Anti-floating design of the pit of glass melting furnace

In recent years, the design of glass melting furnaces in glass factories has increased daily melting capacity. The original glass melting furnace is set in a pit, and it has been transformed into two glass melting furnaces or even three glass melting furnaces in a pit. The plan size of the kiln pit is getting bigger and bigger. The plan size of the kiln pit has been designed from about 45m×45m to 85m×50m. The maximum plan size of the kiln pit even reaches 120 m×60 m. Due to the requirements of the production process, the top elevation of the bottom of the furnace pit is generally about -10 m. In addition to the thickness of the bottom of the pit, the depth of the furnace pit reaches -11.5 to -12 m. Such a large and deep pit, if anti-floating design considerations or improper construction measures are considered, engineering accidents such as pit floating, floor swelling, floor cracking and water seepage are extremely likely to occur. For example, after the construction of the melting furnace pit of a glass factory in Guangdong, the furnace Before the equipment was installed, due to the sudden heavy rain, the pit as a whole floated. Due to the restriction of the backfill around the pit, the floor of the pit was raised 350 mm upward and wide cracks appeared on the side wall of the pit; another example is the melting of a glass factory in Fujian. After the completion of the construction of the kiln pit and before the installation of the kiln equipment, it was also in the rainy season. During the equipment installation, it was found that the pit floor continued to settle and cracks as the load of the regenerator increased, which seriously affected the masonry of the regenerator And the installation of the furnace. Problems like this happen from time to time, seriously affecting the progress of the project and equipment installation, and leaving hidden dangers for future glass production. This article will discuss the reasons for the floating accident of the furnace pit, the factors to be considered in the anti-floating design, and the countermeasures of the anti-floating design.
1. Reasons for the floating accident of the furnace pit
For some time, in the engineering accidents caused by water buoyancy in multiple furnace pits, the main problem is: some designers are not clear about the basic concept of pit anti-floating design, and some designers have an understanding of the nature and function of groundwater insufficient. Common problems in actual engineering include the following situations:
1) Insufficient understanding of the anti-floating water level, large deviations in the value of the anti-floating water level, ignoring the overall anti-floating calculation of the pit structure, and lack of attention to the anti-floating measures during the construction of the furnace pit, thinking that it has tens of thousands of tons of dead weight The pit loaded by the furnace cannot float.
2) The local anti-floating of the furnace pit lacks sufficient attention. In the process of pit design, only the strength, deformation and cracks of the pit floor, retaining wall and other structural components are checked, but the local floating resistance of the pit is not checked. The overall anti-floating ability of the furnace pit in the regenerator and the furnace column range may be relatively high, but other parts of the pit floor may cause partial anti-floating failure due to insufficient upper dead load, thus causing partial upper pits. Warped, tilted or damaged.
3) Lack of a correct understanding of the various working conditions of the melting pit in the melting section, lack of sufficient understanding of the construction process of the melting pit, the installation process of the melting equipment, the completion of the installation of the melting equipment, and the cold modification of the melting equipment. Awareness.
4) Cracks and leakage in the pit of the melting furnace are attributed to the effect of temperature stress or the quality of concrete pouring, and the force of ignoring the groundwater is far greater than the design load of the pit structure itself.
5) Water-proof curtains or combined support of piles, anchors, and shotcrete are used for deep foundation pit support, and there is impervious foundation soil (bedrock, hard clay) under the foundation floor of the pit, which is suitable for surface water caused by natural rainfall. The possible water buoyancy effect is not considered.
In addition, some designers did not make clear requirements on the anti-floating measures that should be taken during construction in the structural design instructions. Some on-site construction workers lacked understanding and attention to precipitation during the construction of the pit, did not take precipitation measures or did not reach the water level. Dewatering was stopped when the water level was controlled by the design, which eventually caused the entire furnace pit to float up during construction. In addition to lack of engineering experience, the main reason for the above phenomenon is the lack of understanding of my country's current technical specifications and regulations. For example, Article 4.4.3, paragraph 8 of the Ministry of Construction's "Regulations on the Depth of Construction Engineering Design Documents" stipulates the "basement anti-floating (waterproof) design water level and anti-floating measures, precipitation requirements during construction and conditions for termination of precipitation, etc. "It should be clearly stated in the structural design description: "Technical Specifications for Basement Waterproofing" clearly stipulates in Chapter 10, "During open-cut basement waterproofing construction, the groundwater level should be lowered to the lowest elevation of the project bottom 500 mm, and the effect of precipitation should continue. Until the backfill is complete". We should strictly implement these standards and regulations. Only in this way can we do a good job of anti-floating design and anti-floating construction of the furnace pit.
2. Factors to be considered in anti-floating design
The anti-floating design of the melting furnace pit should consider the geological conditions of the site soil, the perennial groundwater level, the type of groundwater, the anti-floating water level, local hydrological conditions, seasonal rainfall, and the water permeability of the pit base soil. Here we mainly discuss the types of groundwater and how to determine the anti-floating water level.
2.1 Types of groundwater
According to groundwater burial conditions, groundwater can be divided into three categories: upper stagnant water, diving and confined water.
1) Upper stagnant water: the stagnant water existing on the partial water-impermeable layer below the ground (such as clay and other soil layers with little water permeability). Its distribution range is limited, and only after a lot of precipitation and snowmelt can it gather more water, it is a seasonal or temporary water source.
2) Diving: underground water with free surface buried above the first continuous and stable water barrier below the ground. The elevation of the water surface for diving is called the groundwater level. Because rainwater and surface water can directly infiltrate the ground to recharge diving, its distribution, water level, and water quality are related to the climate and change greatly. The shape of the diving surface fluctuates with the terrain, and the depth of burial varies from place to place. The diving water level is often lower than the upper stagnant water.
3) Pressurized water: Groundwater filled with a certain hydrostatic pressure between two stable water barriers. There are water barriers above and below the confined water, and its buried area is inconsistent with the replenishment area. The confined water mainly relies on atmospheric precipitation and replenishment of river and lake water through diving.
2.2 Determination of the anti-floating water level of the furnace pit
In actual projects, in the survey reports provided by some survey units, the anti-floating water level of the furnace pit is vague or not mentioned, and the designers are not clear about the concept of the anti-floating water level of the pit, and lack seriousness in the geological survey report. When studying and analyzing, the following three kinds of problems often appear:
1) The survey report did not specify the anti-floating water level of the furnace pit, and only the visible water level of the exploration hole was described in the report.
2) The geotechnical survey report only mentions the highest and lowest water levels throughout the year. Designers often do not have an anti-floating design concept. For safety reasons, the highest water level is often used as the design anti-floating water level.
3) When the furnace pit of the glass factory is close to rivers, rivers, lakes and construction sites with permeable layers, the impact of the maximum flood level of the river for the design service life is not considered, and only the anti-floating water level is proposed according to the general site, and proceed accordingly Anti-floating design of the furnace pit.
The determination of the anti-floating water level of the furnace pit is a very complicated issue. The complex and variability of groundwater in the site soil and the difference in geological conditions of the site soil make it difficult to determine the anti-floating water level of the furnace pit. However, the anti-floating water level is a decisive parameter in the anti-floating design of the furnace pit, which must be taken seriously and selected correctly.
First of all, survey and design personnel should conduct survey and analysis in accordance with the relevant regulations of the "Geotechnical Engineering Survey Code" (GB 50021-2011) and the "High-rise Building Geotechnical Engineering Survey Code" (JGJ 72-2004). Among them, according to Article 8.6.2 of the "Regulations for Geotechnical Engineering Survey of High-rise Buildings", the comprehensive determination of the groundwater anti-floating waterproof level of the site should meet the following requirements:
(1) When there is confined water in the site and there is hydraulic connection with diving, the confined water level should be measured and its influence against the floating waterproof level should be considered.
(2) When long-term water level observation data is available, the site's anti-floating waterproof level can be determined by the highest water level measured. If there is no long-term water level observation data, it should be determined comprehensively based on the highest water level measured during the survey period and combined with factors such as site topography and landform, groundwater recharge, drainage conditions, etc. .
(3) When considering the anti-floating protection during the construction period, the anti-floating waterproof level should be determined in consideration of atmospheric precipitation or the highest water level in a hydrological year.
In addition, designers should also conduct necessary demonstrations and analyses for the following special circumstances: First, when groundwater burial conditions are complex, groundwater varies widely, regional recharge and drainage conditions may have major changes or engineering needs, special demonstrations should be made. ; Second, on the banks of rivers, rivers, and lakes where there is a head pressure difference, and there is a permeable layer, the anti-floating water level of the pit should be determined according to the highest flood level in the design base period; the third is for the southern regions with abundant rainwater. Particular attention should be paid to the correction of the anti-floating water level in the original survey report after the ground elevation changes. In many areas in the south, the anti-floating water level is sometimes directly determined as the outdoor ground elevation.
3. Conclusion
The anti-floating design of the furnace pit needs to attract the attention of every designer, because once the anti-floating design is not considered properly, the loss to the project is huge and it is difficult to deal with. The correct design concept should be: According to different geological conditions, different anti-floating water levels, and different construction conditions, comprehensively consider the use of safe, economical and reasonable design schemes.