Seepage phenomenon analysis below a hydraulic structure using Ansys

1. Finite Element Method
Homework III
Seepage phenomenon below a hydraulic structure
The main tasks are to draw out
conclusions about the flow net aspect (flow
lines and equipotential lines) in both cases, to
calculate the uplift pressures acting on the
bottom of the structure and to assess the
discharge dropdown by using the grout curtain.
According to the Fourier law, the heat flux q = - [K] grad T, analogous with the Darcy law. So for this
exercise as 2D solid element should be used: element PLANE55 (or 55) in the ANSYS elements library (the so
called 2D Thermal Solid). This is a quadrilateral element (4 nodes by default), with only one degree of
freedom per node, i.e. the nodal temperature (which in our exercise will mean the hydraulic head). The
material property that characterizes the seepage phenomenon is the permeability (thermal conductivity in
the thermal field problem).
Building the model
For this exercise another 2D solid element should be used: element PLANE55 (or 55) in the ANSYS
elements library (the so called 2D Thermal Solid). This is a quadrilateral element (4 nodes by default), with
only one degree of freedom per node, i.e. the nodal temperature (which in our exercise will mean the
hydraulic head).
The model meshing displayed on the next page will be used. Also the Ansys model along with nodes
numbering will be annexed to this document.
N 5m H2 1m 0.1N 1.5m
L 22m N 27m Hv 4m 0.2N 5m
H 10m 0.5N 12.5m b 2m
H1 3m 0.1N 3.5m B 7m 0.2N 8m
Paul Ionescu
Gr. 1 N=5

2. The analysis in absence of the
grout curtain will be done using this
code:
/PREP7
ET,1,55
MP,KXX,1,1E-4
N,1,0,0
N,2,4,0
N,3,7,0
N,4,8.5,0
N,5,9.5,0
N,6,11.5,0
N,7,12.5,0
N,8,13.5,0
N,9,15,0
N,10,16.5,0
N,11,17.5,0
N,12,18.5,0
N,13,20,0
N,14,23,0
N,15,27,0
NGEN,2,15,1,15,1,0,2.5
NGEN,3,15,16,30,1,0,1.5
NGEN,3,15,46,60,1,0,1
NGEN,3,15,76,90,1,0,1.5
NGEN,3,15,106,120,1,0,1
TYPE,1
MAT,1
E,1,2,17,16
EGEN,14,1,1
EGEN,9,15,1,14
NSEL,S,NODE,,136,140,1
D,ALL,TEMP,16
NALL
NSEL,S,NODE,,145,150,1
D,ALL,TEMP,14
NALL
NSEL,S,LOC,X,0
NSEL,A,LOC,Y,0
NSEL,A,LOC,X,27
NSEL,A,NODE,,140,145,1
SAVE
/SOLU
SOLVE
FINISH
To simulate the presence of the grout courtain we will declare a new material also (MP,KXX,2,1E-10) and we
change material for the elements forming the curtain using this code :
The final code:
/PREP7
ET,1,55
MP,KXX,1,1E-4
MP,KXX,2,1E-10
N,1,0,0
N,2,4,0
N,3,7,0
N,4,8.5,0
N,5,9.5,0
N,6,11.5,0
N,7,12.5,0
N,8,13.5,0
N,9,15,0
N,10,16.5,0
N,11,17.5,0
N,12,18.5,0
N,13,20,0
N,14,23,0
N,15,27,0
NGEN,2,15,1,15,1,0,2.5
NGEN,3,15,16,30,1,0,1.5
NGEN,3,15,46,60,1,0,1
NGEN,3,15,76,90,1,0,1.5
NGEN,3,15,106,120,1,0,1
TYPE,1
MAT,1
E,1,2,17,16
EGEN,14,1,1
EGEN,9,15,1,14
NSEL,S,NODE,,136,140,1
D,ALL,TEMP,16
NALL
NSEL,S,NODE,,145,150,1
D,ALL,TEMP,14
NALL
NSEL,S,LOC,X,0
NSEL,A,LOC,Y,0
NSEL,A,LOC,X,27
NSEL,A,NODE,,140,145,1
ESEL,S,ELEM,,201,243,14
MPCHG,2,ALL
EALL
ESEL,S,ELEM,,201,243,14
MPCHG,2,ALL
!All dimensions are in meters

3. Post processing
-Equipotential lines /SHOW,,,1 PLNS,temp
Equipotential lines – model without grout curtain
Equipotential lines – model without grout curtain

4. -Seepage velocity PLVECT,tf,sum
Velocity vectors - flow lines – model without grout curtain
Velocity vectors - flow lines – model with grout curtain

5. 0
0.5
1
1.5
2
2.5
3
3.5
4
9 11 13 15 17 19
Pressure(m)
Nodal x coordinates (m)
Hypothesis 1 - without grout curtain
0
1
2
3
4
9 11 13 15 17 19
Pressure(m)
Nodal x coordinates (m)
Hypothesis 2 - with grout curtain
0
0.5
1
1.5
2
2.5
3
3.5
4
9 10 11 12 13 14 15 16 17 18
Pressure(m)
Nodal x coordinates (m)
with vs without grout curtain
With
Without
-Hydraulic head values
An uplift pressure diagram will be drawn in both hypotheses of the exercise, in order to emphasize the grout curtain’s
effect. Finally, the discharge dropdown due to the grout curtain will be assessed by calculating the discharge in both hypotheses. The
inflow line is defined by the same nodes used to apply the upstream hydraulic head H1.
Using the PRNS or PRES commands with label = tf (thermal flux), after selecting the appropriate nodes, prints the list of
seepage velocities.
Uplift pressure diagrams
nsel,s,node,,140,146,1
prns,temp
NODE TEMP
140 16.000
141 15.328
142 15.175
143 15.006
144 14.748
145 14.439
146 14.000
NODE TEMP
140 16.000
141 14.488
142 14.479
143 14.450
144 14.370
145 14.233
146 14.000

6. Discharge assessment
Q1
0.60866 0.72900
2
4
0.72900 1.1205
2
3
1.1205 1.6688
2
1.5
1.6688 3.1759
2
1





10
5
 9.964 10
5

m
3
s
Q2
0.68113 0.77982
2
4
0.77982 0.95151
2
3
0.95151 1.0159
2
1.5
1.0159 1.0297
2
1





10
5
 8.017 10
5

m
3
s
nsel,s,node,,136,140,1
prns,tf
NODE TFX TFY TFZ TFSUM
136 -0.17973E-19 -0.60866E-05 0.0000 0.60866E-05
137 0.56778E-20 -0.72900E-05 0.0000 0.72900E-05
138 0.14452E-19 -0.11205E-04 0.0000 0.11205E-04
139 0.15818E-18 -0.16688E-04 0.0000 0.16688E-04
140 0.16802E-04 -0.26950E-04 0.0000 0.31759E-04
NODE TFX TFY TFZ TFSUM
136 0.12176E-19 -0.68113E-05 0.0000 0.68113E-05
137 0.40949E-19 -0.77982E-05 0.0000 0.77982E-05
138 0.75902E-19 -0.95151E-05 0.0000 0.95151E-05
139 0.29362E-19 -0.10159E-04 0.0000 0.10159E-04
140 -0.23360E-19 -0.10297E-04 0.0000 0.10297E-04
Without grout curtain
With grout curtain