Comparison of Gantt Chart Scheduling Method and Linear Scheduling Method in Scheduling Multiple Utility Line Construction Project

Introduction

It is not unusual for utility contractors to construct multiple utility lines with several crews working simultaneously. Although popular scheduling methods, such as Gantt chart with Critical Path Method (CPM), is really beneficial to the construction industry, it is still criticized while applied to scheduling linear construction projects. Linear scheduling method is beneficial to linear construction in its presentation on productivity and project progress corresponding to true location. However it is not addressing the scheduling in multiple utility lines. Multiple utility lines intersect with each other at some locations as designed. Each line at the intersected location has its own elevation. Crews are assigned to work on each line. These crews may arrive in the intersected location simultaneously which cause work space conflicts. The following will present the process of the construction scheduling of HCC utility construction company,  compare both scheduling methods in scheduling multiple utility line, describe the problems that are faced by HCC utility construction company, identify limitation of both methods, and further research area is addressed.

HCC is one of the largest utility contractors in the North Florida area. It was founded in 1974. The company’s participation in both the public and private sectors and its diverse mix of project types and sizes have contributed to the Company's revenue growth and profitability. The business expanding brings challenges of construction management to HCC, especially in planning and scheduling stage. The construction management personnel tries to avoid work space conflicts and time of rework, improve its crew productivity, to maximize its profit. The company applies several popular planning and scheduling methods to solve the challenges from scheduling. These methods include linked bar chart and line of balance method. In order to describe the application and limitation of these scheduling methods, the following sections would first introduce the planning and scheduling procedure of HCC and then compare the applications of these methods.

Construction Operation of HCC

Usually HCC splits an awarded contract into its several parts: sanitary sewer system, storm drainage system, potable water system, fire and irrigation system, power sleeve line, and gas line. HCC has several types of crews to perform these separated jobs. Each type of crew has expertise experience on its work.

Figure 1 shows the operation flow chart for the HCC. The estimators do quantity taking off for the projects, and after it is checked by the project engineer they call suppliers for quotations. The bookkeeper maintains the as-built drawing using the daily report from superintendents for pay-app purposes as well as other paper work such as change orders if the as-built quantity is not same as the planned. The accounting department takes care of all the accounts payable and receivable with the help of the front desk receptionist.

Quantity Estimates for the Project

The company uses the unit-price contract type. The following are regular bid items:

After review the engineering drawing, the estimator takes off the quantity of each type of bid item. For utility projects, one of the most important jobs is to estimate the quantities of soil to be moved and backfilled. For this strenuous job, the HCC has developed a spreadsheet for making this job easy, and compiles the information from the engineering drawings:

Besides the information from spreadsheet, the cost information needed for estimating are:

By extracting the information from spreadsheet, the cost of the material, labor, equipment, etc can be generated. The total cost is the sum of the material cost, equipment cost, labor cost, and overhead and benefit. The bidding sheet will be prepared by estimators. The next step is to decide the project duration.

Estimate Project Duration

Several items must be known in order to estimate project duration:

The quantity of work for a project.

The first item is known through estimating. The second item is to make sure what resources are available. The HCC has eight main line crews (water sewage crew) and five miscellaneous crews. Some crews may already have their work on hand done. Other crews almost have their work done. Management personnel will know which crew will be available at the required time. The following describes the equipment and crew production rates.

Estimate equipment production rate

The equipment production rate is determined by several factors such as optimum production rate, equipment capacity factor and soil swell (load) factor. For different brands of equipments, the company usually calculates the production rate from the operator’s manual which the manufacturers give to the company. The equipment optimum production rate is calculated as the following:

(Equation 1)

However, the equipment production rate is affected by other factors such as soil swell factor, equipment capactiy factor, therefor equipment production rate is determined by the following equation:

Equipment Production Rate = Equipment Optimum Production Rate × Equipment Capacity Factor × Soil Swell (Load) Factor      (Equation 2)

Estimate the Production Rate for Each Crew from the Time Sheet

The last item before scheduling a project is to calculate the crew production rate from the historical data. The company collects data from the superintendents’ dairy reports. The data collected from this daily report are:

The company collects data for each type of pipe in order to estimate the productivity of crews. The productivity of the crews is affected by pipe depth, pipe size, the number of structures (manholes), structure depth, working condition (city or country), and skills of crews. These parameters affect productivity as follows:

The duration of a project is determined by:

                    (Equation 3)

Where:

: duration of a project;

: duration of constructing utility line i of a project;

i: a specific utility line;

t: the total number of utility lines of a project;

The duration of a utility line is estimated by:

      (Equation 4)

Where:

: duration of constructing utility line i of a project;

: duration of installing manholes of a project;

: duration of constructing main pipe of a utility line i for a project;

: duration of constructing service pipe of a utility line i for a project;

 i: a specific utility line;

If the contract gives the duration and the contract duration is shorter than the duration estimated from the above equations, then the management personnel may add more crews to this project.

Information Requirements of Project Monitoring and Control

After the company is awarded a bid and a contract is signed as shown in Figure 1, the project monitoring and control phase will start. The construction management personnel requires the schedule can easily or directly tell:

The above introduces the HCC’s background, organizational structure, its project operation model, and information requirements of project monitoring and control. The following will introduce how a project is scheduled in HCC and the problems faced by HCC.

Unit 97 Project

The Villages is a huge community development project located in Ocala, Florida. It includes many units, such as Unit 97, Unit 98, and Unit 99, etc. Each Unit is composed of several hundred housing lots. The Villages Unit 97 is a typical project that HCC usually works on. HCC undertakes the construction of the underground utilities of the Unit 97. The utility lines include: Storm Drainage, Sanitary Sewer, Fire/Irrigation Water, and Potable Water. The Roads and Lot Grading are subcontracted out.

The work on each utility line of the Unit 97 project is described as follows (Table 4 shows the summary of project information):

Estimating Construction Duration for the Utility Lines

The first attempt to build a schedule consisted of dividing the project into four main lines since the project has four main components: (1) Sanitary Sewer Line; (2) Storm Drainage Line; (3) Potable Water Line; and (4) Irrigation/Fire Water Line. The following section will describe how to estimate the construction duration of the sewer line, the storm drainage line, the potable water line, and the irrigation/fire water line.

Estimate of Sewer Line Construction Duration

The duration of sewer line construction is affected by the rate of production. It consists of three components: the construction durations for the main sewer line, for the manhole construction, and for the service line.

Duration of Main Sewer Line-- The Unit 97 project only uses 8” diameter PVC as its sewer pipe.  In order to determine the rate of productivity which is affected by the depth of the sewer pipe line, the whole main sewer line is categorized into different depth ranges:

0 – 6’

6 – 8’

…..

26’-28’

28’-30’

The depth and length of the main sewer line is itemized based on these categories in Table 5. In the fourth column, the total duration for laying the main sewer line is determined to be14.13 days.

Manhole Construction Duration-- The duration of manhole construction is also affected by the depth of the installation. The deeper the installation, the slower the productivity is. Therefore the rate of production is categorized into various depth ranges as for the main sewer line. The quantities for the manholes are computed based on these depth ranges.By applying similar method described in estimating duration of main sewer line, the total duration for manhole construction is 4.375 days.  

Estimate of Service Line Installation Duration—The duration for Table 6 shows the predicted duration for installing the service lines.

The total duration for laying the sewer line is computed by:

= 14.13 + 4.375 + 7.27 = 25.77 days

Here i represents the sanitary sewer line.

Estimate of Storm Drainage Line Construction Duration

The estimate of the storm drainage line construction duration consists of two components: the duration for the construction of the main storm drainage line, and that for the manhole construction. By using the similar method as described in sewer line construction, The duration for laying storm drainage line is computed using the following equation:

= 17.80 + 6.70 + 0 = 24.50 days

Here i represents the storm drainage line.

Estimation of Potable Water Line and Irrigation/Fire Water Line Construction Duration

The construction of the potable water line and the irrigation/fire water line is not as complicated as the sewer line and storm drainage line. The lines are covered by 4 feet thick dirt. The construction durations for the construction of the potable water line and the irrigation/fire water line are determined by the production rate of each of these two lines. Table 7 lists the predicted construction duration for these two lines.

Table 8 lists the information for each line based on its length, crew allocation, rate of production, estimated duration. Three crews are assigned to the Unit 97 project. The crew one works on the storm drainage line and crew two works on the sanitary sewer line. Both of crews start their work on the same date (day 0). Crew three works on two lines, the potable water and the irrigation/fire water lines. It works first on the potable water line and then on irrigation/fire water line.

Scheduling Unit 97 by Linked Bar Chart

All of the above information is used to develop the Linked Bar Chart schedule and Linear Schedule Method for the Florence Path of Unit 97 project. The Linked Bar Chart is shown in Figure 2 and the linear schedule method diagram is shown in Figure 3.

The linked bar chart schedule shows repetitive activities (Excavate, Lay pipe, and Back fill) in the storm drainage line and sanitary sewer line. It illustrates how laborious the linked bar chart method is in planning the repetitive projects. Although it is time-scaled chart, the logical links of repetitive activities of each section do not make much sense in the presentation:

The locations of utility line sections are described in repetitive activities in Gantt Chart. It does not graphically tell the location of the utility line sections. The longer the utility line, the more repetitive activities in Gantt Chart.

Althought the repetitive activities show the location of utility line scetions, it does not tell the length of each utility line section. In other words, the user can not get direct work quantity from the Gantt Chart. The Gantt Chart can not reflect the production rate of utility lines. Although Gantt Chart can roughly identify the progress of the project, it can not acurately tell the user the information of the progress, such as the utility length that the crew has already finished. 

Furthermore, in the case of two or more pipe lines intersecting somewhere on the site, where the elevation of one utility pipe is lower than the other, if the crews arrive at this kind of location simultaneously, obviously only one crew can continuously work and the other crews have to stop and wait until the first one to finish. The Gantt Chart can not indicate these interference locations and predict the possible interruption time.

Scheduling Unit 97 by Linear Scheduling Method

Since the Gantt Chart scheduling method can not give such information as HCC needs, HCC tries using Linear Scheduling Method to retrieve information for project Unit 97. By using the data that is listed in Table 8 such as the start time and finish time for each utility line, the estimated project duration is 26 days. Storm drainage and sanitary sewer lines are undertook by three separated crews and they started from day 0. The potable water and the irrigation/fire water lines are constructed by the same crew. The potable water line is installed first. After it is completed, the same crew will start installing the irrigation/fire water line. The horizontal axis represents the time in day unit. The vertical axis represents the location or line length in feet. Only four utility lines appear in the Figure 3:

There are no repetitive activities in the Figure 3.  User can identify through linear scheduling:

The relationship between locations of utility line sections and planned construction time. Linear scheduling method illustrates at what time how long the utility line would be completed. It establishes the time-location coordinates.

·         The production rate for each utility lines.

·         The work flow of the crews. For example, the crew 3 works on the potable water line and then on the irrigation/fire line.

·         The acurate progress of the project through the time-location coordinates instead of the rough completion percentage indicated in Gantt Chart.

Although the Linear Schedule Method clearly presents the production rate of each utility line, the start time and finish time, and accurate progress for the individual schedule date, but it can not illustrate the interference locations, and predict the interference time in the case of two or more pipe lines intersecting somewhere on the site. The interference slows down the construction progress, interrupts the work continuity of the crews, and therefore lowers the production rate which may even stop the work of crews, delay the project, and incur cost overruns.

Conclusion

This paper describes in detail the construction planning and scheduling procedures of the utility construction company HCC. It presents the processes and techniques of construction duration estimation for each utility line under various construction environments. In order to meet the specific scheduling requirements from the construction management personnel, which are:

The company tried two scheduling methods – linked time-scaled Gantt Chart and Linear Scheduling methods, and compared these two methods based on the information extraction requirements. The comparison is listed in the Table 9.  Although linear scheduling method can satisfy the information requirements from construction management personnel from many respects compared to the Gantt Chart method, it still does not solve the work space conflicts among utility lines which are caused from the elevation differences of lines at the intersected location. A methodology is expected to be developed by further research. This method should be able to identify accurately the intersected location, estimate the construction interference time, and adjust the schedule to avoid the construction interruption. It would help keep the continuity of crew work, and avoid the delay of construction and cost overruns

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