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This time, the revision of the foundation design specification has caused everyone to fight against the floating level, and we **This article is for the bigwigs**, and the copyright belongs to "Aon-COFCO Design".
Editor's note
In order to implement the "strong technology" measures, Aon-COFCO Design has successively set up professional research centers in several design and production departments to engage in professional research on deepening design, underground space, water supply and drainage technology, etc. In this issue of "Craftsman's Words", we select the discussion content of the structure group of the Underground Space Research Center of the First Architectural Design and Research Institute on the selection of anti-floating water level under the new anti-floating standard.
Summary:
With the development of the national economy, the anti-floating design of underground engineering has gradually received attention in recent years, and the "Technical Standard for Anti-floating Engineering of Construction Engineering" JGJ476-2019 came into being. The determination of the anti-floating water level is a key and difficult point in recent years, which often causes controversy, and if the water level of the anti-floating design is not properly determined, it is easy to leave potential safety hazards to the project [1]. This article introduces the relevant knowledge of anti-floating water level from many aspects, so that designers can choose the anti-floating water level more reasonably.
Survey norms against floating water level regulations
1.Code for geotechnical investigation GB50021-2001 (2009 edition) 41.13. Investigate and demonstrate the impact of groundwater on the project and the environment during the construction period. For important projects with complex conditions, special research should be carried out when it is necessary to demonstrate the change of water level during use and to propose anti-floating waterproof level.
2.Geotechnical investigation standard for high-rise buildings JGJ T 72-2017
8.6.1. The anti-floating evaluation of the basement should include the following basic contents:
1) Analyze and put forward reasonable suggestions for anti-floating waterproofing;
2) According to the anti-floating waterproof level, combined with the buried depth of the basement, the weight of the structure, etc., put forward suggestions on the problems related to anti-floating.
8.6.2. The comprehensive determination of the anti-floating waterproof level should comply with the following provisions:
1) The anti-floating waterproof level should be the highest water level that may be encountered in the basement from the construction period to the whole service life.
2) The anti-floating waterproof level during the construction period can be ...... according to the highest water level of the site measured during the survey
3) When the underground structure is adjacent to large surface water bodies such as rivers, lakes, rivers and seas, and has a hydraulic connection with the groundwater of the site, the anti-floating waterproof level can be comprehensively determined according to the high water level of the surface water body once in a hundred years and its wave height, combined with the underground drainage pipe network, etc., and according to the local experience.
4) For the low-lying areas in the city, it should be determined according to the situation that the streets may be flooded during the heavy rainstorm, and the outdoor floor elevation can be taken for the areas with high groundwater level in the south and the saturated foundation soil in the south.
8.6.3. When the construction site is located in the slope area and the height difference is large, or the groundwater occurrence conditions are complex, the change range is large, and the regional recharge, runoff and discharge conditions during the basement use may change greatly or the project needs to be carried out, a special demonstration should be carried out and a special consultation report on the anti-floating waterproof level should be provided.
The selection of anti-floating waterproof level in the new specification
1. The division of anti-floating fortification zones
When the elevation of the site and its surroundings or the vertical design of the site is quite different, it is advisable to use different anti-floating waterproof levels according to the division of anti-floating fortification zones.
2 Set up a waterproof position
a) During the construction period, the anti-floating waterproof level shall take the highest value of the following groundwater level:
In Tianjin, precipitation is generally required during the construction period, and anti-floating water level is not the focus. During this period, the focus of anti-floating is to grasp the time of stopping precipitation and the number of wells sealed in the large area.
b) The anti-floating waterproof level during the use period shall be taken as the highest value of the following groundwater level:
c) The anti-floating waterproof level of the site under special conditions should be the maximum value of the construction period, the use period and the following elevations:
1) Low-lying sites with the possibility of inundation from the perspective of the entire urban area are designed to be above the outdoor floor5m high-rise;
2) Sites with flat terrain, weak permeability grade and above and poor drainage, are designed for outdoor floor elevation(The foundation pit fertilizer groove adopts the soil backfill with strong permeability, the compactness is poor, the surface sealing effect is poor, and the surface infiltration such as groundwater seepage or rainfall will be stored in the fertilizer tank, and the water head difference is formed due to the absence of discharge conditions, similar to a water basin.) Many engineering accident cases have proved that this condition will produce greater buoyancy to the foundation floor, in view of the fact that there is no suitable method for this kind of buoyancy calculation, from the perspective of engineering safety, the design, construction and Party A must pay attention to the material, permeability and compactness of the fertilizer tank backfill, so as to avoid future troubles;Due to the high elevation of buildings in mountainous areas, the anti-floating water level is very low, and the general survey will not be anti-floating, so it is more prudent to avoid the occurrence of water basin phenomenon.
3) When the groundwater in different vertical design elevation zones can be discharged by itself in the next elevation zone, it is the elevation of the next elevation zone.
Problems that should be paid attention to in the selection of anti-floating water level in the design
According to the meteorological and environmental monitoring reports in recent years, there is a lot of precipitation in Tianjin, and groundwater exploitation is gradually decreasing (most of the mining in Jizhou District is seasonal exploitation of agricultural water), especially the groundwater level in the central urban area and the four districts around the city is rising slowlyThe general characteristics of the distribution of shallow groundwater in Tianjin are in the piedmont plain in the north, and the general flow direction of shallow groundwater is from north to southIn the southern plains, shallow groundwater flows from north-west to south-east. In general, the shallow water runoff is sluggish, and surface water bodies such as rivers, depressions, and reservoirs are often the local discharge zones of shallow groundwater, but rivers recharge groundwater during the flood season, and groundwater is generally discharged to the Bohai Bay. [2]
1 Attention Problem:
a) Due to the current anti-floating waterproof level work has just started, there are many imperfect or even non-standard behaviors, in order to ensure the integrity and reliability of the technical consultation results, when the anti-floating water level proposed in the survey report deviates from the normal value, it is recommended that the design unit treat it carefully, and if necessary, it should be recommended that Party A adopt the method of expert review to evaluate the floating water level;
b) The impact of the artificial lake built near the site on the construction of the project cannot be ignored, and the groundwater level may rise after long-term impoundment, which should be considered when determining the water level;
c) The scale of urban engineering construction is large, and the groundwater level drops due to the precipitation of the foundation pit, so it is necessary to consider the impact of the restoration of the groundwater level on the building after several years;
d) When the ground height difference of the proposed site area is large, it should be treated as a slope site. Due to the significant difference in water level and the complex flow direction of groundwater, the anti-floating waterproof level should not be considered according to the unified water level, and should be treated separately by partition or different buildings
e) Pay attention to whether the time of the survey report is a dry period or a wet period, and the anti-floating water level given by the general survey is a certain empirical value floating on the current water level. Generally, Tianjin is the dry season, and September to October is the wet season
f) When designing an unfamiliar site, it is recommended to use survey, survey of surrounding projects, etc., to comprehensively evaluate the value of the anti-floating water level in the survey report.
2 Anti-floating prevention and control measures:
a) Impervious zones such as clay layers should be set on the surface around the basement to prevent rainwater from infiltrating during concentrated precipitation;
b) The site should be set up with seepage wells, drainage blind ditches and drainage ditches to form an organized drainage system of interception ditches, drainage ditches, drainage ditches, etc.;
c) The backfill of the foundation pit fertilizer tank should be made of micro-permeable materials such as layered compacted plain soil, ash soil or plain concrete, and permeable materials such as graded gravel should be strictly used to avoid the "water basin effect".
d) The interfaces, ditches, culverts, etc. of water supply and drainage pipelines should take anti-leakage measures;
e) When a large area of water pool is set up in the landscape of the overburden on the roof of the garage, its influence on the reduction of the self-weight of the overburden should be considered.
Comparison of engineering anti-float design of the old and new standards
Original standard
The new standard
Since the anti-floating safety factor is directly linked to the anti-floating engineering design grade, the change of the anti-floating combination coefficient. In Tianjin, the implementation of the new norms, the basement needs to be set up with uplift piles;Under normal circumstances, the cost of anti-floating increases by about 30%.
A few thoughts on the anti-float specification
1）3.0.1 How to define the characteristics of Grade A construction projects, and who will define them?
2）3.0.5. How to define whether the geotechnical engineering investigation documents meet the requirements of anti-floating design and construction, if determined by the design, there is a suspicion of unclear responsibility and ultra vires.
3）2.1.16. Anti-floating plate: a plate-like structural component with waterproof and anti-seepage function that is set up to ensure the use function of the construction project and bear the role of buoyancy. 7.2.3-3 The thickness and reinforcement of the anti-floating floor shall meet the requirements of deformation, cracks, minimum reinforcement ratio and anti-seepage, and the thickness shall not be less than 350mm. In Tianjin, the main building is multi-storey, and the foundation adopts a beam and plate structure, and the thickness of the foundation floor should also meet 350mm
4) Anti-floating specification 63.Article 3: When calculating the anti-buoyancy force, the upper part of the structure and the outer wall of the underground structure pick out the standard value of the self-weight of the filling material on the structure, and the saturation gravity should be calculated below the anti-floating water level, and the natural gravity should be calculated above the anti-floating water level. The bulk density of the soil in Table A of the load specification continues: 16kn m for dry soil;Wet soil 18kn m;Very wet soil 20kn m;When anti-floating, the bulk density of the soil is considered to be 18kn m in the calculation of anti-float before the new specification, and 16kn m is used in some design institutes. After the new specification, it is necessary to pay attention to the saturation gravity and natural gravity of the soil. The overlying soil above the roof of the basement is generally backfilled with plain soil of good quality and treated with simple tamping, and its state is close to that of plain fill in the natural state. By counting the natural gravity of 1-b (plain fill) soil layer in 40 engineering survey reports in Tianjin, the following conclusions are drawn:
The average is: 1893kn/m³。Therefore, without considering the influence of landscape soil covering, it is recommended that in the anti-compressive and anti-floating design, the bulk density of the soil cover should be 185kn m considered.
Wrapping up
Anti-floating is often one of the major problems that are easily ignored in structural design and lead to engineering accidents. Therefore, it is an effective measure to determine the reasonable anti-floating design water level, clarify the anti-floating level, and improve the anti-floating safety. In addition to reading the investigation report carefully, you should visit the site in person when you are in an unfamiliar site, especially in low-lying areas.