Running 4,300km north to south along South America’s western coastline, Chile is one of the region’s fastest growing economies and remains one of the most attractive investment markets in South America ranking number 16 in the Arcadis Global Infrastructure Investment Index.

Like its economy its infrastructure is growing as improving transport, connectivity, and power generation capacity is vital to service the demand. “The market is quite active now, as there are some really interesting projects under development and coming in. There are road projects, metro projects, hydropower projects and mining projects,” says José Miguel Galera, managing director of consultant Subterra, which has been designing tunnels in the country for more than 20 years and does 50 per cent of its business in Chile.

The firm is currently involved in three major road tunnel projects. The second stage of the Santiago Centro Oriente Project is currently under construction. This project involved the expansion of the east-west connections across the city. An existing 5.5km tunnel provides some capacity but another four tunnels are required to serve the growing traffic demand. The most high profile of these is the new 1.1km Túnel Kennedy, which is due for completion at the end of 2017 and was the subject of a detailed report in Tunnels and Tunnelling International in May.

For its design the priority has been the minimisation of induced settlements on the surface. The new road will run partially beneath a golf course alongside the existing Kennedy Avenue. To ensure the existing infrastructure was not affected the designer undertook numerical modelling focusing on the stresses acting on the support elements and the induced settlements.

Construction of the tunnel is being undertaken by Sacyr Chile and Gesival under a USD 500M contract. The excavation method is a bespoke version of NATM, or sequential excavation method, which is necessary due to the extra-large cross section of the tunnel, which measures 22.9m across and is 13.5m high. Depending on the geometry the top heading is excavated using two or three side drifts, while benching was always excavated using three phases. The excavation has been done using hydraulic excavator, backhoe and wheel loader.

Subterra is also undertaking design and construction supervision on Tunnel Chamisero 2, part of the north-west motorway known locally as Radial Nororiente, where a second 1,600m tunnel is being built alongside an existing passage to increase capacity.

But the largest scheme that it is working on is the missing link of the Americo Vespucio highway, which is a privately financed concession road running around the perimeter of Santiago. The first 65km of ring road is operational but the most challenging eastern section is now in the design phase. “It has been divided into two contracts. Americo Vespucio Oeste 1 (AVO1) is a 9km tunnel and we are the designers in joint venture with Louis Berger from North America. The second one is just 5km and concessionaires are bidding for it,” says Galera. This includes a Chinese consortium, which adds a new dimension to the tunnelling industry as Chinese firms to date have not undertaken this type of work in the country.

The 9km section involves 3km of conventional drill and blast tunnelling beneath a hill known as the Cerro San Cristóbal and the other 6km is a mixture of cut and cover, the traditional German method and NATM. The concessionaire for the project is OHL Concesiones in consortium with Sacyr Concesiones and the route runs through five communities and beneath the Mapocho River. Construction is scheduled to start in January 2018.

NEW HORIZONS

But perhaps the most exciting road tunnel for Chile will be the Paso de Agua Negra road tunnel running between Chile and Argentina and promoted by a joint governmental organisation known as Ebitan. “All tunnellers are positioning ourselves for this really interesting project. They called for prequalification at the end of 2016 and just two months ago it was published that 10 JVs have presented their candidature and four are Chinese. The other six are from the rest of the world,” says Galera.

The scheme involves construction of twin 14km road tunnels and is intended to improve regional connectivity particularly between Argentina and Chilean ports. Despite the enormous length of the Argentinian and Chilean border the number of crossings is small and largely consists of minor roads. The existing Agua Negra route for example is a minor road used just in summer as it runs along an altitude of 4,780m meaning that snowfall blocks the route in winter months.

The new tunnel will be around 1,000m lower and is planned as a twin tube due to its length and height. A second passageway offers many safety advantages and by having two equal tubes natural air circulation is enhanced reducing energy costs. It was also decided that each tunnel as a single lane is much safer from a driver perspective. With two tunnels and one-way direction each, light vehicles might keep a good speed and are not delayed by heavy traffic taking the outer lane.

The two tunnels will be 40m to 50m apart and descend from Argentina into Chile with the Argentinian entrance 4,085m above sea level and the Chilean entrance at 3,620m giving the tunnels a slope of 3.37 per cent. Each of the road lanes will be 7.5m wide with space either side for pedestrians. The internal height of the tunnels will be 4.8m and emergency tunnels for people will connect both main tunnels at 250m spacings along the whole length. Vehicular interconnection galleries will be located every 1,550m.

The industry is now waiting to hear who the successful prequalifiers will be and whoever ultimately wins this project will undoubtedly take on lessons from other tunnelling projects that have been carried out in the Andes including a raft of hydropower schemes. The most recent of these are the Alto Maipo project and the Los Condores project, which both include significant tunnelling works for construction of the headrace tunnels and other infrastructure.

ROCK HARD

AES Gener and Antofagasta Minerals are responsible for the USD 2bn Alto Maipo hydropower project situated in the Andes approximately 70km south of Santiago. The project involves around 67km of tunnelling to transfer water from the Maipo River into two caverns where the flow will generate 531MW of hydroelectric power.

The country has generally favoured traditional methods of drill and blast and NATM, but this scheme has been a testing ground for the use of TBMs in Chile. As far as testing grounds go, the Andes are a tough place to start.

Challenging conditions have contributed to a number of issues on the project, which is running over budget and has led the client to terminate the AM-CO 610/620 contracts awarded to contractor the CNM joint venture. This was a pairing of Germany’s Hochtief through local subsidiary Hochtief Construction Chilena and Italy’s Cooperativa Muratori & Cementisti di Ravenna (CMC), and was cancelled in June due to what AES Gener states in its Q2 financial report for 2017 were “breaches of contract by CNM”.

This led the client to take control of the works to continue progress and Robbins is running its TBM until a replacement contractor is appointed. At the same time, the client has drawn on the USD 73M Letters of Credit posted by CNM and CNM and the Alto Maipo client have now entered into judicial and arbitration proceedings. Both AES Gener and CNM declined to comment on the current situation.

However, AES Gener states in its Q2 report that termination of the contract triggered a technical default and the project finance disbursements have stopped until a replacement contractor is found. Negotiations for this are ongoing and could see the main construction contract restructured. Depending on the forward strategy, and potential new delivery date project costs are likely to increase.

CHALLENGING TUNNELLING

On Alto Maipo the CNM joint venture was undertaking mechanised tunnel boring in two key sections using the same 4.13m diameter Robbins open gripper type hard rock TBM fitted with 17 inch Wedge Lock back-loading cutters and 267kN nominal load capacity each. The intention was that it would first be used to bore an initial 7km section, which was at the south end of AM-CO 620 contract, before being disassembled and moved back along the tunnel to then be used to bore a 3km section to the north of the contract area. In a white paper describing the project Robbins tunnelling experts Carlos Lang, Mark Belli and Pablo Salazar explain the progress and the issues encountered underground.

Tunnelling began in June 2015 making good progress with 106.33m achieved in 15 days thanks to the good to fair quality of the rock mass, composed of andesitic breccias with signs of alteration. But the rock quality deteriorated as the work progressed with sub-horizontal joints and faults continuously detected leading to continuous over break and the need to employ the full remit of rock support (Type I- to Type IV) and ultimately installation of the bespoke McNally support system used for Robbins TBMs.

This system prevents rock movement in the critical area immediately behind the cutterhead support. Shield fingers are switched out with a curved assembly of pockets with rectangular cross-sections. The pockets extend axially back from the rear side of the cutterhead through the support, in the area where roof drills could work.

“Standard ground support measures were applied to handle these sub-horizontal joints in the initial drill and blast excavation but this became a serious problem for a small 4.13m open type TBM that in principle had been selected for good rock

conditions. Under difficult ground conditions the TBM operation demanded continuous manual scaling and manual mucking in order to remove loose material caused by wedges formed from these sub-horizontal joints in the tunnel crown,” write the Robbins authors, who also note that the sub-horizontal structures contained along the excavation were not outlined in the geological baseline report.

These difficult ground conditions led to a number of improvements to be made to the TBM including an extension of the side supports and the vertical front support to better face extreme squeezing ground conditions; a new probe drill set up; a mechanical system for removal of the large blocks in the invert area, a new dewatering system, new storage areas and a more user friendly shotcrete robot set up to reduce rebound. To accelerate progress the client has now ordered a second open gripper main beam TBM to use on the project.

On the Los Condores 150MW hydropower project owned by Enel Generatión Chile, a 4.56m double shield TBM is being used for a 12km intake tunnel, under construction by contractor Ferrovial Agroman. This was launched in February 2016 and has bored more than 1.2km of tunnel in sandstone, tuff and pyroclastic breccias. Again the ground conditions have been challenging and unexpected, and a very large inflow of water and running ground has made it impossible to consolidate the ground from inside the tunnel leading to the need to construct a bypass tunnel to continue work.

METRO EXPANSION

Meanwhile in Santiago the metro system has been the beneficiary of significant investment in recent years. Originally opened in 1975, the client Metro de Santiago is just about to open a newly constructed Line 6, which will connect seven municipalities into the public light rail system. There are more than 15 km of underground tunnel, and 10 stations, of which four will be combined stations with existing operational lines and two will connect with the urban train between Santiago and Nos.

At the same time the new 22km extension to Line 3 is also approaching completion. Like Line 6, the project will have fully automated trains and was constructed using NATM. The circular diameter of the tunnels is 25m, with 15m diameter ovoidal access galleries and stations.

According to the client Metro de Santiago the main construction challenges of Lines 6 and 3 have been delivering the new lines without affecting existing operations, particularly in the nine locations where the new lines meet the existing ones, and in addition to that the construction of the combination station Ñuñoa with tunnels of Line 3 on the tunnel of Line 6.

A total of 700,000m3 of concrete has been required for construction, with two-thirds of this shotcrete and one-third reinforced concrete.

Looking ahead the next major tunnelling opportunities on the city’s metro system are expected to be extension of Line 2 and a new Line 7.