Heathrow Airport Multi-Storey Car Park 1A (1995–1996)
Introduction
The Heathrow Express (HEX) Rail Link provides a fast direct connection between the airport and central London. A principal component was the new underground station in the airport’s Central Terminal Area (СТА). This involved three large diameter bored tunnels, constructed in close proximity (Figure 6.1). Within the СТА, the observational method (OM) was being applied on a global basis to a range of structures that were exposed to tunnclling-induced settlement. These included three multi-storey car parks (MSCP) 1, 1A and 3 (Figure 6.1). Only MSCP 1A suffered any visible damage. This building is effectively two structures separated by a central movement joint. The south western half sits on pad footings where it is located above the London Underground Piccadilly Line tunnels, while the north-eastern half of the building is founded on bored piles. It provides a unique case history of a very adverse building response to tunnclling-induced settlement in which relatively small ground movements caused substantial local damage. This applied to both types of foundation. The unexpected sensitivity also highlighted an important limitation in the use of the OM which was further compounded by an outlier beyond the general approach to managing risks of damage to buildings. All this arose despite the OM being implemented by an experienced team and in an ‘OM aware’ environment. The case history provides a dramatic comparison with that of the Mansion House in Chapter 3.
Key Aspects of Design and Construction
Overview
MSCP 1A is a five-storey structure of reinforced concrete (r.c.) construction and measuring 60 m by 142 m overall in plan (Figures 6.1- 6.3). The spatial relationship between the СТА station tunnels, the Piccadilly Line and the building is shown in Figures 6.1 and 6.4. Although the building
Figure 6.1 Layout of HEX СТА Station showing multi-storey car parks MSCP I, IA and 3.
has the appearance of a single structure, the foundations are different in each half. The south-western half of the car park together with its lift tower is founded on pad footings and was completed in October 1964. The north-eastern half of the car park and its lift tower have piled foundations and were completed in April 1965. Both lift towers are separated structurally from the car park.
The London Underground Piccadilly Line tunnels run beneath the southwestern half of the building as shown in Figure 6.1 and were constructed in this area between 1971 and 1976 (Cooper et al., 2002). These tunnels have a diameter of 3.8 m and are at about 13 m below existing ground level (Figure 6.4). The FIEX СТА station comprises two 230 m long platform tunnels and a central 300 m long concourse tunnel. The FIEX tunnels pass beneath MSCP 1 and MSCP 3, which are r.c. frame structures on pad footings being two and five stories, respectively (Figure 6.1).
Geology
Ground conditions across the site are relatively uniform consisting approximately 6 m of Terrace Gravel over London Clay - although there is some doubt regarding the depth and presence of the gravels below the north-eastern half of MSCP 1A, which may account for the use of piles. Ground water level is approximately 1 m above the top of the London Clay that, in this area, has a thickness of around 60 m (Powderham and Rankin, 1997). Except for the upper levels of the shafts and escalators, the FIEX construction is within the clay that, at the depth of the running tunnels, has a shear strength of around 100 kN/m2.
Tunnel Construction
The original tunnel construction for the СТА station was undertaken using sprayed concrete linings (SCL) as the primary support. This system was adopted following its successful use in tunnels around the world and on a trial tunnel for this project. However, in October 1994, some eight months after commencing the work, a major collapse of the tunnels at the СТА occurred. Some settlement was suffered by the Piccadilly Line tunnels, although the effects on MSCP 1A were minor with settlements in the range of 2-3 mm.
As part of the recovery operation, a 30 m deep, 60 m diameter cofferdam was constructed enclosing the majority of the collapsed area (Powderham and Rust D’Eye, 2003). The СТА Station tunnels were constructed from access shafts just north of the cofferdam. The 150 metres of partially collapsed concourse tunnel that had been filled with concrete following the incident were re-excavated using pneumatic breakers and lined with 8.0 m diameter cast iron segments. The remainder of the СТА Station tunnels was constructed with precast segmental linings. The 9 m diameter shields incorporated hydraulic forepoling, face gates and tables in the top half of the face to reduce settlement and a backhoe excavator in the lower half of the face. Pilot tunnels with a diameter of
3.3 m were also driven ahead of the main shields to further aid control of settlement (Deane et al., 1997).
With regard to the induced settlement of car park MSCP 1 A, the most crucial dates concern the construction of the upline tunnel which was closest to the car park. The pilot tunnel for this commenced 9 August 1995 and was completed beyond the zone of influence of the car park by 4 October 1995. The tunnel enlargement progressed past the car park on 22 December 1995 and was completed on 15 March 1996. Comparative dates for the downline tunnels were for the pilot: 3 November 1995 to 8 December 1995 and the enlargement: 8 March 1996 to 31 May 1996. Breaking out of the concrete for the concourse tunnel started from the southern-most shaft. Tunnelling commenced in both directions from this shaft from mid-January 1996. The concourse tunnel was completed past the car park in August 1996.
Compensation grouting was used to limit settlement during construction of the tunnels. The grouting was carried out from the south escape shaft to limit tilting of the southern lift tower, both pre collapse during 1994 and during the second half of 1995. Compensation grouting was also carried out from the north shaft following similar concerns regarding the northern lift tower. This commenced in February 1996. The resulting heave is particularly evident around chainage 110 (see Section 6.4.4).
Car Park Structure
The main structure of MSCP 1A is an in situ r.c. frame with flat slabs spanning monolithically between the beam and column construction. The columns are generally set out on a grid measuring 7.85 m by 5.49 m, with the longer dimension parallel to the tunnels. However, the outer line of columns on rows A and J is set 3.67 m from the edge of the building and is 9.68 m from the adjacent rows В and H. The columns are similar in both halves of the building and at the ground floor level are typically 0.46 m square.
As noted, the building foundations comprise two distinct types. In the south-western half of the building above the Piccadilly Line tunnels, the structure sits on discrete r.c. pads, which are founded just below ground level on the gravels. The pads are typically 2.9 m square and 0.84 m thick. The other half of the building is founded on piles. The r.c. pile caps, which are constructed at the same level as the pad footings, are rather more variable in size but the dimensions are typically 2.31 m square and 1.07 m thick with an arrangement of five piles beneath the pile caps.
While several of the pile caps are structurally tied together with r.c. ground beams running below the ground floor slabs, these connections are mostly in the central zones of each half of the building and, more particularly, where such r.c. ground beams are present they are aligned parallel to the СТА tunnels (see Figure 6.2). The pile caps of the north-eastern lift tower are also structurally tied together with r.c. ground beams aligned at right angles to the HEX tunnels. The significance of these structural details is discussed in Section 6.5.5.
Figure 6.2 Plan of MSCP IA showing foundation layout and limited locations of ground beams.
