Developing A Luoi

19 June 2009



Work is well underway on the A Luoi hydro project in Vietnam’s Thua Thien Hue Province, with civil construction contractor Cavico Corporation building the project’s extensive tunnel system. IWP&DC talked to the Vietnamese company to learn more about the work involved in the 170MW scheme


The 170MW A Luoi hydro power plant is being developed on the A Sap river in the A Luoi district of Thua Thien Hue Province, Vietnam, by the Central Hydropower Joint Stock Company. The project, which is scheduled to begin operation in 2011, is expected to help improve the ecological environment in the area, promote infrastructure development, and improve cultural and social life for local communities.

The project’s infrastructure includes:

• Ascent dam: a 50.5m high, 200m long concrete gravity dam, el 55m asl.

• Reservoir with a surface area of approximately 10km2. The power pool is nearly 11km long and lies along the valley of the A Sap and Ta Rinh rivers and Rao Lai stream.

• A 2.2km long diversion channel.

• A single portal type intake.

• A 158.5m high surge tank.

Tunnelling and excavation work

Vietnamese civil construction contractor Cavico Corporation was contracted to carry out the complete tunnel and excavation works on the scheme. The company was awarded two contracts. The first (package 60/XL-AL) was signed on 29 July 2007 and involved construction of the intake, temporary tunnels No 1 and No 2, and the headrace tunnel from the intake to the rear of tunnel No 1. The contract was valued at US$31.4M and work was scheduled to take 40 months. The second contract (package 61/XL-AL, valued at US$22.3M) was signed on 29 June 2007 and involved construction of temporary tunnels No 3 and No 4, surge tank, and the headrace tunnel from the rear of tunnel No 2 to the tunnel outlet. Work on this package was schedule for 1187 days (approximately 39 months).

The tunnel work involves building the 11,626m long headrace tunnel which consists of: a 6204m section covered by armoured concrete (H4.7m, Dia 4m); a 1057m section covered by steel and concrete (H4.7m, Dia 3.2m); a 4100m section without covering (H6.5m, Dia 6.5m); and a 265m vertical shaft with an outer diameter of 4.8m and internal diameter of 3.2m, covered by concrete. A 1526.46m long temporary tunnel is also being built split into four sections as mentioned above: Tunnel 1 (496.92m); Tunnel 2 (374.49m); Tunnel 3 (405.21m); and Tunnel 4 (249.84m). The project’s surge tank is 150m in length with an outer diameter of 5m and an inner diameter of 4m.

Excavation works on the project include excavating and reinforcing the portals, building the cofferdam and the barrow pit for the surge tank. The total volume of material to be excavated is 1.9Mm3, including soil and stone. Details of the excavation work are included in Table 1, with the main equipment used for the project works shown in Table 2. The construction material used at site includes: reinforced concrete, cement, sand, steel, cobble, ashalar stone and granite set, explosive P113.

Geology

The A Luoi hydro power project is located in the Truong Son sequence of the Viet Laos folding system. The project is situated on two architecture zones: Long Dai zone (Hue sub-zone) and A Vuong–Se Kong zone (A Vuong sub-zone). The boundary between the two zones is the breaking deep level of II Rao Quan - A Luoi.

Geologically, the headrace tunnel is divided into four zones. Stage 1 from K0 +00 - K0 +500 is deep underground and has been excavated in the powder stone zone and clay mortar of the A Vuong system. It is in areas affected by the Rao Quan A - Luoi breaking system and is foliated very strong and slim.

Stage 2, from K0 +500- to K3 +150, is located in metamorphism aqueous rocks of the A Vuong system and granite type of the Dai Loc complex system. It is directly affected by the deep break system causing origin rock roots to be strong. At 300m deep the tunnel breaks phase III, on the South West slope angle of 75-850. Break has a destroyed zone width, and broken chips are 100m wide.

In addition, the tunnel breaks in level IV located on the same breaks of level III, with a destroyed zone width 10m on average. Boundary between the changed aqueious rock granite in stage K2 +550 - K2 +580 and K3 +450 - K3 +480, has been changing, and is subject to compression pressure, crinkle, and strong chink. Original stone is outside the destroyed zone, the chink zone is IIA aqueous rock, IIB zone of aqueious rock and granite. The geological conditions of this stage are very stable.

Stage 3 from K3 +150 - K5 +500 is located in aqueious rock and the A Vuong system of complex systems, Ben Giang-Que Son. The tunnel stage from K5 +050 passes through level III breaks, and in addition the tunnel passes 10 breaks of level IV, with a destroyed zone width of 2m.

Section 4 from K5 +500 to K11 +600 is located deep under the natural ground from 80-400m (on average 300m). This stage passes 16 breaks of IV level with an influence average 10m (only origin stone was broken chips).

In stages 1, 3 and 4 the geological conditions of the headrace tunnel are not convenient for construction – there are weak stones because the destroyed activities of break have caused strong chink. In these zones, reinforced concrete has been used with an Anchor plate and steel mesh combination.

Project timeline

The project work has been divided into a number of stages. From June 2007 to January 2008, most of the construction work involved excavation. From February to May 2008, underground excavation work continued, while some construction work above ground begun. Excavation works on the headrace tunnel began in May 2008, and are scheduled for completion in May 2010, with concreting and the remainder of the works expected to be complete in June 2011.

Construction and excavation works had to be carried out on a continuous basis in order to overcome obstacles created by the mountainous A Luoi site conditions.

In order to ensure schedules are met, Cavico work under the following basis: three shifts are scheduled each day; work is carried out over all weekends and holidays; all equipment is mobilised and material delivered on time; a salary and bonus scheme has been implemented for workers meeting schedules.

Almost two years after start of construction, more than 30km of roads of have been constructed together with more than 20km of transmission lines. With regards to the tunnelling work, almost 86% of the excavation has been completed, with a further 7km to be finished this year. Almost 1.5Mm3 of rock and soil has been excavated.

Dam foundation works have been completed and the cofferdam has been built. Approximately 3000m3 of concrete has been used.

For further details visit www.cavicocorp.com



Tunnelling methodology

The tunnel excavation and support at A Luoi was carried out following the New Austrian Tunnelling Method (NATM). This method uses the advantage of the rock’s capability to support itself, by careful measures and deliberate guidance of the forces during the re-adjustment process from the primary to secondary state of stress-strain which takes place in the surrounding area of the excavation. The rock mass will be able to support the stress-strain as long as local progressive loosening will be limited by rock support like shotcrete and rockbolts. In more jointed rock mass the shotcrete has to be reinforced by steel mesh or steel fiber and the length and number of rock bolts have to be increased. In the fault zones it may become necessary to use rockbolts and rock anchors in order to improve the triaxial state of stress in rock mass itself, enabling the support to withstand the secondary shear stress. The application of the NATM requires deformation monitoring and measurement by means of survey and special devices.



Tables

Table 1
Table 2

Tunnelling Tunnelling
Excavation work Excavation work


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