A review of selected major losses

Heathrow Express Link, London, UK, 1994

The Heathrow Express Link connects central London and Heathrow Airport. The running tunnels were driven with tunnel-boring machines (TBMs) whilst the two stations at the airport were to be built using the New Austrian Tunnelling Method, which involves the application of shotcrete. A number of trial tunnels had been driven successfully and a few tunnel sections subsequently constructed without any problems when, on 21 October 1994, disaster struck. At first, cracks were discovered in the shotcrete lining of one of the three headings, followed by large-scale concrete spalling and subsidence craters on the surface. These problems gradually spread to the other two headings. Finally, all three tunnels caved in one after the other, taking several buildings with them.

Bolu tunnel, Turkey, 1999

Construction work on the Bolu tunnel forming part of the Anatolian motorway was already far advanced when, on 12 November 1999, the area around the town of Düzce (northwest of Bolu) was shaken by an earthquake. It had a magnitude of 7.0 on the Richter Scale and was particularly notable for its unusually high horizontal accelerations.

Besides damaging a motorway bridge section, the quake mainly affected the tunnel named after the city of Bolu. This tunnel was being driven using the New Austrian Tunnelling Method and was in a known fault zone. Although the tunnel had resisted a previous earthquake in August 1999 (100 km west of Bolu), the November quake resulted in a longish section collapsing. The tunnel had been designed to withstand the earthquake strains encountered up to that time but could not cope with the enormous horizontal accelerations.

Taegu underground, South Korea, 2000

A serious accident occurred on 22 January 2000 during the construction of an underground line in Taegu. Following the failure of a diaphragm wall, part of a station excavation pit caved in. The debris buried a local bus: three passengers were killed and the driver was seriously injured. There was considerable damage to neighbouring buildings.

The accident was found to have been caused by a load case that had not been considered in the design phase and was due to unforeseen soil conditions. Large variations in the water table set in motion the previously unexplored gravel and sand banks, triggering the load case which exceeded the design strength of the diaphragm wall.

Tseung Kwan O underground line, Hong Kong, 2001

The Tseung Kwan O line is an extension of the Hong Kong underground network. The tunnels were already complete and the electromechanical work in the underground stations and tunnel tubes far advanced when the region was struck by a severe typhoon. Besides causing torrential rain and devastating winds, it also generated a flood wave that reached the coast on the morning of 6 July 2001.

The roof of the running tunnel between the stations Hang Hau and Tseung Kwan O had an opening for transporting materials into the tunnel tubes. Although the opening was surrounded by a concrete wall to prevent water from entering, it was overcome by the masses of water and the construction site was flooded. As there were no cross-bulkheads, 75% of the new underground line was inundated. The main damage was to the electromechanical equipment such as switchgears, transformer stations, cabling, signalling equipment, and the platform screen doors, escalators, and lifts.

Socatop tunnel, Paris, France, 2002

The Socatop project (Société de Construction de l'Autoroute de Traversée de l'Ouest Parisien) is part of the extension work on the A 86. Its main section is a tunnel with a diameter of 11 m, driven using a mix-shield TBM. The tunnel accommodates two decks each with three lanes of traffic and air extraction and ventilation systems in the crown and invert areas. One of the special features of construction was that the lower carriageway slabs were immediately laid by a travelling formwork running 600 m behind the TBM. Material was taken to the TBM by service trains running under the lower carriageway slabs in the invert area.

On 5 March 2002, one of these trains was on its way to the working face when it caught fire. The tank of the diesel locomotive was soon engulfed in flames, with the result that the train was automatically stopped. The crew attempted to extinguish the fire but could not prevent the fire from spreading to the spoil conveyor and the ventilation duct, the travelling formwork, and the tunnel's concrete lining. The dense smoke and intense heat blocked the way back to the portal, forcing the workers to take refuge in the TBM's compressed-air chamber. The TBM itself was not damaged by the fire, as it was protected by a sprinkler system at the end of the back-up train.

Shanghai underground's Pearl Line, People's Republic of China, 2003

The central element of Shanghai underground's new line 4 is the tunnel that takes it below the Huangpu River on the way from the economic centre of Pudong towards the city centre. The two parallel tunnel tubes had already been driven using earth-pressure-balance TBMs when a disaster occurred during the construction of an emergency cross passage below the river in the vicinity of the river bank. Shortly before the breakthrough of the cross passage at a depth of approx. 35 m, there was a massive inrush of material and water, which the miners at the face were unable to control. Although they managed to get to safety, the surface subsided over a large area, seriously affecting neighbouring buildings. A number of high-rise office blocks were severely damaged, others collapsed altogether or had to be demolished because the risk of collapse was too great. A dyke was also badly damaged. For a time, there was a threat of flooding, as the Huangpu River runs very high at that particular time of year. After the ground gave way, the tunnel tubes subsided a few metres and were flooded; the tunnel lining was fractured. The loss was found to have been caused by failure of the ground-freezing unit that had been installed to protect the excavation work for the cross passage.

Circle Line, Singapore, 2004

Contract 824 of the Circle Line in Singapore consisted of running tunnels to be driven by earth pressure balance TBMs and station structures and running tunnels to be built using the cut-and-cover method. The retaining walls of the excavation pits, 40 m deep in some parts, were formed by diaphragm walls and nine levels of horizontal struts. A jet-grouted base slab served as the sealing blanket.

On 20 April 2004, a construction pit was being excavated directly beside the six-lane Nicoll Highway when there was a disastrous collapse. The excavation pit, which was about 35 m deep at the time, collapsed along a length of over 100 m. Four construction workers were killed, the highway collapsed and had to be closed for several months while the damage was being repaired.

A number of causes were identified: use of an inappropriate soil simulation model which over-estimated the soil strength at the accident site and underestimated the forces on the retaining walls within the excavation; an error in the design of the strut-waler support system with the connections being under-designed; and deviations in actual construction, which further aggravated the under-designed conditions.

Orange Line, Kaohsiung, Taiwan, 2005

A two-line underground system is being built in Kaohsiung. Under Contract O2 of the Orange Line, station structures and running tunnels had already been completed and only a cross passage with a pump sump at its lowest point (approx. 40 m) still had to be constructed. There were only a few centimetres still to be excavated when, on 5 December 2005, there was a massive inflow of water and huge amounts of sand. In spite of the workers' rescue attempts, the cross passage and a large section of the running tunnel caved in. The collapse resulted in considerable subsidence and structural damage in a road tunnel above. The loss was caused by defective jet-grouting of the ground in the immediate surroundings of the pump sump.

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