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General measures to reduce uneven settlement of building foundations
2019-11-26
The foundation of a building is the soil layer that directly bears the load of the structure. The foundation should have good stability and settle uniformly under load to ensure even settlement of the building. If the soil distribution of the foundation is uneven and not properly treated, it will cause uneven settlement, affecting the normal use and safety of the building. In mild cases, this may cause cracks in the upper walls or tilting of the house; in severe cases, it can lead to building collapse, endangering life and property safety.
The foundation of a building is the soil layer that directly bears the load of the structure. The foundation should have good stability and settle uniformly under load to ensure even settlement of the building. If the soil distribution of the foundation is uneven and not properly treated, it will cause uneven settlement, affecting the normal use and safety of the building. Minor effects include cracking of the upper walls and tilting of the house, while severe cases may lead to building collapse, endangering life and property.
The main causes of uneven settlement of foundations are: first, poor accuracy and reliability of geological survey reports. Some surveys are not conducted according to regulations, such as inaccurate hole placement or insufficient depth during probing; some copy data from adjacent buildings. These issues easily cause errors in analysis, judgment, and design by designers, potentially leading to uneven settlement or structural damage. Second, design problems. Excessively long buildings, complex shapes with many recesses and corners, lack of settlement joints in appropriate locations, and insufficient overall stiffness of the foundation and building can all cause excessive uneven settlement. Third, construction issues. Failure to carefully inspect excavation pits, disturbing foundation soil before foundation construction, storing large amounts of building materials or soil around completed buildings, and poor masonry quality can all cause uneven settlement after construction. Based on work experience, the author proposes the following targeted suggestions to reduce uneven settlement of building foundations.
Construction Measures
Ensure the authenticity and reliability of survey reports. Geological survey reports are the main design basis for designers and are a specialized science that cannot tolerate any falsification. Therefore, it is necessary to improve the professional skills, political quality, and professional ethics of geological survey personnel, strengthen their sense of responsibility, and ensure surveys are conducted according to regulations and actual conditions, so that the reports are authentic and reliable.
Keep the building shape simple. For houses built on weak foundations, the floor plan should be as simple as possible, avoiding recesses and corners, because the main foundation parts intersect and stress concentrates there. Complex structures are prone to larger settlements.
Set settlement joints. Settlement joints should be set at plane turning points, places with significant height or load differences, where foundation soil compressibility differs significantly, and when the building's length-to-height ratio is too large. Settlement joints should vertically separate the building from foundation to roof with a certain gap width to prevent collisions caused by uneven settlement.
Maintain clear spacing between adjacent building foundations. When constructing a new building next to an existing one, or when there are significant height or load differences between adjacent buildings, a certain gap distance should be left to avoid additional settlement caused by overlapping foundation pressures.
Control building elevation. The original elevations of building units, underground pipelines, industrial equipment, etc., will change as the foundation settles. Therefore, measures can be taken in advance to raise the indoor floor and underground facilities' elevations according to estimated settlement amounts.
Structural Measures
Enhance the stiffness of the superstructure. When the overall stiffness of the superstructure is high, it can improve uneven foundation settlement. Even if the foundation settles somewhat, large cracks will not form. Conversely, if the superstructure stiffness is weak, even small foundation settlement can cause cracks. Therefore, the overall stiffness of the building must be strengthened in design.
Reduce additional pressure on the foundation base. Reducing additional pressure on the foundation base can reduce foundation settlement and uneven settlement. Reducing the building's self-weight can reduce foundation pressure and is an important way to prevent and mitigate uneven settlement. Lightweight materials such as porous bricks or other light walls can be used; lightweight structures such as prestressed reinforced concrete, light steel structures, and various lightweight spatial structures can be selected; foundation types with lighter self-weight and less soil cover, such as shallow wide foundations, foundations with semi-basements or basements, and indoor raised floors, can be chosen. Additionally, using larger foundation base areas to reduce additional pressure can decrease settlement.
Strengthen foundation stiffness. The overall integrity of the foundation plane should be enhanced by setting necessary strip footings for connection and adding reinforced concrete beams where soil or load changes occur. Depending on the foundation and building load conditions, reinforced concrete ribbed strip footings, strip footings under columns, raft foundations, box foundations, pile foundations, etc., can be used. These foundations have high overall stiffness, can expand the bearing area, and adjust uneven settlement.
Foundation design should control deformation values and must verify the final settlement and eccentricity of the foundation. The final settlement should be controlled within the limits specified in the "Code for Design of Building Foundations." When natural foundations cannot meet the settlement deformation control requirements, technical measures such as installing prefabricated reinforced concrete short piles must be taken.
Construction Measures
If during construction the foundation soil is found to be too hard or too soft and does not meet requirements, or if voids, dry wells, hidden channels, etc., are found, local treatment should be carried out based on the principle of making settlement as uniform as possible to reduce uneven foundation settlement.
During excavation, do not disturb the foundation soil. Usually, about 0.2 meters of soil is left at the bottom and removed manually during cushion construction. If the bottom soil is disturbed, it should be excavated and backfilled with sand or gravel and compacted. Attention should be paid to the impact of piling, well-point dewatering, and deep foundation excavation on nearby buildings.
When a building has parts with different heights and weights, the high and heavy parts should be constructed first, allowing them to settle before constructing the low and light parts, or the main building should be constructed first, followed by auxiliary buildings. This can reduce some settlement differences.
Around small and lightweight completed buildings, large amounts of building materials and soil should not be piled up to avoid additional settlement caused by ground loading.
Due to the complexity of foundation distribution and the limited number of exploration points, foundation pit inspection should be taken seriously. Potential risks of uneven settlement in foundation soil should be discovered and eliminated as much as possible before foundation construction to compensate for deficiencies in engineering exploration.
In industrial and civil buildings, it is necessary to accurately monitor building settlement and promptly detect adverse sinking phenomena to take measures ensuring safe use of the building and provide data for future reasonable design. Therefore, settlement monitoring must be conducted during construction and after commissioning.
Overall, the cost of the foundation generally accounts for about 20% to 25% of the total project cost. For high-rise buildings or when foundation treatment is required, the cost is even higher. Once a quality accident occurs in the foundation, it has a significant impact on the building, and the repair work is much more difficult than that of the superstructure, sometimes even irreparable. However, if sufficient attention is paid in design, construction, and other aspects, and practical measures are taken, uneven settlement can be effectively prevented and controlled, ensuring the quality and safety of the construction project.
