The Hai Van Pass Tunnel project is being implemented to improve the main north-south road corridor in Central Vietnam, aptly named National Highway No.1. The existing Hai Van Pass, on the section linking Thua Thien-Hue Province and Danang City (Figure 1), is approximately 20km of low standard highway with numerous tight bends and a steep gradient that crosses the pass at an elevation of 475m. Traffic along this road is frequently disrupted by broken down vehicles and landslides. Many lives are lost due to accidents along the route each year.
To avoid this dangerous section of road the Vietnamese government decided to upgrade the highway – a plan that included the construction of a tunnel beneath the existing pass. Results of feasibility studies that had been underway since 1996 convinced the government of Japan to provide a loan through the Japan Bank of International Cooperation (JBIC) to finance the project’s approximate US$150M construction cost, of which just over half is attributable to the tunnel works.
The executing agency for the project is the Project Management Unit No. 85 (PMU85) under the country’s Ministry of Transport who, in 1997, appointed a JV of Nippon Koei and Louis Berger International, in association with the Transport Engineering Design Incorporation (TEDI) of Vietnam for consulting engineering services for the project. Detailed design started in February 1998 and was substantially completed by the end of 1999.
Hai Van project layout
The project comprises 12.2km of two-lane highway including the approach roads with eight viaducts and the 6,274m long tunnel section up to and in excess of 1000m beneath the Hai Van massif (Figure 2). In addition to the 75m² finished cross-section main traffic tunnel, a 16m² cross-section parallel evacuation tunnel is being excavated concurrently, 30m to the east, connected by emergency crosspassages at 400m intervals (Figure 3). A longitudinal ventilation system is provided for the traffic tunnel with fresh air supplied and exhaust removed near the mid point via a ventilation adit. Three electrostatic precipitators clean the air within the tunnel. The evacuation tunnel is scheduled for enlargement sometime within the next few years to provide twin unidirectional road tunnels, depending on the route’s future traffic growth, and is therefore unlined in the initial phase of the project. The ventilation adit is also being used to provide additional construction access to the central section of the main drives.
The project is divided into a number of contract packages. The first two contracts for construction of the tunnel civil works on the project were awarded in September 2000. The $45.7M Package 1A went to a JV of Hazama and Cienco 6 for the north tunnel section, which comprises 3857m of main and evacuation tunnels and the 1888m ventilation adit. At $27.9M, Package 1B was awarded to a JV of Dong Ah and Song Da for the south tunnel section, which comprises 2417m of main and 2429m of evacuation tunnels. Subsequent contract packages awarded comprise of: Package 2A (US$5.3M) “Lang Co Bridge Section” for the approach road and viaduct at the north end of the tunnel and Package 2B (US$4.5M) “Southern Highway Section” for the approach roads and bridges at the south end of the tunnel. Package 3 (US$20.6M) “electrical works”. Package 4 (US$23M) “mechanical works”. Package 5 (US$7.2M) “110KV transmission line and 110/22KV substation”. Package 6 (US$2M) “maintenance equipment” and Package 7 (US$413,000) infrastructure development in the resettlement area at the north end of the project. The outstanding approximate US$14M of the US$150M is attributed to consulting engineering services.
Tunnel and geology
The main tunnel is designed as a waterproofed drained single tube. After initial drill and blast excavation and primary NATM support, the tunnel is sealed with a 2mm thick PVC waterproof membrane, followed by a cast 300mm minimum thickness secondary un-reinforced concrete lining. At the north section Hazama gained approval to substitute the PVC membrane with a 0.8mm thick EVA membrane, to date both systems have proved equally successful in sealing the tunnel lining. Any water is collected in the fabric installed beneath the waterproof membrane to be channelled into the side collector drains and the main centre drain (Figure 3).
The locations of the tunnel portals were selected to optimise the tunnel profile against construction costs and place the portals in the best locations to minimise the soft soil excavation at the portals. Another consideration for the south portal was to drive the main tunnel directly beneath a sacred Banyan tree at the portal, this has been achieved to good affect.
Excavation of the tunnels is predominantly through good quality granite with a uniaxial compressive strength of 80MPa-100MPa. The NATM type support has been determined by the RMR for this rock mass, which averages at around 70, therefore the support is mostly Type I, which is 50mm of shotcrete and bolting determined locally by the rock jointing. However, for the first 100m at each portal the alignment is through highly weathered to completely decomposed granite, with virtually no uniaxial compressive strength, and with erosion deposits covering the valley floors.
Early problems
Tunnel excavation commenced in mid 2001 from both the north and south portals. Excavation was to be carried out through the initial soft ground from both portals as a 59m² top heading, 35m² bench and a 26m² invert. When the competent ground was reached at the north, the contractor had chosen to adopt a 90m² full face excavation, whilst the southern contractor was to divide the face into a 56m² top heading, followed within 500m by a 34m² bench.
Unfortunately, Package 1B had only progressed approximately 30m from the southern portal in the soft ground section of the main tunnel when a major collapse occurred, in September 2001 (T&TI, Jan’ 02).
From the initial breakout at the portal until the 30m point was reached, the ground water encountered in the tunnel face had increased considerably. This led initially to a small loss of ground above the tunnel face that rapidly chimneyed upwards, with a large quantity of sand and boulders filling the excavation, to form a crater in the portal slope. The collapse caused no injuries and the cavity was grouted to stabilise the tunnel face whilst the solution for completing the excavation was developed.
Fortunately the tunnelling crews that were constructing the parallel evacuation tunnel had managed to negotiate the initial poor quality soil and had progressed into the harder more stable granite. The contractor was therefore able to recover the delay, after proving the rock condition at the location of the main tunnel, by excavating an additional cross passage from the evacuation tunnel back to the main tunnel’s alignment, 180m from the portal. This enabled the contractor to set up two additional faces to work from – one to carry on northwards and the other to drive back through the soft ground. This allowed the rock excavation in the main tunnel to resume on schedule leaving the soft ground section to be completed off the critical path.
To investigate the extent of the soft ground and to provide drainage of the ground water in the collapse area, a 16m² cross sectional area pilot heading was excavated from the stabilised face, just in from the portal, through the completely decomposed granite. Excavation of this pilot continued for a length of approximately 50m until firmer ground was hit. High ground water pressures were encountered that caused some flow of ground in the face where pockets of sand were encountered.
Additional consolidation grouting was then carried out from the pilot tunnel within the area disturbed during the collapse. The pilot tunnel was then enlarged, in five stages, to the full 59m² top heading. This was carried out during the area’s dry season, taking advantage of the considerable reduction of the ground water table and allowing the enlargement to be carried out without further ground stability problems.
Simultaneously with the works in the pilot tunnel from the portal, the main tunnel top heading excavation started from the additional cross passage back towards the portal. Connection with the pilot tunnel occurred at the end of April 2002, after which the pilot tunnel enlargement was completed. The full top heading was eventually finished on 4th July 2002. This was soon followed by the completion of the bench and invert to enable lining to commence from the south portal during October 2002, as had originally been scheduled.
Recent progress
Since the recovery from the early collapse at the south portal, the rock quality encountered has been as expected and therefore excavation has progressed at reasonable rates from both tunnel portals without any significant incident. From the south, the Dong Ah/Song Da JV is currently driving the main tunnel using an Atlas Copco Rocket Boomer as its primary machine with three Atlas Copco Boomer 352’s and a Tamrock HS205 completing additional duties. Each top heading blast consists of three cut holes reamed to 105mm and 157, 45mm diameter blast holes, all 4.5m long. The blast holes are packed with 5kg each of Orica powergel and 2kg per hole for the perimeter giving a total charge of 575kg per round or approximately 1.8kg per m³ of rock excavated. Mucking was originally carried out by two Russian Chain loaders but, due to poor availability, these were replaced in March 2003 by a Volvo 180D loader, which has worked well. The bench is being mucked by a Cat 330 excavator.
The JV is applying the primary shotcrete lining using a Meyco Suprema with an Aliva 285 machine standing by as a replacement.
On the north drive the Hazama/Cienco 6 JV is carrying out the full-face excavation also using an Atlas Copco Rocket Boomer supported by Boomer 352s with a Furukawa face shovel loading out onto 40T dump trucks.
Both tunnelling contractors have achieved similar excavation rates, of some 150m/month, through the good rock tunnel sections. If maintained, these rates are sufficient to meet the original contract schedule targets, however neither contract has been able to consistently achieve their target rates in all grades of rock.
Due to the longer tunnel drives on contract package 1A, higher tunnel excavation rates are required from the north portal than from the south portal. The package 1A contractor had also planned to excavate a considerable part of the main and evacuation tunnels from the base of the ventilation adit in order to meet the contract schedule. Excavation progress in the ventilation adit has been delayed, where higher than expected water inflows were encountered in a number of fault zones in the rock. Water inflows, at a rate of approximately 90l/sec, from a fault zone 600m from the tunnel portal resulted in inundation of the tunnel during February 2002. This led to the delay of approximately two weeks, during which time additional pumps were mobilised from Japan, until the contractor was able to regain access to the tunnel face. As a long term solution remedial grouting works are now being undertaken, prior to lining the adit, to reduce the water inflows in the tunnel to levels that the tunnel drainage system can cope with. Excavation of the adit continued through the wet conditions, although the contractor was unable to achieve his target production rates and therefore completion of the adit was delayed by approximately seven months from the scheduled date. This has had a knock on effect to completion of the main and evacuation tunnels, as a greater percentage of the works are now required to be completed via access from the north portal.
The package 1A contractor has not been able to increase the excavation rates from the north portal to compensate for the delay in the ventilation adit. Therefore Hazama/Cienco 6 has subcontracted Dong Ah/Song Da to continue excavation from the south beyond the contract boundary for an estimated length of 450m, to recover their tunnel excavation delays. This, when combined with the planned acceleration of the tunnel finishing works, will recover sufficient time to meet the original project schedule targets.
The end in sight
At the end of August 2003 there remains 510m of rock between the two tunnel faces, which with the current progress rates should see connection of the tunnels from the north and south in late October 2003. Works will then be required to continue at a hectic pace to complete the lining and pavement, in conjunction with the electrical and mechanical systems, to permit the overall completion and commissioning of the project, ready for opening the tunnel to traffic at the start of 2005.
Related Files
Fig 2 – Longitudinal section of the Hai Van Tunnel project
Fig 1- Plan map of the project alignment including contract packages
Fig 3 – Typical cross section of the main 75m2 tunnel with the 16m2 emergency tunnel running 30m to the east
