Project scheduling and plan review: Practice example

The following five-step guide allows the civil engineer to schedule a project (and interpret the output of time management software packages).

Graphical representations of a timeline of projects and tasks adopt the following steps:

Step 1: Access and review the preprepared method statements (of the respective element, subelement and civil engineering work classi- fication items) of the tasks that make up the project (as discussed in Chapter 2).

Step 2: Confirm task precedence(s) and establish respective earliest and latest start and finish times.

Assess opportunities for flexible float in the start and end times of individual activities, without affecting overall project duration. Determine the critical path of activities based on an absence of float.

Step 3: Access and review an off-the-shelf software package able to repre- sent a project’s scheduled time frame.

Prepare a Gantt chart of project duration(s).

Step 4: Review method statement (likely task time) durations estimated in step 1 towards acknowledgment of risks, to assess the worst, pes- simistic time if all goes terribly, as well as the best, optimistic time if all goes perfectly.

Use this range of potential durations to assess standard devia- tions from an overall expected time.

Step 5: Use standard deviations prepared in step 4 towards assessing the chances of being able to meet revised time frames imposed in the future by stakeholders.

A scenario might involve a civil engineering project for the supply and installation of 25 standard galvanised mesh rock-filled box gabions of 1 m3

each (with a CESMM4 reference3 _{of ‘X410’ for miscellaneous work associ-}

ated with earthworks) that requires localised rock excavation, cage assem- bly sub-base preparation and then subsequent preparation and installation of gabion(s).

Step 1

Acknowledge phased substructure works in the general scheme of the proj- ect (Figure 3.6).

Break down a method statement of task resources (and respective dura- tions) into

• Substructure element • Earthworks subelements

• Method statement items related to 25, 1-m3 _{rock-filled gabions}

• Excavation of rock fill, approximately 3 weeks • Cage assembly, estimated at 2 weeks

• Sub-base preparation estimated at 1 week

• Gabion installation estimated at 1 week, and 1 week to test

Step 2

Establish earthworks’ activity precedence(s) and the subsequent earliest and latest start and finish times for rock excavation, cage assembly, sub- base preparation and gabion installation and testing based on task duration times determined from step 1.

Determine and complete activity node information (see Section 3.1.3), where T = task times and durations generated previously from step 1 method statement(s).

A ‘forward-run’ through a network of tasks helps to fill-in the blanks for ES = Maximum earliest finish of all predecessors

EF = ES + Activity duration time

A ‘backward run’ through a network of tasks gives LF = Minimum LS of all following activities LS = LF − Activity duration time, with

Float (TF) = LS − ES or LF − EF; if 0, the task is deemed critical and is highlighted

Gantt chart

Task Duration Start Finish

1 Substructure (phased) Mon Nov 7 Thu Aug 3 1.1 earthworks 6 wks Mon Nov 7 Fri Dec 16

Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Figure 3.7 shows a precedence diagram drawn for interrelated gabion installation tasks.

Step 3

After cumulative task precedence(s), input the information into a relevant bar chart spreadsheet or scheduling software package. Figure 3.8 applies to gabions.

Step 4

Using task duration times (step 1), calculate the potential for deviation from timelines by assessing potential expected duration variance(s) related to the civil engineer’s updated (risk) analysis of an on-site activity’s worst possible and alternatively best possible time to complete a task.

Action by highlighting critical tasks and tabulating is found in Table 3.1.

EF=3 LF=3 Ref: 3 LF=3 Ref: 2 ES=0 T=3 LS=0 Excavate rock TF=0 ES=0 T=2 LS=1

Assemble steel cage TF=1

ES=0 T=1 LS=2

Prepare sub-base fill TF=2 ES=3 T=2 LS=3 EF=5 LF=5 Install gabion TF=0 Ref: 4 ES=5 T=1 LS=5 EF=6 LF=6 Inspect/test/end TF=0 Ref: 5 EF=2 EF=1 LF=3 Ref: 1 Figure 3.7 Precedence. 1 Substructure (phased) 1.1 earthworks 5 wk Nov 1 3 wk Nov 1 2 wk Nov 1 1 wk Nov 1 2 wk Nov 22 Dec 8 Nov 22 Nov 15 Nov 8 Dec 9 1.1 excavate rock 1.2 assemble cage 1.3 prepare sub-base 1.4 install gabion

Item Duration Start Finish Nov Dec

Step 5

Use standard deviation and expected times in step 4 to assess if a new maximum deadline imposed by client-stakeholders is possible. In this case, assess the chances of meeting a new, imposed maximum 6-week limit for the installation of gabions.

• Use critical path earthworks items (excavate rock and install gabion) and calculate project variance by adding critical path item variances together.

• Project variance = 0.25 + 0.25 = 0.5.

• Calculate project standard deviation such that.

• Project standard deviation = √0.5 = 0.707 or 0.7 of a week (or 4 work- ing days out of a 5-day working week).

Recall that in a normal curve for gabion installation (Figure 3.9), the most likely time to install gabions is 5 weeks, but that there is a 50%

Table 3.1 Critical tasks

Task Best time (BT), in weeks Likely time (LT), in weeks Worst time (WT), in weeks

Expected time (in weeks), T = (BT + 4LT + WT)/6 Potential variance (in weeks), V = [(WT − BT/6)]2 Excavate rock 2 3 5 (2 + 12 + 5)/6 = 3.17, ≈3.5 [(5 − 2)/6] 2 ₌ 0.25, 0.25 Make cage 1 2 3 (1 + 8 + 3)/6 = 2, 2 [(3 − 1)/6] 2 ₌ 0.11, 0.11 Dig base 0.5 1 2 (0.5 + 4 + 2)/6 = 1.08, ≈1.5 [(2 − 0.5)/6] 2 ₌ 0.06, 0.06 Install gabion 1 2 4 (1 + 8 + 4)/6 = 2.17, 2.5 [(4 − 1)/6] 2 ₌ 0.25, 0.25 Normal curve for gabion completion

5 weeks Expected time to install 20 m³ of rock gabions Standard deviation, 0.707 weeks

chance of completing installation in less than 5 weeks and a 50% chance of completing installation in more than 5 weeks.

If a new maximum of 6 weeks is imposed by the client, rather than the expected 5 weeks, the number of standard deviations from the mean will give the chances of completing by this new maximum deadline. Therefore, to calculate the chance of success:

Chances = (New due date − Expected date of completion)/Project stan- dard deviation

= (6 − 5)/0.707

= 1.41 standard deviations

The civil engineer should retain the value of 1.41 standard deviations and then recall and seek to apply normal curve area tables (with reference to engineering financial first principals calculation reminders and normal curve area tables8_{) to assess the chances of meeting the new 6-week dead-}

line imposed by the client for the work on-site.

If the civil engineer recalls and consults their normal curve areas tables, then the figure of 1.41 standard deviations corresponds to a value of 92% of the area under the normal curve (see Figure 3.10) and, thus, it can be stated that there is a 92% chance of completing installation in 6 weeks.

If the client demands a 99% assurance of completion, then, once again, consultation with the normal curve areas table is required, which will indi- cate that 2.33 standard deviations, beyond the mean of 5 weeks, allows a 99% certainty of gabion installation.

Installation chance: Area under the normal curve 92% chance for Gabions in less than 6 weeks

5 Expected time to install 20 m³ of rock gabions

6 weeks

In other words

99% chance of installation = Likely date + (Tables − Value × Standard deviation)

99% chance of installation = 5 weeks + (2.33 × 1.41) weeks 99% chance of gabion installation = In just over 8 weeks

The civil engineer can then communicate (with relative arithmetical confidence if nothing else) that gabion installation is expected to take 5 weeks and that there is a 92% chance to install the gabions before 6 weeks, and also a 99% chance that the gabions will be installed before 8 weeks.

The scenario above shows how civil engineers can use their knowl- edge of task resourcing requirements to plan and schedule a timeline for a project and also attempt to predict the chances of meeting specific deadlines.

The discussion in Section 3.2 builds on the scheduling techniques pre- sented, to show how the civil engineer might use task resourcing (require- ments) data to speed up a programme of works or to address project delays and bring schedules back on track.

3.2 RESCHEDULING TECHNIQUES TO IMPROVE