Taking the Long View on Short Tunnels
Tunnel construction is risky by nature, primarily because geological features that affect tunneling operations can’t always be completely defined in advance. Each tunnel presents the contractor’s mining crew with a new learning curve in regards to the behavior of the ground. On projects that are primarily trench jobs, with limited lengths of actual tunnel work, the typical general contractor spends a brief amount of time evaluating ground conditions, relying on the experience of a local tunneling subcontractor and data gathered from just a few onsite borings. These short crossings compound the risks involved with tunnel construction because they are usually at locations where unplanned remedial work is a costly prospect—if it can be done at all.
To address the risks involved on short tunnel projects, contractors and their clients must apply the same strategies and preventive measures used for larger projects.
Why some projects fall short
Consider a predictable pipe-laying project that crosses a railroad, river, or major highway. This is no time for the owner to discover that their contractor might lack the means to successfully address short tunnel construction. Hiring a design firm with tunneling expertise on-staff and a proven record of experience on a wide range of tunneling projects can supplement the abilities of the contractor, mitigate the risks involved in construction, and help prevent significant cost overruns.
In places where “short tunnels” are needed, the opportunity for geotechnical investigation is usually limited. Unlike projects where miles and miles of tunnels are being constructed and relocating a boring a few hundred feet is not critical, these projects are being completed around (or more precisely under) a huge obstacle, such as a highway. Because of this, investigators can only probe beneath the surface in a few places, and the owner ultimately receives a less reliable geotechnical data report.
When unforeseen problems are encountered during the excavation—changing soil conditions, anomalies such as voids or sinkholes, or unexpected boulders—the cost to complete the project can skyrocket. Although the cost of a short tunnel in a contract is typically included as a lump sum, it is executed at a significant hourly rate due to equipment and labor costs. Consequently, removing boulders or even changing the configuration of the boring machine partway can quickly lead to a six-figure change order.
Anticipating the unknown
To address the risks involved on short tunnel projects, contractors and their clients must apply the same strategies and preventive measures used for larger projects. Owners can manage risks by prequalifying subcontractors to ensure that they meet key minimum requirements for financial solvency and technical expertise. They can also create a more flexible bid, comprised of several bid items, to enable risk sharing between contractor and owner. Bid documents can be placed in escrow, so that they remain protected up until the point where access or revision is needed.
Another useful approach is to include a geotechnical baseline report as part of the contract documents. This report provides an interpretation of ground conditions for bidders, allowing each of them to bid on the same tunneling conditions. Implemented correctly, these precautions can help project leaders prepare for the uncertainty inherent in short tunnel excavation, thereby reducing associated risk for all involved.
Mike Gilbert, vice president, has more than 30 years of geotechnical engineering experience and specializes in tunneling.