3.2.1　Main Technical Measures of Shallow Overburde...

3.2.1　Main Technical Measures of Shallow Overburden Construction

1）Construction of shallow overlying soil with shield in arrival section

In the initiation and arrival areas of shield，due to shallow overburden and low vertical pressure，it is easy to“float upward”when shield is advancing，and mud is easy to leap out of the ground to“slurry”and destroy the balance of slurry，which makes it more difficult to control the cut water pressure.In addition，it is easy to cause the collapse of the top soil，so it is difficult to control the settlement of the ground surface.

It is difficult to control the shield axis and land subsidence in the shallow overburden section.It is necessary to take effective soil reinforcement measures and optimize the construction parameters to establish the mud water balance of the shield machine quickly and reliably in order to avoid the problem of arriving in the construction because of the initiation.The end is strengthened by high pressure rotary jet grouting pile，and the end layer is strengthened by freezing method.After the reinforcement construction of the end layer，the reinforcement area is coring vertically and several horizontal holes are arranged evenly at the entrance of the tunnel to detect the reinforcement effect.If there is any problem，continue to freeze until the freezing requirement is met to ensure the safety of shield.

2）Construction of shallow overlying soil shield across the bottom of the river

For the construction area where the overlying soil in the river is less than 1 times the diameter of the shield，it is easy to break through the overburden，resulting in mud leakage and even river water invading the inner part of the shield.Under this kind of working condition，the following four aspects are mainly adopted to ensure the smooth progress of the construction.

（1）Measures to prevent slurry leakage（leakage prevention）at the tail of the shield.

①Improve the quality of synchronous grouting.Before each ring is pushed forward，the grouting slurry must be sampled and tested to strictly control its initial setting time and slurry thickness.in the grouting process，the grouting pressure should be reasonably mastered so that the grouting quantity and grouting speed can match with the shield tunneling speed.

②Keep the incision water pressure stable.Keep the incision water pressure stable and make it slightly lower than the grouting pressure，beware that a large number of slurry forward channeling affects the grouting effect.

③Water-stop sponge is pasted outside the segment.In order to prevent the leakage of the tail of the shield，the water-stopping sponge is glued to the outer arc surface of the segment，so that the tail of the shield can be blocked by water absorption and expansion when the slurry is leaking.

④Uniform enough pressure injection of shield tail oil.The oil injection in the tail of shield was combined with automatic and manual mode，and the amount of grease in the tail of shield was increased appropriately，which was initially determined as 120 kg per ring.When the partial leakage of shield tail was carried out，manual mode was used to replenish the oil in the leakage part in time.

⑤To assemble segments in the center to ensure uniform clearance at the end of the shield.The segment is centered to prevent the clearance between shield and segment from being too large and too small，resulting in uneven clearance of shield tail and reducing the sealing effect of shield tail，and even damaging the shield tail brush.The difference between shield tail clearance and shield tail clearance is controlled within 20 mm during construction.

（2）Prevent grouting（anti-spillage）measures at river bottom.

①Strictly control the fluctuation range of notch water pressure，which is controlled at-0.2 to+0.2 kg/cm2.

②Strictly control the amount of soil produced，in principle，according to the theoretical unearthed amount unearthed，can properly under-excavate，keep the soil dense，so as not to infiltrate into the soil and enter the shield.

③Strictly control synchronous grouting pressure and install safety valve in grouting pipeline to avoid the overlying soil due to high grouting pressure.

④If mechanical failure or other causes cause shield to stop pushing，measures should be taken to prevent shield movement.

When pulping occurs at the bottom of the river，the main countermeasures taken are as follows：

When it is found that the grouting at the bottom of the river can be pushed forward without reducing the water pressure of the excavation surface，the propulsion speed can be properly accelerated，the assembly efficiency can be improved，and the shield machine can pass through the grouting area as soon as possible.

When the slurry is serious and cannot be pushed forward，the water pressure of the excavation surface is reduced to（soil pressure+water pressure）balance；increase the density and viscosity of slurry；in order to make the shield machine push forward，check the amount of sand drilled and confirm if there is any over-digging；after a certain distance of excavation，sufficient post-wall grouting is carried out；return the water pressure of the excavation surface to the normal state and carry on the normal excavation.

When it is found that the river water flows into the tunnel from the tail of the shield，the current situation should be analyzed first，and the strength should be organized to deal with the emergency.In the process of emergency，polyurethane is injected into the leakage part accordingly.At the same time，drainage work shall be arranged to ensure the smooth discharge of the river water entering the shield.

（3）Prevent the settlement of soil layer at the bottom of the river（prevent collapse）.

①Set the cut water pressure according to the design value，and make corresponding adjustment according to the change of the water level of the Yangtze River at the propulsion time，strictly control the fluctuation value of the mud water pressure，so as to prevent the cut water pressure from supporting the soil on the excavated surface and cause the soil mass collapse；Prevent the high water pressure of the cut，and cause the soil mass collapse due to excessive disturbance to the soil mass.

②Strengthen the observation of soil settlement at the bottom of the river.To strengthen the settlement observation of soil at the bottom of the river，when it is found that the settlement of the bottom of the river is greater than 5 cm，the amount of synchronous grouting should be properly increased，and the grouting should be replenished if necessary.

③Strengthen mud management to prevent over-digging.Strengthen mud quality control，mud delivery and discharge monitoring，properly increase mud specific weight and control its viscosity，ensure mud film quality，strengthen mud delivery and discharge monitoring，calculate the relationship between excavation speed and mud discharge in advance，find abnormal mud discharge，adjust parameters in time to prevent collapse caused by over-excavation.

④The stability of the front soil is checked by using the soil detection device.During the advancing process，when the amount of dry sand is too large，the frequency of the formation detection device shall be increased，so as to master the collapse of the front soil mass of the cut in time，and the parameters shall be adjusted in time according to the specific construction conditions to make the data of dry sand close to the theoretical value and reduce the possibility of front soil collapse.

（4）Measures to prevent tunnel floating（anti-floating）.

①During the construction period，the tunnel axis is strictly controlled，so that the shield can push along the design axis as far as possible，and each ring can correct the deviation evenly and reduce the disturbance to the soil.

②To improve the quality of synchronous grouting，it is required that the slurry has a short initial setting time，so that it does not crack after encountering mud water，and the slurry is required to have certain fluidity，can evenly fill the tunnel for one week and fill the building voids in time.

③When it is found that the floating capacity of the tunnel is large and the spread range is long，it is necessary to immediately apply the grouting measures to the built tunnel in order to cut off the path of slurry loss.

④On the basis of synchronous grouting，combined with polyurethane grouting，a hoop is formed around the tunnel，and a hoop is played every 10 m，so that the tunnel longitudinal water stop isolation zone is formed，so as to slow down and restrict the tunnel floatation，so as to control the deformation of the tunnel.

⑤Strengthen the monitoring of longitudinal deformation of tunnel，and correct the grouting according to the monitoring results，such as adjusting the grouting position and grouting quantity，preparing fast setting and early strength slurry.

⑥Strengthen the monitoring of topographic settlement at the bottom of the river.