Abstract

Shuibuya 233m high CFRD at Qingjiang River is the highest of its kind in the world. The settlement of dam and crack resistance of face slab must be controlled strictly. Based on detailed studies made to the dam structure, construction materials and its foundation treatment, the dam is possessed with the following main features: (1) the main rockfill is inclined from the cross of dam axis with crest toward downstream in a slope of 1:0.2; (2) the design indices for dry density and porosity of material on each zone is of high standard; (3) the adoption of standard plinth with extended seepage-proof board; (4) a downstream RCC cofferdam is integrated with the dam; (5) fully use of materials from structure excavation;(6) only partial removal and ramming is adopted for riverbed cover.

Key Words: Shuibuya Project CFRD Structure Foundation treatment

1 Geology and topography

Shuibuya dam is located at a “S”-shaped part about 800m long in the middle stream of the Qingjiang River. The river valley at the damsite is unsymmetrical, slopes of both banks are steep in topography, the overall angle of slope on the left bank is about 52°, and the average slope angle at the right bank is 35°. The riverbed elevation at the damsite is about 193~107m, and the valley width at the dam axis at crest elevation 409m is about 560m. Bazigou Gulley located at the upstream on the right bank of the dam has materially impact on the layout of plinth.

The strata of the dam site slopes gently dip upstream and slant toward the left bank at 7~12°. Major strata include Maokou Group (P1m) and Qixia Group (P1q) of the lower Permian. Longtan Group (P2l) of the upper Permian also distributed on elevation above 395m at the left abutment with the Quaternary debris, collapsing sediment, flood sediment and a few artificial deposits distributed at bank slopes and the riverbed.

The riverbed at the damsite is generally flat with thin coverage, but also with some deep trenches. The thickness of coverage is around 1.48~21.7m, which is thick around the centerline and gradually reduced toward both banks. The riverbed coverage is comprised of washing debris, collapsing sediment, flood sediment and artificial deposits. Riverbed washing debris is composed of scree stratum, grit stratum with piece rock together with grit silt and clay lens strata. Geological survey shows that the riverbed sand scree stratum is of loose to middle density structure, with low loading capacity on washing, low deformation modulus and high compressibility.

2 Dam zoning and rockfill material design

Shuibuya CFRD is 233m high with crest elevation at el.409m, crest width 12m, dam axis length 608m, the river direction length from upstream plinth to downstream RCC cofferdam 616.2m, dam filling volume about 15.3 million m³.

1.2.1 Dam zoning

Under the premise of safety operation, in considering of the working state, stress distribution, source of material and hydraulic features of the dam, proper zoning on dam filling materials is carried out to take the fully utilization of material from structure excavation and to simplify construction. The principle for dam zoning is: 1) the part upstream the dam axis is the main loading zone, higher deformation modulus is required for rockfill in this zone, lower value can be adopted for the part downstream the axis; 2) material in each zone shall meet the requirement on hydraulic transit and the permeability coefficient shall be kept on increasing from upstream to downstream; 3) take full utilization of material from structure excavation to reduce the cost; 4) satisfactory with mechanical construction. The dam zoning is shown as following (see Fig.1):

Fig.1 Shuibuya Dam Zoning

(1) IA zone (fine sand cover): Its function is to block any crack on the slab and to restore the impermeability by entering the crack with flow in case partial crack or damage in water stop system. The crest elevation of fine sand material is 263.0m, horizontal width 3.0m.

(2) IB zone (cover zone): Its function is to protect fine sand material not be scoured and eroded. The crest elevation of cover zone is 265.0m, crest width 15.0m, upstream slope 1:2.5.

(3) IIA zone (filter zone): Filter zone is located underneath the slab. Its function is to provide even and flat support surface for slab, to counter-filtering upstream curing material and control the seepage in case partial crack or damage in water stop system. Filter material is equally arranged in 4.0m wide. A 2.0m thick filter material also placed on the contact with foundation rock in upstream 1/3 head range.

(4) IIAA zone (fine material zone): In order to provide more compact, even and flat support surface for the bottom copper water stop at perimeter joint, and enhance the control of filter material on seepage, such a zone is arranged underneath the perimeter joint. The zone is shaped in trapezoid, minimum thickness at top is 3.0m, downstream slope 1:1.4.

(5) IIIA zone (transient zone): It is for the hydraulic transient between filter material and main rockfill. It is arranged in equal width of 5.0m horizontally. To prevent void in the contact with bank slopes, a 2.0m thick transient zone is arranged at the contact of main rockfill with dam foundation.

(6) IIIB zone (main rockfill zone): It is the main support for water load located in the upstream of dam axis. The boundary of main rockfill and secondary rockfill is in the slope of 1:0.2 from dam axis down to the riverbed.

(7) IIIC zone (secondary rockfill zone): To take full utilization of material from structure excavation, secondary rockfill is arranged at the downstream of main rockfill from el.225.0m to el.380.0m. The crest is 29.0m wide and the boundary slope with IIID zone is of 1:1.4.

(8) IIID zone (downstream rockfill zone): For secured dam drainage and the stability of downstream dam slope, downstream rockfill zone is placed in the scope below el.225.0m from the secondary rockfill zone and 10m to the downstream dam slope. The downstream slope is protected with 1.0m thick stone blocks paved by dry masonry.

To secure the stability of downstream dam slope and to facilitate overtop during flood season in construction period, a RCC cofferdam is built at the downstream toe as a part of the dam.

2.2 Design of dam material

(1) Filter material (IIA): Filter material requires sound gradation, low compressibility, high shear strength, adequate permeability and good construction performance. Filter material is processed from limestone of Maokou Group, maximum diameter dmax ≤80mm, the content of d<5mm 35%~50%, the content of d<0.1mm 4%~7%, permeability coefficient 10-2~10-4 cm/s, design dry density 2.25 g/cm3, porosity 17%. Layer thickness for compaction 40cm.

(2) Fine material (IIAA): The maximum diameter of fine material is smaller than filter material so as to function as counter-filtering on curing of cracks. The fine material is obtained by removing particles with d> 40mm from filtering material. dmax ≤40mm, the content of d<5mm 35%~60%, the content of d<0.1mm 5%~10%. Layer thickness for compaction 20cm.

(3) Transient material (IIIA): Transient material is taken from the blasting limestone of Maokou Group and tunnel excavation from the 4th and 5th formation of Qixia Group (P11q, P51q). dmax ≤300mm, the content of d<25mm less than 50%, the content of d< 5mm 20%~30% and the content of d<0.1mm less than 5%. Design dry density 2.19 g/cm3, porosity 18.8%, permeability coefficient 100~10-1 cm/s. Layer thickness for compaction 40cm.

(4) Main rockfill (IIIB): Limestone of Maokou Group with high compression modulus and high shear strength is used for the main rockfill. dmax <800mm, the content of d<0.1mm less than 5%, and continuous gradation is required, permeability coefficient shall be higher than 100 cm/s. Design dry density 2.18 g/cm3, porosity 19.6% . Layer thickness for compaction 80cm.

(5) Secondary rockfill (IIIC): Limestone of Qixia Group from structure excavation is used for the secondary rockfill. dmax <800mm, the content of d< 5mm 20%, the content of d<0.1mm not higher than 5%, and continuous gradation is also required. Design dry density 2.15 g/cm3, porosity 20.7%. Layer thickness for compaction 80cm.

(6) Downstream rockfill (IIID): Downstream rockfill can be limestone from Qixia or Maokou Group, dmax <800mm, the content of d<0.1mm less than 5%, permeability coefficient shall be higher than 100 cm/s. Design dry density 2.10 g/cm3, porosity 20.7%. Layer thickness for compaction not higher than 80cm.

3. Design of plinth, slab and water stop

3.1 Plinth

In order to reduce plinth excavation at both banks, the structure of a standard plinth with extended seepage-proof board is adopted in Shuibuya dam. The foundation of the plinth is of weak load-relief zone or treated strong load-relief zone. The thickness of plinth (standard plinth plus extended seepage-proof board) is defined by the allowing gradient of permeability. Standard plinth is generally 6~8m wide, 0.6~1.2m thick, and the extended seepage-proof board is generally 4~12m wide, 0.3m thick. A settlement joint with water stop is furnished between the standard plinth and the extended seepage-proof board. The plinth is divided into riverbed, left and right bank three sections.

Single layer two-way of reinforcing bar is placed in the plinth concrete, reinforcing rate is about 0.35%, the reinforcing net is connected to foundation rock through anchors.

To eliminate possible crack on plinth concrete, during the construction of standard plinth, a 2m wide groove is left in every 12~16m, then is backfilled with low heat inflation cement when the temperature of plinth concrete is basically stable.

3.2 Slab

The thickness of slab is defined by equation T=0.3+0.0035 H (H is the height to dam crest) with minimum thickness 0.3m at the top and maximum 1.1m at the bottom. The slab is 16.0m wide at compressive zone and 8.0m at tensile area, and is divided into 38 construction blocks, total area is 134 thousand m². The slab is adopted with single layer two-way of reinforcing. The reinforcing rate at slope direction is about 0.4% and about 0.35% in the direction of dam axis. Enhanced reinforce bar is placed around perimeter joint as well as tensile construction joints.

In order to reduce horizontal and vertical restraints of filtering material on the slab, a layer of emulsified asphalt is sprayed on the filter. High efficient additives such as enhancing denser are mixed into the concrete to improve the performance of slab against crack. The compressive strength of concrete below el. 346m is of R90350# and R90300# above el.346m. Tensile limit is not less than 10^-4, classification in anti-seepage is of S12 and D250 for anti-freezing.

3.3 Water stop

Three water stops is adopted at perimeter joints below el. 350m, where the bottom and middle water stops are of copper water stop, the top water stop is comprised of a rubber stick banded with rubber ripple water stop and is filled with flexible materials in between. Flexible material is also placed above the ripple water stop and is sealed by reinforced rubber strip on the surface. For perimeter joints above el.350m, only the same structured bottom and top water stops are placed. The external surface of perimeter joints is placed with non-sticky fine material for self-curing.

As it is difficult to predict precisely the compressive and tensile features of vertical joints on the slab and to secure the integrity of water stop system, all vertical joints are designed based on tensile, namely bottom copper water stop and top water stop of flexible material are placed. For slab contacted with perimeter joint below el. 350m, middle water stop is also installed for vertical joints of the slab in the range of 5m to perimeter joint so as to ensure the tightness of the middle water stop in the perimeter joint.

4. Foundation Treatment

4.1 Disposal of Riverbed Cover

The riverbed cover is of loosen to middle dense structure, washing sediments such as gravel and grit are of low loading capacity (Pk 280~770kPa), low deformation modulus (E₀ 24.3~73.25 Mpa). There are also some grit silts and clay lens in the riverbed cover of the straight section downstream the dam axis. These lens strata are also featured with multiple layers of distribution. Based on investigation, it is decided to remove the riverbed cover within the scope of 180m down from riverbed plinth line and 100m upstream of the downstream cofferdam. Part of the cover remained will be compacted so as to reduce dam settlement.

Based on site compact test, it is decided to use a 2.2m diameter, bottom area 3.8m² and 20.8t weight rammer in a ramming span of 5m x 5m, falling distance of 15m and 312tm of ramming force as the construction parameters. The riverbed cover will be rammed for 2 passes, the first pass will be rammed in span by 8~10 times; the second pass will be rammed with low energy in full range by 2~4 times. For quality control, the settlement between last two strikes shall not be greater than 5cm.

4.2 Curtain Grouting

Shuibuya dam foundation is comprised of karstified strata of Maokou Group and Qixia Group limestones and interbeds of marlite in lower Permian. Faults and load relief zone are developed on both banks and with multiple karst passages. Terranes in the damsite area is flat with weak interbeds such as shear zones well developed. As the curtain of riverbed section is carried out on the plinth which is only 0.6~1.2m thick, it is of grouting with heavy cover. Comparing with more than 200m of head and such complicated geological condition, it is a challenge for both design and construction of the curtain.

Under the premise that no uplift damage to the plinth is secured, it is very important to increase the grouting pressure, especially the pressure for shallow foundation. Based on site grouting test, with the consolidation of anchor to enhance the effect of consolidation grouting, optimized construction method and process for grouting, the final formation of curtain can be secured.

Consolidation grouting is required for the foundation of both standard plinth and the extended seepage-proof board before curtain grouting. In order to ensure the effect of consolidation grouting, the grouting is carried out in two steps, and the grouting hold for the foundation of standard plinth is arranged in cinquefoil, span 2m, depth 5~7m. The grouting hole for the consolidation of extended seepage-proof board is also arranged in cinquefoil with 2m span and 5m in depth. The maximum pressure for the first section (contact section) of consolidation grouting shall be 0.3~0.5Mpa.

The curtain-grouting hole for the foundation of plinth is also arranged in cinquefoil with a span of 2m. 4 rows of holes are arranged in the lower part of the plinth, where row 2 and 3 are of main curtain grouting holes need to be constructed in three steps, row 4 is of auxiliary curtain grouting holes, which will be constructed in two steps. The pressure for curtain grouting is 1.5~4Mpa.

5. Features in the Dam Design

During the bidding design of Shuibuya dam, the design scheme in feasibility study is further optimization and finalized with the flowing main features:

(1) Main rockfill is inclined from the cross of dam axis with crest toward downstream in a slope of 1:0.2, which is favorable for even settlement and the control of the total settlement in the dam;

(2) The design indices for dry density and porosity of material on each zone is of high standard, which is favorable for the control of settlement;

(3) The adoption of standard plinth with extended seepage-proof board is favorable for dam filling speed and the reduction in plinth excavation;

(4) Downstream RCC cofferdam integrated with the dam is good for both the stability of downstream slope and the protection in case of overtopping;

(5) Take the full use of materials from structure excavation which shows the advantage of local material supply and design the dam according to local availability in the design principle of local material dam by using 8 millions m3 of excavation from the spillway;

(6) Based on scientific test, partial removal and ramming of riverbed cover is adopted.