The construction industry, which consumes a large quantity of fossil fuels, has been tasked with reducing airborne emissions. The use of traditional open cut methods and equipment for the installation of underground utilities has been a common practice in the construction industry for decades. Recently, there has been a growing trend towards adopting minimally-intrusive trenchless methods, particularly in congested urban environments. Recognition of the urgency to curb emissions worldwide has led to an increase in research efforts aimed at developing methods to quantify and reduce emissions. Current carbon quantification approaches focus mainly on the effect of added emissions due to traffic delays during construction road closures. While these models provide excellent information, it is imperative that competing utility installation methods also be assessed to determine their ‘environmental friendliness’. As a result, Arizona State University and Vermeer Corporation developed a commercially-available emissions calculator tool called 'e-Calc' to aid stakeholders in calculating and comparing anticipated emissions between competing technology options.

The emissions calculator tool – e-Calc – has default equipment for four different utility construction methods and can be tailored to meet individual project characteristics. To demonstrate the effectiveness of the e-Calc, a case study of a wastewater infrastructure project in Los Lunas, New Mexico, comparing the impact of emissions from traditional open cut construction to trenchless pipe replacement (pipe bursting), is included below.

Model development background

The Environmental Protection Agency (EPA) is an agency of the United States Government that is charged with the responsibility of protecting human health and safeguarding the environment including air, water, and land. The EPA sets national standards for environmental protection using environmental assessment, research, and education. EPA (1997) provides the reports and relevant background data in arriving at EPA approved ‘emission factors’. Emission factors for different equipment using different fuels (i.e. gasoline, diesel, natural gas) operating at different power are provided in various sections of AP-42 (EPA 1995).

The EPA has collected data on operation characteristics of different construction equipment and categorised engines based on their power. Emissions data are published for different categories of engines performing different activities. The different emission control standards enforced by the EPA have resulted in a reduction of sulfur and nitrogen emissions. To determine emissions from equipment and vehicles, emission factors can be calculated based on EPA test data. This approach aids in estimating pollution using equations that are applicable for a specific construction operation.

EPA document AP-42 provides documentation on six airborne pollutants:

1. hydrocarbons (HC); 2. carbon monoxide (CO); 3. nitrogen oxide (NOX); 4. particulate matter (PM); 5. carbon dioxide (CO2); and 6. sulfur oxide (SOx).

These six pollutants are calculated in e-Calc for a given utility project based on actual equipment used (or projected to be used) on a given project. Nitrogen oxide, carbon monoxide, and hydrocarbons are considered important because they are precursors of ozone. Carbon dioxide and nitrogen oxide are principal greenhouse gases.

Emissions calculator tool (e-Calc)

E-Calc was developed in Microsoft Excel using Visual Basic coding and estimates emissions from underground utility projects based on EPA-approved methodology. Required input data can be obtained from daily progress reports or productivity estimates, while equipment-specific information should be acquired from contractor inventory records.

Non-road equipment data includes: power; model year; engine technology; useful hours and cumulative hours to date; fuel characteristics such as type and sulfur content; and activity characteristics such as representative equipment cycle, power used and hours of use. The data required to calculate emissions generated from on road transportation equipment includes: model year, gross vehicle weight, mileage, fuel characteristics such as type and sulfur content; and activity characteristics such as altitude of operation, number of trips, one way distance and return distance.

As with any computer tool, the accuracy of output information depends on the accuracy of the input data. The calculator is a tool intended as a guide for contractors, engineers, and owners to obtain an ‘estimate’ of the environmental impact of their proposed underground utility project during the planning stage. The tool provides a comparison of emissions generated from two possible installation methods with default information available for four typical utility construction methods:

1. horizontal directional drilling; 2. trenchless pipe replacement; 3. trenching; and 4. traditional open-cut.

The tool is relatively user-friendly and can be applied to any construction process that incorporates machinery and equipment.

Open cut versus pipe bursting emissions

The following case study on a project with pipe bursting and a traditional open cut option demonstrates e-Calc and compares emissions generated from two different utility construction methods. A utility contractor that employs both construction methods provided a breakdown of task durations and company equipment inventory details. It should be noted that the project was eventually completed using pipe bursting as the preferred method for the case study installation. Equipment utilisation and activity data were collected onsite by monitoring the construction operation.

The project consisted of upsizing a 200 mm clay wastewater line to a 250 mm high density polyethylene (HDPE) line in the Town of Los Lunas, 25 kilometres north of Albuquerque, New Mexico. The installation depth was 2.1 metres and length was 106 metres spanning between two manholes. There were two marked 100 mm service laterals along the alignment.

Trenchless pipe replacement option

Trenchless pipe replacement involves excavation of an entry pit for pulling the new pipe and service pits for reconnecting the service lateral. Service pits at the lateral locations provide access for reconnection to the main after the installation. The existing pipe was burst using a pneumatic method of pipe bursting.

The crew commenced working on the entry pits and service pits one day prior to the actual pipe replacement operation. Initially, the existing wastewater line was inspected using CCTV and the lateral connections were identified and marked on the site. The lateral crossings required two service pit locations. The entry pit for pulling in the new 250 mm HDPE pipe was excavated near one of the manholes. Twelve meter sections of HDPE pipe were fused together using a butt fusion technology onsite. Traffic flow was restricted to one lane along the length of the alignment. Excavated materials from the entry and service pits were used during the backfilling operation.

The entry pit and service pits were excavated using a Volvo BL70 backhoe. A winch was placed at Manhole 1 (MH 1) and the new pipe was installed from Manhole 2 (MH 2). The winch was positioned above MH 1 and the winch cable was pushed towards the entry pit. The bursting head was connected to the winch cable once it reached the entry pit location. The other end of the bursting head was connected to the new 250 mm HDPE pipe using a swivel. The swivel helps to prevent any torque transferring from the bursting head to the pipe during installation. The existing 200 mm clay pipe was burst by the bursting head while the new product pipe was simultaneously pulled through the expanded borehole created by the bursting head. At the end of the pull, the head was disconnected from the pipe. The head was then adjusted to move inside the existing pipe to exit at the entry pit. Minimal backfill and road restoration activities were required at the excavation locations. Backfill was performed in 300 mm layers and a hand-held compactor was used to compact the soil at the service pits. At the entry pit, a soil compactor with a drum size of 900 mm was used for compaction.

The construction operation at the site of the pipe bursting for upsizing the existing 200 mm clay pipe to 250 mm HDPE pipe was studied. The actual equipment operating times and usage were recorded for calculating emissions. Project details (Figure 2) were inputted into the emissions calculator tool to determine the estimated emissions.

Open cut construction option

In an open cut method of construction, the entire alignment of the new pipe requires excavation to facilitate pipe placement and in comparison to the pipe bursting method, a large site area, is required for movement of onsite equipment. For purposes of comparison, the contractor’s estimator was consulted to provide project productivity estimates if the project had been completed using open cut methods. Since the contractor performs both open cut and pipe bursting projects in New Mexico, the details on activity durations were readily available from their estimating database. Details of the non-road and on-road equipment required for the open cut construction were obtained from the contractor’s equipment inventory.

The site activity commences with excavation of a 1,200 mm trench width for the entire stretch of the alignment. Since the excavation is 2.1 metres deep, shoring is required to be placed along the entire trench alignment. For the purpose of dust control, a water tank of 4,000 US gallon capacity is required to spray water at the site. Excavated material is used to backfill the trench. Similar equipment to those for the pipe bursting option is used for compaction and paving. As with pipe bursting, twelve metre sections of HDPE pipe were fused using a butt fusion technology onsite.

The construction operation at the site for replacing the existing 200 mm clay pipe with a 250 mm HDPE pipe using tradition open cut was studied. The actual equipment operating times and usage were recorded for calculating emissions. As before, project details (Figure 3) were inputs into the emissions calculator tool to determine the estimated emissions.

Comparison of emissions

The total emissions calculated from the two utility methods are compared in Figure 4. The results reveal the emissions from the open cut option to be approximately 79 per cent greater than those generated from the pipe bursting operation. The total project time including mobilisation and demobilisation was three working days for the pipe bursting option, while the estimated duration for completing the project specifications using open cut was seven working days.

In addition to time, cost, and social benefits, trenchless methods such as pipe bursting provide a better environmental benefit as evident by the major reduction in airborne emission compared to open cut. It is anticipated that future project requirements will include a component of emission assessment in addition to cost during the design and method selection.

Conclusions and recommendations

The results revealed that emissions generated from open cut were approximately 79 per cent greater than those from the pipe bursting option. This is a result of the increased productivity and reduced equipment requirements offered by the trenchless option. It is envisioned that quantification of emission data will be a factor in deciding between construction methods on future projects in addition to cost given current environmental sensitivity.

It is recommended that the emissions calculator tool be used on future utility construction projects to further gather information comparing the generated emissions of various construction technology options. This could aid in setting target acceptable emission levels for specific projects. To date, e-Calc has been used on twelve underground utility projects and four different utility construction methods.

This article is a condensed version of a paper titled, “Methodology for Calculating the Carbon Footprint of Underground Utility Projects” published in the Proceedings of the 2009 International No-Dig Show, Toronto, Ontario Canada. Please visit the Trenchless International website for references.