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)