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The engineering characteristics of concrete faced rockfill dam of

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					       ENGINEERING CHARACTERISTICS OF CONCRETE
   FACED ROCKFILL DAM OF GONGBOXIA HYDROPOWER
                    PROJECT ON THE YELLOW RIVER

                              Wang Junli       Wu Zengmou
          (North West Investigation, Design and Research Institute, Xi’an, 710065)

Abstract: The retaining dam of Gongboxia hydropower project is a reinforced concrete faced
rockfill dam with a maximum height of 132.2m and a crest width of 10.0m . The dam site is
located at higher seismic region, where the canyon is asymmetric, and it is cold with the
temperature changing dramatically, the rock property varies with complexity, and the
excavated materials differ greatly in quality.
These characteristics have been adequately considered in design, and the domestic and
oversee advanced technology and experiences have been applied. These technology and
experiences include: setting strong pervious zone, concrete extruding wall, high plinth wall at
left and right banks; equalized rising of embankment fill in full section, and once continuous
construction for concrete face slab, etc. These methods guarantee the advanced design of the
dam, speed up the construction progress, and ensure the construction quality. The application
of these technology and methods in Gongboxia project is very successful.
Key words: concrete faced rockfil dam; engineering characteristics; Gongboxia
hydropower project

 1 General Description of the Project
    Gongboxia hydropower project is located on the main stream of the Yellow river
where Xunhua-Sala Autonomous County and Hualong-Hui Autonomous County in
Qinghai province are intersected at boundary. The site is 25km away from the Xunhua
County and 153km from Xining city.
    The project is rated as large project (Grade Ⅰ) regarding generation as the main
purpose, and irrigation and water supply as additional utilization. The watershed area
at the upstream of dam site is 143619km2, the mean annual runoff is 226×108m3 and
the normal water level is 2005.00m. The design and check flood level is 2005.00m
and 2008.28m respectively. The total reservoir capacity is 0.63billion m3, and
regulating reservoir capacity is 75million m3 as a daily regulating reservoir. The
installed capacity of the station is 1500MW, with the guaranteed output is 492MW,
and the mean annual generation is 5.14b kWh.
    The project is composed of three parts: the dam, the diversion and power system,
and the release structure. Based on the topographical and geological conditions, and
requirements of being applicable to construction and operation, the layout of project is


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determined as follows after comparing the alternative schemes: concrete faced rockfill
dam on the riverbed, diversion and power system at the right bank, (which consists of
headrace channel, intake of concrete dam, open penstock, ground powerhouse and
330kv switchyard), flood discharge tunnels at the left and right banks, spillway at the
left bank, anti-seepage system of face slab at the right bank and water intake for
irrigation at the left and right banks.
     Gongboxia project commenced in August 8, 2002 with the construction of
diversion tunnel started in July 1, 2000, and the river closure in March 18, 2002. The
first unit has put into operation since September 20, 2004, and the project is predicted
to complete in 2006. The foundation excavation began in August 15,2001, the filling
of the dam began in August 1,2002. By October 22, 2003, the dam body has been
filled to the elevation of 2005.50m(the bottom of the parapet wall). The casting of the
concrete face slab started in March 15,2004, and it has been totally completed by June
3, 2004. The impounding of the reservoir started in August 8, 2004.

2 Layout of the Concrete Faced Rockfill Dam

2.1 Natural Condition
   The yellow river at the dam site strikes north east 30°to 50°with a straight and
plain river course. During the mean water period, the water level is at the elevation of
1900m with a 40-60m wide water surface and 12-13m in depth. In this stream, the
coverage of the riverbed is from 5 to 13m. The valley is asymmetric. Below the
elevation of 1980.0m at the right bank is rock slope. Above the elevation of 1940m,
the slope is from 40°to 50°, and below the elevation of 1940.0m, the slope is very
steep and almost stands vertically. Above the elevation of 1980.0m is Grade Ⅲ
terrace which is composed of sandy soil and gravel layer. At the left bank, except the
Grade Ⅱ terrace which is covered by the slope alluvium debris at the elevation of
1930.0m and 1950.0m, the others are all rock slope with an average gradient of about
30°, but there is 10m steep slope along the river. At the dam site, main rocks are:
Pre-sinian gneiss, mica-quartzite schist; Caledonian granite; purplish red sandstone of
Cretaceous system; red sand and gravel of Tertiary system; sandy soil and gravel of
Quaternary system, etc. The basic seismic intensity at the site is 7 degree, and the
design preventive intensity of the dam is 8 degree.

2.2 Layout of Dam Body
   The dam is located at the main riverbed and is a reinforced concrete rockfill dam.
The dam axis strikes NW316°35´13.2´´. The crest elevation is 2010.00m; the
maximum dam height is 132.20m; the crest length is 429.0m; the crest width is 10.0m.
For the concrete face slab which is a dry masonry revetment, its upstream slope is
1:1.4, the downstream slope is from 1.1.5 to 1:1.3, and is provided an “zigzag” access
road which is 10.0m wide. The overall dam slope at the downstream of the dam is
1:1.79. There is a “L” shaped 5.8m high parapet wall provided for connecting with the
face. The bottom elevation of the wall is 2005.0m. Since the valley is narrow, the
intake of the powerhouse gets close to the right dam abutment, and the spillway is


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close to the left dam abutment. So a 38m and a 50m high plinth wall are provided to
connect with the dam face at the positions where the abutment is connected with the
intake of powerhouse and the left dam head is connected with the spillway
respectively. (see Figure 1).
   The dam is divided from upstream to downstream as follows: the soil inclined
blanket zone(1A) at the lower part of the upstream face slab and weighted blanket
zone(1B), concrete face slab, cushion layer (2A, 3m thick), cushion zone (2B),
transitional zone(3 A, 3m thick), main rockfill zone (3BⅠ-1 strong pervious zone ,
3BⅠ-2 rockfill zone and 3BⅡ sandy gravel zone.) and the secondary rockfill zone at
the downstream(3C). The details are shown in the standard profile (Figure 2).

3 Main Characteristics of Concrete Faced Rockfill Dam

3.1 Strong Pervious Zone in Main Rockfill Zone
   The main rockfill zone is a part of main supporting system of the dam, and also be
used as drainage for the dam.( The permeability coefficient is K≥10-1cm/s). In the
original design, the main rockfill materials come from excavated materials (it is




                            Fig.1   Layout of the Project




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                          Fi g. 2 Dam St andar d Pr of i l e




required to be slightly to weakly weathered lower granite and slightly to weakly
weathered schist. The schist content shall not be more than 30%. The maximum grain
size is 800mm. The content of grain which is smaller than 5mm shall be less than 8%).
After an in-situ large compaction test, it is demonstrated that the excavated materials
contain too many fine grains after compaction, and the permeability coefficient is too
small(K≤10-2cm/s), therefore it cannot satisfy the requirement. In order to guarantee
a free drainage way and meanwhile fully make use of the excavated materials, the
strong pervious zone(3BⅠ1) which is composed of slightly to weakly weathered
granite and schist excavated from the borrow is provided at the upstream of the main
rockfill zone. It is required that the schist content will not be more than 30%; the
maximum grain size shall be 800mm; the content of the grain which is smaller than
500mm size shall be less than 8%; the permeability coefficient shall be K≥10-1cm/s;
the porosity≤22.5%. For the 3BⅠ2 rockfill zone, the content of the grain which is
smaller than 500mm shall be less than 20%; the permeability coefficient K≥
10-1cm/s; the porosity ≤22.5%. Therefore, the excavated materials can still be used
for the most of 3BⅠ zone, which makes 2/3 fill materials come from the excavated
materials of the project.

3.2 Cushion Layer Adopting the Extruding Wall for Slope Stabilization
   The application of extruding wall can construct the side wall and cushion materials
at the same time, omitting dam slope trimming, slope compaction, slope protection
and etc.; avoiding extra-fill of cushion materials; speeding up the placement speed;
improving the compactness of slope cushion materials and reducing the scouring to
cushion materials during the construction, as well as facilitating dam slope protection
and water retaining during flood season. By calculation, the compression stress of


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extruding wall shall be 20MPa ~ 36MPa downslope during operation. Axial
compression stress shall be 22MPa ~ 36MPa, greater than the compression strength of
wall body. Therefore, the extruding wall has been crushed during operation, its
performance is the same as that of the cushion. The deflection of face slab is 27.3cm
and 28.1cm respectively with or without extruding wall. The tensile stress is 3.46MPa
and 3.52MPa downslope, and compression stress is 6.8MPa and 7.1Mpa respectively.
The calculated value with extruding wall is smaller, therefore, it is considered that the
extruding wall has no adverse effects on the stress-strain of face slab. The design of
extruding wall is reasonable and has no effect on safety operation of face slab.
   The upstream slope 1:1.4 of wall is similar to that of dam, taking 10cm as top
width. The wall height is the same as the placement thickness of cushion materials.
The inner gradient of concrete extruding wall is 8:1.
   The concrete extruding wall shall reach the following criterion:
(1) Not exceeding 5MPa at 28d compression strength, the 2~4h compression strength
    index shall be such that the squeezed-type wall cannot collapse when the cushion
    materials are compacted by vibration.
(2) Controlling its elastic modulus index in the range of 5000 ~ 7000MPa, which is
    better less than 5000MPa.
(3) Controlling its density index in the range of 2.0~2.25t/m3, approaching to the
    compactness degree as much as possible.
(4) Controlling its permeability coefficient in the range of 10cm-3/s, which will be
    consistent with that of cushion layer (semi-permeable body) as much as possible.
(5) Painting a thin coat of asphalt emulsion on the surface of extruding wall, so as to
    reduce the constraints on concrete face slab, and prevent the face slab from
    cracking.

3.3   Equalized Rising of Embankment Fill, Providing Conditions for
Continuous Construction of Face Slab
   In view of the regulating of large reservoir upstream, the project reduces the water
discharge during flood season, so the weir can be used to retain water all the year. The
equalized rising is adopted for embankment fill, so as to reduce uneven settlement of
dam body and the effects on face slab. By calculation, the settlement is reduced from
137.3cm to 99.69cm ~103.6cm by the application of the wholly-section filling
method.
   By about 5-month self-settlement after dam is placed to elevation 2005.50m
(bottom of wave wall), the concrete pouring of face slab starts, in order to reduces the
effects of dam post-settlement (including rheology) on face slab. All of this creates the
advantageous conditions for continuous construction of face slab.
   For Gongboxia hydropower project, the once-pouring block of concrete face slab
reaches 218m long, which is the longest one for projects under construction. By
calculation, the stress and deflection of once pouring face slab are both less than those
in 2-stage construction (deflection reducing from 23.3cm to 20.5cm, downslope stress
reducing from 10.04MPa to 6.32MPa). Both stress-strain conditions are basically
identical. The displacement values of peripheral joints are slightly reduced. It is


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shown that once pouring of face slab is favorable for dam safety. However, it is noted
that the crack control measures of concrete face slab shall be taken (focusing on
concrete materials, technology, curing and reducing constraints of foundation).

3.4 High Plinth Wall
    The high plinth wall is an important part of seepage control system for face slab,
which is directly related to the anti-seepage reliability of face slab and dam safety. For
Gongboxia hydropower project, concrete high plinth walls, at left and right abutment,
are 38m and 50m high at maximum respectively. The foundation of high plinth wall is
located on weakly weathered rock (for high plinth wall at left bank, part of foundation
is strongly weathered rock), with local outcropping of schistose xenolith.
    The high plinth wall of left bank located at the right side of the spillway, which is
also used as the right guide wall of the spillway, is a concrete gravity retaining wall,
with a top elevation of El. 2010.0m. Meanwhile, it acts as part of the access road from
the dam crest to the intake tower of spillway tunnel at the left bank, with a total length
of 50.0m. On the basis of the access requirement, the top of the wall is 8.0m-wide,
which includes the 1.5m-wide cantilever bracket. Both of the side faces of the wall are
twisted. The left side of the wall is gradually changed from vertical slope to the slope
of 1:0.5, and the right side face changes from the 1:0.5 slope to the vertical. The high
plinth wall of right bank located at the left side of intake is also a concrete gravity
retaining wall, with a top elevation of El. 2010.00 ~ 1952.417m and a total length of
84.99m. According to the structural requirements, the width of the wall top is 4.0 ~
13.21m, and the slope 1:1.5 along the direction of length. The height of the wall is
gradually decreased. The right side face of the wall is a vertical plane, with the slope
of 1:0.6 for the left side. The foundation varies from El. 1960.0m to El.1950.0m.
Grouting drainage gallery is set within the walls, and the consolidation grouting and
curtain grouting are implemented at the bottom of the wall.
    Besides the lateral pressure of embankment rockfill in the construction period, the
high plinth wall shall resist the enormous water pressure during the impounding stage
without large deformation. Damages to the sealing of peripheral joints between the
face slab and the high plinth wall or the cracking to the curtain grouting under the
wall shall be avoided, so the normal operation of the face slab would not be affected.
Since the forces borne by the high plinth wall are complicated, the computing analysis
shall be considered as a whole. The calculating results indicate that smaller tensile
stress regions exist within the body of left and right walls. However, the tensile stress
values are very small, with the largest value of about 0.1MPa. The compressive stress
in the wall is also smaller, and the anti-sliding stability of the wall has higher safety
margin.

3.5 Earthquake-resistant Measures
  The results of static and dynamic stress deformation analyses indicate that the
amplification for the reaction acceleration of earthquake at the dam crest reaches 3.7,
and the dynamic reaction is larger. Local collapse might occur at the upper part of
downstream dam slope. Therefore, the following earthquake-resistant measures


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should be taken:
(1) Change the gradient distribution of downstream dam slope. In order to decrease
    the seismic damage to the dam crest, gradients are divided into three degrees:
    1:1.5 for the top part, 1:1.4 for the middle and 1:1.3 for the bottom.
(2) Grouted masonry revetment is used for the downstream slope above El.1980.00 of
    dam for the purpose of protection.
(3) In order to increase the structural strength of dam crest, transition material (3A)
    are used above El. 1995m. Ties are embedded in the dam body above
    El.1980.00m and connected with grouted masonry revetment, so as to strengthen
    the anti-seismic capability of the crest. The embedded ties belong to the bars of
    φ25,with the spacing of 2m, distance between the layers of 1.6m and the single
    length of 15m.

4 Conclusion
   (1) Feedback calculation for the observational data during the construction period
indicate that non-uniform settlement of dam body is relatively small, with the total
settlement amount of not greater than 1% of the dam height upon impoundment. From
the observational data, it can be found that the settlement deformation of dam body
grows slowly and uniformly with normal regularity. The above mentioned conditions
show that the dam design is reasonable, safe and successful.
   (2) In the design of Gongboxia dam, topographic and geologic conditions of the
project are closely incorporated and the domestic and overseas advanced techniques
and experiences are fully absorbed. Setting of strong pervious zones, concrete
extruding wall and high plinth walls at both banks, equalized rising of dam
embankment, continuous construction of concrete face slab, etc. are adopted in the
process of design and construction, which guarantee the advanced design of dam,
speed up the construction progress and ensure the construction quality. The utilization
of these techniques and measures are successful in Gongboxia hydropower project.

Brief Introduction of the Writers:
   Wang Junli (1960-): Professorial senior engineer, vice-Chief Engineer of
Northwest Investigation Design and Research Institute. Project Manager of
Gongboxia Hydropower Project. Mainly engaged in the design and technical
management of hydropower station.
   Wu Zengmou (1939-): Professorial senior engineer, former Design Chief Engineer
of Gongboxia Hydropower Project (1984 ~ 2001). Mainly engaged in the design and
technical management of hydropower station.




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