Application of Linear Scheduling in Water Canal Construction with a Comparison of Critical Path Method

Abstract

Critical path method (CPM) still remains the most commonly used scheduling technique, despite many studies confirming its shortcomings for scheduling repetitive construction projects. This research evaluated the case study of an alignment-based precast water canal erection project, which was originally planned with CPM and analysed the suitability of using linear scheduling method (LSM) for the same project. The case study project was scheduled using both CPM and LSM tools and the results were compared in terms of estimated total duration and resource cost. The results showed that LSM produced a saving of 10 days in total duration and 20.07% in estimated resource cost over CPM. LSM also proved to be the better tool in terms of other schedule attributes like resource assignment, levelling, visualisation, etc., for alignment-based projects. LSM can be highly efficient for scheduling different types of repetitive construction and offers benefits like better workflow and continuous resource usage.

Headnote

Abstract: Critical path method (CPM) still remains the most commonly used scheduling technique, despite many studies confirming its shortcomings for scheduling repetitive construction projects. This research evaluated the case study of an alignment-based precast water canal erection project, which was originally planned with CPM and analysed the suitability of using linear scheduling method (LSM) for the same project. The case study project was scheduled using both CPM and LSM tools and the results were compared in terms of estimated total duration and resource cost. The results showed that LSM produced a saving of 10 days in total duration and 20.07% in estimated resource cost over CPM. LSM also proved to be the better tool in terms of other schedule attributes like resource assignment, levelling, visualisation, etc., for alignment-based projects. LSM can be highly efficient for scheduling different types of repetitive construction and offers benefits like better workflow and continuous resource usage.

Keywords: Linear scheduling method, Critical path method, Scheduling, TILOS, Precast construction

(ProQuest: ... denotes formulae omitted.)

INTRODUCTION

The fundamental trait of any project is to have a defined beginning and end in time, which makes it a temporary endeavour (Project Management Institute, 2017). Project Management Institute specifies five broad components that constitute the life cycle of a project management process: initiating, planning, executing, monitoring and controlling and closing (Project Management Institute, 2017). These principles naturally apply for to managing construction projects, which is becoming a complex task every day due to increasing variables and uncertainties to be accounted for, especially during the planning stage. According to Yamín and Harmelink (2001), construction companies are trying to gain a competitive advantage by achieving more sophistication and specialisation in executing specific types of construction. Managing specialised projects requires more intensive scheduling tools that need to be advanced than those typically used in the conventional projects. Project scheduling is principally a complex decisionmaking process since it involves numerous activities and resource allocations that need to be optimised properly (Xu and Zhang, 2012).

The critical path method (CPM) is a commonly used scheduling technique in construction which is deemed powerful for scheduling and using project control functions (Bansal and Pal, 2009; Kastor and Sirakoulis, 2009). CPM has its application in the construction industry since the 1960's (Burns, Liu and Feng, 1996) invariably in all kinds of projects (Hegazy, 2005; Shi and Blomquist, 2012). Using software systems for developing plans and schedules has become a prevalent practice in construction projects across the globe (Olivieri et al., 2019). Software packages like Primavera, Microsoft Project (MS Project), Asta Power Project, etc., are commonly used for this purpose and all these packages follow CPM logic in schedule generation (Hegazy and Menesi, 2010; Bragadin and Kähkönen, 2016; Olivieri et al., 2019). In a way, it is the popularity of these software packages that enabled the widespread use of CPM scheduling in construction (Olivieri, Seppänen and Granja, 2018). However, the major criticism placed against CPM is that it is not suitable for scheduling projects with repetitive activities (Harris and Ioannou, 1998; Hegazy and Kamarah, 2008; Koskela et al., 2014) that will have long and exhaustive schedules (Jongeling and Olofsson, 2007; Lu and Lam, 2009). Many researchers have pointed out the limitations of CPM in generating continuous workflows (Arditi, Tokdemir and Suh, 2002; Olivieri, Seppänen and Granja, 2018), balancing of crews (Russell and Wong, 1993; Hamzeh, Zankoul and Rouhana, 2015) and continuous utilisation of resources like material, equipment and labour required in a project with repetitive tasks (Mattila and Park, 2003; Benjaoran, Tabyang and Sooksil, 2015). Besides, the fact that same set of activities and information will be repeated in a project containing repetitive activities, a CPM schedule for such a project will get cluttered with the same information again and again (Ammar, 2019). This might result in a confusing project plan.

Repetitive projects occupy a significant share of global construction and meticulous project planning is an indispensable requirement for them. Repetitive projects may be defined as the continuous construction of multiple similar units (Ammar, 2019). Repetitive construction projects may be grouped into two categories: (1) Point-based projects (e.g., multi-unit housing projects, high rise buildings, etc., that have vertical alignment) and (2) Alignment-based or distancebased projects (e.g., pipeline construction, highway projects, etc., that have horizontal alignment) (Agrama, 2006; Duffy, 2009). According to (El-Rayes and Moselhi, 1998), the repetitive activities can be further categorised as "typical" and "atypical" activities. Typical category activities are assumed to have identical durations along with all units and atypical activities are assumed to have variable durations.

Linear schedules (LS) are proved to be effective alternates for scheduling repetitive projects (El-Rayes and Moselhi, 1998; Arditi, Tokdemir and Suh, 2002). Among the commonly used variants of LSs, line of balance (LOB) or vertical production method (VPM) is adapted for point-based projects and linear scheduling method (LSM) is suited for distance-based projects (Yamín and Harmelink, 2001; Duffy, 2009). LSM is nothing but a graphical representation of the project activities with distance or location on one axis and time on the other axis. It can be defined as a visual representation of a repetitive project's construction plan depicting the logic and relation between the activities of the project (Mattila and Park, 2003). LSM improves continuous workflow significantly better than CPM, controls the production and provides faster response to delays and interferences. Similar to CPM, where the activities on non-critical path contain floats after the critical path is determined, LSM too allows rate floats on its non-controlling activities or non-controlling segments, after evaluating the controlling activity path (Olivieri et al., 2019). Repetitive projects like pipelines, highway, canal projects, etc., involve continuous and linear activities, which need to be constructed along the horizontal alignment of the facility. While scheduling these projects, CPM divides the whole process into discrete activities that are sequenced in order of their performance. However, the major concern in such projects is to assess and arrive the optimum production rates for the timely completion. LSM offers efficient scheduling of these projects by focusing on repetitive work activities and the production rates to identify any possible setbacks in the construction process (Matila and Park, 2003). In CPM, critical path is defined as the longest time-consuming path throughout the network, whereas in LSM, the controlling path is defined on the basis of the least time interval, coincidence interval and the least distance interval between two consecutive activities. Harmelink and Rowings (1998) developed a computerised linear scheduling model in conjugation with an Aut°CAD-based programme to identify the controlling activity path and compared the results with CPM. He concluded that LSM provides a realistic controlling activity path by considering changing constraints in buffers thus providing accurate production rate details of linear activities which could not be achieved with CPM.

The major focus points for LSM application in highway construction projects include determination of production rates, identifying activity interruptions, buffers, calendar considerations and allocation of project resources. LSM also provides realistic and reliable information to plan the method of construction and nature of work, identifies the risks better than the bar chart thus helping to optimise the construction cost and time. LSM's most important benefit is the ease with which it transforms a comprehensive work schedule to location-based segments, thus making it easier to monitor the progress of the project's linear activities (Johnston,1981).

Despite its proven utility for planning repetitive construction projects, LSM does not find widespread application in real-world for various reasons (Agrama, 2011). One of the major reasons seems to be the contractual specifications in favour of CPM. In a survey conducted by Galloway (2006), more than 60% of the respondents confirmed contractual obligation as the reason to opt for CPM schedules for their projects. Other reasons for schedulers to prefer CPM over LSM in repetitive projects are better familiarity with CPM analysis, the existent popular software packages following only CPM logic, the legal validity of CPM in delays and claims due to contractual conditions and lack of awareness and training in using LSM (Yamín and Harmelink, 2001; Olivieri et al., 2019).

The main objective of this study is to apply LSM in a real-time alignment-based repetitive construction project and also provide a comparison of adapting CPM planning for the same project in terms of perceived total duration and planned cost. A precast water canal construction project was chosen as a case study for this purpose and the erection schedule of the project was prepared using both LSM and CPM tools. The results of both these scheduling techniques are compared in terms of savings in total planned duration and estimated resource cost of the project.

BACKGROUND

LSM

LSM is a graphical technique used for scheduling projects with continuous resource utilisation demand like roads, tunnels, pipeline construction, etc. (Duffy et al., 2012). The name "linear scheduling method" is particularly denoted for scheduling horizontal repetitive projects that have linear geometrical alignment (Agrama, 2011). LSM represents the project activities in the form of a 2D graphical chart with location or distance on one axis and time on the other. For alignment-based horizontal linear projects like road construction, the distance or location is represented on the horizontal axis and time on the vertical axis. For projects with vertical linearity like high-rise building construction, the axes are interchanged. Such linear schedules are usually termed as VPM or LOB method (Duffy, 2009). The controlling activity path in LSM is recognised based on the time-distance relationships among the activities, which is very similar to that of a CPM critical path (Harmelink and Rowings, 1998; Agrama, 2011).

Lucko (2007) provided a mathematical approach to understand the concept of LSM in a simplified manner in terms of singularity function has been described by using Macaulay brackets in a transportation project widening of a segment located in Northern Michigan taking time and location as buffers.

... Eq. 1

... Eq. 2

... Eq. 3

Where x is variable, a is segment length that serves cut off value, n is the order of the activities and C is an integration constant. These singularity functions are effective as they contain segments of different properties within one functional expression can be differentiated and integrated using standard rules. They provide a reliable mathematical description for the discontinuous process. This technique is based on geometry and algebra which can be evaluated manually by project managers easily. An important factor while using this technique is to select buffer for example location, time as required by that project. Equations are evaluated in sequential order as LSM is flexible in relating the activities to each other and usually suffice to use a sequence with time and location buffers. This method can accommodate infinite segments of activities, each with their production rate requiring basics mathematical skills yielding complete and precise results for any linear schedule. This application replicates the intuitive nature extending the indepth analysis of the graphical representation of a linear schedule beyond CPM capabilities. In view of the differential section, this situation keeps the activities and their buffers mathematically intact throughout the analysis. In terms of singularity function, start and end times, and their efficiency are simple and distinguished (Lucko, 2008).

Lucko and Orzco (2009) extended the concept of rate float by distinguishing its existence in terms of time and location buffer and combination of both. Float types can be calculated using singularity functions. These mathematical models described activities and their buffers over a continuous range. Float at any location can be determined accurately, equipping schedulers to assess the impact of delays on linear or repetitive construction projects. Rate float indicates possible changes in the production rate of a non-controlling activity to fall under the controlling activity path. To avoid this, a two-stage schedule model integrating LSM and constraint programming was developed for linear project resource-levelling (Tang, Liu and Sun, 2014). Considering two concepts of rate float, the amount of work accomplished by a resource per unit time and amount of work that can be accomplished during unit time overall activities are optimised. As constraint programming strategies like backtracking, testing, the forward check is provided with no additional constraint is required for changing buffer making it a more flexible and quality model for linear scheduled projects.

While LSM has been existent for several years, it is application in real-time repetitive projects is comparatively limited. There are some evidences of LSM applications in highway, pipeline, residential and tunnel projects. In one of the earliest applications of LSM, Johnston (Johnston, 1981) applied LSM in a highway project using different line patterns like line, block, shaded and bar to represent the different activities involved in the highway construction process on the horizontal axis and time duration on the vertical axis. The schedule also included production rates, buffers, calendar consideration and resource allocations. Harris and Ioannou used a modified LSM for scheduling a repetitive housing project and computed the controlling activity path duration based on the activity production rates (Harris and Ioannou, 1998). For a hypothetical bridge project scenario, Liu and Wang (2007) attempted to create a constrained programming based LSM model. Duffy et al. (2012) adopted LSM using the software tool (Velocity 1.0) for scheduling a real-time pipeline project of 750 km long in the USA with varying production rates owing to different project variables. In one of the recent studies, Rzepecki and Biruk (2018) used a simulation method to schedule the repetitive activities of a multi-storey residential building. Table 1 provides the details of additional case studies across the world related to LSM application in different types of construction, identified through literature review. Some cases of LOB application are also mentioned to understand the practicability range of linear scheduling.

As also shown in Table 1, there are different software tools that have been used by different researchers for creating the time-distance representations of LSM schedules. Successful application of LSM requires a suitable software package for efficient calculation and schedule updating (Duffy, 2009). The functionality of these software programmes varies on the level of scheduling and project control requirements. Some of these tools are either add-ins to the existing CPM based programmes in the market or having only basic scheduling functionalities. For managing large repetitive projects, stand-alone LSM based tools integrated with additional project control functionalities are needed. In this regard, Kim et al. (2019) suggest a few integrated LSM-based programmes like MAGNET Project, TILOS and Vico Office for Time that offer augmented functionality beyond basic scheduling. They also did a comparative study of the above three software tools and concluded that TILOS offers all round project management functionalities including alignmentbased scheduling, auto-update and tracking of activities, clash detection etc. and is better suited for repetitive civil engineering construction projects. TILOS also offers the advantage of creating the time-distance LSM diagram in a CAD-type interface and generating resource and cost data along with the linear schedule (Duffy et al., 2012). Based on the above aspects, TILOS was chosen for modelling the continuous nature of the precast canal construction project using alignmentbased linear scheduling.

The literature review reveals that LSM has the potential to be applied to a range of repetitive construction projects. But there are only limited attempts of LSM application in alignment-based repetitive construction beyond highway construction and there is no evidence for the application of LSM in the construction of a water canal, which involves horizontal repetitive activities. In this context, it was decided to investigate the application of LSM in a precast water canal construction project and do a comparison with adopting CPM for the same project.

CASE STUDY

Research Methodology

The principal aim of this research was to show the effectiveness of the LSM over CPM in scheduling any type of alignment-based repetitive project, for which a precast water canal construction project was chosen as the case study. A deductive research approach was adopted with the research goal of verifying the potential advantages that LSM offers over conventional CPM for the selected case study. The flow of this research study is shown in Figure 1.

A real-world water canal project to be built using the precast construction technique was selected as a case study to validate the application of LSM. It is basically a stormwater drainage canal to be located in Bengaluru, India. For the research study, construction of a major segment of the canal was considered which was about 184.32 m length. The plan view and section view of the canal structure are shown respectively in Figures 2 and 3.

The proposed water canal construction consisted of the erection of precast elements like column, beam, hollow core slab, side slab and roof slab. The dimensions, shape and alignment of the elements were designed by the consultant owing to the site conditions and specification requirements of the client. The number of precast elements to be erected for the canal construction included 84 columns, 42 beams, 328 side slabs and hollow-core slabs and 574 roof slabs: 1,028 elements in total. The canal construction was divided into five zones, viz., Zones A, B, C, D and E, for the ease of planning and coordination. The break-up of the number of elements to be erected in each zone is given in Table 2.

The project was at the initial planning stage when this study was taken-up. From the shop drawings of precast elements to be erected, the WBS of the project was formulated and the scheduling process was started. There were no contractual requirements to mandate the use of CPM schedule, but the contractor's planning team were originally set to adopt CPM schedule and were hesitant to go for LSM as they had less familiarity with the technique and its efficacy. So, we decided to schedule the project using both CPM and LSM parallelly and do a comparison of the total planned duration and estimated resource cost to provide a convincing case for LSM.

For the comparison study, the following constraints were considered for both CPM and LSM schedules:

1. Only resource loaded activities were taken.

2. The productivity of an individual resource was fixed and obtained from the standard productivity chart of the contractor.

3. The schedules were generated was based on parameters like total duration of each activity, maximum resource availability, the number of mobilisations and demobilisations needed and the number of activities and logic links.

4. Calendar and working hours were fixed. No overtime was considered.

5. The additional allowance given for LSM schedule was that the sequence of locations could be conveniently changed, wherever it was not mandatory to follow the sequential order (for example, the sequence of structural erection tasks was not altered because they had to go in order).

Creation of Erection Schedule Using CPM

The precast canal erection for all the five zones was to be done on two bank sides that were named as KGA side and Century side. The WBS of the project is shown in Figure 4.

Levels 1.1 to 1.5 of the WBS indicate the erection tasks for the five zones while 1.6 to 1.10 indicate the beam and roof slab erection and other finishing tasks. The sub-levels include the erection tasks for the two banks KGA side and Century side. From the WBS, the erection activities to be carried out on the two sides were arrived. The erection duration was calculated on the piece-count basis with the number of pieces erected per day was assumed based on historical data and expert opinion. Table 3 shows the list of activities, erection count and their durations.

The erection schedule was created in MS Project by entering the activities, durations and other relationship related constraints. The project followed a 24-h working time with two shifts on weekdays and a half day working on Saturdays. Resources for the activities are assigned with their rates taken from "Delhi Schedule of Rates" (Central Public Works Department, 2019). Figure 5 shows the snapshot view of the activity and Gantt chart window and Figure 6 shows the resource allocation in MS Project.

Creation of Erection Schedule using LSM

The alignment-based erection schedule of the precast water canal was created using TILOS software application. The time-distance diagram of the linear schedule was created based on the geographical distance between the members in all five zones which was taken from the architectural plan drawing. Table 4 shows the geographical distance between the members in different zones.

TILOS has the inbuilt feature to automatically calculate the duration and work rate of the activities based on the geographical distance between them. The logic is that the length of an activity is proportional to its quantity or the amount of work needed. So, the duration of the activity is also proportional to its length. In simple terms, the longer the distance of an activity is, the longer will be its duration. The project calendar, activities, constraints and resource allocations entered in TILOS were all the same as followed in MS Project. Figure 7 shows the resource allocation details and Figure 8 shows the activity list with work and duration parameters calculated by TILOS. The line type, pattern and colour help in differentiating the tasks according to their nature. The time distance diagram of the water canal project was generated with distance plotted on the x-axis at a unit interval of 5 m and y-axis denoting the time at a unit interval of two days. Figure 9 shows the timedistance diagram of the project.

RESULTS AND DISCUSSION

Project Duration

The number of activities needed for the precast water canal erection was the same (45 activities) for both CPM and LSM schedules. But the total project duration as calculated using the CPM method was 52 days and the same project activities when modelled through LSM resulted in a total duration of 42 days. The convention of activity focussed predecessor successor relationship between the sequential precast segments was the basis of CPM duration calculation. Additionally, due to the logical constraints and varying production rates of the activities, waste time is created between a few activities which disabled the continuous workflow of the erection process. Hence, the project network demanded more duration when modelled with CPM planning. In the case of LSM, the geographical distance between the segments to be erected continuously was the basic consideration and as such the production rate of erecting segments was modelled based on their location in the erection plan. This enabled planning for a continuous workflow and avoidance of the waste time created due to CPM logical constraints, thus making the project duration as much as 10 days shorter in comparison with CPM planning. The problems of lack of workflow and substantial wasted time between activities with CPM and the evidence of better workflow with LSM have also been confirmed in the study conducted by Oliveri and his team (Olivieri, Seppänen and Granja, 2018).

Estimated Resource Cost

Table 5 shows the estimated resource cost for both CPM and LSM schedules.

The estimated resource cost in LSM is 20.07% cheaper than that of the CPM schedule. Except for the resources needed for excavation activity, all the resources of the LSM schedule take shorter durations than CPM schedule to complete the equivalent tasks. LSM achieves continuous workflow by synchronising the activity durations based on the geographical distance between the erection tasks. This allows for continuous resource usage and avoidance of resource idling. The continuous workflow also reduces the mobilisation and demobilisation time of the resources which helps in lesser time consumption and faster completion of the task. In LSM, the resource scheduling is done on the basis of availability of the resource, which makes the resource levelling easier, so the resource levelling and scheduling go hand-in-hand. The resource allocation basically does not meddle with the work progression tasks. Whereas in CPM, resource scheduling for an alignment-based project such as precast water canal erection only considers the logical relationship of the tasks which makes it difficult to adjust the resources based on their availability. This resource assignment which relies on the succession of the task movement meddles with the work progression and warrants the requirement of resources for longer times. It is for these reasons, why the estimated resource requirement time and ensuing cost are substantially lower in LSM planning than in CPM.

Based on the observations made during the schedule development of the alignment-based water canal erection project, a comparison of how the different schedule attributes fared under CPM and LSM is presented in Table 6.

The key for effective implementation of LSM is dependent on its focus on certain important aspects of construction management. In this regard, a framework for effective implementation of LSM in repetitive projects is recommended as an outcome of this study, which is shown in Figure 10.

CONCLUSIONS

This study adopted two different scheduling methods for planning the erection of a precast water canal project and compared them based on their estimated project time and resource cost. The project schedule in CPM gave an estimate of 52 days to complete the erection process, while LSM schedule estimated 42 days which is 10 days, i.e., 19.23% earlier than CPM. In terms of estimated resource cost also, LSM provided a savings of 20.07%. The study also found two important shortfalls of CPM, viz., lack of continuous workflow and inability to schedule available resources for continuous work, but these problems were effectively resolved by LSM. LSM had the edge over CPM in terms of other schedule attributes like resource allocation, levelling, visualisations, etc., for this case study project.

There are previous studies that explored the usage of LSM in repetitive projects like highways, residential buildings, etc., but this research study considered the possibility of applying LSM in a precast water canal construction project and demonstrated that LSM can be the better planning tool for such projects in all aspects, where the conventional practice was to use CPM tool. But LSM usage is not common even in projects where repetitive elements are there, due to many reasons like lack of familiarity, training, contractual obligations and the perceived risk of using a new technique (Zhang, 2015). For such projects, this analysis may be crucial in promoting LSM adaptation, suggesting that LSM is a convenient tool to learn and use. The significant advantage of LSM over CPM is its virtual-aided features and enabling effective communication among the project members. The fundamental limitation of the study is that only a small portion of the waterway construction was considered for testing the rationality of the LSM application. In addition, it must be analysed how influential LSM will be for a bigger quantum of work, where additional constraints such as fluctuating locations, unique activities and logical relationships and variable production rates might play a role. Also, the LSM schedule in this study did not take into account the project control features like creating baselines, project updating, tracking, etc. needed for future practical variations possible during execution and it is to be seen that how those elements can be incorporated in the LSM schedule. The future studies could address the adequacy of LSM application for these specifications and expand the use of LSM to a variety of construction projects. As specified earlier, despite having a broader scope, LSM usage is not very widespread in construction and needs a rigorous campaigning initiative. To promote the usage of LSM in construction, more opensource linear scheduling software programmes need to be developed and academia should also step in to conduct extensive workshops and training to the industry professionals on effective usage of LSM in construction.

Sidebar

First submission: 12 August 2020; Accepted: 22 May 2021; Published: 30 June 2022

To cite this article: Prasanna Venkatesan Ramani, Ponnambalam Selvaraj, Shanmugapriya T. and Anshul Gupta (2022). Application of linear scheduling in water canal construction with a comparison of critical path method. Journal of Construction in Developing Countries, 27(1): 189-212. https://doi.org/10.21315/jcdc2022.27.1.11

To link to this article: https://doi.org/10.21315/jcdc2022.27.1.11

School of Civil Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, INDIA

Footnote

*Corresponding author: [email protected]

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