Advanced microtunnelling shaft construction
We specialize in advanced microtunnelling shaft construction planning and select the most appropriate shaft design based on geotechnical conditions, groundwater considerations and available workspace all while holding safety to the utmost standard. With our global experience, our team draws upon best practices used in Europe and is one of the few companies in North America providing precast segmentally lined caisson. Our experienced team offers an efficient approach to shaft construction in tunnelling projects and has successfully constructed shafts of various diameter and depth using jacked caisson, underpinning techniques, soldier piles, secant piles and sheet piles.
What is shaft construction?
Many pipeline crossing projects include microtunnelling shaft construction. Trenchless tunnelling, such as microtunnelling, and pipeline installation requires a pit or shaft dug from the ground surface into a tunnel for ventilation or access to the tunnel. Shaft construction, or shaft sinking, is a vertical or near-vertical tunnel excavated from the ground surface. A pipeline crossing project typically requires two shafts, a drive shaft and a reception shaft. The drive shaft is an entry point for launching trenchless technology equipment such as a pipe jack and pre-fabricated pipe segments. The drive shaft may incorporate a thrust wall to spread reaction loads to the ground, and an entry ring to control inflows of groundwater and soil at the portal. The drive shaft must be large enough to accommodate the thrust wall, hydraulic jacking system, microtunnelling machine or pipe segment and entrance ring. The reception shaft is the exit point for the pipeline and is smaller in size as it only needs to accommodate the size of the microtunnelling machine. Most shafts are circular as this shape is naturally structurally stable and requires fewer reinforcements.
Shaft construction size
The size of shaft required for a project depends on a variety of factors with the aim of using existing shafts where possible or determining the minimum dimensions to reduce impact to the surrounding area. Considerations include the size of the pipeline to be constructed, the space required for a tunnel boring machine, traffic patterns, groundwater tables, location of the shaft and above ground features.
Shaft construction techniques
There are multiple microtunnelling shaft construction techniques that can be used. As part of a feasibility study, our engineers will determine the best method based on the project’s needs, ground conditions and surrounding area. In some cases, our engineers work with local experts who understand the ground conditions and history in their community.
Jacked caisson shaft construction uses pre-fabricated hollow boxes or cylinders made of wood, steel or concrete that are sunk into the ground using a hydraulic jacking machine to build the retaining wall. This method is specifically designed to keep water out of the construction zone and can be used in wet conditions, when groundwater is present or for underwater projects. It is also an effective shaft construction method when ground conditions are soft and sandy. Caissons are extremely tough and strong enough to withstand deep water pressure. This method is efficient, effective and creates immediate shafts.
The underpinning technique is used when ground water is minimal. The shaft is constructed by excavating the ground and installing precast concrete segments and is often used in harder ground conditions.
Soldier pile shaft construction uses vertical steel H piles and horizontal wood or concrete lagging to create the retaining wall. Soldier piles can be temporary or permanent depending on the project’s needs, but are primarily used for temporary construction. This method is efficient and cost effective, and can accommodate various shaft size and dimensions.
Secant pile shaft construction uses a series of overlapping primary and secondary long columns, piles, to create a retaining wall. Typically, the primary piles are not reinforced while the secondary piles are reinforced with a concrete fill. Secant piles are usually smaller in diameter (three feet) and are an effective method to build a watertight shaft. They provide greater wall stiffness than other methods and can be used in difficult ground conditions.
Sheet pile shaft construction uses pre-fabricated steel sheet sections that interlock to create the retaining wall. They are typically installed in the ground using vibration. For harder ground conditions and deep installations, predrilling may be required. Sheet pile shaft construction is widely used and the piles come in a variety of shapes and sizes. This method can be difficult in ground conditions with cobble stones and boulders.
Shaft construction advantages
Minimal impact on the surrounding area and ground surface
Eliminates the need for scaffolding
Supports microtunnelling in considerable depths
Can be easily sunk in urban, densley populated areas
Time and effort for remediation is relatively low