Sustainable site practices and construction emissions

Most of a building's environmental footprint is fixed in design, but the construction site itself has impacts the contractor directly controls: soil eroded into streams, sediment washed into storm sewers, dust and noise affecting neighbors, diesel exhaust affecting workers and the community, damaged trees, and the greenhouse gas emissions of equipment, temporary heat and power. These impacts are regulated, they are visible to the public and the owner, and several are prerequisites in green building rating systems. This lesson covers erosion and sediment control, stormwater, dust, noise, idling and other nuisance controls, and then measuring and reducing a contractor's own emissions using the Scope 1, 2 and 3 framework introduced in Lesson 2. HSE-251 covers spill response and waste regulations, which are not repeated here.

Erosion and sediment control

When vegetation and topsoil are removed, rain and wind can erode exposed soil quickly. Sediment washed off a site clogs storm sewers, smothers fish habitat, carries nutrients and pollutants into waterways and can bring heavy fines. Erosion control prevents soil from moving in the first place, for example by limiting the area disturbed, phasing work, and covering bare soil with mulch, blankets or temporary seeding. Sediment control captures soil that does move before it leaves the site, using silt fences, sediment traps and basins, check dams, inlet protection and filter socks. Erosion control is usually cheaper and more reliable than sediment control, so good plans emphasize it.

In the United States, the Clean Water Act requires construction sites that disturb one acre or more (or are part of a larger common plan of development) to obtain a National Pollutant Discharge Elimination System (NPDES) stormwater permit. Most states issue their own construction general permits; in the remaining areas, EPA's Construction General Permit applies. Permit holders must prepare a stormwater pollution prevention plan (SWPPP) describing erosion and sediment controls, pollution prevention measures and inspection schedules, inspect regularly and after storms, keep records and stabilize the site at completion. Many cities add their own grading and erosion control ordinances.

Most failures are maintenance failures. Controls installed at mobilization are often damaged by equipment, filled with sediment or removed for access and not replaced. A good practice is to assign a competent person to inspect weekly and after every significant rainfall, keep a log with photos, and fix problems within a set time. Rating system prerequisites for construction pollution prevention generally require exactly this: a plan, implementation and records.

Stormwater and low impact development

Permanent stormwater management is mainly a design matter, but builders install it and protect it during construction. Low impact development (LID) manages rainfall close to where it falls, using bioretention cells and rain gardens, permeable paving, green roofs, infiltration trenches and rainwater harvesting, rather than piping it away quickly. LID features are vulnerable during construction: sediment can clog bioretention soils and permeable paving before the building is even occupied, and heavy equipment can compact areas meant to infiltrate. Builders should fence off infiltration areas, install LID features late in the job after upstream areas are stabilized, and keep construction runoff away from them. CON-111 (lesson 9) covered green and blue roofs.

Dust, noise, light and idling

Construction affects neighbors through dust, noise, vibration, light and traffic, and complaints can lead to stop-work orders, fines and strained relationships with the owner and the community. Dust is controlled by watering haul roads and stockpiles, covering loads, wheel washing, limiting vehicle speeds, using water or vacuum attachments on saws and grinders, and stopping dusty work in high winds; silica dust is also a serious worker health hazard regulated under occupational health and safety rules. Noise is controlled by respecting municipal noise bylaw hours, choosing quieter equipment, siting generators and compressors away from neighbors, using acoustic barriers and planning noisy work for less sensitive times. Light from site lighting should be directed downward and shielded.

Idling diesel and gasoline engines waste fuel, add emissions and expose workers and neighbors to exhaust. Many municipalities and some provinces and states limit idling to a few minutes, with exceptions for equipment that needs to run to work, and many owners and contractors set stricter site rules. Practical measures include idling limits in site orientation, auto-shutdown features on newer equipment, warm shelters for workers in winter so that trucks are not left running, and monitoring fuel use per machine.

Construction greenhouse gas emissions

A contractor's own emissions come mainly from diesel in excavators, loaders, cranes, generators and trucks; gasoline in pickups; propane, diesel or natural gas for temporary heat; and grid electricity for temporary power, lighting, hoists and electric tower cranes. In Canada, temporary heat during winter construction is often the largest source after equipment. Lesson 3 included an estimate of these emissions in module A5. Under the Greenhouse Gas Protocol, a contractor's fuel use is its Scope 1, its purchased electricity is Scope 2, and its purchased materials, subcontractor activities, waste hauling and employee commuting are Scope 3.

# Marlowe Builders' Scope 1 and 2 emissions on Cedar Row from monthly site logs (illustrative)
import random
random.seed(9)
months = 18
winter = {4, 5, 6, 7, 8, 16, 17}             # months needing temporary heat
factors = {'diesel': 10.21, 'gasoline': 8.78, 'propane': 5.72}  # kg CO2 per gallon (EPA factors)
grid = 916 * 0.4536 / 1000                    # kg CO2e per kWh, RFCW subregion (eGRID2023)
log = {'diesel': 0, 'gasoline': 0, 'propane': 0, 'kwh': 0}
for m in range(months):
    heavy = 1.8 if m < 5 else 0.7             # excavation and structure use more equipment
    log['diesel'] += random.gauss(370 * heavy, 40)
    log['gasoline'] += random.gauss(119, 13)
    log['propane'] += random.gauss(950, 105) if m in winter else 0
    log['kwh'] += random.gauss(9000, 1000)
s1 = {k: log[k] * f / 1000 for k, f in factors.items()}
s2 = log['kwh'] * grid / 1000
for k, v in s1.items():
    print(f"Scope 1 {k:9s} {log[k]:8,.0f} gal {v:6.1f} t CO2e")
print(f"Scope 2 electricity {log['kwh']:8,.0f} kWh {s2:5.1f} t CO2e")
total = sum(s1.values()) + s2
print(f"Scope 1 + 2 total: {total:.0f} t CO2e; at an internal price of $100/t: ${total * 100:,.0f}")

Scope 1 diesel       6,703 gal   68.4 t CO2e
Scope 1 gasoline     2,173 gal   19.1 t CO2e
Scope 1 propane      6,157 gal   35.2 t CO2e
Scope 2 electricity  160,414 kWh  66.7 t CO2e
Scope 1 + 2 total: 189 t CO2e; at an internal price of $100/t: $18,938

Marlowe Builders' Scope 1 and 2 emissions on Cedar Row come to about 189 metric tons. Diesel equipment is a little over a third of the total, propane temporary heat almost a fifth and grid electricity, on an Ohio Valley grid, about a third. Compared with the roughly 2,600 metric tons in the materials Marlowe buys (its Scope 3), the site emissions are small, but they are entirely within the contractor's control, and at an internal carbon price of $100 per metric ton they would carry a notional cost of about $19,000.

An internal carbon price, a notional cost per metric ton used in decisions, helps make emissions visible alongside cost. ACCT-321 (lesson 10) shows how a carbon price can be built into performance measures, and BUS-230 (lesson 12) covers measuring Scope 3 emissions in supply chains. Contractors increasingly report their emissions to owners and lenders, set reduction targets and use the data in bids, and some public owners now ask for construction emissions plans.

Reducing construction emissions

Equipment emissions can be reduced by choosing the right size of machine, maintaining it well, limiting idling and, increasingly, using electric equipment. Battery-electric compact excavators, loaders, telehandlers, scissor lifts and aerial platforms are now commercially available, and electric tower cranes and hoists are common where grid power is available. Electric equipment is quieter and has no exhaust, which helps on urban sites and indoors, but it needs charging infrastructure and adequate temporary power, which must be planned early with the utility. Renewable diesel (hydrotreated vegetable oil) can be used in many existing diesel engines and reduces life cycle emissions substantially, although its tailpipe emissions are similar and its supply and price vary. Newer engines meeting the latest emission tiers reduce particulate matter and nitrogen oxides, which matters for health, but do not reduce carbon dioxide much.

Temporary power should come from the grid wherever possible. A diesel generator typically burns about 0.3 liters of fuel per kilowatt-hour, emitting about 0.8 kilograms of CO2e per kilowatt-hour, which is far more than most grids. Connecting temporary power early, using battery storage to cover peaks, and right-sizing generators when they are unavoidable all cut emissions and noise. Temporary heat can be reduced by enclosing and insulating the building envelope early, using insulated tarps and curing blankets, heating only the areas being worked, using electric heat where the grid is clean, and, with care and the manufacturer's and engineer's approval, using the permanent heating system with protection against dust.

Transport and deliveries can be reduced by consolidating deliveries, using local suppliers, planning just-in-time deliveries to avoid repeat trips, and, on large urban projects, using construction consolidation centers. Worker commuting can be reduced by encouraging carpooling and transit, which also eases parking on tight urban sites.

Tree protection and site ecology

Existing trees take decades to replace, provide shade and stormwater benefits and are often protected by municipal bylaws. Their roots extend well beyond the drip line, and most damage comes from soil compaction, grade changes and trenching in the root zone, not from direct contact. Tree protection zones should be fenced before any work starts, kept clear of stockpiles, parking and washout, and inspected regularly. Where work in the root zone is unavoidable, an arborist should advise on methods such as hand digging, air spading or boring. Topsoil can be stripped, stockpiled and reused for landscaping rather than exported and replaced, and invasive species should not be imported in fill or on equipment.