How to determine the anti-floating water level of blind ditch drainage to reduce the basement water level

At the same time, according to the type of groundwater, the groundwater level of each layer and its variation range, and the conditions of recharge and drainage, the anti-floating fortification water level is provided. In Article 8.6.2, it is stipulated as follows: When there are long-term water level observation data, the measured highest water level can be used as the anti-floating fortification water level of the site; When there is no long-term water level observation data or lack of data, it is comprehensively determined according to the highest water level measured during the survey and combined with factors such as topography, groundwater recharge and drainage conditions; When there is confined water in the field and there is hydraulic connection with diving, the confined water level should be measured and its influence on the floating fortification level should be considered; When only the anti-floating fortification during the construction period is considered, the second level of anti-floating fortification can be determined according to the highest water level in a certain hydrological year. 1.4 The Beijing standard Code for Investigation and Design of Building Foundations in Beijing Area (DBJ 0 1-50 1-92) has made special provisions on groundwater in Chapter 4, requiring that the building foundation should determine the groundwater type and groundwater static level in the construction site; Article 4. 1.2 stipulates that the investigation of construction land in cities and suburbs should provide the highest groundwater level in the past years and the highest groundwater level in 3-5 years. When there is a lack of long-term observation data, the highest groundwater level in the past years can be estimated according to the well water level data of field investigation; Article 4. 1.5 stipulates; For basements or structures with strict waterproof requirements, the fortification water level can be designed according to the highest water level in the past years; For basements or structures that are not strictly waterproof, the fortification water level can be determined with reference to the highest groundwater level in 3-5 years and the measured static water level during the survey. The local standard of Hubei Province, Technical Code for Building Foundations (DB42/242-2003)1.4.11Article 3 stipulates that if there are long-term hydrological observation data and historical water level records, the historical water level can be used to calculate the groundwater force. If there are no long-term hydrological observation data and historical water level records, the highest stable water level in wet season can be used to calculate the groundwater force; Article 4 stipulates that when there is confined water in the site and it is hydraulically connected with the groundwater, the buoyancy effect of groundwater level on the basement shall be calculated according to the highest mixed water level of confined water and groundwater; Article 6 stipulates that the buoyancy of the basement under the action of stable water level should be calculated according to the hydrostatic pressure. Properly reduce the buoyancy under temporary high water level in cohesive soil, and the reduction coefficient is proposed by the exploration unit, but not in sandy soil. 2. Academician Zhang Zaiming of China Academy of Engineering discussed the mechanism of groundwater. The book Groundwater and Building Foundation Engineering written by Academician Zhang Zaiming makes a scientific and detailed exposition on the mechanism of groundwater. In the academician's thick book, the following contents are extracted: 1) There is very little groundwater that is really at rest, and most of the water in the soil is in a flowing state (seepage). Seepage is a complex three-dimensional subject, and the seepage phenomenon of saturated soil and unsaturated soil is very different in engineering properties. 2) Pores in soil are the places where groundwater is stored and the channels through which groundwater moves. The analysis of pore water pressure obtained from seepage analysis is the basis of the analysis of groundwater effect on construction projects. 3) The historical highest water level and recent highest water level cannot be directly provided as anti-floating water levels. In order to provide an objective design of anti-floating water level elevation, it is necessary to have long-term observation data, understand the occurrence form and movement law of groundwater in each layer, conduct seepage analysis, obtain the pressure of groundwater on the basement, and provide anti-floating water level elevation according to the maximum pressure of the basement. In other words, correctly determining the groundwater pressure at the bottom of the foundation is the premise of providing the anti-floating water level elevation in architectural design. 4) The water pressure in the basement is not completely determined by the water level, but also related to the existing form of water. Two examples are given in the book, example 1: the buoyancy determined by the highest water level and the highest water level in recent years are 92kPa and 69kPa respectively, while the buoyancy obtained by seepage analysis is only 36 kPa. Example 2: the buried depth of basement is11m. According to the traditional design method, the anti-floating design water pressure is about 10t/㎡, while the foundation bottom water pressure obtained by seepage analysis is only about 1t/㎡, with a difference of 10 times. It shows that the method of determining water pressure according to the traditional still water state is overestimated and wasted. 4.5) As Academician Zhang said, it is scientific to take the pore water pressure at the basement as the elevation of anti-floating water level. The pore water pressure of basement is related to the water level and the circulation conditions of water in soil. Take the website as an example. Now it has been dug to 2.85 meters. According to the water level data provided by the survey, there is a water pressure of about 3t/㎡, which makes drainage difficult, but the fact is that there is no groundwater at all. 6) The connection (infiltration) of groundwater in soil is not only related to the nature of soil itself, but also related to human external factors. During the construction period, the surrounding area is empty, and it rains and accumulates water. When it rains and stops pumping, it is easy to cause the basement to be partially or completely connected with the surrounding accumulated water. When its weight is less than the buoyancy of water, the basement will be lifted, so we should pay serious attention to the anti-floating construction. 7) The monitoring data released by the Geological Environment Monitoring Station in Wuhu City, Anhui Province shows that from 2006 to 2007, the groundwater subsidence funnel disappeared and the groundwater environment was restored. In the mid-1950s, due to the exploitation of groundwater in the eastern suburb of Wuhu, a regional groundwater drop funnel was formed in the 1970s. To 1995, the maximum monthly drop of groundwater level is 0.93m ~ 3.03m, forming an oval groundwater drop funnel with an area of 70 square kilometers, and the groundwater level in the center of the drop funnel is-18.96 m. The deterioration of geological environment in Wuhu City has attracted great attention from the local government and the Anhui Provincial Department of Geology and Mineral Resources, and timely measures have been taken to strengthen monitoring management, gradually close some production wells, reduce the daily output of single wells, and gradually restore the groundwater level. According to the monitoring data of Wuhu Geological Environment Monitoring Station over the years, in 2006-2007, the groundwater level was basically stable, the water level rose to 1.6 1 m, the groundwater falling funnel disappeared, and the groundwater environment in Wanli area was restored. It shows that the current average water level in Wuhu is 1.6 1 m. According to the actual excavation elevation, it has reached 2.85 meters, and no groundwater has been found. It can be seen that the data of 1.6 1m is in line with the actual groundwater level. 8) The above analysis shows that it is reasonable for our unit to suggest that the exploration unit discount the stable water level according to the pore water pressure of the basement and the on-site soil conditions, so as to provide the anti-floating water level elevation.