IMPETUS FOR EVOLUTION

Trenchless installation methods like slurry pipejacking are important to performing the fast and safe installation of underground utilities while having minimal impact on the project surroundings.

Slurry pipejacking originally comes from the more traditional applications of sewerage and water tunnel construction. Today, new no-dig methods offer planners, power grid operators and construction companies a wide range of potential solutions to build their infrastructure networks. The range of applications includes trenchless cable or pipeline routes, outfall and intake tunnels for seawater desalination, or pipeline landfalls.

Use of the tunneling technique covers the geological spectrum and is flexible on diameters, drive lengths, and depths of installation. In the past, from a geotechnical point of view, though, its applicability was limited in hard rock conditions and in the non-accessible diameter range, respectively.

Today, utility infrastructure projects must cope with a wide range of geological settings, including small overburden, weathered rock, rock-soil transitions as well as fractured and intact hard rock with high strength and abrasivity. Slurry microtunneling now plays a wider role to help designers and contractors meet project requirements. Technological development has continued with major developments, such as innovations in machine performance, cutting tool design, peripheral equipment, and digital solutions.

Together, these technical developments have been pushing back the boundaries for small-diameter slurry microtunneling TBMs (MTBMs) in hard rock.

NEW ROCK GENERATION: AVN 800 HR

As utility tunneling calls for longer drives, especially in the small diameter range, technologies have developed to extend the application range of slurry microtunneling. While this has created more possibilities for how to, practically and economically, design and create underground assets there has been the continued challenge of ground conditions for small diameter tunneling, notably in hard rock.

For that purpose, Herrenknecht developed a new MTBM – the new AVN 800 HR – for small diameter tunneling in hard rock conditions. The machine is equipped with a stronger main bearing, higher torque, triple the thrust jacking loads, and greater penetration rates in hard rock.

The design of its cutting wheel and tools have been fundamentally changed, employing an adapted multi-ring, conical cutting wheel design using tungsten carbide inserts (TCI) cutters. The equipment also offers high wear resistance and, as shown on utility projects already internationally, the machine can perform drives up to 200m in length without interventions.

After tests, the AVN 800 HR had its first operational trial drive on a 34m-long bore in Clara mine, Germany. Rock strengths up to 140MPa were successfully handled. Peak performance of the machine was 35mm/min.

Technical improvements on the AVN 800 HR:

• For rock with high UCS: up to 200MPa
• High wear-resistance of TCI cutter discs
• Extended drive length: up to 200m (219yd)
• Torque: 55kNm
• Increased rotation speed of cutting head: up to 26 rpm @ 260 l/min
• Compact jacking frame to achieve a small launch shaft of Ø 3.2m (10.5ft)
• Operation by standard control container C20 possible
• Flushing ring (with remote-controlled ball valve): fine particles kept from annulus
• Extension kit OD 1,295mm with rock cutting wheel, up to 80MPa possible

See Figures 1a and 1b.

A project example with AVN 800 HR was the Water Main Improvement Project in Hong Kong, in 2020, where the new machine design handled abrasive hard rock conditions (see box panel-1).

For the smaller diameters of AVN 400 to AVN 600, a new rock cutterhead generation has been designed with tricone cutters, having a comparable wearresistance to the TCI offer on the AVN 800 HR. The new generation of small diameter slurry machines will enhance possibilities for microtunneling in rock, working with alternative methods like E-Power Pipe® and Direct Pipe® – and helped by a new jet pump system for slurry discharge, down to 450mm (18”) diameter – to give wider options on projects, such as for underground cable or pipeline installations.

It should be noted, however, that the AVN 800 HR is not restricted to hard rock geology but can be equipped with a standard or mixed soil cutting wheel as ground conditions vary on different drive sections, and alignments, of a utility tunnel.

Another trend in trenchless, small diameter tunnels for alignments to go deeper, especially for sewers. There are two key reasons for this trend: first, to get below the increasing number of utility services in urban settings; second, municipalities want more gravity flow in their sewers rather than building and operating so many pumping stations.

However, going deeper increases the likelihood of encountering hard rock and higher groundwater pressures for the MTBMs to handle.

More variables in small diameter tunneling mean more data to record, manage and analyze. In that regard, digitalization offers notable help. For example, digitalization can help to evaluate key data to support wear prediction and tool change planning, and so help enable improved tunneling performance.

Design of technological systems can also find benefit from digitalization. Following several small-diameter hard rock MTBM projects, for example, Herrenknecht developed a device to control the roll of the MTBM in hard rock sections, as will be discussed below.

ROCK, SOIL, INVESTIGATIONS AND SELECTIONS

As with all trenchless projects, geotechnical investigation and assessment form the basis of selecting the most appropriate no-dig methods and tools, and to establish the relevant parameters to support predictions on performance and wear. Yet such geotechnical values are often insufficiently considered in small diameter rock excavations.

Rock classification and weathering

From the perspective of strictly geological classification, rocks are classified based on their origin and their stratigraphic and lithologic properties, respectively. As an engineering material, rock can be defined as lithified or indurated, and crystalline or non-crystalline.

Rock can be encountered in mass presence or large and small fragments, all of which have consequences for design and construction options compared to those that are due to soil values. Classification of igneous, metamorphic, sedimentary, and pyroclastic rocks can be applied, but more important are the physical properties of the rock and the rock mass, and the engineering descriptive criteria.

When subject to chemical or mechanical weathering, rock generally decreases in strength. The weathering processes change the physical and chemical properties of the rock as, gradually, it is changed more toward soil – which is also called sediment and could be compacted to eventually form sedimentary rock.

Many small-diameter utility tunnels, due to their generally small overburden, are constructed in weathered rock.

Rock, therefore, can be described based on its petrographic, mineralogical, or geotechnical properties. For small-diameter tunneling, geotechnical properties especially are important.

After a successful borehole campaign, sampling at the depth of the proposed tunnel or of characteristically representative rock, tests of the samples in laboratories establish geotechnical, such as: uniaxial or unconfined compressive strength (UCS – ­c); tensile strength (TS – ­t); and, point load index (PLI). These values

strongly influence the design and application of MTBMs. They are key factors for machine type recommendation, cutterhead design and cutter selection, and so are also necessary to help predict performance of the tunneling operation.

The deformation modulus or the modulus of elasticity (E-Modulus, or Young’s modulus) is of less interest for the appropriate selection and design of a MTBM.

Rock abrasivity can be characterized by: the Cerchar abrasivity index (CAI); the drilling rate index (DRI) / cutter life index (CLI) / bit wear index (BWI); and, equivalent quartz content (Equ). The former is industry standard and considered easy to obtain. But for the second group – DRI / CLI / BWI – only a few labs can measure those, making them less common for use in design and application of small-diameter tunnels.

When rock at a project site is mostly sedimentary, another interesting geotechnical property is slaking. A few tests (L.A. Abrasion apparatus value (LAV), slake durability index test, and LCPC abrasivity test) describe and measure the disintegration behavior of rocks, which helps in the layout design of slurry circuit and sizing the slurry treatment plant (STP).

Some rock types tend to swell in combination with water. This geological feature is important to measure and understand, for example for the design of the overcut and selection of the right bentonite mixture, respectively. Quantification tests can include: pressure swelling and swelling displacement tests (HUDER & AMBERG, HENKE & KAISER, PAUL (DGEG), IRSM, THURO & SPAUN).

With such reliance in construction on these important geotechnical parameters, the site investigation campaign with boreholes must be both sufficient and well planned. The locations of rock/ground sampling must be carefully planned against the proposed tunnel alignment, and there be enough performed, and at a suitable frequency, to support generation of quality data for the overall planning. Investigations should also include photos of the boreholes and note of the detailed geotechnical profile, thereby allowing the geological and geotechnical information to be inter-connected between the sampling points.

Additional to evaluating the geotechnical values of samples, the whole rock mass can be characterized – or classified – from a geotechnical perspective by adding information about the fracturing of the body, and information on groundwater presence.

Rock mass classification is the assessment of geological conditions for construction engineering – how the geomechanical behavior of the whole could affect the stability of structures and cavities to be excavated in the rock. The most important rock mass classification systems are:

• ­Rock Quality Designation (RQD) ­
• Rock Mass Rating (RMR, BIENIAWSKI1974 (Council for Scientific and Industrial Research (CSIR) Geomechanic Classification) ­
• Q-system, BARTON et al. 1974 (NGI-index) ­
• Geological Strength Index (GSI), HOEK 1995

MTBM DESIGN FOR HARD ROCK

All rock and rock mass properties have to be taken into account when choosing the correct MTBM type, including designing the best-suited cutting wheel, tools, main bearing, anti-roll measures, and to predict the tunneling performance. Components and their capacities must be adjusted in relation to each other and optimized for the optimal benefit of the whole functioning system. For increased performance and capacity, bottlenecks need to be found and addressed.

It is well known that for small-scale, small-diameter projects it is not possible to determine all geotechnical parameters. The most important and minimum parameters should be: ­

• Strength: UCS; TS, PLI ­
• Abrasivity: CAI ­
• Rock mass: RQD, (Q) ­
• Information about groundwater (pressure, chemical composition, etc.,)

According to the most important single parameter, the UCS, an application range of slurry MTBMs is recommended with diameters (see Figure 2).

Cutting wheel design for hard rock

The cutting wheel’s tooling composition is the most critical part in rock applications, especially selecting tool sizes and arrangements based on rock properties. The tool arrangement is also related to cutting wheel diameter for generation of reasonable rock chip sizes to be handled by the discharge system (see Figure 3).

Wear protection plates and hard facing play a key role in cutting wheel design. For hard rock microtunneling, new cutting wheels have been developed that provide: extra wear protection, like TCI cutters with hard facing and sandwich wear plates on the rim; high performance bearing of the cutters for higher loads; and, a stable solid structure to take the higher loads on the cutters.

A project example of a dedicated hard rock cutting wheel is on the machine design – for a larger AVN, the AVN 2500 – used on a water pipeline project in New Zealand (see box panel-2).

Hard rock tools and selection

There are three main types of cutting tool for MTBMs (see Figure 4):

• Disc cutters – for microtunneling these are normally double- or triple- disc cutters with decreased spacing (compared to large TBMs) enabling faster rock chipping. But more rings on cutters reduces the available thrust per ring, which decreases the probability of creating tensile cracks for chipping. So, small-diameter hard rock AVN machines need high jacking force plus a strong main bearing and cutter bearing. ­
• TCI or button cutters – these are recommended if the problems noted above cannot be solved with a technical solution, or if there is high requirement on the lifetime of the cutters exists. These exert a point load force on the rock, resulting in many small chips. The TCI cutter is considered especially wear-resistant, which is of benefit for long drives in small diameters where cutterhead interventions are not possible. ­
• Milled tooth cutters – these fill a niche for performances in low strength, ductile-behaving rocks which are difficult to chip. Conventional cutters tend to mark or create grooves into such rock faces but do not create tensile fractures to result in chips, whereas tooth cutters penetrate deeply and can lever out loose pieces. Also, soft to mixed ground tools (chisels, rippers, knives, scrapers) can work in low-strength rocks.

Calculating wear

While wear prediction for small-diameter hard rock MTBMs is a largely undeveloped field with very little academic research, and the scarce information available is kept within the companies involved in the industry, it is noted that this is based on empirical and semitheoretical approaches. Several lab tests help to quantify the wear of rock on steel (e.g., Cerchar, L. A., or LCPC abrasivity tests). Several attempts were made to convert the resulting values into cutter lifetime.

The NTNU Trondheim, in Norway, developed an alternative approach based on the brittleness, surface hardness, and wear capacity of a rock. The cutter life index (CLI) is calculated by use of surface hardness and the wear capacity on cutter ring steel quality. However, this value is rarely determined on a global scale, involves complicated testing, and is uncommon outside Scandinavia, especially for small-diameter projects.

Therefore, semi-theoretical wear calculation for small-diameter tunneling projects is based on the Cerchar abrasivity index (CAI), which gives a good indication of abrasivity.

A first order estimation of the wear caused by a measured lab and can be confirmed or corrected by the wear calculation based on the CSM model. For small MTBMs, corrections need made for the smaller diameter of the cutter wheel, and maybe also made for different material and the number of rings.

Smaller cutters also result in much less wear volume.

Cutting wheel design for a small diameter MTBM is, therefore, substantially different to that for a large TBM cutterhead – such as with the share of centre and caliber areas on the cutterhead surface increasing as machine diameter reduces. So, wear can be very irregular and precise prediction by cutter type is not possible. Therefore, average values for cutting tool sets are recommended.

With this uncertainty, investment in high quality, wear-resistant cutters is advisable, and – if applicable – to reduce the intervals for cutterhead intervention, especially early on a project.

Main bearing and main drive

The design of the main bearing is considered, for many reasons, to be crucial to the success of a tunneling operation. The main bearing is the critical link between the cutting wheel and the jacking system, including the steering cylinders. It transfers jacking loads via the cutting wheel to the tunnel face, doing so while handling torque generated by the main drive and the rotation speed of the cutting wheel to chip the rock. Further, it seals off the excavation chamber against atmospheric pressure in the tunnel.

To minimize risk of failure, especially on projects with hard rock geology and where used equipment can be allowed, given the importance of the main bearing it is advisable to consider refurbishment or replacement prior to commencing the drive.

Steering cylinders

The steering cylinders transfer the thrust force to overcome the ground water pressure and the contact force of the cutters to chip the rock. Their stroke needs to be long enough to facilitate curved drives.

Anti-roll solutions

Good lubrication reduces friction between the shield and the rock, although in doing so there tends to be machine roll and which, in small diameters especially, can result in roll at low torque. But the MTBM needs all torque available as well as high revolution and high thrust forces, needed for its small cutterhead and multiring cutters to have sufficient thrust force per cutter to chip the rock. Therefore, in hard rock microtunneling, the roll must be countered by an anti-roll mechanizm.

From the technical point of view, the anti-roll mechanizms can be divided into two types: grippers (located just behind the telescopic station) and roller disc cutters (mounted on hydraulic cylinders inside the shield). These are illustrated in Figure 5.

Jacking and telescopic stations, and pipes

The main jacking station is located in the launch shaft and pushes the slurry MTBM and the product pipes toward the target shaft. The thrust force must be transferred effectively along this linear string, from the launch shaft through the installed pipe and any intermediate equipment forward to the MTBM (see Figure 6).

However, thrust capacity is often limited by the design and loads of the pipes, leading to limited alignment length or the more frequent use of intermediate jacking stations.

Intermediate jacking stations are important, able to boost thrust, especially for long drives, to ensure the MTBM can move even when the friction load on the pipe is too high, or the pipes reach their jacking capacity.

Also, for longer drives, a telescopic station can be used, helping to maximize thrust loads but also sensitively control machine advancement without overloading the cutting wheel tools. A telescopic station consists of a ring of hydraulic cylinders directly behind the cans, enabling the frictional loads of the MTBM to be separated from the tunnel alignment.

Jacking pipes need to be strong enough to transfer all jacking loads to the MTBM at the front end of the pipe string. Various types of jacking pipe materials are used, including steel, reinforced concrete (RCP) and reinforced plastic (FRP). Pipe joints also transfer jacking loads, from pipe to pipe, and so are a further critical component – especially in curved alignments where only part of the pipe cross-section transfers the thrust load.

Bentonite lubrication in hard rock

Control of the skin friction is a key factor for hard rock pipejacking project success, and, as such, special attention is paid to bentonite lubrication. The longer the drive or the larger the diameter, the more the focus is on lubrication. Uncontrolled distribution of bentonite along the tunnel route or tearing of the lubrication film can lead to a significant increase in jacking forces.

For continuous lubrication, Herrenknecht has developed an automated volume-controlled bentonite lubrication system. A project where it was used is in northwest France, on the new 20km-long (12.4 miles) Landivisiau gas pipeline connection for a new power plant in Brittany. The route runs through gneiss, granite and schist with rock strengths of up to 185MPa, and under the River Élorn and a railway with low overburden. The alignment was varied – a gradient of 17%, a curve radius of 700m (765.5yd), and elevation difference of 26m (28.4yd) between the entry and exit points, and 40m (43.7yd) between the entry and deepest point. The tunnel is 530m long (580yd). Groundwater pressure is up to 4 bar.

For such a challenging drive with a 250kW AVN 1800 (OD 2,185mm), bentonite lubrication was important to control friction between the pipe and rocks, and so reduce the required thrust force at the main jacking station and successfully achieve breakthrough (see Figure 7).

Exploration and injection drilling

Small diameter systems can also accommodate probe drilling ahead of the face, and in hard rock projects also handle grout injection. While probing/grouting are common in large diameter TBMs, it is now also possible to equip MTBMs (≥ 1,800mm) with such facility to establish geology ahead of the machine.

One such small diameter project where probing/ grouting equipment was used is in Chile, for seawater inlet and brine outlet tunnels connected to the Los Pelambres desalination plant that conveys clean water to a copper mine located in a drought area.

The tunnel works constructed a 345m-long (377yd) intake tunnel and 543m-long (594yd) outlet tunnel on the Chilean coast. Tunneling involved use of an AVND 2000 and an AVN 1800 (OD 2,500mm), fitted with an exploration and injection drill. Geology was mostly strong diorite and gabbro (UCS up to 250MPa). At a fault zone, the probe drill was installed and ground injection performed, securing the ground to facilitate safe inspection and change of tools on the cutting wheel (see Figure 8).

CONCLUSION

There has been a growing trend toward smaller diameters and longer drives in trenchless tunneling, which has placed a greater emphasis on the design and selection of efficient MTBM equipment. Hard rock tunneling in small diameters presents specific challenges in combination with slurry pipejacking machines but important technical progress has been made.

Geotechnical investigation needs to be as detailed as possible for MTBM use in hard rock projects.

To further increase performance, it is important to gather and analyze relevant performance data which will be the basis for a continuous improvement process. Advancing digitalization will improve the possibilities of data acquisition and evaluation.

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1 REFERENCE PROJECT: HONG KONG – WATER MAINS IMPROVEMENT

The pipejacking project in Hong Kong is part of the larger water mains improvement project with an overall length of 56km (35 miles), involving installation of new pipelines, and surveying and rehabilitation of existing pipelines with diameters of up to DN 1400. The project is owned by the Water Supplies Department.

Victory Trenchless Eng. Co. Inc (VTEC) was awarded a contract to install a 107m-long (117yd) section of pipeline on Wood Road in the Wan Chai area. Pipejacking was selected as the preferred method to install the section on 153m (167yd)constant radius. Geology along the alignment is granite with UCS up to 200MPa.

A 3m-long (9.9ft) steel pipe was selected for the installation, able to support the higher jacking loads. An additional benefit of the welded steel pipes for hard rock is the capability to counteract the roll.

Due to the hard rock condition and small diameter (only OD 960mm (37.8”)), an AVN 800 HR was selected. The cutterhead was equipped with conical 317mm (12.5”) cutters with TCI inserts. Due to the high abrasivity of the rock, the cutting wheel was armored with additional protection plates, especially in the rim area and additional hard facing in all critical zones. The machine had installed power of 90kW, enabling cutting wheel rotation of up to 26rpm @ 260 l/min and a maximum torque of 55kNm.

As typical for Hong Kong, space on site was limited which resulted in challenges, not only for the machine assembly but regular pipe extension and welding, with an average welding time of 2-3 hours. An extremely compact launch shaft design was chosen (3.5m x 6.5m or 11.5ft x 21.3ft). The works were undertaken in 2020.

During the initial period of the drive and learning curve, the cutting wheel drive was adjusted, including replacing the hydraulic pump in the control container to enable a higher cutting wheel rotation speed of up to 26rpm. The pipejacking was completed over 60 days, including several idle days. Average daily progress was 4m. Given the abrasivity of the rock, there was only moderate wear of the cutting tools (see Figure 9).

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2 REFERENCE PROJECT: HUNUA 4 WATER MAIN, NEW ZEALAND

The Hūnua 4 watermain project is the largest water pipeline project undertaken in New Zealand for many years. The project involves over 30km (18.6 miles) of 1.3m to 1.9m-diameter (4.25ft-6.25ft) steel pipeline in Auckland. A Herrenknecht AVN 2500 (OD 3,025 mm) was tunneling in basaltic rocks with a UCS up to 135MPa. Thanks to a dedicated hard rock cutting wheel design (see Figure 10), an average penetration rate of 8mm/ rev was accomplished. This resulted in a best weekly performance of more than 137m (150yd) and a record drive length of 1,296m (1417yd), which was the longest single drive in the southern hemisphere by a pipejacking MTBM of this size.