Civil - Construction Planning And Scheduling
Precedence Definition for Site Preparation and Foundation Work
Suppose that a site preparation and concrete slab foundation construction project consists of nine different activities:
A.Site clearing (of brush and minor debris),
B. Removal of trees,
D.Grading general area,
E. Excavation for utility trenches,
F. Placing formwork and reinforcement for concrete,
G.Installing sewer lines,
H. Installing other utilities, I. Pouring concrete.
Activities A (site clearing) and B (tree removal) do not have preceding activities since they depend on none of the other activities. We assume that activities C (general excavation) and D (general grading) are preceded by activity A (site clearing). It might also be the case that the planner wished to delay any excavation until trees were removed, so that B (tree removal) would be a precedent activity to C (general excavation) and D (general grading).
Activities E (trench excavation) and F (concrete preparation) cannot begin until the completion of general excavation and tree removal, since they involve subsequent excavation and trench preparation. Activities G (install lines) and H (install utilities) represent installation in the utility trenches and cannot be attempted until the trenches are prepared, so that activity E (trench excavation) is a preceding activity. We also assume that the utilities should not be installed until grading is completed to avoid equipment conflicts, so activity D (general grading) is also preceding activities G (install sewers) and H (install utilities). Finally, activity I (pour concrete) cannot begin until the sewer line is installed and formwork and reinforcement are ready, so activities F and G are preceding. Other utilities may be routed over the slab foundation, so activity H (install utilities) is not necessarily a preceding activity for activity I (pour concrete). The result of our planning are the immediate precedences shown in Table 1-1.
With this information, the next problem is to represent the activities in a network diagram and to determine all the precedence relationships among the activities. One network representation of these nine activities is shown in Figure 9-5, in which the activities appear as branches or links between nodes. The nodes represent milestones of possible beginning and starting times. This representation is called an activity-on-branch diagram. Note that an initial event beginning activity is defined (Node 0 in Figure 9-5), while node 5 represents the completion of all activities.
Alternatively, the nine activities could be represented by nodes and predecessor relationships by branches or links, as in Figure 1-6. The result is an activity-on-node diagram. In Figure 9-6, new activity nodes representing the beginning and the end of construction have been added to mark these important milestones.
These network representations of activities can be very helpful in visualizing the various activities and their relationships for a project. Whether activities are represented as branches (as in Figure 1-5) or as nodes (as in Figure 1-5) is largely a matter of organizational or personal choice. Some considerations in choosing one form or another are discussed in our website.
It is also notable that Table 1-1 lists only the immediate predecessor relationships. Clearly, there are other precedence relationships which involve more than one activity. For example, "installing sewer lines" (activity G) cannot be undertaken before "site clearing" (Activity A) is complete since the activity "grading general area" (Activity D) must precede activity G and must follow activity A. Table 1-1 is an implicit precedence list since only immediate predecessors are recorded. An explicit predecessor list would include all of the preceding activities for activity G. Table 1-2 shows all such predecessor relationships implied by the project plan. This table can be produced by tracing all paths through the network back from a particular activity and can be performed algorithmically. For example, inspecting Figure 1-6 reveals that each activity except for activity B depends upon the completion of activity A.