Trench cutter technology has been used successfully in many civil engineering projects and has the potential to become a sustainable technology for mining projects as well
Trench cutter technology, which has typically been used in civil engineering applications, has been used in recent times in various bulk sampling and trial mining applications. This article presents the key features of the cutter technology and the applications in exploration and mining so far and shows steps to develop the system into a sustainable, selective mining technology for various commodities.
Trench cutter applications in civil engineering
Trench cutters are widely used in civil engineering for the installation of diaphragm and cut-off walls. Diaphragm walls act as retaining walls to allow for open excavation pits (eg for the installation of underground car parks, subway lines or building foundations). Cut-off walls underneath and inside dams, including tailings dams, act as a permanent water barrier to avoid seepage and the collapse of the dam. To form the walls, trenches are excavated by the trench cutter in primary and overlapping secondary panels under support of bentonite slurry to final depth and are then backfilled with concrete or soil-cement mix, replacing the slurry. During excavation, bentonite slurry (in civil engineering) or water (in mining) is mixed with the crushed soil and rock and pumped to the surface, where it is separated in a recycling unit. The cleaned slurry or water is pumped back to the cutter excavation in a closed loop to minimise consumption.
Diaphragm walls are normally up to about 100 m deep with a width of 800 mm to about 1500 mm. The increased safety requirements in dam construction and the demand for the rehabilitation of older, existing dams require cutter systems to reach a depth of 250 m and wall widths to be up to 2500 mm.
Trench cutter technology
Development of the cutter technology for diaphragm and cut-off walls for civil engineering applications started at BAUER in 1984 with a newly developed trench cutter for a cut-off wall project in Bavaria, Germany. This was the Brombach dam, where sandstone with fractures had to be sealed.
Based on this first experience, BAUER has continuously developed the trench cutter system over the last 30 years and nowadays has more than 300 units working around the world.
Key components of a trench cutter are two counter rotating cutter wheels that can be fitted with different teeth arrangements for excavation of different types of soils and rocks. The wheels are driven by a gear system with a vertical axis on which a hydraulic motor allows for extremely fine control of the speed and torque of the cutter (Figure 1). A patented flipper tooth cuts underneath the gear shield to avoid having the entire cutter sitting on the centre rim.
Crusher plates mounted on the suction box between the cutter wheels pre-crush the cobbles, boulders and rock to a maximum of 75 mm so that they fit through the openings and consequently through the submersible pump mounted on top of the suction box. Rocks with a strength of up to 200 MPa have been cut by the BAUER trench cutter.
The cutter wheels and pump are mounted in a heavy steel frame to allow for sufficient load on the bit for good penetration. In addition, the long cutter frame is equipped with hydraulically operated steering plates to control verticality or planned inclination in both directions. Both delivery and hydraulic hoses with electric cable are automatically fed into the trench, and for depths greater than 80 m, they are coiled up on drums positioned on the back of the rig (Figure 2). A very efficient double-drum, single-line winch system allows for fast lowering and retrieving of the cutter for inspection and maintenance. To allow the rig to be easily moved from one position to the next, the entire system is mounted on a crawler-based carrier that also provides sufficient engine power for the entire unit.
Cutter technology in exploration and mining
Offshore alluvial deposits
In 1993, BHP Diamonds, which had offshore concessions for diamondiferous gravels in Namibian waters, approached BAUER to evaluate the suitability of the cutter technology for the extraction of a large amount of bulk samples in water depths of up to 200 m, penetrating about 5 m into the seafloor.
Based on proven key cutter components, a special offshore sampling cutter was designed within a landing frame to safely position the cutter on the seafloor to extract controlled samples. The BC 50 cutter had an eight inch submersible pump and a sampling footprint of 1.5 × 3.0 m. Hose reels for mud and hydraulic hoses with heave compensation, together with power generators and a control unit, were mounted on the MV Geomaster with dynamic positioning.
Onshore alluvial deposits
Further trials with a BAUER cutter were conducted with DeBeers of South Africa in 1998 to prove the safe recovery of diamonds in onshore alluvial deposits. The test pit was filled with clay, sand and cobbles, and red tracers with the same specific gravity (SG) as diamonds and artificial diamonds were placed within the material at specific locations.
Utilising a standard BC 30 cutter and a standard BE 250 separation plant, the material recovered was screened manually, which turned out to be difficult due to the huge amount of material and the similar colour of sand and artificial diamonds. While all but one of the red tracers was recovered, only a limited amount of diamonds were retrieved. Overall, the tests confirmed that a cutter can recover diamonds with an SG of 3.52 to the satisfaction of DeBeers.
Open pit mines
The life of an open pit mine normally ends when the strip ratio of ore compared to waste rock becomes uneconomical. However, a significant amount of ore must be left behind, especially in volcanic deposits with the root reaching far down into the earth’s crust. Going underground is only a viable option in a limited number of mines with very high ore values and volumes. The high development cost of an underground mine, as well as the higher underground mining costs, often prevent further mining to greater depth.
The same commercial constraints occur with small orebodies (eg kimberlite pipes), where high stripping ratios prevent open pit mining, even from the beginning. To overcome this limitation, BAUER and BHP Billiton, together with NUNA Logistics, successfully tested the cutter system in a large-scale trial mining exercise in the Misery pit of the Ekati mine in Canada’s North-west Territories in 2003 (Figure 3).
Mining sequences are now being developed to suit each individual orebody and to maximise the extraction of ore to a depth of 250 m.
Dykes, veins and seams
Dykes, veins and seams of various ores such as kimberlite, gold or coal often constitute a challenge to miners. The relatively small size of the orebody over great length and only little exposure to surface makes it difficult to mine economically. Open pit mining of such orebodies often does not last long due to the high stripping ratio. Mountaintop removal mining, as frequently practiced in the US and Canada and especially when used for almost vertical coal seams, is associated with deforestation and pollution of watercourses and will be phased out in the near future. Underground mining is expensive to develop due to the tight working conditions in the fissures or the high ratio of waste rock to ore.
Koidu Limited, a BGS company, holds diamond mining leases in Sierra Leone for kimberlites, mostly kimberlite dykes. A joint decision was made to use trench cutter equipment for these dykes to greater depth. Despite the remote location of the leases in eastern Sierra Leone, a test was carried out with a BC 33 cutter in 2010 (Figure 4). The cutter width chosen was 800 mm, based on Koidu’s assumption of a dyke width between 800 and 1000 mm.
A sustainable selective cutter mining system
To investigate the application possibilities of a selective cutter system in mining, an intensive study was performed by Delft University in the Netherlands to select suitable orebodies based on conditions such as compressive strength, deposit geometry, associated rocks, abrasivity and ore value.
As a result, the most appropriate orebodies for cutter mining were identified as:
- kimberlite pipes and dykes
- metallurgical coal in steep stratiform beds
- gold turbidite-hosted veins
- uranium hydrothermal veins
- diamond sediment-hosted pockets
- gold sediment-hosted pockets
- rare earth residual enrichment
- deep sea seafloor massive sulfide deposits.
Selective cutter mining system
The selective cutter mining system will concentrate on the excavation of the vertical or near-vertical orebody only.
The advantages of such a system are:
- mining of ‘uneconomic’ small orebodies is viable
- increase of mine life after open pit mining ends without increasing the environmental footprint
- no underground development required, leading to a much lower installation cost
- added value to the mine
- increase of mine life without any additional infrastructure
- time to mine is extremely short
- minimum dilution of ore, almost a one to zero strip ratio
- primary crushing incorporated in the system
- equipment can be used on several orebodies or mine sites.
The original cutter, which was developed in the civil engineering environment, has to cope with a high variety of soil and rock conditions on a single, relatively small job site. However, in mining, rock conditions are fairly homogenous over a long mine life, meaning that the cutter system can be optimised to suit the individual orebody.
Based on the properties of the orebody, future improvements of the cutter system may include:
- teeth configuration on the cutter wheels
- quality and design of teeth to fit specific ore properties
- improvement of gear boxes
- improve power installed for optimum performance of all components
- increase weight on bit by weight of cutter frame to 200 t and more
- improve pump capacity up to 1200 m³/h as required
- increase footprint of cutter size to approximately 4 × 4 m
- increase depth reach of cutter to 300 m and more
- automation of the cutting process, including recording and analysing rig and progress data.
A possible improvement for dyke, vein and seam mining may include a cutter head with a push-out system to
the side walls and a thrust cylinder for the cutter head.
The system also incorporates increased mine safety due to:
- mechanised, hands-free operation
- ore being conveyed/pumped to a separation plant
- minimised trucking/rehandling
- no additional road and bench maintenance
- no underground operation
- no or limited blasting.
A sustainable mining technology
Mining has always been at the centre of various interests. Economic viability, environmental impact and social acceptance are key factors that have to be balanced to achieve a sustainable mining solution (Figure 5).
With its low (or no) additional environmental footprint and optimised economics, cutter mining may be accepted more readily by authorities and the public than conventional mining. Based on these three pillars, the cutter mining system helps to satisfy the growing demand for the much-needed commodities in our modern society that has become more focused on sustainability.
The BAUER cutter system has been a proven technology in civil engineering for more than 30 years, with applications in different types of soil and rock conditions around the world. The system has also been applied at several bulk sampling and trial mining operations on and offshore. By optimising the system for higher productivity in mining, the BAUER cutter can make a significant contribution to sustainable mining.
Adams W M, 2006. The future of sustainability: re-thinking environment and development in the twenty-first century, report of the IUCN Renowned Thinkers Meeting, 29–31 January.
BAUER Maschinen GmbH, 2015. Brochure, BAUER Mining Solutions, 905.733.2 8/2015.
BAUER Maschinen GmbH, 2016. Brochure, BAUER Trench Cutter Systems, 905.679.2 1/2016.
Olive R, Wonnacott J and Schwank S, 2004. Dykes to access Canadian diamonds, the DIAVIK experience, ANCOLD Bulletin, 126:147–155.
Schwank S, (Bauer Maschinen Gmbh), 2003. Method and apparatus for excavating soil material, Canadian Patent CA2438634 A1.