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Six Sigma innovation: Transforming a Victorian tunnel for modern trains

The Crossrail project is the biggest construction project in Europe and Bechtel is supplying project management and execution capability as the project delivery partner (PDP) within a fully integrated team. The scope consists of 13 miles (21 kilometers) of twin bore tunnels and seven new stations. 

The project runs through the center of London and joins train services from east and west of the city including a link to the UK’s largest airport, Heathrow. The work has to be done in a heavily congested area with a number of other metros, rivers and road tunnels in place. An unusual and critical challenge of the job would to modernize the Connaught Tunnel, a Victorian tunnel built for Victorian rail use, to meet the space needs for the new electrified trains.

The challenge

Crossrail uses overhead lines to power the trains, reducing the available space in the Connaught Tunnel, a cut-and-cover tunnel built in 1878, for steam train use. Therefore, the base of the tunnel needs to be deepened by approximately 6.5 feet (2 meters). The deepening of the tunnel is referred to as “invert replacement” and it will place in the east and west ends of the Connaught Tunnel—a total of aproximately 360 feet (110 meters). The east portion of the effort of nearly 250 feet (75 meters) is on the critical path for development and will be addressed first. This work has to be completed as per the project baseline schedule of 30 weeks and at the same time prevent any risk of tunnel collapse during construction. Two early trials of invert replacements took an average of 44.5 days to complete against 21 days included in the project schedule.

  • Image 1: The Victorian Tunnel
  • Image 2: Crossrail design with overhead power lines and invert to be removed and replaced. Invert to be deepened by 6.6 feet (2 meters) for the overhead line equipment (OHLE)

The objective: Reduce the cycle time of invert replacements

Site space and access is narrow and limited. Any attempts to make improvements to cost and/or schedule would be minimal if implemented after commencing site works. The invert replacement process is challenging, slow, expensive, and cumbersome involving numerous serious risks. Strict engineering constraints has been established limiting the constructability options that could be considered while attempting to reduce schedule.

A team composed of members from all stakeholders in the project and led by Bechtel's Six Sigma experts, solved the challenge.

Due to the known schedule risk from this complex and repetitive process, a Six Sigma process improvement project (PIP) was kicked off prior to commencing the works. The team wanted to identify improvements embedded from the start to improve the schedule.

The details

One strict engineering requirement was that the concrete needed to achieve full 28-day strength prior to removing the support of excavation to avoid a potentially catastrophic tunnel collapse due to earth pressures. This requirement would have an impact on the schedule and could not be modified. The PIP team had to come up with ways to reduce the schedule without compromising safety and cost. Careful planning was needed.

Process mapping

An early step undertaken by the team was to map the invert replacement process as anticipated to take place. This step proved to be helpful as it clarified expectations and compromises.

High-level process map for invert replacement

Early risk identification and mitigation

A Failure Modes and Effect Analysis (FMEA) workshop was held to ensure that the agreed solutions were implemented on site removing re-occurring risks. Some examples include:
Removing props too early                  Implement a tagging system for the props after “permit to unload” is granted
Damaged props           Have additional props on site. Incorporate an early warning system to prevent props from getting hit
Rebar will not fit           Use a bright colored template bar to check one rebar per bundle upon material delivery. Use the template to ensure correct bars are being used on installation
Equipment failure           Open an account with nearby equipment rental
Poor quality of concrete finish           Perform an advance trial and agree on methodology

Simulation modeling

By this point, the scope and challenges were identified. It was time to determine whether the proposed plan would work as expected and whether it would support Crossrail’s schedule. Since there was no production data available, iGrafx software was used to create a model to simulate the process based on the proposed plan.

In iGrafx, the process map is drawn; and resources, work hours, task durations, and constraints are specified to incorporate real-life circumstances as close to reality as possible.

Simulation baseline results

The baseline total time predicted by the simulation for replacing the 250 feet (75 meters) of inverts was 38.5 weeks. The schedule for these works was set at 30.5 weeks. Something needed to be done. One of the first things that became apparent was that none of the resources were utilized to maximum capacity. 

Highlighted on the table below is the excavator which was utilized 70 percent of the time and was also one of the resources in greatest demand. What-if analysis conducted using the model highlighted a large reduction in excavator utilization where a second one was added.

The reason behind this anomaly, is that the excavator was used everywhere in the process map, even for short durations, some of which were not considered relevant until identified such as moving rebar. Some of the activities were: breakout, lifting, spoil removal, and even 2 hours of blinding. In conclusion, the excavator was the bottle neck; it was not being utilized optimally.

The evolution of the excavator

The original plan was to have one 22-ton excavator per section. This large excavator was needed for lifting the props and walers. The simulation showed that an additional excavator would reduce total duration. The team needed to decide what type of excavator to add. A mockup was setup on site (outside the tunnel), to measure the range of the 22-ton excavator. It was confirmed that the excavator would not be able to fit inside the narrow tunnel and perform the tasks per the original plan.

A revised plan was created with two 14 ton excavators for each tunnel section. In order to understand what the excavator was being used for, the simulation was run splitting the excavator in three categories: (1) Breakout (2) Lifting, and (3) Spoil removal.

In conclusion, 50 percent of the time the excavator was planned to be used for lifting; 30 percent for breakout; and 3 percent for spoil removal (most of the spoil removal is intended to take place before and after the shift—therefore excluded from the model). In addition, the excavator was “waited for” the most for lifting operations; while the number of instances it was “waited for” breakout and muck away were minimal.

This valuable information was used to make educated decisions. The addition of a second excavator was not necessarily the most optimal solution. Originally, it was thought that the smaller excavator (14 tons) would not be big enough to lift the props and walers, and would not reach far enough to excavate sections of the invert. Vinci/Coffey investigated alternative options for lifting equipment as opposed to the second excavator such as spider cranes.

It was concluded that the 14-ton excavator would be big enough for lifting operations, but not enough for excavating. Therefore, excavation was performed exclusively by BROKKs, freeing up the excavator for the lifting and spoil removal activities. 

The Results

By completing this Six Sigma PIP to reduce invert replacement cycle time prior to commencing site works at the Connaught Tunnel, the team mitigated quality, schedule and budget risks.

In the end, the schedule was reduced by 13 weeks, including an additional five weeks against the project schedule. The additional five weeks were gained as a result of the contractor engaging Yellow Belts to improve focused activities and remove more waste that was already included in the productivity assumptions. In addition, major risks were mitigated prior to starting work such as realizing that the 22-ton excavator would not perform the tasks as expected.