Application of super large and deep diameter assembly shafts in soft soil layers

Considering the background of the Shanghai Jing’an Smart Garage Project, first, we introduced the world’s largest vertical shaft boring machine, which was suitable for vertical shaft excavation of diameter 12∼23 m. Then, we expounded the design points and key technologies of super large and deep diameter assembly shafts in soft soil layers. Finally, we verified the applicability of the vertical shaft boring technology in soft soil layers in Shanghai through on-site measurement results. It was observed that this technology had a minimal impact on the surrounding environment, indicating its potential for widespread applications in the coming years.


Introduction
In recent years, China has issued a series of strategic plans for the development and utilization of deep underground space, and Shanghai has been seeking development space from a depth of 40-100 m underground.Effectively and reasonably utilizing limited deep underground spaces, urban corners, and scattered plots in densely populated urban centers and old urban areas was a challenging problem that needed to be solved by the builders in the current bustling urban renewal and construction.
The vertical shaft structure was the channel and throat for the development of deep underground space, which was an important carrier for the development of deep underground space.The vertical shaft structure could be widely used in underground mechanical parking garages, shield tunneling or pipe-jacking working shafts, subway air shafts and escape shafts, super deep-water storage wells, underground logistics, and other projects in central urban areas.Achieving efficient urban development was an inevitable direction for promoting new infrastructure as well as digital and intelligent urban development, and it played a vital role in the development and utilization of underground space.
The limitations of traditional construction technologies, such as the sinking method and cut and cover method by diaphragm wall, were increasingly prominent due to the site limitations, environmental protection, and excavation depth.The vertical shaft boring technology is a more advanced technology in the field of deep excavation today, which possesses characteristics such as high prefabrication ratio, fast construction speed, extensive excavation depth, and less disturbance to the surrounding environment.This concept has become increasingly popular in the development of deep underground spaces.The vertical shaft boring technology was still in the initial exploration stage in China, and there were very few engineering cases for soft soil formations with an outer diameter greater than 15 m and an excavation depth greater than 50 m.The super deep and large diameter assembly type vertical shaft structure faced prominent problems such as structural anti-floating, sensitive environmental impact, small spacing mutual influence, and underwater bottom sealing in soft soil layers.There was a lack of systematic and mature design theory and calculation methods, and there was no professional specification to refer to, which undoubtedly brought massive challenges to designers.Therefore, it was fundamental to study the design theory of super deep and large-diameter assembly shafts in soft soil layers.
The paper referred to past engineering experience and relies on the underground intelligent garage of the vertical shaft boring method in the Jing'an District.At the same time, we discussed the difficulties as well as solutions in the structural design process of the vertical shaft boring method in soft soil layers.Finally, the method was well-validated in current engineering field.

Project overview
The vertical shaft boring method adopted concepts such as undrained excavation, overall suspension sinking of the shaft, mechanical arm combination with milling barrel to cut the formation, and prefabricated assembly of the shaft.Further, it addressed the shortcomings of traditional shaft sinking resistance, uncontrolled posture, and substantial environmental disturbance.At present, there are some implementation cases of the vertical shaft boring method, such as in Nanjing, Shanghai, Guangzhou, and other places in China.However, the outer diameter is approximately 12.8 to 14.0 m.There were fewer engineering cases for assembly shafts with an outer diameter exceeding 15 m.
The Jing'an Smart Garage Project adopted a new type of composite cutter head vertical excavation technology, and it utilized sporadic old and unused land plots in urban areas to explore urban space downwards.Parking lots are added in scattered areas of high-density urban areas to create intelligent, efficient, and intensive parking spaces, alleviate parking difficulties in the central metropolitan area, and provide innovative measures for improving living conditions.
The excavation diameter of the proposed #1 and #2 shafts in this project was 23 m, and the maximum excavation depth was approximately 50.95 m.The main underground body of the project consisted of two vertical circular shafts and a 10-kV substation.The substation was connected to the underground floor of the circular shaft through two connecting channels.There were gas pipelines, rainwater pipelines, street lights, and high-voltage lines distributed in the proposed site.
The inner diameter of the vertical shaft was 21 m, and it composed of top cast-in-place segments, prefabricated segments, and a steel blade foot.The anti-friction mud space is designed between the segment and the soil layer.The designed ground elevation was +3.80 m, and the buried depth of the bottom plate was 43.95 m.
The uplift piles were arranged around the outside of the shaft, and the anti-floating system was composed of the top ring beam and the shaft wall.The thickness of the bottom sealing concrete was equal to or greater than 6.6 m.The overall structural design scheme as shown in Figure 1.

Equipment and construction technology
On November 29th, the 'Dream' of the world's largest diameter shaft jointly boring machine developed by China Railway 15th Bureau Group and China Railway Construction Heavy Industry, was officially launched.The 'Dream' integrated the functions of excavation, slag discharge, support, and guidance, and it possessed three excavation functions-alternating, synchronous, and advanced support.Variable diameter excavation was suitable for vertical shaft excavation under various working conditions such as soft soil, sand layers, rock formations, low water, and rich water geological conditions with a diameter of 12~23 m, the maximum excavation depth was 80 m.
To better control the construction risks of super large and deep-diameter vertical buried shafts in this project, which also adapts to the characteristics of Shanghai's soft soil layer, the undrained sinking process was used.During excavation, the blade feet were extended into the soil, which formed presupport, and the soil was only excavated inside the blade feet without over-breaking.In the presupport mode, the shallow soft plastic to soil layer can smoothly sink by relying on its own weight.During the sinking process, the sinking speed and slope were controlled by hanging steel strands.In deep hard plastic clay and dense silt layers, the friction is reduced, and the pressure is maintained through sidewall mud sleeves.When sinking was difficult, an equipped sinking jack could be used for downward pressure and sinking assistance.During the entire excavation process of the shaft, dynamic correction of the shaft posture was achieved through automated monitoring and real-time feedback.The shaft posture could be adjusted by adjusting the length of the lifting system steel strand, and the inclination of the shaft could reach 0.7 ‰.

Overview of engineering geology.
According to the geological survey report, the soil layers from top to bottom on the site include ① layer of miscellaneous fill, ②1 layer of silty clay, ②3 layer of clayey silt, ④ layer of muddy clay,⑤ 1 layer of silty clay, ⑥ 1 layer of silty clay, ⑥ 2 layer of silty clay mixed with sandy silt, ⑧1 layer of silty clay, and ⑧2-1 layer of gray silty clay and clay inter-bedding.The impurities with larger particle sizes such as broken bricks in the first layer of miscellaneous fill can easily make it difficult or impossible for the vertical shaft to sink, and it may damage the edge of the vertical shaft.The characteristic of ②1 and ②3 was average strength, low permeability, moderate shear strength, and bearing capacity.The characteristics of ④ included high moisture content, high compressibility, low shear strength, and bearing capacity, and it possesses certain rheological and thixotropic properties.It was easy to cause sudden sinking, uncontrolled sinking, and over-sinking during vertical shafts.It was recommended to take effective measures in design and construction to prevent its adverse effects.The characteristic of ⑤ 1 was silty, soft plastic, with high compressibility.The characteristic of ⑥ 1 was hard plastic, high strength, low permeability, and moderate shear strength and bearing capacity.The characteristics of ⑥2 included average strength, low permeability, moderate shear strength and bearing capacity, and locally mixed with sandy silt, which possessed high permeability, and it was prone to phenomena such as piping and sand flow.The characteristic of ⑧ 1 was plastic to soft plastic, with medium to high compressibility, average engineering properties, with moderate shear strength and bearing capacity.
The groundwater types that affected the project shaft were pore phreatic water and confined water in the ⑧ 2-2 layer and ⑨ 1 layer.

Verification calculation of the basal heave stability
Due to the implementation of an undrained sinking technology, the stability of confined water in traditional processes could be effectively avoided.However, due to the difference in water and soil pressure inside and outside the well, the stability of the excavation surface soil could not be easily ensured when it was located in a soft soil layer.At present, the mainstream calculation theory is based on the Prandtl theory of the ultimate bearing capacity of foundations [4] but it does not consider the assumed frictional resistance between 1333 (2024) 012053 IOP Publishing doi:10.1088/1755-1315/1333/1/0120535   H  q   S R q the sliding surface soil, and its application in structures with evident spatial effects such as vertical shafts possessed certain limitations.The application in super large and deep vertical shafts possessed significant limitations.
Therefore, the project was handle as a planar problem, and based on the classical soil pressure theory, the following formula was applied to calculate the basal heave stability: F s  r where, F s was safety factor, q r was weighted average unit weight of soil between AB surface and soil surface (kN/m 3 ),  was the weighted average weight of soil between ground surface and AB surface (kN/m 3 ), H was excavation depth of vertical shaft (m), R was the width of AB plane of sliding soil (m), and S was the total frictional resistance between soil on BN surface (kN).Based on the aforementioned theory, empirical calculations could meet the requirements for the basal heave stability throughout the entire sinking construction process.To ensure safety, the proportion of mud in the shaft should not be less than 1.15 when crossing sandy strata.

Verification calculation of anti-floating stability
The anti-floating stability during the water drainage inside the shaft is after the completion of the bottom sealing.This was important to the successful implementation of the vertical shaft boring technology.Therefore, it was necessary to verify the anti-floating stability during the construction and normal service stages.
In the construction phase, when the completed bottom concrete and the water drainage in the draft occurred, the anti-floating stability of the structure was the most unfavorable.The anti-floating stability of the structure considers the uplift pile, the weight of the bottom sealing concrete, the weight of the segment structure, and the weight of the ring beam.Among these, the uplift pile was the main anti-floating component.Because the surrounding soil of the shaft was disturbed in the sinking stage, a certain reduction coefficient was considered according to the most unfavorable consideration.Then, the anti-floating safety factor should not be less than 1.05.In the normal use stage, the shaft side wall friction resistance and uplift pile were considered to resist floating together.Then, the shaft wall side wall friction resistance was considered within the depth range of the shaft, and the uplift resistance of the lateral uplift pile was considered below the shaft blade foot.At this time, the anti-floating safety factor should not be less than 1.15.

Sinking verification
The load during the suspension of the vertical shaft wall to control sinking was relatively complex, and the relationship between loads during the sinking process changed regularly at the same time , and the vertical load and its changes were the primary factors.Due to the use of undrained sinking technology, the vertical loads on the shaft wall mainly included the self-weight of the shaft structure, the self-weight of the main engine, the frictional resistance of the side wall, end resistance, and side resistance of the blade foot, the buoyancy of the shaft wall, and the vertical suspension force [5] and [6].
Referring to the design and construction specifications for the open caisson, the sinking coefficient of the vertical shaft should meet the requirement that the value is greater than 1.05.When the sinking coefficient was greater than 1.15, the sinking stability coefficient must be checked and calculated accordingly.When the sinking stability check could not be satisfied, the sinking stability should be accurately controlled by the suspension system and the auxiliary sinking system.

Vertical shaft structure design 4.1. Top ring beam design
According to the overall design, the power distribution room should be set up between the two shafts as the connection channel, and some of the top ring beams need to be chiseled out in the later stage.Considering the safety of the structure and the convenience of construction, the top ring beams that needed to be chiseled out were designed as steel-concrete composite beams.The concrete part of the top ring beam adopted C40 concrete, and the steel beam part adopted the Q355B steel.
After the processing of the steel beam in the factory, it is transported to the site and then poured in the concrete beam as a whole to form a composite beam.The steel beam was not filled with concrete.Then, the steel beam could be cut off as a whole after the main structure of the shaft and the power distribution room were completed.

Blade foot and bottom sealing concrete design
The vertical shaft of the project was super wide and deep, and confined water aquifers were distributed below the bottom plate, including ⑧ 2-2 silty sand, ⑨ 1 silty sand, and ⑨ 2 fine sand.The bottom sealing concrete played a crucial role in the drainage stage of the vertical shaft and was a key factor determining the success or failure of the entire project.According to traditional theory, the bottom sealing thickness of the project would also be super deep and thick, and the risk of the bottom sealing construction system was extremely high.
To adapt to the construction requirements of the super large tonnage suspension system, soft soil strata with super deep and large diameter vertical shafts in the project, we improve the stiffness and construction accuracy of the blade foot structure, and avoid the problem of difficulty in meeting the accuracy of pipe segment assembly during on-site pouring construction; It was necessary to adopt a reasonable blade foot construction form to reduce the thickness of the bottom sealing concrete and thus reduce the risk of the bottom sealing concrete construction system.
Then, to achieve the aforementioned objectives, a combined vertical shaft blade foot structure was designed to adapt to the project, which included the first assembly unit and the second pouring unit.The first assembly unit included a steel blade foot and a prefabricated segment connected to it, which spliced to form a pipe joint.A distinctive feature was that it also includes a second pouring unit connected to the first assembly unit as a whole.The second pouring unit includes an integrated blade foot lining, connecting beam, and bottom ring beam.The blade foot lining, connecting beam, and bottom ring beam were all made of concrete pouring.The blade foot lining set on the inner side of the steel blade foot and prefabricated pipe segments, and the bottom ring beam was further inward from the blade foot lining and connected to the blade foot lining through multiple connecting beams.The bottom sealing concrete was connected to the side wall with a 1-m high groove, forming a "bottle stopper" effect through the bottom ring beam.Through these measures, the thickness of the bottom sealing concrete in the middle of the span could be controlled at 8.5 m, and the thickness of the bottom sealing concrete at the connection with the wellbore could be controlled at 6.6 m.

Segment structure design
The segments were designed with staggered assembly.The outer diameter of the lining ring was ∅ 22600 mm, the inner diameter was ∅ 21000 mm, the thickness was 800 mm, and the ring width was 2.0 m.The lining ring is composed of 10 segments.The lining ring was designed as a wedgeless ring with shear pins.The segments were connected with T39 diagonal bolts in the circumferential direction and M48 long bolts in the longitudinal direction.The lining ring was assembled in staggered joints.
In an ideal state, the circular shaft was subjected to uniform soil and water pressure around the shaft, and the axial force.Further, the bending moment was small.Then, considering that the outer diameter of the shaft of this project was 22.6 m and there were unfavorable factors such as the difference in the properties of the surrounding soil, the inclination of the sinking stage and the uneven overload of the ground crane during the construction process led to the uneven circumferential force of the circular shaft structure, and the eccentric load was often used in the design.
According to the domestic "Code for Design of Reinforced Concrete Vertical Shaft Structures in Water Supply and Drainage Engineering" (CECS137:2015) [8], the uneven load caused by uneven soil quality is simulated based on the assumption that the difference in the internal friction angle of the soil at two points of 90 degrees apart was 4-8 degrees.According to the Japanese design and construction guidelines, a comprehensive eccentric load applied to the structure to a comprehensively simulation of the effects of various unfavorable factors mentioned previously.
The standard value of the internal force of the structure under the most unfavorable working conditions (per linear meter) is as follows: the maximum bending moment was about 647.8 kN * m, and the maximum axial force was about 2747.8 kN.

Interface design between substation and the shaft
According to the overall plan, the substation was connected between the two shafts to the main structure of the shaft through a channel.The top plate of the connecting channel and the top ring beam were connected as a whole using a connector.
The connecting channel floor, side wall, and the main structure of the shaft were separated.The 10 预留通道柱 现浇管节 bottom plate utilizes existing uplift piles as an anti-floating measure for the structure.
We installed overhanging brackets in the wellbore of the passage area, and we connected Ushaped deformation joints with the substation structure.We used a buried belt and steel plate water stop as waterproof measures.

Structural waterproofing design
The waterproofing of the vertical shaft structure follows the principle of "prevention first, blockage second, multiple lines of defense, and comprehensive treatment."The waterproof level was Level 2 based on the domestic standards.The strength grade of waterproof concrete for segment structure was C60, and the impermeability grade was P12.The single piece leak detection standard for pipe segments were as follows: under a water pressure of 0.8 MPa, maintain the pressure for ≥ 3 h, and ensure a water seepage thickness ≤ 5 cm.The following measures were taken for waterproofing the joints of prefabricated pipe segments: 1.The lining joint adopted double layers of EPDM elastic rubber sealing pads with the same material and cross-section as waterproofing measures.2. The upstream surface of the external sealing gasket was equipped with a sand strip, which was made of water expanding rubber.Due to the fact that the circumferential handhole of this project is located on the outer curved surface of the pipe segment, the waterproofing at the handhole was relatively weak.If a sealing pad was still installed on the outer side of the joint according to the conventional practice, leakage water may bypass the outer sealing pad and flow into the interior along the handhole.Therefore, an inner sealing pad required to be added.
The durability of elastic sealing pads primarily relies on stress relaxation tests for verification.According to the results of existing engineering stress relaxation tests, the stress relaxation of the sealing pads used in this project mainly occurs in the first eight days, and after 36 days, the stress relaxation basically reaches stability, with a stress relaxation of 25%.According to the requirement of water tightness of 1 MPa, after 25% stress relaxation, the waterproof performance of 0.75 MPa may be achieved, which is still greater than the maximum buried depth water head of this project (0.5 MPa).
The joint between the cast-in-place bottom plate and the pipe segment adopts double layers of water swelling sealing adhesive combined with embedded grouting pipes as waterproofing measures.The upstream surface of the cast-in-place bottom plate adopts a pre-laid waterproof membrane P (nonasphalt) as the waterproof layer, with a thickness of 1.5 mm.The coil material should directly engage with the bottom plate, and it should set a no fine aggregate concrete protective layer.

Key technologies for super deep and large diameter assembled vertical shaft segments
At present, the world's largest shield tunnel segment in Tuen Mun to Chek Lap Kok Link in Hong Kong, which inner and outer diameter of 15.6 m and 17.0 m, respectively.The outer diameter of the vertical shaft segment in the project was 22.6 m, and this facilitated the need to design the world's largest prefabricated segment.
The following measures were taken in the project without setting a secondary lining to ensure the reliability of the connection form of the prefabricated vertical shaft structure under uneven lateral soil pressure: 1.The longitudinal bolts separated from the shear pins, and 40 grade 8.8 M48 straight bolts were used to connect the rings, in addition to 40 shear pins.2. The blocks were connected by three 8.8 grade M39 circumferential diagonal bolts.

Key technologies for bottom sealing of super large and deep assembled vertical shaft
In the design phase, which is intended to reduce the thickness of the back cover and reduce the risk of the entire construction system, various structural forms suitable for the bottom sealing of this project were compared and selected based on the combination with the characteristics of this project.Finally, the bottom ring beam structure was adopted as an impressive innovation.By analyzing the different failure modes of bottom sealing concrete, a set of calculation methods suitable for bottom sealing of super large and deep vertical shafts has been developed.
During the implementation process, it was necessary that the underwater bottom-sealing concrete be continuously poured over the entire bottom area of the vertical shaft.The continuous pouring of such a large volume of underwater concrete has brought great challenges to construction engineers.In practice, a set of construction techniques for sealing the bottom of super large and deep vertical shafts had been formed.

Solution of the basal heave stability
In the design stage, according to the characteristics of the circular shaft, on the basis of the ultimate bearing capacity theory of Prandtl foundation, a theoretical basis for the calculation of the anti-heave stability was established.
In the design phase, the frictional resistance of sliding surfaces was considered to provide a theoretical basis for the calculation of the basal heave stability, which is based on the characteristics of circular vertical shafts and the Prandtl theory of the ultimate bearing capacity of foundations.
The following measures, were taken during the specific implementation process: 1.We strengthen the weak foundation below the ring beam with cement soil mixing piles and strictly control the construction quality of soil reinforcement within the soft soil layer.2. Adopting an undrained excavation method to ensure the water level and specific gravity of the mud, strictly prohibiting over-excavation, and ensuring that the blade foot possesses a certain depth of insertion.During the implementation of the project, under the condition of not affecting the normal operation of the equipment, efforts should be made to under-excavate and ensure that soil was left within a certain range of the cutting edge.3. The lifting system was used to control the sinking speed, the amount of each excavation, and the amount of sinking with the steel strand to prevent sudden sinking.4. The uplift piles were arranged along the top ring beam on the outside, which could play the role of isolation piles in the sinking stage to ensure the stability of the soil on the excavation surface.

Anti-floating system
Anti-floating safety was the key to the success or failure of this process.Compared with traditional processes, the assembled vertical shaft was greatly optimized in terms of structural self-weight due to the constraints of assembly construction technology.Relying solely on self-weight to maintain antifloating stability was not practical, and other anti-floating design and construction measures needed to be considered.The following measures were taken in the project: 1.The uplift pile was arranged around the outside of the shaft, and the top ring beam and the shaft wall were arranged on the top of the pile to form the anti-floating system.2. After the bottom sealing was completed and reached the design strength, the outer side of the well wall was grouted and reinforced.After the cement slurry on the well wall reached the design strength, the pumping operation in the well could be carried out.3. Information-based construction was used in the project, which could monitor the sinking and uplift of the vertical shaft in real-time, and weighting treatment was carried out when abnormal situations were found.

Analysis of on-site monitoring data
The The monitoring results show that when the vertical shaft sunk to the design elevation, before the construction of bottom sealing concrete, the maximum inclination of the surrounding piles was approximately 11.74 mm, the maximum inclination of the soil was 13.24 mm, and the maximum surface settlement was 8.06 mm.The impact on the surrounding environment is controllable, and the degree of impact was less than the deformation design control index of the foundation pit with a firstclass environmental protection level in Shanghai.
The measured data of inclination measurement showed that the maximum deformation position of the pile and soil was not located at the bottom of the well, which is located within a range of 30-40 m below the site level.The reason for the analysis was that there was a deep ⑧1 silty clay layer in the area.During the sinking process, the anti-friction mud on the side wall could not balance the lateral water and soil pressure well, causing the surrounding soil to move towards the gap of the mud sleeve.The surrounding strata moved, thus causing surface subsidence.In the project, due to the reinforcement effect of anti-uplift piles around the shaft wall, the movement trend of a large area of soil was effectively isolated, resulting in less settlement feedback to the surface.
Thus, compared to the excavation method of underground continuous wall enclosure, this construction method had a smaller environmental impact, construction period, and occupied area, and it possessed good economic value.Further, the reduction of the friction and wall protection effect of the mud jacket were ensured.The construction method was worth promoting.

Conclusions
Considering the background of the Shanghai Jing'an Smart Garage Project, we first introduced the world's largest Vertical Shaft Boring Machine, which was suitable for vertical shaft excavation of diameter 12~23 m.Then, the design points and key technologies of Super large and deep diameter assembly shafts in soft soil layers were expounded.Finally, the great applicability of vertical shaft boring technology in soft soil layers in Shanghai, was verified through on-site measurement results.The main conclusions and recommendations are as follows: (1) Based on the Prandtl foundation, the ultimate bearing capacity theory and the classical earth pressure theory were appropriate to calculate the basal heave stability of the excavation surface.(2) The effect of surrounding uplift piles on maintaining the stability of shaft walls and excavation surface soil was seen in the process of sinking, which was particularly necessary for the application in soft soil layers in Shanghai.(3) It was crucial to reduce the friction and protect the wall of the side wall mud for the vertical shafts boring method.The circulation of the side wall mud should be maintained during the whole construction process.(4) In the soft soil layer of Shanghai, it was necessary to adopt an undrained excavation method.To reduce the impact on the surrounding environment, it is necessary to ensure the mud water level and specific gravity, and strictly prohibit over-excavation.(5) From the perspective of current construction efficiency and the impact on the surrounding environment, the application of super deep and large-diameter assembly shafts in soft soil in Shanghai was successful.

Figure 2 .
Figure 2. Anti-heave stability of the shaft bottom.

Figure 3 .
Figure 3. Mechanical model for anti-heave stability of the shaft bottom.

Figure 6 .
Deformation and internal force verification calculation of the top ring beam.

Figure 10 .
Figure 10.Connected aisle of the substation room and shaft.

Figure 11 .
Figure 11.Site photos of the dual sealing gasket.

Figure 12 .Figure 13 .Figure 14 .
Figure 12.World's largest lining segment ringThe lining ring was composed of 10 pipe segments, and each segment weighed approximately 27.4 T, which posed high risk by using the traditional lifting process of carrying pole beams.The lifting process of grippers was innovatively adopted in the project.

Figure 15 .Figure 16 .
Figure 15.Measured results of pile and soil inclinometer around assembly shaft during boring.

Table 1 .
Soil physical and mechanical parameters of the stratum.