HOLDING the key to the future management and development of the Yellow River, the second largest river in China, is the 1800MW Xiaolangdi project. Commissioning of the first unit is scheduled for the end of 1999, but the origins of the project can be traced back over 60 years ago.

The present Yellow River Conservancy Commission (YRCC) had identified Xiaolangdi as a significant component in controlling the Yellow River, or ‘Huanghe’ as it is called locally. The site had originally been considered for this purpose in 1935 and drilling started in 1953. However, progress was slow until November 1990, when the State Planning Commission instructed YRCC to proceed with the detailed design; calling tenderers for the construction work. In order to facilitate the purchase of equipment and materials through international competitive bidding, financing of the foreign currency component of the project was requested from the World Bank. Xiaolangdi is now the largest World Bank-financed hydroelectric project in China.

The Yellow River Water and Hydroelectric Power Development Corporation (YRWHDC) is responsible for the US$4.2bn project which was designed by the Reconnaissance, Planning, Design and Research Institute (RPDRI) in Zhengzhou, Henan. Site supervision falls under the remit of the Xiaolangdi Engineering Consulting Company (XECC), and all of the above companies fall under the umbrella of the Ministry of Water Resources of China.

The Canadian International Project Managers (CIPM) Ltd are acting as consultants to the employer. The work related to the underground power structures was awarded in 1994 to Dumez, Philip Holzman and Construction Bureau 5 named Xiaolangdi Joint Venture (Lot 3). The remaining underground works were awarded to a joint venture of Zåblin, strabag, Wayss and Freytag, Del Favero, Salini and Bureaus 7 and 11 named CGIC JV (Lot 2). Spie Batignolles have since replaced Del Favero. The 51.8 x 106m3 volume main dam and associated structures are being constructed by a joint venture formed by Impregilo, Hochtief, Italstrada and Bureau 14 called Yellow River Contractors (Lot 1).

Benefits to be derived from the Xiaolangdi project include flood and ice control, sediment management and water supplies for the purposes of irrigation and hydroelectric generation. Each one of these requirements demands a different design approach which makes the Xiaolangdi project layout very complicated and distinctive, when compared to other projects.

The powerhouse complex and all water conveyance tunnels are located in the left bank, which resulted in a complex layout and closely spaced underground excavations.

One of the major components of the project involved diverting the Yellow River. This took place on October 28, 1997, and became a national event in the company of the diversion of the Yangtze at the Three Gorges on November 8, 1997.

Sediment control

A unique characteristic of the Yellow River is its very high sediment content, which reaches more than 900kg/m3 during peak floods. The average sediment load is 37kg/m3 which compares to 4.3 in the Ganges, 2.8 in the Indus and 1.4 kg/m3 in the Nile. Each year, a total of 1.6 billion tonnes of silt and fine sand are presently carried to the lower reach, adding a new 0.1m layer to the river bed. A large part of the land downstream of Xiaolangdi is protected by 1400km, 9-11m high dykes, which must be maintained and raised constantly to prevent flooding. The main dam, a conventional inclined core earth and rockfill dam, is designed to take advantage of the rapid silting up of the 12.6 x 109m3 volume reservoir. The impervious core extends through the upstream cofferdam and only after one year of reservoir operation will it be further lengthened by the natural silt blanket below el 185. The photograph on p33 shows a seldom visible zoning of a fill dam, including an upper extension to a slurry cut-off wall constructed with cast-in-place concrete. Seepage control under the upstream starter cofferdam is provided by a jet-grouted cut-off wall. The depth of the alluvium in the river bed, consisting of sandy gravel with large cobbles, reaches 60m.

The three 14.5m inside diameter, 16.1-16.5m outside diameter concrete lined orifice tunnels are first used for diversion. The 23.4m wide by 42m high gate chambers are located approximately 300m downstream of the intake towers. Three pre-stressed concrete lined sediment tunnels are located just above the orifice but below the power tunnels to keep the intakes clear. The three free-flow tunnels are located above the power tunnels. The total discharge capacity at full supply level of all outlet tunnels and service spillway, excluding the six power tunnels, is 17 327m3/s.

The design team has faced important challenges related to enhancing the ability of various hydraulic structures and mechanical parts to resist the abrasiveness of the suspended sediments. Design demands the use of high strength silica fume concrete (70 MPa at 28 days) in the tunnels and water passages to increase erosion resistance. The sediment tunnel concrete liners are pre-stressed to resist internal pressure.

In order to help dissipate the high kinetic energy, which will be associated with the periodic release of high sediment content and high pressure water outflow, special diameter restrictions (orifices) have been designed, model tested and will be installed in the large diameter orifice tunnels. Many of these special design features were developed from Chinese experience gained on other plants and similar, but not necessarily as difficult, conditions encountered in other countries.

Power facilities

The underground powerhouse complex consists of:

• A 251.5m long, 26.2m wide by 61.44m high powerhouse.

• A 150m long, 15.2m wide by 18.3m high transformer chamber.

• And a 15m long by 15m wide draft tube gate chamber.

Six draft tube tunnels discharge into the draft tube gate chamber and the three 12m wide by 19m high tailrace tunnels exit from the gate chamber.

With the project being mainly utilized for flood control, management of sediment, irrigation and general water supply, the generation power is subject to operating rules other than those set by power demand. The full capacity of the plant, six 300MW Francis turbines will mainly be utilized during peaking operation and during the flood season. At other times operation of the plant may be limited to a discharge of 400m3/s, which will be utilized using two units. During the flood season the units will operate with sediment- laden flow, under extremely hostile conditions.

A number of technical innovations, including the use of fully corrosion-protected tendons and testing and the use of improved grouting methods should be noted. In China, cementitous grout is considered as adequate protection against corrosion for tendon wires. Fully protected tendons were first used at the Ertan project and then very extensively at Xiaolangdi. The roof of the fourth highest underground powerhouse cavern in the world is supported with rock bolts and corrosion protected tendon rock anchors (Wang and Solymár, 1997). Also extensive use of corrosion protected tendon rock anchors was made in the support of the 120m high intake and plunge pool slopes (Gao et al, 1997). Figure 2 shows the partly completed plunge pool.

Chinese standards are based on single packer settings at or just below the surface and high grouting pressures. Maintaining the single packer setting and high pressures, the introduction of stable mixes, GIN pressure/volume correlation (Lombardi and Deere, 1993, Lombardi, 1997) and new refusal criterion, resulted in considerable time and materials being saved. The Chinese grouting method, refined at Xiaolangdi, basically consists of drilling a 2-5m deep, slightly larger diameter hole than used for grouting, and inserting an orifice pipe with a valve at the top.

The pipe will remain in the ground after completion of the top few metres and forms a deep packer with the hardened grout seal between the pipe and the rock. Drilling and grouting continues in 5 or 10m depth intervals from inside of the orifice pipe. The grout is allowed to set only for a few hours before the next section is drilled. The pumping pressure is increased with depth to maximum of 1.9 MPa at 70m. The 40 to 50kg/m cement absorption refusal criteria (Ewert, 1985) was adopted after intensive field testing. In about 95 per cent of the cases, the flow rate was less than the specified 1 Lugeon in the check holes drilled between holes where the above refusal criterion was met.

Success of the jet grouting method in alluvium, containing large numbers of up to 400-500mm diameter cobbles and boulders, was an open question until completion of extensive field tests. The method was found to have so many advantages, compared with open excavation, that it will now be used to attenuate the effects of an unfavourable rock profile found below the river bed under the left abutment of the dam. Column diameter of the exposed jet grouted test columns varies between 1.3-2.2m, depending on the coarseness of the material treated and withdrawal speed of the jet grouting pipe.

Underground openings

A great number of large diameter underground openings were excavated in a relatively short time. All underground structures were excavated using the new Austrian tunnelling method. Rock support has been modified during excavation to suit site conditions. Use of lattice girders, steel ribs and alluvial bolt spiles was made in fault zones and at some portals.

Alluvial anchors were also installed in some collapsed sections of the orifice tunnels and in fault affected rock mass. Generally, either short epoxy or fast setting cement grout cartridges were used for anchorage of the medium strength, locally made, fully grouted rock bolts.

Fully grouted dowels were extensively used in tailrace tunnels and in the powerhouse side walls. Approximately 14 000m of 500kN steel solid bar anchors were installed for the support of the suspended concrete crane beams, and to increase the shear resistance of the clay intercalation in the upstream wall of the powerhouse. About 1890m of Swellex rock bolts were used in all excavations which is less than 0.18 per cent of the total. This cannot be described as a large quantity (Liljeström, 1996).

At this stage of the project it seems as if breaking up the work between three different groups of international and Chinese contractors has worked well. A number of local subcontractors were hired, increasing the participation of local companies in the project. Inevitably, logistic problems were encountered. These included handling material by one contractor being placed in temporary stock piles by another contractor, coordination of the distribution of the designated local materials, and planning of the diversion between the Lot 2 contractor (underground works including diversion tunnels, plunge pool and temporary coffer dams) and Lot 1 contractor (main dam), has been a challenge for the staff of the engineer and contractors.

A remarkable achievement

The work of Lot 2 fell one year behind schedule, partly due to a large number of rock falls and collapses recorded in the orifice tunnels at the start of the project, and partly to other events also in dispute. Three collapses occurred and the delay associated with the reconstruction of the failed sections appeared to be almost insurmountable at the time of the events. Excavation was nearly at a standstill for two months while the situation was assessed. A repair programme was prepared by the contractor and modified/approved by the engineer.

Acceleration measures, including assigning a new subcontractor for the underground works, some reorganization and better cooperation between the engineer and contractor, helped to reach the target date for river diversion. Concrete production rates reached more than 115 000 m3/month from the planned 90 000. This alone is a remarkable achievement.