By Glenn M Boyce, PhD, PE
Shafts are the doorways to the underground, 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 tunneling project.
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 best to use on your project.
The most common shaft construction methods, from simplest to complex, are:
- Trench boxes and speed slide rails
- Soldier piles and wood lagging (or steel plates)
- Liner plates
- Precast segments
- Conventional excavation with rock dowels and shotcrete
- Sheet piles
- Secant piles
- Drilled shafts
- Cutter soil mixing
- Slurry walls
- Ground freezing
Some of these methods only work in soils, some only work above the groundwater, and some are restricted by the depth of construction. It is not unusual for methods to be used in combination. This article provides a brief discussion of shaft design and a summary of the different shaft construction methods available and their limitations.
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. For transit tunnels, shafts can be used for access, elevators, ventilation, transit stations and utility drops. It is difficult to determine exactly what size shaft the contractor will need because you will not know the proposed means and methods and the exact equipment that will be used. When surface space allows, assume the contractor may need to increase the footprint of the shaft.
The big design question is: Can the shaft be circular? A circular shaft is structurally stable. The earth loads on a circular shaft place the shaft support in ring compression. The benefit of a circular shaft is that the reinforcement in the structural elements can be reduced and the need for internal support is eliminated.
Traffic patterns and traffic control may also influence allowable shaft size. Some cities require shafts to have traffic plates placed over them at certain times of the day because of rush hour. Other cities want concrete barriers around shafts for public safety. Fall protection provisions must be provided at all shafts.
The laydown area around a shaft can require the contractor to conform to some unusual requirements, like working under bridges or restricted access. Aboveground features such as light poles, electrical transmission lines, other utilities, structures and vegetation may unfavorably impact the laydown area required. Consider noise and vibrations from the shaft construction on adjacent neighbors.
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
There are 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.
Trench boxes and other pre-engineered systems, such as speed rail systems, are normally limited to 20 ft deep and should be dewatered if the shaft invert is below the groundwater table. Trench boxes and other pre-engineered support systems are designed to meet OSHA requirements based upon soil type. Shoring needs to be in contact with the ground. Speed slide rails, similar to trench boxes, are a series of telescoping boxes, one inside the other. The boxes support the ground on all four sides, as compared to a trench box, which is open at two ends.
Soldier piles with wood lagging are a common method of shaft construction, allowing for flexibility in shaft size and dimensions. They can be used with dewatering and in combination with other shaft construction methods, and can be removed and reused. Soldier piles are drilled into place at the design spacing and depth. Excavation is sequential, with wood lagging installed between the soldier piles. The depth of a solider pile shaft is limited by the cantilever depth available from the ground type. The earth between the soldier piles can also be supported by steel plates inserted from the surface.
Liner plates are used extensively in the Midwest, typically with dewatering, and can be constructed to withstand hydrostatic pressure after completion of the shaft. Individual plates are bolted together to form a circular ring. Plate length is 3.14 ft, orounting the number of plates used to form the circular ring gives you the diameter of the shaft. The plates are added to the previous ring of plates as the shaft is excavated. Joints are typically staggered. Ring beams are added as required to the inside to provide internal shaft support. The ring beams are usually reclaimed and reused as the shaft is backfilled. Liner plates can also be reused after shaft completion.
Precast segments are used extensively in Europe. The segments are similar to the ones used in the running tunnels. The segments are bolted and gasketed together to form a ring. Rings are added at the top when used as a caisson or at the bottom when the ground has stand-up time. Adding segments at the bottom is similar to using liner plates.
Conventional excavation with rock dowels and shotcrete is a method used in rock or ground with stand-up time. The ground is excavated in lifts and supported with reinforced shotcrete and rock dowels. The shotcrete provides a sprayed-on concrete hardened surface to support the ground. The shotcrete is reinforced with fibers or wire mesh and multiple layers. The dowels are installed by drilling an inclined hole, inserting a steel rebar, and then grouting the rebar in-place. The spacing of the support for a conventionally excavated shaft can be varied vertically and horizontally to meet the actual ground conditions encountered. Dewatering is typically needed to help deal with the hydrostatic pressure.
Sheet piles are a typical method of shaft support. The sheet piles are driven into the ground or can be vibrated into place. Predrilling can be done for harder ground and for deeper installations. When using sheet piles, the interlock needs to be inspected to ensure watertightness. If there is a small leak, the contractor can weld the joint between two sheet piles. Sheet piles can be difficult to install in ground with cobbles and boulders. One disadvantage is the noise and vibration generated when sheet piles are installed. They come in different shapes and sizes with different design properties. Typically, sheet piles are used to support a rectangular shape and, in that case, are braced with internal walers and struts.
Secant piles are small-diameter (3 ft) concrete columns drilled side-by-side, an effective way to build a watertight shaft. As depth increases, the drill holes used for construction tend to drift. Extra care is needed to maintain verticality with the deeper secant piles. Secant pile shafts typically max out at a 70-ft depth, although some are now being drilled to 115 ft. The shaft may require surveying of the drill holes. Contractors can install a second ring of piles if windows form from drifting secant piles.
The piles are installed one at a time. While the concrete of the first pile is still green, a second pile is drilled. Piles are placed in an overlapping pattern and filled with concrete until a circle is completed. The overlapping piles provide the vertical watertightness. The center of the shaft is excavated with conventional excavation equipment.
Drilled shafts are used for smaller diameter shafts (<10 ft). A drill rig with an auger is used to excavate the shaft opening. Slurry keeps the bored hole stable until a steel casing can be inserted and grouted in-place. Many times a corrugated metal pipe (CMP) is used instead of a steel casing as a cost saving measure.
Cutter soil mixing is a new method of shaft construction. Like slurry walls, panels are created. The panels are a mixture of soil and cement. Depth is limited to 90 ft. The problem with the deeper panels is the heat generation from the increased amount of cement in the ground.
Slurry walls are a series of excavated panels. A panel is excavated and remains stable and open with the use of slurry. After panel verticality is checked, a reinforcement cage is lowered into the excavated panel/slot. The slurry is displaced with tremie concrete and allowed to harden. Primary panels are installed first. After the primary panels are in place, the secondary panels are excavated between the primary panels. The overlapping panels provide the vertical watertightness. Slurry walls can be dug in any configuration. Care is required to obtain acceptable verticality. Verticality of 0.5 percent is typical. Slurry wall panels can extend to depths of 200 ft.
Ground freezing requires the presence of groundwater or partially saturated ground. The advantage is that you only need to drill simple boreholes from the surface in the needed configuration, which allows shaft construction in unstable ground. Ground freezing may also experience drill hole drift. If the water is saline or moves quickly, its freezing point may be lower than standing water and the saline water cannot be frozen with a brine solution. Liquid nitrogen may be required to drop the temperature low enough to freeze the salt/saline water or moving water. Ground freezing can be expensive and unpredictable, and the process can also freeze other liquids used in tunneling applications.
Caissons are typically constructed from the surface, in the wet, and without dewatering. They are excavated from the inside, and the dead weight forces the caisson down as soil is excavated near or from the bottom. The caisson is cast-in-place at the surface. The shaft bottom is excavated to allow the weight of the caisson to drop into the ground. Once the caisson reaches a set height, the concrete forms are added to the top. Concrete is placed in the forms to create the next top section (lift) of the caisson. Metal angles act as a guide to help maintain the verticality of the caisson.
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 contractors in your area to find out the types of shaft construction used with the local ground conditions.
Glenn Boyce is a Senior Associate with Jacobs Associates in Walnut Creek, Calif. (Photo by Sue Bednarz, Jacobs Associates)