Diaphragm wall drilling technology is a construction process that forms continuous trenches through multiple single wells and connects multiple trenches to form a wall. It is widely used in engineering, serving as cut-off walls, retaining walls, anti-scour walls for underwater structures, side walls of metro sections, shaft linings of large-diameter shafts, etc. It integrates multiple functions such as support, seepage prevention and load bearing, making it one of the core construction technologies in the field of underground engineering.

      Ⅰ Development History and Application Scope

     Diaphragm wall technology was first applied by ICOS S.p.A. of Italy in 1950, with the first underground continuous cut-off wall for a reservoir constructed using a grab-type drilling rig, marking the official entry of this technology into engineering application.

     After two decades of development, the technology had formed a standardized construction process in Europe. Thanks to its remarkable technical advantages, it has been widely recognized and trusted by the engineering community. At present, its application is no longer limited to large-scale projects; it has also been commonly adopted in small and medium-sized high-rise buildings. For instance, the underground works of White Swan Hotel, a landmark building in Guangzhou, adopted this technology. To date, diaphragm wall technology has been extensively applied in the following engineering fields:

    1.Earth-retaining and seepage-proof structures for municipal and industrial facilities such as underground oil depots, reservoirs and sewage treatment plants;

    2.Retaining walls for underground space projects including underground complexes, railway stations, commercial buildings, and railway and highway parking lots;

     3.Supporting retaining walls for structures such as vertical shafts and maintenance shafts of water pipelines;

     4.Underground continuous cut-off walls for water conservancy projects such as reservoirs, lakes and rivers;

     5.Supporting and seepage-proof structures for transportation projects such as overpass underlayers and underground passages.

     In general, diaphragm walls can be constructed in advance as supporting and seepage-proof structures for all projects requiring deep underground excavation. This technology enables vertical excavation of foundation pit walls, effectively eliminates potential water seepage, reduces excavation volume and construction land occupation. Meanwhile, the wall can be directly used as part of the main engineering structure, solving many technical problems in underground engineering construction. It has turned deep underground excavation projects previously considered unfeasible into reality, providing efficient and reliable technical support for underground engineering construction.

    The application of diaphragm wall drilling technology in China began in the early days after the founding of the People's Republic of China. Initially, percussion drilling was adopted for trench forming, followed by pouring reinforced concrete to form continuous walls, which were mainly used for seepage control in water conservancy projects. Later, it was gradually extended to various engineering fields such as foundations of high-rise buildings, foundations of large bridges, and inlet and outlet sections of cross-river tunnels.

     With the rapid development of engineering construction and continuous advancement of science and technology in China, breakthroughs have been made in the trench forming methods, construction techniques, wall thickness and depth of diaphragm walls. New construction technologies and machinery have emerged continuously, gradually replacing the traditional percussion trench forming process, and significantly improving construction efficiency and engineering quality.

     At present, mature trench forming technologies include the grab-type trench forming method and the multi-head drilling rig trench forming method. Among them, the multi-head drilling rig can drill a section of trench in one go, greatly enhancing construction efficiency. In addition, the reverse-circulation full-face drilling method has been gradually promoted and applied, which not only effectively solves the construction difficulties of wall thickness and depth, but also enables the wall foot to be embedded into bedrock, further improving the bearing capacity and seepage resistance of the wall.

     It is worth highlighting the breakthrough in wall-stabilizing slurry technology: currently, after drilling and trench forming, only cement and other chemical admixtures need to be mixed into the slurry to solidify the slurry into a wall, eliminating the need for slurry replacement before concrete pouring. The application of this technological achievement has greatly shortened the construction period of diaphragm walls, significantly reduced engineering construction costs, and promoted the large-scale application of diaphragm wall technology.

     Ⅱ Drilling Methods

     In diaphragm wall drilling construction, geological conditions and slurry quality directly affect the determination of panel length. The drilling methods are mainly divided into two categories: the panel method and the shaft array method, with specific construction techniques as follows.

     1. Panel Method

     (1) A guide trench is laid on the ground as the positioning reference and boundary control structure for the panel, providing guidance during drilling. The panel is always filled with slurry maintained at a specified liquid level, and the static pressure of the slurry is used to stabilize the trench wall and prevent collapse.

     (2) The width of the guide trench shall be 30~50 mm larger than that of the drilling tool to ensure smooth passage of the tool. The depth of the guide trench is generally controlled within 10 m, adjusted according to geological conditions and construction requirements.

      (3) The determination of panel length shall comprehensively consider the following three core factors:

       1) Influence of geological conditions and groundwater occurrence on trench wall stability;

       2) Weight and size of the steel reinforcement cage, and rated lifting capacity of hoisting equipment;

       3) Engineering design requirements for wall stress, seepage prevention and overall rigidity.

     There are four main panel arrangement methods, as shown in Figure 1:

Figure 1 Panel Arrangement Methods

A. Single-section Type    B. Two-section Type
C. Three-section Type D. Four-section Type

      2.Shaft Array Method

    The shaft array method is a construction technique that forms panels through continuous drilling and overlapping drilling of multiple single wells, ultimately composing a diaphragm wall. There are four main shaft arrangement types, as shown in Figure 2:

Figure 2 Arrangement Methods and Drilling Sequences of Shaft Array Method

A. Staggered Type B. Straight Lap Type C. Tangent Type D. Spaced Type

      (1) Staggered Type: The centers of shafts are not on the same straight line. Its core advantage is that a thicker wall can be formed with smaller-diameter drillings, making it suitable for projects with high requirements on wall thickness. However, this type has the problem of easy obstruction during steel reinforcement cage lowering, and drilling deviation is likely to occur when the wall is embedded into bedrock.

       (2) Straight Lap Type: There is a partial overlapping area between adjacent shafts, with the lap length determined by design requirements. It effectively ensures the continuity and integrity of the wall, with excellent seepage resistance.

     (3) Tangent Type: Adjacent shafts are only tangent without overlapping areas, featuring high construction efficiency. However, the wall continuity and seepage resistance are relatively weak, making it suitable for supporting projects with low seepage control requirements.

      (4) Spaced Type: A certain distance is reserved between adjacent shafts. After the completion of main shaft drilling, auxiliary shafts are drilled in the spaced areas to connect the shafts into trenches. It is suitable for projects with complex geological conditions requiring staged control of trench wall stability.

      The numbers of shafts in Figure 2 represent the construction sequence of drilling. The core purpose is to ensure the stability of the trench wall and surrounding strata during drilling through a reasonable drilling sequence, and avoid trench wall collapse caused by construction disturbance.

     Among the above four arrangement types, except for the staggered type which has problems such as obstruction of steel reinforcement cage lowering and deviation in bedrock drilling, the other three types have no such hidden dangers. Considering wall thickness, continuity and seepage resistance comprehensively, the straight lap type is the optimal choice.

      3. Drilling Sequences

    Diaphragm wall drilling sequences are mainly divided into two methods: horizontal drilling and vertical drilling. The selection of either method shall be comprehensively determined based on stratum lithology, performance of drilling equipment and construction efficiency requirements.

     (1) Horizontal Drilling Method: The drilling rig moves along the panel axis. Each drill pipe connection completes the drilling operation of one horizontal layer. After the entire layer is drilled, the drill pipe is extended for drilling of the next horizontal layer. It is suitable for projects with uniform strata and moderate drilling depth.The construction schematic of the horizontal drilling method is shown in Figure 3.

Figure 3 Schematic Diagram of Forward Horizontal Drilling with Reverse Circulation Drilling Rig

     (2) Vertical Drilling Method: Also known as vertical driving, it refers to staged drilling in accordance with the shaft number sequence. Odd-numbered shafts (1, 3, 5, 7...) are drilled first, followed by even-numbered shafts (2, 4, 6, 8...). Spaced drilling reduces construction disturbance and ensures trench wall stability, making it suitable for construction in complex geological conditions and collapse-prone strata. The construction schematic of the vertical drilling method is shown in Figure 3.