4.3.2　Post-wall Pulp Management

4.3.2　Post-wall Pulp Management

With the progress of shield construction，stratum deformation is closely related to many factors such as stratum soil conditions，groundwater conditions，tunnel section，tunnel setting depth and construction technical parameters.When the assembled segment lining is removed from the shield tail，due to the original space occupied by the shield steel shell，the gap left for the assembly operation of the lining and the adhesion of some soil to the shield shell during the shield tunneling，there is a ring gap between the back of the lining ring and the actual excavation of the tunnel wall，so that the soil is in a state of no support，and this gap is the gap at the shield tail.

The size of the shield tail gap is determined by the thickness of the shield steel shell and the shield tail operation space，generally in 10-20 cm.Because the segment and surrounding rock soil are temporarily suspended without support when the segment is out of the shield tail，the soil moves to the gap of the shield tail to produce deformation or local collapse，and the loose range of the surrounding rock is expanded，which is an important reason for the deformation and settlement of the stratum caused by shield construction.

Shield tunnel is a kind of stable structure which is composed of segment lining and surrounding rock.Filling the gap of shield tail uniformly and compactly is the precondition to ensure the uniform action of earth pressure on segment lining.Although the gap can be controlled by post-wall grouting，the grouting material，grouting time，pressure，grouting amount and grouting position of post-wall grouting will affect the deformation of the stratum.

The purpose and function of post-wall grouting in shield construction mainly include preventing stratum deformation，improving tunnel impermeability and ensuring the early and late stability of segment lining.If the shield tail gap is not filled in time，it is bound to cause stratum loss，cause stratum deformation，and make adjacent surface buildings settlement or tunnel itself offset.Therefore，the most important purpose of post-wall grouting is to fill the gap of shield tail in time to reduce the stress release and formation deformation.After consolidation and hardening of grouting slurry behind the wall，it generally has certain impermeability，which can be used as the first waterproof line of the tunnel to improve the overall impermeability of the tunnel.The slurry with certain early strength is used to fill the gap of shield tail in time to ensure the early and late stability of segment lining.

4.3.2.1　Slip Casting Methods

1）Synchronous grouting

In the process of shield tunneling，grouting at the same time is the most commonly used grouting method to fill the gap of shield tail in time.Grouting is carried out through the grouting pipe installed at the tail of the shield（generally No.3 shield body），and the built-in methods are generally convex and concave（Fig.4.20c）.What kind of method is from the shield design.The convex type can reduce the outer diameter of the shield to a certain extent，thereby reducing the thickness of the gap at the tail of the shield.However，the shield shell is not round，which is not conducive to the entry and exit of the shield，and is easy to wear when the soil strength is high.Although the concave type increases the outer diameter of the shield and the gap between the shield tail to a certain extent，it is not easy to wear and tear，and has better adaptability to the stratum.

2）Timely grouting

After the shield tunneling rings，when there is a large number of gaps in the shield tail，the gap in the shield tail is grouted through the grouting hole reserved on the segment（Fig.4.20b）.Because this grouting method cannot quickly fill the gap at the shield tail，it increases the disturbance to the soil and is not conducive to the control of land subsidence.Moreover，since the early segment is suspended after it is out of the shield tail，it is unfavorable to the stress of the segment and prone to dislocation.The grouting method is generally used only when the geological conditions are good and the surface settlement requirements are low.

3）Secondary grouting

When the primary grouting effect is not ideal，it is necessary to supplement the early grouting through secondary grouting.This method is generally used in the abnormal settlement of the tunnel，the deviation of the tunnel or in some special sections（near the entrance and exit station of the shield and the contact channel），and can also be used for the reinforcement of the main body crossing buildings and underground pipelines.As shown in Fig.4.20b，timely grouting or secondary grouting can be carried out through the reserved grouting holes on the segment.

In the actual shield construction，the choice of grouting method behind the wall is generally determined by the shield.In some special cases，it is conceivable not to use the grouting pipe provided by the shield for grouting，but to use another purchased grouting device to grout through the reserved grouting holes on the segment.

4.3.2.2　Pulp Engineering Characteristics

In order to achieve the purpose of post-wall grouting and meet the requirements of shield construction，in addition to the grout injection must be quickly and fully filled with shield tail gap，the slurry engineering characteristics of post-wall grouting must meet the following requirements：

（1）Good filling properties and is not easy to flow to areas other than the shield tail gap（not missing into the digging surface and surrounding rock soil）.

（2）The slurry should have good fluidity and be less analytical.

（3）The slurry shall have an appropriate consistency and is not easily diluted by groundwater.

（4）The material is stable and not easy to layer，so that it can be pressed over long distances.

（5）After the wall slurry is filled，the restraint tube should be condensed early，and the early strength is best comparable to the strength of the original soil.

（6）The volume shrinkage and permeability coefficient of the slurry after hardening should be small.

（7）Pollution-free，cheap.Among them，the most important engineering characteristics are filling，fluidity，strength and shrinkage.

The post-wall grouting materials in shield construction can be divided into two types：single slurry and double slurry，and single slurry can also be divided into inert slurry and hard slurry（Fig.4.21）.Inert slurry is slurry which has no cementitious material such as cement in slurry preparation material，long setting time，low early strength and late strength.generally consists of lime，fly ash，sand，bentonite（clay），water，additives，etc.The hard slurry is the slurry mixed with cement and other cementitious materials，generally by cement，fly ash，sand，bentonite（clay），water，additives and so on in the mixer，with a certain early strength and late strength of the slurry.The setting time can be adjusted in a certain range according to the mixing ratio.The double liquid slurry is mixed by liquid A（single liquid slurry）and liquid B（generally water glass）at a certain volume ratio.According to the actual needs，the setting time can be adjusted from several seconds to tens of seconds.The early strength and late strength of this slurry are generally higher than those of single slurry.

The longer the setting time is，the more likely the slurry leaks to the excavation surface and loses in the soil.At the same time，it is also easy to be diluted by groundwater and mud channeling from the excavation surface，which is difficult to restrict the segment and control the formation displacement.However，when the shield tail gap is not fully filled，the setting time is too short to make the slurry lose liquidity，and it is easy to cause pipe blockage.When the construction is not smooth，this situation is more likely to occur，which requires higher construction control management.

Each kind of post-wall grouting has its own advantages and disadvantages due to different slurry types and injection methods，as shown in Table 4.3.The selection of slurry material types and injection methods is closely related to geological conditions，tunneling methods，construction conditions and cost control.Single liquid grouting and timely grouting can be considered in areas with low ground protection requirements，good geological conditions and certain self-stability of soil or rock.The soft strata and ground protection requirements are higher，especially in the areas with important buildings or underground pipelines，double slurry and synchronous grouting should be preferred.If the construction management level is high，single slurry can also be used，but synchronous grouting must be used，and the consistency of the slurry is large，water retention is good，which can restrict the segment and maintain the early stability of the tunnel.In the formation rich in groundwater or caves，double liquid slurry and synchronous grouting method with short setting time are preferred to prevent the dilution or loss of slurry by groundwater.In the construction of slurry pressurized shield，in order to prevent the influence of slurry on slurry or the dilution of slurry after slurry channeling，double slurry and synchronous grouting should be adopted.

From the perspective of cost control and ease of operation，single slurry is superior to double slurry，and inert slurry is superior to hard slurry.From the grouting effect，the double slurry is superior to the single slurry，and the hard slurry is superior to the inert slurry.In the current construction of large section cross-river slurry pressurized shield，inert slurry has also achieved good results.Therefore，the choice of slurry type and injection method should be determined according to the specific situation.

The unreasonable slurry engineering characteristics of post-wall grouting will lead to poor grouting effect，which is mainly manifested in poor slurry fluidity，short setting time，poor physical stability，serious segregation，excessive shrinkage after solidification，and low early strength after solidification.At the same time，it will also cause insufficient filling，unevenness，grouting pipe blockage of the shield tail gap during the implementation of post-wall grouting，as well as water conversion of the slurry at the top of the shield tail gap after the implementation of post-wall grouting，insufficient control of stratum loss，and the failure of segment lining to form a community with surrounding rock soil，which will cause tunnel uplift and axis deviation，thereby causing surface subsidence.

4.3.2.3　Pulp Control

Post-wall grouting is an important measure to prevent stratum settlement.The construction control parameters of post-wall grouting include grouting pressure，grouting amount，grouting location selection and grouting rate，among which grouting pressure and grouting amount are the main control parameters in the implementation of post-wall grouting.During tunneling，it is necessary to not only fully grout，but also control the grouting pressure to avoid the grouting pressure exceeding the high roof to break the overburden.There are two ways in construction：one is to control grouting by grouting pressure，and the other is to determine whether the grouting process ends by preset grouting amount.Since the grouting pressure is affected by many factors，at present，under the premise of ensuring a certain amount of grouting，the upper limit value of grouting pressure is usually set to determine whether the post-wall grouting is completed.

The setting of the grouting pressure is first determined by the engineering characteristics of the slurry.The slurry must overcome the groundwater pressure，earth pressure and pipe friction during the injection process to be pressed into the gap of the shield tail.The greater the grouting pressure，between the lining and the formation denser the void filling，the more stable the surrounding formation.However，the grouting pressure should not be too large，otherwise the splitting grouting will occur，the formation will be bulged，and the shield seal brush will be easily broken or the slurry will be sucked into the pressure chamber，and the force on the lining segments will not be very strong during the construction period.In particular，in the case of a single grouting hole，it is more likely to cause damage to the joint portion or to endanger the water proof performance.Therefore，the value of grouting pressure is quite critical，and the optimum value of grouting pressure should be determined on the basis of geological conditions，sheet strength，equipment performance，slurry characteristics，pressure chamber soil and water pressure.The formation of a uniform slurry layer around the tube sheet is a prerequisite for the final requirement of post-wall grouting.However，injecting excess slurry may cause the soil to cleft and tend to appear in soft clay formations.In general，when the grouting pressure is increased，the amount of injection will increase accordingly.When the thickness of the overburden increases，the grouting pressure will increase accordingly，and there is a certain positive correlation.

In the implementation process of post-wall grouting，only grouting amount is used as the control index，which will cause insufficient grouting amount or uneven filling in special sections（geological mutation，cutter head overexcavation，small curvature construction，etc.）.If the grouting amount is high，it will lead to surface uplift.The grouting pressure setting is too large to cause the slurry to enter the pressure chamber along the periphery of the shield shell，resulting in surface uplift，breakdown of shield tail seal，segment cracking or dislocation.The unreasonable selection of grouting hole position and corresponding grouting pressure results in uneven or insufficient grouting filling，segment dislocation，tunnel floating and axis deviation.Therefore，the grouting pressure and grouting quantity should be strictly controlled when the shield passes through the embankment.During the construction，the construction gap is filled in time by synchronous grouting to reduce the deformation of soil in the construction process and ensure the stability of embankment buildings.In addition，according to the monitoring situation，when the synchronous grouting cannot meet the requirements，the double liquid slurry（cement slurry and sodium silicate）is poured through the secondary grouting hole reserved for the segment，so that the soil is consolidated and stabilized in a relatively short period of time and the embankment is reinforced.

4.3.2.4　Post-pulping Simulation of the End Wall of the Shield

The grouting process behind the shield wall is to use a fluid-like grouting material with a certain fluidity，using a grouting pump to grout.The pressure and grouting amount are pumped into the annular shield tail gap outside the segment，and the filling of the shield tail gap is ensured to be even and full，thereby reducing the stress release and deformation of the formation.After the wall grouting material gradually undergoes consolidation compression and cementation hardening，the grouting pressure gradually dissipates and returns to the state equal to the formation stress.The simulation of grouting process actually includes the simulation of grouting material and the simulation of grouting pressure.This section summarizes the grouting rate and filling rate by summarizing the research work related to the predecessors.The concept of a post-grouting simulation method that is more in line with the actual grouting process.

1）Discussion on the concept of“grouting rate”and“filling rate”

Through the investigation of relevant domestic literature，many scholars have mentioned“grouting rate”and“filling rate”in their studies.However，in the field of engineering，technology and scientific research，these two words are confused and often confused with each other.Fig.4.22 is a schematic diagram of the actual filling effect of the grouting behind the shield wall.Because the outer diameter of the shield cutter head and the shield shell is larger than that of the lining ring，there will be a shield tail gap between the planned（ideal）excavation boundary and the segment ring in the project.The volume Vgap1 of this shield tail gap in the project is known as：

At the time of construction，the actual excavation diameter of the shield is larger than planned due to shield rectification or other construction factors.The（ideal）excavation has a large diameter，so the actually produced shield tail gap volume is larger，and this book is defined as Vgap2.Due to the complexity of the shield construction control，it is difficult to obtain the actual shield gap volume Vgap2 of a certain ring by direct calculation，which is often determined according to the management of the amount of slag combined with the construction experience.Therefore，define the empirical over-extraction coefficient α：

According to the construction situation of the Shield Tunnel of the Wei San Road in Nanjing and the Slender West Lake Tunnel in Yangzhou，the α is 1.1-1.3.Under the condition of good support pressure and attitude control，α takes a small value；the shield attitude is poorly controlled，α takes a larger value.

After the slurry is injected into the shield tail clearance，the diffusion process is very complex due to the influence of slurry properties，groundwater level，construction technology，grouting pressure and formation conditions.A large number of laboratory tests and field measurements show that the slurry is not evenly distributed in the gap，and is not only distributed in the shield tail gap.The diffusion mode of slurry in the pores and strata around the segment will experience the process of filling，infiltration，compaction and splitting，so there will be permeable slurry in the surrounding strata.And in some parts（especially the top of the lining）strata squeeze into the shield tail gap，making this part of the shield tail gap has not been filled with slurry.Therefore，the total grouting amount is the sum of the lost slurry penetrated into the stratum and the slurry filled in the shield tail gap.

Define the slurry loss coefficient δ，so that：

In the formula，δ is related to slurry properties，formation gradation and grouting pressure，and the range is from 0.30 to 0.50.

Grouting rate and gap filling rate：

β1—grouting rate；

Qinject—the total grouting volume of a ring can be controlled and accurately measured during construction；

β2—gap fill rate；

Qfill—The amount of slurry filled in the shield tail gap is not easy to be measured directly.The slurry loss coefficient δ can be obtained indirectly by indoor similar model test or theoretical calculation.

Thus，the concepts of“grouting rate”and“gap filling rate”are clarified，and the relationship between the two is established.

Taking the Wei San Road crossing channel in Nanjing as an example，according to the construction situation，α=1.21 and δ=0.42.The outer diameter of the segment is 14.5 m and the diameter of the excavation is 15 m.Then Vgap1=3.14 x（152-14.52）/4=1.58 m3，when the grouting volume is controlled to 18-22 m3，the grouting rateβ1is 155.4％-190％，and the corresponding gap filling rateβ2is 74.5％-91.1％，which is corresponding to the relevant grouting parameters given by Wu Shiming，Zhu Caihui and Zhu Hehua.

2）“Equivalent generation”optimization

In order to reflect the influence of shield tail clearance，soil squeezing into the shield tail clearance and the distribution of grouting body behind the wall on the settlement and deformation of strata，Zhang Yun et al.put forward the concept of“equivalent layer”：in numerical simulation，the soil，grouting body behind the wall and disturbed surrounding rock naturally squeezed into the shield tail clearance are generalized into an annular homogeneous elastic material layer with equal thickness after the segment is removed from the shield tail，which makes the settlement and deformation of the actual soil layer around the segment and the surface behind the grouting body remain unchanged.After putting forward the method of using“equivalent layer'”instead of wall grouting for numerical simulation，many scholars have carried out a lot of research.However，most of the literature did not consider the filling rate of grouting，and did not unify the grouting in numerical simulation with the grouting amount and filling rate in practical engineering.Moreover，the consolidation-cementation hardening effect of slurry with time and the dissipation of grouting pressure behind the wall were less considered.Therefore，based on the concept of equivalent layer proposed by Zhang Yun，a series of optimizations are carried out to make this abstract“equivalent layer”not only reflect the filling rate，slurry consolidation and cementation and grouting pressure dissipation，but also unify with the grouting rate and grouting effect in practical engineering.

The optimized equivalent layer is shown in Fig.4.23.The grouting body adopts the“three-stage consolidation-cementation hardening”unit，and the three-stage dissipation pressure is applied to simulate the grouting pressure behind the wall.The low modulus elastic element is used to consider the outer circumference of the grouting as an unfilled void（the thickness is determined according to the filling rate）.

3）Grouting unit and hardened slurry unit

When a shield tunneling ring is completed and grouting behind the wall of the ring is completed，the slurry is in a plastic flow state，and the segment lining and surrounding rock soil are mainly subjected to liquid-plastic slurry pressure.As the shield continues to move forward and time goes on，the slurry has the effect of consolidation，drainage and cementation hardening at the same time，and gradually becomes a solid state.A protective layer is formed around the segment，and the surrounding rock stress is acted on the lining.The strength and elastic modulus of grouting materials increase with the consolidation-cementation of grout.In order to simulate the mechanical properties of grouting materials at different stages and improve the efficiency of numerical model calculation，the“three-stage consolidation-cementation equivalent layer”method is adopted.After grouting，the plastic flow-like material composed of slurry and soil is formed outside the segment lining，which is approximately a homogeneous elastic material layer of equal thickness for simplified simulation.Considering the strength and elastic modulus of grouting material increases with time，the elastic modulus of three stages is used to calculate.According to the study of Han Yuewang et al.，according to the actual consolidation-cementation process of grouting material behind the wall，the material is divided into the following three kinds：

（1）The initial slurry body，regarded as a flow-shaped slurry that has just been injected into the tail of the shield，with an elastic modulus of 1.0 MPa.

（2）Short-term hardened slurry，considered to be after 24 h hardening of the slurry，elastic modulus 5.0 MPa.

（3）Long-term hardened slurry，considered to be after 28 days hardening of the slurry，elastic modulus takes 300 MPa.

According to the excavation step material transformation to simulate the solidization and bonding process of the wall after the slurry body：1 step after excavation using the initial slurry simulation injection，1 excavation step after transformation into a short-term hardened slurry body，4 excavation steps after the use of long-term hardening slurry simulation.

4）Grouting pressure simulation

When grouting behind shield tail wall，it is necessary to exert certain pressure on the slurry to distribute the slurry around the segment.If the grouting pressure is too large，it may lead to segment dislocation cracking，bolt shear，tunnel seepage accident.Grouting pressure is too small，it is easy to lead to insufficient grouting or uneven，is not conducive to the full filling of slurry，resulting in large settlement，so the selection of grouting pressure is very important.Usually，the grouting pressure is 0.1-0.2 MPa larger than the stratum resistance around the segment.In this paper，the initial grouting pressure difference of 0.2 MPa is selected according to the hydrogeological conditions and actual construction conditions of Nanjing Wei San Road crossing river channel and Yangzhou Shouxihu tunnel，and the surface force of a certain vertical gradient is acted on the lining segment and the surrounding rock of the tunnel as the grouting pressure behind the wall.When the three-stage grouting pressure is selected，the initial grouting pressure is 0.2 MPa larger than the initial formation stress at the corresponding position，and it decreases by 0.1 MPa after one excavation step，and it decreases to be equal to the initial formation stress at the corresponding position after four excavation steps.

5）Determination of the thickness of the equivalent layer

The joint type（4.1）and the formula（4.2）can be used to derive the actual shield gap thickness：

Then the thickness of the grout after grouting：

The thickness of the void unit is：