1.2　Common Technical Difficulties in Large Diamete...

1.2　Common Technical Difficulties in Large Diameter Underwater Shield Tunnel Construction

Due to China's vast territory and large differences in geological conditions，in addition to the Shanghai area and individual tunnels crossing the muddy clay stratum，the large-diameter slurry shield projects that have been built and are under construction in China have almost encountered the composite stratum of fine sand gravel，upper soil and lower rock（Fig.1.1 is the composite stratum map of Nanjing Weisan Road crossing the river channel）due to many unpredictable factors during the construction of crossing the river and the seabed.The construction of large-diameter underwater shield tunnels is facing great risks.If the control is not good，serious deformation of the stratum and instability of the excavation face will occur，which will bring great risks to the project.

The construction of large diameter underwater shield tunnel technology in China is mainly faced with the problems of initiation and arrival，crossing the embankment，shallow overburden construction，crossing the high water pressure permeable composite stratum in the middle of the river，long distance tunneling and tool replacement，crossing the channel in the middle of the river.At present，China has become the country with the largest number of shields in the world，and the number of large section slurry shields is considerable.In the future，slurry shield construction method will be widely used in many major tunnel projects，especially in tunnel projects across the river and sea.Therefore，combined with the current slurry shield engineering examples in China，this book analyzes and summarizes the common problems，providing theoretical support for the formation of systematic，comprehensive and standardized construction technology methods.

1）Initiation and arrival

The initiation and arrival stage of shield mainly includes two parts：soil reinforcement and construction technology at the end of the tunnel.The problems in the construction are mainly manifested in the poor reinforcement effect of the shield in and out of the tunnel end stratum，the water gushing at the tunnel entrance when the shield enters and out of the tunnel，the difficulty of shield attitude control，the failure of establishing a good pressure balance as soon as possible，and the sealing failure of the tunnel entrance.Poor reinforcement effect of the portal will lead to a large amount of mud overflow and sand gushing.When the shield enters and exits the hole，due to the high pressure head of deep groundwater in the foundation pit，it is prone to inrush or piping problems.In the past，grouting method was the main method for stratum reinforcement at the initiation and arrival parts of shield tunnel.However，in recent years，with the increase of buried depth and section of shield tunnel，high-strength reinforcement is urgently needed.In addition，due to the time-consuming removal of diaphragm walls and other underground walls，the reinforced strata are required to remain stable for a long time.

The construction technology of shield initiation and arrival includes two parts.The starting section mainly includes four aspects：auxiliary engineering construction，negative ring segment and box culvert assembly，door breaking and starting tunneling parameter control.The arrival of shield has its particularity and importance relative to the construction of interval tunnel，mainly including：stratum reinforcement，shield positioning and receiving portal position review measurement，portal processing and installation of portal ring temporary sealing device，installation of receiving base，etc.Through the above construction technology，the deviation correction between the central axis of the shield machine and the design axis of the tunnel and the position of the receiving gate is gradually completed，and the tunneling is controlled to control the settlement and segment assembly，so as to prevent the displacement of the shield receiving base and the surface settlement caused by the breakdown of the shallow overburden layer.

2）Shallow overburden construction

When the tunnel passes through the shallow soil section of river bottom，due to the requirements of the tunnel design line，the overlying soil layer of the tunnel will be shallow.When the overlying soil weight and river pressure cannot resist the buoyancy of the tunnel caused by grouting during the excavation of the tunnel，the tunnel will float or roof fall，which brings immeasurable disasters to the project.There are many examples of similar problems encountered in shield tunnel construction at home and abroad.In the construction process，it is of great significance to solve the balance between the soil around the shield and the shield itself，which is of great research significance to the shallow overburden problem caused by the tunnel crossing the inland river.

There are three main risks and problems in the construction of shallow overburden of tunnels under water（river，lake and sea）by shield method：water flowing through the roof of shallow overburden，tunnel floating，sand flowing and piping.In the case of shallow overburden and high head pressure，the earth pressure balance in front of the large cutterhead is not easy to establish.Water sand often enters the shield through the opening of the large cutterhead and the tail of the shield from the cracks in the disturbed soil，causing the shield to be flooded，and such accidents of machine damage and casualties occur from time to time.The upper and lower forces of the shield advancing under the water area are not balanced，the shield posture is raised，the pressure slope is difficult，the tunnel is floating，and the axis is difficult to control.After the assembled tunnel ring is removed from the tail of the shield，the tunnel floats because the upper pressure load and the weight cannot resist the buoyancy caused by groundwater.If the corresponding reinforcement measures are not taken，it is easy to cause local tunnel cracking and water leakage.Construction in liquefiable soil layers such as sandy soil and sandy silt，due to shield driven cutter head rotary cutting extrusion disturbance，plus high head pressure（sometimes up to 0.3-0.5 MPa）.Liquefied sand leaks into the tunnel along the shield tail and tunnel joint with groundwater.If measures are not taken in time，local foundation hollowing，tunnel subsidence，spiral fracture and tunnel failure may occur.

3）Crossing embankments and other buildings

When shield tunneling in the stratum，it will inevitably disturb the stratum，cause stratum settlement，and then affect the stability of the surrounding buildings，so that the adjacent buildings have settlement，inclination and even cracking，which has become one of the most noteworthy problems in the shield construction process.At present，relevant studies have shown that the main modes of building damage caused by shield tunnel construction include uneven settlement damage，damage caused by ground curvature，damage caused by inclination，and damage caused by horizontal deformation.Usually，the compression and negative curvature of the surface appear at the same time，and the tensile and positive curvature of the surface occur at the same time.In practice，the deformation of buildings is often a combination of various forms.Among them，the curvature，inclination and horizontal deformation caused by differential settlement are more obvious to the damage of buildings，which will appear distortion，subsidence，cracking and even collapse.In the process of shield tunneling，the influence of shield construction disturbance on buildings is restricted by many factors，including engineering geological conditions，the form of building foundation，the position relationship and relative distance between buildings and tunnels.In particular，when the large-diameter slurry shield is constructed in the urban river-crossing tunnel，the river bank of the city is often the central area，which is an important barrier for urban flood control and is related to the safety of people's lives and property.Therefore，it is essential to ensure the stability of the embankment during the shield construction.

Embankment has potential safety hazards such as ground subsidence，cracks，collapse and seepage in flood season.Its harm mainly has two aspects：one is that the looseness of embankment soil reduces the safety of seepage resistance，especially the voids and voids above the shield will form seepage channels，which are prone to piping.Second，the physical and mechanical indexes of soil are reduced due to soil loosening，which affects the anti-sliding stability of embankment slope and further affects the embankment safety.Therefore，the use of corresponding embankment reinforcement means is the key to safe construction.

4）High Water Pressure Permeable Composite Strata Crossing the River

In recent years，the common characteristics of large-scale cross-sea cross-river tunnel projects at home and abroad are large diameter，which often passes through coarse sand，gravel，sandy silt and other granular strata rich in groundwater.In the harsh working conditions，especially in a large number of water-bearing gravel layer，no cohesion，very unstable soil layer and shallow overburden tunnel engineering，as well as large diameter shield and ground deformation requirements are particularly high area construction，most of the slurry shield construction.Slurry shield is a kind of sealed mechanical shield.In the construction process，the slurry needs to meet the following three requirements：①reliable mud film is formed on the excavation surface to stabilize the excavation surface.②Slag soil was successfully transported to avoid sediment.③Slurry separation was completed by screening equipment.The stability of excavation face is maintained by the following three factors：①Mud pressure balances soil pressure and water pressure.②The impervious mud film is formed on the excavation surface，so that the mud pressure can effectively play a role.③The slurry penetrates into a certain stratum from the excavation surface，and the stratum on the excavation surface increases the cohesion.With the continuous excavation of the excavation surface，it is required that the mud pressure always acts on the excavation surface in the form of a surface.In this process，the formation of impervious cement film on the excavation surface is the core factor.The stratum in the middle section of the river crossing by slurry shield is generally complex.Therefore，according to different geological conditions，the corresponding construction technology is used to ensure the safe and efficient tunneling of shield.

In low permeability clay and silty clay strata，the formation pore is small，and the mud is easy to form mud film on the excavation surface.However，in the high permeability formation such as fine sand and gravel sand，mud easily penetrates the formation and directly seeps out，and cannot form mud film on the excavation surface，which will lead to a large number of mud filtration，mud pressure is difficult to maintain，thus inducing excavation surface instability and other accidents，the consequences will be very serious.At the same time，large size particles in the formation are unfavorable for carrying slag，but the separation of slurry is relatively easy.Therefore，how to form a good mud film on the surface of high permeability stratum is an urgent problem to be solved in the cross-river tunnel project.

5）Long distance tunneling and tool replacement

In most shield projects with complex geological conditions，tool wear occurs in different degrees.For example，when shield tunneling in sand formation，because of the characteristics of small cohesion，large internal friction angle，poor flow plasticity，large permeability coefficient，high quartz content，easy to appear cutter head，cutting tool，screw conveyor wear，cutter head torque and thrust increase，abnormal excavation parameters，difficult to unearth and difficult to control land subsidence and a series of risk problems，such as serious safety accidents will be caused by a series of risk problems，such as small cohesion，large internal friction angle，poor flow and plasticity，large permeability coefficient，high quartz content，wear，torque and thrust of cutter head，abnormal excavation parameters，difficult to unearth and difficult to control land subsidence，and so on.In recent years，more and more attention has been paid to the problems caused by shield tunneling in sand strata，especially in long distance excavation.

The normal wear of the tool is the result of the normal use of the tool.The wear is mainly due to the contact and sliding friction between the blade surface of the tool and the rock and soil body，which makes the blade surface of the tool lose continuously.The main phenomenon is the uniform wear of the alloy block.However，when the formation quartz content is high，the wear coefficient of the cutter will greatly increase，and the single cutter heading mileage and service life will be greatly reduced.In addition，in the composite strata of upper soft and lower hard，upper soil and lower rock，due to the unevenness of the section，the abnormal wear of the cutter such as tooth collapse，eccentric wear and tool fracture seriously affects the development of shield technology.On the other hand，due to the technical personnel is not clear about the cutting mechanism of the cutter and the matching law with the stratum，there is a serious situation of abnormal wear of the cutter due to improper configuration of the cutter，such as the hob configured in Guangfo subway cutting soft soil layer and the occurrence of eccentric wear accidents.The improper selection of cutting tools is due to the section shape of the cutting tool is not suitable for cutting strata，such as Nanjing Wei Qi Road Yangtze River Tunnel crossing sandy cobble strata，due to the sharp blade of the scraper，resulting in a large number of fracture of the alloy block of the cutting tool.

With the continuous application and development of shield technology in China，the river-crossing tunnel has the characteristics of large diameter，long distance，high water pressure and soft and hard excavation surface.Long-distance tunneling in upper soft and lower hard strata puts forward higher requirements for the life of the cutter.It is inevitable to repair or replace the cutter in the tunneling process.However，due to the complexity and variability of the geological conditions and the limitations of the surrounding conditions，it is often difficult to implement atmospheric cabin entry by reinforcing the soil in front of the excavation.With the characteristics of no need for stratum reinforcement，strong geological adaptability，low requirements for the surrounding environment and small influence，the technology of pressurized cabin entry solves the problem of tool change under harsh conditions.The so-called pressure into the cabin refers to the pressure of compressed gas to replace the mud pressure or soil pressure in the pressure cabin.According to the principle of compressed air construction，the excavation face is supported by air pressure，and the operator enters the pressure cabin under a certain air pressure environment for operation.Although there are many successful cases of large-diameter slurry shield construction in China，the research on the cutter replacement technology with pressure in ultra-high water pressure（above 0.68 MPa）and strong permeable strata is still in its infancy.There are still many difficulties in the operation of pressurized cabin，such as the stability control of excavation face under air pressure，personnel safety and welding repair operation.

6）Crossing the channel in the middle of the river

The slurry in shield construction is not only responsible for maintaining the stability of excavation face，but also responsible for carrying slag.The selection and control of slurry parameters are of great significance to the normal and safe construction of slurry shield.Especially in the construction of the punching section，due to the shallow overburden thickness，the slurry parameters must be strictly controlled to maintain the stability of the excavation face.When the shield punching section passes through the silty sand layer，due to the single particle size and poor gradation of the silty sand layer，the slightly disturbed stratum is easy to be liquefied.It is necessary to form a dense mud film on the excavation surface to maintain the stability of the excavation surface and ensure the safety of the excavation surface.However，the particle size of this kind of fine sand stratum is small，and the mud is not easy to penetrate into the formation to form a permeable zone，which can only form mud skin in the surface siltation.Since the mud is prone to damage under disturbance，in order to ensure the stability of excavation face in such strata，it is necessary to ensure the formation of dense mud with high strength and strong anti-disturbance ability on the surface of excavation face.