1512
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping core. The foundation is formed by
alternation of volcanic and inter-stratified sedimentary rocks.
Figure 3. Central cross-section of Almagrera dam before closure.
The dam corresponds to the downstream borrow material
type, and was raised 5 times adding material into the
downstream side. The slide shows the original dam axis and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
The downstream slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) for the last two phases. According to a report
delivered before the 5th phase construction, the filter criteria
were not fulfilled. This way, after the 4
th
phase, the dam rather
behaved as a homogeneous dam. During the 5
th
phase, an
inclined sand and gravel filter was placed between the 4
th
and
5
th
phase shells using normalised filter criteria. A downstream
foot drain was placed below the inclined filter and the
downstream shell, protected by a non-woven geotextile. Leaks
ranging from 1 up to 16 m
3
/h appeared in the downstream slope.
4.1
Simulation of closure operations
A mechanical model for all the operations involved during
closure was entrusted to the main author. A finite element
model was set up to reproduce all the steps that are being taken
during closure:
1. Finding the initial safety factor.
2. Upstream water drawdown produced by pumping the
contaminated reservoir water.
3. Upstream filling with coarse waste material.
4. Long term stability. Calculation of tailings settlements.
5. Seismic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
reinforcement from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have been carried out with 15-node elements. A Mohr-Coulomb
materials model has been used; this is a model of perfect, non-
associated plasticity
Table 3 shows the calculation parameters.
4.2
Tailings
Figure 4 shows the tailings thickness inside the reservoir and
the thickness of Las Viñas material that will be placed above.
The hatched area is the area that should be treated with band-
shaped drains. Band-shaped drains should be always placed
when the tailings thickness is ≥15 m.
Table 3. Calculation parameters
Soil
type
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Core
SC
18
30º 19.8
10
-8
50
Filter
SP-SM 1
35º 20
10
-5
50
Quarry run
GC
6
33º 20.2
6.5*10
-5
30
Rockfill
15
31º 21.9
9.5*10
-7
60
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
Rock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
ML
1
29º 13.2
5.1*10
-9
0.52
Medium tailings ML
1
32º 19.7
5.1*10
-9
1.0
Las Viñas
Material
1
30º 20
1.2*10
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas material that will be placed above.
Table 4 shows the maxima settlements in tailings obtained
using several hypotheses.
Table 4. Maxima settlements in tailings.
Remainder
settlement
after construction
(m)
Test
Tailings
thickness
(m)
Fill
thickness
(m)
Maximum
settlement
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculations
According to the inventory of tailings dams and ponds in
extractive industry in Spain (ITC 2000) it is clear that
Almagrera dam is class 1 (height greater than 15 m) but only
category C (moderate damage only incidentally affecting lives).
So, the first row of the safety factors in Table 2 must be
accomplished.
Compacted rockfill reinforcement was projected to fulfil
with the safety factors specified by the Spanish Regulations
(Figure 5).
Figure 5. Reinforcement needed to fulfil the safety factors of Table 2.
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
or
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping core. The foundation is formed by
alternation of volcanic and inter-stratified sedime tary rocks.
Figure 3. Central cross-section of Almagrera am before closure.
The dam corresponds to the downstream borrow material
type, and was raised 5 times adding material into the
downstream side. The slide show the original dam axis and th
states of the dam at the end f 3rd, 4th and 5th heightenings.
The downstream slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) for the last two phases. According to a report
delivered before the 5th phase construction, the filter criteria
were not fulfilled. This way, after the 4
th
phase, the dam rather
behaved as a homogeneous dam. During the 5
th
phase, an
incline sand and gravel filter was placed between the 4
th
and
5
th
phase shells using normalised filter criteria. A downstream
foot drain was placed below the inclined filter and the
downstream shell, protected by a non-woven geotextile. Leaks
ranging from 1 up to 16 m
3
/h appeared in the downstream slope.
4.1
Simulation of closure operations
A mechanical model for all the operations involved during
closure was entrusted to the main auth r. A finit element
model was set up to repr duce all the steps that are being taken
during closure:
1. Finding the initial safety factor.
2. Upstream water drawdown produced by pumping the
contaminated reservoir water.
3. Upstream filling with coarse waste material.
4. Long term stability. Calculation of tailings settlements.
5. Seismic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
reinforcement from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have been carried out with 15-nod elements. A Mohr-Coulomb
materials model has been used; this is a model of perfect, non-
associated plasticity
Table 3 shows the calculation parameters.
4.2
Tailings
Figure 4 shows the tailings thickness inside the reservoir and
the thickness of Las Viñas material that will be placed above.
The hatch d area is the area that should be treated with and-
shaped drains. Band-shaped drains should be always placed
when the t ilings thickness is ≥15 m.
Table 3. Calculation parameters
Soil
type
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Cor
SC
18
30º 19.8
10
-8
50
Filter
SP-SM 1
5 20
0
-5
Quarry run
GC
6
3
0.2
6.5*10
-5
30
Rockfill
15
31 21.9
9. *1
-7
6
Selected rockfill
1
35 20
5.1*
-3
60
Weathered rock
50
20º 20.5
1.4* 0
-6
300
Rock
250 20
1.4
1.3 0
-
1
4
S ft tailings
ML
1
9º 13.2
5.1
-9
0.52
Medium tailings M
32 19.7
.1 10
-9
1.0
Las Viñas
Material
0º 20
1.2 0
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas material that will be placed above.
Table 4 shows the maxima settlements in tailings obtained
using several hypoth ses.
Table 4. Maxima settlements in tailings.
Remainder
settlement
after construction
(m)
est
Tailings
thickness
(m)
Fill
thickness
(m)
Maximum
settle ent
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
.19
5
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculations
According to the inventory of tailings dams and ponds in
extractive industry in Spain (ITC 2000) it is clear that
Almagrera dam is class 1 (height greater than 15 m) but only
catego y C (moderate damage only incid ntally affecting lives).
So, the first row of the safety factors in Table 2 must be
accomplished.
Compacted rockfill reinforcement was projected to fulfil
with the saf ty factors specified by the Spanish Regulations
(Figure 5).
Figure 5. Reinforcement needed to fulfil the safety factors of Table 2.
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping core. The foundation is formed by
alternation of volcanic and inter-stratified sedimentary rocks.
Figure 3. Central cross-section of Almagrera dam before closure.
The dam corresponds to the downstream borrow material
type, and was raised 5 times adding material into the
downstream side. The slide shows the original dam axis and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
The downstream slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) for the last two phases. According to a report
delivered before the 5th phase co str ction, the filt r criteria
were not fulfilled. This way, after t 4
th
phase, the dam r ther
behaved as a homogeneous dam. During the 5
th
phase, an
inclined sand and gravel filter was placed between the 4
th
and
5
th
phase shells using normalised filter riteria. A downstream
foot drain was placed below the inclined filter and the
down
shell, prote ted by a non-woven geotextile. Leaks
rangin fro 1 up to 16 m
3
/h appeared in the downstream slope.
4.1
Simulation of closure operations
A mechanical model for all the operations involved during
closure was entrusted to the main author. A finite element
model was set up to reproduce all the steps that are being taken
during closure:
1. Finding the initial safety factor.
2. Upstream water drawdown produced by pumping the
contaminated reservoir water.
3. Upstream filling with coarse waste material.
4. Long term stability. Calculation of tailings settlements.
5. Seismic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
reinforcement from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have been carried out with 15-node elements. A Mohr-Coulomb
materials model has been used; this is a model of perfect, non-
associated plasticity
Table 3 shows the calculation parameters.
shaped drains. Band-shaped drains should be always placed
when the tailings thickness is ≥15 m.
Table 3. Calculation parameters
Soil
typ
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Cor
SC
18
30º 19.8
10
-8
50
Filter
SP-SM 1
35º 20
10
-5
50
Qu rry run
GC
6
33º 20.2
6.5*10
-5
30
Rockfill
15
31º 21.9
9.5*10
-7
60
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
Rock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
ML
1
29º 13.2
5.1*10
-9
0.52
Medium tailings ML
1
32º 19.7
5.1*10
-9
1.0
Las Viñas
Material
1
30º 20
1.2*10
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas material that will be placed above.
Table 4 shows the maxima settlements in tailings obtained
using several hypotheses.
Table 4. Maxima settlements in tailings.
Remainder
settlement
after construction
(m)
T st
Tailings
thickness
(m)
Fill
thickness
(m)
Maximum
settlement
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oed meter
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculations
According to the inventory of tailings dams and ponds in
extractive industry in Spain (ITC 2000) it is clear that
Almagrera dam is class 1 (height greater than 15 m) but only
category C (moderate damage only incidentally affecting lives).
So, the first row of the safety factors in Table 2 must be
accomplished.
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping core. The foundation is formed by
alternation of volcanic and inter-stratified sedimentary rocks.
Figure 3. Central cross-section of Almagrera dam before closure.
The dam corresponds to the downstream borrow material
type, and was raised 5 times adding material into the
downstream side. The slide shows the original dam axis and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
The downstre m slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) for the last two phases. According to a report
delivered before the 5th phase construction, the filter criteria
were not fulfilled. This way, after the 4
th
phase, the dam rather
behaved as a homogeneous dam. During the 5
th
phase, an
inclined sand and gravel filter was pla ed between the 4
th
and
5
th
phase shells using normalis d filter criteria. A downstream
foot drain was placed below the inclined filter and the
downstream shell, protected by a non-woven g otextile. Leaks
ranging from 1 up to 16 m
3
/h appeared in the downstream slope.
4.1
Si ulation of closure operations
A mechanical mod l for all the operations involved during
closure was entrusted to the main author. A finite lement
model was set up to reproduce all the steps that are being t k n
during closure:
1. Finding the initial safety factor.
2. Upstream water drawdown produced by pumping the
contamin ted reservoir water.
3. Upstre m filling with coarse aste material.
4. Long term stability. Calculation of tailings settlements.
5. Seismic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
reinforcement from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have be n carried out with 15-node elements. A Mohr-Coulomb
materials model has been used; this is a model of perfect, non-
associated plasticity
Table 3 shows the calculation parameters.
4.2
Tailings
Figure 4 shows the tailings thickness inside the reservoir and
the thickness of Las Viñas material that will be placed above.
The hatched area is the area that should be treated with band-
shaped drains. Band-shaped drains should be always placed
when the tailings thickness is ≥15 m.
Table 3. Calculation parameters
Soil
type
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Core
SC
18
30º 19.8
10
-8
50
Filter
SP- M 1
35º 20
10
-5
50
Quarry run
G
6
33º 20.2
6.5*10
-5
30
Rockfill
15
31º 21.9
9.5*10
-7
60
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
Rock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
ML
1
29º
3.2
5.1*10
-9
0.52
Medium tailings ML
1
32º 19.7
5.1*10
-9
1.
Las Viñas
Material
1
30º 20
1.2*10
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas material that will be placed above.
Table 4 shows the maxima settlements in tailings obtained
using several hypotheses.
Table 4. Maxima settlements in tailings.
Remainder
settlement
after construction
(m)
Test
Tailings
thickness
(m)
Fill
thickness
(m)
Maximum
settlement
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculations
According to the inventory of tailings dams and ponds in
extractive industry in Spain (ITC 2000) it is clear that
Almagrera dam is class 1 (height greater than 15 m) but only
category C (moderate damage only incidentally affecting lives).
So, the first row of the safety factors in Table 2 must be
accomplished.
Compacted rockfill reinf rcement was projected to fulfil
with the safety factors specified by the Spanish Regulations
(Figure 5).
Figure 5. Reinforcement needed to fulfil the safety factors of Table 2.
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class
ategory
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4
MAG ERA TAI I GS
l agrera tailings da has a height of 35 above foundation
at axis. The height above the lo est foundation level is 37.3
(Figure 3).
It has an upstrea sloping core. he foundation is for ed by
alternation of volcanic and inter-stratified sedi entary rocks.
Figure 3. Central cross-section of Al agrera dam before closure.
The dam corresponds to the downs re m borrow material
type, and was raised 5 times adding material into the
downstream side. The slide shows the original dam axis and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
he downstream slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) f r the last two phas s. According to a report
delive ed before the 5th phase constructi n, the filt r criteria
were not fulfilled. This way, after the 4
th
phase, th dam rath r
behaved a a homogeneo s d m. uring the 5
th
phase, a
incl e sand and gravel filter was placed between the 4
th
and
5
th
pha e shells using normalised filter criteria. A do nstream
foot drain was placed below the in lined filter and t e
downstream sh ll, protected by a non-woven geotextile. Leaks
ranging from 1 up to 16 m
3
/h ppear d in the downstr am slope.
4.1
S mulation of closure operations
A m chanical model for all the operations involv d during
closure was entrusted to the main author. A finite element
model was set up to reproduce all the steps that are being taken
during closure:
1. Finding the initial safety factor.
2. Upstrea water drawdown produced by pumping the
contaminated reservoir ater.
3. Upstream filling with coarse waste material.
4. Long term stability. Calculation of tailings settle ents.
5. Seis ic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
reinforce ent from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have been carried out with 15-node elements. A Mohr-Coulo b
aterials odel has been used; this is a model of perfect, non-
associated plasticity
Table 3 sho s the calculation para eters.
shaped drains. and-shaped drains should be always placed
hen the tailings thickness is ≥15 m.
Table 3. Calculation parameters
Soil
type
US S
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Core
SC
18
30º 19.8
10
-8
50
Filter
SP-SM 1
35º 20
10
-5
50
Qua ry run
GC
6
33º 20.2
6.5*10
-5
30
ockfill
15
31º 21.9
9.5*10
-7
60
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
ock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
L
1
29º 13.2
5.1*10
-9
0.52
Medium tailings
L
1
32º 19.7
5.1*10
-9
1.0
Las Viñas
aterial
1
30º 20
1.2*10
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
iñas material that will be placed above.
Table 4 shows the axima settlements in tailings obtained
using several hypotheses.
Table 4. Maxi a settle ents in tailings.
Re ainder
settle ent
after construction
(m)
Test
Tailings
thickness
(m)
Fill
thickness
(m)
Maxi u
settle ent
(m)
o
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
a calculations
ccording to the inventory of tailings da s and ponds in
extractive industry in Spain (I C 2000) it is clear that
Almagrera da is class 1 (height greater than 15 m) but only
category ( oderate da age only incidentally affecting lives).
So, the first ro of the safety factors in able 2 ust be
acco plished.
l . f t f t r r ir f r t ili
i
i l i l ti .
ti
l
t
r
r l
i
t l
tr
1
r
.
.
.
r
.
.
.
r
.
.
.
l
t ili
i t
ti
at i .
i t
t l
t
ti l
l i .
(Figure 3).
t
tre l i
.
ti i
lt r ti l i
i t
t ti i
i
tary rocks.
Figure 3. Central cross-section of Almagrera dam before closure.
sponds to the downstream
teri l
t
,
i ti
i
t i l i t t
t
i .
li o t i i l
i
t e
t t t
t t
, t
t i t i
.
t
l
.
:
t t t i
,
:
t l t two ph .
i t
t
li e bef t t
t ti ,
ilt r it i
t l ill . i
, t t 4
th
, t
the
behave as a homogeneous dam. During the 5
th
ph se, an
i li
l ilt
l
t
the 4
t
th
ll i
li ilt it i .
t
foot drain was l
l t i li
ilt
t
dow t
ll,
t t
eotextile. Leaks
in m to 16 m
3
/h p ared in the downstream slope.
.
i l ti f l
ti
A mechani l
l
ll t
ti
i
l
i
closure
t t t t
i t . i it l
t
l
t t
o ll t t t t
i t
i l
:
. i i t i iti l
t t .
.
st
t
i t
t i t
i t .
.
illi it
t t i l.
.
t
t ilit . l l ti t ili
ttl
t .
.
i i
l l ti , i l i
i
ti
t ili
li
ti .
.
, i i t
t
t it t
i
t
t .
l i
.
d and the calculations
a ried out with 15-n de ele nts. A Mohr-Coulomb
t i l
l h
; t i i model of perfect, non-
i t l ti it
l
t l l ti
t .
.
ili
i
t t ili
t i
i i t
ervoir a
t e t i
i
t i l t at will be placed above.
t e
i t
t t
l t t it
i .
i
l l
l
t taili t i
i
.
l . l l ti r t r
il
t
'
’
/
3
/
re
º
.
-8
ilt r
-
3 º
1
-5
50
Quarry r
3 º 2 .
.
-5
fill
º
.
.5 0
-7
0
l t r fill
º
. 10
-3
t r r
º
.
. 1
-6
º 2 .
. *1
-6
0
4
ft t ili
2 º
.
. *10
-9
.
i t ili
1
º
.
5. *
-9
.
i
t ri l
1
3 º
. *1
-4
Figure 4. Tailings thickness inside the reservoir
t t i
f
iñas m t ri l t t ill l
.
Table 4 shows the maxima settlements in tailings obtained
i
l
t
.
l .
i ttl
t in tailings.
i
r
settl
t
after construction
(m)
t
ili
t i
(m)
ill
t i
(m)
i
ttl
t
(m)
No
r i
Drains
25
5.5
3.3
2.5
0.42
Oedometer
0
7
.
.4
0.19
2
5.
.
.
.
i
e
ft t ili
.
.
.
.
.
.
.
i
i
t ili
.
.
.
.
l l ti
According to th inventory f tailings d ms an
i
e t ti i
t i
i
it i l t t
Almagrera
i l 1 ( i t
t t
t l
t
t
l i i e t lly
ti li
.
, t i t
t
t
tors in Table 2 must
accomplished.
t
ill i
t
j t t l il
it t
et
t
i i t
i
l ti
i
.
i r . i f r
t
t f lfil t f t f t r f
l .
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping c r . T foundation s formed by
alternation of volcanic and inter-stratified sedimentary rocks.
Figure 3. Central cross-section of Almagrera dam before closure.
The dam corresponds to the downstream borrow material
type, and was raised 5 times adding material into the
downstream side. The slide shows the original dam axis and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
The downstream slope was 1.7 (H): 1 (V) up to the third phase,
and 2 (H): 1 (V) for the last two phases. According to a report
delivered before the 5th phase construction, the filter criteria
were not fulfilled. This way, after the 4
th
phase, the dam rather
behaved as a homogeneous dam. During the 5
th
phase, an
inclined sand and gravel filter was placed between the 4
th
and
5
th
phase shells using normalised filter criteria. A downstream
foot drain was placed below the inclined filter and the
downstrea shell, protecte by non-woven geotextile. Le ks
ranging from 1 up to 16 m
3
/h appeared in the downstream slope.
4.1
Simulation of closure perations
A mechanical model for all the operations involved during
closure was entrusted to the main author. A finite element
model was et up to reproduce all the steps that are being taken
during closure:
1. Find ng the initial afety factor.
2. Upstream water drawdown produced by pumping the
contaminat reservoir water.
3. Upstream filling with coarse waste material.
4. Long term stability. Calculation of tailings settlements.
5. Seismic calculation, including consideration for tailings
liquefaction.
6. If necessary, finding the new safety factors with the
shaped drains. Band-shaped drains should be always placed
when the tailings thickness is ≥15 m.
Table 3. Calculation parameters
oil
type
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
C re
SC
18
3 º 19.8
0
-8
5
Filter
SP-SM
35º 20
10
-5
50
Quarry run
GC
6
3º 20.2
6.5
-5
30
Rockfill
5
1º
1.9
9.5
-7
6
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
Rock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
ML
1
29º 13.2
5.1*10
-9
0.52
Medium tailings ML
1
32º 19.7
5.1*10
-9
1.0
Las Viñas
Material
1
30º 20
1.2*10
-4
10
Figure 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas material that will be placed above.
Table 4 shows the maxim settlements in tailings obt ined
using sev l hypotheses.
Table 4. Maxima settlements in tailings.
Remainder
settlement
after construction
(m)
Test
Tailings
thickness
(m)
Fill
thick ess
(m)
Maximum
settlement
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.3
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculations
Table 2. Safety factors required for tailings dams in Spanish legislation.
Actions
Class Category
Normal Accidental Extreme
1
A or B
1.4
1.3
1.2
2
C or D
1.3
1.2
1.1
3
C or D
1.2
1.1
1.0
4 ALMAGRERA TAILINGS DAM
Almagrera tailings dam has a height of 35 m above foundation
at axis. The height above the lowest foundation level is 37.3 m
(Figure 3).
It has an upstream sloping core. The oundation is formed by
alternation of volcanic and inter-stratified sedimentary rocks
Figure 3. Central cross-secti n of Almagrera dam before closure.
Th dam corresponds to he downstream borrow material
type, and was raised 5 times adding m terial into the
down tream side. The slide shows the riginal dam ax and the
states of the dam at the end of 3rd, 4th and 5th heightenings.
The dow stream slope was 1.7 (H): 1 (V) up to the third phase,
nd 2 (H): 1 (V) for the last two phases. Accor ing to a report
delivered before the 5th phase constructio , th filte criteria
wer not fulfilled. This y, after the 4
th
phase,
dam rather
behaved s a homogeneous dam. During the 5
th
phase, an
inc n d sand and gravel filter was placed betwe n the 4
th
and
5
th
phase shells using normalised filt r criteria. A downst eam
foot drain was placed below the inclined filter and the
downstream shell, protected by a non-woven geot xtil . Leaks
ranging from 1 up to 16 m
3
/h appeared in the downstream lope.
4.1
Simulation of closure operati s
A mechanica model for all th operatio s involved during
closure was entrusted to the main author. A finite element
model was set up to reproduce all the steps that are being take
ur ng closure:
1. Finding the ini ial safe y factor.
2. Upstre m water drawdown prod ced by pumpi the
contaminated reservoir water.
3. Upstream filling with coarse waste material.
4. Long erm stability. Calculatio of tailings settlements.
5. Seismic calculati n, including consid ration for tailing
liquefaction.
6. If necessary, finding the new safety factors with the
reinforcement from step 3.
Plaxis 2D-9.02 program has been used and the calculations
have been arried out w th 15-node elements. A M hr-Coulomb
materials odel has been used; this is a model of perfect, non-
associ ted pl sticity
Ta l 3 shows the calculation paramet rs.
4.2
Tailings
Figure 4 shows the tailings thickness inside the reservoir and
the thick ess of Las Viñas material that will be placed above.
The hatched area is the area that should be treated with band-
shaped drains. Band-shaped drains should be always placed
when the tailings thickness is ≥15 m.
Tabl 3. Calculation parameters
Soil
type
USCS
c'
kPa
Φ’
γ
kN/m
3
k
m/s
E
MPa
Core
SC
18
30º 19.8
10
-8
50
Filter
SP-SM 1
35º 20
10
-5
50
Quarry run
GC
6
33º 20.2
6.5*10
-5
30
Rockfill
15
31º 21.9
9.5*10
-7
60
Selected rockfill
1
35º 20
5.1*10
-3
60
Weathered rock
50
20º 20.5
1.4*10
-6
300
Rock
250 20º 21.4
1.3*10
-6
10
4
Soft tailings
ML
1
29º 13.2
5.1*10
-9
0.52
M dium tail ngs ML
1
32º 19.7
5.1*10
-9
1.0
Las Viñas
Mat ri l
1
30º 20
1.2*10
-4
10
Figur 4. Tailings thickness inside the reservoir and the thickness of Las
Viñas m terial that will be placed above.
Table 4 shows the maxima settl ments in tailings obtained
using sev r ypothes s.
Table 4. Maxima settlem nts in tailings.
Remainder
settlement
after construction
(m)
Te t
Tailings
thickness
(m)
Fill
thickness
(m)
Maximum
settlement
(m)
No
drains
Drains
25
5.5
3.3
2.5
0.42
Oedometer
20
7
3.4
2.4
0.19
25
5.5
2.5
1.9
0.32
Piezocones
Soft tailings
20
7
2.6
2.6
0.15
25
5.5
1.3
1.0
0.16
Piezocones
Medium
tailings
20
7
1.3
0.9
0.07
4.3
Dam calculatio s
According to the inventory of tailings dams and ponds in
extractive ind stry in Spain (ITC 2000) it is clear that
Almagrera dam is class 1 (height greater than 15 m) but only
category C (moderate damage only incidentally affecting lives).
So, the first row of the safety factors in Table 2 must be
a complished.
Compacted rockfill r inforceme t was projected to fulfil
with t e safety fact rs speci ied by the Spanish Regulations
(Figure 5).
Figure 5. Rein orcemen needed to ulfil the saf ty factors of Table 2.