Digging Deep

by Balfour Beatty

Imagine you’re tasked with digging a tunnel 30 feet underneath one of the busiest streets in a major metropolitan city. Now imagine that you must complete the work by hand, tunneling 700 linear feet with little more than jackhammers to chip away rock and handcarts to remove dirt and debris.

Our team and trade partners on the North End Improvements project in Dallas, Texas, utilized this exact method to enable critical wastewater utility access for a premier mixed-use tower development. Adding mere weeks to the planned schedule, the team and our incredible trade partners tunneled through nearly 3,500 cubic feet of Dallas soil and bedrock. Through collaborative solutions, the team kept Dallas traffic moving, prevented utility disruption and kept our workers safe.

Creative Solutions

Placing new utility tunnels typically involves traditional trenchwork – digging out a trench the full length of the line, then backfilling the space and replacing any excavated pavement. On North End Improvements, however, this route was simply not an option for our client or the City of Dallas. The existing, 18-inch wastewater line served a large area and could not be disrupted. Additionally, the street above is one of the city’s most heavily trafficked.

Our project team and trade partners North Star Trenchless and John Burns Construction instead turned to the creative but ambitious solution of complete, end-to-end tunneling. Large-scale industrial tunneling equipment like augers and boring machines could handle the project scope, but these methods are too imprecise to avoid disrupting the road above or the critical utility infrastructure nearby.

“Once we decided to explore the possibility of tunneling, we collaborated quickly with our partners to assess the geotechnical reports and develop a plan to complete the tunnel by hand,” says Senior Safety, Health and Environment Manager Drew Heffner. “Our chief concern at every step was to protect our workers and protect the soil stability and utilities around the tunnel.”

To that end, the team planned for just ten linear feet of excavation per day in 18-inch sections. The steady and deliberate pace enabled the team to take every safety consideration into account, including soil stability, lighting, ventilation, fire risk and emergency rescue procedures.

Safe Spacing

Traditional trench digging is a dangerous enough process. Tunneling by hand, in small and enclosed spaces at all times, demands even more stringent safety procedures to ensure safety for our workers and commuters on the roadway above. And ensuring safety on that active roadway was the most apparent above-ground step.

The team created four traditional trenches that provided access to segments of the tunnel. The access trenches were surrounded on all sides by active traffic, so the team deployed extensive concrete barriers and worked closely with the city on strategic and limited lane closures to protect our workers and any drivers and pedestrians.

The area is also highly traveled by foot, so the team also supplemented their concrete partitions with high fences, warning signage and other mobile barricades to prevent passersby from accessing a potentially dangerous area. At all times, the team communicated with nearby businesses to maintain the integrity of the project’s safety boundaries and ensure that pedestrian rerouting was as logical and convenient as possible for residents and local business clientele.

“Working in close and constant communication with our client, the city and nearby businesses was more than just an important part of our Zero Harm planning,” says Safety, Health and Environment Director Jordan Webster. “It was also a way to go the extra mile and serve all stakeholders’ needs, preventing disruptions to daily business in the area as much as possible.”

Safe Environment

During excavation, maintaining workers’ ability to breathe clean air remained a constant imperative. In addition to the limited airflow of a small working space, the dust particulate, jackhammer exhaust fumes and other pollutants created by excavation required comprehensive and creative solutions.

The team first turned to forced air – displacing dangerous contaminants and stale air by pushing clean air into the excavated tunnel. Although it was an important component of the overall safety plan throughout the project, forced air became less effective as the tunnel stretched farther out from each access point.

“Some sections of the tunnel could reach 300 feet between access points, and at that distance, forced air isn’t enough to ensure worker safety,” Drew says. “We enhanced our forced air approach by opening up pilot holes at each access trench, providing another escape route for dangerous air.”

The access trenches’ mid-road locations also necessitated constant air monitoring, as any idling vehicles nearby could dump poisonous carbon monoxide and hydrocarbons directly into the open tunnel. Again, forced air and mechanical ventilation prevented fumes from entering the tunnel and gave workers a steady stream of breathable air, while our teams on the surface used sensors at each tunnel head to vigilantly monitor air quality.

Safe Soil

Our project teams and trade partners also ensured soil, tunnel and trench stability at every phase of planning and completion, creating a safe working environment in which our crews could dig, remove dirt and debris and lay new pipeline in confidence.

In order to place the new 30-inch wastewater line, the tunnel workers first had to excise the existing 18-inch, steel-cased pipe with a cutting torch. Working three feet at a time, the team reinforced each new section with steel tunnel liner plate, creating a 46” tunnel space before performing any further excavation. This repeatable formula enabled the team to constantly reassess progress and safety conditions.

The team encountered a variety of below-grade materials and conditions, including granular backfill and solid rock, that all required different approaches to safety. Our steady approach, constant radio communication with workers in the tunnel and collaboration with North Star Trenchless ensured that our workers remained safe, and our tunnel remained structurally sound.

“We are immensely proud of the exceptional efforts of our trade partners, North Star and John Burns,” says Senior Project Manager Brent Johnson. “Digging this tunnel by hand was an incredibly complex and difficult challenge, and one that these teams performed with unparalleled safety and skill.”

The projected team also contended with a relatively high water table in this area of Dallas, situated directly over the Trinity Aquifer. As excavation proceeded, the team maintained a 24/7 water pumping operation to remove groundwater and keep the site dry and stable.

Of course, a no less crucial part of safety planning is to prepare for the unforeseen, so the team also engaged local first responders to assess the site during planning, review the team’s safety plans and become familiar with the project site should emergency evacuation have become necessary.

The team also maintained a status board at all times with the names of descended workers. Every worker was always accounted for, and had an emergency situation arisen, first responders at the site would immediately have known who they were looking for and where.

A Team Triumph

When presented with a novel challenge, our project team dug deep to create innovative solutions.

The North End Improvements utility hand tunneling ultimately stands as a testament to our team’s commitment to serve our clients, communities, people and partners. Whether above ground or below, we always prioritize safety for our people and excellence for our clients.