Comparison of Carbon Footprint of Trenchless and Open-Cut Methods for Underground Freight Transportation

Abstract

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. The construction industry is a major producer of such emission due in part to the magnitude of operations and the vast array of equipment. The proposed underground freight transportation (UFT) uses unmanned vehicles to move freight through tunnels and pipelines between terminals. This paper presents a comparison of carbon footprint for conventional open-cut and trenchless technology methods, particularly tunneling in rural area. The paper considers building a freight pipeline in a proposed route from Huntsville to Madisonville, Texas, under existing right-of-way and with a pipe diameter of 8 to 13 ft. The objective of this paper is to quantify carbon emissions produced by construction equipment for hauling excavated soils during pipeline construction. Trenchless technologies with minimum surface and subsurface disruptions offer a viable alternative and result in lesser carbon emissions compared to open-cut method. The findings of this paper will assist the pipeline construction industry in technology selection to minimize environmental impacts.

Full text

Comparison of Carbon Footprint of Trenchless and Open-Cut Methods for
Underground Freight Transportation
Razieh Tavakoli1; Mohammad Najafi2; Amir Tabesh3; and Taha Ashoori4
1Ph.D. Student, Graduate Research and Teaching Assistant. E-mail:
razieh.tavakoli@uta.edu
2Professor and Director. E-mail: najafi@uta.edu
3Ph.D. Student, Graduate Research and Teaching Assistant. E-mail:
amir.tabesh@uta.edu
4Ph.D. Student, Graduate Research and Teaching Assistant. E-mail:
taha.ashoori@uta.edu
1,2,3,4 CUIRE, Dept. of Civil Engineering, Univ. of Texas at Arlington, P.O. Box
19308, Arlington, TX 76019.
Abstract
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human
activities. The construction industry is a major producer of such emission due in part
to the magnitude of operations and the vast array of equipment. The proposed
underground freight transportation (UFT) uses unmanned vehicles to move freight
through tunnels and pipelines between terminals. This paper presents a comparison of
carbon footprint for conventional open-cut and trenchless technology methods,
particularly tunneling in rural area. The paper considers building a freight pipeline in
a proposed route from Huntsville to Madisonville, Texas, under existing right-of-way
and with a pipe diameter of 8 to 13 ft. The objective of this paper is to quantify
carbon emissions produced by construction equipment for hauling excavated soils
during pipeline construction. Trenchless technologies with minimum surface and
subsurface disruptions offer a viable alternative and result in lesser carbon emissions
compared to open-cut method. The findings of this paper will assist the pipeline
construction industry in technology selection to minimize environmental impacts.
Key Words: Trenchless Technology - Pipeline Construction - Underground Freight
Transportation (UFT) - Carbon Footprint - Open-cut Method
INTRODUCTION
The term carbon footprint is commonly used to describe the total amount of
carbon dioxide (CO2) and other greenhouse gas (GHG) emissions for which an
individual or organization is responsible. Footprints can also be calculated for events
or products. Carbon footprints are typically calculated to include all greenhouse gases
(GHG) and are expressed in tones of CO2 equivalent (tCO2e). Other forms of
calculating footprint include CO2 only and express the footprint in tCO2 (tones of
CO2) (Chilana, 2011). It is important that construction projects minimize the
possibility of creating hazardous conditions to the public and workers. Some
construction activities that contribute to air pollution include, land clearing, operation
of diesel engines, demolition, burning, and working with toxic materials. It is
important to conserve energy and protect the environment and the quality of life. This
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paper presents a comparison of open-cut1 and tunneling methods regarding carbon
footprint to build a 25-mile, pallet size underground freight transportation (UFT)
between Huntsville to Madisonville, Texas (Figure 1).
Figure 1. UFT Huntsville to Madisonville route
(Source: Google Earth)
BACKGROUND
Construction projects are generally carried out to support economic growth
and/or the social welfare of society. However, during the construction phase, the
community surrounding the construction site often finds itself subjected to negative
effects such as traffic impairment, noise, dust and subsequent economic losses
(Kamat, 2011). It has long been accepted that open excavation is capable of causing
major disruption to commerce and the general public (Najafi and Gokhale, 2005). A
key advantage of “trenchless” construction method is the ability to install new and
rehabilitate existing underground assets with limited disruptions to traffic and
business activities, reduced damage to existing paved surfaces, fewer adverse
environmental impacts and less disruption to normal life patterns of the people living,
working and shopping around the construction zone (Apeldoorn, 2000). Social costs
for pipeline type projects are typically much less for trenchless technology type
methods than they are for the conventional open cut techniques.
1 Cut-and-cover or open trench excavation.
Huntsville
Madisonville
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Underground freight transportation (UFT) is a class of automated
transportation systems in which vehicles carry freight through pipelines and tunnels
between terminals. Being able to use a part of the underground space of the existing
highways, will greatly facilitate the construction of such pipelines and tunnels and
reduce their construction costs (TxDOT, 2016). The UFT system will increase the
freight transportation capacity and decrease the social and environmental impacts of
the conventional transportation methods. Pipe diameter of two single-track2 tunnels is
8 ft (as shown in the following section for open-cut method) and diameter of one
twin-track tunnel is 13 ft as shown in Figure 2 for UFT standard-size pallets.
Figure 2. One twin-track system for standard-size pallets
(TxDOT, 2016)
METHODOLOGY
This paper estimates CO2 emissions for open-cut and tunneling methods for
UFT construction. Statistical data is used to calculate the quantity of CO2 emissions
to determine the magnitude of environmental impacts of both methods. A potential
UFT route is considered for 25-mile distance from Huntsville to Madisonville, Texas,
in rural area. Figure 3 shows the methodology of calculating the total carbon
footprint.
2 Two parallel tunnels (Two single-track tunnels with 8-ft diameter each).
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Figure 3. Methodology
OPEN-CUT METHOD
Historically, open-cut methods have been used for utility construction instead
of trenchless technology. Although, the open-cut method in the past was considered
to be the most economical way of laying pipes, there are other considerations that
may make open-cut unjustifiable. For example, the UFT concept assumes that the
pipe segments will be buried only a few feet underground (Liu, 2004), but large pipe
diameters will increase trench depth and a wider trench at the surface cause huge
amount of soil excavation and backfill. Open-cut construction for the UFT project is
possible as an option at locations where minimal disturbances to traffic, surface
development and the environment exist. In the open-cut method, most construction
efforts and resources are spent on trench excavation, shoring, dewatering,
embedment, backfilling and compacting, and reinstating the surface (Najafi and
Gokhale, 2005). Figure 4 illustrates the optimum UFT trench cross section option
based on trench width, soil type, and safe excavation depth (TxDOT, 2016).
Identify
construction
Choose the right
equipment
Calculate volume
of earthwork
Check equipment
output per hour
Calculate amount
of CO2 per hour
Calculate amount of
produced CO2 per CY
Calculate total amount of
produced CO2 per CY
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Figure 4. Trench cross sectional area (321 ft2)
Table 1 shows the quantity takeoff (QTO) for a 25-mile, pallet size UFT
construction by open-cut method.
Table 1. General QTO of Open-cut Method.
Item Volume Comment
Excavation 1,569,333 BCY or
2,510,933 LCY
60% Swell Factor Backfilling 1,364,543 LCY
Hauling 716,494 BCY or
1,146,390 LCY
Open-cut Construction
The following equipment was selected for open-cut construction:
• 3.5-CY excavator (one excavator every 5 miles) to excavate the trench.
• 3-CY bucket, front-end wheel-mounted loader for loading the stockpiled soils
in trailer trucks.
• 20-CY trailer dump trucks.
• Front-end loader for loading dump trucks and backfilling.
• 300-horsepower dozer for hauling the soil.
• 16.5-CY trailer truck, 50 mph average speed with a total cycle of two miles to
haul the flowable fill materials.
• 420 hp pipe layers to lay pipe sections in 30-min intervals assuming each pipe
section is 6 ft.
Since the excavation is in a rural area, the working time for trench excavation
and hauling soils assumed to be 16 hours per day (two 8-hour shifts). Table 4
shows the calculated emissions of construction equipment for open-cut method.
The following assumptions are made:
• Hauling distance is 20 miles.
• Loader cycle time is 140 sec.
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• Soil swell factor is 60%.
• Compaction factor for clay soil (limestone) is 90% (TxDOT, 2016).
• Amount of produced CO2 is 22.2 lb per gallon for trucks (Chilana, 2011).
• Energy consumption of truck is 8mpg (Franzese and Davidson, 2011).
• Average speed of truck is 50 mph.
The total CO2 produced using open-cut method is approximately 5,379 tons
for construction of 25-mile UFT.
Table 4. Information of Construction Equipment for Open-cut Method.
Equipment Power
(HP)
Productivity
(RSMeans)
Produced
CO2/hr
(gr)
Produce
d CO2
(gr/CY)
Volume
CO2
(U.S. tons)
3.5 CY
Excavator 200
115.5
(BCY/hr)
172,689
1,495
1,569,333
BCY
1,606
300 HP Dozer,
50-ft Haul 300
128.125
(BCY/hr)
160,890
1,256
716,494
BCY
992
200 HP
Loader, 50 ft
Haul
(backfilling)
200 243.75
(LCY/hr)
107,260
440
1,364,543
LCY
662
Front End
Wheel
Mounted
Loader 3 CY
Bucket
(Filling trucks)
156 196.88
(BCY/hr)
83,663
425
716,494
BCY
336
20 CY Truck,
Cycle 20
Miles
340
180
(LCY/hr)
62,936 180
716,494
LCY
276
16.5 CY
Truck, Cycle 6
Miles Hauling
the Flowable
Materials
285
350
(LCY/hr)
62,936 132
716,494
LCY
142
Pipe layers 420 12 (ft/hr) 112,623 9,385
(gr/ft)
13,200
ft 1,365
Total 5,379
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TRENCHLESS TECHNOLOGY
Trenchless technology and tunneling, as a trenchless technology technique,
includes all the methods of pipeline construction and renewal with minimal surface
and subsurface excavation (Najafi, 2013). Large diameter tunnels such as those
constructed by a tunnel boring machines (TBMs) and drill-and-blast techniques are
larger versions of pipeline construction. For the success of trenchless construction
projects, selection of appropriate equipment and tools are critical. Such factors as site
restrictions, design requirements (pipe strength), existing underground utilities, above
ground structures, obstructions on the installation path, soil conditions,
drive/reception shafts distances, required accuracy, as well as costs are all important
(Tabesh et al., 2016). Trenchless technology has the following advantages:
• More friendly to environmental: Less soil is disturbed. Impacts on adjacent
organisms and water bodies are reduced.
• Less disruption: Traffic delays are reduced or eliminated.
• Higher speed of installation: Trenchless construction often takes less time.
• Enhanced safety: Many safety concerns are associated with steep-excavation
slopes, work inside trench boxes, and worker exposure to traffic may be
eliminated or reduced with trenchless methods.
• Less interference with existing utilities: Trenchless projects can be planned to
go under or around existing utilities.
• Fewer unknowns: Minimal ground disturbances result in fewer contingencies
associated with subsurface conditions during installation of pipelines
(Tavakoli et al., 2017).
Tunnel Boring Machine (TBM)
The Tunnel Boring Machine ( TBM) is assembled inside a launch shaft at
one end of the tunnel alignment from where it initiates the boring operation
through the ground. The front of the TBM is equipped with a cutter head
having a number of mounted cutting wheels. The cutter head is designed to suit
the geological conditions expected during the tunnel drive. The excavated soils
(spoils or mucks) are transported back through the tunnel to the launch shaft or
accessible shaft locations where they can be raised to the surface and removed from
the jobsite by dump trucks (TxDOT, 2016). As the tunnel is excavated, reinforced
precast concrete segments are installed behind the TBM to form the tunnel lining
and provide the jacking mechanism for the TBM to move forward.
It is assumed that TBM has an average production rate of 100 ft per 20-hour
shift to allow 4 hours for maintenance. Various types of TBMs have specific power
requirements. Some factors considered for design of TBM are machine diameter,
length of the machine, segment layout, quantity of segments in ring, ground
conditions, minimum segment width and thickness (Lovat, 2017). Considering TBM
power requirements for different diameters, the average the total power for a 13-ft
tunnel is found to be approximately 3,200 kWh. The electric power packs using
hydraulic motor produce 5 grams of CO2 per kWh (EIA, 2017). The total CO2
emission is approximately 16,000 gr per hour. Table 2 shows the total cutter head
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power for different TBM diameters. Table 3 shows quantity takeoff of a 25-mile,
pallet-size UFT construction by tunneling method.
Table 2. Total Power of TBM for Different Tunnel Diameters.
(Adapted from Robbins, 2017)
No. Tunnel Diameter Cutter Head Power Total Power
1 11.5 ft 1,340 kW (1,836 hp) 2,010 kW
2 13 ft 2,100 kW (2,816 hp) 3,200 kW
3 14.1 ft 2,345 kW (3,143 hp) 3,517 kW
4 14.1 ft 960 kW (214.5 hp) 1,440 kW
5 15.1 ft 1,260 kW (1,690 hp) 1,890 kW
6 15.8 ft 2,275 kW (3,050 hp) 3,413 kW
Table 3. General QTO of Tunneling Method.
Item Quantity Unit Comment
Number of Main Shafts 1 Ea. Entry and Exit shafts
Number of Access Shafts 24 Ea. Every 1-mile for spoil
removal
Depth of Shafts 50 ft
Access Shaft Excavation 8,889 BCY
Main Shaft Excavation 4,630 BCY
Tunnel Boring Volume 648,584 BCY
Total volume of excavation 662,103 BCY
Tunneling Construction
The following assumptions were selected for tunneling:
• The soil swell factor is 60%.
• TBM production rate is 100 ft per day.
• Average hauling cycle is 20 miles to dump sites in rural areas.
• 20-CY trailer trucks are used and idle time is 15 min.
• Loader bucket size is 3 CY and Loader cycle time is 130 seconds.
• Productivity of crane is adjusted based on productivity of excavator.
Table 5 shows information about equipment used for tunneling. The total CO2
produced using trenchless method is approximately 887 tons for construction of 25-
mile UFT.
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Table 5. Information of Construction Equipment for Tunneling.
Equipment Power
(HP)
Productivity
(RSMeans)
Produced
CO2/hr
(gr)
Produced
CO2
(gr/CY)
Volume
Produced
CO2
(U.S. tons)
3.5 CY
Excavator 200
115.5
(BCY/hr)
172,689
1,495
13,519
BCY
14
Luffing Jib
Crane
20
kWh
115.5
(LCY/hr)
9317.6 81
21,630
LCY 2
Front End
Wheel
Mounted
Loader 3 CY
Bucket (Filling
trucks)
156 196.88
(BCY/hr)
83,663
425
13,519
BCY
6
20 CY Truck,
Cycle 20 Miles 340
180
(LCY/hr)
62,936 180
1,059,365
LCY
408
TBM 3200
kWh 5 (ft/hr) 16,000 640 648,584
BCY 457
Total 887
CONCLUSIONS
Carbon footprint analysis is becoming more and more popular in every
industry due to increasing concerns on global warming and greenhouse gas (GHG)
emissions. Construction industry needs to identify the potential benefits of carbon
footprint analysis and for every project. To analyze the carbon footprints, trenchless
and open-cut method of UFT was analyzed for energy consumption and CO2
emission. The open-cut method is often used to remove large volumes of soil during
construction of UFT. The disposal of this material requires hauling equipment which
drives up the cost. This paper presented a comparison of open-cut and tunneling
methods regarding carbon footprint to build a 25-mile, pallet size underground freight
transportation (UFT) between Huntsville to Madisonville, Texas. Analysis of carbon
footprint for this project showed that the total CO2 produced using trenchless
technology method is 887 tons and for open-cut method is 5,379 tons, an
approximately 6 times increase, resulting in more environmentally friendly
construction operations. In addition to carbon production for trenchless technology
and open-cut methods, other parameters and aspects of building an underground
freight transportation system must be considered, which were not part of scope of this
study.
ACKNOWLEDGMENTS
This research was conducted under a grant from Texas Department of
Transportation (TxDOT) under Project number 0-6870. The authors would like to
thank TxDOT for funding this innovative project.
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LIST OF ABBREVIATIONS AND ACRONYMS
BCY Bank Cubic Yards
ft Foot
HP Horsepower
GHG Greenhouse Gas
QTO Quantity Takeoff
TCO2e tones of CO2 Equivalent
TCO2 tones of CO2
CO2 Carbon Dioxide
KWh Kilo Watt Hour
LCY Loose Cubic Yard
TBM Tunnel Boring Machine
TxDOT Texas Department of
Transportation
UFT Underground Freight
Transportation
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