关于地基承载力如何取得合理值.doc

How to get the reasonable value of bearing capacity of foundation1. Introduction With the rapid development of city construction, city land increasingly tense, whether on the ground or underground space are required to maximize the use of large, complex shape, the upper load size disparity of buildings have constantly appeared, easily lead to the uneven foundation deformation. The results of the investigation shall be applied directly to the construction project, in order to facilitate the understanding and the rational use of specification, we on engineering survey information in determining the bearing capacity and the values were discussed and put forward the treatment principle.2Characteristic value of sub grade bearing capacity determinationThe bearing capacity of the foundation for the foundation design is crucial, code for design of building foundation ( GB50007-2002 ) cancelled code for design of building foundation. GB J 7-89 about bearing capacity table articles and appendix. But in the practical work some industries and local codes, regulations also retains the bearing capacity of the foundation of experience and experience formula table. With the large number of complex architecture, the building s upper load often difference is bigger, easy to cause the uneven foundation deformation, design is needed when using a variety of ground and foundation type or consider the use of sub grade and foundation and superstructure of deformation calculation, solve the uneven settlement of foundation and upper structure effects of the problem, not simple according to data statistic value, look-up table to determine the bearing capacity of the foundation.Therefore, determination of bearing characteristic value, the reasonable selection of engineering characteristics of representative value at the same time, should be fully taken into account the effect of geological environment on the ground. For example, a venue of the two engineering distance of about 20 meters, belong to the same engineering geological unit, the water content of soil index, void ratio, compression modulus difference, project 1, project 2 main compression layer spt the measured average value N were 22 hits, but the characteristic value of sub grade bearing capacity for FA 220 kPa, 160 kPa.From the above results can be obtained in determining the bearing capacity characteristic value, select the engineering characteristics testing for the value, the selection principle is different, project 1 select the standard values for the representative value engineering 2, slightly higher than the minimum value of selected as representative of value. Evident in the same engineering geological unit selection of representative value of different engineering properties, to determine the bearing capacity characteristic value is relatively large, so the scientific and reasonable selection of engineering characteristics of representative value is to determine the characteristic value of sub grade bearing capacity in the key.From the geological environment on the foundation s influence, is also one of the key factors. For example from the underground water level impact analysis, the engineering of geomorphic units of the Yellow River alluvial I terrace, 5 meters high river terrace. Bearing layers of sand, groundwater depth is 2 meters, the groundwater of Quaternary pore water, groundwater fluctuation and atmospheric precipitation and the Yellow River have close hydraulic connection. Determination of bearing layer characteristic value of sub grade bearing capacity to fully consider the buried depth of groundwater table and water level variation factors. Through the water level and water level above two groups of plate loading test results to determine the characteristic value of sub grade bearing capacity were 280kPa, 460kPa.Engineering practice shows that the engineering properties of diversity, engineering geology and hydrologic geology condition, engineering and technical personnel in the determination of characteristic value of sub grade bearing capacity should be objective, scientific, reasonable selection of engineering property index value, taking the geological environment on the foundation of effect factors, so to determine the characteristic value of Sub grade bearing capacity is the safety in real.3The foundation bearing capacity calculationNational standard code for design of building foundation. the underside of the base pressure calculated by the following equation:Pk ( Fk+ Gk ) / A Pk fa ( 1)On building foundation width and depth of the modified characteristic value of sub grade bearing capacity of FA press type calculation amendments:FA = FAK + BB ( B-3 ) + gamma ETA gamma M D ( d-0.5 ) ( 2)At present, many high-rise buildings exist in whole structure of building with podium and underground garage is located adjacent to the two cases. On the characteristic value of sub grade bearing capacity correction is often overlooked, but simply by building the base width and depth is corrected and estimation, led to the survey results conclusion mistakes or unreasonable.For the first case of main wing foundation loading capacity of whole structure depth correction, the bottom surface of the base within the scope of the load, according to the sides of the overloading consideration, when the overload work foundation width width of two times, can be converted into the soil thickness as overloading the depth of foundation based on both sides; when overloaded unequal, according to the small value consideration.On second kinds of buildings and the underground garage is located adjacent to the bearing capacity of foundation depth correction, are advised by the stress condition of considering adverse combination. Often there are upper load size disparity, structure stiffness and structural changes, appear very easily the uneven foundation deformation, design is needed when using a variety of ground and foundation type. For example, high-rise building, the main building for the 15-28 layer, the layer number difference is bigger, the raft foundation, foundation depth of 7.2 meters, the basal pressure 500kPa; on both sides of the main underground garage, foundation depth of 6 meters. The main building foundation bearing layer is the layer of sand, characteristic value of sub grade bearing capacity for 160kPa. If not considering the surrounding underground garage stress conditions, characteristic value of sub grade bearing capacity according to (2) type revising calculation results for 691kPa.Located in the basement level layer of sand, based upon the depth and width of the modified characteristic value of sub grade bearing capacity, without considering the surrounding underground garage stress condition of natural foundation can meet design requirements.According to the actual situation on both sides of the underground garage foundation embedded depth of 6 meters, has changed the building foundation and the stress state, the stress condition by unfavorable combination of consideration, the main building foundation depth of D should be 1.5 meters underground garage, on foundation bottom surface above the load according to the load conversion for the corresponding soil the thickness of h as a basis for calculating buried depth, its main building foundation embedded depth is D+h = 2.5 meters are corrected, by (2 ) type revising calculation results for 395kPa.Based upon the depth and width of the modified characteristic value of sub grade bearing capacity, in considering the surrounding underground garage stress condition, is located in the basal layer of fine sand, can not meet the design requirements of natural foundation.Clearly two different understanding and Consideration on the foundation, use is different, the first consideration of natural foundation can meet the design requirements, design should be adopted. The reason is not considered on both sides of the underground garage foundation embedded depth of 6 meters of free surface, i.e., on both sides of the main airport the actual stress condition. Second according to the actual stress condition considered, considering building foundation side limit permanent extinction, based on both sides of the overload factor, the calculation results can not meet the design requirements of natural foundation, foundation reinforcement, in order to ensure the safety of buildings.4A comprehensive analysis of reliabilityIn 2002 the state of code for investigation of geotechnical engineering ( GB50021 ) revised after the implementation of. Can a country on engineering survey has put forward higher requirements, the reconnaissance unit not only confined to provide geological data, but also relates to the rock and soil remediation, transformation and utilization of comprehensive analysis, in order to reflect the investigation should serve for the whole process of Engineering construction.At present the data of geotechnical engineering investigation, is still in the traditional engineering geological survey report on the level of geotechnical engineering, comprehensive analysis of the content is less, the lack of rock and soil characteristic parameter back analysis results. Regardless of the original data sets and reliability or not, simply on the physical and mechanical indexes and statistical data, directly using the results of analysis and evaluation; the use of several methods are presented the bearing capacity, no analysis that the reliability of the method, the comprehensive value given to a minimum value or slightly higher than the minimum value, this is clearly inconsistent with the rock analysis of the basic principles of geotechnical engineering.With the constantly raise the level of design and engineering quality requirements stricter, especially on the foundation design B project, should consider the test method is feasible and the geological environment effect analysis data, reasonable and representative, as well as the actual engineering combination of load and stress conditions, combined with similar engineering geotechnical parameters the analysis results to select engineering property index, identify and select the characteristic value of sub grade bearing capacity.地基承载力如何取得合理值一、前言 随着城市建设的高速发展，城市用地愈来愈紧张，无论地上还是地下空间均要求最大限度地利用，众多体型复杂、上部荷载大小相差悬殊的建筑不断出现，很易导致地基的不均匀变形。勘察成果须直接应用于建设工程，为便于理解和合理使用规范，我们对工程勘察资料中承载力确定和取值进行了分析并提出处理原则。 二、地基承载力特征值确定 地基承载力对于基础设计至关重要，建筑地基基础设计规范(GB50007-2002)取消了建筑地基基础设计规范GBJ 7-89有关承载力表的条文和附录。而实际工作中一些行业和地方规范、规程还保留地基承载力经验表以及经验公式。随着众多复杂大型建筑的出现，这些建筑的上部荷载往往差别较大，易导致地基的不均匀变形，设计时需要采用多种地基和基础类型或考虑采用地基与基础和上部结构共同作用的变形计算，解决不均匀沉降对基础和上部结构的影响问题，不能简单地按数据统计取值，查表来确定地基承载力了。因此，确定地基承载了特征值时，在合理的选取工程特性的代表值同时，须充分考虑地质环境对地基的影响。例如某场地的两个工程相距约20米，隶属同一工程地质单元，其土工试验指标含水量、孔隙比、压缩模量差异不大，工程1、工程2主要压缩层标准贯入试验实测平均值N均为22击，但是地基承载力特征值fa分别为220 kPa、160 kPa。 从上面的结果可得出在确定承载力特征值、选取工程特性测试代表值时，两者选取的原则不同，工程1选取的标准值为代表值，工程2选取略高于最小值作为代表值。明显看出在同一工程地质单元选取的工程特性代表值不同，对确定承载力特征值影响比较大，所以科学合理的选取工程特性代表值是确定地基承载力特征值的关键。 从地质环境对地基的影响来看，也是关键因素之一。例如从某工程地下水位的影响分析，该工程地貌单元属黄河冲洪积级阶地，阶地高出河床5米。持力层为中砂，地下水位埋深为2米，地下水为第四系孔隙潜水，地下水的升降与大气降水及黄河有密切的水力联系。确定持力层地基承载力特征值要充分考虑地下水位埋深和水位变幅因素的影响。通过水位以下和水位以上两组平板载荷试验结果确定地基承载力特征值分别为280kPa、460kPa。 工程实录表明因工程性质多样，工程地质与水文地质条件不同，工程技术人员在确定地基承载力特征值时要客观、科学、合理地选取工程特性指标代表值，充分考虑地质环境对地基的影响因素，这样确定的地基承载力特征值才是安全真实的。三、地基承载力修正计算 国标建筑地基基础设计规范基础底面的压力按下式计算： Pk(Fk+Gk)/APkfa(1) 对建筑物基础宽度和埋置深度修正后的地基承载力特征值fa按下式计算修正： fa=fak+bb(b-3)+dm(d-0.5)(2) 目前众多高层建筑存在着主裙楼一体结构和地下车库紧邻的两种情况。在对地基承载力特征值修正时往往被忽视，而只是简单地按建