February 21, 2008

The basic assumption of location-based planning is that working continuously results in higher productivity. Multiple crews working in the same location also result in decreased productivity and increased risk because resources might leave the site and return late.

In my PhD research I collected progress data from three projects weekly. All the projects showed the same result. If multiple crews are working in the same location, work either slows down or stops altogether. Slowdowns and stoppages happened even if the tasks did not have technical logic link with each other. Many tasks can be done in free sequence but they can not be done at the same time in the same location. For example floor covering work or plasterwork interferes with any other trade working in the same area.

Figure below shows a typical example from one of the case projects showing interference. Notice that the actual slopes of all of the tasks are very close to planned slope but only if the tasks can keep the complete location to themselves. Immediately when tasks happen at the same time in the same location, work either stops or slows down considerably. Average productivity loss is 20 to 50 % depending on the particular mix of tasks. In a project of 15 000 m2, I identified over 350 issues where productivity was lost because of these conflicts. It was also scary to notice from meeting memos that subcontractors had the same crew on site but achieved 50 % of production rate when there were conflicts. Think about the effects on subcontractor bottom line...

What are the implications for planning?

• CPM is not a valid planning technique because slowdowns and stoppages happening in locations even without technical precedence have larger effect than a delays on critical path (will be shown in my PhD)
• Buffers are necessary to keep trades far enough from each other
• Weekly location-based controlling is required to achieve productivity benefits and to prevent trades from clashing

For subcontractors, this will affect the bottom line directly. For General Contractors, project contractual milestones become easier to achieve because each task can be produced faster. Schedule risk of the project will decrease because slowdowns and stoppages are by nature unpredictable. Some of the time saving is required for buffers but at least 10 % duration reduction is possible without increases in risk level. Alternatively the same duration can be achieved with much lower risk.