Although more a scheduling approach than a delivery system, we discuss phased construction, or fast-tracking, here because several of the delivery systems are better suited than others to this method of planning and carrying out the project schedule. The basic idea is that some portions of the design work occur concurrently with field construction, thus achieving an overall savings in total project duration. Recall the traditional designtenderbuild approach in which the entire project is fully designed prior to the calling for tenders for the construction phase. Under this method, construction does not begin until the end of the tendering and contractor selection efforts, which necessarily must follow completion of design.
Contrast this approach with a scheme under which enough design work is completed to begin field work. An example might be a building project that is designed and built in several parts. After the foundations are designed, foundation construction begins. Design for the balance of the structure proceeds concurrently with that first construction effort. The structural frame design is completed while foundation installation work is underway, after which the structural frame is erected. The process continues, with design and construction occurring in overlapped phases. Figure 2.8 is a bar chart that compares the time schedule for a building project designed and constructed using the traditional designtenderbuild approach with that for a similar project using phased construction. For this hypothetical project, the total design time, 12 months, is the same under both approaches; similarly, the duration of construction, from start to finish is the same, 26 months. The 12-month difference in total project duration results from being able to start the foundation work at the beginning of the fifth month under the phased approach, whereas that work would not start until the beginning of the seventeenth month using the traditional method. The phased-construction approach would be well suited to the separate prime contracts delivery system, under which the owner, or the owners project manager, coordinates the several contractors and assures that the design schedule proceeds sufficiently to provide documents for the various construction phases. In Figure 2.8, we include a tender period for each construction phase, as required by this separate-prime-contracts delivery system. The designbuild delivery system would also be compatible with a phased-construction approach; in this case, the designbuild organisation would oversee the coordination of all phases of design and construction and it is likely that fewer formal tendering periods would be needed.
An obvious risk inherent in the phased-construction/fast-track approach is that the early design work must make assumptions about the outcomes of later design work; that early work may be incorporated into the finished construction before completion of all design and changes may be needed if those early assumptions were not accurate. A simple but basic example relates to foundation design. If the building is completely designed prior to tendering (that is, if it is not fast-tracked), the foundation design will be based on actual design loads of the structure, weather envelop, equipment, furnishings and any other building components, as well as live loads. However, to save time, under phased construction the foundation design will be completed before all of these loads have been determined and some assumptions will be required. If the foundations are not sufficient for the loads in the final structural, mechanical and other design, extra costs will ensue from the required foundation strengthening or replacement.
Clough and Sears (1994) offer some helpful cautions and comments about phased construction:
. . . fast-tracking has received harsh criticism because it emphasizes time rather than quality and can sometimes take longer than the usual, sequential process when applied to complex projects. It has been pointed out that the final construction cost is unknown at the start of the fast-tracked construction and if bids for subsequent phases of the work come in over budget, redesign options to reduce cost are very limited . . . For the process to be truly effective . . . the best possible cooperative efforts of the architect-engineer and the contractor or construction manager are required. It is also necessary that the owner have a well-thought-out program of requirements and be able to make immediate design decisions.
