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Construction methods for underground waterproofing projects I. Construction of structurally self-waterproofing concrete 1. Materials and mix proportions for waterproof concrete: The type of cement to be used should be selected in accordance with the design requirements, and its strength grade should not be lower than 32.5. The natural gradation of the coarse aggregate should be appropriate, with particle sizes ranging from 5 to 40 mm; the clay content must not exceed 1%. Requirements for the mix ratio of waterproof concrete: The required impermeability strength in the trial mixing should be 0.2 MPa higher than the design value ； The cement content shall be not less than 300 kg/m3; when active admixtures are used, it shall be not less than 280 kg/m3 ； The sand ratio should be between 35% and 45%, while the water-cement ratio should not exceed 0.55 ； The slump should not be greater than 50 mm. 2 Key construction points ⑴ The formwork for waterproof concrete works should be flat, with tight joints to prevent leakage of concrete. It is generally not advisable to use bolts or wires to pass through concrete walls in order to secure formwork; when it is necessary to use bolts to do so, waterproofing measures must be taken. If tool bolts are used, a water-stop steel plate should be welded in the middle of the bolt, and shims should be placed at both ends of the bolt. ① Method using tool bolts: Use tool bolts to fix and tighten the waterproof bolts. When removing the formwork, remove these tool bolts, and then seal the bolt grooves tightly with sealing material and polymer cement mortar. Method for using bolts as anchors: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/e8c12bddd3358e62c8da29173f7420c0.jpg ② Method of using bolts with plugs: Grooves are created around the bolts on both sides of the structure; after the formwork is removed, the bolts are cut to the bottom of these grooves, and then the grooves are filled using expanded cement mortar. Tool bolts are used for the basement walls: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/c7b5f71aa1fda81120b2dad84f57ad46.jpg ③ Method of using embedded sleeves with welded water-stop rings: Sleeves are installed around the tie bolts; these sleeves also serve as supports, and shims are placed at both ends of the sleeves. After demolding, the bolts are removed, and the interior of the sleeve is filled tightly with expanded cement mortar; the depressions left by the shims are also filled using the same method. Schematic diagram of embedded sleeve support: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/ef3ca7c008fe0ded004889a6eeb9ed80.jpg ⑵ To prevent water from flowing along the rebar, the protective layer around the rebar on the water-facing side must be at least 50 mm thick. The rebar in the base slab must not come into contact with the concrete cushion; rebar should not be fixed to the formwork using nails or wire. It is strictly prohibited to use rebar as padding for the protective layer, in order to prevent water from seeping in through the rebar. ⑶ Before pouring the waterproof concrete, there should be no bleeding or segregation, and the free fall height should not exceed 1.5 m. ⑷ Mechanical vibration should be used for waterproof concrete to ensure thorough compaction. ⑸ For each 500 m3 of continuously poured concrete, 1 set (6 specimens) of impermeability test specimens shall be retained, with no less than 2 sets per project. ⑹ Waterproof concrete should be cured naturally, with a curing period of not less than 14 days. 3 Waterproofing construction measures ⑴ Construction joints Construction joints are weak points in terms of waterproofing, and as few as possible should be left during construction. The concrete for the base slab should be poured continuously, and no vertical construction joints shall be left in the walls. Horizontal construction joints in walls should not be placed at the points where shear and bending forces are greatest, or at the junction of the base slab and the wall. The lowest horizontal construction joint should be at least 200 mm away from the surface of the base slab, and at least 300 mm away from the edges of any openings passing through the wall. Joint types for construction joints: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/3dba04fd4a871a69306f9f4afbe376d5.jpg ⑵ Treatment of pipes passing through walls: Water supply and drainage pipes, heating pipes, and cable conduits that pass through the exterior walls of basements must be properly waterproofed; otherwise, water leakage is very likely to occur along the pipe roots. To ensure ease of waterproofing work and pipe installation, the pipes should be placed 25 cm away from interior wall corners or protruding areas, with a spacing of more than 30 cm between pipes. The direct-burial type is suitable for pipes with a diameter of <5 cm that pass through walls: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/c40c62e93a673ecf39ae54b81088f64b.jpg. The sleeve type is suitable for pipes with a diameter of >5 cm that pass through walls: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/f6b50da73c730a704cd02f3c758aaaf9.jpg. The wall-mounted box type is suitable when multiple pipes need to pass through a wall simultaneously: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/aa0e0d826075903a7c717b370580cacc.jpg. When pouring concrete, it is not allowed to strike or vibrate the pre-installed pipes that pass through the walls; likewise, when backfilling soil outside the wall, it is not allowed to press or hammer on the pipes that extend outside the wall, to prevent leaks due to damage to the waterproofing measures. ⑶ Embedded parts: The iron components embedded in the inner walls or floor of underground spaces are fixed using hangers or specialized tools to prevent water from seeping into the space through those components. Embedded iron components are subject to significant stress; in order to prevent disturbance of the surrounding concrete and damage to the waterproof layer, the thickness from the end of the embedded component to the outer surface of the wall must be at least 25 cm. If this thickness is not achieved, local reinforcement should be applied. Bolts pre-embedded in the basement floor: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/4a642006ab65ebee2d583bb889a66be9.jpg ⑷ For expansion joint water stop strips, embedded rubber (plastic) water stop strips are currently the most commonly used type. Embedded rubber (plastic) waterstop: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/1ca9bdff55304a13aca80b4f7f265c91.jpg Removable rubber waterstop for expansion joints: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/adc15d4246e94559b1418e872146ccee.jpg Adhesive neoprene sheet waterstop for expansion joints: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/1df0197d6fb291278ff78dc9ae55bd38.jpg ⑸ Post-cast strips: For post-cast strip 1, the waterproof layer remains continuous; an additional layer is added along with an external waterstop: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/a58360e2b95b34e78afc472622c16b70.jpg For post-cast strip 2, the cushion layer is thickened locally and additional rebar is installed; differential settlement can cause the cushion layer to form a slope without tearing the waterproof layer: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/57bfbb77516810539522fda7059ee189.jpg For post-cast strip 3, the bottom slab can be thickened locally, with a waterstop also being installed. Since the thickness increase of the base slab generally does not exceed 25 cm, it is not appropriate to use embedded water stop strips; instead, external water stop strips should be used: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/d43b107cf3bf0d9b9830a3b410cfa00f.jpg II. Construction of a rigid waterproof layer using waterproof mortar A rigid waterproof layer is suitable for use as a waterproofing layer in underground brick and stone structures, or as a reinforcement layer in waterproof concrete structures. It has poor resistance to deformation, and therefore is not suitable for use in structures subjected to severe vibration loads, or in areas prone to corrosion, high temperatures, and repeated freezing and thawing. 1 Substrate preparation: Proper preparation of the substrate for a rigid waterproof layer is very important. This preparation involves cleaning, watering, scrubbing, and leveling, so as to keep the substrate surface moist, clean, smooth, solid, and slightly rough. Ribs and uneven areas larger than 1 cm should be chiseled into a gentle slope, washed clean with water, and leveled in layers using plain lime and cement mortar. Treatment of uneven concrete surfaces: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/ae4574a3f5428b7833e465462af49618.jpg For the honeycombed holes on the concrete surface, loose stones should first be removed, the surface should be washed clean with water, and then plain lime mixed with cement mortar should be applied alternately until the surface is level with that of the base layer. Treatment of honeycombing in concrete substrates: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/ab9f525b67f99e58d1615125587ec910.jpg For masonry constructed with lime mortar or cement-based mixtures, the joints should be carved to a depth of 1 cm, with right angles inside those joints. Cleaning the joints in brick masonry: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/cc7b0c3e1b993234ad026b554f0a8faa.jpg For concrete construction joints, a V-shaped groove should be created along those joints; after rinsing them with water, a base layer of plain mortar is applied, followed by compacting and leveling with cement mortar. Treatment of construction joints in concrete structures: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/1cf1d6bfd1a81654870d2b7d327289b9.jpg 2 Construction of cement mortar waterproof layer: The cement mortar waterproof layer is created by applying pure cement paste and cement mortar in layers in a cross-pattern; the number of application layers is determined by the design, with 5 layers being the most common approach. The first layer is a plain gray layer with a thickness of 2 mm. First, apply a layer of cement paste 1 mm thick, then use an iron trowel to spread it back and forth; the cement paste fills the pores in the surface of the base layer. After that, apply another layer of cement paste 1 mm thick as a leveling layer. Once applied, brush the surface of the cement paste once in sequence using a wet brush. The second layer of cement mortar, with a thickness of 6–8 mm, is applied after the first layer of cement mortar has begun to set; it is ideal for the cement mortar to be applied in a thin layer, accounting for about 1/4 of the thickness of the first layer. The above layers alternate. The construction joints of the waterproof layer should have a sloped, stepped shape, with the overlaps between these sections being arranged in layers according to the sequence of construction. The stubble should generally be left on the ground, with a distance of 20 cm from all corners. Treatment of joints in the waterproof layer: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/c4681a72a6d2fb2cac7b4795c00455c2.jpg Layered and overlapping application of the cement mortar waterproof layer: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/0ca19558838395b25f881da9497850e7.jpg The waterproof layer at internal and external corners should be applied in a rounded shape, with a diameter of 5 cm for internal corners and 1 cm for external corners. III. Construction of flexible membrane waterproofing layers: Flexible waterproofing layers are implemented using membrane materials. In current waterproofing projects for underground structures, polymer-modified asphalt waterproof membranes and synthetic polymer waterproof membranes are used. The disadvantage of flexible waterproofing layers is that it is difficult to repair them once leaks occur. The laying methods for constructing membrane waterproofing layers can be divided into two types, based on the sequence of construction relative to the underground waterproofing structure: the external application method and the internal application method. Construction of the external application method for basements: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/425c64435b44efbd70c03725a0e4b6d7.jpg 1 The external application method involves laying the vertical waterproofing membrane directly on the outer surface of the building’s exterior wall. Construction procedure: Pour the base layer → Build the permanent protective wall → Construct a 300mm high temporary protective wall → Apply a cement mortar leveling layer on the wall → Install an additional waterproof layer at the corners → Apply a waterproof layer to the base slab → Pour concrete for the base slab and walls → Apply a cement mortar leveling layer to the exterior walls of the waterproof structure → Carry out the installation of the vertical waterproof layer → Conduct inspection, then apply the protective layer. Method of overlapping membrane waterproofing layers: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/8a551f6aa92c5268716a2aef173502e0.jpg Method of joining membrane waterproofing layers: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/86709bd45ed3e60241c1faf9ac333b99.jpg The external protection with internal application method involves constructing the entire permanent protective wall after pouring the concrete base layer, and then laying the membrane on top of both the base layer and the permanent protective wall. Schematic diagram of the external protection with internal bonding method: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/6defaee2b4b7f4dc8fff9dd913658a14.jpg Panoramic view of the construction of the underground structure using the external protection with internal bonding method: http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/01a5ded8a746706ade2fde34bbe39de1.jpg http://img.civilcn.com/d/file/zhishi/gcgl/2020-01-13/e428c6641cc3a92178f68024c8f5f9d1.jpg