Heathrow rises from the mire and shines - TunnelTalk
Heathrow rises from the mire and shines JUN 1996
Shani Wallis, TunnelTalk
There is now only a limited amount of excavation left to do and despite the long delay of almost a year between getting NATM work underway again after the collapse, the delay on the original project programme has been limited to 6 months. Trains on the Express Link are scheduled to start running between Paddington and the CTA and T4 stations at Heathrow on 1 June 1998.

Whatever jinxed the job in the beginning, all appears to be well under control at Heathrow Airport in London. The tunneling now being carried out for the Express Rail Link project looks faultless. The application of men and machinery and project resources is impressive and the attention to detail is meticulous.
While an event on the scale of the tunnel collapse which occurred on the project back in October 1994 will always focus the mind. The question remains - if it is being achieved now why wasn't it back then? The answer to that though, is the subject of an in-depth investigation by the British government's Health and Safety Executive (HSE). The report of which is unlikely to appear for several months yet. Material evidence is still being collected from the site and the possibility of legal proceedings as a result remains a Sword of Damocles over the project and the parties involved.
Therefore, it was made perfectly clear to journalists attending an all-party-organised site visit and technical briefing in late June 1996.that questions about the causes of the collapse and the costs thereof would not be answered. The journalists however were treated to a thorough briefing on the project's current progress and to visit underground into various sections of the on-going works.
Knowing exactly how the current quality of workmanship and construction supervision differs from what was going on before the collapse may never be known perhaps, but it was explained in some detail how what is now being built is different from the original.

Giant shaft rescue
To start, the most obvious, most extensive and clearly most expensive, modification to the original scope of works is the enormous 60 m diameter x 30 m deep cofferdam being built in the Central Terminal Area (CTA - Terminals 1, 2 and 3) to almost totally encompass and remove the extent of the collapse and the disturbed ground adjacent. Only by standing at the edge of this huge hole can its scale be appreciated. "The upper 20 m through soft water bearing ground and gravels is supported and stabilized by 1.2 m diameter Secant piles which interlock on 1.06 m centres," explained Mr. Alan Powderham, Managing Director of the Foundations and Geotechnical Division of Mott MacDonald and the lead designer's representative on the so-called Solutions Team convened immediately following the collapse to engineer a safe and economic way forward. "The London Clay below is supported by 900 mm diameter contiguous piles which step in from and continue a further 20 m below the Secant piles. "A substantial capping beam at ground surface forms a rigid concrete collar linking all the piles together.
"Originally, recovery ideas were instinctively based on excavating a large box of about 65 m x 65 m," said Mr. Powderham. "However, a rectilinear structure would have required between 5-6,000 tonne of propping. The circular concrete-lined cofferdam eliminated this propping and reduced the amount of excavated material by a substantial 30,000 m3."

Once the giant cofferdam was chosen as the method of recovering the collapse situation, piling by specialist subcontractor Stent Construction began in early October 1995. Piling went very well, it was said, and was completed in early February 1996. Central excavation in 1 m lifts began in early March 1996 and was completed by mid-July 1996. Once exposed, each lift was lined with the shaft's 1 m thick in-situ concrete lining, the central dumpling of earth contributing to shaft stability until later excavation.
Excavation of the core actually progressed so well that the initial 1 m lifts were increased to 1.2 m to speed progress. The construction teams also reduced the original 7-day cycle per lift to 5 days. Another technique adopted to expedite the programme was the idea of 'portalisation'. Immediately outside the cofferdam perimeter are four 32.5 m deep access shafts sunk down onto the station tunnel alignments. The two to the south are 10.7 m in diameter and are permanent ventilation shafts. The two to the north are 15 m in diameter and were added to allow early access for continued station excavation at the CTA and possible permanent uses for them are being studied. All four however were sunk to allow station tunnel excavation works to continue without having to wait for full excavation of the cofferdam. This was important in efforts to gain early access for the track laying progress.

'Portalisation' was the idea of completing the short lengths of station platform tunnel short lengths of station platform tunnel from these access shafts back into the cofferdam and creating the in-situ concrete portal structure before full excavation of the cofferdam had reached base level. At the end of June 1996 as we visitors gazed with amazement into the big hole, core excavation had reached lift 19 of 25. The top of the portal structures of the two station tunnels from the south were exposed and the equipment left buried in the 70 n/mm2 concrete and foam concrete used to immediately backfill all the excavated space after the collapse was being uncovered. As the cofferdam excavation had gone down. The original 28 m deep x 19 m diameter access shaft had been broken out and removed and it was now time to unearth the collapsed tunnels and dig out the evidence. Teams of independent observers representing the client added to the hive of activity in the bottom of the hole, watching as each bucket load of dirt was removed and logging all possible indications as to why the tunnels collapsed in the first place.
Given the size, cost, quality of construction and time taken to build it, surely the cofferdam was not going to be back-filled and forgotten like a structure that is, but should never have been? No. It is to be incorporated into the final design of the CTA station. In addition to accommodating the station's lifts and escalators, the huge hole actual provides potential for some 5,000 m2 of valuable space which is being safeguarded for possible development of underground car parking or office space. The 255 anchor piles to be sunk into the floor of the cofferdam to hold down its base slab are also designed to carry the weight of a possible seven-story building above. If BAA, the private owners of Heathrow Airport and principal client for the current Express Link Project, decides, this could be a top-quality hotel or office development built right in the heart of the airport and on the rapidrail transport link into London. The potential is there and development studies are underway according to Mr. Rod Hoare. Group Rail Strategy Director for BAA.

Tunneling Culture
Following a look-see at the cofferdam, the group of visiting journalists was taken underground into the main crossover of the twin-tube railway tunnel alignment, a structure which will be largest span NATM cavern ever built in London Clay. At its widest the cavern will be 13.33 m high x 16 m wide. At its narrowest it is 9 m high x 16 m wide. At its narrowest it is 9 m high and 10 m wide. The crossover with its narrow-waisted bow shape, is 98 m long and for the sake of scale was compared to the Channel Tunnel's UK NATM crossover chamber which was 160 m long x 15.4 high and 21.2 m in span (256 m2) in its primary lined state. Located on the south side of the CTA station, the crossover was accessed from the 6 m-diameter shield-driven and precast concrete segmentally-lined running tunnel which was driven from the Sealand Road access shaft. The sequenced excavation of the chamber to full cross section then progressed from the enlarged central section. The NATM excavation phases progressed in 1 m steps and on a top heading-bench, top heading-bench and double invert sequence. Average advance at each of the many concurrent NATM faces was said to be 1 m/day working two 12 hour shifts/day, seven days/week. While excavation continues, one of the running tunnel shields is working its way towards the crossover from one of the access shafts outside the cofferdam perimeter. Once it has broken through it will be turned and relaunched to complete the parallel running tunnel back toward the cofferdam stopping in the adjacent access shafts. The short length between the access shafts and the cofferdam has been excavated using the 'portalisation' system described above.
As was the case on the Jubilee Line Extension project for London Underground, all current NATM operations in London Clay were suspended following the collapse at Heathrow. Without any immediate explanation as to why the NATM work collapsed at Heathrow on October 21, 1994. The attention, along with questions concerning workmanship, operation, quality assurance certification et cetera, turned to possible inadequacy of design. The method itself was not con-demand and while several reviews of the applicability of the technique in London Clay were commissioned [1,2] and parts of the NATM works on the Jubilee Line.

Project had to be converted from NATM to traditional methods for programming reasons [3], the technique has survived and continues to be used on all original NATM projects in London.
The collapse however did lead to many NATM design and excavation sequence modifications. Many would say that the changes were as a result of knee-jerk reactions to the collapse but nonetheless, they have been imposed. Together with increasing substantially the robustness of the original designs, the changes have also removed a large portion of the risk which is, or was, expected to be monitored, managed and controlled under the original design and excavation phases for the various NATM projects in London. The collapse appears also to have introduced a greater appreciation of the method among all concerned, from senior company management to those at the face. There is undoubtedly greater attention to detail not only among the operatives, but also towards the batching and quality control of shotcrete; the recording, collating and interpretation of geotechnical instrumentation data; and the presence on site, at all times and in sufficient numbers, of qualified and experienced engineers.
A major point emphasized during the briefing to the technical press at Heathrow in June was the 'culture' developed for the project perhaps from the beginning but certainly re-enforced since the collapse. This culture, it was explained, centres on five main principals:
A single team approach to eliminate the lateral 'them' and 'us' demarcations between the parties involved
Supportive management to eliminate the horizontal divisions between levels of management from foremen and work force to the most senior managers
A pro-active and continual process of risk management not just at the beginning of the job but reassessment throughout the full construction period
Team building with front line focus groups dealing with any problems which may arise on a daily basis and bringing more serious problems more immediately to the attention of more senior staff
And promotion of total 'can do' positive thinking. The single-team approach to project completion was described by Mr. Hoare as "totally unusual and very. Very different to other approaches used in the past."

If there were any rifts between the major players after the collapse they have certainly been pulled together. It is a united project from which responded to questions by the collapse may journalists and given that the collapse may well yet be the subject of legal proceedings. This attitude is likely to change in the short term. The continuation of the construction civil works is the result of a confidential agreement between BAA, the client and Balfour Beatty, the UK construction company which won the tunneling contract for the project back in January 1994 as the lowest of six bidders and for a contract award price of £60 million. Media speculation has put the cost of the tunneling contract now at more than £200 million but this has not been substantiated. It is also reported that Allianz, the all-party. All-risk project insurer will cover this extra cost but again this is not confirmed. Balfour Beatty's contract is extensive covering the total 8.8 km of shield-driven. Segmentally-lined running tunnels and the two underground stations at CTA and Terminal 4. Tarmac Construction has completed the 50 m of surface works from the tunnel alignment to the rail with the existing Great Western main-line railway from Paddington Station in London to the west. Tarmac is also the track-laying contractor and the Laing Bailey joint venture has been awarded the mechanical and electrical equipment installation contract.
Claims from third parties arising from the consequences of the collapse were said to be minimal. "A few private citizens claimed compensation for having missed their flights due to the fact that one of the car parks was closed immediately after the event," explained Mr. Hoare for the Heathrow Express (HEX) project team. "There is also a claim currently being considered from London Underground arising from the fact that the Heathrow extension of the Piccadilly Line to Terminal 4 was closed for 10 days. The trains terminating at the CTA station. Other than that, the airport didn't actually close at all and there was no impact on airlines or other third party operators in the airport."
The unified response to the collapse by the major parties was made possible; it was said, by the New Engineering from of contract adopted for the project by BAA. The terms and conditions of the NEC (New Engineering Contract) document eliminates the clauses which, under other more commonly used contract documents, create an adversarial climate among the parties involved. The comment was made that under an ICE 5th Edition type of contract, for example, the work at Heathrow might still be on hold waiting for some kind of break-through which would allow things to move forward. This is perhaps an exaggeration but the point is clear. "The NEC allowed a "common sense" approach to contracting," said Mr. Hoare-and it is, all parties to the contract were involved in the Solutions Team established within days of the event, and resumed after a brief pause for thought.

The current 'seamless' approach to project management builds on an organization shuffle undertaken back in June 1994, four months before 'the event'. In the early days, Taylor Woodrow was the 'construction manager' to BAA for the project. This role has since been absorbed into the HEX (Heathrow Express) project team in which there are some 40 to 50 Taylor Woodrow employees. BAA also strengthened its involvement in team by increasing its staff numbers. Mott MacDonald continues in the team as the principal designer for the job and Balfour Beatty is the team's tunnel construction partner.
Geoconsult, the specialist NATM tunnel designer involved on the project is also part of the team. Originally Geoconsult was engaged by Balfour Beatty as the designer of the primary lining shotcrete support when this was considered as 'temporary works' and therefore the responsibility of the contractor. The responsibility for the primary lining design however has since shifted from Balfour Beatty to the HEX team. Mott MacDonald is now responsible for the design of the primary lining. As such Geoconsult is now a specialist sub-consultant to Mott MacDonald. There is a team of Geoconsult engineers in the on-site engineering staff with one appointed to each NATM shift.
NATM Redesigns
Restart of NATM works at Heathrow after the collapse-as was also the case with the works at waterloo and London Bridge Stations for the Jubilee Line Extension [3]-was as a result of an extensive review of the technique and its construction sequences by the HSE. Not until a series of design and construction sequence modifications were adopted would HSE allow NATM work to resume by submitting its now familiar 'letter of no objection'. These reviews and revisions took some time and it was not until September 13, 1995, some 11 months after the collapse in October 1994, that NATM at Heathrow for the Rail Link project resumed. According to Mr. Alan Finch, project Director for consultants Mott MacDonald, the main changes fall into seven categories:
The thickness of the initial immediate shotcrete support was increased by 50 mm on average.
The shape of the tunnel profile was changed slightly and the lattice girder rings extended through the invert
The quality of the shotcrete was re-examined to confirm consistency of the mix and the quality of the end product
The excavation sequences were reviewed and consecutive excavation of three parallel NATM tunnels in the stations areas was prohibited. The excavation of station tunnels at CTA have reverted to shield-driven and hand-driven excavations through the disturbed zone and concrete-filled NATM excavations of the collapse. At T4 the two outside station platform tunnels must be complete with their in-situ concrete final lining before excavation of the central concourse tunnel is allowed to begin
Initially, face bolts were introduced to stabilize larger face excavations. In a 9 m high x 8 m wide NATM station platform tunnel face at T4 (for example) a pattern of about 20.18 m-long face bolts were installed. The tunnels then advanced 8 to 10 m before the next set of face bolts were installed. Initially, fiber glass bolts were used but the splinters of fibre glass produced as the excavator broke them out produced a spoil which required special disposal according to UK regulations. Steel bolts were then substituted and the ends burned off as they were uncovered by the advancing excavation. Face bolting is no longer necessary in current NATM works.
The application of compensation grouting was reviewed and more strict guidelines imposed
A more structured system of collecting, collating and interpreting geotechnical data recorded by instrumentation was introduced.
NATM advances
Although some might suggest that the above changes place unnecessary constraints on the basic philosophy of NATM and its use in soft ground, despite this, BAA as a buyer of a NATM construction has allowed some major advances with the technique in the UK. The most significant is the use of shotcrete not only as the immediate

support but also as the final lining. Wet shotcrete using MBT equipment and admixtures provides the final lining in the main crossover chamber, in the 'turnouts' of the running tunnels into and from the stations and in other non-public areas of the stations.
The final permanent lining shotcrete is designed to the same load bearing criteria and specifications as an in-situ concrete lining. It is 350-400 mm thick and reinforced with 40 kg/m3 of Dramix steel fibre, the fibre is included principally to help limit shrinkage cracks rather than increase load bearing capacity. A trip to the terminal 4 (T4) turnout from the CTA station saw the application of permanent shotcrete lining in action.
To control the application, the tunnel length is divided into 2.5 m wide bays using metal stop-end strips fixed to the inner shotcrete shell. The steel-fibre reinforced final shotcrete is then built up in sections around the profile starting with the first side-wall blocks up from the in-situ concrete floor slab. Surprisingly the 350-400 mm thickness of shotcrete over a 2.5 m x about 3 m area was going on all at once without any mesh or reinforcement other than the steel fibres. The final lining has little by way of additives except those required to assist wet shotcrete pumpability. Despite this, Mr. Alan Myers, project director for Balfour Beatty Civil Engineering, explained that the final steel fibre reinforced shotcrete was acquiring early strengths of some 40 n/mm2 within 7 days and continued to gain strength with time achieving some 54 n/mm2 at 28 days and 61 N/mm2 at 120 days.
In the T4 turnout, the visitors-at the end of the shotcrete cycle-witnessed the operatives grading the shotcrete by pulling a leveler over the finished section using the stop ends as guides rails. On to this is sprayed a 50 mm thick layer of cement and sand gunite or mortar to achieve a smooth final finish and cover the protruding steel fibres. Onto this is sprayed a liquid curing agent to ensure a crack-free set of the mortar.
For its wet mix operations, Balfour Beatty is using both MBT equipment and MBT admixture. There are seven Suprema shotcrete pumps now on the project and two Robojet nozzle booms. For the primary shotcrete, the company is using the Delvocrete hydration control stabilizer and the Delvocrete S-51 sodium aluminate with the Rheobuild 3520 superplasticiser. The final permanent lining is a richer mix of Meyco SA 145 alkali-free non-caustic liquid activator with the Rheobuild 2000 PF superplasticiser and the Delvocrete stabliser.
Initially, Balfour Beatty mobilized with dry mix shotcrete using SBS dry-mix equipment and Shotax additives from Lafarge. Wet-mix using MBT technology was introduced in the last quarter of 1995 and is now the principal method used on the project. According to Mr. David Ingle, Balfour Beatty's site agent for the shotcreting works, the change to wet-mix shotcrete. For the primary lining initially, was promoted by several factors, "Wet mix shotcreting is more highly productive. We are applying plus 6 m3/h now instead of the 3 m3/h achieved with the dry mix method. There is less rebound at about 10% for hand held spraying as opposed to 20 to 25%. There is minimal dust when using the wet mix and with the new MBT admixtures now available the quality of the product is superior. High early strengths of up to 25 N/mm2 in 24 h are possible and the shotcrete continues to gain strength with time which is another major advantage of the wet mix over the dry. These cost, heath and quality advantage of the wet mix over the dry. These cost, heath and quality advantages of the wet mix product supported its adoption as the permanent lining where appropriate."

With 350 to 500 mm of immediate shotcrete, reinforced with three layers of wire mesh, plus 350 to 400 mm of final steel fibre reinforced shotcrete. The thickness of the lining in the larger excavations is now some 700 to 900 mm in total. However, where appropriate and in smaller NATM tunnel cross sections, a combined initial/final lining design concept will be used. Such a composite final lining is currently being used at Heathrow on the baggage tunnel facility between T4 and the CTA terminals. For that project, the contractor (Miller Civil Engineering) and its design and construct partners (Betonund Monierbau of Austria) and with consultants Ove Arup as independent design checkers for BAA, has completed the 8 m diameter NATM chambers at either end of the shield-driven tunnel - which is lined with an expanded steel-fibre reinforced precast concrete segmental lining with a 'CombiShell' shotcrete lining in which a certain capacity of the initial support shell is considered contributory to the final lining of the NATM works on the baggage tunnel project is 275 mm [4].
Settlement Control
During the media briefing at Heathrow, Alan Finch of Mott MacDonald went on to discuss the compensation grouting techniques used to limit ground movement and settlement as the tunnels came through under major airport structures. As designed, the railway alignment takes the tunnels directly beneath the terminal building. The airport runways and taxiways and roads railways and car parking facilities. At T4, the platform station tunnels pass narrowly under the floors of the underground ticketing hall of the London Underground system's Piccadilly Line station and under a foul sewer pumping station. Clearance under the ticketing hall is 5 m and only 2 m under the pumping station.
"Under all structures, we have limited the horizontal tube-a-manchette arrays for compensation grouting to a minimum of 5 m above the advancing Express link tunnels," explained Mr. Finch. "However, because of the limited distance between the tunnel and the Underground ticketing hall, we decided not to use concurrent compensation grouting. Instead we injected grout through the arrays before tunnel excavation began and pre-heaved the floor by 15 mm. The systematic face control during excavation of these tunnels and there was a contingency but this was not required. Excavation of the tunnels was completed back in January and February 1996 and since then the floor of the hall has returned to its original ground 'zero' level.

The final steel-fibre rein-forced permanent lining shotcrete is applied in 2.5m wide bays and between stop ends made of metal strips

Settlement under all other structures, it was said on several occasions, "has been less than that predicted". Settlement and the instrumentation installed to monitor the inevitable movement caused by ground excavation is now an obvious focus of project attention. How this differs from the situation before the collapse was not discussed, but the instrumentation installed to monitor movement during the excavation of the huge cofferdam. For example, is comprehensive. All reading from these instruments are logged daily and for some more frequently. The data is gathered at a central on-site monitoring station and meeting are held daily to discuss the readings and their interpretation. Data gathered from an extensive array of instrumentation installed for excavation of the cofferdam indicated a maximum movement of the piled wall of 13 to 14 mm once excavation had reached about 24 m deep. The capacity of the piled wall, according to Mr. Powderham of Mott MacDonald is 70 mm. An amber level of 50 mm had been set for the point at which contingency measures would have been implemented had they been necessary.
In short
To recap, all is now going extremely well at Heathrow. A better standard of NATM work and workmanship overall would be very difficult to find anywhere in the World. A full team effort attitude towards completing the project is evident. The hourly paid work force on the tunnelling job is at its peak of some 900 (in May 1996 and up from 400 workers in October 1995) which, when added to the senior management staff and project employees appointed by BAA, Balfour Beatty, Mott MacDonald and other companies associated with the project, totals about 1,250. By June 1996, 800m of tunnel was left to excavate. The two 2.8km long twin tube running tunnels from the transition portal with surface works near the existing Paddington-Reading railway line to the CTA station are complete. The 2.2km long single track running tunnel from the double crossover south of the CTA station to the T4 station is also complete. The only tunnelling left to do is the two running tunnel legs to link the large cofferdam structure to the double crossover facility and the second 900m running tunnel link from the T4 turnout into the T4 station. Meanwhile, by June 1996, all NATM excavation of the two platform tunnels for the T4 station are complete with excavation of the central NATM concourse about to start. Excavation of the T4 turnout is complete and NATM excavation of the double crossover chamber was about 75% complete. In the CTA area, the two station platforms are complete and excavation of the concourse through the concrete-filled collapsed zone has started. The section of tunnel adjacent to the Sealand Road shaft near T4 incorporates the NATM trial tunnel which was designed and supervised by the Dr. Sauer Company, excavated by the Kier/Kunz joint venture, and completed three years ago to substantiate the choice of NATM for the CTA and T4 stations [5].
It has often been said that it takes a collapse for all parties involved to appreciate the classicism of NATM and the limitations of any type of tunnelling or civil engineer. It would be nice to believe that the approach to the project currently evident at Heathrow was there before the collapse but there is no doubt that 'the event' has certainly focused the minds. It can only be hoped that the current standards of work and attitude toward civil engineering design and contracting can be sustained and promoted throughout the UK tunnelling industry without any relaxation.
1. Safety of NATM with particular reference to London Clay: The UK Health & Safety review: HSE Books PO Box 1999, Sudbury, Suffolk CO10 6FS. Price £25.00
2. Sprayed Concrete Linings (NATM) for tunnels in soft ground. The Institution of Civil Engineer's (ICE's) guidelines. Thomas Telford Services Ltd 1 Heron Quay London E14 4JD Price £17.50
3. 3. The Jubilee Line Extension Volume 2 supplement of World Tunneling: Distributed with the February 1996 issue
4. Heathrow baggage tunnel: World Tunneling October 1994. P319. Tunnels & Tunneling, July 1995. P17 and Tunnels & Tunneling, June 1996. P11
5. NATM trial tunnel at Heathrow: World Tunneling. June 1992. P242

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