Abstract: With the increase in the service life of construction projects, quality problems of foundations are gradually exposed. Reinforcing the foundations of existing buildings is a good solution to such problems. Based on the author's years of construction experience, this paper analyzes the causes and proposes solutions for reference.

　　The foundation is a key part of a building. If quality problems occur in the foundation of an existing building, or if the existing building is added with stories or expanded, the existing foundation needs to be reinforced. Therefore, correctly analyzing the reasons for reinforcement and reasonably determining foundation reinforcement measures to ensure the quality of the building is an arduous task faced by construction management personnel.
　　I. Investigation must be done before foundation reinforcement
　　Before reinforcing the foundation of an existing building, the hydrogeological conditions at the construction site must be understood. A foundation investigation report can reflect the site's hydrogeological conditions in detail and provide quality assurance for foundation reinforcement construction. Without a foundation investigation report, the construction personnel's judgment of the site's hydrogeological conditions may deviate from the actual situation, leading to wrong construction decisions.
　　To do a good job in foundation investigation, it must be clear that the purpose of the investigation is to provide true geological data to facilitate architectural design. Then, according to the building's use requirements and the site's characteristics, the drilling depth should be determined to ensure that the bottom of the hole sits on the designed bearing stratum. If the drilling depth is too shallow, it will affect the calculation of foundation bearing capacity and foundation settlement. Therefore, an appropriate drilling depth must be determined. In addition, it is necessary to ensure that the number of investigation pits and boreholes is sufficient to fully express the characteristics of the soil layers, avoid quality accidents, and prevent increased project costs.
　　II. Reasons for foundation reinforcement of existing buildings
　　(1) The reasons for foundation reinforcement of existing buildings include insufficient bearing capacity of the building foundation, the impact of adjacent new building construction, lowering of the groundwater level, unqualified pile foundation quality, too few investigation points in foundation surveying, and incorrect analysis of soil layers. These factors cause cracks in building walls, foundation settlement, and other quality problems, making the building unable to meet functional requirements and seriously threatening user safety. If the existing building is demolished and rebuilt, huge funds will be wasted, which is very uneconomical in terms of economic benefits. Adopting foundation reinforcement not only can remedy foundation problems but also save costs. Therefore, many construction units use foundation reinforcement to solve this problem.
　　(2) Reconstruction, expansion, and story addition of existing buildings increase the superstructure load, and the original foundation bearing capacity cannot bear the new load after reconstruction, expansion, or story addition.
　　III. Measures for foundation reinforcement of existing buildings
　　In the construction of foundation reinforcement for existing buildings, according to the conditions of the existing foundation, reinforcement methods such as the pile-type underpinning method and the beam enlargement method can be used.
　　1. Pile-type underpinning method
　　Scope of application of the pile-type underpinning method: When reinforcing the foundation of an existing building, the pile-type underpinning method is more suitable for foundations of sandy soil, plain fill, clayey soil, miscellaneous fill, and loess.
　　Classification of the pile-type underpinning method: The pile-type underpinning method includes three types: pier-type pile underpinning, pit-type static pressure pile underpinning, and grout filling method.
　　(1) Pit-type static pressure pile underpinning
　　Pit-type static pressure piles are mainly used for the reinforcement of foundations of sandy soil, plain fill, miscellaneous fill, and loess.
　　1) Before construction of pit-type static pressure pile underpinning, first determine the pile positions according to the superstructure load on the foundation to be underpinned, try to avoid weak wall parts such as doors and windows above, and investigate the groundwater level at the bottom of the pile foundation. If the groundwater level is too high, construction must be carried out after dewatering.
　　2) During construction, the pit should be excavated near the inner or outer side of the building to be underpinned and reinforced, so that the pit is as close as possible to the building. If the soil of the pit wall is soft soil, the pit wall needs to be supported, and then a horizontal guide pit is excavated below the base.
　　3) After placing the first pile section into the guide pit, install the load-measuring device and jack, and press each pile section in with the jack. Determine the length of each pile section according to the jack stroke; the pile body can be precast reinforced concrete square piles or steel pipes. During construction, technicians must record the pile settlement value and pressing force, and correct the pile verticality at any time.
　　4) When the pressing force reaches 1.5 times the standard value of the single pile vertical bearing capacity, the corresponding pile penetration depth meets the design requirements, and the jack can be removed. If steel pipe piles are used, concrete can be poured into the pipe and then cast integrally with the existing foundation.
　　(2) Pier-type pile underpinning method
　　Pier-type piles are mainly used for the reinforcement of various foundations such as reconstruction of existing buildings and bridge projects, as well as cracking caused by uneven foundation settlement. They have the characteristics of simple construction, low cost, and no obstruction to the use of the building during construction.
　　When using the pier-type pile underpinning method for reinforcement, first set a pier-type pile foundation under the foundation of the existing building. The soil at the base must be hard and dense, and then place the prepared reinforcement cage into the pier-type pile foundation. When placing the reinforcement, care must be taken not to insert the end of the reinforcement cage into the soil of the pit wall. After placing the reinforcement cage, perform foundation grouting. Grouting construction must be carried out in two stages, and the grouting pressure must be controlled. The pressure value for the first stage can be set at 0.5 kPa, and for the second stage at 2 kPa. The second-stage grouting should be carried out when the grout injected in the first stage just begins to set. Before construction with this method, the groundwater level at the base must be investigated. If the groundwater level is below the base, this reinforcement method can be used. Otherwise, it cannot be used.
　　(3) Grout filling method
　　The grout filling method is mainly used for the reinforcement of strip foundations of existing buildings on cohesive soil, artificial fill, and muddy soil.
　　During construction, under the existing foundation, at certain intervals, excavate short pits about 1 m long and slightly wider than the existing building strip foundation, with depth reaching the designed bearing stratum, and pay attention to supporting the pit walls. After the pit is excavated, pour concrete immediately, stopping when the concrete is 7 cm from the base of the existing building. After 24 h, when the concrete has fully settled, fill the gap between the new and old foundations with cement grout mixed with accelerator and expanding agent. Note that the cement should be ordinary Portland cement or slag cement with a grade higher than PS325.
　　2. Concrete jacket reinforcement method
　　When cracks occur in the foundation of an existing building due to insufficient bearing capacity, the concrete jacket method is generally used to reinforce the foundation.
　　When using the concrete jacket method for reinforcement, widen each side of the foundation by 20 cm, and the reinforcement added in the widened part should be welded to the main reinforcement in the existing foundation. Before pouring concrete, the existing foundation should be roughened and cleaned, and crushed stone or gravel should be spread on the original foundation to a thickness of about 15 cm. The poured concrete should be mixed with a slight expanding agent, and the concrete strength should not be lower than C30.
　　IV. Precautions for foundation reinforcement
　　(1) After the foundation of an existing building has been in use for a period of time, the bearing capacity of the foundation soil will change significantly compared with before. Therefore, before foundation reinforcement, the existing foundation should be reviewed, and technical parameters such as foundation bearing capacity should be determined through on-site investigation. If on-site investigation is inconvenient, reference can be made to the foundation bearing capacity of neighboring houses, and the bearing capacity value of the existing building foundation can be appropriately increased. For expanded foundations, the bearing capacity after the foundation base area is enlarged must be checked. If the check result does not meet the requirements, the foundation must also be heightened.
　　(2) To enable the reinforced foundation to effectively bear the superstructure load, the following measures must be taken to ensure that the new and old foundations are tightly connected.
　　1) The surface of the existing foundation must be roughened, washed with water, and then coated with a thin layer of plain cement slurry.
　　2) The reinforcement in the newly added foundation should be firmly welded to the reinforcement in the existing foundation, or short rebars can be planted in the existing foundation and welded to the reinforcement of the newly reinforced foundation.
　　3) When excavating the existing foundation, it is not allowed to dig a trench along the entire length of the foundation; excavation should be done in sections at intervals to avoid exposing the foundation soil for too long and causing foundation settlement.
　　4) For the widened part of the foundation, a cushion material with high compressive strength should be laid underneath and compacted densely.
　　V. Conclusion
　　In summary, foundation reinforcement of existing buildings is a very complex technical task. During the construction of foundation reinforcement for existing buildings, we must carefully analyze the specific problems according to the actual site conditions, reasonably determine the reinforcement scheme for the existing building foundation, and achieve the purpose of solving problems and saving costs.