San Cristobal Tunnel Breakthrough in Santiago de ChileThe San Cristobal tunnel will connect Huechuraba and Recoleta, two municipalities north of Santiago de Chile, with Providencia and Las Condes. The 1.8 km tunnel under Cristobal Hill is part of a road connection including the construction of 2.2 kilometres of connecting roads to Americo Vespucio-El Salto interchange, Kennedy-Lo Saldes interchange, and to Avenida El Cerro, totalling four kilometres. The project forms part of the Americo Vespucio-El Salto-Kennedy road bypass awarded to concessionaire Sociedad Concesionaria Túnel San Cristóbal S.A. in November 2004. Click cl/25.The San Cristobal tunnel comprises two one-way tubes carrying two 3.5 m-wide lanes each, with variable average width, and 1.5 m shoulders. The gradient is 2.5%. Speed will be limited to 80 km/hour.The second tunnel broke through on 14th April, 2007 on road C2, which goes from Providencia to Huechuraba. This concluded the excavation phase, which generated the evacuation of 549,000 cubic metres of rock corresponding to 70,000 truck loads. The other tunnel on road C1, in Huechuraba-Providencia direction, broke through on 13th March. Tunnelling of the San Cristobal tunnel began in May last year and the tunnel is expected to open to vehicular traffic on 21st June, 2008. Click here.Now that excavation is completed, crews are concentrating on secondary work inside the tunnel such as culverts, drainage pipes, pavements, surfacing and wall lining, then all equipment will be installed. Click here. Structural designFor the construction of both one-way tunnels, which total 3,656 metres, the structural design methodology was used, which basically uses very refined tensile deformation calculations. These calculations receive feedback during construction from convergence measurements to verify and improve the calculation predictions. The principles of the structural design are based on good knowledge of the ground characteristics, using for that the Bieniawski classification to estimate the properties of the rock mass. This is how that, thanks to the characterization of the face during construction, the typical designed support is chosen. Then using convergence measurements, the real tensile deformation behaviour of the ground is verified and compared to the designs. In case of noticeable differences, the characteristics of the support are modified. GeologyThe drill and blast method, mechanised or mixed, the advances in metres and the types of supports used during construction of the tunnel (bolts, TH-29 steel arches, sealing off of the perimeter with 3 cm-thick shotcrete and support layer of variable thickness, reinforced concrete invert and 5 cm-thick minimum shotcrete lining) vary according to the rock type.Therefore, the first step consisted in analysing the types of materials that compose the geology of San Cristobal Hill. A succession of andesite, tuff, volcanic breccia, intrusive rock, unconsolidated Quaternary deposits (soils), alluvial deposits, and valley-bottom deposits was detected. The conclusions show that the rock composition at the north side (Americo Vespucio) and the south (El Cerro) feature noticeable differences. Core drills and geoelectric surveys were performed to evaluate the rock quality that varies according to humidity, as well as compressive and fracture strengths. In the north, the rock has great consistency while in the south fractured compounds of poor quality predominate.Sandvik jumbosThe detailed ground survey and application of new technologies allowed fast progress during construction of the San Cristobal tunnel. Rapid excavation can be explained by the use of high technology underground drill rigs and explosives, mechanical and mixed, according to the ground features. Read E-News Weekly 6/2007 & 48/2005.For the San Cristobal tunnel, three Sandvik drill rigs have been acquired, two of which are Axeras T11 and a Paramatic H 205-90. Their operation is fully computerised. The drilling patterns are entered in the memory and they are executed perfectly. There is no margin of error by human action. In case of last minute change, the operator can intervene manually. The three booms achieve perfect drills, the quiet machinery is equipped with advanced computerised programmes and powerful rock hammers, and requires only one operator. The average drilling patterns included 100 blast holes, 4 m long. The jumbos drilled all the blast holes in one and a half hours. The output achieved by the jumbos greatly explains the fast advance of the project, with about eight daily metres of tunnel dug by two blasts every 24 hours. Such advances have been achieved at the north side, ahead of the original expectations. Visit www.miningandconstruction.sandvik.comA jumbo Axera T11 was mobilised at each portal at both headings. Two blasts per day were performed at each face on average, which means that each jumbo had to take care of the two attacks from each side. The drilling cycles lasted approximately three hours. To fulfill the drilling phases, the jumbos worked 12 hours per day approximately. The third jumbo, the H 205-90, is employed for bolting to secure outside slopes and for back-up in case of breakdown of the Axeras but in fact, failures were rare. ExplosivesTwo excavation methods for the tunnels were defined. In the south area, a hydraulic hammer and excavator were mainly used. The tunnel was supported with TH-29 steel ribs and shotcrete. This method was implemented until rock of better quality was encountered so that explosives and reinforcement with bolts and shotcrete could be used.At the north side however, due to the greater consistency of the rock mass, the drill and blast method was preferred. Full-face drilling patterns (70 sq m) with 96 to 102 blast holes were implemented, which produced advances ranging from 1.8 to 4.6 metres depending on the noise restrictions, rock quality and proximity to faults.The explosives are emulsions for the column charge, Tronex Plus supplied by Enaex for the bottom charge and Softron for the contour holes. Three types of detonators were used, according to the blast, i.e. Nonel MS and LP series non-electric delay detonators, as well as detonating cord and fuse for the standard blasts (with no noise or vibration restrictions); Nonel EZTL, detonating cord and fuse for controlled blasting (vibration restrictions) with discharge blast after blast; and lastly Nonel EZTL for silent blasting (noise restriction) as well as Nonel shock tubes and fuse. Visit www.enaex.com, www.orica.com and www.dynonobel.com The explosive Tronex Plus was used specially in the north sector of the project, where the bedrock encountered by geologists is of great consistency. A Tronex Plus charge can open 30 to 40 cm holes in walls. It is a light coffee brown semigelatine dynamite that smells sweat and sour. It is a high performance, very versatile and very safe product when it is handled by experts. However, it contains nitroglycerin, and can be extremely dangerous as a result of its extreme instability if it is not stored safely.Three different detonators were used, because one of the great challenges of the project consisted in reducing the acoustic impact. The emulsions are similar, the difference lies in the detonators and for this reason the contractors tested all the available technologies not only to make blasting as efficient as possible but also to reduce the noise level.In the north sector, there are residential zones near the tunnel in Bosques de La Piramide (1,600 m) and El Salto (700 m), in addition to Huechuraba business park at 600 metres. Initially the controlled blasting was restricted to daytime work hours. As the tunnel progressed, it was possible to increase the explosive charge, so that eventually blasts without restrictions were possible. Nevertheless, time restrictions were imposed to avoid greater inconvenience to residents, because the reduction of the background noise causes an increase in the perception of the blast. Visit www.tunelsancristobal.cl and www.elsalto-kennedy.cl 22/07.