6.1.1　Shield Tool Type and Cutting Principle

6.1.1　Shield Tool Type and Cutting Principle

6.1.1.1　Shield Tool Type The performance of shield excavation is mainly guaranteed by the selection and arrangement of cutters.Different types of cutters are arranged on the cutter head of the shield，so that the shield can adapt to the excavation of various strata from soft soil to hard rock.Mastering the correct selection and use method as well as reasonable arrangement on the cutterhead is crucial to improve the tunneling efficiency of shield and create good economic benefits.

In terms of tool shape，Fig.6.1 shows the front view and side view of the four commonly used digging tools（tooth cutter，roof cutter，mosaic cutter and disc cutter）.Among them，tooth cutters and roof cutters are mainly used for excavating weak strata such as sand，silt and clay.The mosaic cutter and disc cutter are mainly used for excavation of hard strata such as gravel layer and rock stratum.

The tool can also be classified according to the purpose of the cutting and setting position，as seen in Table 6.1.

Shield cutting tools can be divided into two categories：rolling tools and cutting tools.Rolling tools include single edge hob，double edge hob，three edge hob，etc.Cutting tools include cutters，tooth cutters，scrapers，advance cutters，profiling cutters，central fishtail cutters，shellfish cutters，etc.For the excavation of different strata，the cutters of shield usually adopt different forms：when the excavation stratum is hard rock，the disc cutter is used.When the stratum is soft rock，the tooth knife is used.When the stratum is soft or broken soft rock，the cutter or scraper can be used.

1）Hob

In hard rock excavation，the use of rock cutter.The feature of rock breaking by hob is to achieve the purpose of rock breaking by the impact crushing and shear crushing generated by the rolling of the cutter.The hob is mainly composed of cutter ring，cutter body（sleeve），cutter shaft，bearing，seal，end cover and other components（Fig.6.2）.

According to different shapes，hobs can be divided into disc hobs，ball-tooth hobs and wedge-tooth hobs.According to different installation positions，it can be divided into central hob，positive hob and profiling hob.According to the material of blade，it can be divided into all-steel hobs and cemented carbide inserts.According to different structure forms，it can be divided into single-edged hob，double-edged hob，three-edged hob，four-edged profiling hob，two-joint hob，three-joint hob and eccentric hob.Some hobs for shield are shown in Fig.6.3.

Three-edge and four-edge hobs are only used in the center of the cutterhead to adapt to the small radius rotation of the cutterhead center.The double-edged hob is usually used in mudstone，shale and other soft rock，which requires small blade pitch.The double-edged hob can meet the geological requirements.When the strength of rock mass is high，too small blade distance will lead to low tunneling efficiency on the one hand，and on the other hand，the double-edged forced synchronous rotation will aggravate wear under the condition of non-ideal leveling of the excavation face，and one of the blade wears and the whole blade fails.Therefore，the double-edged hob is not recommended in the medium and hard rock strata.

2）Cutter

The cutter is a soft soil cutter，which is installed on both sides of the cutter head opening groove to cut the soil layer on the palm surface.When the cutter rotates with the cutterhead，the axial shear force and radial cutting force are generated on the soil of the excavation face.The cutter head and blade are inserted into the stratum，and the soil of the excavation face is continuously cut down.The cutter is generally applicable to loose strata such as sand，pebble and clay with particle size less than 40 mm.The cutter arrangement on the cutterhead generally has concentric circles and Archimedes spiral arrangement.At present，Archimedes spiral arrangement method is widely used.

3）Tooth cutter

When the shield tunneling in the case of soft rock，due to the lack of effective adhesion between the disc cutter and the rock face，the disc cutter cannot roll，resulting in string grinding.If the disc cutter cannot roll，it will lose the effective rock breaking function.Therefore，some disc cutters should be replaced by replaceable tooth cutters in soft rock strata，and the rock breaking trajectory is the same as that of the hob.The rock breaking principle of shield cutter is similar to that of cutter，and there are two cutting edges on the cutter，and the cutter head can break rock when the cutter head is rotating or reversing.Some shield cutters are shown in Fig.6.4.

4）Scraper

The scraper is a tool for cutting soft rock and soil layer or scraping slag soil，and its main role is to clean up rock debris and edge slag soil.The cutting mechanism of the scraper is similar to that of the cutter，and the soil under cutting is scraped into the soil cabin.According to the different installation position，the scraper can be divided into positive scraper and edge scraper（peripheral scraper，diameter cutter）.The disadvantage is easy to wear failure.Some scrapers for shield are shown in Fig.6.5.

5）Fishtail knife

The fishtail cutter is installed at the center of the shield cutterhead，which is usually used in sandy cobble stratum or clay stratum with high strength.It is used to first scrape the soil at the center of the cutterhead，improve the fluidity of the soil at the center，prevent it from consolidating into mud cake，and reduce the cutting resistance of other scrapers and reduce wear.The central knife can be divided into fishtail knife and horn knife，and the fishtail knife can also be divided into integral and combined types.The fishtail knife is shown in Fig.6.6.

The size of the fishtail knife is large，generally 1200-1500 mm long，400-500 mm high，200-300 mm higher than the ordinary scraper.The design and arrangement principle of the fishtail cutter is that the fishtail cutter is not on a plane with other cutters，so the fishtail cutter first cuts the soil and loosens the original stratum.The root of the fishtail cutter is designed as a cone，which adds a turning motion to the soil cut by the fishtail cutter when the cutter rotates.This not only solves the cutting problem of the central part of the soil，but also improves the fluidity of the cutting soil.

6）First knife（leading knife）

It is arranged between the panel and the spoke，and the cutting trajectory of the scraper.The first knife is usually 40-50 mm higher than the scraper or the cutter，which does not play a direct cutting role.It contacts the stratum before the cutter，ploughing soft rock and soil layer.The cutting tool first cuts the soil before cutting the soil，and cuts the soil into blocks to create good cutting conditions for the cutting tool.The use of advance cutter can generally significantly increase the flow of cutting soil，greatly reduce the torque of the cutter，reduce the cutting resistance of the scraper，improve the cutting efficiency of the cutter，and reduce the wear of the cutter.First knife is divided into replaceable and non-replaceable two types.The leading knife is shown in Fig.6.7.

7）The rim bell knife

It can be regarded as the peripheral advance cutter，which is used for the right angle cutter head around.The cutting outer diameter is slightly larger than the outer diameter of the shield，which reduces the shield propulsion resistance，prevents the wear of the cutter head，and cleans the soil residue at the bottom of the cutter head to improve the tunneling efficiency.When the shield passes through the sandy cobble stratum，especially the sandy cobble stratum with large particle size，if the hob cutter is used，due to the loose soil debris，large deformation will occur under the excavation and extrusion of the hob，which greatly reduces the cutting effect of the hob，and sometimes even loses the cutting crushing ability.The disc ring shell cutter is arranged on the front face of the disc ring of the cutter，which is dedicated to cutting sandy pebbles.The problem of cutting soil（sandy cobble）by shield machine can be well solved by using circle scallop cutter.The circle scallop knife is shown in Fig.6.8.

8）Copying knife（super-cutting knife）

Shield generally designed two profiling cutters（one spare），arranged at both ends of the spoke.During construction，according to the requirements of over-excavation amount and over-excavation range，the profiling cutter can be extended and retracted from both ends of the spoke radially to achieve the purpose of over-excavation cutting.The maximum extension of profiling knife is generally 80-130 mm.The profiling cutter is designed for curve propulsion，turning and rectification.It is installed on the edge of the cutter head，and the extension of the cutter is controlled by a hydraulic cylinder.The ordinary section does not work，and the cutter head extends during curve propulsion，turning and rectification.Some profiling cutters for shields are shown in Fig.6.9.

6.1.1.2　Shield Cutter Cutting Principle

1）Cutting principle of the hob

The research on the cutting mechanism of the hob was first started from the mining and coal breaking.It was considered that the broken line of the coal seam after the hob cutting was circular，and the cutting force of the hob was calculated based on this assumption.The main ways of rock breaking by the hob were extrusion failure，shear failure and tension failure.The extrusion rock breaking of the hob mainly occurs on the path of the hob.The rock mass is directly subjected to compressive stress through the blade surface of the hob.The rock mass surface is subjected to excessive compressive stress，which leads to damage and produces fine broken rock particles.The shear rock breaking mainly occurs outside the trajectory line of the hob blade，and the rock mass is damaged due to the shear stress exceeds its shear strength.The main rock breaking method of hob is tension failure，and the failure of rock mass conforms to the tension failure theory.The roller exerts compressive stress on the rock mass，which makes the rock mass broken and cracks appear inside the rock mass.Then，due to the elongation and coalescence of cracks，the rock mass produces excessive plastic deformation and finally tensile failure，as shown in Fig.6.10.

By comprehensively considering the three rock breaking methods of tension，extrusion and shear of the hob，and combining with the tool selection，cutting parameters，rock type and linear cutting machine experimental data，Ozdemir et al.of Colorado University of Mining and Technology proposed the CSM model of semitheoretical and semi-experimental，which was used to calculate the force of the hob（Fig.6.11），providing a basis for the selection of the hob.However，the model only considers the normal force and rolling force of the cutter，and does not consider the lateral force of the cutter.Therefore，the force of the cutter calculated based on the model is smaller than the experimental test value：

Ft—the resultant force of the hob.

φ—the contact angle between the hob and the rock.

T—the width of the hob tip.

R—hob radius.

P—the pressure in the crushing zone.

P0—the basic pressure of the crushing zone directly under the hob，and the strength of the rock（rock compressive strength σc and rock shear strength σt），hob size（hob radius R，tool tip spacing S and one excavation amount p），the blade shape（the hob tip width T，etc.）is related，that is，P0=f（σc，σt，S，T，R，p）.

ψ—the pressure distribution coefficient of the tool tip，ψ=-0.2 to 0.2（if the hob is V-shaped and sharp，ψ=0.2；if the width of the hob tip is large，ψ=-0.2；in general，ψ=0.1）.

θ—blade angle.

However，the single-blade hob and the double-edged hob are not exactly the same in the same kind of rock.The single-blade hob is destroyed by multiple crushing of the rock until the tensile crack occurs in the rock mass.The more regular cracks，the larger the lump block is；for the double-edged hob，the rock is broken by the synergy between the hobs.The hob has a greater pressing force on the rock mass，and the rock is easily broken and produces fine gravel and rock shovel.The energy consumption of rupture is high，and only complete lenticular rock ridges appear in extremely hard rock.Therefore，in the general rock mass，the single-edged hob should be preferentially arranged to reduce the energy consumption of rock breaking；and the double-edged hob should be arranged in the extremely hard rock to strengthen the pressing force of the hob and improve the rock breaking efficiency.

Different rock properties also affect the cutting mechanism of hob.The discrete element method is used to simulate the cutter cutting process.It is found that the rock joint characteristics have a significant impact on the cutter rock breaking.The smaller the joint spacing is，the greater the impact on the penetration of the cutter is.The cracks produced by the rock are easy to develop along the structural plane of the rock mass.Similarly，the rock confining pressure also has a significant impact on the cutting performance of the hob.The increase of confining pressure hinders the development of cracks to a certain extent.In the process of rock intrusion by the hob under different confining pressures，it is found that the broken form of the rock gradually changes from a single lenticular rock mass to three irregular rock fragments.

2）Cutting principle of the scraper

The early research on scraper cutting mechanical model mainly referred to the experience of soil cutting processes such as bucket teeth of excavators in earthwork engineering and plough teeth in farming and forestry.Among them，the most influential one was the flow model of Japanese scholars Yano Shintarou and Tomiaki Jirou.The soil excavated by cutting tools has different flow patterns，which are mainly related to cutting angle，speed，thickness and soil properties.There are four common flow patterns：flow pattern，fracture pattern，shear pattern and exfoliation pattern，as shown in Fig.6.12.Flow cutting mainly occurs in low-intensity strata such as silt stratum，water-rich clay stratum and silty soil stratum.With the movement of scraper，the soil generates continuous shear deformation from the blade.Shear cutting mainly occurs in the clay and silty soil layers with relatively high strength.When cutting，the soil first generates compression deformation，and then the cutting soil moves along the blade surface of the tool until shear failure occurs.Fractured cutting occurs in sand stratum.The soil generates compression deformation at the front of the blade and maintains stability for a certain time.With the cutting progress，the soil at the blade generates cracks and destroys，and the formed slag soil is small block.The stripping cutting occurs in the ordinary gravel and sandy cobble stratum，and the scraper plays the role of stripping sandy cobble.

As the most commonly used force model in scraper cutting，Mckyes-Ali model divides soil failure into central failure zone and two half-moon failure zones，as shown in Fig.6.13.The logarithmic spiral failure line is simplified to a straight line，and the cutting force of the tool is derived.Domestic research on scraper cutting model is mostly based on Mckyes-Ali model，by assuming that the scraper vertical cutting soil or ignoring the weight of soil，so as to improve the calculation model.