6.1.3 Field Excavation Test Study
In the process of shield construction，different cutters should be scientifically and reasonably selected according to different geological conditions to prolong the service life of cutters，which will have an important impact on the progress and benefit of the project.Combined with Nanjing Yangtze River Tunnel Project，this section mainly optimizes the design of scraper and summarizes the long-distance construction experience of gravel stratum.
6.1.3.1 Replaceable Scraper Improvement Program
Based on the analysis of the insufficient design of the initial scraper，the engineering performance of the scraper corresponding to different improvement schemes is studied，and the geological adaptability design points of the refined scraper in the gravel sand stratum are further verified.The influence of tool parameters and tunneling parameters on the torque of the cutterhead is analyzed.Three sets of scraper improvements are shown in Figs.6.17，6.18 and 6.19 and Table 6.2.
Compared with the original scheme，the three schemes are basically in line with the principle of“reducing the front angle，increasing the rear angle，increasing the alloy section，and passivating the alloy blade”.At the same time，the width of the alloy block is increased，and the impact resistance is enhanced from the size and specification.In order to be suitable for the alloy section form and increase the area of the alloy and the blade body，the combination mode of the alloy and the blade body is changed to the surface type.Scheme I embeds a small alloy behind the main alloy，which can effectively delay the secondary wear of the rear corner.Scheme Ⅱperipheral cutter head is designed as parallel double edge，so that it has both forward and reverse cutting ability.Scheme Ⅲ blade radius is directly passivated to R=10 mm，which is the first application in China（Beijing railway diameter line scraper is improved to the second application）.The above is the design innovation of the three schemes，and the specific engineering performance must be tested in the actual excavation.
Fig.6.17 Changing the squeegee improvement scheme
Fig.6.18 Changing the squeegee Ⅱ improvement scheme
Fig.6.19 Changing the squeegee Ⅲ improvement
Table 6.2 Scraper parameters and design description of different schemes
6.1.3.2 Scraper Replacement Experiment
Three sets of scraper improvement schemes have distinct characteristics in the design，but their applicability and process reliability still need to be tested by practical engineering.The improved scraper is first applied to the left line tunnel.The improved scraper and the original scraper are installed at the same time at different cutter positions of the cutter head.In the tunneling process，the improved scraper is gradually adjusted according to the tunneling effect and the performance of the cutter，and the left line shield is successfully penetrated.
6.1.3.3 Statistical Comparison of Normal Wear Coefficients
The normal wear monitoring value（the wear value of uncracked alloy tooth）obtained during the whole tool changing process before and after the improvement of the scraper was used to deduce the wear coefficient.According to the proportion of various strata in the tool changing interval，the wear coefficient of the cutter in different strata was calculated.The wear resistance of different types of scrapers was tested，and the calculation results were used for reference in future projects.
The regression of wear coefficient in different intervals is shown in Figs.6.20，6.21，6.22，6.23，6.24，6.25，6.26，6.27 and 6.28.The starting mileage in Figs.6.21，6.22 and 6.23 is ring 647 due to the alternate tool change of different tool numbers.The proportion of various strata in each tool change interval is shown in Table 6.3.
Fig.6.20 578-647 rings original cutting tool
Fig.6.21 647-680 rings original cutting tool
According to the proportion of different types of strata in different cutter replacement intervals，combined with the statistical results of 0-578 rings in Table 6.3 and Figs.6.20～6.27，the normal wear coefficient of each type of scraper in different strata is calculated.The effective statistical intervals of improved No.Ⅰand improved No.Ⅱ are only one and two，respectively，and the wear coefficient of this type of scraper in the corresponding stratum cannot be calculated.Since the alloy blades of the improved No.Ⅰ and the original cutter are both sharp designs，the improved No.Ⅱ and the improved No.Ⅱ are both sharp designs.No.Ⅲis designed for passivation treatment，so it is assumed that the wear coefficient of the improved No.Ⅰ and the original tool is equal in the fine sand stratum，and the wear coefficient of the improved No.Ⅱ and the improved No.Ⅲ is equal.According to experience，it is assumed that the wear coefficient of the improved No.Ⅰ in the round gravel layer is twice its wear coefficient in the gravel layer.On the basis of the above two assumptions，the classification wear coefficient of each type of scraper in different strata is calculated.According to the current situation of the advance cutter in the right-line cabin opening，it can be concluded that the left-line advance cutter is also completely invalid.Therefore，all statistical calculation results are based on the failure of the advance cutter，which can be used as a reference for the prediction of tunneling wear of similar projects without the advance cutter（Table 6.4）.
Fig.6.22 647-719 rings original cutting tool
Fig.6.23 647-742 rings original cutting tool
Fig.6.24 742-792 rings coefficient of wear
Fig.6.25 792-885 rings coefficient of wear
Fig.6.26 792-885 rings coefficient of wear
Fig.6.27 1006-1513 rings coefficient of wear
Fig.6.28 Comparison of wear coefficient of each type of scraper in different stratum
Table 6.3 The proportion of all types of strata in the tool changing interval
Table 6.4 Statistical results of wear coefficient of each type of scraper in different stratum（x 10-3 mm/km）
It can be seen from the statistical data that the wear coefficient of the original scraper in silty clay and fine sand stratum is close to the corresponding empirical statistical data，but the wear coefficient increases rapidly to 0.079 mm/km in gravel sand stratum，which is about 2.47 times of the corresponding empirical wear coefficient.The wear coefficient of the original scraper in the fine sand stratum is normal，indicating that the hardness parameters are normal.In the gravel sand stratum，not only the cracking and overall shedding are serious，but also the normal wear coefficient is too large.The fundamental reason is that the alloy blade is too sharp，and the normal wear process has been accompanied by fine cracking，which also proves that the alloy is too sharp in the gravel or pebble stratum.
The improved No.Ⅰ has improved the design of the alloy blade.The normal wear performance of the alloy blade in the gravel layer is better than that of the original tool，but the wear coefficient in the round gravel layer is as high as 84 x 10-3 mm/km，which is about 1.4 times of the corresponding empirical wear coefficient.The fundamental reason is that the design of the alloy blade takes too much consideration of its cutting performance and the passivation degree is not enough，which is not conducive to crack prevention and normal wear，and the blade tip is quickly flattened after short-distance tunneling.
The normal wear coefficient of the improved No.Ⅱ is the smallest in all types of cutting tools，and is far less than the empirical wear value，which shows the rationality of the alloy edge design and the high hardness of the alloy.At the same time，the cutting tool also shows the characteristics of easy cracking，indicating that the high hardness of the alloy leads to its brittleness and insufficient bending strength.It does not conform to the principle of selecting the alloy performance parameters in the gravel stratum.It is suggested to reduce the hardness of the alloy and improve the bending strength.
The wear coefficient of the improved No.Ⅲ is moderate，which is slightly less than the empirical wear coefficient，and about 70 cutters are used in the whole tool change process.Only 4 cutters appear alloy cracking，and the rest are normal wear.In the gravel layer，the general tool change distance reaches 200 ring，and the longest reaches 263 ring，which is much higher than the average 50 ring of other types of scrapers.The replaced cutters of the improved number Ⅲ maintain good shape，and can still be used in practice.The improved No.Ⅲ tunneling process has the most application and stable performance，which not only directly verifies the good geological adaptability of“blunt knife”in gravel and round gravel strata，but also shows that the material selection of this type of tool alloy is reasonable in taking into account both hardness and bending strength.
Table 6.5 The suggested wear coefficient of sludge-shield scraper under the condition of no advance cutter k（x 10-3 mm/km）
According to the engineering performance of different types of improved cutters，it is suggested that the design and material selection of similar engineering scrapers can refer to the improved NO.Ⅲ，and based on the statistical value of wear coefficient in sandy soil，and referring to the statistical value of wear coefficient of original cutters in clay and silt，the recommended value of wear coefficient under similar engineering conditions is proposed，as shown in Table 6.5.
6.1.3.4 Evaluation of Each Improved Blade Design
Although the engineering performance of the improved scraper Ⅰ and Ⅱ is better than that of the original tool，there are still serious alloy cracking and overall shedding in practical engineering applications.The overall performance of the improved scraperⅢ is excellent，but there are also improvements.The reasons are analyzed，and the shortcomings of the design and process are found.It has a direct reference for the design of the scraper in gravel-sand cobble formation.Break probability statistics of three types of scrapers in each tool change interval are shown in Table 6.6.
It can be seen from Table 6.6 that the fracture of improved scraper Ⅰ alloy is common，but there is no overall shedding，indicating that the brazing quality control is greatly improved compared with the original tool.The fracture of the improved scraper Ⅱ alloy is better than that of the improved scraper I，but some of the tools have the whole alloy falling off，and the brazing process and structural design have defects.In previous inspections，only four blades of the improved scraper Ⅲ appeared alloy cracking，and the good rate was above 90％.The gravel sand gravel stratum had strong geological adaptability，but the wear condition showed that the structural design could be further optimized.According to the typical shapes of three kinds of tools after wear or fracture，the shortcomings of tool design or process are analyzed.On this basis，the optimization design is proposed.
Table 6.6 Improved blade collapse design
6.1.3.5 Scraper Optimization Design
According to the practical engineering application performance of scraper，the scraper is further optimized based on the improved No.Ⅲ.The front angle is changed from 5°to 0°，and the rear angle is changed from 5°to 15°，so as to reduce the tool wear.The main alloy tooth is changed from rectangle to L shape，and the amount of residual alloy is reduced，so that it not only has excellent engineering performance，but also meets the principle of saving and application.The proposed improved tool has not been used in engineering practice，and the actual engineering effect needs to be tested.