Tube Line’s Use of Six Sigma and Innovation to Deliver Savings
Being economic and efficient underpins Tube Lines’ approach to innovation and change. A quarterly Change Challenge Cup recognizes business process improvements by individuals and teams. The Six Sigma program, established since 2004, identifies opportunities for improving process effectiveness and in this particular case study shows how an innovative solution can drive savings.
Applying Six Sigma to Earth Structures
Tube Lines is responsible for around 80 km of cuttings and embankments along the Jubilee, Northern and Piccadilly lines. Each earth structure has to be assessed to establish its condition and remediation work carried out where the condition of a structure is found to have deteriorated. After carrying out a review of the assessment process, Nick Whelan, the Black Belt, and the Six Sigma team found that the techniques for measuring movement of the slopes were not entirely accurate, resulting in remedial work being undertaken on some structures unnecessarily. This was costing Tube Lines over £1.5 million in unnecessary maintenance costs.
Looking over the Historical Measurement Data for Tube Line’s Six Sigma Project
The primary metrics for this Six Sigma project were the meters of the asset going through to detailed assessment. The secondary metrics were progress towards benchmarks, variation of asset condition assessment on inspection and the cost of maintenance per meter of asset.
Consistency of measurements was ranked as the most important impact on processes. Tape measures and compasses were used to measure slopes. Other important factors were information gathering and the passing of that information. This is contained within the inspection form; from this the data is taken that dictates the scores. Of primary importance are the inspectors who carry out these surveys and their understanding of what the information is to be used for.
There was a vast amount of data, so the Six Sigma team calculated the minimum asset sample size using the standard deviation. Over the years this sample slope appears to have varied in length by 14m. The angle of slope has varied by 7 degrees and height has changed by 7m. Direction has changed and the form of the slope has varied. Data fluctuation was repeated across all 36 slopes examined.
This was emphasised when the Six Sigma team plotted the data on separate graphs using trend analysis. The measurements are around the average, suggesting that we are measuring around the true reading and not on it. The trend shows a gradual fall in the length of slope over the years. (Click on diagram to enlarge.)
Here we see another slight fall in the angle over the years but again the measurements seems to be around the average. (Click on diagram to enlarge.)
These recurring themes throughout all the historical measurements reinforced the suspicion that the data was not accurate. The data was analyzed using scatter plots and correlations between the measurements were not as distinct as they should be.
The same triangular slice of a slope was measured time after time. It was expected that the measurements would be repeated year in, year out, giving a virtually straight line of plots showing far greater correlation. However, from these results it seems the measurements of the height, angle and length of the triangle are not related. The Six Sigma team had to find out why.
Understanding the Variation in the Historical Data for the Six Sigma Project
Track marker plates show direction of travel and meterage at each point of inspection. These are at 100m intervals, at the axis of the transect, and are moved by the track gangs although normal wear and tear sometimes means they are missing. When they are moved it means we start at a different point than the previous year. If they are missing the distance is often paced out, which does not give us the accuracy that is required. Both of these scenarios would give us the variation seen in the historical measurement.
To add to this variation, the axis of the transect is the start point of the inspection. London Underground standards state that we can measure from 5m either side of the axis, giving a further 10m in which to compound our variation of measurement. When the movement stated above is taken into account there is great potential for error. If track plates are missing and our pacing is 10m out of true, we could measure 20m from the intended point of inspection. Within this distance the angle, length and height of the slope will appear to have changed, but we are now actually measuring a different slope. Essentially the measurements will indicate that the slope has changed but in reality it may not have.
The other factors to be considered are the tools used to measure these slopes. Presently these are a tape measure and a compass with inclinometer. The tape measure is occasionally caught in vegetation, which exaggerates the measurement. The inclinometer is around 80mm long and therefore insufficient to gain an accurate measurement of the angle of a 20m slope. (Click on diagram to enlarge.)
Coming Up with the Six Sigma Project’s Solution
Failure mode effects analysis was used to examine the faults and then work out the most suitable remedy along with a decision matrix to verify the best fix. After much discussion and robust trials the Six Sigma team chose the smart rover global positioning system manufactured by Lecia. This uses the Russian and American satellite systems and gives measurements within 30mm of accuracy.
It also has the capability to provide the height, length and angle of the slope without further calculation. We can store the inspection form on the device so it can be transferred straight to Tube Lines’ systems or via GPS. Multiple slopes can be measured using one device, therefore cutting down on the time previously taken both carrying out the inspections and transferring the data.
Comparisons showed the old system was only 16 percent capable of measuring height and angle and 25 percent capable of measuring length when it was compared directly with the solution. The solution has reduced variation by 95 percent enabling our asset engineers to rely on the data and be sure that any variation truly warrants further inspection.
Results from the Six Sigma Project
The result is that the team is now able to target remediation of the earth structures where it is actually needed, and reduce the cost of maintenance per meter of cutting or embankment. Groundbreaking improvements were made with an estimated £2.6 million saving from the company’s budget. The process is now being used as the new benchmark by others in the transport and engineering industry.
The Six Sigma team entered this case study in its own inaugural Change Challenge Cup competition and won second prize in the last quarter of the 2008 competition, the judges noting that they were impressed by the level of savings achieved and the accurate record of our assets that the process delivered.
Innovation and process improvement have always played an important part in Tube Lines with a strong Six Sigma program sponsored by its chief executive. The use of Six Sigma across the business is reaping tangible benefits, including reduced cycle times for our station upgrades, lower project costs generally across the board and an improvement in the quality of the work we are delivering.