BC Trench Cutter Technology
BC trench cutter technology is the most advanced technology for cut-off walls in hydroelectric power schemes.
On the way to becoming an industrial nation, it is imperative for India to satisfy the ever increasing demand of electric power. Tremendous changes in the current world economy influence the supply of coal and oil for thermal power plants. Nuclear power has lost a lot of its attraction, especially since Fukushima. As it appears that these common resources are reaching a peak to supply energy to a more demanding market, it is essential for a safe and steady growth of economy to strengthen alternative sources of energy. More focus is put on clean and green power sources such as sun, wind and water. India, thanks to the mighty Himalayas, is blessed with immense hydel power potential. The development of this vast power potential has been given top priority by the Government of India. A two-digit number of hydroelectric projects are currently under construction, a much higher number of future projects are in the feasibility, design or approval stage.
Dams are very important structures in all hydroelectric power plant schemes. They serve the purpose of storing and retaining water to generate electricity. Dams are often constructed as earth-fill dams or rock-fill dams.
Controlling seepage
The formation of an effective water barrier within the soil and rock formations beneath dams is one of the most important tasks in dam construction. Such measures are intended to control seepage beneath the dam in order to minimise water losses from the reservoir and, more importantly, to ensure the stability of the dam against internal erosion.
The oldest method of providing a barrier beneath dams, still widely used today, is the grout injection technique. In this process, boreholes of approximately 100 mm diameter are drilled into the underlying soil and rock formations and injected with cement grout to seal the pores in the surrounding soil or fissures in rock beneath the foundations of the dam.
The impermeability of such grout curtains is governed by the spacing of the grout holes, and soon reaches its economic and technical limitations in alluvial soils especially when they are mixed with stones and cobbles.
As a result of this problem, alternative processes have been developed during which soil is excavated and the resulting void backfilled with a barrier material of well-defined properties (?plastic concrete?). The most important method for constructing a positive cut-off wall is the use of the diaphragm wall technology. It has been used successfully since many years on numerous jobsites all over the world.
For many years, walls were constructed with rope suspended mechanical or hydraulic grabs.
There are, however, some limitations in the grab system. The excavation output reduces with increasing trench depth as a result of the intermittent excavation process. It is difficult to form a leakage-free continuous wall for big depth excavations and, last but not least, it is difficult to excavate hard soil formations.
The development of the hydraulic trench cutter system made it possible to overcome these problems and it opened up new horizons for the use of the diaphragm wall technique.
? The advantage of the trench cutter technology is the possibility of adjusting the system to depth (30-150 m) and thickness (600-1,500 mm).
? It works very well in mixed soil conditions from loose sand to cemented layers and even in rock layers.
? High productivity is a major advantage of the trench cutter system.
? The capacity of working within stringent verticality tolerances is an important quality feature.
? The excavated material allows visual control of embedment in impermeable soil at the toe of the wall.
The BC Trench Cutter System
The trench cutter is an excavating machine that operates on the principles of reverse circulation. It is made up of a heavy steel frame (1) with two gearboxes (2) mounted at the bottom of the frame. Cutting wheel drums fitted with a series of teeth are fixed to the gearboxes; they rotate in opposite directions, break up the soil and mix it with the bentonite suspension (3). As the cutter penetrates, soil, rock and bentonite are conveyed towards the openings of the suction box (4). From where they are pumped by a centrifugal pump (5), located right above the cutter wheels, through the slurry pipe incorporated in the cutter?s frame, via the mast head into the slurry conveying system to the desanding plant. Here solid soil and rock particles and liquid bentonite are separated and the latter is pumped back into the trench.
The torque output of the cutter wheels in combination with the weight of the cutter is sufficient to cut into any type of soil and to crush cobbles, small boulders or rock. The verticality of the trench cutter and thus the trench alignment are generally measured on two-axis by means of two independent inclinometers (6). Adjustment of verticality in the two directions is carried out by a system of steering plates (7).
Tough projects
The underground conditions in the Himalayan valleys, which are preferred locations for the construction of hydroelectric power plants, are well known for being extremely difficult for excavation. Due to geological and climatic conditions they contain stones, boulders and erratic blocks within the soil matrix.
BAUER has gained a long list of track records where it has been proven that extreme soil conditions can be tackled best by deploying a BC trench cutter as lead equipment, assisted by hydraulic diaphragm wall grabs and even by ?old- fashioned? heavy chisels.
Applying a DHG hydraulic grab as assistance tool for the trench cutter becomes feasible whenever the use of the trench cutter to crush stones or boulders into small fragments would result in a high energy consumption and unproportionally high wear and tear on the cutting teeth.
The reason for using the combination of multiple systems is the utilisation of specific advantages and strengths of different excavation techniques. Grabs and chisels can only work effectively when they are operated by modern heavy-duty crawler cranes.
BAUER has accumulated a lot of experience in designing, manufacturing and using hydraulic trench cutters for cut-off walls since its first use in 1984 on a dam project in Germany. Many specialised companies all over the world have used or are using this modern techno?logy successfully.
An outstanding example is the construction of a plastic concrete cut-off wall with a record depth of 120 m in Peribonka, Canada (2006).
Experience in India
A breakthrough for this method in India was reached with the construction of the first cut-off wall for the 280 MW Dhauliganga hydroelectric power plant in 2002.
The cut-off wall is located at the upstream toe of the dam and it is constructed in the diaphragm walling technique. The thickness is 1 m and it reaches a maximum depth of 70 m with a total area of 8,000 m?. The initial design concept had foreseen to stop with the cut-off wall on top of the bedrock and close the wall with a grout curtain below the wall toe. The client could be convinced that it is possible to socket the wall into bedrock, thus eliminating the need for a grout curtain.
The dam axis is located at a V-type valley with very steep side slopes. The bedrock is formed of biotite gneiss and augen gneiss with bands of mica schists. The valley deposits are mainly fluvial deposits formed of sand and gravel with some bands of marl type layers. The main characteristic however is the presence of boulders throughout the whole valley section. The boulders are found at any depth with a size ranging from 20 cm to several metres.
A combination of the reverse circulation cutter technique with BAUER trench cutter BC 40 and standard hydraulic grabs and heavy chisels was selected as combined system. The experience which was gained during this project shows that it was in fact the right decision to rely on a combined system.
In 2008, another challenging cut-off wall project was executed on the Parbati Hydroelectric Project (District Kullu, Himachal Pradesh). Similar configuration of equipment with cutter, hydraulic grab and chisel was used for the wall construction. As on most projects in the Himalayan region it was again a big challenge for all parties involved to deliver a good product within the given timeframe. In the course of excavation, many stone and boulder obstructions had to be penetrated. Boulders mainly consisted of gneiss (with UCS of 100 MPa) and quartz (with UCS up to 200 MPa). The wall had to be socketed into meta-volcanic bedrock with very steep side slopes.
The maximum wall depth was 46 m and the total volume of cut-off wall was 3,400 m? which was constructed in five months, one month ahead of schedule.
Navayuga Engineering Company is currently involved in the construction of the 1,200 MW Teesta III hydroelectric package which is a run of the river scheme in the North Sikkim district. In cooperation and with prior consultation with BAUER Maschinen, the final decision was taken to build the cut-off barrier below the 60 m high dam with the BC trench cutter system.
The latest reference for the use of BAUER diaphragm wall equipment on a challenging hydel project with extreme geological difficulties is the HE Project Phununtsangcchu in Bhutan.
On this project, a COW is built underneath the upstream cofferdam with a volume of 7,500 m3.
As the soil contains major layers of very uniform gravel layers, it was decided to perform a full pretreatment grouting programme along the wall axis. The grouting programme lasted from March 2012 to May 2012.
The timeframe for the COW is very tight, as the wall has to be completed by end of November 2012. Therefore two units of BAUER cutters, one grab unit and one chiseling unit have been mobilised by BAUER and Larsen & Toubro.
The project is another very good example for the flexibility and adjustability of the cutter system to changing conditions on site:
? The maximum depth was predicted with 80 m. The exploratory holes (for grouting) proved that in some areas it is necessary to extend the depth to 100 m.
? Rock strength was predicted with 60 MPa in the documents. In reality boulders have been encountered with UCS 100 MPa.
? The amount and size of boulders is double the quantity as predicted.
? On the left bank the inclination of the rock flanks is nearly vertical in some areas, which requires a massive amount of 20-30 m of vertical rock cutting.
Although all these adverse factors were unknown at the start of the project, they can be handled due to the capacity of the BC cutter system and the experience of the specialists on site.
Bauer BC Cutter System: Factors for success
? provable shape of COW
? economy
? output
? accuracy
? reliability
? environmental compatibility
On the way to becoming an industrial nation, it is imperative for India to satisfy the ever increasing demand of electric power. Tremendous changes in the current world economy influence the supply of coal and oil for thermal power plants. Nuclear power has lost a lot of its attraction, especially since Fukushima. As it appears that these common resources are reaching a peak to supply energy to a more demanding market, it is essential for a safe and steady growth of economy to strengthen alternative sources of energy. More focus is put on clean and green power sources such as sun, wind and water. India, thanks to the mighty Himalayas, is blessed with immense hydel power potential. The development of this vast power potential has been given top priority by the Government of India. A two-digit number of hydroelectric projects are currently under construction, a much higher number of future projects are in the feasibility, design or approval stage.
Dams are very important structures in all hydroelectric power plant schemes. They serve the purpose of storing and retaining water to generate electricity. Dams are often constructed as earth-fill dams or rock-fill dams.
Controlling seepage
The formation of an effective water barrier within the soil and rock formations beneath dams is one of the most important tasks in dam construction. Such measures are intended to control seepage beneath the dam in order to minimise water losses from the reservoir and, more importantly, to ensure the stability of the dam against internal erosion.
The oldest method of providing a barrier beneath dams, still widely used today, is the grout injection technique. In this process, boreholes of approximately 100 mm diameter are drilled into the underlying soil and rock formations and injected with cement grout to seal the pores in the surrounding soil or fissures in rock beneath the foundations of the dam.
The impermeability of such grout curtains is governed by the spacing of the grout holes, and soon reaches its economic and technical limitations in alluvial soils especially when they are mixed with stones and cobbles.
As a result of this problem, alternative processes have been developed during which soil is excavated and the resulting void backfilled with a barrier material of well-defined properties (?plastic concrete?). The most important method for constructing a positive cut-off wall is the use of the diaphragm wall technology. It has been used successfully since many years on numerous jobsites all over the world.
For many years, walls were constructed with rope suspended mechanical or hydraulic grabs.
There are, however, some limitations in the grab system. The excavation output reduces with increasing trench depth as a result of the intermittent excavation process. It is difficult to form a leakage-free continuous wall for big depth excavations and, last but not least, it is difficult to excavate hard soil formations.
The development of the hydraulic trench cutter system made it possible to overcome these problems and it opened up new horizons for the use of the diaphragm wall technique.
? The advantage of the trench cutter technology is the possibility of adjusting the system to depth (30-150 m) and thickness (600-1,500 mm).
? It works very well in mixed soil conditions from loose sand to cemented layers and even in rock layers.
? High productivity is a major advantage of the trench cutter system.
? The capacity of working within stringent verticality tolerances is an important quality feature.
? The excavated material allows visual control of embedment in impermeable soil at the toe of the wall.
The BC Trench Cutter System
The trench cutter is an excavating machine that operates on the principles of reverse circulation. It is made up of a heavy steel frame (1) with two gearboxes (2) mounted at the bottom of the frame. Cutting wheel drums fitted with a series of teeth are fixed to the gearboxes; they rotate in opposite directions, break up the soil and mix it with the bentonite suspension (3). As the cutter penetrates, soil, rock and bentonite are conveyed towards the openings of the suction box (4). From where they are pumped by a centrifugal pump (5), located right above the cutter wheels, through the slurry pipe incorporated in the cutter?s frame, via the mast head into the slurry conveying system to the desanding plant. Here solid soil and rock particles and liquid bentonite are separated and the latter is pumped back into the trench.
The torque output of the cutter wheels in combination with the weight of the cutter is sufficient to cut into any type of soil and to crush cobbles, small boulders or rock. The verticality of the trench cutter and thus the trench alignment are generally measured on two-axis by means of two independent inclinometers (6). Adjustment of verticality in the two directions is carried out by a system of steering plates (7).
Tough projects
The underground conditions in the Himalayan valleys, which are preferred locations for the construction of hydroelectric power plants, are well known for being extremely difficult for excavation. Due to geological and climatic conditions they contain stones, boulders and erratic blocks within the soil matrix.
BAUER has gained a long list of track records where it has been proven that extreme soil conditions can be tackled best by deploying a BC trench cutter as lead equipment, assisted by hydraulic diaphragm wall grabs and even by ?old- fashioned? heavy chisels.
Applying a DHG hydraulic grab as assistance tool for the trench cutter becomes feasible whenever the use of the trench cutter to crush stones or boulders into small fragments would result in a high energy consumption and unproportionally high wear and tear on the cutting teeth.
The reason for using the combination of multiple systems is the utilisation of specific advantages and strengths of different excavation techniques. Grabs and chisels can only work effectively when they are operated by modern heavy-duty crawler cranes.
BAUER has accumulated a lot of experience in designing, manufacturing and using hydraulic trench cutters for cut-off walls since its first use in 1984 on a dam project in Germany. Many specialised companies all over the world have used or are using this modern techno?logy successfully.
An outstanding example is the construction of a plastic concrete cut-off wall with a record depth of 120 m in Peribonka, Canada (2006).
Experience in India
A breakthrough for this method in India was reached with the construction of the first cut-off wall for the 280 MW Dhauliganga hydroelectric power plant in 2002.
The cut-off wall is located at the upstream toe of the dam and it is constructed in the diaphragm walling technique. The thickness is 1 m and it reaches a maximum depth of 70 m with a total area of 8,000 m?. The initial design concept had foreseen to stop with the cut-off wall on top of the bedrock and close the wall with a grout curtain below the wall toe. The client could be convinced that it is possible to socket the wall into bedrock, thus eliminating the need for a grout curtain.
The dam axis is located at a V-type valley with very steep side slopes. The bedrock is formed of biotite gneiss and augen gneiss with bands of mica schists. The valley deposits are mainly fluvial deposits formed of sand and gravel with some bands of marl type layers. The main characteristic however is the presence of boulders throughout the whole valley section. The boulders are found at any depth with a size ranging from 20 cm to several metres.
A combination of the reverse circulation cutter technique with BAUER trench cutter BC 40 and standard hydraulic grabs and heavy chisels was selected as combined system. The experience which was gained during this project shows that it was in fact the right decision to rely on a combined system.
In 2008, another challenging cut-off wall project was executed on the Parbati Hydroelectric Project (District Kullu, Himachal Pradesh). Similar configuration of equipment with cutter, hydraulic grab and chisel was used for the wall construction. As on most projects in the Himalayan region it was again a big challenge for all parties involved to deliver a good product within the given timeframe. In the course of excavation, many stone and boulder obstructions had to be penetrated. Boulders mainly consisted of gneiss (with UCS of 100 MPa) and quartz (with UCS up to 200 MPa). The wall had to be socketed into meta-volcanic bedrock with very steep side slopes.
The maximum wall depth was 46 m and the total volume of cut-off wall was 3,400 m? which was constructed in five months, one month ahead of schedule.
Navayuga Engineering Company is currently involved in the construction of the 1,200 MW Teesta III hydroelectric package which is a run of the river scheme in the North Sikkim district. In cooperation and with prior consultation with BAUER Maschinen, the final decision was taken to build the cut-off barrier below the 60 m high dam with the BC trench cutter system.
The latest reference for the use of BAUER diaphragm wall equipment on a challenging hydel project with extreme geological difficulties is the HE Project Phununtsangcchu in Bhutan.
On this project, a COW is built underneath the upstream cofferdam with a volume of 7,500 m3.
As the soil contains major layers of very uniform gravel layers, it was decided to perform a full pretreatment grouting programme along the wall axis. The grouting programme lasted from March 2012 to May 2012.
The timeframe for the COW is very tight, as the wall has to be completed by end of November 2012. Therefore two units of BAUER cutters, one grab unit and one chiseling unit have been mobilised by BAUER and Larsen & Toubro.
The project is another very good example for the flexibility and adjustability of the cutter system to changing conditions on site:
? The maximum depth was predicted with 80 m. The exploratory holes (for grouting) proved that in some areas it is necessary to extend the depth to 100 m.
? Rock strength was predicted with 60 MPa in the documents. In reality boulders have been encountered with UCS 100 MPa.
? The amount and size of boulders is double the quantity as predicted.
? On the left bank the inclination of the rock flanks is nearly vertical in some areas, which requires a massive amount of 20-30 m of vertical rock cutting.
Although all these adverse factors were unknown at the start of the project, they can be handled due to the capacity of the BC cutter system and the experience of the specialists on site.
Bauer BC Cutter System: Factors for success
? provable shape of COW
? economy
? output
? accuracy
? reliability
? environmental compatibility
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