Last year, BAUER Maschinen GmbH developed an individual solution for the requirements of Meichle + Mohr GmbH from Immenstaad on the north shore of Lake Constance: the prototype of the BAUER Perforation cutter (Fig. 1).
The Perforation Cutter is a piece of auxiliary equipment for underwater extraction and is based on proven, worldwide-established diaphragm wall cutting technology. By deriving the technology used in specialist foundation engineering, new possibilities of increasing the productivity of floating dredgers when hard soil layers occur arise. During the operation at Meichle + Mohr GmbH, for whom the prototype was custom-designed and manufactured, it became apparent that the BAUER Perforation Cutter can be used not only for the originally planned application of perforation harder soil layers at various depths in order to create access points for the hydraulic grab of the floating dredger. It turned out that the Perforation cutter can likewise also significantly contribute to increase the safety in underwater extraction.
At Meichle + Mohr GmbH‘s gravel plant in Radolfzell on the shore of Lake Constance, a floating dredger with a twelve cubic meter grab is used to extract gravel in underwater extraction. The subsoil is interbedded with hard layers and cannot be loosened with the dead weight of the 22-ton hydraulic grab. In preliminary talks with the plant management, Edgar Engler and Herman Beil reported that these heterogeneous, hard layers of soil occur throughout the entire extraction area at various levels.
The hard layers are made of conglomerate (so called “Nagelfuh”), a sedimentary rock made of coarse components such as gravel, stones,cobbles and boulders, embedded in a cemented matrix. Tests revealed strength of around 3 MPa. In many areas, the hydraulic grab of the floating dredger could not properly penetrate these hard layers. The bucket just scraped over these layers without being able to loosen them – the achievable feed rate of the floating dredger decreased massively. The solution: the new Perforation cutter, whose prototype was custom-designed and developed by BAUER Maschinen GmbH in 2014 for these requirements.
The BAUER Perforation Cutter
Originating from the idea of Bauer machines for the CSM method, which were first introduced in 2004, which, in turn, were based on the Bauer trench cutters, BAUER Maschinen GmbH first worked on developing the idea of installing trenches in the existing hard layer with the help of a rope-guided cutter – to perforate the layer, so to say. These trenches were subsequently providing access points for the floating dredger to penetrate the hard layer and excavate it. This should make it possible to extract the the deeper lying sand or gravel layers.
The main part of the Perforation Cutter‘s cutting unit is made of the cutter frame containing two cutter gearboxes, hydraulic motors, various measuring sensors, weight plates, hydraulic cylinders and other equipment (Fig. 2). The cutter gearboxes transfer the rotational movement generated by the hydraulic motors in the cutter frame to the cutter wheels. Each gearbox can transfer a torque of 57 kNm to the cutter wheels. Various sizes of exchangeable cutting wheels allow the adjustment of cutting width. The machine delivered to the client is equipped with mixing wheels for a cutting width of 800 mm. The mixing wheels are equipped with cutting teeth (Fig. 3), which get highly stressed during cutting. That is the reason why the teeth have tungsten carbide tips and can be easily replaced. The trench is 2.4 m long. The cutter wheels loosen the hard soil layers; additionally, during cutting, flushing water and flushing air is introduced into the trench through two pipes that end between the two gearboxes. This allows the cutter to perforate the soil more easily. The two hydraulic cylinders mounted on the side of the cutter frame help stabilizing the Perforation Cutter at the bottom of the lake.
The cutter is lowered into the water using two steel ropes. All supply lines are positioned between the steel ropes. In consultation with the client, the drum that is used for the steel ropes as well as the supply lines has been designed for a maximum working depth of 60 m. The drum can rotate by 180 degrees and is mounted on a tower segment. This enables the Perforation Cutter to reach three positions without having to reposition the pontoon on which it is mounted. In this way, two trenches with a distance ideal for the width of the floating dredger grab can be installed. In the third position, the cutter can be put on the deck of the pontoon and can, for instance, be maintained.
The pontoon can be fixed to the floating dredger and can be positioned using it. The control system of the Perforation Cutter is integrated into the cab of the floating dredger (Fig. 4). It is thus possible to operate the excavator as well as the Perforation Cutter comfortably from the same place. The touch screen shows the rig operator all the important equipment data, which can also be recorded.
On client‘s request, the Perforation Cutter is operated by an electrically powered hydraulic power pack that uses the same voltage as the floating dredger and is located on the pontoon of the Perforation Cutter. In addition, on the pontoon are a water pump for flushing water and an air compressor for flushing air.
Possible applications and practical experience
The application originally planned and desired by the client was to perforate hard, relatively horizontal soil layers in order to be able to excavate the sand or gravel layers underneath it (Fig. 5). However, shortly after the BAUER Perforation Cutter was commissioned, the client discovered other possible applications as well: The Perforation Cutter is not only used to cut through deeper, hard layers of soil, but it is also used to cause the very precipitous lake folds that are partially under the water surface to slide down (Fig. 6). This cannot be done using the hydraulic grab of the floating dredger, since it is designed in such a way that the floating dredger cannot guide it close enough to the shore.
Until now, the quarry lake was expanded by excavating sand or gravel from the lower parts that the hydraulic grab could reach. This method however involves the risk that the steep slope suddenly slides and buries the excavator bucket under it. Compared to the floating dredger, the Perforation Cutter, owing to its smaller draft, can be brought closer to the shore in order to cause the sand or gravel that is relatively close to the water surface in the upper area to slide. Thus, the water depth close to the shore can be increased to an extent that the floating dredger can come closer to the shore and can safely extract the sand or gravel.
Another possible application of the Perforation Cutter was discovered because of the conglomerate layer that is relatively close to the water surface in certain areas. The backhoe excavator could previously only reach the surface of this layer from the shore. Until now, the quarry lake could be expanded and sand or gravel could be safely extracted only by blasting the conglomerate layer. However, blasting is expensive, complex and, at times leads, to standstills in the extraction of sand and gravel. Without blasting, undercuts are formed due to the extraction of sand or gravel with a floating dredger, and this leads to large overhangs of the conglomerate layer. These overhangs constitute a big safety risk, since they can break anytime in an uncontrolled manner and can cause the hydraulic grab of the floating dredger to get buried under them, or can at least suddenly cause strong waves.
In order to prevent this and, at the same time, to avoid blasting, the conglomerate layer was excavated as far as possible from the top using a backhoe excavator. The resulting water depth was sufficient to place the Perforation Cutter over the conglomerate layer (Fig. 7). Subsequently, individual, closely spaced trenches were cut in the conglomerate overhang using the Perforation cutter. The overhangs can thus be broken in a controlled manner.