There are a variety of methods for installation of shafts, depending on the basic parameters like size, depth, geology and hydrogeology.

Sinking shafts below the groundwater level can be an extremely difficult process especially in unstable heterogeneous soils where the conventional methods in this field sometimes have to be supported by additional measures like soil stabilization.

In the most common methods, the shaft installation is performed in three different steps.

•    Building the wall, often below the ground water level.
•    Soil excavation, which may need the lowering of the groundwater level.
•    Preparation and pouring of the base, usually in dry condition.

With the introduction of VSM technology, Herrenknecht has a proven method that enables all three individual operations to be performed safely and without lowering the ground water level. The first two operations are performed in parallel followed by the pouring of the base. This leads to higher performance rates. Moreover, no personnel are required at any time in the shaft, so that the new VSM method is setting new safety standards.

It is characteristic to all of all the conventional methods such as sheet wall, bore pile or diaphragm wall that after the wall has been successfully installed, soil excavation is performed in most cases by means of conventional technologies, which could include the use of personnel on the shaft floor, and it is normally also necessary to lower the groundwater.

VSM Shaft Sinking Technology


The VSM equipment consists of three essential components. First is the excavation unit, which systematically cuts the soil that can have strength of up to 80 MPa (11,600 psi) or more. Therefore, the VSM is equipped with a cutting drum on a telescopic boom that allows excavating under the cutting edge and, where required, an overcut.

In one case, the VSM had to excavate through several meters of unexpected rock in the region of up to 200 MPa (29,000 psi). Although this is possible for the VSM, high cutter consumption should be considered.
The excavation process is carried out below the groundwater level and is fully remote controlled from the surface.

Secondly, a slurry discharge system removes the excavated soil. A submerged gravel pump is located directly on the cutter drum casing. It transports the water and soil mixture through a slurry line to a separation plant on surface. A centrifuge unit can be added to the separation plant to remove fine particles. This improves the transport of the excavated soil and ensures clean shaft water which can be more easily disposed of when emptying the completed shaft.

Thirdly, component is the so-called lowering unit. The lowering unit stabilizes the entire shaft construction against uncontrolled sinking by holding the total shaft weight through steel strands and hydraulic jacks. Only when the excavation under the cutting edge of the shaft is completed, the complete lining can be lowered uniformly and precisely.

The whole operation takes place from the surface and is controlled by the operator from the control container. All machine functions are guided remotely without the necessity to view the shaft bottom or the machine.
The shaft lining is installed at the surface and is in most cases made up with precast concrete segments. These are, in general, comparable with tunnel linings, however, bolts and connectors can be handled from outside the shaft.

A second alternative is an in-situ concrete casting of the shaft walls. Here, the slower progress of works of lining is compensated by a “continuous” structure without joints and the possibility of integrating entire entry and exit structures for microtunneling activities in the walls of the shaft.

The VSM technology has been continuously developed since introduction in 2003, and is thus also opening new fields of application.

Start and target shafts for microtunneling activities have up to now been sunk within  an internal diameter range of 6 to 10 m (21 to 33 ft) and to 85 m deep (279 ft). The entry and exit areas in the shaft wall can be prepared with glass fiber reinforcement if required.

To connect an already existing sewer through the soft and unstable soil of St. Petersburg a VSM system sunk several shafts up to a depth of 84 m with overcoming difficult soil condition such as big boulders mixed in soft clay.

Another successful field of application is the excavation of ventilation shafts for subway systems. This inner-city application requires handling under very constricted space conditions and the prevention of settlement, which are two requirements the VSM technology can meet. Ten ventilation shafts having diameters of 4.5 m have been sunk in very restricted inner-city space conditions down to depths of 45 m in the city of Naples, Italy.

Two further machines have worked in Spain on several shafts for the high speed train system.

U.S. Debut: Ballard Siphon Project


A VSM machine has recently successfully completed its first project in the United States. The Ballard Siphon Project in Seattle, where a 77-year-old wooden sewer line is being replaced under the Lake Washington Ship Canal. The tunneling project will be realized with a Herrenknecht EPB segmental lining machine.

For the excavation of the launch shaft, the VSM was chosen by the contractor James. W. Fowler due to its advantages regarding safety, noise emission and cost efficiency. The 125-ft deep, 9-m inner diameter shaft was finished in about four weeks excavation time and the average excavated depth per shift was 6 to 7 ft.
The ground conditions encountered were soft soils including sand, gravel and clay coupled with a very high water table. The small overcut between the shaft wall and the soil is lubricated by a bentonite mixture and grouted after the shaft had reached final depth. The concrete base slab was poured with the lowering of the water table.

The possibility to create an overcut of up to 500 mm once the shaft had reached its final position was especially helpful to increase the volume of the concrete slab on the shaft bottom. This helped as well to balance the shaft and soil weight against buoyancy forces on the concrete slab.

Upon completion of the excavation the VSM was recovered by the three shaft winches and dismantled within one week. The VSM will now be facing its next challenge: the excavation of two shafts of about 120- and 33-ft diameter for the Ala Moana harbor crossing project in Honolulu.

Summary


The VSM technology is at present an economical alternative for a diameter range of 4.5 to 10 m and for depths or water pressures of up to 85 m. Sinking performances of 1 to 5 m per day are reached, depending on the diameter and the geology. Unmanned sinking and the preparation of the lining at the surface have also set new standards for safety. New machine concepts allow the execution of shafts up to 16 m (52.5 ft) in diameter or 140 m (460 ft) in depth with the VSM technology.

There is potential for the increased use of the VSM technology in city centers with very limited space and high demands to avoid disturbing the ground or the infrastructure at the surface. The results from the already completed shafts have demonstrated the accuracy, safety and reliability of the VSM method.

Peter Schmäh is a member of VSM executive board, business unit utility and Sebastian Berblinger is VSM product manager.

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