Technically speaking, the factors affecting the anti-floating evaluation of underground engineering include:I. Meteorology and Hydrology
Regional rainfall is the recharge source of groundwater and an important factor in the anti-floating evaluation of underground engineering. Annual rainfall, maximum rainfall, maximum daily rainfall, monthly rainfall, etc. all affect changes in groundwater. Rainfall in each region has its own patterns, such as rainy-season rainfall and periodic changes in annual rainfall (it is said that the Beijing area has a 30-year minor cycle and an 80-year major cycle). Using annual average rainfall, maximum rainfall, etc. will all have a greater impact on the anti-floating evaluation. The distribution of rivers and lakes in a region directly recharges groundwater, and their water levels, water volumes, maximum flood levels and their changes are undoubtedly important factors to be considered in groundwater level evaluation.
II. Site Topography, Engineering Geology, and Hydrogeological Conditions
The site topography and landforms, the types, distribution, and engineering characteristics of rock and soil layers and their changes have a relatively large impact on groundwater recharge, runoff, and discharge. Among these, topography and landforms change with the natural environment.In addition to the current groundwater conditions, the historical high groundwater level, the highest water level in recent years and its changes over time are also important factors in the evaluation and prediction of the anti-floating water level.
With the development of industry and agriculture, population growth, and urbanization, human demand for groundwater has become increasingly large. Groundwater recharge in the Beijing area can no longer meet demand, which has led to a decline in groundwater levels and the formation of relatively large groundwater depression cones. The development and utilization of urban underground space and construction dewatering have also accelerated the decline in groundwater levels.In order to alleviate water shortages in northern regions and protect the water environment, in recent years the South-to-North Water Diversion has increased the water supply in the Beijing area, and further through the ecological water replenishment projects of the Yongding River and Chaobai River, as well as the increase in annual rainfall in the Beijing area in recent years, the groundwater level in the Beijing area has rebounded significantly.
Human activities such as mountain cutting and land creation and the construction of water conservancy projects can greatly change the occurrence state of groundwater. These factors are sometimes not easy to determine, so it is necessary to promptly collect regional population, industrial and agricultural development, and engineering construction planning data for systematic analysis.
IV. Foundation Location and Construction Impact
Relative to regional planning and development, the construction project itself also has a relatively large impact on the anti-floating evaluation.
For example, excavation and fill in site planning, surface drainage systems, foundation embedment depth, and excavation and backfill all affect the groundwater level. When multiple groundwater levels exist, the foundation embedment depth has a significant impact.The permeability and compactness of backfill materials after foundation excavation are also a factor that changes the local groundwater level. Many engineering cases have proven that because the permeability of backfill materials is significantly greater than that of the original soil and the compactness is insufficient, poor site drainage conditions lead to anti-floating failure. Therefore, we must pay attention to the impact of construction project execution.。
Regarding the Regulations on TrenchesLet us first look at the requirements of relevant codes and standards for trench backfill.(JGJ3-2010) 12.2.6 High-rise building basement (1) Technical Specification for Concrete Structures of Tall Buildings: The peripheral backfill shall use graded sand and gravel, sandy soil, or lime soil, and shall be compacted in layers.(2) Standard for Anti-floating Design of Building Engineering JGJ476-20196.5.5-3 Backfill of foundation trenches shall use layered compacted cohesive soil, lime soil, or poured ready-mixed fluidized solidified soil, plain concrete, and other low-permeability materials:
V. Design Service Life of Construction Projects
The anti-floating water level evaluation is intended to ensure the safety of a construction project throughout its entire life cycle, so the design service life of a construction project is naturally an important factor affecting the anti-floating evaluation. Generally, the longer the design service life of a construction project, the higher the expected anti-floating water level.The factors affecting the anti-floating evaluation mentioned above are mainly technical factors.In fact, project importance, tolerance for anti-floating failure, and economic factors also affect the anti-floating water level evaluation.
In analysis and evaluation, we should:Collect long-term regional construction and water conservancy planning data, and comprehensively analyze the site meteorology, hydrology, topography and landforms, site drainage conditions, engineering geological conditions, and hydrogeological conditions;Accurately ascertain the measured groundwater level at the site, collect data on dynamic water levels in the past 3 to 5 years and historical maximum groundwater levels, and fully consider factors such as site environment and its changes, the South-to-North Water Diversion and ecological water replenishment, and project importance;
And consider necessary redundancy to comprehensively analyze and evaluate the anti-floating design water level.
The evaluation results should clearly specify the standards on which they are based and the evaluation conditions, and propose follow-up measure requirements to ensure the evaluation conclusions. These follow-up measure requirements include construction drainage, permeability and compactness of trench backfill soil, and other related requirements and recommendations (for narrow spaces, new technologies such as fluidized concrete can be used).
Speaking of anti-floating evaluation, let me add:
Engineering anti-floating evaluation needs to consider many factors, including the current groundwater level, historical maximum water level, highest water level in recent years and future hydro-meteorological and topographic environment changes, and the impact of human activities. Considering that groundwater levels have strong timeliness, the evaluation should consider the impact of spatiotemporal changes in groundwater levels and retain appropriate redundancy. Even if no groundwater is observed during investigation, the anti-floating impact should still be evaluated.
Mandatory standards do not stipulate that the anti-floating water level must be provided by the investigation unit, but its impact should be evaluated, and recommendations for anti-floating measures and relevant parameters required for design should be proposed.
For areas with large topographic relief where anti-floating is affected, different anti-floating water levels may be provided by zone or anti-floating water level zoning may be adopted.
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