Development and Experimental Study of a Novel Diaphragm Wall Joint with Retractable Shear Studs

Abstract

Diaphragm walls are widely used for deep foundation pit support and permanent underground structures. The joints between adjacent panels are critical weak points, significantly influencing the overall deformation and stress distribution of the structure. To address the insufficient shear and tensile capacity of existing diaphragm wall joints, this study proposes a novel rigid joint incorporating retractable shear studs. The joint features a straightforward and constructible design, primarily comprising retractable shear studs, H-section steel, and shear stud pop-out limit plates. By withdrawing the limit plates inserted into the H-section steel, the retractable shear studs mounted on the web automatically extend along their axis, penetrating into the adjacent reinforcement cage to form an intrusive lap joint. This mechanism effectively enhances the integrity and load-bearing capacity at the joint. To validate its mechanical performance, large-scale specimens featuring this new joint were fabricated and subjected to shear and tensile tests. The experimental results demonstrate that, compared to traditional H-section steel joints, the peak shear and tensile strengths of the proposed joint are increased by approximately 10 times and 16 times, respectively. These findings fully verify the excellent mechanical performance of the novel diaphragm wall joint structure.

1. Introduction

A diaphragm wall is constructed by sequentially excavating deep trenches in the ground, placing reinforcement cages into the panels, and casting concrete, ultimately forming a continuous underground wall with retaining or load-bearing functions [1,2,3]. Such walls have been extensively employed in projects involving deep foundation pit support and permanent underground structures [4,5,6,7,8,9,10]. In recent years, their application has expanded to areas including bridge foundations and suspension bridge anchorages [11,12,13,14,15,16,17,18,19,20,21]. The connection between adjacent wall panels—the joint (illustrated in Figure 1)—constitutes a structural weak point, significantly affecting overall deformation and stress distribution [22,23,24,25]. In deep and large foundation pits, as well as in permanent structures subjected to complex loads, the inadequate bearing capacity of joints has become a key factor limiting the overall performance and safety redundancy of diaphragm walls.

Current engineering practice categorizes diaphragm wall joints primarily into flexible and rigid types [26,27]. Flexible joints, exemplified by circular pipe joints, offer advantages of simple construction and low cost. However, their mechanical performance is poor; they can transfer only limited shear force and possess negligible bending and tensile resistance. Rigid joints mainly include H-section steel joints, cross perforated steel plate joints, and parallel reinforcement lapping joints. H-section steel and cross steel plate joints aim to improve connection strength by welding steel sections to enhance concrete interlock. Among these, the H-section steel joint is one of the most widely used rigid forms. The parallel reinforcement lapping joint seeks to enhance shear and tensile capacity through the non-contact, parallel lapping of horizontal bars extending from adjacent panels, though it demands high construction precision during cage installation.

A fundamental deficiency of the aforementioned rigid joints is the independence of the reinforcement cages in adjacent panels, which prevents the formation of invasive overlapping. Consequently, loads cannot be effectively transferred between the cages. Therefore, the joint’s load-bearing capacity depends primarily on the localized shear and tensile strength of the concrete, failing to fully mobilize the reinforcement’s capacity, which results in deficient shear and tensile performance.

Shear studs are common connectors in concrete structures designed to transfer shear forces [28,29,30], ensuring reliable load transfer and composite action between different concrete pours. However, directly applying conventional shear studs to diaphragm wall joints presents challenges. If the studs are too long, they obstruct the installation of the reinforcement cage. If they are too short, they cannot penetrate into the adjacent cage to form an invasive lap, leaving the joint strength reliant on concrete properties.

To address these issues, this paper proposes a novel diaphragm wall joint based on retractable shear studs. This joint achieves intrusive lapping between reinforcement cages of adjacent panels via the retractable studs, effectively enhancing the integrity and load-bearing capacity of the joint. Simultaneously, it offers advantages of straightforward structure and convenient construction. To validate its mechanical performance, large-scale specimens incorporating this new joint were fabricated and subjected to shear and tensile tests, comprehensively verifying the excellent mechanical properties of the proposed structure. The following sections detail the joint design, experimental methodology, results, and analysis.

2. Development of the Novel Diaphragm Wall Joint with Retractable Shear Studs

2.1. Design of the New Diaphragm Wall Joint

Addressing the challenge that existing diaphragm wall joints cannot achieve intrusive lapping between reinforcement cages of adjacent panels, this paper designs a novel rigid joint based on retractable shear studs. As shown in Figure 2a, the joint structure mainly consists of retractable shear studs, H-section steel, and shear stud pop-out limit plates. The core concept is as follows: by pulling out the limit plates inserted into the H-section steel, the retractable shear studs installed on the web automatically extend along their axial direction (Figure 2b), thereby penetrating into the adjacent reinforcement cage to form an intrusive lap joint (Figure 2c). This mechanism substantially enhances the joint’s load-bearing capacity.

The structural composition of a single retractable shear stud is shown in Figure 3. It primarily comprises a front-end nut, a tapered rod, a tapered outer sleeve, a spring, and a rear-end cap. The working principle is as follows: when the front end of the shear stud is compressed, the spring at its rear end is compressed, causing the stud to retract into the tapered outer sleeve along its axis. When the front-end constraint is released, the shear stud extends from the tapered outer sleeve under the force of the spring. The taper of the stud’s tapered rod matches that of the outer sleeve. Therefore, when the tapered rod extends, a wedging effect occurs between it and the outer sleeve, thereby locking the rod in place.

The installation process of the retractable shear studs on the H-section steel is shown in Figure 4. The specific steps are: (a) Weld the outer sleeve onto a hole on the web of the H-section steel. (b) Insert the tapered rod and spring into the outer sleeve in sequence, then seal the sleeve with the rear-end cap. (c) Install the front-end nut on the tapered rod. (d) After compressing the shear stud into the tapered sleeve, insert the limit plate into the H-section steel. The limit plate is connected to the H-section steel via a C-shaped locking structure, allowing it to slide vertically. When the limit plate is withdrawn from the H-section steel, it sequentially triggers the extension of the shear studs from their sleeves.

2.2. Construction Process of the New Diaphragm Wall Joint

The construction process for the proposed joint is illustrated in Figure 5 and detailed as follows:

• Weld the new joint onto the reinforcement cage of the preceding diaphragm wall panel. After inserting the shear stud limit plates, the shear studs at the joint are in a retracted state, concealed within the leading panel.
• After casting the concrete for the preceding panel, excavate the trench segment for the subsequent panel. Lower the reinforcement cage of the adjacent panel vertically.
• After the reinforcement cage is positioned, pull the shear stud limit plates upward. The shear studs then extend from the preceding panel and penetrate into the reinforcement cage of the subsequent panel, forming an intrusive lap.
• Cast concrete into the trench segment. After the concrete hardens, a rigid joint is formed between the two panels, integrating them.
• Repeat steps (1) to (4) until the entire diaphragm wall structure construction is completed.

Figure 5. Schematic diagram of the construction process for the novel rigid diaphragm wall joint based on retractable shear studs.

Figure 5. Schematic diagram of the construction process for the novel rigid diaphragm wall joint based on retractable shear studs.

In actual engineering, the limit plates protect the shear studs before withdrawal, preventing impacts during cage lowering that could deform the studs and hinder extension. To prevent water ingress into the shear studs before concrete casting, which could cause corrosion, a flexible waterproof sealant can be applied in advance at the seams between the limit plates and the H-section steel. This measure does not affect plate withdrawal but effectively blocks water entry.

To ensure that the extended shear studs smoothly penetrate the reinforcement cage, the retractable studs should be positioned as centrally as possible within the cage mesh during structural design. Additionally, positioning control is required during cage lowering. However, in practice, the mesh size in diaphragm wall reinforcement cages typically ranges from 15 to 30 cm. The diameter of the front-end nut of the retractable shear stud is only 2 cm. Consequently, the reinforcement cage provides ample space for the extended studs, meaning this new joint does not demand high positioning precision during installation.

Regarding the equipment and method for withdrawing the limit plates, since the plate length matches that of the lowered reinforcement cage, on site, the same crane used for hoisting the cage can be employed to withdraw the plates. For deep diaphragm walls requiring multi-segment splicing of the reinforcement cage, the limit plates can also be designed as a multi-segment spliced type. Furthermore, considering that friction between a long limit plate and the shear studs can be significant, PTFE (Teflon) sheets can be attached to the front of the shear stud nuts to reduce friction and facilitate smooth plate withdrawal.

2.3. Advantages of the New Diaphragm Wall Joint

The proposed joint design offers several distinct advantages:

• Straightforward and reliable structure; convenient construction: Simply withdrawing the limit plates allows the shear studs to automatically extend and penetrate the adjacent reinforcement cage, forming an intrusive lap joint. Moreover, each shear stud operates independently; even if individual studs fail to engage effectively, the impact on the overall joint capacity is minimal.
• Good compatibility with existing methods; ease of promotion: The construction process for this new joint is highly compatible with that of common H-section steel joints, differing mainly by the added step of withdrawing the limit plates. Therefore, it is easy to promote and implement.
• Favorable overall economy: This joint only adds a certain number of retractable shear studs to a conventional H-section steel joint. The retractable shear stud structure is simple and low-cost, yet it significantly enhances the joint’s load-bearing capacity. This improvement can effectively optimize the overall diaphragm wall design, leading to potential savings in total project cost.

3. Shear and Tensile Mechanical Tests of the Novel Diaphragm Wall Joint

3.1. Joint Specimen Fabrication

To verify the mechanical performance of the proposed joint, large-scale diaphragm wall joint specimens were fabricated and subjected to shear and tensile tests. A total of four specimens were made: two experimental group specimens featuring the novel joint with retractable shear studs and two control group specimens with conventional H-section steel joints.

The overall dimensions and structural layout of the specimens are shown in Figure 6. Considering that the prototype size of diaphragm walls is too large for testing (typical wall thickness ranges from 800 mm to 1500 mm), a 300 mm thick local wall section was selected as the test object. This dimension is sufficient to accommodate the novel joint structure containing four retractable shear studs. Since the shear and tensile loads at the prototype joint are uniformly distributed along the thickness direction, the test results of the partial wall specimen can faithfully reflect the stress transfer paths between the shear studs and the concrete, as well as the main failure modes at the joint. Additionally, compared to the thicker prototype wall (where core concrete experiences stronger three-dimensional confinement), the partial wall specimen reflects the joint’s performance under relatively weaker lateral constraint, constituting a conservative testing scenario relevant for practical engineering design.

The overall size of the shear test specimen is 1200 mm × 300 mm × 300 mm. To facilitate shear stress loading, two joints are symmetrically positioned in the middle of the specimen (Figure 6a,b). The overall size of the tensile test specimen is 1600 mm × 800 mm × 300 mm. To facilitate tensile loading, the specimen consists of two T-shaped parts spliced together, with the joint located in the middle (Figure 6c,d).

All specimens were cast using C35 concrete. For the reinforcement cages, the main reinforcement consisted of 8Φ20 deformed steel bars, and stirrups used Φ10 steel bars. The stirrup spacing was 10 cm, and the cover thickness was 2 cm.

Figure 7 shows the specific dimensions of the novel joint structure with retractable shear studs used in the specimen. The total length of a single shear stud tapered rod is 200 mm, made from No. 45 steel. When extended, the protruding length is 100 mm. The front-end diameter of the tapered rod is Φ13.5 mm, and the rear-end diameter is Φ20 mm. The front steel nut diameter is Φ20. The H-section steel was fabricated by welding 3 mm thick Q345 steel plates. Considering the test’s primary purpose was to verify mechanical performance, limit plates were not installed during specimen fabrication.

Photos of the specimen fabrication process are shown in Figure 8. The H-section steel, retractable shear studs, and reinforcement cages were processed sequentially. To obtain strain data from the shear studs during testing, resistance strain gauges were attached at the root of the extended section of the studs during fabrication (Figure 8c). Finally, after assembling all components, formwork was set up for concrete casting (Figure 8e,f).

Figure 9 shows photos of the completed joint specimens. To prevent damage during transportation and hoisting, temporary reinforcement protection using steel rods and channel sections was applied on both sides of the joint during fabrication. These channel sections were removed before formal testing.

3.2. Test Setup and Procedure

Figure 10 illustrates the loading devices and schemes for the shear and tensile tests. For the shear test, steel force-transfer blocks were placed on the top and bottom sides of the specimen joint. Loading was then applied from top to bottom through a large gantry reaction frame system to generate shear load at the joint (Figure 10a). For the tensile test, a specialized dual-hydraulic cylinder synchronized push-loading device was employed (Figure 10b). This system ensures synchronized application of equal-magnitude thrust on both sides of the specimen, generating pure tensile stress at the middle joint and avoiding eccentric tension.

Displacement control was used for loading in both tests, with a rate of 0.2 mm/min. During testing, laser displacement sensors and embedded strain gauges were used to record the load-displacement data at the joint and the strain data of the shear studs in real time. Simultaneously, cameras recorded the failure characteristics. Testing ceased after complete specimen failure.

4. Analysis of Joint Mechanical Test Results

4.1. Shear Test Results

4.1.1. Joint Shear Mechanical Performance and Failure Characteristics

Figure 11 and Figure 12 present the shear test failure characteristics of the conventional H-section steel joint specimen and the novel retractable shear stud joint specimen, respectively. From Figure 11, it is observed that, under shear load, the middle concrete section of the conventional joint specimen was entirely pushed downward. However, the concrete itself did not fail; only obvious friction marks existed at the contact area between the concrete surface and the steel section. This indicates that the shear strength of the conventional H-section steel joint relies solely on the bond and contact friction between the concrete and the steel.

From Figure 12, it can be seen that, for the novel joint specimen, under shear load, the concrete at the joint exhibited significant shear failure characteristics. Shear cracks first initiated at the root of the extended section of the shear studs, then propagated diagonally upward. Eventually, penetrating shear cracks formed, causing the specimen to lose its load-bearing capacity. This demonstrates that the novel joint fully mobilizes the load-bearing capacity of the concrete, reinforcement cage, and H-section steel, resulting in significantly enhanced shear strength.

Figure 13 shows the load–displacement curves obtained from the shear tests for the two joint types. The shear load-bearing capacity of the novel joint specimen is markedly higher than that of the conventional joint specimen. For the conventional joint specimen, during initial loading, its load–displacement curve rises slightly, with a calculated initial stiffness of approximately 22,571 kN/m. This stage involves the combined action of bond and friction at the joint. When displacement reached 3.52 mm, the curve attained its peak strength of only 46.8 kN. Subsequently, the curve began to decline as the friction transitioned from static to sliding and the bond began to fail. When displacement reached 10 mm, the curve entered a plateau at about 35 kN, indicating that only sliding friction remained.

For the novel joint specimen, its load–displacement curve initially exhibits a linear ascending trend (segment OA), indicating elastic behavior, with an initial stiffness of approximately 44,614 kN/m. When displacement exceeded 11 mm (point A), the curve slope decreased, marking the transition into a non-linear ascending stage (segment AB). This point A represents the yield point, coinciding with the initiation of micro-cracks in the joint concrete. At a displacement of 17.37 mm (point B), the curve reached its peak strength of 485.3 kN. Thereafter, the curve gradually declined with fluctuations (segment BC), signifying crack propagation and interconnection, leading to the failure stage. Taking the displacement at point A as the yield displacement (11 mm) and the displacement corresponding to 0.85 times the peak strength as the ultimate displacement (24.12 mm), the ductility ratio of this novel joint specimen is calculated as 2.19. This value is not particularly high, likely attributable to the use of No. 45 steel for the shear studs, a material with relatively low elongation.

Table 1. Main mechanical performance index values from shear tests for two different types of diaphragm wall joint specimens.

Joint Type	Shear Peak Load (kN)	Shear Peak Disp. (mm)	Initial Stiffness (KN/m)
New Joint	485.3	17.37	44,614
H-section Steel Joint	46.8	3.52	22,571

presents the main mechanical performance indices from the shear tests. The peak shear strength of the novel joint is 10 times that of the conventional joint. The corresponding peak shear displacement is 5 times greater, and the initial stiffness is 2 times higher. These results clearly indicate that the intrusive lap formed by the retractable shear studs enables full shear load transfer between panels, significantly improving joint shear capacity.

4.1.2. Analysis of Joint Shear Bearing Mechanism

Figure 14 shows the variation curves of shear stud strain versus loading displacement during the shear test (monitoring points at the root of the extended section). In the initial loading stage, each stud is in a state of tension on the upper side and compression on the lower side, indicating initial bending deformation. When displacement exceeds about 6 mm, the lower side of each stud gradually transitions from compression to tension (while upper side strain gauges failed), signifying the onset of shear deformation at the stud roots.

Integrating all aforementioned test results, we can analyze the stress characteristics and failure mode of the novel joint specimen under shear load (as shown in Figure 15). After the shear stud pops out, half of its length remains fixed within the leading panel. When the extended part of the shear stud is subjected to downward shear load, the leading panel wall and the H-section steel provide reactive support to the shear stud, resisting the shear load at the joint (Figure 15a). Therefore, during initial loading, the extended part of the shear stud bends slightly downward under bending moment. As the load increases, the shear force acting at the root of the extended part continuously increases, causing the shear stud to undergo significant shear deformation. The final failure mode involves concrete shear cracks initiating at the root of the extended studs, propagating diagonally upward, and finally forming penetrating shear cracks leading to loss of capacity (Figure 15b).

4.2. Tensile Test Results

4.2.1. Joint Tensile Mechanical Performance and Failure Characteristics

Figure 16 and Figure 17 show the tensile test failure characteristics for the conventional and novel joint specimens, respectively. For the conventional joint (Figure 16), under tensile load, cracks appeared at the middle joint and rapidly propagated through, pulling the specimen apart. However, the concrete on both sides did not fail, indicating that its tensile strength relies solely on the bond and friction between concrete and H-section steel, resulting in very weak tensile capacity.

For the novel joint specimen (Figure 17), under tensile load, minor cracks first appeared at the middle joint (Figure 17a). As load increased, the cracks gradually propagated through the entire joint cross-section (Figure 17b), yet the joint continued to bear load. Only when the crack width increased to about 15 mm did the joint completely lose capacity (Figure 17c). During loading, four distinct metallic fracture sounds were heard. Post-test disassembly revealed that the joint concrete did not fail, but all four shear studs fractured at their connection with the front-end nut (Figure 17d). This confirms that the retractable shear studs fully undertook the tensile load.

Figure 18 shows the load–displacement curves from the tensile tests. The tensile capacity of the novel joint is significantly higher than that of the conventional joint. For the conventional joint, its tensile load–displacement curve rose slightly to a peak strength of only 2.86 kN at a displacement of 2.29 mm, with an initial stiffness of approximately 1249 kN/m. The curve then gradually declined to near zero at 10 mm displacement. For the novel joint, the initial loading stage (segment OA) showed non-linear growth due to the tightening of tiny gaps. The curve then rose linearly (segment AB), indicating elastic behavior where the shear studs began fully carrying tension, with a stiffness of about 8960 kN/m. At a displacement of 7.02 mm, the specimen reached its peak tensile strength of 47.41 kN. Subsequently, the curve exhibited sudden drops at points B, C, D, and E (at displacements of 9.41 mm, 11.17 mm, and 13.22 mm), before finally tending to zero.

Figure 19 shows the variation curves of shear stud strain versus loading displacement during the tensile test (monitoring points located at the root of the extended section of each shear stud). Each time the load–displacement curve in Figure 18 showed a sudden drop (at points B, C, D, and E), the strain data of one shear stud suddenly returned to zero. This reveals that the four drops correspond to the sequential fracture of the four shear studs due to uneven force distribution.

Table 2. Main mechanical performance index values from tensile tests for two different types of diaphragm wall joint specimens.

Joint Type	Tensile Peak Load (kN)	Tensile Peak Disp. (mm)	Initial Stiffness (KN/m)
New Joint	47.41	7.02	8960
H-section Steel Joint	2.86	2.29	1249

summarizes the main tensile test results. The peak tensile strength of the novel diaphragm wall joint specimen with retractable shear studs reached 47.41 KN, which is 16 times that of the conventional H-section steel joint. The corresponding peak tensile displacement is 7.02 mm, which is 3 times greater. And the initial stiffness is 7 times higher. This is because the intrusive lap formed by the studs enables full tensile load transfer between the reinforcement cages, significantly improving joint tensile capacity.

4.2.2. Analysis of Joint Tensile Bearing Mechanism

Integrating the test results, the stress characteristics and failure mode under tensile load can be analyzed (as shown in Figure 20). When subjected to tension, the front-end nut transfers tensile stress to the tapered rod. This causes mutual compression between the rear half of the rod and the leading panel wall, creating a wedging effect. Thus, the rear half receives a reaction force from the wall and friction from the outer sleeve, resisting the tensile load. Since the connection between the stud front-end and the nut is the weakest point, the final failure mode involves fracture at this connection (Figure 20b), causing loss of capacity. Clearly, improving the strength of this connection could further enhance the joint’s tensile capacity.

5. Discussion and Conclusions

To address the insufficient shear and tensile capacity of conventional diaphragm wall joints, this study proposed, developed, and experimentally evaluated a novel rigid joint incorporating retractable shear studs. The design rationale, construction procedure, and mechanical performance, as revealed through shear and tensile tests, have been presented. The principal findings are summarized as follows:

• The novel diaphragm wall joint based on retractable shear studs provides a rapid and effective solution for establishing an intrusive lap between reinforcement cages of adjacent panels. The joint features a straightforward structure, convenient construction, and is highly compatible with conventional H-section steel joint practices, making it readily suitable for widespread promotion and application.
• The new joint exhibits significantly superior load-bearing capacity compared to the conventional H-section steel joint, with its shear and tensile strengths increased by a factor of approximately 10 and 16, respectively. Consequently, its application will substantially enhance the overall safety redundancy and risk resistance of the diaphragm wall as an integrated structure, while also effectively broadening its range of application, such as ultra-deep foundation pits and anchorage foundations for ultra-long-span suspension bridges.
• The failure modes and underlying load-transfer mechanisms have been preliminarily elucidated. Under shear, load is transferred via a fixed-end cantilever action of the studs, leading to progressive concrete shear failure. Under tension, the load is carried primarily by the studs through a wedging action, with failure occurring via sequential fracture at the front-end nut connections. These insights establish a solid foundation for further optimization of the joint design.

Limitations and Future Work: Despite the promising results, this study has some limitations. As only comparative mechanical tests on four joint specimens have been conducted so far, these results primarily provide qualitative validation of the effectiveness of the new joint. To further optimize the design and application of this novel joint, future works will focus on: (a) developing and validating a numerical model based on experimental data to perform parametric studies (e.g., on stud number, layout, and length) for design optimization; (b) experimentally investigating the joint’s performance under complex combined loads and comparing it with other joint types to further elucidate its load-transfer mechanisms; and (c) pursuing practical implementation in engineering projects to optimize construction procedures and define its application scope.