Diaphragm wall engineering technology
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I. Trenching and pouring of diaphragm walls: Starting in 1963, experimental studies on diaphragm walls using the trench wall method were conducted to build underground structures such as subway stations. Geological drilling equipment was used along with grab machines as trenching tools, and mud was prepared to protect the trench walls. In strata dominated by clay, a trench 0.8 meters wide, 3 meters long, and 20 meters deep was dug, after which concrete was poured to form an underground wall. In 1974, trenching tests for underground walls were conducted in the fill layers along the Huangpu River, using a conventional grab to dig trenches and a modified kaolin mud prepared through research as a substitute for bentonite for wall stabilization; trenches with a width of 1 meter, a length of 5 meters, and a depth of 30 meters were dug, and they remained stable for 1 year without collapsing ； At a distance of 1 meter from the edge of the groove, the surface soil has settled by only 30 millimeters in total ； At the same time, an ultrasonic wall detector for measuring the verticality of groove walls was developed. After 1976, in conjunction with subway test projects, trenching equipment such as rod-type hydraulic grabs, guide-plate type hydraulic grabs, and cable-grab types that push soil between two pre-drilled guide holes was developed, enabling the vertical accuracy of the trench walls to reach 1/300 to 1/500. Through further research, the mud mix design, trenching equipment and processes were improved, as well as construction techniques such as lifting the rebar cage, pouring concrete, joint treatment, and mud separation and dewatering, along with structural design. By 1980, a relatively complete set of construction techniques for diaphragm walls had been developed using domestic design methods, processes, and equipment. In 1984 and 1989, two sets of hydraulic grab trenching units manufactured in Japan were introduced successively for use in constructing the underground walls of the underground substation project at People’s Square, achieving an average vertical accuracy of 1/850. Following digestion and absorption of these advancements, in 1990 a hydraulic grab trenching unit equipped with a inclinometer and deviation correction device was developed; the vertical accuracy of the trenches formed was less than 1/500, and could even reach 1/1000, thereby taking the technology for constructing diaphragm walls to a new level. II. Tests and applications of diaphragm walls in deep foundation pit projects. From 1981 to 1982, test shafts using diaphragm walls were constructed and on-site tests were carried out at the location planned for the northern head shaft of the Caobaolu Subway Station. Before and during excavation, through in-situ tests on earth pressure and wall stress, data reflecting the variation patterns of water pressure on the inner and outer sides of the diaphragm wall, the internal forces within the wall, and its deformation were obtained ； In strata consisting of water-bearing silty clay interbedded with thin layers of silt, measures such as advance dewatering using well points were also taken to improve the stability of the foundation pit; in addition, a reverse filter layer was installed beneath the floor of the subway station to eliminate buoyancy forces beneath that floor ； Improve trenching equipment, wall inclinometers, mud preparation methods, and mud separation processes, to gradually master the design and construction techniques for diaphragm wall deep foundation pits. The research findings were applied to the construction of the Shanghai Railway Station subway station from 1984 to 1986. From 1986 to 1989, during the construction of the rectangular tunnel buried beneath the road on East Yan’an Road and the Caobao Road subway station, a construction method for excavation and support that took into account spatial and temporal effects was developed. Preloading was applied to the supports, and during the excavation process, horizontal and vertical settlement patterns of the foundation pit, as well as the displacement of nearby buildings and roads, were monitored; based on this information, the construction parameters for excavation and support were adjusted in a timely manner. At the same time, the layered grouting method is used to reinforce the foundation at the bottom of the pit, improving the safety against heaving and thereby protecting the safety of nearby high-rise buildings and underground pipelines. Further test studies were conducted during the construction at Caobao Road Station and Xujiahui Station on the subway, yielding technical data on the correlations between the geological characteristics of the foundation pit, the conditions for soil reinforcement, construction parameters, design parameters of diaphragm walls, the safety factor against heave of the foundation pit, and the vertical and horizontal movement of soil layers. This helped to establish, to a preliminary extent, control standards for ground settlement around deep foundation pits with diaphragm walls, based on the environmental conditions surrounding such pits. The 9 underground stations on Shanghai Metro Line 1 built between 1991 and 1993 all had design standards for deep foundation pits using diaphragm walls and methods for controlling settlement established based on the environmental conditions in which they were located, with strict adherence to construction technical regulations. By employing prediction and prevention techniques for ground movement around the foundation pits, and by utilizing construction monitoring data, grouting is carried out to protect underground pipelines and to control ground settlement on the outside of the diaphragm walls, thereby ensuring the safety of the deep foundation pit projects related to the diaphragm walls in each subway station as well as the safety of the surrounding environment. III. Top-down construction method for the diaphragm wall lining structure. In 1986, in the construction of Shaft No. 1 for the Yan’an East Road Tunnel, a top-down construction method was developed for the stepped excavation of circular deep foundations using diaphragm walls, along with the layered pouring of reinforced concrete structures for these diaphragm wall linings, in a top-to-bottom manner (similar to the method used in vertical mine excavations). This approach provided experience for the underground structures of underground power plants built between 1990 and 1991, as well as for similar projects. From January 1991 to May 1993, the structures of three Line 1 subway stations on Huaihai Road were constructed using the top-down method, from the roof slab and middle slab to the base slab. The underground diaphragm walls on both sides together with temporary support piles in the middle bore the entire vertical load above the middle slab, and their settlement and differential settlement were both below the design limits. Taking into account the restraining effect of the top floor slab on the diaphragm wall as well as the spatiotemporal effects, an excavation support method was adopted in which the upper supports were lowered one by one as the soil layers below were excavated, which reduced the amount of supports used by half while keeping the wall displacement and ground settlement within 2–3 centimeters. To control the differential settlement of multiple diaphragm walls, connecting steel plates are also used at the joints between the walls, thereby giving the walls a certain degree of integrity; they can function as side wall structures for a single layer to meet the required performance standards. This construction method, which involves excavating soil beneath the roof within the area enclosed by diaphragm walls and constructing the floor slabs and base slabs in reverse order, reduced the road closure time required for the construction of three subway stations on Huaihai Road by over a year. It also helped to control ground settlement in the vicinity effectively, ensuring the normal operation of all nearby multi-story buildings and underground pipelines, thereby yielding significant social and economic benefits.
1 Characteristics of underground diaphragm walls: Underground diaphragm walls are continuous walls constructed underground using special trenching equipment; they are commonly used for soil retention, water containment, seepage prevention, and load bearing. Diaphragm walls were first used in projects in Milan, Italy, in 1950, and have seen rapid development over the past 50 years. With the development of urban construction and industrial transportation, there is an increasing number of subways, high-rise buildings, bridges, heavy industrial facilities, and large-scale underground structures. For example, some newly built or expanded underground projects are located adjacent to surrounding streets or in close proximity to existing buildings ； In some projects, due to soft soil foundations, pile driving can affect the safety of nearby buildings and generate noise ； In some other projects, it is difficult to implement measures such as wellpoint dewatering due to environmental constraints or the complexity of hydrogeological and engineering geological conditions. In such situations, the use of diaphragm walls for support offers clear advantages. Diaphragm walls have been widely used and developed due to the following advantages: 1. Reducing the environmental impact of construction projects. Minimal vibration and low noise during construction ； It allows construction to take place in close proximity to adjacent buildings and underground pipelines, making it easier to control settlement and displacement ； 2 Underground diaphragm walls have high wall stiffness and good integrity, resulting in minimal deformation of both the structure and the foundation; they can be used for ultra-deep retaining structures as well as for the main structural elements ； 3 The underground diaphragm wall is a monolithic continuous structure; with the cast-in-place walls having a thickness of generally ≥60 cm, a large reinforcement cover layer, good durability, and also excellent water resistance ； 4 Top-down construction can be employed, which enhances construction safety, accelerates the construction pace, and reduces costs. Diaphragm walls also have their own disadvantages and areas that need further improvement, mainly including: 1. The treatment of waste soil and slurry. In addition to increasing construction costs, improper handling can lead to new environmental pollution ； 2 Geological conditions and suitability for construction. The most suitable strata for diaphragm walls are soft-plastic and plastic clay layers. When the stratum conditions are complex, it also increases the construction difficulty and affects the project cost ； 3 groove walls collapsed. A sharp rise in the groundwater level, a rapid drop in the level of the retaining wall mud, the presence of weak or loose layers as well as sandy interlayers, improper properties of the mud or its deterioration, and poor construction management can all cause the collapse of the trench walls. The collapse of the groove walls can, in mild cases, lead to an excess amount of concrete in the wall structure and dimensions that exceed the allowed limits; in severe cases, it can cause settlement or collapse of adjacent areas, posing a threat to the safety of nearby buildings and underground pipelines. 2 Applicable conditions for underground diaphragm walls. Underground diaphragm walls are a structural solution that is more expensive than bored cast-in-place piles and deep mixing piles. The applicable conditions for their use in foundation engineering are as follows: 1 The depth of the foundation pit ≥ 10 m ； 2 Soft soil foundation or sandy soil foundation ； 3 When constructing foundation pits in dense building complexes and strict restrictions are required on ground settlement around them as well as settlement of buildings, diaphragm walls are advisable ； 4 When the enclosure structure is integrated with the main structure and used as part of it, diaphragm walls are advisable when high requirements are placed on water resistance ； 5 Projects that employ top-down construction methods, with the lining and retaining wall forming a composite structure. 3 Classification of underground diaphragm walls Underground diaphragm walls are classified according to the material used in their construction into soil walls, concrete walls, reinforced concrete walls (cast in place and precast), and composite walls (a combination of precast reinforced concrete panels and cast-in-place concrete, or a combination of precast reinforced concrete panels and self-setting cement-bentonite slurry) ； Based on their wall-forming method, they are classified into pile row type, wall panel type, and combined pile-wall type ； Based on their purpose, they are classified into temporary retaining walls, impermeable walls, and diaphragm walls that serve as both the main structural element and a temporary retaining wall. A row of pile-type diaphragm walls is essentially a diaphragm wall formed by connecting bored cast-in-place piles side by side. Its design and construction can be classified as bored cast-in-place piles, and it will not be discussed in detail in this chapter. 2 Wall-type diaphragm walls are constructed using specialized equipment; deep trenches are dug underground with slurry wall protection, and concrete is poured underwater to form the diaphragm wall. 3 Pile-wall composite diaphragm wall, which is a diaphragm wall that combines the aforementioned pile-array type and wall-plate type diaphragm walls for use.