INTRODUCTION
Shafts are the doorways to the underground tunnel, serving as the location at which all material enters and exits. They vary in size and depth, and their design and construction are key to the successful completion of any tunnelling project. Shafts are quite often larger, deeper, more complex and, in some cases, more expensive than the cost of the actual tunnel itself.
When designing a shaft, ask yourself these four key questions: Where is the groundwater table? What type of ground will be excavated? How much working space is needed? How deep is the tunnel horizon? The answers to these questions determine which shaft construction methods are feasible and how best to use on your project.
Shaft Design
Before determining your shaft construction method, decide your minimum shaft size. During design, the minimum dimensions are typically determined by the physical layout of the final structure to be constructed or space needed for launching a tunnel boring machine (TBM). For water and wastewater tunnels, final structures will include drop shafts, access shafts, pump stations, gate valves and surge chambers.
Shaft construction necessitates the implementation of deep excavation safety controls, such as physical barriers to prevent people or objects falling into the shaft, excavation stability and support and continuous monitoring of the shaft atmosphere for hazardous gases.
Even with a watertight shaft, a sump and sump pump should be installed. Water can enter the shaft from minor leaks, rain or launching of the TBM. Providing power and backup power to the sump pump is important to prevent flooding of the shaft. A project can be down weeks or months if a shaft and tunnel are allowed to flood.
Shaft Construction Methods
We have several different shaft support methods available. A watertight shaft support system should be used below the groundwater table. Watertight shoring systems will not require dewatering to lower the groundwater table. Dewatering can be an expensive process. Discharging the water can be an even greater challenge particularly in an urban environment. The most common shaft construction methods that we can deliver, from simplest to complex, are:
- Trench boxes and speed slide rails
- Soldier piles and steel plates
- Precast segments
- Conventional excavation with rock dowels and shotcrete
- Sheet piles
- Secant piles
- Drilled shafts
- Caissons
Summary
Many different shaft construction methods are available. The key to deciding on a method is determining if there is a groundwater table presence. If so, use one of the watertight construction methods. Also, see if you can design your shaft with a circular shape to minimize or eliminate the need for wall reinforcement and internal bracing. If you are not sure which shaft support method to use, consider contacting us to find out the types of shaft construction that should be used in your local ground conditions.
