Study on Seepage Behavior and Control of Longtouqiao Earth Dam

Study on Seepage Behavior and Control of Longtouqiao Earth Dam
CHENG Shengguo ZHANG Zhenhua
Geological Hazards Mitigation Institute, Three Gorges University, P.R.China, 443002
[email protected]
Abstract seepage behavior and control is the focus of earth dam, and is one of major factor of
accidents. In the paper, based on seepage of homogeneous earth dam of Longtouqiao, engineering
geology prosperities of dam site area, physical and mechanical prosperities of material filling dam , dam
operation and management status were studied. Program of seepage control was made according to state
criterion of earth dam design, and we adopted technique of blocking in foreside and draining on back.
The works of programs are analyzed and optimized by technique and economic and determined the
optimal programs. Effect reconstructed is rehabilitated to analysis the seepage control of dam, and the
result shows that the measure is significant. It can be applied and is very important to other project.
Key words Earth dam, seepage, control, programmer, finite element
1 Introduction
China is the most of the number of dam in the world. 85288 reservoirs have been built. It includes 445
large reservoirs, 2782 middle reservoirs and 82061 small reservoirs [1]. These reservoirs and dams had
played a vital role in national economic construction and disaster protecting. But most of the small
reservoir built in the 50-70 age and are low in engineering standards. Security issues are prominent. The
accident of dam-break occurred when annual flood season is beginning, and the seepage problems are
one of the main reasons [2] - [4]. In the paper, seepage behavior, engineering geology prosperities of dam
site area, physical and mechanical prosperities of material filling dam, dam operation and management
status is studied, and control programs were made according to state criterion of earth dam design,
combining to seepage of homogeneous earth dam of Longtouqiao. Program of seepage control was
made according to state criterion of earth dam design, and we adopted technique of blocking in foreside
and draining on back. The works of programs are analyzed and optimized by technique and economic
and determined the optimal programs. Effect reconstructed is rehabilitated to analysis the seepage
control of dam, and the result shows that the measure is significant. It can be applied and is very
important to other project.
2 Longtouqiao reservoir of the dam seepage
Longtouqiao Reservoir is in Qilichong county, Zhijiang City, Hubei Province and is a mainly to
irrigation, flood and farming. Dam is homogeneous. its length is 415.0 m, maximum height is 21.63 m
and filter dam height is 116.00m. Dam was found on Cretaceous sandstone and Q2 Netted silty clay
layer. Physical and mechanical meet the request of dam. But abutment base is not completely clear and
poor quality of filling, the leakage has been caused. Seepage safety assessment is C-class and need to
adopt necessary measures to control.
Dam site area bedrock of reservoir is sandstone, and its surface is strongly weathered rock, Physical and
mechanical property of dam Area rock samples is shown in Table 1.
Table 1 Rock Physical and Mechanical Parameters of Longtouqiao
Density Porosity
Compression elastic modulus
Water
Lithology
Gravity
KN/m3
(%)
absorption (%) strength Mpa
(104 Mpa)
（
）
（ ）
Strongly Weathering sandstone
2.0
21.9
8.80
1.65
3.15
3.5
Weak weathering sandstone
2.58
25.5
1.90
0.30
6.58
5.1
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Water-pressure test shows that permeability coefficient of dam soil is between 7.87 × 10-5 and 1.98 ×
10-4. field water content is 21.9~ 26.1%, dry density is 1.59~1.76g/cm3, natural void ratio is 0.719~
0.471, liquid limit is 32.0~48.8%, plastic limit is 18.7~22.0%, plasticity index is 13.2~27.3%, cohesion
is 35.4KPa, angle of internal friction is 15.70, compression coefficient is 0.29MPa, the permeability
coefficient is shown as Table 2 by water-pressure test and laboratory test.
Table 2 Permeability coefficient of soil and proposed value
proposed value
Water-pressure test
laboratory penetrate test
K cm/s
K cm/s
K cm/s
（ ）
1.63×10-4
5.81×10-4
2.90×10-4
Soil
Dam body
Silty clay
Anti-filter dam
（ ）
（ ）
3.62×10-5
7.55×10-5
×
2.78×10-4
2.90×10-4
1.15 10-4
3 Seepage control program and seepage calculation
3.1 Seepage control program of dam
Controlling programs of dam seepage are blocking in foreside and draining on back. Solutions are as
follows.
Program 1 is fracture grouting and drainage prism. Program 2 is plastic concrete impermeable wall and
drainage prism. Program 3 is jet grouting and drainage prism.
Table 3 amount of works and investment of three programs
amount of works
Items
Units
Program 1
Program 2
Program 3
Fracture grouting
Impermeable wall
Jet grouting
m
5643
2
m
90
9165
m
6280
3
Investment
10 Yuan
95.9
826.4
345.4
Comparison shows that the program 1 is the smallest investment, simple construction, small impact on
the structure of the dam and reliable impermeable.
3.2 calculation profile and operating mock
The largest section of dam was selected to be calculation profile. The section is as follow:
133.13
1:2.
50
.8
1:2
128.00
126.00
1:3
1:3.5
119.00
1:4
dam body
122.00
1:3.
25
QS4
116.00
0
1.
1:
1:2.
5
Strongly Weathering sandstone
middle weathering sandstone
Figure 1 impermeable reinforced the dam cross section map
According to rolling-earth dam design guidelines(SDJ218-96), operating mock water level combination
are as follow:
(1) Upstream normal water level and downstream corresponding minimum water level;
(2) Upstream design flood level and downstream corresponding level;
(3) Upstream check flood level and downstream corresponding level;
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(4) The most adverse operating mock of upstream slope while water level descended.
Where, design flood level is 131.68 m, check flood level is 132.20m, normal water level is 130.70m,
water level descent is from normal water level 130.70m to minimum water level 118.50m, and depth of
dam foundation is twice of dam height when seepage of dam is calculated.
3.3 Theory and calculation program
Seepage calculation of dam made use of plane finite element method. Since soil of earth dam is fully
consolidated, incompressible flow equation is used and infiltration coefficient is isotropic, stability
seepage function meets the following equation:
Initial border: h
t =0
∂
∂h
∂
∂h
(k ) + (k ) = 0
∂x ∂x ∂y ∂y
= h( x, y )
Head border: h Γ 1 = h( x, y, t )
Border traffic: k
(1)
(2)
(3)
∂h
= q (h, x, y, t )
∂h | Γ2
(4)
Where:
Γ1 and Γ 2 are the border of flow, q is flow function, h is head function, t is time, x and y is
coordinates axis.
3.4 calculation parameters and results
Dam was filled by Clay and a few gravel stones. natural water content of clay is 21.9~26.1%, natural
porosity is 0.47~0.72, dry density is 1.52 ~ 1.76g/cm3, optimum water content is 16.6%, the maximum
dry density is 1.77g/cm3, permeability coefficient is 1.63×10-4~3.62×10-5 cm/s. In the program 1,
seepage of dam was controlled by fracture grouting and anti-filter dam was rebuilt. The thickness of
fracture grouting mud wall is 0.3~0.8m, we can choice 0.5m when seepage of dam is calculated.
Permeability coefficient of mud wall is 1×10-6cm/s, Permeability coefficient of anti-filter dam is
1×10-2cm/s. The permeability coefficient of rock and clay of dam are shown in Table 2. The calculation
results are as follow:
Figure 3 seepage calculation result of design flood level
Figure 4 seepage calculation result of normal water level
838
Figure 5 seepage calculation result of check flood level
Table 4 percolation gradient and seepage quantity of profile
Level(m)
unit seepage quantity(m3/d.m)
operating mock
Before reinforcement
After reinforcement
upstream downstream
Design flood level
Check flood level
Normal water level
131.68
132.20
130.70
107.95
107.95
107.95
1.20
1.27
1.08
0.61
0.63
0.57
4 Conclusion
(1) Longtouqiao reservoir has run 40 years. The leakage of dam is very serious. Seepage safety
assessment is C, and need to be controlled by necessary measures.
(2) Programs of blocking in foreside and draining on back were made to control seepage. Fracture
grouting and drainage prism are Optimal program.
(3) After the dam was reinforced, seepage behavior improved remarkably. The unit seepage quantity is
reduced about 50% and meet the requirement of state criterion.
Acknowledgement
This research was supported by Scientific Research Fund of Hubei Provincial Education Department
(Project No. Q20081305).
References
[1] Li Lei, Lu Yunqiu, China's reservoir and dam safety management practices and facing challenges [J],
China's water conservancy, 2003, (11): A
[2] Niu Yunguang, dam safety and reinforcement[M], Beijing: China WaterPower Press, 1981
[3] Nu Naihua, Niu Yunguang, incident and security of dam[M], Beijing: China WaterPower Press, 2001
[4] Nu Naihua, Niu Yunguang, earth dam [M], Beijing: China WaterPower Press, 2001
[5] Zhao Jiacheng, Wang Congfeng, Jiang Lihan, study on reinforcement of medium and small reservoir
[J], Journal of China Three Gorges University, 2005, (04).
[6] Niu Yunguang, reinforcement technology of earth dam [J], dam and security, 1995, (3)
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