The Yacambu-Quibor irrigation system (close to Venezuela’s fourth largest city, Barquisimeto) consists of one of the most famous tunnels in South America (Figure 1). Not because it is amoung the longest, at a respectable 24.3km, or because of its 1200m overburden and challenging geology – though they are indirectly responsible for its fame. Simply because Yacmbu has been in progress now for around two and a half decades, many of its miners have spent their entire working lives on the project, some of them introduced to it by their fathers.
In the 1950’s, the Quibor valley in the south of Venezuela, was seen as a ‘salad bowl’ with its rich lime soil and abundant water supply allowing three or more harvests a year. However the rapid expansion of the nearby industrial city Barquisimeto, accelerated desertification of the area as the natural aquifers were being drained more quickly than they could refill. To alleviate this problem, the Venezuelan Ministry of Public Works in 1955 put forward a proposal to utilise the Yacambu river (just 25km’s away) which acts as a major catchment channel for the Andean foothill in which it is situated. The water coarse deposits millions of cubic metres of water in to the Atlantic Ocean. All that was needed was a dam to catch it and a tunnel to allow the passage of the water!
The project took 20 years to get past planning stages; finally, in 1975, work commenced. However 27 years and seven contracts later, the project is still underway, most of the contractors falling foul of amongst other things, the complex geology.
The tunnel passes through the north eastern section of the Andean range, produced by the collision of the Caribbean, South American and Nasca tectonic plates. This section of the range has undergone at least four phases of deformation. Basically the geology is split into the Moran formation and the Villa Nueva formation, both consisting of metamorphosed argillaceous type rocks with a propensity to rapid weathering on exposure to air or water. This is combined with a measured in-situ stress regime in which σh=1.5σv and just for good measure the tunnel alignment passes through two major fault zones (Turbio and Bocono) which consist of fragmented, weakly cemented argillaceous rocks.
The tunnel is effectively divided up into two parts (figure 2); the first is a 2.2km inclined 4.5m diameter adit which intersects the main tunnel at around chainage 17000 (gradient 10o) and is referred to as the Ventana Inclinada (VI). The second is the main 24km long, 4.5m diameter tunnel, which was being driven from the portal de salida only (work on the portral de entrada is due to recommence after refurbishment of the existing tunnel is completed). At various points in the tunnel’s excavation it has been driven from up to four headings (Portal de Salida-PS, Portal de Entrada-PE, Ventana Inclinada in the direction of Salida-VIDS and Ventana Inclinada towards Entrada-VIDE).
Ground support system
The recent ground support system (used on Contract 6) was the culmination of the previous 27 years’ experience. The ground itself is classified Class A, representing (relatively) high strength, 50-80MPa rock with little to no graphite foliations, down to Class D representing fault gouge material (Bocono Fault). Correspondingly, different ground support regimes were defined for each type of ground encountered from “spot bolting” to the “worst case” support that were, until recently, being installed in very weak ground comparable to class D.
The ground in front of the face was reinforced with an arch of grouted spiles three metres long, with the top heading excavated using air driven picks. This excavated area allowed shotcrete (150mm) with two layers of mesh, rockbolts and the top half of the rib (with sliding joints) to be installed. After the remainder of the round was excavated more meshed shotcrete was applied to the invert section and the compressible arch completed before more rockbolts (Swellex 3m bolts) were installed (the ribs with sliding joints were placed approximately one metre apart and over the next four to five days reduced in circumference by about 600mm). After four or five days the relaxing effect of the ground had effectively destroyed the previously placed (preliminary) shotcrete. At this point movement had normally slowed/stopped to allow the addition of another layer of 250mm of meshed shotcrete. The excavation was fully monitored and convergence pins installed every 3m (two rounds of excavation) approximately 4-5m back from the face of the top heading.
This excavation cycle was painfully slow and problems were reaching critical levels regarding low UCS core strengths for the sprayed concrete. Therefore the contractor, having previously been in contact with the Underground Construction Group belonging to Master Builders Technologies (MBT) (who specialise in sprayed concrete solutions) decided to contact them. The MBT engineers assessed that the shotcrete design was far from optimal for the long-term durability of the tunnel.
Concrete mix design
As a consequence of the growing use of sprayed concrete as a permanent construction material, demands on its durability have increased. The use of traditional accelerators in high dosages has led to serious problems in sprayed concrete, often only a short time after it has been applied. This is well documented and readers are directed to the proceedings from the international symposiums on sprayed concrete (especially the Gol, Norway 1999 proceedings). However the addition of traditional accelerators is not the only factor that can negatively affect the durability of sprayed concrete (and the addition of an appropriate accelerator can actually help improve the durability). Other sprayed concrete factors can be split into external and internal:
External Factors – Movement of the rock mass or aggressive ground water are only two examples of factors that can reduce the durability of applied sprayed concrete. Additional factors such as low humidity or excessive ventilation can result in shrinkage cracking. Cracks not only represent mechanical damage that reduces the static performance of a structure, but also open the door to chemical attack.
Internal Factors – Mix design is one of the major factors affecting the durability of sprayed concrete: The choice of the binder components and binder content play a key role. In order to obtain a good sulphate resistance one can either select a sulphate-resistant cement (with a low C3A content) or increase the overall cement content. Additional contribution can of course be gained by using microsillica or blended cements (with limestone or fly-ash etc.).
Another key role is played by the water binder ratio. A high water content not only increases the extent of shrinkage cracking that will occur, but also causes the concrete to have increased porosity. This increased porosity has an effect similar to that of cracking, only on a smaller scale, the pores allow the penetration of water and gas. The addition of suitable additives and admixtures can improve the concrete quality. In order to reduce shrinkage, cracking fibres can be added that distribute internal stresses and bridge micro cracks, thus preventing crack propagation.
The alkali aggregate reaction is another threat to durability. Major contributors of alkalis to the concrete are found in the cement, mixing water and of course admixtures. Since it is difficult to avoid alkalis in the cement and water, the construction chemicals industry has developed alkali-free accelerators (<1%Na2O). They replace conventional aluminates and waterglass based products, contributing no additional alkalis to the concrete mix and causing no significant decrease in final strength.
Curing is also of vital importance to achieve high quality, durable, sprayed concrete and for a multitude of reasons curing by conventional methods (water curing or the application of curing agents) is difficult when applied to sprayed concrete. Because of this, admixtures have been developed that act as internal curing agents providing improved hydration, less cracking, better chemical sulphate resistance and improved interlayer bonding.
Yet another factor is the compaction of the sprayed concrete during application. A good technique (e.g. distance and angle of spraying jet) and adequate equipment are instrumental in achieving a durable sprayed concrete.
The most promising approach to achieve high quality durable sprayed concrete is obtained by a combination of countermeasures: low water binder ratio, correct type of cement, addition of microsillica, addition of fibres, the use of alkali-free accelerators and high performance water-reducing admixtures, proper curing and good working practices.
Conclusion
At Yacambu, the problems of slow progress and reduced compressive strength were related and the two problems could be addressed by removing at least one of the mesh layers from the support design. The double layer of mesh was slowing down the cycle time due to its tricky installation and because of a shadowing effect. This is where a zone of poorly compacted concrete is created by mesh layers casting a shodow on the substrata. The effect increases exponentially with the addition of more mesh layers. The in-situ shotcrete quality was being reduced especially in the soffit. The removal of one of the mesh layers represented a deviation from the ground support design and was therefore a concern to the designers. So a solution was needed that would appease the designers without sacrificing the savings in time or quality. MBT suggested a structurally rated polypropylene fibre (made by US firm SI) that could be dosed easily and would not pose any operational problems (such as “balling”) in the restricted diameter sections where the on-site shotcrete equipment was used. After testing, both in the laboratory and with core/beam samples taken from site, the designer approved the change and it was applied. Cycle times improved dramatically as did the quality of the placed shotcrete. Shortly after the change to polypropylene fibres was applied the expiration period of Contract 6 was reached and the project put out to tender yet again, only to be retained by the current contractor Dell Acqua. The author is hopeful that, with only 6km to go and good ground expected, seven will prove to be a lucky number and that, finally, some water will start to flow.
Related Files
Figure 2
Figure 1
