Tunneling vs. Traditional
Installing an underground pipeline is a major undertaking, regardless of the size of the pipeline or the size of the city. Traditional methods of open cut or cut and cover are appropriate and cost-effective for shallow depths less than 25 feet and if there is adequate space for construction. However in congested urban environments or very deep installations, trenchless construction methods may be more appropriate or the only viable alternative.
Halff experts understand the trenchless construction methods of tunneling, microtunneling (MTM), horizontal directional drilling (HDD) also known as directional boring, pipe ramming (PR), pipe jacking (PJ), moling, horizontal auger boring (HAB) and other methods for the installation of pipelines with minimal excavation. Large diameter tunnels, such as the Mill Creek project described below, can be constructed by a tunnel boring machine (TBM), and drilling and blasting techniques are larger versions of subsurface construction. By using trenchless tunneling, we eliminate or greatly reduce the disruption associated with cut-and-cover trenches. Today, tunneling provides the most appropriate and cost-effective method for very deep pipelines, pipelines crossing environmentally sensitive areas or congested urban environments. Halff’s tunneling insight and ability facilitated the analysis and design of the Mill Creek project.
Mill Creek/Peaks Branch/State Thomas Drainage Relief Tunnel
Mill Creek, Peaks Branch and State Thomas are three prominent drainage basins in Dallas. Combined, these basins consist of 5,700 acres of commercial and residential land use. Fair Park, Baylor Hospital, Deep Ellum, Old City Park, the “M” streets and IH 30 are in these basins.
The drainage systems in these areas consist of underground storm sewers that were first constructed in the 1930s. These drainage systems are undersized, and these areas are subject to significant flooding during large storms. The May 5, 1995, storm caused significant flooding at Fair Park and IH 30. Storms also caused significant flooding in these areas in March 2006 and September 2007.
The City of Dallas selected Halff to develop and evaluate alternatives for providing flood relief to these areas and to develop construction documents for the selected alternative. Halff prepared and evaluated alternatives for conventional cut-and-cover relief systems and deep tunnel relief systems. The selected alternative is a 5-mile long, 30- foot diameter deep tunnel that will be constructed by a tunnel boring machine (TBM) in the Austin Chalk at depths ranging between 100 and 150 feet below ground.
Utilizing a deep tunnel for this project has several advantages over a cut-and-cover system. Cut-and-cover systems create significant neighborhood traffic disruptions and impacts to local businesses as pavements and utilities are demolished to accommodate construction.
A cut and-cover system would also require significant right-of-way acquisition and associated relocations. The tunnel will be constructed at depths well below existing improvements such as buildings, roads and utilities, which will significantly reduce street and utility reconstruction costs, right of-way acquisition costs and disruptions to neighborhoods and businesses.
The major elements of the Mill Creek/ Peaks Branch/State Thomas Drainage Relief Tunnel include:
• Surface inlets and near-surface storm sewers to intercept and convey overland flow
• Five large inlet structures that will receive stormwater from existing storm sewers and new inlets
• Five drop shafts varying in size from 12 to 25 feet in diameter, connecting the inlet structures to the deep tunnel
• A 40-foot diameter outfall shaft
• A 30,000-gpm dewatering station
The project design has been completed and construction is expected to commence in 2016.
The benefits of tunneling can outweigh those of traditional cut-and-cover methods for very deep pipelines, pipelines crossing environmentally sensitive areas or congested urban environments. Protecting the environment as well as cutting costs means that this method is a viable alternative. Halff has a multi-disciplined team of engineers, modelers and planners who work together to help clients achieve long-lasting results and the greatest value for our clients’ investments. If Halff Associates can assist your team, please call Executive Vice President Jessica Baker, PE, CFM, PMP, at (214) 217-6692.
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