Abstract: Taking a project in Nanjing as an example, this paper introduces the design method, construction technology, and related technical measures of II-shaped joints for diaphragm walls. In the project, the II-shaped joints and the diaphragm wall panels were constructed separately, which improved the construction efficiency of the new joint diaphragm wall. Through the optimization of the joints, the difficulty of panel construction was reduced, the overall quality of the diaphragm wall was more easily ensured, and the construction cost was lowered, achieving good results.
Keywords: underground engineering; diaphragm wall; steel column; joint; construction technology
0 Introduction
Diaphragm walls are a widely used technology in underground engineering, introduced in China in the 1950s. With the continuous development of the construction industry, diaphragm walls have been increasingly applied in large deep foundation pit projects such as building construction and subways. In recent years, diaphragm walls have shown a trend toward greater depth and thickness. However, for ultra-deep diaphragm walls, traditional construction methods encounter many difficulties and may even be infeasible. How to properly construct ultra-deep diaphragm walls under water-rich, soft soil, and thick sand layer conditions is a problem worthy of attention.
1 Project Overview
The Nanjing Press Media Building project is located in the Hexi area of Nanjing. The foundation soil layers consist of miscellaneous fill, mucky silty clay, silty sand, and weathered rock. The groundwater level is high. The designed depth of the diaphragm wall is over 60 m, making it an ultra-deep diaphragm wall. The wall must pass through all soil layers and embed into weathered rock. The total designed length of the diaphragm wall is 397.705 m, with a two-walls-in-one design and a wall thickness of 1.0 m. The designed reinforcement cages are divided into four structural forms: A, B, C, and D. The longest reinforcement cage for the first panel is 59.05 m. The top elevation of the designed reinforcement cage for the diaphragm wall is -2.050 m, and the designed bottom elevations of the panels are -60.500 m and -61.100 m. The original design adopted the I-beam form of the steel joint method, using the skip-panel method. The panel forms include 8 L-shaped panels and 61 "straight" panels, totaling 69 panels. The design involved mechanical grab trenching, integral connection and lowering of reinforcement cages and I-beams, followed by concrete pouring to form the diaphragm wall.
2 Construction Difficulties
According to the construction experience of similar projects and diaphragm walls in the Hexi area of Nanjing, the reinforcement cage with I-beam joints, constrained by the I-beams at both ends, forms a rigid whole for the first panel. During lowering, it is prone to being unable to descend due to hard obstruction by the trench wall. The greater the depth, the more likely cage sticking occurs. Once the depth exceeds 45 m, the difficulty of lowering increases sharply, and it is very difficult to lower to the design elevation. Problems encountered during the process exceed imagination, and the measures taken often come at the expense of the construction quality of the diaphragm wall. Moreover, serious quality and safety accidents are highly likely, causing adverse social impact, affecting project quality, schedule, and corporate image, with increased difficulty in later handling and incalculable accident handling costs.
3 Optimization of Diaphragm Wall Joint Construction Scheme
To effectively solve the difficulties in ultra-deep diaphragm wall construction, after analysis, research, repeated exploration, and reference to diaphragm wall practices in other projects in the Hexi area of Nanjing, it was decided to optimize the original I-beam joint design to an II-shaped steel column joint, and to optimize the original integral lowering of the reinforcement cage and joint to lowering the steel column joint first, followed by trenching for the panel. With this construction method, the lower structural reinforcement of the panel reinforcement cage can be omitted. The joint and the panel reinforcement cage are constructed in three separate parts, reducing the single lifting weight, reducing the amount of steel used and the specifications of the supporting roads. The original design joint construction sequence is shown in Figure 1, and the optimized joint construction sequence is shown in Figure 2. 4 Construction Process Flow of II-Shaped Joints for Diaphragm Walls
1) Before diaphragm wall construction, guide wall construction is first carried out. The dimensions and support requirements of the guide wall are shown in Figure 3. After the guide wall construction is completed and the strength requirements are met, the joint steel column construction begins. First, the center position of the steel column is set out, its position is determined, and clear marks are made on the guide wall concrete.
2) Mud is supplied from the mud pit. The center point of the hydraulic grab is aligned with the steel column setting-out position and excavation begins. After the hydraulic grab excavates to the design trench bottom elevation, excavation stops, and a rotary drilling rig is used to continue drilling 1.4 m downward at the center point to form the steel column hole.
3) The lowest section of the steel column is lengthened by 1.0 m during fabrication. After installation in place, it is 0.4 m above the hole bottom, in a suspended state. The top of the steel column is temporarily fixed to the guide wall. The verticality of the steel column is tested by ultrasonic method, and based on the test results, the steel column is finely adjusted to meet the design requirement of verticality < L/300.
4) A tremie pipe is installed inside the hollow cavity of the steel column, and concrete for the bottom embedment end is poured. The tremie pipe is withdrawn, and soil on both sides of the steel column is gradually and evenly backfilled to the top elevation of the guide wall. Then the tremie pipe is installed again to pour the remaining concrete inside the steel column.
5) The construction process of the above II-shaped diaphragm wall joint is as follows: ① First grab of the trenching machine, constructing the trench position for the square steel column installation (see Figure 4a); ② The rotary drilling rig drills 1.4 m downward at the center at the design trench bottom elevation to fix the bottom end of the square steel column (see Figure 4b); ③ The top end of the square steel column is temporarily fixed to the guide wall, and the bottom end is 0.4 m above the bottom of the embedment hole. Based on ultrasonic testing data, the square steel column is adjusted until the verticality is qualified, then the top end is fixed. 5 m3 of concrete is poured inside the steel column to embed the bottom end. After the embedment concrete reaches final setting, soil is evenly backfilled on both sides of the steel column to the guide wall height, and then concrete is poured inside the square steel column to the design height (see Figure 4c).
5 Construction Technology of II-Shaped Joints for Diaphragm Walls
5.1 Steel column joint fabrication and butt joint requirements The steel column joints are cut from 12 mm thick steel plates, factory-welded into 20 m sections. Each steel column joint consists of 3 sections. The steel column extension is performed on site. The butt welds are in a Z-shape, i.e., the flange plate welds are horizontally staggered. The butt joint method is shown in Figure 5. The forming quality standards of the joint steel columns are shown in Table 1. 5.2 Panel construction for lowering steel columns
5.2.1 Mud preparation
Mud mix ratio: bentonite, about 8% of water mass; CMC (sodium carboxymethyl cellulose), 0.05% of water mass; soda ash (dispersant), about 0.3% of water mass. The mud is prepared using a mud mixer. First, CMC is soaked and diluted in water, then added to the mixing drum in the specified amount and mixed with bentonite and soda ash. After mixing, it is stored in a fresh mud pit. To fully exert the effect of the mud, it must be stored in the fresh mud pit for 24 h after mixing before use. The properties of the newly prepared mud are shown in Table 2.
5.2.2 Positioning and trenching
The excavation length of the panel is 2.8 m, and the standard panel spacing is 6.42 m. First, mark points with red paint at the panel division positions, then drill holes with an electric drill at the marked positions and embed steel bar heads as the trenching center points. The allowable deviation for positioning at the division points is +10 mm.
5.2.3 Trenching construction quality requirements
The center of the steel column is used as the center for the trenching machine excavation positioning. The trenching depth is determined based on the measured top elevation of the guide wall. The trenching machine excavates according to the operating procedures. After cleaning the bottom sediment, trenching is completed. The trenching quality acceptance standards are shown in Table 3. During trenching, the mud quality in the trench is tested every 2 h. The control standards and testing methods for circulating mud and waste mud during construction are shown in Table 4. Unqualified mud must be adjusted promptly to ensure the stability of the panel soil.
5.2.4 Steel column embedment hole construction
After the grab reaches the design trench bottom elevation, a rotary drilling rig is used to drill 1.4 m downward at the corresponding position of the steel column to form an embedment hole with a diameter of 1 m and a depth of 1.4 m.
5.3 Steel column lowering construction process requirements
5.3.1 Steel column lifting
The flanges on both sides of the web within 1 m of the steel column bottom are cut off. A QUY160 crawler crane is used to lift the steel column and align it with the center of the panel for lowering. The bottom 1 m of the steel column is embedded in the hole drilled by the rotary drilling rig, with the bottom end 0.4 m above the hole bottom. The steel column joint is fixed to the guide wall (as shown in Figure 6), keeping the steel column joint in a suspended, vertical, and stable state.
5.3.2 Fixing method
After the steel column is lowered, before fixing, the verticality of the steel column is tested by ultrasonic method. If the test shows that the verticality deviation exceeds the standard, the steel column is lifted and the position of the steel column joint is readjusted according to the test results until it meets the standard, and then fixed.
Openings are made on the four side flange plates at the top of the steel column joint according to the calculated positions. I-beams are passed through the four openings and spanned on the guide wall. To prevent displacement of the steel column joint, L-shaped angle steel is firmly fixed to the guide wall with expansion bolts, and then [10 channel steel is used to horizontally tie all steel column joints along the diaphragm wall direction (see Figure 6). 5.3.3 Sediment cleaning
After fixing, the tremie pipe and hole cleaning equipment are installed. The positive circulation method is used for hole cleaning and mud replacement. Attention is paid to timely mud replenishment. The mud level inside the column must not be lower than 2 m below the guide wall.
5.3.4 Pouring embedment concrete
After hole cleaning is completed, 5 m3 of concrete is poured to fill the bottom of the panel, so that the bottom end of the steel column is embedded in the concrete. After pouring, the actual embedment depth and effect of the steel column are determined by checking the concrete surface heights inside and outside the column.
5.3.5 Column trench backfilling
After the bottom sealing concrete reaches final setting, clay is evenly backfilled on both sides of the steel column. An excavator is used for slow backfilling, keeping the backfill height on both sides uniform and synchronous, up to the guide wall top elevation. During backfilling, attention is paid to timely recovery of the mud in the trench and removal of impurities in the mud through a desander.
5.3.6 Pouring concrete inside the column
After backfilling is completed, concrete is poured inside the column. First, the tremie pipe is installed to the bottom, then the positive circulation method is used to remove the mud sediment at the bottom. Before pouring, the sediment thickness inside the column is ≤100 mm. Finally, underwater concrete is poured to the design elevation. The panel construction of the diaphragm wall is discussed separately.
6 Conclusion
The diaphragm wall project of the Nanjing Press Media Building was completed in May 2012. Through the testing pipes installed in the II-shaped steel column joints, the verticality of the steel column joints was tested, and the results all met the design requirement of L/300. At the same time, through the optimization of the joints, the difficulty of panel construction was reduced, the overall quality of the diaphragm wall was more easily ensured, the construction progress was accelerated, safety hazards and lifting risks were greatly reduced, and the configuration of lifting machinery and temporary roads was reduced, lowering costs. The implementation of the II-shaped joints for diaphragm walls achieved good results and received unanimous praise from the industry.