4.4.2 Embankment Reinforcement Technology
There are potential safety hazards such as land subsidence，cracks，collapse and water seepage in flood season in the embankment.The hazards are mainly in two aspects.First，the loose soil in the embankment reduces the safety of seepage resistance.In particular，the gaps and cavities 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.
1）Foundation reinforcement
In the process of shield tunneling，the foundation of flood dike is reinforced according to the monitoring situation.The commonly used foundation reinforcement methods include stirring method and grouting reinforcement method.The stirring method uses special mixing machinery to mix the foundation soil and cement slurry in situ into cement soil.After 2-3 months of curing，the strength of cement soil can generally reach 0.5-1 MPa.The foundation strength after cement-soil reinforcement will be greatly improved，which is an effective foundation reinforcement method.The reinforcement technology of this method is mature and reliable without earthwork，but a certain amount of cement is needed.The grouting reinforcement method uses special machinery to force the cement slurry into the parts that need reinforcement at a certain pressure.The cement and the reinforcement parts jointly form a composite foundation，which greatly improves the foundation strength.The advantages and disadvantages of this method are the same as stirring method.
The Nanjing Yangtze River Tunnel was drilled and reinforced with sag grouting in the construction of the flood control embankment on the north bank of the Yangtze River at RK3+73.The scheme is to pre-bury the PVC grouting pipe at the angle of the water surface of the levee within 25 m around the shield axis.The grouting pipe is at an angle of 30°to the vertical plane，and is 3 m away from the top of the tunnel with a spacing of 1 m.If the displacement of the levee is found to be abrupt or the crack width is increased，the construction must be stopped.At the same time，the foundation is reinforced with spiral bored piles during construction，the pile diameter is 40-60 cm，and C30 concrete is used，which is arranged in plum blossom shape.It can be designed and constructed according to the specifications，as shown in Fig.4.27.C30 concrete should be used No.425 slag cement or ordinary cement，plus medium or coarse sand（with less than 5％mud），mixed stones（pebble or gravel，large diameter is 5-32 mm，with less than 2％mud）.
Fig.4.27 Schematic diagram of spiral pouring pile
The construction steps are as follows.
（1）The drill is in place：The drill must remain stable when in place.In order to accurately control the depth of drilling，a control scale should be made on the rack or on the tube for observation and recording during construction.
（2）Drilling：straightening the frame bout，on the good pile bit，start the machine drilling，unearthing，to reach the control depth after stopping drilling，drilling.
（3）Check hole quality，including drilling depth measurement and aperture control.The depth of hole and the thickness of virtual soil were measured by sounding rope（hammer）or portable lamp.The thickness of virtual soil equals the difference of drilling depth.In general，the thickness of virtual soil should not exceed 10 cm，so as to prevent the borehole diameter expansion caused by drill pipe sloshing，resulting in disturbance soil attached to the hole wall and increased fallback soil at the bottom of the hole.
（4）Pore bottom soil cleaning.After drilling to the predetermined depth，the soil must be cleared at the bottom of the hole，and then stop rotation.The drill pipe shall not be twisted.The soil scattered on the ground during drilling must be removed at any time.
（5）Move the rig to the next bit.After the hole inspection，the construction record of the pile hole should be filled.Then cover the hole cover and prevent driving or walking on the cover.Finally，remove the rig to the next level.
2）Dike crack reinforcement
In the study of embankment reinforcement，the reinforcement technology of weak parts such as cracks is described in detail，and the commonly used methods are excavation backfill and filling grouting.These methods are relatively mature in technology，and have been applied in many dam flood control and emergency projects at home and abroad.The effect is good，and the crack treatment is relatively mature in technology.
（1）Excavation backfill.Through analysis and research，it is concluded that the surface and embankment cracks caused by shield tunneling are developed from the surface to the bottom，and the development depth is not large.This development characteristic of cracks provides conditions for the excavation and backfill of cracks.In engineering，the large-width cracks can be treated by excavation backfill.The implementation method is as follows：a trench is excavated downward along the crack，and the depth is 0.3-0.5 m below the crack.In order to facilitate the combination of backfill soil and embankment soil，the slope of groove slope should meet the relevant requirements，and the shape of groove slope can be made into steps.When backfill is carried out，the filler with good compactness and close combination with the original embankment is used，and the water content of backfill soil meets the design requirements.Stratification backfill ramming，backfill height should be slightly higher than the crest elevation，prevent rainwater infiltration，affect backfill effect.
（2）The cross wall is separated.The cross-wall cut-off method is suitable for the treatment of cracks crossing the embankment.The treatment method is to excavate trenches perpendicular to the cracks along the direction of the embankment in the center of the embankment.The excavation requirements of the trenches are the same as those before.The layered backfill compaction is carried out with soil materials with good water insulation performance and close combination with the original embankment.It should be noted that if the crack has communicated with the river water，it may lead to a large number of river water escaping along the groove during excavation.The closure should be made in the embankment first，and the filling should be completed.After the embankment is stable，the embankment is allowed to contact with the river water.
（3）Grouting treatment.Grouting technology is a method to use pressure to inject grout that can be consolidated into the embankment through drilling，filling weak surfaces such as cracks and pores of the embankment，and realizing the treatment of embankment cracks.In the treatment of embankment crack grouting，the commonly used grouting methods include curtain grouting，high pressure jet grouting，cone penetration grouting，etc.The cracks on the embankment are generally not deep into the foundation，and the water retaining height is not high.Clay slurry can be used for filling and filling，and cone penetration grouting is generally used.The cone penetration grouting belongs to the filling grouting，which is a method of injecting the slurry into the hidden danger area by the weight of the slurry to block the hidden danger.The grouting range of the affected embankment extends 50 m outward.The arrangement of grouting holes，row spacing，hole spacing，grouting depth，grouting materials and so on need to be determined according to the damage to the embankment caused by shield tunneling.
As a sharp weapon for urban rapid transportation across rivers，super-large diameter slurry shield will inevitably cross the river embankment.Whether it can smoothly cross the embankment not only affects the construction progress，but also relates to the life safety of the people.Through the successful experience of Nanjing Yangtze River Tunnel Project，it is proved that through detailed construction investigation，reasonable tunneling parameters（slurry pressure，slurry index，grouting pressure，grouting amount，mortar index，etc.），fine operation management（shield tail protection，segment assembly，etc.）and complete emergency measures，the ultralarge diameter slurry shield can completely complete the crossing of Yangtze River embankment or similar embankment without damage，which provides a reference for other large diameter shield underwater crossing embankment construction.