Today, there is a worldwide trend towards the rehabilitation and installation of new underground utility infrastructure. Urban development has facilitated the need to expand current networks, while simultaneously upgrading existing ones to conform to this growth. The concept of sustainability is rapidly becoming an important societal consideration for selection of construction methods and materials.

The World Commission on Environmental and Development (WCED) defines sustainable development as “development which meets the needs of the present without compromising the ability of future generations to meet their own needs” (WCED 1987). According to Bourdeau (1999), enhancement of sustainability can be realized by focusing on three aspects: 1) minimizing environmental impact; 2) maximizing economical benefits; and 3) minimizing socio-cultural impact. This triple bottom line approach forms the basis; however, research on the development of effective measurement for sustainability in infrastructure development is still in its infancy. Assessment of can be approached by either focusing on the development stage or systemic sustainability performance during the life cycle. A combination of the two approaches is also a possible way to assess the entire sustainability of an infrastructure development project.

Koo and Ariaratnam (2007) developed a methodology for assessing the sustainability of infrastructure using a Sustainability Assessment Model (SAM) that contains forty-seven sustainability indicators. SAM is a decision support model for use when multiple alternatives are possible for a project. It is based on three major steps in the framework: 1) identifying sustainability indicators; 2) assessing selected indicators in appropriate measurement; and 3) eliciting a decision based on the assessment results. The model framework is illustrated in Figure 1.

Watermain Replacement Case Study

The full paper goes on to provided a case study to illustrate the method, which is only briefly summarised here.

Project Description

The main objective of this case study is to verify the applicability of sustainability assessment by applying SAM to a typical municipal capital improvement project. A section of watermain system located in the City of Scottsdale, Arizona, is used for this case study. Currently, 807.7 m of existing 200 mm diameter Asbestos Cement Pipe (ACP) watermain system is providing drinking water supply to local residents and businesses. The case study assumes that this ACP watermain system is required to be replaced as a result of it being under capacity due to urban growth and deterioration by aging. Three alternatives combining different construction materials, methods, and pipe layout are proposed for consideration. The three project alternatives are: 1) new installation of ductile iron pipe using conventional open cut with shield (Alternative I); 2) replacement of existing watermain to high density polyethylene (HDPE) pipe using the pipe bursting method (Alternative II); and 3) new installation of fusible polyvinylchloride (PVC) pipe using the Horizontal Directional Drilling (HDD) method (Alternative III). Selection of each distinctive alternative will result in different impacts during the construction and life cycle stages. These impacts are detailed by the sustainability indicators and are evaluated through relative significance ratings and absolute value comparisons.

Pipe bursting and HDD are two trenchless technologies that require significantly less excavation, disturbances, and completion time than the conventional open cut method. The pipe bursting method is defined as the replacement of the original pipe by fragmenting the existing conduit and installing a new pipe or equal or larger diameter in its place. Two major working pits, the machine pit and insertion pit, should be of an adequate size to negotiate working space for machine operation, and maximum bending radius of flexible pipe, or section length of sectional rigid pipe. Horizontal Directional Drilling, a trenchless underground pipeline installation process, is performed in three distinct phases: 1) pilot bore; 2) reaming; and 3) pipe pullback. All three phases are accomplished through an underground bore connecting two end sides; the drilling head entry side and the exit side. Most installations can be completed from the surface allowing proper set back distance from the designed depth of bore.

Utilizing these two trenchless methods in an urban environment provides tremendous benefits, including minimizing disturbance and reducing the project schedule. However, the conventional open cut method is still used in practice, especially for typical underground infrastructure projects with favourable surface entry.

Sustainability Assessment Process

The assessment process consists of six assessment modules as shown previously in Figure 1. The six assessment processes are: 1) Analytic Hierarchy Process (AHP); 2) real cost estimation; 3) pollution estimation; 4) energy estimation; 5) time estimation; and 6) natural resource depletion impact analysis. A single assessment method cannot assess all sustainability indicators that have multi-criteria characteristics. The assessment process is based on readily available data and various estimations at the front end stage for the future life cycle stages. The modelling boundary and scope are illustrated in Figure 2. The first step is to define the project descriptions and objectives. The second step involves proposing design alternatives that are able to satisfy project objectives. This paper focuses on twelve sustainability aspects and appropriate sustainability indicators are accordingly selected from the predetermined forty-seven indicators. Assessment results are presented in subsequent sections for each assessment category. To complete the sustainability assessment process, these six independent results are integrated through a normalization process provided by the weighted sum model (WSM). The paper goes on to detail the methods employed in determining each of the six assessment modules before making a conclusion.

Decision Making Process

The decision making process determines the most sustainable alternative. This decision is based on the assessment results presented in Table 2. These eight assessment results, including three air emission estimations, are incomparable attributes that can only be treated independently. A Multi-Criteria Decision Making (MCDM) technique was used in order to integrate the independent attributes into a single attribute for final prioritisation.

The Weighted Sum Model (WSM) was utilized in this scenario. The initial normalization process requires using consistent sign convention to reduce arbitrary errors that occur from using two signs. The result of AHP is already a normalized value and the highest value, which has priority over the others. All other results, including cost, energy, pollution, and depletion impact, are low value priority values. When a sign convention follows a high value priority in the normalization process, an equation can be used to normalize other values, transforming them from low value priority to high value priority.

The final process of WSM decision making requires the application of weight factors to each normalized value so that each value can be assigned relative significance towards the overall sustainability prioritisation. The sum of the weight factors is equal to one; hence the sum of the overall sustainability prioritisation values must be equal to one.

The final result of WSM and the sustainability prioritisation are presented in Table 3. The summation of each column indicates relative sustainability among the proposed alternatives. Pipe bursting with HDPE pipe was found to be the most sustainable alternative. This decision is the final result of the overall SAM process.

One consideration when using WSM for sustainability prioritisation is the uncertainty of the weight factors. The final decision can differ if different weight factors are applied in the last process of the WSM. Regardless of the significance of the weight factors in WSM, judgment of weight factors can be subjective and can vary by personal preference, knowledge, external influence, or any other conditions in which decision makers may have bias. Therefore, a sensitivity analysis for the final decision should be performed to ensure that the threshold of the current decision is acceptance. Sensitivity analysis will improve the reliability of the decision by providing ranges of weight factors that verify the possibility of a decision revision. An iterative loop function with proportional weight factor change is one of the possible methods that could be utilized for the sensitivity analysis.

Conclusions and Recommendations

This paper presented an application of a model to determine the feasible infrastructure construction option considering sustainability principles using a case study of a watermain replacement project. Three alternatives, using different combinations of construction materials and methods, were used to demonstrate the model application. The Sustainability Assessment Model (SAM) considers combinational integrations between quantitative and qualitative sustainability assessment factors. SAM uses three main processes including the determination of sustainability indicators, application of six assessment methods, and final decision criteria in evaluating the most sustainable option. The six assessment methods include AHP, real cost, pollution, energy, time, and natural resource depletion; covering major interest areas based on the current definitions and objectives of sustainable development. Each alternative option has unique sustainability characteristics; hence the six assessment methods produce their own assessment results. In the results of the case study example, the selection of ductile iron pipe using open cut construction resulted in the least natural resource depletion impact. However, using a Multi-Criteria Decision Making (MCDM) technique to determine final prioritisation of the options resulted in the selection of pipe bursting using HDPE pipe as the most sustainable option followed by Horizontal Directional Drilling with PVC pipe. As hypothesized, open cut using ductile iron pipe was deemed to be the least sustainable option.

To truly embrace sustainability, changes must be made to the status quo of current infrastructure development practices and developing approaches for rewarding construction methods and materials employing sustainability principles should be adopted. Minimizing environmental impact, maximizing economical benefits, and minimizing socio-cultural impacts should be quantified for application to infrastructure development. It is anticipated that municipal owners will adopt models such as SAM for determining their public works infrastructure projects rather than just based on lowest cost estimate.

This article is a simplified version of a paper Sustainability Concepts Applied To Trenchless Construction Projects given by Dr Samuel T Ariaratnam, PhD at the Trenchless Technology in the Asia Pacific Conference hosted by the CHKSTT in Macau, November, 2007.