DISPLACEMENTS OF THE XINGÓ CONCRETE FACE ROCKFILL
DAM - BRAZIL
Ricardo José Barbosa de SOUZA1, Aurélio Alves de VASCONCELOS2, Alberto Jorge C CAVALCANTI3
Companhia Hidro Elétrica do São Francisco – CHESF
Rua Delmiro Gouveia 333, Bongi Recife PE CEP 50761-901 Brazil.
1, ricardob@chesf.gov.br
2, aurélio@chesf.gov.br
3, alberto@chesf.gov.br
Abstract: THE Xingó Hydroelectric Project is located on the São Francisco River, between the states of Alagoas
and Sergipe in the Northeast region of Brazil. The main dam is a concrete face rockfill embankment type
(CFRD), with maximum height of 150 m. The upstream slab thickness varies between 0.30 m and 0.70 m and it
was built with vertical contraction joints, 16 m apart, in two construction stages. The rockfill embankment was
entirely built with gneissic and granitic rock made available by excavation for the foundation of the concrete
structures. The performance of the dam during construction and operation was accompanied through monitoring
instruments installed in three main sections. The reservoir impound began June 10, 1994 and reached its
maximum normal water level on Nov. 15, 1994. To monitor the vertical and horizontal displacements of the
rockfill embankment three sections were chosen, representing the right abutment, the highest section in the
riverbed and the left abutment. Fifteen years after the reservoir impound The largest vertical displacement of the
superficial marks occurred in the central section of the dam, having reached at the crest about 615 mm.
Inspections detected a crack 20 m deep close to the plinth, a set of transversal fissures 26 m deep at the slab L6
with opening of up to 35 mm and crushing of the concrete in the upper portion at the joint L12/L13. This paper
presents the dam performance during construction, reservoir impounding and operation stages.
Key Words:
1
Instrumentation, rockfill, CFRD, Xingó
Introduction
The Xingó Hydroelectric Project is located in the Northeast region of Brazil, on the São Francisco
River, between the states of Alagoas and Sergipe. The power plant has an installed capacity of 3,162
MW and the installations allow for future expansion of up to 5.000 MW. The project belongs to
CHESF - Companhia Hidro Elétrica do São Francisco. Engineering works, including preliminary,
final and detailed design were carried out by Promon Engenharia Ltda.
The main dam is a concrete face rockfill embankment type (CFRD), with maximum height of 150
m, crest length of 850 m and rockfill volume of 12,900,000 m3. The upstream concrete slab has its
thickness varying between 0.30 m and 0.70 m, vertical contraction joints at each 16.0 m, and it was
executed in two stages, the first until el 70 m and second from el 70 m to el 138 m. The project
includes, on the right bank, four 16.0m-diameter unlined diversion tunnels, a bottom outlet
structure located in one of the tunnels – for the purpose of maintaining a minimum downstream
flow during the reservoir impounding –; a concrete gravity dam with ten water intakes; six 9.5mdiameter steel penstocks; a powerhouse with six 500 MW Francis units; a 500 kV switchyard; and
four saddle dikes in the reservoir area. A surface spillway, with two sloping chutes and with 33,000
m3/s capacity, is located on the left bank. A general view of the project is presented in Figure 1.
The regional geology includes a crystalline embasement, from the medium and lower preCambrian ages, consisting of a complex of igneous and metamorphic rocks. Intrusions of granitelike rocks occurred in the upper pre-Cambrian. Gneissic rock, with schist or granitic structure
varying from place to place, is the most dominant rock type. At the site, the river flows in a canyon,
and the lateral plateau is about 135m above the riverbed. The overburden at the abutments,
constituted by coluvial and residual soils is up to 4.0m thick. The thickness of the weathered rock
varies from place to place, achieving a maximum of 15.0m.
Figure 1. General view
The foundation of the dam is of gneissic rocks, with structure sometimes schistous, sometimes
granitic. The plinth is 0.5 m thick and variable width ranging from 4.0 m to 7.7 m and fixed in
unaltered rock through 32 mm anchors at each 1.7 m. The upstream concrete slab has its thickness
varying between 0.30 m and 0.70 m, vertical contraction joints at each 16.0 m, and it was executed
in two stages, the first until el 70 m and second from el 70 m to el 138 m. A general view of the
project is presented in Figure 1.
2
Rockfill features
The rockfill embankment was entirely built with gneissic and granitic rock made available by
excavation from the foundation of the concrete structures. The rockfill was in general founded on
rock, after removal of the small overburden layer, in both abutments. At the riverbed, some sand
deposits were left in place, but sand removal was carried out in the upstream and downstream regions
of the embankment foundation.
The construction of the dam was concluded in the first half of 1994 and the reservoir filling
occurred from 10/June/94 to 15/Nov/94. The features and the zoning of the main dam are presented
below and in Figure 2.
Zone I, beneath the upstream concrete slab, has a variable width, from 6.0 m at the lowest
foundation level to 4.0m at the crest. The width is doubled at the contact with both abutments. The
material for Zone I was obtained by grizzling, 4" bars spacing, weathered rock available from the
superficial portion of the rock excavations. The material is impermeable to semi permeable, with
35% to 60% in weight passing nº 4 sieve and 4% to 12% passing in the nº 200 sieve. It was
compacted in 0.4 m layers with 6 passes of a 6-ton vibratory roller.
Zone II, a transition for Zone I, has a constant width of 4.0m and is flared to 8.0m at the contact
with the foundation. The material for Zone II is selected fine rockfill with a maximum diameter of
0.4m. The material has 35% to 70% in weight passing the 1" sieve and 3% to 8% passing the nº
200 sieve. It was compacted in 0.4m thick layers, with 6 passes of a 10-ton vibratory roller.
Zone III corresponds to the upstream third and consisted of unaltered rockfill, compacted in 1.0m
thick layers, with 4 passes of a 10-ton vibratory roller, applying150 liters of water per cubic meter
of rockfill. The material presented up to 40% passing in the 25mm sieve and up to 3% passing in
the # 200 sieve.
Figure 2. Main section with rockfill zoning of the dam
Zone IV, corresponding to the downstream portion, consisted of unselected rockfill, compacted in
2.0m thick layers, with 4 roller passes, without addition of water. Weathered rock, available from
the upper portion of the spillway excavation, was used mainly in the left abutment and partially in
the riverbed portion of the embankment. The material presented from 15% to 60% passing in the
25 mm sieve and 2% to 7% passing in the # 200 sieve. Figure 2 shows the geometry of the dam and
the zoning of the rockfill.
3
Displacements monitoring design
To monitor the vertical and horizontal displacements of the rockfill embankment three sections were
chosen, representing the right abutment, the highest section in the riverbed and the left abutment,
corresponding to Stations 40+00, 49+00 and 62+00. The instrumentation of the upstream slab
concentrated on the observation of the differential displacements between the slab and the plinth, the
opening of the joints between slabs in the area of the abutments, and the observation of the
temperatures and stresses. After the reservoir impound an additional instrumentation was installed to
monitor the area where cracks were appeared in the slabs.
Settlements of the rockfill embankment are being monitored through settlement hydraulic cells, of
the Swedish box type. Additionally, two magnetic settlement gauges were installed on riverbed,
one in Zone III, other in Zone IV with plates at each 4 m and 6 m respectively. The horizontal
displacements are measured in the same positions of the settlement cells, through multiple rod
extensometers with KM horizontal stems. Superficially, the horizontal and vertical displacements
are being measured through topographical marks in the top of the slab and slopes including marks
in the instrumentation reading cabins. The global control of the seepage flow through the dam and
foundation is made by a triangular flow meter, MV01, installed in the top of the downstream
cofferdam. The Figures 3, 4 and 5 shows the instrumentation of the rockfill embankment in the
right bank, in the riverbed (highest section) and in the left bank.
Figure 3. Typical section and instrumentation plan on the right bank
Figure 4. Typical section and instrumentation plan on the riverbed
Figure 5. Typical section and instrumentation plan on the left bank
4
Monitoring instrumentation data
4.1
Construction Period
The settlements measured during the construction period, in the highest section of the dam, Station 49,
reached the values of 1700 mm in Zone III and 2900 mm in Zone IV. Considering such settlements
recorded by instrumentation and performing a retro-analysis, the following average deformability
moduli were obtained for the rockfill embankment, Souza et al [5]:
Zone III - E = 32 Mpa
Zone IV - E = 20 Mpa
The maximum horizontal displacement observed in section 49, at the end of the construction
period, was of 590 mm measured at the top and 530 mm measured inside the embankment.
The following occurrences were verified during the construction of the dam, regarding the behavior
of the rockfill embankment:

At the beginning of the construction 2nd stage of the slab, between el 70 m and el 135 m, voids
were observed, of the order of some centimeters, between the upstream slab and the rockfill
embankment of Zone I, at el 70 m, top of the 1st stage. Those voids were filled with mortar and
cement grout, before the construction of the slab, above el 70 m.

Some superficial cracks were observed in the exposed face of Zone I, before the construction of
the upstream slab second stage, between el 70 m and el 135 m, around el 115 m at the left
abutment. Cracks were filled with fine sand applied with water and the slope face was recomposed
with material from Zone I.
4.1
Reservoir impound and Operation Periods
The reservoir impound started on June 10, 1994, with water level at el 40 m, increasing quickly to el
120 m on June 16 and to el 134 m on July 13, where it remained stable until November. The filling
restarted on November 11 and the reservoir reached its maximum level at el 138 m on November 15,
1994 and since then it has been oscillating between 138 m and 137.5 m. To monitor the behavior
during the reservoir impound and operation periods, the displacements of the construction phase were
reset.
The displacements measurements of CFRD during the 15 years of operation, including the period
of filling the reservoir, are presented in Figures 6 to 11.
5
Measurements analysis
From the measurements analysis, fifteen years after the reservoir impound, we noted the following
observations and occurrences:

The largest vertical displacement of the superficial marks(surface benchmarks) occurred in the
central section of the dam, station 49 - Figure 7, having reached at the crest about 615 mm at
MS11 and 515 mm at the mark of the reading cabin CL-6 at El. 110 m;

The maximum vertical displacement measured inside the rockfill by settlement cells was 550 mm,
El. 110 m - CR18 - Station 49, and the maximum measured in Zone I was 520 mm, El. 85 m CR14, Figure 7, central section of the dam;

The maximum horizontal displacement was of 385 mm at MS22 at the crest, and internally it was
of 310 mm at KM6P1 in the central section of the CFRD, Figure 10;

During the first year of operation the behavior of the dam, in terms of displacements and seepage
flow, was perfectly normal, being observed a tendency of stabilization of the measurements;
cr19
cl08
cr20
ms16
cr21
ms19
cr22
ms24
Figure 8. STATION 62 – Vertical displacements
cr23
cl 06
cl07
14/09/09
14/09/09
18/03/09
19/09/08
23/03/08
25/09/07
29/03/07
cl02
cr09
18/03/09
19/09/08
23/03/08
25/09/07
cl 05
29/03/07
30/09/06
03/04/06
05/10/05
08/04/05
cl01
cr08
30/09/06
03/04/06
05/10/05
08/04/05
10/10/04
13/04/04
16/10/03
19/04/03
cr05
cr07
10/10/04
cl 04
cr 18
13/04/04
16/10/03
19/04/03
21/10/02
21/10/02
24/04/02
26/10/01
29/04/01
31/10/00
cr03
cr06
24/04/02
cr 14
cr 17
26/10/01
29/04/01
31/10/00
04/05/00
06/11/99
10/05/99
11/11/98
cr02
ms21
04/05/00
cr 11
cr 15
06/11/99
10/05/99
11/11/98
15/05/98
16/11/97
20/05/97
21/11/96
25/05/96
27/11/95
31/05/95
02/12/94
05/06/94
vertical displacement (mm)
cr01
ms07
15/05/98
16/11/97
cr 10
ms 22
20/05/97
21/11/96
25/05/96
27/11/95
31/05/95
02/12/94
05/06/94
vertical displacement (mm)
14/09/09
18/03/09
19/09/08
23/03/08
25/09/07
29/03/07
30/09/06
03/04/06
05/10/05
08/04/05
10/10/04
13/04/04
16/10/03
19/04/03
21/10/02
24/04/02
26/10/01
29/04/01
31/10/00
04/05/00
06/11/99
10/05/99
11/11/98
15/05/98
16/11/97
20/05/97
21/11/96
25/05/96
27/11/95
31/05/95
02/12/94
05/06/94
vertical displacement (mm)
650,0
600,0
550,0
500,0
450,0
400,0
350,0
300,0
250,0
200,0
150,0
100,0
50,0
0,0
cl03
Figure 6. STATION 40 – Vertical displacements
650,0
600,0
550,0
500,0
450,0
400,0
350,0
300,0
250,0
200,0
150,0
100,0
50,0
0,0
ms 11
Figure 7. STATION 49 – Vertical displacements
650,0
600,0
550,0
500,0
450,0
400,0
350,0
300,0
250,0
200,0
150,0
100,0
50,0
0,0
KM7P1
MS16
KM7P2
MS19
KM7P3
MS24
KM8P1
KM8P2
Figure 11. STATION 62 –Horizontal displacements
CL07
14/09/09
18/03/09
19/09/08
23/03/08
25/09/07
cl02
CL08
14/09/09
18/03/09
KM5P3
19/09/08
23/03/08
25/09/07
KM5P2
29/03/07
29/03/07
30/09/06
03/04/06
05/10/05
cl01
KM3P2
30/09/06
03/04/06
05/10/05
KM5P1
08/04/05
08/04/05
10/10/04
13/04/04
16/10/03
KM2P2
KM3P1
10/10/04
13/04/04
16/10/03
KM4P4
MS11
19/04/03
19/04/03
21/10/02
24/04/02
26/10/01
29/04/01
KM2P1
KM2P3
21/10/02
24/04/02
26/10/01
KM4P3
cl06
29/04/01
31/10/00
31/10/00
04/05/00
06/11/99
10/05/99
11/11/98
KM1P3
KM1P4
04/05/00
KM4P2
cl05
06/11/99
10/05/99
11/11/98
15/05/98
KM4P1
cl04
15/05/98
16/11/97
20/05/97
21/11/96
KM1P2
MS21
16/11/97
20/05/97
21/11/96
25/05/96
MS22
KM6P2
25/05/96
27/11/95
31/05/95
02/12/94
05/06/94
KM1P1
MS07
27/11/95
31/05/95
02/12/94
05/06/94
Horizontal displacement (mm)
Horizontal displacement (mm)
14/09/09
18/03/09
19/09/08
23/03/08
25/09/07
29/03/07
30/09/06
03/04/06
05/10/05
08/04/05
10/10/04
13/04/04
16/10/03
19/04/03
21/10/02
24/04/02
26/10/01
29/04/01
31/10/00
04/05/00
06/11/99
10/05/99
11/11/98
15/05/98
16/11/97
20/05/97
21/11/96
25/05/96
27/11/95
31/05/95
02/12/94
05/06/94
Horizontal displacement (mm)
390,0
360,0
330,0
300,0
270,0
240,0
210,0
180,0
150,0
120,0
90,0
60,0
30,0
0,0
cl03
Figure 9. STATION 40 –Horizontal displacements
390,0
360,0
330,0
300,0
270,0
240,0
210,0
180,0
150,0
120,0
90,0
60,0
30,0
0,0
KM6P1
Figure 10. STATION 49 –Horizontal displacements
390,0
360,0
330,0
300,0
270,0
240,0
210,0
180,0
150,0
120,0
90,0
60,0
30,0
0,0

The specific vertical displacements, registered by the surface benchmarks, were practically the
same in the three sections in November of 1995, one year after the filling of the reservoir. In
November 2008 the increment of the vertical displacements in related to November 1995 was of
40% at Station 40, 49% at Station 49 and 90% at Station 62, see Table 1. These values indicate the
influence of the rockfill wetting by the seepage flow at the left abutment that even affected the
central section of the dam.
6
Station.
Height
(m)
Vertical
displacement
Nov
1995
(mm)
Vertical
displacement
Nov
2008
(mm)
Specific
Vertical
displacement
Nov1995 (%)
Specific
Vertical
displacement
Nov2008 (%)
RATIO
2008/1995
(%)
40
113
277
388
0,2451
0,3434
40,10
49
150
385
575
0,2567
0,3833
49,31
62
86
257
489
0,2988
0,5686
Table 1: Specific vertical displacements 1 year and 14 years after filling
90,29
Cracks in the upstream slab
Between September and November of 1995, the instruments installed at Station 62 registered a fast
intensification of the vertical displacements, with increase of speeds of up to ten times, Figure 12. The
increase of the settlements speed coincides with the increase of the seepage flow and with the
observation of damages in the slab.
Station 62 - Vertical Displacements MS e CL - Flow MV
ms16
ms19
ms24
MV01
29-Mar-07
25-Sep-07
30-Sep-06
5-Oct-05
3-Apr-06
8-Apr-05
10-Oct-04
16-Oct-03
13-Apr-04
21-Oct-02
19-Apr-03
26-Oct-01
24-Apr-02
29-Apr-01
4-May-00
0,00
31-Oct-00
20,00
0,0
6-Nov-99
40,00
50,0
11-Nov-98
60,00
100,0
10-May-99
80,00
150,0
16-Nov-97
100,00
200,0
15-May-98
120,00
250,0
21-Nov-96
140,00
300,0
20-May-97
160,00
350,0
27-Nov-95
180,00
400,0
25-May-96
200,00
450,0
2-Dec-94
220,00
500,0
31-May-95
240,00
550,0
Flow (l/s)
cl08
600,0
5-Jun-94
Settlement (mm)
cl07
Figure 12: Total flow and vertical displacements, MS and CL – Station 62
The intensification of the flow through the upper part of the left abutment would have drained the
water towards the river channel and to downstream, due to the conformation of the foundation,
causing rockfill wetting and the sudden increment of vertical and horizontal displacements. Divers,
down to a depth of 40m, firstly performed the investigations of the upstream slab, at the left
abutment area, in 1996. These inspections detected a crack 20 m deep close to the plinth in the base
of slab L5, a set of transversal fissures 26 m deep at the slab L6 with opening of up to 35 mm and
crushing of the concrete in the upper portion at the joint L12/L13, Figure 13.
Figure 13: Location of damages at the left abutment slabs
In a second stage the underwater inspection with divers and robot identified several points of
suction in the cracks of the slab. Three crack treatment campaigns were accomplished in these
places through the placing of fine silty sand in 30-kg bags applied with the aid of divers. The sand,
with granulometry of 99.8% #1.2 mm; 99.5% #0.6 mm; 90.4% #0.3 mm; 26.1 #0.15 mm, in terms
of percentage that passes through sieve was thrown directly on the cracks, until clogging and
stopping the suction. The cracks in the slabs are in the same area where the surface cracks
happened in Zone I at the end of the construction period. The causes are associated
essentially to the protuberance of the rock massive in the upstream side creating a strong
unevenness between Zones III and IV areas of the dam. The treatment with placing of silty
sand directly on the points of suction of the slab was effective in the reduction of the flow. In
consequence a reduction was observed in the rates of the measured displacements at the left
abutment.
6
Conclusions
The main conclusions regarding the performance of the concrete face rockfill dam are:
 The sudden increase in the vertical displacements, starting from September 1995, was
probably caused by the infiltrations through the cracks in slabs L4 / L5 and L6, resulting in
the wetting of the rockfill of Zone IV of the dam, especially at the left abutment;
 The specific vertical displacements ranged from 0.34% to 0.56% in November 2008 and
still do not show a trend of stabilization;
 The long term slow deformation of the rockfill embankment varied ranged from 40% to
90% since November 1995 up to November 2008;
 The follow-up through the instrumentation of the dam was crucial to detect and correct the
problems;
 The cracks in the slabs of the left abutment are associated with the foundation topography,
with the plinth much more elevated in relation to the downstream toe of the dam;
 The treatment with silty sand was effective in the reduction of the flow through the slab
cracks. In consequence a reduction was observed in the rates of the measured
displacements at the left abutment;
7
Acknowledgements
The authors wish to thank Companhia Hidro Elétrica do São Francisco - CHESF for their support and
the authorization to publish the data presented in this work.
8
Bibliography
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face rockfill dam”, XX ICOLD International Congress on Large Dams, Beijing - China.
[2] Souza, R.J.B., Cavalcanti, A.J.C.T., Vasconcelos, A.A., Silva, P. N., (2007) – “Performance of the
Xingó concrete face rockfill dam”, III Symposium on CFRD Dams honoring J. Barry Cooke,
Florianópolis – SC- Brazil.
[3] Souza, R.J.B., Silva, S.A., Silveira, J.F.A. (1999) – “Xingó concrete face rockfill dam behavior of
the dam on the left abutment”, II Symposium on CFRD, Florianópolis – SC- Brazil.