Almost all tunnelling projects require shafts, either as start and reception shafts for the tunnelling process or for inspection, ventilation and rescue purposes. There is also a trend towards infrastructure installations at ever-deeper depths, driven by, among other things deep sewer construction projects that aim to avoid pumping stations as well as the need to build new installations below existing infrastructure
The Vertical Shaft Sinking Machine (VSM) was originally developed by Herrenknecht for the mechanised construction of deep launch and reception shafts for micro-tunnelling. After starting design and testing in early 2004, the first Herrenknecht VSM equipment went into operation in Kuwait and Saudi Arabia in 2006. The machine concept, fully remote-controlled from the surface, as well as its implementation on site proved to be an efficient solution right from the start for the safe and fast realization of shafts especially in difficult, inner-city environments without lowering the groundwater table.
The benefits of VSM
The VSM technology masters the main challenges associated with shaft sinking: inner-city shaft structures demand safe working conditions for surrounding buildings and the environment, especially potential ground settlement. There is an increased requirement to avoid the lowering of groundwater during the construction of shafts in order to avoid the associated settlement, which can affect a wide area. Deep shaft construction companies often encounter difficult geological conditions such as high groundwater pressure combined with layers of hard and soft material. In addition, deep shafts need special attention for the safety of the operating personnel.
Simultaneous excavation and ring building facilitate high advance rates and shortened overall project duration. At the same time, continuous performance ensures overall high planning reliability for all stakeholders.
The lining of a VSM shaft consists of either precast segments or cast-in-situ concrete. As the lining installation is completed on the surface, high-quality installation can be accomplished, leading to greater accuracy of the shaft structure. In most cases, a secondary, time-consuming lining is not required, resulting in a reduced wall thickness of the shaft and, thus, less soil excavation.
As the water level in the shaft is maintained close to the groundwater level outside the shaft, water flow is prevented which otherwise could cause ground movement and lead to a high risk of settlement. The Herrenknecht VSM can be applied below groundwater with a hydrostatic pressure of up to 10 bar and in heterogeneous soil and hard rock of up to 140MPa compressive strength.
All installations, including the lining erection, are remotely controlled from the surface. No personnel have to enter the shaft until it has reached the final depth and is fully secured. In general, mechanised shaft sinking requires less personnel and machinery on-site, which leads to minimised risk exposure.
Measures for groundwater lowering are not necessary, as the VSM machine concept is designed for operation under groundwater. As the VSM technology applies a high degree of accuracy of shaft construction, the shaft lining thickness can be reduced to a minimum, which reduces the amount of excavated soil.
VSM machine components
The VSM consists of two main components; the excavation unit and the lowering unit. The excavation unit systematically cuts and excavates the soil and consists of a cutting drum attached to a telescopic boom that allows excavation of a determined overcut. The lowering unit on the surface stabilises the entire shaft construction against uncontrolled sinking by holding the total shaft weight with steel strands and hydraulic jacks. When one excavation cycle is completed, the complete lining can be lowered uniformly and precisely.
A slurry discharge system removes the excavated soil and a submerged slurry pump is located directly on the cutting drum casing. It transports the slurry to a separation plant on the surface. The whole operation is controlled by the operator from the control container on the surface where all machine functions are remote-controlled.
VSM job site preparation
Depending on the space conditions on-site, the VSM components can be positioned flexibly to suit local circumstances. As most sites are located in heavily built-up urban areas, the access for logistics is limited. However, special concepts to relocate components such as the separation plant are available.
After preparation of the site surface, a concrete ring foundation has to be installed in a pre-excavated pit. This foundation bears the loads of the VSM and serves as a support for the lowering units, which guide and hold the shaft at all times. The size of the ring foundation depends on the ground conditions and the size of the shaft. Connection bolts for various VSM components such as lowering units, recovery winches and energy winch tower are also integrated into the ring foundation.
After pre-excavation and ring foundation, the installation of the cutting edge and the first segment rings are the next steps. The cutting edge is designed to cut the shaft profile in soft and loose soil conditions. Its design depends on the shaft diameter and wall thickness. It can be integrated as the first concrete segment ring or welded onto the shaft lining as a separate steel ring.
The first 5m of the shaft lining constitutes the so-called start section, which has a stronger steel reinforcement to be able to take the loads and reaction forces of the machine during excavation and shaft sinking. Furthermore, the shaft lining is equipped with cast-in steel plates, onto which the brackets for the machine arms of the VSM are welded. As the VSM employs a sequential partial face excavation technique there is no torque transmitted into the shaft structure.
The excavation unit arrives on-site in three parts: telescopic boom with the cutting drum, machine main body, and the adapter parts to the required shaft internal diameter. The lowering unit consists of the strand jacks and the coiled steel strands on a drum. The number of strands depends on the total predicted weight of the shaft including the machine weight and the estimated buoyancy and friction forces. The strand jacks are bolted to the ring foundation by anchor bolts. Coming from the strand drum, the strands are fed through the strand jack, lowered through the outer annulus of the shaft wall and connected to the cutting edge. When all strand jacks are installed and connected to the cutting edge the strands can be tensioned and carry the loads.
Now, the preassembled excavation unit can be lifted into the start section. The VSM is secured by hydraulically activated locking bolts. When the VSM is in place, the recovery winches are installed and connected to the three arms of the machine. The recovery winches are used to recover the VSM for required maintenance or for final machine recovery.
Next, the energy chain tower is installed and the excavation unit is connected to the hydraulic and electrical supply as well as to the feed and discharge lines.
Before excavation starts, a calibration of the VSM in reference to the projected alignment of the shaft is required to ensure the accurate action of the cutting boom.
Stored data, together with the position of the cutting boom, is shown on a graphic display, giving the operator full control of the excavation and sinking process. The excavation unit can be operated in three different overcut options, which requires the installation height measured from the level of the cutting edge to be adjustable.
In its highest position, the excavation unit is not able to create an overcut under the cutting edge, which is important in soft or unstable soil to maintain surrounding stability.
When an overcut is required, e.g. in stable or cohesive soil, the excavation unit works in its lowest position. In this case, the annulus should be stabilised by a bentonite-water suspension. In addition, each segment can be equipped with bentonite nozzles for lubrication, which can also later be used to grout the annulus.
The standard installation is to connect the segments in ring number three and five with the bentonite mixing unit right from the beginning and to lubricate from these two segment rings at the shaft bottom during the sinking. The stabilisation of the annulus together with the controlled sinking of the shaft by the strand jacks minimises the risks of settlement.
During the excavation and sinking process, the shaft is kept full of water to balance the level of the groundwater table in the surrounding geology. The cutting drum cuts and crushes the material to a granular size that can be handled by the pumps (pump capacity: 200-400m³/h). A slurry circuit transports the excavated material from the shaft to a separation plant on the surface.
The telescopic boom allows varying inner diameters between 5.5 and 18m with a reinforced frame structure. Excavation is possible even in water depths up to 85m. The cutting arm moves radially from the centre to the outside of the shaft with an additional telescopic extension of 1m. With a rotation of +/-190 degrees the cutting boom covers the whole cross-section of the shaft. The cutting speed and the movement of the boom can be varied to achieve the best excavation rate.
In most cases, the shaft lining consists of precast concrete segments installed at the surface. This so-called ring building is comparable to segmental lining in tunnelling. The ring is built at the surface by crane. The number of segments depends on the shaft diameter. Ring building work includes the proper connection of the rings by anchors and bolts, which can be handled from outside the shaft. The excavation process of the VSM is not affected by the ring building process increasing shaft sinking performance.
Alternatively, in-situ concrete casting of the shaft walls is another solution, especially for larger shaft diameters where segment handling becomes more difficult. In this case, the progress of shaft construction works is slowed down by the necessary time to build the formwork and the setting time of the concrete structure. The benefit of in-situ casing is the ‘continuous' structure without joints and the possibility to integrate entire entry and exit structures, e.g. for micro-tunnelling activities in the shaft walls.
After reaching the final depth, the bottom plug has to be installed. Usually, the VSM is used to excavate the required overcut. When this final excavation is done, the VSM can be disconnected, recovered by the recovery winches and lifted to the surface and out of the shaft by a crane. The bottom plug is cast with underwater concrete. The shaft annulus is then grouted through the lubrication lines to stabilise and anchor the shaft to the surrounding ground. Finally, the shaft water can be pumped out and the shaft is completed.
The current and worldwide trend to construct more and more infrastructure underground, metro, road and railway, as well as a large variety of utility lines, promotes a growing demand for the construction of shafts.
With increasing depth and groundwater levels, conventional shaft construction methods are reaching their technical and economical limits. Herrenknecht has developed the solution: The Vertical Shaft Sinking Machine, developed by Herrenknecht, is the solution to these problems. Its efficiency and benefits in terms of budget, construction time and occupational safety have led to a total of approx. Since its first deployment in 2006, 75 VSM projects with a total depth of 3.8km where shafts have been successfully sunk.
Ventilation and emergency shaft applications
In Spain, for the high-speed rail link from Barcelona to the French border, a total of four shafts were built in Girona by a Herrenknecht VSM as ground stabilisation shafts before tunnelling works (four shafts, ID 5,250mm, depth 20m). The same VSM built one additional shaft in Barcelona for ventilation and as an emergency exit (ID 9,200mm, depth 47m).
The major challenge for the construction contractors were extremely confined working conditions. For example, one of the shafts in Girona was located between two rows of houses with a spacing of only 12m. Here, the VSM's ability to work under limited space conditions in inner cities proved to be a major benefit.
Due to the lack of ground stability in the centre of the city of Girona, only small, lightweight cranes could be used and this led, in turn, to a complete reorganization of assembly logistics: The main components of the VSM10000 were delivered just in time and assembled directly in the shaft start section.
A total of 13 ventilation and emergency shafts (ID 4,500/5,500mm and depth up to 45m) for the subway line were sunk in Naples, Italy, by a Herrenknecht VSM.
The site was located in a densely built-up area in the inner city with high levels of traffic. The required job site footprint was approximately 300m² in the narrow streets of Naples. Noise exposure for the residents had to be kept at a low level. Because the excavation of the shafts and their lining with precast concrete segments could be realised simultaneously, the production of the shafts could be finished quickly with performance rates of up to 5m per day. Due to its modular setup, the VSM was rapidly disassembled and transported to the next site after completing each shaft.
Two large shafts with a 10m inner diameter were sunk in Honolulu, Hawaii. These 36m deep shafts were to be used as launch shafts for a pipe jacking project. Cast-in-situ was the preferred lining method in order to handle the necessary thrust forces in the shaft wall when launching the pipe jacking machine. Moreover, fibreglass reinforcement simplified the launch process for the TBM. In Hawaii, the VSM successfully handled a challenging geology comprised of hard basalt as well as coral that would have been problematic for conventional methods.
Sewage collector shafts
The deepest VSM shaft under groundwater to date was completed in St. Petersburg, Russia, where a total of four shafts were realised to depths ranging from 65 to 83m. In St. Petersburg, the Herrenknecht VSM technology proved to be especially efficient in the face of tight time schedules. Together with the customer, Herrenknecht assembled the machine in nine days following site preparation, and successfully finished the first shaft of 83m depth and an internal diameter of 7.7m in 50 working days.
Stefan Frey is product manager VSM and Peter Schmäh is a member of the Executive Board, Business Unit Utility Tunnelling, Herrenknecht AG