In the process of building structure construction and use, the quality problems of foundation and foundation engineering cause cracks in the building wall and ground, which affect the use, make people feel unsafe and even make the building collapse. In recent years, there is an upward trend. According to statistics, the quality problems of foundation and foundation engineering account for 2 1% of the total number of accidents. In the design and construction of building structures, it is generally believed that the most difficult to control is not the superstructure, but the foundation and foundation engineering of the project. Although the superstructure of a building is extremely complex, it is widely used in computers. Today, they are basically predictable and mastered in design and construction. Generally speaking, people can only get a little information from the test of several borehole soil samples before design, and only after construction can they know the surface information from the borehole results at the bottom of the trough. As for the deeper and more comprehensive situation, they can't fully grasp it, and often handle it by experience, which will lead to mistakes and even damage to the building after completion. Moreover, the foundation is an underground concealed project, which is difficult after the completion of the construction project.
Accidents in foundation engineering may be caused by the interaction of factors such as survey, design, construction, manufacture, installation and use. Among these factors. Some factors lead to unexpected accidents. Other factors may lead to consumption accidents. From a safety point of view, sudden accidents are dangerous. Therefore, it is more universal to study and discuss the causes of foundation engineering accidents. Local and empirical, the lessons learned from its analysis are the knowledge wealth that construction engineers and technicians need to accumulate continuously. It is a subject worthy of attention to take effective measures to prevent accidents in basic projects.
Second, the causes and prevention methods of foundation and foundation engineering accidents
(1) Accidents caused by engineering geological exploration errors The engineering investigation report should fully reflect the engineering geological and hydrogeological conditions of the construction site to prevent engineering accidents of the foundation and foundation. First of all, we should have a comprehensive and correct understanding of the engineering geological and hydrogeological conditions of the site. The key is to do a good job in engineering investigation, and to reasonably determine the purpose and task of engineering investigation according to the characteristics and building conditions of the construction site. Exploration is an important sequence of design, which can never be ignored or done casually without considering whether it is applicable. Especially for complex and weak foundations, we should be more cautious. Even if it is a single-story general building, we should explore it.
Accident example: A warehouse building in a city is a two-story building with a straight plane, with a length of 47.28m from east to west, a width of 10.68m from north to south and a height of 7.50m m. The central part of the warehouse is a stairwell, and there are two large rooms in the east and west, with a length of 10.89m and a width of10.20 m. This building has been used for one year. Cracks were found in the second floor wall on the west side of the warehouse. Since then, the number of cracks has increased and the width of cracks has expanded. According to detailed investigation and statistics, there are 33 large cracks, some of which are over 1.80m in length and over 10 ~ 30 mm in width, and there are many ground cracks. Six years later, it was found that the crack was 3.20m long and 8 ~ 10 mm wide, and the inside and outside were connected. It shows that the settlement of warehouse has developed for more than 6 years.
Cause analysis of the accident: The original survey error is the main cause of the accident. Although the original survey report paid for drilling data, only the inclined holes 4 1# and 46# in the warehouse are 5. 10m and 5.35m deep respectively, and the other five holes are only 2m, far less than the depth of the foundation compression layer. What's more noteworthy is that there are two holes that pass through the organic soil and peat layer, but they are not recorded or explained in the report. It is simply suggested that the calculated foundation strength fk= 100KN/M2, which is the root of the serious quality problems in this warehouse. For this rough survey report, the designer did not ask for a supplementary survey. In addition, according to the code, the length-height ratio L/H should be less than or equal to 2.5 for residential buildings with three or more floors. Although this example is a two-story masonry structure, the length-height ratio L/H=47.28/7.5=6.3, and the secondary value "25" causes the whole building to be too small and the ability to adjust the uneven settlement of foundation is too weak. The designer did not take effective structural measures to strengthen the rigidity of the superstructure, which is also an important reason for the wall cracking.
Lessons should be learned: first, the engineering investigation work is rough; Second, improper foundation selection and treatment methods failed to make the house located on a relatively unified natural or artificial foundation; Third, the overall stiffness of the superstructure is weak. These three lessons are often said: "the situation is unknown, the determination is not great, and the method is not good."
In addition, attention should be paid to the selection of drilling depth during exploration. Because the drilling depth must meet the design requirements, if it does not meet the design requirements for compression thickness, or is not greater than the soil layer where the pile is located, it will be impossible to correctly calculate the settlement of the foundation or the correct bearing capacity of the pile, and it will also fail to meet the foundation design requirements. Therefore, the appropriate drilling depth must be determined according to the design requirements. If the number of exploration is insufficient, the number of drilling holes and exploration pits is small, and the drilling depth is not enough, the unevenness and inconsistency of bedding can not be expressed, which may cause warping and bending of buildings, resulting in cracks, causing harm and waste. ...
(2) The accidental foundation bearing capacity caused by excessive earth pressure at the bottom of building foundation exceeds the foundation bearing capacity is a key index of building foundation design. All kinds of foundations have a certain limit to bear the load from the foundation. Beyond this limit, the first thing that happens is that the building has a large uneven settlement, which leads to the cracking of the house. If it exceeds this limit too much, it may slide or sink sharply due to the shear failure of the foundation soil, leading to the collapse or serious damage of the house. The following is a world-famous accident example: Tescon barn in Canada, with a plane distance of 59.44m, a height of 3 1.00m and a width of 23.47m It is a cylindrical silo with a volume of 5 silos per row 13. The granary foundation is an integral reinforced concrete raft foundation with a thickness of 6 1cm and a buried depth of 3.66m,191year, and the construction was started and completed in the autumn of 19 13. The self-weight of the granary is 20,000 t, which is equivalent to 42.5% of the total weight after being filled with grain. Since the end of 2003, the granary has been filled with grain, which is carefully distributed in June 5438+10. When the granary is filled with 3 1822m2 of grain, it is found that the granary sinks, and the settlement in one hour reaches 3 1.5cm. The structure tilted to the west, and within 24 hours, the whole barn fell, with a dumping degree of 26.53. The west end of the barn sank 7.32m and the east end rose1.52m.. According to the test results of foundation trench excavation of adjacent projects, the bearing capacity of foundation calculated in this design is 352KPa, which has been applied to this granary. The site of the barn is located in the glacial lake basin, and the surface of the foundation is a modern sedimentary layer with a thickness of 3 m; Below the surface is a glacial sedimentary clay layer with a thickness of 122m. 1952. Some boreholes were drilled at a distance of 0/8.3m from the barn/kloc-. Measured from the undisturbed clay model of borehole, the average water content of clay layer increases from 40% to 60% with the increase of depth. The unconfined compressive strength decreased from 1 18.4 KPa to 70.0 Kpa, with an average value of100kpa; . The average liquid limit ωl= 105%, plastic limit ωp=35%, and plastic index as high as IP=70. Tests show that this layer of soil is highly colloidal and plastic. According to Karl Terzaghi formula, the bearing capacity f of foundation is calculated. If the average unconfined compressive strength of clay layer is 100 Kpa, the bearing capacity f of foundation is 278.6 KPa, which is less than the basement pressure of 329.4 KPa when the granary foundation is destroyed. If calculated by Qu Min =70.0 KPa, it is f= 193.5 KPa, which is far less than the actual foundation force when the granary foundation slides.
The main cause of the accident: The barn in Tezcone was damaged because the barn was designed blindly without prior investigation, and the design load far exceeded the bearing capacity of the foundation soil, which led to a serious accident of sliding damage of the whole foundation of the barn.
Experience and lessons: The overall shear failure accident of foundation leads to serious engineering accidents, which must be highly valued by civil engineers and technicians. Designers should be cautious about the recommended value of foundation bearing capacity provided by the engineering investigation report and strictly calculate the actual earth pressure of foundation. If they have doubts about the recommended value of the investigation report, they can do load tests to verify it. When building large and medium-sized projects on natural foundations, the constructors should check the rationality of the design foundation bearing capacity. Once the foundation has a large settlement or inclination, it must be stopped immediately and studied with the exploration, design and use units. Take necessary measures to prevent catastrophic damage to foundations and buildings.
(three) accidents caused by the influence of old structures such as culverts and ancient tombs in the foundation. After foundation trench excavation, many local anomalies may be encountered. For example, there are abolished structures such as culverts, ancient tombs and old foundations. In the foundation soil, culverts and ancient wells are often filled with loose construction waste or silt soft soil, forming local weak parts, which may cause serious local subsidence of the foundation. Cause the upper wall or structure to crack; In case of hollow structures such as ancient tombs and air-raid shelters, it may cause collapse accidents; As for the old foundation, garbage septic tank and other structures, they are often much more solid than the surrounding natural foundation, forming a sudden change in soft and hard, which will also cause the superstructure to crack. Therefore, it is very important to find local anomalies during planing and inspection.
Accident example: The cast steel workshop of a factory is 66.75m long and 39m wide, with the same height and three spans bent. The column foundation is a reinforced concrete cup foundation, with a general burial depth of 2 m, a foundation compaction dry density ρd≥ 16g/cm3, and a compaction impact depth of 0.3 ~ 0.4m The main structure of the factory building is completed. Structural cracking accident was found before the crane was installed: the ground on the east side of the building cracked, the crack length was 15m, and the maximum width was 50 ~ 60mm. The east side of the south wall is cracked, the maximum width of the crack is 20mm, and the reinforced concrete ring beam is also pulled apart, with more than 20 cracks. 20mm away from the southeast corner of the factory building. Six foundations in the southeast of the factory building sank. The average sinking speed is about 3 ~ 4 mm per month. Cause analysis of the accident: First, the burial exploration depth was not 6 ~ 7 m according to the design requirements. The actual depth of tomb exploration is only 2 m, and supplementary exploration was carried out after the accident. Within the range of 10 column base in the southeast corner, 1 1 wooden coffin was found under or beside the foundation. The top of the wooden coffin is about 1.5 ~ 2.0m from the bottom of the foundation. Some wooden coffins are empty, while others are filled with mud. Second, the factory has not been investigated in detail. According to the exploration data of foundation pit near the factory building in the initial exploration stage, the foundation soil is designed blindly according to the bearing capacity 150 kPa. The actual foundation soil is not natural sedimentary soil, but filled soil, and the bearing capacity of foundation soil is only 100 ~ 120 kPa. A little experience: In foundation construction, old structures such as underground ditches and ancient tombs are often encountered. The most important thing at this time is to find a way to figure out the situation. In addition to the necessary investigation and excavation, it is very necessary to consult local people and workers with an open mind and conduct subtle investigation and research. Then we can make practical treatment methods.
(four) accidents caused by corrosion and piping of building foundation.
1, when there is groundwater in the building foundation and the following conditions are met, corrosion and piping accidents may occur:
(1) Karst caves may appear in limestone areas due to long-term groundwater. In the karst cave development area, foundation dissolution will occur.
(2) When the particle size of mountain residual soil or residual soil is quite different. Corrosion or piping may occur under the action of groundwater flow.
(3) If the foundation soil is poorly graded and the groundwater velocity is high, fine particles in the foundation soil may be washed away, resulting in piping.
All projects built in the above areas should be carefully investigated in engineering geology. If the foundation has the above-mentioned corrosion problems, another site should be chosen, because the measures against the above-mentioned corrosion accidents are not easy and the cost is very high.
Accident example: Alabama water purification plant in the southeastern United States was built beside a hill, and a foundation trench with a depth of 6m was dug to build a sedimentation tank and a filtration building. One month after the factory was completed and put into use. One morning, the operator heard a loud noise. With a series of rumblings, like a long-distance cannon, the filter building was violently shaken and cracked, and half of the building was tilted from top to bottom.
Cause analysis of the accident: the foundation soil of the water purification plant is residual soil, the bedrock is limestone, and cracks are developed. During the construction, the construction unit accidentally broke the water main with a diameter of 457, and as a result, the large water tank with a capacity of 226 was emptied, making a lot of water seep into the ground. After the local foundation is soaked in water, the small particles are washed away by the water due to the disparity in the particle size of residual soil, resulting in corrosion, piping and other accidents, resulting in a large karst cave at the bottom of the sedimentation tank, and a large gap between the sedimentation tank foundation and the foundation, with a width of 15 ~ 30. The water purification plant was completely destroyed and unusable.
Lessons to be learned: Civil engineering technicians should realize that groundwater is closely related to the design scheme, construction method, construction period, investment and use of construction projects. If groundwater is not handled properly, engineering accidents may occur.
2, the main effects of groundwater are:
(1) Foundation depth-The foundation should be buried above the groundwater level and below the frozen soil thickness, which is related to the capillary water in the soil.
(2) Construction Drainage-When the foundation is buried below the groundwater level, the foundation trench excavation and foundation construction must be drained. If the drainage is not good or the foundation groove is trampled, it will cause hidden dangers.
(3) Groundwater fluctuation-the decline will cause uneven settlement of buildings, and the rise will cause soft flowers in clay layer, subsidence of collapsible loess and water absorption and expansion of expansive soil layer.
(4) Dissolution and piping-The existence of groundwater in limestone area will cause dissolution. In the area with confined water, if the foundation trench digs out the water-resisting layer above the confined water, there may be a lot of water gushing to soak the foundation.
(5) Floating of hollow structures-The buried depth of pools, oil tanks and open-pit underground works exceeds the groundwater level for a long time, which may float and affect the use.
Third, the conclusion
When a major foundation accident occurs, the most important thing is to analyze the causes of this quality accident. Only by correct analysis can we find the original crux of the accident. Conduct fair arbitration and clarify the responsibility for the accident; Only by correct analysis can we find the lessons to be learned in the future and turn negative factors into positive ones; Only by correct analysis can we formulate corresponding preventive measures to nip in the bud. Errors in structural design, construction technology and use are mostly subjective. Errors can be avoided when the provisions and corresponding requirements of exploration, design and construction standard documents are strictly observed. Engineering designers should pay attention to the following aspects when designing foundations:
First, the foundation design should be based on the use requirements of buildings, structural types and soil conditions of the site, combined with the specific conditions of the site. On the premise of applicability and economy, it is necessary to ensure that the main load-bearing structure of the building will not crack or be damaged during normal use.
Secondly, foundation engineering accidents are a common problem in current construction projects. In order to nip in the bud, the relevant personnel should "suit the remedy to the case" according to the foundation conditions, and do a good job in exploration, overall layout, foundation type selection and design calculation.
Third, the relevant personnel should not only study the existing engineering accidents as a "lesson from the past", but also learn from the existing successful experiences and methods to continuously improve the technical level and ensure the engineering quality.
Fourthly, in the earthquake area, the foundation accident of expendable foundation can not be ignored and should be repaired in time, otherwise it may turn into a catastrophic engineering accident under the action of earthquake.
Fifth, it is suggested that laws and regulations to prevent accidents in basic projects be compiled so that relevant parties can attach importance to this work.
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