Critical Path Analysis at Network Rail
A Network Rail case study

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Page 4: Co-ordinating people via Critical Path Analysis

Major construction projects involve a lot of separate steps. For example, in constructing the viaduct, the 96 pre-fabricated metal decks which made up the main structure of the viaduct were specially made off-site and transported by rail from Ulverston.

Some activities in a project can be carried out simultaneously - for example, while the metal decks were being made, the piers supporting the viaduct were repaired at the same time. Other activities need to be carried out in a sequence, for example, you can't lay the new rail track before the old track has been taken up. We say such activities are 'dependent' upon the completion of the previous one.

Critical Path Analysis

a plan of what needs doing

Critical Path Analysis involves setting out a diagram showing all of the activities involved in a particular project and how they relate to each other. The following network diagram shows the main sequence of activities involved in carrying out the project.

The process

It would be too complicated to show all of the steps in the construction of the viaduct because there were so many operations involved. However, the process can be illustrated by taking an example of part of the process.

In order to simplify this we shall assume that:

A - This part of the process starts with the fabrication of the steel which is expected to take four months.

B - This is followed by site preparation which takes two months. This is dependent upon activity A.

C & D - After site preparation the walkways can be built (two months) followed by the decks (two months). Activity C is dependent upon activity B and activity D is dependent upon activity C.

E - While the walkways and decks are being built the piers can be repaired (which is expected to take five months).This means that activity E is dependent upon activities A and B.

The network diagram

This is illustrated in a new network diagram:

  • The circles in the diagram are called nodes. They show the start or finish of a task. The nodes are split into three sections. In the left hand semi-circle the node number is written.
  • The number in the top right is the earliest starting time (EST). This shows the earliest time the next activity can start.
  • The number on the bottom right is the latest finishing time (LFT). This is the latest that the previous task can finish without delaying the next task. Arrows show the order in which activities take place.

a more detailed plan

For example, in the diagram the earliest activity B (preparing the site) can take place is after 4 months, when the steel is ready. The earliest that pier repairs can start is after 6 months, when the site is prepared, etc.

The critical path

The next step is to set out the critical path. Critical activities are those which most urgently need to be completed on time to ensure the whole project is not delayed. For example, if the pre-fabricated steel did not arrive on time then this would be a serious problem. This shows how important the role of a good Project Manager is. We can illustrate the critical path by highlighting it in red through the arrows on the diagram. This joins up the sequence of activities that will take the longest. The critical path can also be indicated by drawing two parallel lines (shown in blue) across the arrows between nodes. In this case the critical path is through the activities A, B and E. This is because E takes five months whereas C and D together are only expected to take four months. Site managers and contractors must pay particular attention to E (pier repairs) if the whole project is not to fall behind schedule.

Network Rail's Leven Viaduct engineers were dealing with a project which involved hundreds of separate processes. They used spreadsheets and computer models to give them a detailed picture of all of the stages involved. Every day they kept a close eye on where the project had got to. This enabled them to put in extra resources whenever there was a delay, particularly along the critical path.

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