Wang Yunliang, Gu Shouguo
Tangshan Desheng Technology Co., Ltd.
Summary:Corridors are underground structures, and their waterproofing projects have received significant attention. However, specific waterproofing design, material selection, and construction methods still require further research. This article examines waterproofing design and construction methods for the Shijiazhuang Zhengding corridor project as an example.
Keywords:Pipe gallery waterproofing, underground engineering, design, construction, and expansion joints.
The construction of underground comprehensive pipe galleries in China has experienced a significant surge in development since 2015, achieving substantial progress in construction methods, planning and design, construction, and operation management. Due to the underground nature of these galleries, waterproofing has received extensive attention. However, in terms of design, material selection, and construction, the author believes there is still considerable room for further research. This article examines the design and construction of waterproofing for underground pipe gallery projects, using the Shijiazhuang Zhengding pipe gallery project as an example.
1 Project Overview
The "Shijiazhuang Zhengding New District Olympic Street Comprehensive Conduit Project" is located in the starting area of Zhengding New District, Shijiazhuang City, specifically on Olympic Street underground (from Huayang Road to Hengyang Road), with a total length of approximately 2.1 km. The standard segment conduit width is 13.8 m, and the total project investment is 84,409.83 million yuan, with an expected completion date of 2019. Due to its location in the core area of the starting area, this project has been designated as a key infrastructure project in Hebei Province. The design unit for the Olympic Street Comprehensive Conduit Project is Shijiazhuang Municipal Design Research Institute Co., Ltd., and the general contractor is China State Construction Engineering Corporation Ltd. The waterproof solution is provided by Tangshan Desheng Technology Co., Ltd. The Olympic Street Comprehensive Conduit Project employs open excavation and cast-in-place construction methods, with a waterproof protection level of Grade 1. Different parts and nodes have adopted targeted design solutions. As a linear structure, the conduit has numerous expansion joints, with a total of over one hundred expansion joints spaced 20 m apart. Therefore, the waterproofing of these expansion joints is particularly important.
2. Waterproofing Solutions and Key Considerations
2.1 basement Slab
2.1.1 Waterproof Construction
The waterproofing construction method for the underground floor is shown in Figure 1.
2.1.2 Waterproofing Key Points
1) Strengthening measures for expansion joint areas
This project utilizes a composite waterproof construction method, combining a central-embedded waterproof strip with an external waterproof membrane. The specific implementation is shown in Figure 2. This method enhances the connection between the substrate and the main waterproof layer through adhesive bonding, creating a monolithic waterproof layer. This allows for more direct load transfer during structural deformation, and also simplifies the acceptance of the waterproof quality. This waterproof construction method adheres to the relevant provisions in the national construction standard design manual "Underground Building Waterproof Construction" (10J301).
based on tracking numerous projects (residential and infrastructure projects), the author observed that in actual construction, many projects place the reinforcing layer beneath the main waterproofing layer and bond it to the substrate. The likely reason for this is confusion between the reinforcing layer and the interface treatment agent. This approach is "back to front," as the reinforcing layer does not effectively enhance the waterproofing layer. This practice should be corrected.For underground composite pipe galleries, the expansion joints extend vertically from the bottom plate to the side wall and top plate, creating a continuous joint. Therefore, in actual bottom plate construction, the reinforcing layer of the expansion joint extends continuously from the main waterproofing layer to the permanent protective wall, and is leveled with the main waterproofing layer to better integrate with the expansion joint at the side wall, as shown in Figure 3.
2) Permanent protection of the wall
In cast-in-place underground combined pipe gallery projects using the open excavation method, the omission of permanent protective walls is common. While this saves some costs, it increases the risk of damage to the waterproofing layer. Waterproofing layers are typically laid on a base layer and covered with geotextiles and wooden boards as temporary protection, without effective measures. The risk of damage from formwork support, rebar tying, and subsequent construction activities, such as foot traffic and material storage, is unavoidable. When the waterproofing layer is joined, the damage to the waterproofing layer is often severe, making it difficult to join and increasing the risk of leakage. Through communication with all project stakeholders, the construction company has added permanent protective walls to the initial design to ensure the quality of the waterproofing work.
3) Protection against roller joint gaps
The protection of the expansion joint is an integral part of the bottom plate and side wall construction process. It involves permanently protecting the waterproof layer of the wall and the contact area between the wall and the substrate, which constitutes an external waterproof and internal adhesion structure. This requires that the protection wall be loosely bonded, and the waterproof layer must have a stress release condition, as specified in GB 50108-2008 "Technical Specifications for Waterproofing of Underground Engineering." Before pouring the concrete for the bottom plate, remove the corresponding isolation film on the surface of the wall, ensuring full adhesion with the foundation concrete. Then, before pouring the concrete for the side wall, remove the corresponding isolation film.
In this project, the substrate utilizes a double-sided, high-strength self-adhesive waterproof membrane. The surface covering film uses PET-coated aluminum film, which exhibits superior aging resistance compared to conventional covering films. Additionally, at the seams, bricks are used as temporary protection, and a dedicated person conducts daily construction checks to protect the waterproofing layer (Figure 4).
2.2 Side Walls
2.2.1 Waterproof Construction
This project requires a ground-side wall height of 5.4 m. To ensure construction quality and safety, the wall will be constructed with two layers of 1.5 mm thick, high-strength, self-adhesive membrane waterproofing material. This waterproofing material has excellent tackiness and adhesion, providing good bonding with the substrate; it is also lightweight and easy to install. The double-layer construction is a thin, multi-layer approach with overlapping seams, which enhances overall waterproofing. The specific construction details are shown in Figure 5.
2.2.2 Waterproofing Key Points
1) Waterproofing construction of joints with the substrate.
According to the construction process schedule for pipe galleries, the exterior waterproofing layer should be applied after the construction of the side wall and top plate structures are completed. The pre-formed waterproofing material at the bottom plate, which has been compressed, bent, and edged, is prone to protruding, particularly at the bent areas, leading to a high risk of "false adhesion." Furthermore, the adhesion strength of the self-adhesive material is also influenced by the cleanliness of its surface.
When installing the self-adhesive waterproofing membrane for this project, pre-baking the membrane followed by adhesion yields good results. It is important to note the baking distance and angle to avoid burning through the membrane.
2) Enhanced treatment for horizontal construction joints
The horizontal construction joints on the underground wall are a high-risk area for water leakage in building engineering and are also a critical waterproofing point for this project. Waterproofing measures, such as water barriers, are installed within the construction joints. Additionally, a composite waterproofing layer consisting of a high-strength cross-laminated film (double-sided) self-adhesive waterproofing membrane and non-hardening rubber asphalt waterproofing material is applied as a reinforcement treatment, creating a construction that is fully bonded to the substrate.
3) Strengthening of expansion joints
The materials used for waterproofing at the wall expansion joint should be compatible with the general structural deformation. When choosing between "anti-deformation" and "expansion," the latter is preferred. The interior includes polyethylene foam strips, which are sealed with silicone sealant. The exterior waterproofing layer uses a high-strength, cross-laminated self-adhesive waterproof membrane (double-sided) and non-hardening rubber asphalt waterproofing coating. The non-hardening coating has good creep properties, which can act as a buffer layer between the waterproofing layer and the structure, and also create full adhesion with the substrate, preventing water leakage.
4) Seams in rolled materials
In many projects, the edges of wall cladding often remain on the wall, which can negatively impact waterproofing. In this pipe corridor waterproofing project, the edges of the wall cladding are left at 250 mm above the top plate. While damage to the waterproofing layer at the edge of the top plate is possible, it can generally be controlled with minimal protection. Even with some damage, the overall integrity of the wall cladding is superior to the edge-cutting method described above (Figure 6).
2.3 Top Slab
2.3.1 Waterproof Construction
The topsoil of this project is approximately 2,000 mm thick and is located directly beneath the roadside green strip. To address the issue of root penetration, the waterproofing layer consists of a 2.0 mm thick non-hardening rubber asphalt waterproofing coating and a 4.0 mm thick root-resistant SBS modified asphalt waterproofing membrane (chemical root barrier).
2.3.2 Waterproofing Key Points
1) Surface preparation through abrasive blasting.
For coating-type waterproofing, proper substrate preparation is essential for ensuring waterproofing quality. In recent years, the domestic waterproofing industry has introduced shot blasting into civil engineering projects. After shot blasting, the substrate reveals the concrete structure, achieving a clean and solid condition. Structural defects on the top slab are also easily exposed, facilitating waterproofing before application. When applying non-curing rubber asphalt waterproofing coatings on the shot-blasted substrate, their self-healing and creep properties can effectively fill cracks on the top slab. Furthermore, a full-contact waterproofing layer is formed between the coating and the substrate. It is important to note that shot blasting should be combined with project scheduling, and ideally, the waterproofing should be applied the same day as the shot blasting to prevent secondary contamination of the top slab.
2) Handling of expansion joints
This project's original top plate design was a flush joint (Figure 8), which was later modified to a staggered joint with a deformation seam (Figure 9). Regardless of the type of deformation seam, it involves the sealing of the waterproof layer between the top plate flush joint and the side wall upright joint. Since both areas use a composite approach of coatings and materials, the compatibility is good. During construction, the overlap width between the top plate and the side wall should be at least 250 mm (Figure 10).
3) And the jointing at the side wall sections.
Regarding the joint between the wall and the side wall, the focus is on ensuring a tight closure with the expansion joint, as previously described. This project includes adding an additional reinforcement layer to the underside of the top plate, extending it to the side wall. The top plate and the side wall are both 250 mm wide, and utilize a 2.0 mm thick non-curing rubber asphalt waterproofing coating and a polyester non-woven fabric reinforcement. The waterproofing layer formed by the non-curing coating and modified asphalt rolls extends down to 250 mm on the side wall.
4) Seams in rolled materials
The treatment of projecting structures such as feed ports, is identical to that used by Minjian Waterproofing. The waterproofing is raised 500 mm above the ground, and the joint is fixed and sealed using a compression strip. A detail that is often overlooked is that the upper end of the compression strip should have a certain width to allow for the width of the sealant. The sealant should also be a modified asphalt sealant that is compatible with asphalt rolls.
3. Conclusion
3.1 Like other underground comprehensive pipe gallery waterproofing and other waterproofing projects, the key to waterproofing lies in the details, including expansion joints, construction joints, and sealing of overlaps and seams between materials. These areas must be given high priority in terms of waterproofing design and construction.
3.2 Waterproofing works occupy a small percentage in underground comprehensive corridors, but are a crucial and specialized component. Effective communication and coordination among all parties are essential during design and construction to ensure the quality of the waterproofing works.
3.3 Waterproof materials are primarily used as a foundation in waterproofing projects, with a greater emphasis on the adaptability and durability of the waterproofed areas. The success of a waterproofing project largely depends on the construction phase, and meticulous workmanship by specialized teams can minimize potential issues, particularly at critical junctions.
3.4 Strictly implement the "three-check and control" system for construction projects, with self-inspection being of paramount importance. This is crucial for controlling construction quality.
The article was published in "China Construction Waterproof" in 2019.