6.3 Schedules, Sequencing, and Critical Path
Key Takeaways
- A WRE schedule must respect physical dependencies, permits, submittals, procurement, shutdown windows, testing, and restoration.
- Critical path method questions require activity durations plus logic, not just a list of tasks.
- Float belongs to an activity path, and using float on one activity can consume flexibility for later work.
- Wet-weather restrictions, bypass pumping, dewatering, utility conflicts, and disinfection testing can control WRE sequencing.
- The shortest construction activity is not necessarily low risk if it gates startup, public access, or regulatory acceptance.
Sequence Before You Calculate
Project schedules in PE Civil WRE are usually tested through activity identification, sequencing, and critical path logic. The math is straightforward only after the dependencies are correct. Installing a water main before pipe delivery is impossible, but so is putting a new main into service before pressure testing, disinfection, flushing, and bacteriological clearance if those steps are required by the project documents.
A critical path is the longest path through the activity network. It controls the project duration because delay on that path delays completion unless the schedule is changed. Float is the amount of time an activity can slip without delaying the project or a defined milestone.
| WRE activity | Typical predecessor | Why it matters |
|---|---|---|
| Permits and notices | Project award | Work in channels, roads, or wetlands may not start without approval |
| Submittal review | Notice to proceed | Pipe, pumps, valves, and controls need approved data |
| Procurement | Approved submittal | Long-lead equipment can control the job |
| Bypass pumping setup | Approved plan | Existing sewer service must remain functional |
| Dewatering | Excavation area ready | Groundwater can control production and safety |
| Installation | Materials, access, utilities | Crews need clear work fronts and conflict resolution |
| Testing and startup | Installation complete | Failed tests create rework and retesting |
| Restoration | Underground work complete | Pavement and site restoration often lag installation |
Critical Path Workflow
Use the same process every time:
- List activities, durations, and immediate predecessors.
- Draw or mentally build the network.
- Forward pass: early start (ES) is the largest early finish of predecessors; early finish (EF) = ES + duration.
- Backward pass: late finish (LF) is the smallest late start of successors; late start (LS) = LF - duration.
- Total float = LS - ES, or LF - EF.
- Activities with zero total float are critical in a simple network.
Consider a sewer replacement schedule. Activity A, permit closeout and notices, takes 15 days and starts first. B, pipe submittal review, takes 10 days after A. C, pipe procurement, takes 20 days after B. D, bypass pumping plan approval, takes 8 days after A. E, sewer installation, takes 12 days after both C and D. F, testing and CCTV inspection, takes 4 days after E. G, pavement restoration, takes 5 days after E. H, final startup and acceptance, takes 3 days after F. Project completion requires both G and H.
The main paths are A-B-C-E-F-H = 15 + 10 + 20 + 12 + 4 + 3 = 64 days; A-B-C-E-G = 62 days; A-D-E-F-H = 42 days; and A-D-E-G = 40 days. The critical path is A-B-C-E-F-H, with a project duration of 64 days. Pavement restoration has 2 days of float because the path through G is 62 days while the controlling path is 64 days.
WRE Schedule Judgment
The exam may ask for the best sequencing decision, not only the duration. For a water main tie-in, shutdown constraints and customer notices may control the order. For a treatment plant upgrade, temporary process capacity may require building parallel facilities before demolition. For a stormwater channel project, erosion controls and weather windows may be predecessors to earthwork. If a delayed submittal controls procurement, adding a second excavation crew will not shorten the project unless installation has become the controlling path.
A schedule path for a sewer project is A-B-C-E-F-H with durations 15, 10, 20, 12, 4, and 3 days. A competing path A-B-C-E-G has durations 15, 10, 20, 12, and 5 days. If completion requires both paths, what is the project duration?
A pump station replacement requires approved pump submittals before fabrication, fabrication before delivery, delivery before installation, and installation before startup testing. Which activity is most likely to reduce the total project duration if procurement is on the critical path?