The construction industry is the third highest producer of greenhouse gas (GHG) emissions from industrialized countries, placing it under the spotlight in terms of sustainable technologies. At the same time, the role of trenchless construction projects continues to grow as cities rehabilitate ageing pipelines underground and expand their networks into new areas.

According to research from Journal of Green Building with the City of Edmonton and the Consortium for Engineered Trenchless Technologies (CETT) at the University of Alberta to compare actual GHG emissions for hand tunneling versus those of the trenchless pilot tube method (PTM).

Learn more about GHG by reading Understanding Construction Emissions: What’s Causing Greenhouse Gases & How We Can Improve.

What Are Greenhouse Gases?

The most widely used measure of GHG is the emission rate of carbon dioxide (CO2). The Journal of Green Building quote the Global Greenhouse Gas Reference Network Report when they state that statistics show a global increase of CO2 in the atmosphere from 280 ppm to over 380 ppm over the last 60 years.

A rise in GHG is widely accepted as a significant cause of climate change. As a result of these figures and their consequences, individuals, companies and nations have developed initiatives to curb and reverse the trend.

However, CO2 is not the only GHG to be worried about. Particulate matter, hydrocarbon content, SO2, and NOx are all also important measure to bear in mind.

The most well-known international initiative to combat climate change is the Paris Accord, agreed in 2015. The agreement was developed by the United Nations Framework Convention on Climate Change. It sets the ambitious goal of keeping global temperature rise to less than 2º C above pre-industrial levels.

How Construction Contributes to GHG emissions

The Journal of Green Building describe their research methodology base on calculating GHG emissions in the construction industry on the basis of fuel consumption. This includes both construction machinery itself like bulldozers, excavators etc. as well as transportation trucks to get equipment and fuel to and from the site.

When calculating emissions from construction machinery, it is important to take into account the machine loading.

Fuel consumption and therefore emissions vary significantly when idling compared to using equipment at full load. Various studies and models have been developed to assist researchers to make accurate estimates of GHG emissions.

What's the Difference Between Hand tunneling and PTM?

While the purposes of hand tunneling and PTM are both the same in terms of installing an underground pipe, the methods used are quite different.

As the name implies, hand tunneling is manual labor intensive. First a vertical shaft is excavated to the required depth. Then, the tunnel is excavated. Soil is removed from the tunnel to the access shaft from which it is hoisted to the surface and removed. During the process the tunnel is supported and lined.

Mechanical excavation uses a tunnel boring machine (TBM). This is typically necessary for longer tunnel excavations. (Read How Tunnel Boring Machines Work.)

PTM is a combination method using microtunneling, horizontal directional drilling (HDD) and auger boring. A PTM project happens in three stages: pilot tube installation, reaming and installing auger casings and finally product pipe installation. The method was originally designed for small diameter gravity flow pipe installations but can be used for larger pipes too.

New generation machines can eliminate the pilot tube stage and progress directly to auger casing installation.

The Journal of Green Building Case Study

The Journal of Green Building obtained the research during a live construction project in the City of Edmonton in Canada. Both hand tunneling and PTM are used extensively by the city for installing new pipes or replacing old pipes.

In this particular case, a contractor used both methods for the installation of a new 68 cm diameter clay sewer line with an overburden depth of 12.9 m and a length of 60 m. The contractor recorded a breakdown of all construction activities including equipment used and duration.

Emission factors were derived using the EPA standard emissions model. Using the records from the construction contractor along with these emission factors actual emissions were calculated for each piece of equipment and for the project as a whole.

An identical length of pipe at an identical length was installed using each method and total emissions calculated for comparison.

The duration of the PTM project was 15 days with an average working time of 10 hours per day. GHG emissions from construction equipment was far higher than that of transportation, with the biggest individual contributor of CO2 being a site generator. A power pack and crane were also significant emission contributors.

Total CO2 emissions for all equipment and transport over the entire duration of the project was calculated at 42, 160 kg.

Due to the high labor content, a hand tunneling project has a much longer duration. This project took 60 days at an average of 10-hours per day. Once again, the equipment GHG emissions far outweighed the emissions from transportation.

A site crane was the biggest individual CO2 contributor due to the constant use for removing excavated soil from the access shaft. The air compressor was also a high emission source. Total CO2 emissions for all equipment and transport over the entire duration of the project was 55,781 kg.

What We've Learned

This study conducted with the support of the City of Edmonton and the University of Alberta sheds some valuable light on the difference in GHG emissions using different trenchless construction methods. For this project, the Journal of Green Building showed that PTM proved to have 32% lower CO2 emissions than hand tunneling.

These comparisons highlight the need for emission estimates to form part of the method selection process for trenchless projects. By taking advantage of the more efficient methods, the construction industry can contribute to a more sustainable future.