Heathrow Airport Airside Road Tunnel

Introduction

The Airside Road Tunnel (ART), completed in 2005, formed an important strategic element of the airport’s development and addressed two main functions. In the short term, prior to the opening in 2008 of the new Terminal 5 (T5), it provided unrestricted access to remote aircraft stands, thus reducing surface traffic. In the long term, the ART forms an integrated transport route between the Heathrow Central Terminal Area (СТА) and T5 (Figure 9.1).

Key Aspects of Design and Construction

Portal Structures and Tunnels

The ART comprises 1.2 km of twin bored tunnels running east-west between two portal structures (Figure 9.2). The project, costing £140 million, was managed by the owner BAA using an integrated team approach (Powdcrham and Rust D’Eye, 2003).

The portal structures were designed and constructed to meet the requirements of the bored tunnelling programme. A 9 m diameter earth pressure balanced tunnel boring machine (TBM) was used to construct both tunnels (Darby, 2003). To accommodate the installation and removal of this large machine involved excavations up to 17 m deep between large diameter contiguous piled walls. Bored tunnelling was undertaken in two phases between June 2002 and June 2003, with both drives starting from the west and ending at the east portal. The west portal site is constrained by aircraft stands and the interface with the existing Piccadilly Line loop tunnel. This was constructed between 1984 and 1986 and is operated by London Underground (LU). It passes beneath the west portal at two locations (Figures 9.2 and 9.3) and is a 4.1 m diameter precast concrete segmen- tally lined tunnel. At the junction area, the excavation was less than

Figure 9. / Plan of airport.

Figure 9.2 Aerial view showing ART portals in relation to T5 and СТА.

Figure 9.3 ART plan of west portal. The TBM chamber is 17 m deep and measures 20 * 30 m in plan.

5 m above the tunnel crown, with retaining wall pile lengths curtailed so as not to enter the exclusion zone of 3 m around the tunnel set by LU (Figure 9.4). All airport facilities and the Piccadilly Line tunnel had to he kept fully operational throughout the ART construction.

The portal structure comprises a cut and cover section which includes the TBM launch chamber, a junction and a ramp (Figures 9.3 and 9.24). These elements were addressed sequentially for each application of the observational method (OM).

Geology

Ground conditions at Heathrow Airport were well documented following four phases of ground investigation carried out between 1995 and 2000 for the T5 development. A geotechnical database was created to collate all available information. This combined earlier site data with that from the geotechnical investigations following the Heathrow Express (HEX) tunnel collapse in October 1994.

Figure 9.4 ART longitudinal section at west portal.

The ground conditions are typically made ground overlying Terrace Gravel for the first five metres above London Clay. This, extending to a depth of around 60 m, overlies the Lambeth Group and the Upper Chalk. The water table is generally around 2 m below ground level within the Terrace Gravel.

Base Case Design

The base case design for the ART portal structures was undertaken during 2000-2001, with the west portal design being followed by the east - the same order in which they would be constructed. Based on BAA’s key requirements, the aim was to achieve a technical solution that could be constructed to meet the bored tunnelling programme and that was demonstrably safe and economic. It also needed to have a minimal impact on both airfield and LU operations during its construction. Simplicity and robustness were used to enhance safety and facilitate quality and ease of construction. Both portals were designed to utilise the same construction methods, plant and equipment. Sizes of the structural elements were standardised and overall cross-sectional dimensions kept constant where practicable. Summaries of the geotechnical design parameters used for the ART base case are provided in Tables 9.1 and 9.2, where z is depth below top of London Clay surface.

Construction Sequence

For the west portal TBM launch chamber, top-down construction provided the most economical and practical solution. By incorporating as much of the temporary works as possible into the permanent design, costs could be reduced and the construction programme shortened, allowing for the earliest re-instatement of the adjacent aircraft stands. The base case design for the cut and cover sections required one level of temporary propping at the mid-height of the piled walls. This was designed in structural steelwork and, for the TBM chamber, had a total weight of 60 tonnes (Figures 9.5-9.7).

Retaining Wall Design

Design of the retaining walls for the ART portal structures was carried out in accordance with the recommendations of CIRIA Report 104 (Padfield and Mair, 1984), adopting ‘moderately conservative’ parameters as listed in Tables 9.1 and 9.2. While respecting the need for robustness, various economies were used in the design.

Stratum
Terrace Gravel	19.5	38	35	0	30	1 x I0~4	0.4
London Clay	20.0	23	20	8	0.77EU	5 x I0~'° (h) 0.5 kh (v)	1.5

Table 9.2 ART base case design parameters for London Clay.

CIRIA Report 104 recommends:

• • Applying a minimum construction surcharge of 10 kN/m² to the retained soil.
• • Allowing for an over-excavation by the lesser of either 0.5 m or 10% of the retained height.
• • Reducing the undrained shear strength, s_u, to zero over the top 1 m to allow for excavation disturbance and dissipation of excess pore water pressures at excavation level.
• • Reducing the s_u profile by between 20 and 30% to account for potential softening of the soil in front of the wall.

The first three factors were applied, but the s_u profile was not reduced on the passive side. This economy was based on previous experience of similar excavations in London Clay. (Though not in current guidance at the time, this was subsequently recommended in the design guide, CIRIA Report C580 (Gaba et al., 2003)). To take further advantage of the temporary conditions during construction, two further design factors were included that reduced predicted loading and wall movement and increased support. A mixed analysis was selected using effective stresses on the retained side and total stress conditions for the passive support. Also, since the gaps between the contiguous piles would allow seepage, a 50% reduction in pore water pressure on the retained side was assumed before internal faces of the piles were sealed with skin walls.

The reinforced concrete frame at the top of the TBM launch chamber is shown in Figure 9.8. It illustrates the significant constraints to installing contingency temporary propping in an acceptably short period. In the event of adverse movement trends, the three piled walls of the chamber would have to be expeditiously supported. The simplicity of utilising individual single props as applied at Limehouse Link was not applicable here. To satisfy timely contingencies using steelwork propping, the only effective option would have been to install the steel framework (Figures 9.6 and 9.7) fully fabricated and held ready just below the reinforced concrete frame at roof level. While this would have enabled its prompt deployment, it would have defeated the prime objective of the OM to eliminate this temporary propping. Flow this key issue was resolved is discussed in Section 9.3.