Prevent the “Blow Fly” Effect on the Construction Jobsite

I have observed several times that subcontractors have resources on site even though there is no productive work for them. The crews (or rather the individual workers) can do a bit of work here and a bit of work there. But much of the time is spent looking for work or waiting for instructions. In our book, Location-Based Management for Construction (Kenley & Seppänen 2009) we called this the “blow fly effect” (workers buzzing around looking for work).  Naturally, this is very expensive for the subcontractor.  So I have started asking the question of why are they still there. Wouldn’t it be more economical to leave and return when more work was available?  The answer is yes from subcontractor perspective - if they have more productive alternative resources.  From the General Contractor’s perspective, the sensible answer depends on circumstances, but the most common answer is that the resources need to stay on site.


blowfly effect in construction 

Caption: Like flies buzzing around the back porch, construction workers are often found milling around the jobsite looking for any work they can pick up. But which is better: to let them demobilize and risk a timely return or keep them occupied with other tasks?  One best practice is to plan a work backlog for each trade – work in non-critical locations like a parking garage.


Letting the resources leave increases the schedule risk of the project per basic LBMS theory (please see video #3 – the difference between paced and asap tasks in the Schedule Planner video training series).  It may not be possible to get the resources back in a timely manner when work does become available or different workers may show up with their own starting difficulties.  In any case, momentum is lost on the project.  In terms of short disturbances of workflow it probably makes sense to have the workers wait and do small unproductive work.


IF the duration of work stoppage is long, it makes sense to demobilize.  Having resources on site who have nothing productive to work on is a drain on management capacity.  Superintendents do not let the workers stay idle. They are actively looking for work.  This diverts their energy from solving the actual root problem. Productivity claims are also a real risk. Keeping the resources on site adds cost and productivity claim risks. It also causes a lot of issues with starting too many locations at the same time, overlapping work in locations, etc., which in turn decrease the productivity of other subcontractors and may cause the site to become chaotic – not controlled by any plan (see Keep Chaos Out of the Construction Schedule for a discussion of out-of-sequence and out-of-logic work and When Activity on the Construction Site Can Mask Productivity Issues for a discussion of scope confusion and productivity loss when subcontractors are in the same location).


A better solution is to plan work backlog for each trade.  Work backlog can be tasks of small importance; for example, tasks with a lot of float and no real dependencies which can be done flexibly when required.  Work backlog can also be non-critical locations. For example, a parking garage is often not on critical path and workers can be diverted there when they run out of work in critical areas.  Best projects establish these work backlogs explicitly and only use them to balance times of low utilization.  If work on critical production tasks is continuous (good production control!), these work backlog tasks are done after finishing all work on critical tasks.


Be sure you are caught up with all the articles in this series:

Part 1: Beyond Start Dates

Part 2: Get the Subs Involved

Part 3: Manpower and Suspensions

Part 4: Control Actions

Part 5: Planning the LBS

Part 6: Clarifying Scope

Part 7: Look-Ahead Plans

Part 8: Running Parallel Schedules

Part 9: Resource Graphs

Part 10: Production Rates and Location Sequence

Part 11: Using 5D with LBMS in Subcontractor Meetings

Part 12: Deliveries and Lay-Down Areas

Part 13: Starting As Early As Possible Will Hurt Your Project

Part 14: Buffers Are Important for Production Control

Part 15: Implement Control Actions as Soon as Possible

Part 16: In LBMS It Is Possible to Work Too Fast!

Part 17: Keep Chaos Out of the Construction Schedule

Part 18: Additional Types of Out-of-Sequence Work

Part 19: A Lot of Activity, But Little Progress

Part 20: How to Handle Inspections and Design Changes




Tags: lbms, work backlog for each trade, productivity claims risks, location-based management, subcontractor demobilization risk, production control best practice, blow fly effect