j i ji j
n n n ni j i j i j i j
i j i j i j i j
u u u u u us K K K Kt x x x xu
u u u uK K K K
y y y y
1 1 1 1 1, , , ,, , 1 1 1 12 2 2 21, 1, , 1 , 12 2 2 21 1
, ,
1 1 1 1, , , ,1 1 1 12 2 2 2
, , , ,2 2 2 2
2 ( , ) 2 ( , )
n n i j i j i j i ji j i j n n n ni j i j i j i jn n
i j i j
i j i j i j i jn n n ni j i j i j i j
R x y W x y
K K K Ku us s u u u u
t t x x y yu u
K K K Ku u u u
x x y y
2 ( , ) 2 ( , )R x y W x y
1
1,1, ,2
1
1 ,1, ,2
1
1,, 1 ,2
1
1,, , 12
Inter-cells conductivities:
1 12
1 12
1 12
1 12
i ji j i j
i ji j i j
i ji j i j
i ji j i j
KK K
KK K
KK K
KK K
1 1 1 1, , , ,2 2 2 22 2 2 2, , , ,
i j i j i j i j
ij ij ij ij ij
K K K KsA C B D E
x x y y t
1, 1 1 1 1 1
1, 1, , 1 , 1 ,1,
,
1,
2 ( , ) 2 ( , )
ni j n n n n n
ij i j ij i j ij i j ij i j ij ij ij ij i jni j
ni j
ij ni j
us A u C u D u B u A C D B ut u
uE R x y W x y
u
1 1 1 11, 1, , 1 , 1 ,1
,1,
1,
2 ( , ) 2 ( , )ijn n n n nij i j ij i j ij i j ij i j i jn
i jni j
ijij ij ij ij n
i j
EA u C u D u B u u R x y W x y
uu
EA C D B
u
1 1, ,
n ni j i jh u
2 2
2
2 21 22
1
1
2
2h Rx K
Solution
h h Rh h x L x xL K
hxhhLKR
Analysis of Underground Dam: Groundwater Model
1D steady state, unconfined flow with recharge:
The hydraulic head (m)The distance from the origin (left hand side),(m)The hydraulic head at the origin (m) The hydraulic head at L (m)
The Length between h1 and h2 (m)The hydraulic conductivity of the aquifer (m/day)Recharge Rate (m/day)
#
0 10 20 Kilometers
N2
1°1
0'
21°10
'
21°
15'
21°15
'
21°
20'
21°20
'
21°
25'
21°25
'
21°
30'
21°30
'
39°55'
39°55'
40°00'
40°00'
40°5'
40°5'
40°10'
40°10'
40°15'
40°15'
40°20'
40°20'
40°25'
40°25'
#
#
# #
#
#
J113J204J205
J216
J330
TA205
0 10 Kilometers
N
21°
10'
21°10
'
21°
15'
21°15
'
21°
20'
21°20
'
21°
25'
21°25
'
40°00'
40°00'
40°5'
40°5'
40°10'
40°10'
40°15'
40°15'
40°20'
40°20'
40°25'
40°25'
احداثيات المحطاترمز
المحطة
خط الطول
E
دائرة Fالعرض
N
40°07’00" 21°22’00" J113
40°12’00" 21°21’00" J204
40°13’00" 21°21’00" J205
40°07’00" 21°13’00" J216
40°01’00" 21°20’00" J330
40°16’12" 21°22’480" TA205
التغذية الجوفية (مم) 10%
المتوسط (مم) J113 J204 J205 J216 J330 TA205 السنه
30.0 30.0 299.8 247.9 227.0 335.0 389.1 1966
53.2 23.3 232.6 239.8 186.6 297.6 229.4 103.6 338.8 1967
87.2 34.0 339.7 353.4 391.2 338.0 429.6 95.1 430.8 1968
130.8 43.6 436.1 469.7 474.8 492.4 340.8 307.4 531.6 1969
153.6 22.8 227.8 238.0 248.6 226.2 155.2 187.2 311.8 1970
174.2 20.6 205.8 164.5 250.6 283.6 71.0 87.6 377.3 1971
201.7 27.6 275.6 305.1 284.3 298.2 270.0 201.2 294.7 1972
220.0 18.2 182.1 203.2 195.8 180.6 203.4 119.0 190.8 1973
235.2 15.3 152.9 187.0 209.4 188.4 150.5 25.5 156.5 1974
268.9 33.6 336.1 297.1 441.0 403.8 260.6 201.8 412.4 1975
288.9 20.1 200.8 273.4 152.4 237.6 262.4 76.8 202.2 1976
297.1 8.2 81.6 75.5 42.8 118.0 131.0 42.0 80.2 1977
1
1 m
ii
P Pm
050
100150200250300350400450500
1960 1970 1980 1990 2000 2010YEAR
dept
h (m
m)
Average
10%
0
100
200
300
400
500
600
700
1960 1970 1980 1990 2000 2010YEAR
Cum
mul
ativ
e de
pth
(mm
) Cummulative
Linear (Cummulative)
wadiwadi
a
AR RA
م 3000 طول الرسوبياتم 800 عرض الرسوبيات 0.4 )ε المسامية (
م 45 متوسط سمك الرسوبيات 20% α 80% β
م 9 .5 Δ H
مليمتر/ سنه 182متوسط التغذية الجوفية
في السنةكيلو متر
مربع 2.4 مساحة الرسوبياتمتر مكعب /
سنه 43727.6حجم التغذية الجوفية في
السنةمتر مكعب /
يوم 119.8 معدل التغذية اليومي
متر مكعب 43200000التخزين الممكن في
الرسوبياتمتر مكعب 15,552,000 التخزين المتوفر
Stability of Penetration Part of Cutoff Wall
2/31 (1 )'
2 ( 1) .3
SVV
nJustin s formula
V G d g
Actual flow velocityApparent flow velocityArea porosityPorositySpecific gravity of soil particlesSoil particle size (cm)Gravitational acceleration (cm/s2)
SVV
nGdg
a) Stability against seepage failure:
Stability of Penetration Part of Cutoff Wall
Stable unStable
S C
S C
V VV V
Critical velocity, set by the designer according to the soil type CV
Stability of Penetration Part of Cutoff Wall
' ,1
1.5
2 ,
c
w w
w w
ViK
Sichart s formula
iK
hid t
given t calculate d
b) Stability against hydraulic gradient:
Seepage Analysis
Flow lines
Equipotential linesImpervious
Sheet pile wall
DatumA
H
F
G
E
B
DC
H1
H2
Saturated soil
Boundary conditions:
AB: hw = H1
BC and DE: qw = 0
EF: hw = H2
Ah and FG: qw = 0
HG: qw = 0
x2 ¶ ¶2 h +
y2 ¶ ¶2 h =0
Seepage Analysis (cont.)
Spreadsheet Model for Seepage Analysis
10 9 8 7 6 5 4 3 2 1 i=
1 1 1 1 1 0.001 1 1 1 1 j=
1 1 1 1 1 1 0.001 1 1 1 1 1 1
1 1 1 1 1 1 0.001 1 1 1 1 1 2
1 1 1 1 1 1 0.001 1 1 1 1 1 3
1 1 1 1 1 1 0.001 1 1 1 1 1 4
1 1 1 1 1 1 0.001 1 1 1 1 1 5
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.001 0.00001 0.00001 0.00001 0.00001 0.00001 6
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.001 0.00001 0.00001 0.00001 0.00001 0.00001 7
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 8
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 9
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 10
0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001 0.00001
Spreadsheet Model for Seepage Analysis (cont.)
12345678910
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
0.8-1
0.6-0.80.4-0.60.2-0.40-0.2
Spreadsheet Model for Seepage Analysis (cont.)
12345678910
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
0.8-1
0.6-0.8
0.4-0.6
0.2-0.4
0-0.2
Dam HeightThe dam height is determined by the necessary reservoir capacity and
the necessity of drainage facility installation.
Free BoardFree board is chosen to prevent groundwater level rise in the reservoir area
Underground Dam under Construction (Brazil)