Soft soil tunnel construction of Nanjing Metro 1 Line?

The north-south line of Nanjing Metro is the 1 line in the network planning, with a north-south trend. The first phase of the project starts from Maigao Bridge, passing through the bustling business district and urban transportation hubs such as Zhonghua Gate, Sanshan Street, Xinjiekou, Drum Tower and Nanjing Railway Station, forming a rapid rail transit corridor running through the central axis of Nanjing. The total length of the line is 16.92km, of which the ground line is 6. 1 1km, the underground line is 10.8 1km, and the ground line accounts for 36% of the total length of the line. There are altogether 3 stations/kloc-0, including 8 underground stations, and the control center is located in the northeast of Zhujiang Road Station in the city center. See figure 1 for the overall distribution of lines and site settings.

2 General situation of engineering geology and hydrogeology

Nanjing is located in the lower reaches of the Yangtze River, surrounded by mountains on three sides and wading on one side, with undulating terrain. The hills and plains in the city crisscross, and the modern water system (mainly the inner Qinhuai River system and Jinchuan River system) flows through, and an ancient river running through the north and south is buried underground, forming a more complicated landform. Some denuded residual hills in urban and suburban areas are distributed in the northeast, forming three bedrock uplifts, which divide Nanjing into two small basins, north and south, and the two basins are connected into a whole by ancient rivers.

The three sections of bedrock uplift constitute the landform of low mountains and hills, which is mainly composed of denuded residual mountains and eroded accumulation terraces, with depressions or valleys among the mountains, and the terrain fluctuates greatly. Generally, the thickness of overburden in hilly areas is less than 20m, and bedrock in some areas is directly exposed to the surface. The alluvial plain of ancient river channel is mainly composed of floodplain and ancient river bed, with flat terrain and low terrain, and the bedrock is buried deeply, generally 35 ~ 40m. Four buried terraces are generally developed on the alluvial plain of ancient rivers, and the soil layer is mainly silty clay in plastic state, and some clay and silt in soft plastic and flowing plastic state. For different sections of Nanjing subway, as shown in figure 1, Xiaoxing-Zhonghua Gate, Zhujiang Road-Xuanwu Gate, Nanjing Station-Maigao Bridge belong to low mountains and hills, while Zhonghua Gate-Zhujiang Road and Xuanwu Gate-Nanjing Station belong to floodplain sections.

Hydrogeological conditions along the subway, like engineering geological conditions, are controlled by geological landforms. Its groundwater is mainly pore phreatic water or weak confined water, which is shallow and generally below the ground1.0 ~ 2.0m.. Because of the different soil properties of the strata that constitute the aquifer, the permeability of each soil layer is also very different. The deep channel of the ancient river has thick water-bearing sand layer, good water permeability and strong water-abundance, and the maximum permeability coefficient can reach 5× 10-3cm/s(4.32m/d).

3 shallow stratum tunnel construction technology

In view of the complicated and changeable stratum conditions such as ancient riverbed, flood plain and low hills in Nanjing, and considering the surrounding environmental characteristics and economic factors, various tunnel construction methods such as overhead, open excavation, underground excavation and shield are selected for 1 line, as shown in table 1. During the construction of Metro 1 Line, there are two soft soil sections, which are difficult to construct. First, the underwater shield tunnel construction under the shallow buried condition of Sanshan Street-Zhonghua Gate section; The second is the pipe shed construction of long-span tunnel under buildings in soft plastic clay and silt layer between Zhujiang Road and Gulou; The second is the shallow overburden blasting construction of Gulou-Xuanwu Gate.

3. 1 Underwater excavation construction technology of shield crossing shallow overburden

3. 1. 1 Characteristics and Difficulties of Covered Underwater Shield Construction

The tunnel between Zhonghua Gate and Sanshanjie on subway 1 line needs to cross the Inner Qinhuai River, with a width of 16.8m, and the thickness of the shallowest overburden at the bottom of the river is only 0.7mm. The topsoil at the bottom of the river bed contains a large amount of gravel, fill and floating mud, and its permeability is extremely irregular, which brings great difficulties and risks to shield driving, mainly in two aspects:

(1) is easy to cause water inrush accidents. Generally speaking, the thickness of the cover soil is required to be 2 ~ 2.5d (d is the tunnel diameter) for shield propulsion, but the cover soil here is extremely thin, so it is easy to cause surface soil cracking when shield propulsion is carried out under such a thin condition. At the same time, it is directly below the river bed water level, and the water supply is sufficient. Once water inrush occurs, the consequences are unimaginable.

(2) It is difficult to control the axis of shallow tunnel. For the stratum covered by shallow soil here, the buoyancy of the tunnel is much greater than the pressure of water and soil on it. Therefore, in the natural state, it will lead to the floating deformation of the tunnel, and effective measures should be taken to control it.

3. 1.2 Anti-floating Control Technology for Shallow Underwater Shield Construction

The floating of shallow shield tunnel will cause passive damage to the soil above the tunnel lining. As shown in Figure 2, assuming that the water depth is H 1 and the overburden thickness at the top of the tunnel is H2, the limit equilibrium conditions of the soil in the passive area are as follows:

The river depth H 1 here is 2.0m What is the internal friction angle? 12.3, cohesion c is 8.9kpa, saturated gravity γ of soil is 17.7kN/m3, segment outer diameter R 1 is 3.2m, inner diameter R2 is 2.75m, and concrete weight γ concrete is 20 KN/m3. According to the calculation, the minimum cover thickness of H2 is 4.306m. Obviously, the cover thickness here is only 0.7m, which is not enough to balance the buoyancy of the tunnel. In the construction, we use anti-floating plates and uplift piles to solve this problem. As shown in Figure 3, an anti-floating plate with a thickness of 700mm is constructed at the bottom of the river bed above the tunnel, and a cast-in-place pile with a diameter of 600mm and a depth of 15m is drilled below the anti-floating plate, and the pile and the plate are anchored together, thus effectively preventing deformation during and after tunnel construction.

3. 1.3 Water outburst prevention control of shield propulsion

For the control of water inrush in the process of shield underwater propulsion, the methods such as controlling excavation, pressing bentonite slurry, timely synchronous grouting and strengthening forecast are mainly adopted to cross the Inner Qinhuai River quickly and evenly.

(1) Control the amount of excavation. If overbreak and overbreak, it will inevitably lead to a large subsidence of the ground, otherwise it will lead to excessive uplift of the stratum. During the construction, it is mainly to adjust the pressure of the earth bin in front of the shield to make it slightly higher than the earth pressure in the stratum, and calculate the rotating speed and excavation amount of the spiral excavator according to the advancing speed of the shield to avoid over-excavation and under-excavation.

(2) Hydraulic jet bentonite slurry. Earth pressure balance shield machine is used in this construction, because the covering soil there is very thin. During the construction, we injected a proper amount of bentonite slurry in front of the working face through the mud adding system of the shield machine, so as to reduce the cutting resistance of the cutter head and the friction resistance between the shield and the surrounding strata, thus reducing the disturbance of the shield construction to the surrounding strata.

(3) Application of synchronous grouting technology. Through the grouting system of the shield, cement slurry is injected in time when the shield moves to fill the gap between the lining and the surrounding strata after the separation of the shield tail and block the hydraulic passage.

(4) Strengthen forecasting. With the help of the shield propulsion simulation system, through the real-time simulation analysis of the traveling parameters, the laws of parameters such as ground deformation and pressure change of the earth bin are sought, and the possible posture changes in the later stage of the shield are predicted. Combined with the experience of artificial intelligence solidified into the system, the construction parameters are adjusted in time.

3.2 Construction technology of pipe shed in soft clay layer under buildings

In soft rock or waterless conditions, the application of pipe-shed support technology is mature, but for soft clay stratum with high water content, the application of pipe-shed support is rare. Subway 1 Line Zhujiang Road-Gulou Tunnel is close to Zhujiang Road Station. The tunnel is located in the silty clay area about 200 meters long, with a thin layer of silt in some areas, and the soil moisture content is 29.7% ~ 3 1%. The tunnel is horseshoe-shaped (Figure 4) with inverted arch, with a clear height of 5.30m and a clear width of 5. 18m, and a six-story residential building is built above it. Tunnel construction adopts the technology of combining long and short pipe sheds.

3.2. 1 Characteristics and difficulties of pipe-shed construction in soft soil stratum

In the complex stratum with high water content soft clay and thin silt layer, it is difficult to drill long pipe shed, form water-stop curtain and tunnel excavation.

(1) Long-distance horizontal drilling is difficult. Influenced by the deflection and rigidity of drill pipe and the heterogeneity of soil layer, pipe-shed drilling in this kind of stratum is easy to cause borehole deflection and collapse, thus affecting the hole-forming quality of terminal pipe-shed.

(2) It is difficult to form an effective waterproof curtain at one time. Because tunnel excavation is mainly carried out in clay layer, the permeability of clay layer is poor, and the grouting effect is difficult to control.

(3) It is easy to cause large deformation in the process of excavation. The tunnel here is deeply buried, and there are overloaded houses on it, so the ground pressure is high. What's more, the soil here is soft and the water content is high, so it is easy to collapse the stratum due to the untimely quality and support of the pipe shed during construction, endangering the houses above.

3.2.2 Pipe shed construction technology in soft clay stratum with high water content

Pipe-shed reinforcement means burying a certain number of steel pipes around the tunnel to be excavated, and grouting the soil around the pipes to form a waterproof curtain with certain strength. There are two mechanisms of action. One is the beam-arch effect. Because the front end of the pipe shed is buried in the surrounding soil, and the exposed end is erected on the tunnel support, a group of longitudinal support beams are formed around the tunnel to bear the pressure of the soil and restrain the excessive deformation of the soil. The second is to strengthen the soil effect, and the grout injected from the flower tube of the pipe shed is squeezed into the gap between soil particles through the hole wall to solidify the soil, thus improving the elastic modulus and strength of the soil around the hole. In order to form an effective pipe-shed structure under such complicated stratum conditions, during the construction process, through optimization

Determination of (1) pipe shed parameters

For the pipe shed shown in Figure 4, the pressure acting on the top is:

Considering that during the construction of the pipe shed, the supports are generally close to each other and can be in close contact with the core material of the pipe shed, assuming that the pipe shed steel pipe is a continuous beam with equal span and the distance between supports is L, the maximum bending distance of the pipe shed steel pipe is Mmax:

Assume that the inner diameter and outer diameter of the steel pipe are R 1 and R2, respectively, and its bending modulus w is:

On this basis, the maximum tensile stress of the pipeline can be obtained: σ max = mmax/w.

It is generally believed that in soft soil stratum, the pressure of stratum is completely borne by steel pipe, and the grouting and fixing of pipe shed only play the role of curtain water stop. Assuming that the effective thickness of curtain and solid is d, the shear strength of curtain is [τ], and the distance between the pipe centers is b, the grouting and solid thickness of pipe shed must meet the following conditions:

Where k is the safety factor, and 1.5 ~ 2.0 is desirable.

Based on this, the main parameters of pipe shed construction can be effectively determined, including pipe core spacing, pipe diameter, curtain thickness, row spacing between supports and so on. Grouting pressure can be further determined according to curtain thickness and stratum conditions. In this structure, what is the pipe used for the long pipe shed? 108, steel pipe wall thickness is 6mm, pipe shed spacing is 250mm, and support spacing in the tunnel is 500mm. At the same time, according to the current horizontal drilling technology, the terminal deviation can be controlled within 0.5 ~ 1.0m when drilling 40m at a time in the soil. Therefore, the length of the main housing is also determined to be 40m. During the construction, an enlarged drilling workshop is set up every 35m, with a length of 6m and an outer diameter exceeding 700mm compared with the tunnel section, so as to facilitate the pipe shed drilling construction of the subsequent tunnel, as shown in Figure 5.

(2) Application of long and short combined pipe shed

Because the top pressure of the pipe shed is the largest, a long pipe shed is arranged in the range of arch 150 to resist the deformation of the tunnel caused by pressure. The tunnel here is arranged in clay, which is a typical water-rich soft plastic stratum with high viscosity, strong plasticity and easy softening when it meets water. Therefore, the permeability of cement slurry is weak, and it is difficult to completely isolate the hydraulic connection with the surrounding strata through a long pipe shed grouting. In order to ensure the formation of an effective water-stop curtain, advance small conduits are drilled in the center of adjacent large pipe sheds, with a steel pipe spacing of 250mm and a length of 2.5m, so as to ensure the lap length1m.. The small conduit is grouted once every 1m, and the short pipe shed is arranged along the whole section of the circle to combine it with the long pipe shed and the entity (Figure 5), and a water-stop curtain is formed after grouting and plugging.

(3) Strictly control the construction quality of pipe shed.

The construction quality of pipe shed directly affects the waterproofing of tunnel and the stability of soil around tunnel. During construction, the hole arrangement, positioning, installation and grouting of pipe shed should be strictly controlled from the drilling position.

1) drilling control. The technical key of pipe shed construction is to install steel pipes in parallel and accurately to produce arch effect. During construction, first use high-strength rails and standard sleepers to lay tracks. After the rig is in place, clamp the rig with a walker to ensure that the rig can only walk according to the design route. When the direction is fixed, attention should be paid to the downward trend of drill pipe in the process of pipe-shed rotary drilling, especially in soft clay construction. Therefore, a certain angle is arranged in the opening direction, which is between 0.8 ~ 1 after testing, and it is often checked with theodolite and level when drilling. When laying holes, in order to reduce the disturbance of drilling to undisturbed soil and affect the accuracy, drilling and pipe laying are carried out by jumping, and the spacing is double hole spacing.

2) Pipe shed installation control. The pipe shed is made of seamless steel pipe, each section is 4.5m long. The roundness, concentricity and thread accuracy of steel pipes should be guaranteed during processing, and each steel pipe should be distributed along the design axis.

3) Grouting control. After the steel pipe is laid, pressure grouting should be carried out in time, and the gaps around and inside the steel pipe should be filled with cement slurry. Single liquid cement slurry is used for pipe shed grouting in this department. Because it is injected into clay, on the one hand, the water-cement ratio of the material is appropriately increased (here, the cement slurry of 0.8 ~ 1 ∶ 1 is selected); On the other hand, increase the grouting pressure (1.5 ~ 2.0 MPa is selected here) to enhance the permeability and grouting effect. Advance small duct grouting, adopt double liquid grouting, the volume ratio of cement slurry to sodium silicate is 1∶0.5, and block the hydraulic channel in time.

(4) Tunnel excavation control

The excavation shall be carried out in two steps, and the upper steps shall be excavated by 0.5m each time, then the grid steel frame shall be erected, and 25cm concrete shall be sprayed for initial support, and the total length of excavation steps shall be controlled at 6 ~ 7m. For the lower steps, after each excavation of 0.5m, the initial support shall be carried out immediately. During excavation, the arch foot of the upper steel frame adopts job-hopping excavation to stabilize the upper steel frame. For the tunnel face, due to its large exposed area, it is necessary to hang the net and spray 10cm thick concrete in time to stabilize the stratum.

3.3 Construction Technology of Rock Tunnel under Shallow Buried Buildings

3.3. 1 Construction characteristics and difficulties

As mentioned above, it is very difficult to dig a rock tunnel in such a shallow overburden because of the large terrain fluctuation, many lithologic changes and many buildings on the ground.

1) The rock stratum is complex and changeable. The line 1 crosses the rock stratum, and there are four characteristic layers: Zhujiang Road-Xuanwu Gate and Nanjing Station-Tokyo Pavilion. From Zhujiang Road to Xuanwu Gate, with Gulou Station as the boundary, the rock mass in the southern section is mainly purplish red conglomerate, gravelly sandstone and fine sandstone with argillaceous or calcium-iron cementation, and the northern section is mainly purplish red andesite and andesite tuff. From Nanjing Station to Tokyo Pavilion, grayish yellow and gray limestone are distributed near Nanjing Station, and grayish white fine sandstone, timely and feldspathic sandstone are distributed in the northern section.

2) Poor lithology. Within the tunnel distribution range of 1 line, joints and cracks in rock stratum are developed, and the hardness of rock is uneven. Strong weathering, weak weathering and slight weathering are all reflected in the tunnel, and the strength grade of surrounding rock is III-V..

3) Buildings and structures on the ground are dense. In the construction of rock tunnel, the tunnel needs to pass through Zhongshan Road, Zhongyang Road and an underground street crossing successively, and mainly through densely populated areas. Houses are mostly buildings with 4 floors or below and the highest 7 floors, and the foundation forms are mostly strip foundations. There are dense pipelines under the traffic pavement, so the ground will not be greatly deformed during construction.

4) Buried depth of tunnel. Generally, the buried depth is 8~ 18m, and some areas such as Hongshan Park are almost exposed to the ground.

3.3.2 Construction Technology of Shallow Rock Tunnel

In order to minimize the impact of rock tunnel construction on the surrounding environment, in the actual tunnel construction, starting from the total amount control, multi-stage high-precision detonators are used to reduce vibration and control blasting, and the strata with special cracks and low rock strength are pre-reinforced, which has achieved good results.

(1) charging control

Because the 1 line is distributed along the main traffic trunk lines and densely populated areas, and it is very shallow from the surface, if conventional blasting is used, the amplitude and vibration speed are too large, and the ground deformation is large, resulting in the destruction of houses. Generally, the relationship among vibration speed, charge and blasting distance is as follows:

V=K(Q 1/3/R)a( 10)

Where v is the particle vibration velocity (mm/s);

Q—— unit charge or single hole charge (kg);

R—— the distance from the blasting hole to the building (m);

K, a—— the coefficient and attenuation coefficient related to the topography, geology and other conditions of the blasting point;

The value of k is generally 50 ~ 350, and the value of a is generally 1.3~2.0.

Most of the houses here are ordinary brick houses or non-seismic block buildings, and the vibration speed is required to be no more than 2 ~ 3 cm/s. According to the formula (10), the buried depth of the tunnel directly affects the charge of single-stage blasting. According to the formula (10), combined with the buried depth, geology, topography and other conditions of 1 line tunnel, Table 2 shows the charging parameters of typical shallow strata, which are adjusted according to the blasting vibration during construction.

(2) Shock absorption controlled blasting

In order to reduce the blasting vibration speed and avoid resonance caused by simultaneous initiation of multiple blastholes, the vibration waves after explosion of each blasthole should be interfered and cancelled. Generally, the vibration duration caused by single-hole blasting is short. In most cases, the amplitude of only three complete vibration periods (3T) is greater than A/2, and the subsequent vibration attenuation can be ignored. Therefore, when the delay difference of detonator is greater than 3T, resonance will not occur, but the vibration waves of porous blasting cancel each other out. Theoretically, it can be realized by changing the initiation time interval and adjusting the phase difference of the waveform. But in fact, the vibration frequency f of each blast hole is uncertain, so it is impossible to reduce the vibration wave of each blast hole. In actual blasting, in order to generate random interference waves, multi-stage high-precision series detonators are often used. The deviation value of mine pipeline at the same stage is greater than 100ms, and the interval between detonators at different stages is longer. What is the choice of the center hole of this shallow overburden cut? 25mm cartridge, divided into 8 sections, single hole and single section, detonator delay difference 100ms, cutting arrangement adopts cylinder-cone mixed cutting method; The tunneling hole, inner hole and peripheral hole are detonated by non-electric millisecond detonator in 25 stages. See Table 2 and Figure 6 for the initiation sequence.

The excavation method is half-section positive step method. The height of the upper half is 3.3m, the bottom width is 5.98m, and the step length is controlled at about 3m. Using the construction method of breaking the whole into parts, the once exposed area of surrounding rock is small, the time is short, and the amount of explosives is also small.

(4) Smooth explosion suppression control technology.

In order to form a smooth contour surface, the distance between smooth blasting holes is small. Considering that the surrounding rock is generally Ⅲ ~ Ⅳ, the minimum resistance line distance of smooth blasting is W light = 1.2~ 1.5a light, W light = 0.4m The distance between two adjacent smooth blasting holes is 0.2m

(5) using small circulation footage

If the footage is small, the amount of cyclic blasting is small, and the amount of one blasting is small, it is easy to design the blasting network.

(6) Pre-reinforcement in advance

For the stratum with many cracks and low rock strength, this time, the advance small pipe pre-grouting method is adopted to reinforce the rock mass around the tunnel first, so as to improve the elastic modulus and strength of the rock mass and facilitate the stability of the rock mass and tunnel excavation.

Comparative analysis of shield method and pipe shed method

For the application of these two construction technologies, from the construction practice of Nanjing Metro 1 # line, there are certain differences in safety and economy:

safe

From the perspective of construction safety, the construction safety of shield tunnel is far greater than that of pipe shed tunnel, because of its thick shell, good sealing performance and fast and stable support system.

economy

Economically speaking, the shorter the tunneling distance, the more economical it is to adopt the pipe shed method. Generally speaking, for large-diameter tunnels, the length is within 150m. If the formation conditions permit, it is more economical to adopt the pipe shed method. If it is longer than this length, shield tunnel construction technology should be adopted.

Adaptability to stratum

Compared with the pipe-shed method, the adaptability of shield tunnel to soft soil stratum is much better than that of the pipe-shed method.

4 conclusion

Due to the stratum conditions of Nanjing Metro 1 line, the complexity of ground buildings (structures) and the particularity of tunnel distribution, various construction technologies such as shield tunneling, pipe shed excavation, drilling and blasting have been applied and succeeded in 1 line, which has accumulated valuable experience for urban tunnel construction in soft soil areas in the future.

In the practice of subway 1 line tunnel, we have the following experiences:

In the underwater construction of (1) shield crossing shallow overburden, by controlling the pressure and excavation volume of excavation bin and injecting appropriate bentonite slurry, the influence of tunnel advancement on the surrounding environment can be reduced more effectively, which is beneficial to the control of tunnel water prevention and control.

(2) If the overburden is shallow and the buoyancy is high, setting anti-floating plates and uplift piles can not only balance the long-term buoyancy of the shield tunnel, but also prevent the tunnel from generating excessive uplift deformation during construction, which is beneficial to the axis control of the shield tunnel;

(3) The practice of pipe-shed construction in soft plastic stratum shows that, for high water-cut clay stratum, in order to succeed in pipe-shed enclosure, reasonable pipe-shed support parameters must be determined first; Secondly, it is very important to control the installation quality and grouting construction quality of steel pipes in the pipe shed, which is the key to the success or failure of the pipe shed. In addition, in the process of excavation, the excavation method should be reasonably selected, and if necessary, short pipe sheds should be added at local leakage places to form long and short pipe sheds to reduce the impact of excavation on the surrounding environment.

(4) The key of shallow rock tunnel construction technology lies in charge control and reasonable blasting mode. Engineering practice shows that multi-stage high-precision detonator random interference shock absorption blasting can effectively control the ground deformation and reduce the impact of blasting construction on existing buildings and structures.

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