In excavation with slurry microtunnel boring machines (AVN), continuous development plays a fundamental role to overcome the limits of feasibility and meet the growing demands of each project.Over the last few decades, the range of application of microtunnel slurry machines has expanded considerably, being used in increasingly demanding projects for underground utilities, as well as in the installation of pipes and cabling.
In rock conditions, the use of small diameter slurry microtunnel boring machines has been especially limited.With the AVN 800 HR for hard rock, Herrenknecht has developed a new machine concept, equipped with a stronger main bearing, higher thrust force and an adapted cutting wheel design.TCI (tungsten carbide insert) cutters allow lengths of up to 200 meters without intervention, as they offer a high degree of wear resistance. Low Price Self-Priming Water Pompa
To increase the length of the route in small diameter installations, the sludge evacuation pump system with high pressure line (Jet Pump) has been successfully integrated into the AVN sludge microtunneling machine, creating the new AVNS series.This new AVNS series is used for methods such as E-Power Pipe and for small diameter Direct Pipe from 24” and up.In the future, these advances will further push the boundaries of slurry microtunnel boring and alternative methods such as E-Power Pipe and Direct Pipe in difficult terrain, making trenchless solutions even more attractive to stakeholders.
Currently the main focus in the construction of underground services is to complete the works safely, on time and at the expected cost and with minimal impact on the surroundings and environment.This is of special interest to both clients, project managers and contractors.Successfully implemented projects help build public trust and acceptance of trenchless technologies and convince authorities of their benefits.
Microtunnel solutions with slurry machines play a key role in the trenchless industry.Microtunnel boring concepts are increasingly being developed to push the boundaries of what is currently feasible.Pipe jacking and related technologies such as Direct Pipe and E-Power Pipe, which also use the slurry machine concept, are considered for a wide variety of applications: from the installation of water and sewage pipelines to the installation of underground cables. , for intakes, outfalls or crossings under water and traffic routes on the surface.
More and more projects are planned in difficult terrain, which can be hard, solid rocks, soft soils and aquifers or changing conditions along the tunnel route.Machine concepts must be reviewed and new solutions developed to respond to the requirements of projects carried out on complicated terrain and the trend towards increasingly longer layouts in all diameters.At the same time, safety standards have increased in the construction of small diameter tunnels.
This article provides an overview of trends in small diameter slurry microtunnel execution and the latest technological solutions developed by Herrenknecht.As a leading manufacturer of trenchless technologies, Herrenknecht strives to provide the trenchless industry with the highest degree of flexibility in the planning and construction of sustainable utility infrastructure to realize increasingly demanding projects and exceed existing feasibility limits.
Sludge microtunnel boring machines have a wide range of applications.This also includes hard rock formations.The equipment is selected based on thorough geotechnical investigation.In hard rock, special attention is paid to machine performance factors and the design of critical components, such as the cutting wheel, tools, main bearing capacity or anti-spin measures.All components and their capacity must be adjusted to each other.Thus, the success of microtunnel excavation is determined by the best incorporation of all components into a functioning system.To increase performance, it is important to find the bottleneck of the system and increase its capacity.The entire system can be as effective as its weakest component allows.
The design and selection of cutting wheel tools is the most critical part in rock applications.The size and arrangement of the tool are determined according to the properties of the rock.The arrangement leads to reasonable sizes of rock chips that can be handled by the discharge system.Wear protection plates and hard facing are important to protect the steel structure of the cutting wheel from excessive wear.New cutting wheels for microtunnels in hard rock have been developed to provide:
Three main types of cutting tools can be considered in this application.The most common cutting tool remains the standard cutter, which is well known for large diameter hard rock TBMs.If there is a high requirement on the life of the cutters, TCI or button cutters are recommended.This type of cutter exerts a point loading force on the rock, generating numerous small flakes.Crusher cutters may fill a niche in small diameter tunneling based on the observation that standard and TCI cutters have comparatively low performances in rocks of low strength and ductile behavior.
Main bearing and main drive
The main bearing is the fundamental connection between the cutting wheel and the microtunnel boring machine thrust system including the steering cylinders.It not only has to transfer the thrust loads through the cutter wheel to the front of the tunnel, but at the same time has to deal with the turning torque generated by the main drive and the rotation speed of the cutter wheel to break the rockAt the same time, the main bearing seals the excavation chamber against atmospheric pressure in the tunnel.That is why the design of the main bearing is considered crucial to the success of the tunnel operation.To minimize the risk of main bearing failure, especially on hard rock projects where used equipment is expected, repair or even replacement should be considered.
The steering cylinders must be strong enough to transfer the thrust force necessary to overcome the groundwater pressure and the contact force of the cutters to break the rock.In addition, the stroke of the steering cylinders must be long enough to allow the machine to make curves and follow the layout of the planned tunnel.
Figure 2.- Telescopic station, intermediate thrust stations and main thrust frame.
A telescopic station is vital especially for long stretches.It consists of a ring of hydraulic cylinders assembled directly behind the shield of the microtunnel boring machine.With the telescopic station, the friction loads of the microtunnel boring machine are separated from those of the tunnel layout.By operating the telescopic station, thrust loads can be maximized and machine advance controlled more sensitively, without overloading the cutting tools.
An anti-spin unit is another important factor in hard rock excavation.Especially at small diameters, good lubrication creates little friction between the shield and the surrounding rock.Therefore, the microtunnel boring machine tends to rotate even at low torque.However, large thrust forces, high revolutions, and all available torque are required to be able to transfer a thrust force high enough to break rock.This is especially necessary with a small cutting wheel with small multi-ring cutters.From a technical point of view, the anti-spin units can be divided into shoes actuated by 'grippers' cylinders just behind the telescopic station and into cutters mounted on hydraulic cylinders inside the shield.The use of the anti-spin unit ensures that all available thrust can be applied and achieve the best possible performance during excavation.
Figure 3.- Example anti-twist units: gripper version (left) or version with cutter (right).
Intermediate push stations are important, especially for long runs, as they provide an intermediate push force over a very long tunnel section.They ensure that the microtunnel boring machine can move even when the friction in the pipe section is too high or the pipes reach their maximum thrust capacity.They are another key success factor for the execution of projects with long distances and especially in hard rock applications.
The main thrust frame is located in the attack well and its objective is to move the microtunnel boring machine and the driven pipes to the exit well.However, their pushing capacity is often limited by the design and allowable loads of the piling pipe itself, ultimately leading to limited trace length or more frequent use of intermediate pushing stations.
Piping pipes must be strong enough to transfer thrust loads from the main thrust frame or intermediate stations to the microtunnel boring machine in front of the pipe string.Various types of driving pipe materials are being used around the world, including steel, reinforced concrete, and fiber-reinforced plastic among others.Crucial to success is the design, with sufficient safety factor, quality in manufacturing and quality control before and during installation in the attack well.
Another critical component is the pipe joint, which has to transfer thrust loads from one pipe to another and must be strong enough to prevent damage.This is extremely important in curved layouts where only a portion of the pipe cross section can be used for load transfer.
AVN 800 HR machine concept for hard rock
In the past, the applicability of slurry machines has been especially limited in the non-accessible diameter range.With the AVN 800 HR for hard rock, Herrenknecht has developed a machine concept equipped with a stronger main bearing, three times greater thrust force on the cutting wheel compared to the traditional AVN of the same size and a wheel design Adapted cutting using TCI cutters (tungsten carbide inserts).These cutting tools offer a high degree of wear resistance, allowing layouts of up to 200 meters without interventions, as has been demonstrated in international reference projects.
Hong Kong Reference Project, Water Supply Network Improvement Project
For this water main upgrade project in Hong Kong, the Water Supply Department has selected an AVN 800 HR.The project included the installation and commissioning of new pipelines, and the rehabilitation and inspection of existing pipelines with diameters up to DN 1400. The pipe jacking system was selected as the preferred method to install the section at Wood Road in the area by Wan Chai.
Hong Kong contractor VTEC (Victory Trenchless Eng. Co. Inc.) has been awarded to install the 107 meter long section, with a constant radius of 153 metres.The entire tunnel layout ran through granitic rock (grade II) with a maximum UCS of 200 MPa.
Due to the hard rock conditions and small diameter of only 960mm (OD), an AVN 800 HR with 317mm tapered cutters with TCI cutters was selected.Due to the high abrasiveness of the rock, the cutting wheel has been reinforced with additional protection plates, especially in the edge area, and additional hard coating in all critical areas.The selected AVN 800 HR had an installed power of 90 kW, allowing a cutting wheel rotation of max.26rpm @ 260l/min and a maximum torque of 55kNm.
As is often the case with projects in Hong Kong, space to install equipment for tunnel operation is very limited.This led to additional challenges, not only for machine assembly, but also for extending and welding pipes with an average welding time of 2 to 3 hours.An extremely compact attack well design was chosen with dimensions of approximately 3.5 m wide and 6.5 m long.Even with the high abrasiveness of the rock, there was only moderate wear of the cutting tools.
Figure 4.-.Work surroundings, attack shaft and AVN 800 HR after the section.
It took a total of 60 days to complete the stretch, including several days of downtime.The average daily performance was 4m/day.At the beginning of the excavation during the learning period, the cutter wheel drive was adjusted to maximize performance.Adjustment measures included replacement of the hydraulic pump in the control container to facilitate a higher cutting wheel rotation speed of up to 26 rpm.
AVNS sludge machine with sludge evacuation pump with high pressure line (Jet Pump)
The AVNS range of tunnel boring machines uses the 'jet pump' as the sole mud pump.The AVNS concept has been developed to expand the field of application of slurry microtunnel boring machines and allow longer sections in small, inaccessible diameters.Currently, the AVNS system is used for E-Power Pipe and Direct Pipe technology.The 'jet pump' is integrated into the microtunnel boring machine and allows a large capacity for transporting sludge while requiring very little space.The hydraulic unit for supplying the cutting wheel and steering cylinders is integrated into the microtunnel boring machine.This eliminates coupling of hydraulic lines, which would otherwise be typical in small diameter pipe jacking procedures.The cutting wheel is specially designed and equipped with extremely strong cutting tools.The crushing cone located behind the cutting wheel crushes boulders, rocks or other materials to a pumpable grain size.
Figure 5.- AVNS350 XB (Ø 505 mm excavation diameter) with its components.
The first of its kind, an AVNS 350 XB, is used in two-stage E-Power Pipe technology and is designed for a 20-inch digging diameter.It can cover a geological spectrum ranging from loose soils to soft rocks up to 30 MPa, while handling grain sizes up to 100 millimeters.In the past, even quartzite layers with compressive strengths of up to 100 MPa have been successfully overcome.
E-Power Pipe is used for trenchless installation of small diameter pipes at shallow depths.Over long distances of more than one kilometer, both pressure-resistant and non-pressure-resistant pipes can be installed.The two-stage process integrates aspects of proven horizontal directional drilling (PHD) and pipe jacking (microtunnelling) technology.Even with the most diverse project requirements, E-Power Pipe offers the ability to install protective pipes (sleeves) near the surface, as well as other pipes used in trenchless technology, with minimal impact on the landscape.The wide range of applications ranges from the installation of protective pipes for wiring and installation of underground cables for the expansion of the electrical network, to pipes for gas supply, district heating or hydrogen transportation.
Figure 6.- Stages of the E-Power Pipe method for trenchless installation of different pipes in sections with long distances.
The drilling is held mechanically at all times.During installation, if necessary, the pipe is mechanically and thermally connected to the ground by adding filler material.The record installation length is currently 2 km and was achieved in early 2022 in the Netherlands, where a bundle of HDPE pipes for underground cables was installed in a 20" borehole.
Figure 7.- E-Power machine used in the Netherlands, temporary pipe coupling (left) and preparation for the installation of the pipe package.
The AVNS machine concept has also incorporated the well-known Direct Pipe technology to allow one-pass installation of longer pipes under 36 inches.Traditionally, small diameters were limited by the fact that centrifugal pumps were too large to fit inside the pipe.The high pressure line 'jet pump' sludge evacuation pump is essentially the same as that used for the smaller E-Power Pipe system and the same principles and requirements apply for the auxiliary equipment.The umbilical lines that supply the Direct Pipe microtunnel boring machine with power, communication data and all other necessary lines, as well as mud supply and return, have been redesigned for use in small diameters.Most notable here is that the high pressure mud lines are fully interchangeable for different pipe diameters, i.e. they can be used within 24” pipes as well as 60” pipes.
In Poland, the first small diameter Direct Pipe system using jet pump technology has recently crossed two rivers.An AVNS 600 completed two 28” sections for the Poland-Lithuania Gas Interconnector.
The first crossing of 340 meters was completed with the installation of a single section of welded pipes, while the second crossing of 884 meters was divided into two sections of welded pipes.
Anticipated soil conditions at both crossings were sand, silt, clay, some peat and some pebbles.However, the ground conditions encountered at the first crossing were significantly different and changed shortly after to heavy gravel and some boulders for almost the rest of the route.This prototype demonstrated what was considered impossible for decades: the installation of small diameter pipes with an AVN slurry machine is feasible even for long distances.
Figure 8.- Installation on site in Poland for a river crossing with 28” pipe with Direct Pipe.
Direct Pipe is typically used to safely cross rivers.Thanks to further technical development and growing popularity among customers and contractors, Direct Pipe's range of applications has constantly expanded in recent years.Today, Direct Pipe is also increasingly used for the installation of intake and discharge pipelines or cables, especially where strict environmental requirements must be met in coastal areas.New developments in machine technology, for example the jet pump and cutting head design, now make it possible to use Direct Pipe even with small diameters from 24".
Due to public environmental awareness and the need for more and more underground utility installations, smart trenchless installation methods are required for quick and safe installation, with minimal impact on the environment.These are not only sewage and water networks, but also pipes or underground cable installations to connect and use new energy sources, such as offshore wind or hydroelectricity.
Within the range of trenchless technologies, slurry microtunnel boring machines play a key role, not only in pipe jacking.Sludge microtunnel boring machines are also used in other newer technologies such as E-Power Pipe and Direct Pipe.
New trenchless construction methods and the further development of existing technologies offer greater flexibility to parties involved in the planning and execution of trenchless works.Today, trenchless technologies cover the entire geological spectrum, with great flexibility in terms of diameters, lengths and depths in the network sections to be installed.
New technologies like E-Power Pipe show their strengths where long-established methods like PHD reach their limits in terms of feasibility.
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