Yacambu-Quibor Water Tunnel Breaks ThroughThe Quibor valley in Lara state, Venezuela occupies an area of some 434 square kilometres with semi-arid climate. It is situated 25 km south of Barquisimeto, a city of about one million residents, which is the largest food distribution centre in Venezuela. The valley possesses exceptionally favourable conditions for vegetable, fruit and livestock production of high quality and economic value, but while some 21,500 hectares of the valley have potential for cultivation under irrigation only 3,500 hectares are currently in production.Agricultural development has been achieved through the exploitation of local groundwater resources. The rate of groundwater extraction for agricultural irrigation has exceeded the replenishment over a period of almost 40 years. This has led to a major reduction in the potentially exploitable aquifer reserves. In addition to continuously falling groundwater levels, increasing groundwater salinity has also been observed.The existing water supply of the city of Barquisimeto is from wells in other local aquifers and from the Alto Tocuyo surface water system, but all of these sources are proving insufficient to meet the increasing demand, both in the urban and agricultural sectors. The Quibor valley is rich with soils to produce all year but aquifers have been drying up and the water table is at more than 150 metres. In Barquisimeto, water shortage is as high as 1,700 litres per second, affecting one-third of the population.To remedy the situation, the Yacambu-Quibor water system will exploit the waters of the Yacambu river in favour of the agricultural development of Lara state, in addition to improving the potable water supply of the city of Barquisimeto, capital of this western state. The Yacambu-Quibor water system consists in constructing a 162 m-high dam on the Yacambu river that will create a reservoir covering 852 hectares containing 435 million cubic metres of water and a 24.3 km tunnel to channel water from the upper river basin to the arid Quibor valley. The tunnel will transfer 287 million cubic metres of water annually from the Yacambu river to San Jose en el Valle. Two-thirds of the water will irrigate lands and one-third will compensate the water shortage in Barquisimeto.Once the project is concluded, it is estimated that the agricultural offer of the state will increase by 35.8%, since the tunnel will boost the development of agriculture and food products as well as direct and indirect employment. The project is making history as the tunnel was concluded on 3rd August, 2008. Venezuelan president Hugo Chavez is awaited to inaugurate the tunnel in the very first days of August. The tunnel is expected to be operational in 2011. Click ve/15. Visit www.yacambu-quibor.com.veA total of about USD600 million has been invested in the project, of which financial institution Corporación Andina de Fomento (CAF) has contributed over USD280 million, including a USD75 million loan agreement signed on 15th July, 2008 to finance investments in infrastructure (regulation, diversion, supply line and treatment plant) and develop the project's environmental and social activities.History of the projectThe design of the Yacambu-Quibor water transfer scheme was initiated as long ago as 1973. Its tricky construction faced around eight stops, five national evaluations and redesign threats. Contractors failed to complete it, it lacked financing and an international media campaign tried to impose its unfeasibility. Several construction methodologies were implemented, like tunnel boring machines, roadheaders and finally explosives. Excavation began in 1975. The awarded contract specified the use of full-face TBMs. The international consortium began with three machines (two from the two main portals and one from an adit drive).In 1977, the weaker strength of the rock failed to provide the TBM grippers with sufficient grasp. Squeezing ground was another problem. After driving 1,350 metres in 22 months, the contractor decided to quit. That same year, a new consortium takes over the job but work progresses at an even slower pace.In 1979, during the Christmas holidays, tunnelling work was shut down. The rock continued to converge, resulting in one of the TBMs to be completely swallowed by a rapid and intense ground squeezing process. The contract was again terminated with 1,600 metres of the 24.3 km completed. The remains of the machine were removed several years later.In 1980, in awarding a new contract to a new consortium, it was decided to allow the drilling and blasting option. Until 1989, litigation involving international contractors paralized the project. The project was initiated under the responsibility of the Ministry of public works (MOP), then was placed in the hands of state-owned company Sistema Hidráulico Yacambú-Quíbor (SHYQ) that same year and it is today a project of the Ministry of the environment. Visit www.minamb.gob.veIn 1995, a further contract enables the change from roadheader excavation to drilling and blasting.Advance reaches 20 km in 2002, 22.5 km in 2006 and the project finally makes breakthrough in 2008, 33 years after commencement. The tunnel was built according to the original design.GeologyThe geology comprises sandstones, phyllites and limestones in squeezing conditions. The graphitic phyllite has an average compressive strength of about 50 MPa and the estimated GSI value is about 24. At its worst, the UCS of the phyllite is 15 MPa. The graphitic phyllite is a very poor quality rock that causes serious squeezing problems which, without adequate support, result in complete closure of the tunnel just excavated.The Yacambu project was a prime example of tunnelling blind. The geology was largely unfamiliar and unpredictable. With little previous experience, it was unknown how the rock would react to tunnelling, especially under the high stresses of the Andes. To complicate matters, the tunnel would have to be driven through the Bocono geological fault, and extension of the San Andreas fault severing the western part of California from North America. The completed water tunnel crosses the Andean mountain range under a maximum overburden of 1.200 metros.Construction methodsTwo Robbins TBMs were used to excavate 1,613 metres from the entry portal and 1,846.36 metres from the exit portal. Then, the construction method was changed and a Dosco roadheader was used from the entry portal. It bored 2,500 metres (from km 1+850 to km 4+350). Visit www.therobbinscompany.com and www.dosco.co.ukFrom 1995, an Atlas Copco 282 two-boom jumbo has been used on each of the two drives, with R32 1 5/8" drill bits and 2" diameter 3.70 m R32/R38 drill bits. The excavated cross-section is 21.24 square metres on average while the reinforcement and lining represent 8.67 square metres so that the final cross-section equals 12.57 square metres. Visit www.atlascopco.comThe project also requires the construction of a selective intake, consisting of three 3 m-diameter horizontal tunnels at 730, 710 and 690 m above sea level, two vertical shafts (70 m deep and 4.30 m in diameter each) and three 1,500 mm diameter VAG butterfly valves.Several Häggloaders were used for rock transportation: a small 8HR2 and a big 8HRS at the entry portal together with two small 8HR2 at the exit portal. Also, 11 to 14 cu m Häggloader shuttle trains have been employed. Visit www.gia.seRock reinforcement has been secured by Atlas Copco's Swellex bolts (1.50 m, 2 m, 3 m and 4 m), 7 mm welded mesh made in Venezuela and H200 steel arches. Shotcreting was performed with a Putzmeister pump, an Aliva 260 and a Shotcrete Technologies shotcreting robot at each face. The concrete admixtures were supplied by Sika and MBT. Visit www.swellex.com, www.sika.com/stm.htm, www.putzmeister.de and www.shotcretetechnologies.com. To view pictures of the project, click here. 32-33/08.