6.1.2　Analysis of Geological Adaptability Design o...

6.1.2　Analysis of Geological Adaptability Design of Cutterhead and Cutter

The cutterhead is one of the core components of shield tunneling.In general，the cutterhead has the following three functions：cutting rock and soil mass under the combined action of cutterhead torque and propulsion.Support the excavation face during excavation and shutdown.Improvement of rock and soil during excavation.

In many projects，due to the inappropriate design of head is not suitable for the location，there are serious wear of cutter head and surrounding cutters，and large-scale loss and failure of cutter alloy.In this section，combined with the design of existing cutter head of Nanjing Yangtze River Tunnel and the analysis of the advantages and disadvantages of the adopted process，the technical points and parameter selection of geological adaptability design of large-diameter slurry shield cutter head in gravel or sandy cobble stratum are proposed.

6.1.2.1　Analysis of Geological Adaptability Design of Cutterhead

There are usually three types of cutterhead structure，namely spoke type，spoke panel type and panel type，and the corresponding opening rates are usually about 65，35 and 20％.According to the analysis，the shield cutter head in sandy cobble stratum should adopt the spoke cutter head with large opening rate，which is beneficial to the flow of slag and soil and the rapid excavation，reduce the tool wear and improve the secondary excavation distance of the tool.However，for the large-diameter slurry shield，considering the tunnel scale，cutterhead function，slag and soil properties and other factors，the spoke cutterhead is not applicable.The large-diameter slurry shields in China and abroad all choose the spoke panel type or pure panel type cutterhead structure without exception.The cutterhead opening rate is 15-40％.The cutterhead of Nanjing Yangtze River Tunnel Project adopts the spoke panel type structure，and the cutterhead opening rate is about 35％.The analysis is as follows：

（1）The large-diameter cutter head adopts a spoke type structure，and the structural rigidity is difficult to be ensured，and the cutter disc steel ring is easily deformed or even unstable.

（2）Large-diameter slurry shield is generally used in highway tunnels or railway tunnels.The tunnel is large in scale and long in mileage，and most projects are more than 2 km long.When the excavation section contains sand layer or sandy cobble stratum，it is necessary to develop a medium-distance tool change plan.The relatively moderate opening rate of the cutterhead is conducive to the stability of the auxiliary excavation face when the pressure tool change is applied.

（3）When the project is a large diameter slurry shield at the bottom of the water，if the support pressure is improperly controlled in the tunneling process，the slurry is likely to occur over-pressure splitting roof fall failure or pressure collapse failure.The panel can play the role of temporary support excavation face，and win time for taking corrective measures to avoid accidents.

（4）The residual soil of slurry balance shield is in the form of dilute slurry with a specific gravity of about 1.35，and there is no poor flow plasticity of residual soil，which does not affect the tunneling speed.

6.1.2.2　Tool Geology Adaptability Design Analysis

The leading cutter and scraper on the cutterhead are the main cutters，and the center cutter，overbreaker and peripheral scraper have fixed positions on the cutterhead.Therefore，the layout design of the cutter mainly refers to the layout of the leading cutter and scraper.

1）First knife design

The advance cutter of shield tunneling in sandy cobble stratum is the primary position，which is widely used in earth pressure balance shield with mature technology.According to the working principle of the leading knife and scraper，the content of the tool layout design includes the design of the leading amount of the tool and the spacing of the leading knife.The cutting tool advance is the height difference between the cutting tool and the scraper，which depends on the tool wear length and the rock type，and should not be less than the tool wear length.Whether the cutting tool design is reasonable or not directly affects the cutting effect of the cutter head，the stirring effect of the cutter on the slag soil and the life of the scraper.For sand stratum，the fine particle content is high，and the slag soil is easy to be improved and cut.Therefore，the length of the leading knife is shorter than that of the scraper，and the leading amount is generally 10-15 mm.In Nanjing Yangtze River Tunnel Project，the cutter head is equipped with three rows of alloy columns only in the middle of the cutter body.After the wear of the cutter body on both sides，the alloy columns fall off，which leads to further wear of the cutter body as a whole.It is completely unsuitable for the serious wear characteristics of gravel sand stratum.In practical engineering，the upper part of the cutter head is completely worn.According to the surfacing traces，the wear is estimated to be about 90 mm，and the remaining height is about 165 mm，which is lower than the height of the scraper and completely invalid（less than 300 m in the gravel sand stratum of some sections）.The leading knife with the best application effect in sandy cobble stratum is shown in Fig.6.14.

2）Scraper design

The scrapers（fixed scrapers and replaceable scrapers）are installed in the left and right shields of Nanjing Yangtze River Tunnel.When tunneling in the middle and short distance of gravel stratum，the phenomena such as alloy cracking，overall shedding and groove turnover are common.The scrapers used in this project are comprehensively analyzed.Compared with the design of shield scrapers in sandy cobble stratum in China，the design parameters and principles of scrapers suitable for sandy cobble stratum are proposed.

（1）Alloy materials.A large area of tool change was carried out on the left line after heading to the 578 ring.Some tool alloys showed normal wear.The wear amount was taken for the inverse calculation of the wear coefficient，and the alloy performance was compared with the existing empirical wear coefficient.Through comparison，it is found that the reverse wear coefficient of the normal wear of the surrounding tool is larger than that of the empirical wear coefficient，which may be due to the failure of the scraper and the fixed knife around the same trajectory in advance，and the actual number of effective tools is less than the number of calculated tools.The discreteness of the reverse wear coefficient of most tools is small，indicating that the hardness of the alloy is normal.Tool cemented carbide（blade）has two performance evaluation parameters，namely hardness and bending strength.These two parameters restrict each other.Increasing hardness is beneficial to wear resistance，but bending strength will decrease.Shield cutter in sandy cobble stratum is prone to alloy cracking.Therefore，appropriate sacrifice of hardness，improving bending strength and reducing the probability of alloy cracking are the correct ideas to select alloy performance parameters.It is suggested that the hardness of cemented carbide for cutting tools in sandy cobble stratum should be HRA bigger than 86，and the bending strength should be greater than 2000 N/mm2.

（2）The front and rear angle parameters of scraper are taken.The front and rear angles of shield cutters are usually 5°-15°，and the specific values are determined according to different geological conditions.The secondary wear on the back of the cutter in sandy cobble stratum is serious as shown in Fig.6.15，so as to reduce the embedded depth of the alloy.In order to delay this trend，the back angle should not be too small，and it is recommended to take 15°.The scraper in gravel or sandy cobble stratum is mainly peeled and stirred.In order to reduce the alloy cracking，the rake angle should not be too large，and it is recommended to take 5°.The rake and rake angles of the scraper in this project are 10°，which can be further optimized.

（3）Selection of bonding mode，section size and chamfer radius of blade.Scraper cemented carbide and cutter body usually have two kinds of combination，surface-mounted and embedded，both have their advantages and disadvantages.The main problem of scraper in sandy cobble stratum is to prevent alloy cracking，and the direct cutting function requirements are relatively low，so the embedded application is more，but the engineering effect is poor in harsh sandy cobble stratum.Therefore，the application of surface-mounted combination is gradually increasing.

The definition of cemented carbide section size（width x height）is shown in Fig.6.16.When the surface bonding method is used，too small width will lead to weak impact resistance，and too small height will lead to all cemented carbides entering the stratum during tunneling，which aggravates the wear of the tool body at the bottom of the alloy and causes the alloy to fall off.When the embedded bonding method is adopted，the alloy width can be taken as the conventional value（≥15 mm），but the height cannot be too small.Otherwise，the slight wear of the cutters on both sides of the alloy leads to insufficient embedded depth and alloy shedding.

The chamfer radius of the blade reflects the sharpness of the tool，usually R=2 mm.In general，increasing the blade radius will cause the tool to become blunt and reduce the cutting ability.However，in practical engineering，the blade in sandy cobble stratum is too sharp，which often occurs cracking in the initial tunneling.As mentioned above，the sandy cobble stratum is spalling failure，and the requirement for cutting function is not high.In practical engineering，the blade angle radius of the newly improved scraper in Beijing diameter line reaches 10 mm，and the engineering application effect is good.Therefore，it is recommended that the blade chamfer radius of the cutting knife in sandy cobble stratum should be 5-10 mm，and the low value should be taken when the interval contains clay stratum.

Through the above analysis，the design principle of the geological adaptability of the scraper in the gravel sand and gravel stratum is proposed：reduce the rake angle，increase the back angle，increase the alloy，and passivate the blade.