Special features of scheduling tunnel works - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 285

Time‑distance diagram. A tunnel is, except for branched caverns, normally a linear
structure with the built volume expanding linearly, with the technical and operational
dependencies being arranged in a clearly apparent row. For representation in a time-
distance diagram, the tunnel length is shown on the x-axis and time on the y-axis, or
sometimes the other way around (see Fig. 7-3). This depiction can simultaneously il-
lustrate the temporal, spatial and capacity coordination of the activities, so the time-
distance diagram is now predominant for the scheduling and control of the progress of
tunnel construction.
Critical path diagram. A critical path diagram is a graphical depiction of the sequence
of activities as nodes and arrows. This planning tool is predestined for the general plan-
ning of complex construction projects and for the planning of the sequence of complex
processes. Critical path diagrams include the processes of analysis, description, planning,
control and monitoring based on graph theory. Time, cost, equipment and other influential
factors can be taken into account [138].
Critical path planning was first developed in 1956 and 1957 for the space industry and for
production changeovers in chemical plants. The known methods “Critical Path Method”
(CPM) and “Program Evaluation and Review Technique” (PERT) then spread quickly
in the construction industry. After a few setbacks in practical application, attempts were
made to develop systems that better reflected the conditions affecting construction pro-
duction. These systems were also based on node-oriented networks, and some established
names are the “Metra-Potential Method” (MPM), “block network planning” (BKN) and
“precedence diagram”. Theoretical formulations and details of application are not given
here and reference is made to the specialist literature.

7.5 Control methods

General. The tasks of progress planning include the coordination of construction time
and capacity including consideration of cost, process technology, time and specific con-
struction site factors. In the construction phase, the progress of construction is a variable
quantity. This makes it even more important to continuously check the actual progress of
the tunnel drive against the intended progress and against contractually agreed deadlines,
and also to determine the actual cost position on the site.

7.5.1 Control of deadlines
It is safe to assume that the actual progress of construction will not normally coincide with
the planned progress since either the design is changed, quantities are not as estimated,
unforeseen geological difficulties have been encountered or also that not all conditions
were known when the schedule was drawn up. In all these cases, however, the result is that
measures to adapt capacities, dates and dependencies have to be planned and implemented
as construction proceeds.