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1 General Provisions 1.0.1 This code is formulated with a view to implement the national technical and economical policies in design of the building grounds, foundations, that the design can be accomplished with safety and usability, using advanced technology, with economy and rationality, assuring the quality and protecting the environment. 1.0.2 Design of the building grounds, foundations must persist in the principles of suiting measures to local conditions, using local materials, protecting the environment and economizing on the resources; the design shall be painstakingly performed, considering the factors comprehensively as the type of structures, the availability of materials and the construction conditions etc., according to the geotechnical survey data of rocks, soils. 1.0.3 This code is applicable to the design of grounds, foundations of industrial and civilian buildings (including structures). For the design of grounds, foundations in the collapsible loess, the perennial frozen soil and the swelling soil, as well as, under the action of seismic load and vibrating load of machines, that the requirements in the current relevant standards or codes of the nation shall also be complied with. 1.0.4 This code is formulated according to the basic principles of national standard Unified Standards for the Design of Structures (GBJ 68-84) and the requirements of national standard Design of Structures — General Symbols Units of Measurement and Basic Terms (GBJ 83-85). 1.0.5 When this code is used for design, the values of loads shall meet the requirements in the current National Standard Load Code for the Design of Building Structures (GBJ 9-87); the calculation of foundations shall also meet the requirements of the National Standard Code for Design of Concrete Structures (GBJ 10-89) and Code for Design of Masonry Structures (GBJ 3-88). When the foundations are laid in aggressive environment or are subjected to temperature influence, that the requirements in the current relevant mandatory standard of the nation shall also be complied with, and the corresponding protective measures shall be taken. 2 Basic Requirement 2.0.1 According to the severity degree for being possible to destroy buildings (endanger person life, result in economic loss and social influence, as well as the possibility of restoration), hence, the design of grounds, foundations are divided into three design classes, which shall be selected for design from Table 2.0.1, according to the specific conditions. Table 2.0.1 Design Classes for Grounds, Foundations Design classes Consequence Categories of buildings and grounds Class I Very serious Important industrial and civilian buildings; Tall buildings with more than 20 stories; Complicated shaped high buildings more than 14 stories; Buildings with special requirements for deformation of ground; Buildings of single pile subjected to loads above 4000kN Class II Serious General industrial and civilian buildings Class III Not serious secondary buildings 2.0.2 According to the design classes of ground, foundation, as well as, the degree of influence on the upper structure due to the deformation of ground under the effects of long-term loading, hence, the design of ground, foundation shall meet the following require-meats: I The deformation design of ground shall be performed for the buildings belong to Class I, Class II (except those specified in Table 2.0.2); the calculation shall simultaneously meet the requirements of 6.2.1 and 5.1.1 of this code; II The buildings for design Class III with the scope listed in Table 2.0.2, that the checking computation of deformation may not be performed, if one of the following situations occurs, the checking computation of deformation shall still be performed: 1. The standard value for load-bearing capacity of ground for building is less than 130kPa, and with the complicated shape building; 2. The ground stacking load is acted on the foundations and in the vicinity of foundations or the difference of loadings on the neighboring foundations is quite large, when it may possible arouse the large unequal settlements of ground; 3. When the tilting of building may be occurred, if the distance between the neighboring buildings on the soft weak soil ground are too closer; 4. When there exists the earth-fill with comparative great thickness or the earth-fill with non-uniform thickness, its self-weight consolidation has not been completed. Deformation check of Classes II and III buildings at other conditions is not compulsory, provided that meet the requirements of Clause 5, subclause 1. III For the tall buildings, the high-rise structures sustain the effects of horizontal loading frequently, as well as, the buildings and the structures are constructed on the inclined slope or near the side slope, their stability shall also be checking computed; Table 2.0.2 Scope for Buildings of Design Class III, Which the Calculation for Deformation of Ground may not be Performed Conditions for main bearing stratum of ground Standard value for load-bearing capacity of ground fak (kPa) 60≤fak<80 80≤fak<100 100≤fak<130 130≤fak<160 160≤fak<200 200≤fak<300 Gradient of each soil stratum (%) ≤5 ≤5 ≤10 ≤10 ≤10 ≤10 Type of building Masonry load-bearing structure, Frame structure (number of stories) ≤5 ≤5 ≤5 ≤6 ≤6 ≤7 Single story bent frame structure (6m span between columns) Single span Rated hoisting capacity (t) of crane 5~10 10~15 15~20 20~30 30~50 50~100 Span of factory building (m) 412 418 424 430 430 430 Multi-span Rated hoisting capacity (t) of crane 3~5 5~10 10~15 15~20 20~30 30~75 Span of factory building (m) ≤12 ≤18 ≤24 ≤30 ≤30 ≤30 Chimney Height (m) ≤30 ≤40 ≤50 ≤75 ≤100 Water tower Height (m) ≤15 ≤20 ≤30 ≤30 ≤30 Volume (m3) ≤50 50~100 100~200 200~300 300~500 500~1000 Notes: 1 The main bearing stratum of ground denotes the scope of stratum with a depth of 3b (b is the width of foundation base surface) beneath the base surface of strip foundation, with a depth of 1.5b beneath the base surface of independent foundation, and their scope of thickness are not less than 5m (besides the general civilian buildings under two stories); 2 When the standard value for load-bearing capacity of soil stratum is less than 130kPa, in the main bearing stratum of ground, hence, the design of masonry structure, as specified in Table, shall meet the relevant requirements in Clause 7 of this code; 3 The masonry load-bearing structure and the frame structure in Table denote the civilian buildings, for the industrial building may be converted into the corresponding number for stories of civilian building, according to the height of factory building and the loading conditions; 4 The values of the rated hoisting capacity of crane, the height of chimney and the volume of water tower in Table, denote their maximum values. 2.0.3 When according to the load-bearing capacity of ground to determine the base area of foundation and the depth of embedment to determine the number of piles, that the load-effects, which are transmitting to the base surface of foundation, shall be calculated by using basic combination, soil mass deadweight partial coefficients 1.0 and actual gravimetric density. To calculate the foundation deformation, long term effect combination shall be adopted for the load effect transmitted to the foundation base which shall not be counted into wind load and earthquake action. When calculating the earth pressure of retaining wall, the stability of ground or slope, as well as, the thrust of landslide, that the load-effects shall comply with the fundamental combination, but their partial coefficients are 1.0. 2.0.4 For Class I buildings, settlement observation shall be carried out at construction period and service period, and the measured data shall be served as one of the reference of foundation work quality inspection of building grounds. The methods and requirements of settlement observation shall be in accordance with Annex 1 of this code. 3 Classification of Geotechnical Rocks, Soils and Their Engineering Property Indexes 3.1 Classification of Rocks, Soils 3.1.1 The rock, soil, which are used for the grounds of buildings, may be classified into rock, gravelly soil, sandy soil, silty soil, cohesive soil and artificial earth-fill. 3.1.2 The rock shall be the rock mass with firmly joining between its particles and showing as a whole or as possessed some fissures at its joints. The hardness of rocks are classified into hard rock and weak rock. The weathering degree of rocks may be classified into fresh, slightly weathered, moderately weathered and highly weathered. Rocks may be comply with the Annex 2 of this code. 3.1.3 The broken stone soil means that the content of grains with its diameter larger than 2mm are exceeding 50% of the total weight of soil. The broken stone soil may be classified into boulder, block stone, cobble, broken stone, rounded gravel and angular gravel, according to Table 3.1.3. Table 3.1.3 Classification of Broken Stone soils Name of soil Shape of grain Content of grains set Boulder Block stone Rounded shape and sub-rounded shape are predominant Aries shape is predominant Content of grains with its diameter greater than 200mm are exceeding 50% of the total weight of soil Cobble Broken stone Rounded shape and sub-rounded shape are predominant Aries shape is predominant Content of grains with its diameter than 20mm are exceeding 50% of the total weight of soil Rounded gravel Angular gravel Rounded shape and sub-rounded shape are predominant Aries shape is predominant Content of grains with its diameter greater than 2mm are exceeding 50% of the total weight of soil Note: When classifying the broken stone soil shall accord to the content of grains set column from top to bottom, it is determined by the first conformable one. 3.1.4 The sandy soil means that the content of grains with its diameter larger than 2mm is not exceeding the 50% of total weight of soil, with its diameter larger than 0.075mm is exceeding the 50% of total weight of soil. The sandy soil may be classified into gravelly sand, coarse sand, medium sand, fine sand and silty sand, according to Table 3.1.4. Table 3.1.4 Classification of Sandy Soils Name of soil Content of grains set Gravelly sand Content of grains with its diameter larger than 2mm amounts to 25%~50% of total weight of soil Coarse sand Content of grains with its diameter larger than 0.5mm is exceeding the 50% of total weight of soil Medium sand Content of grains with its diameter larger than 0.25mm is exceeding the 50% of total weight of soil Fine sand Content of grains with its diameter larger than 0.075mm is exceeding the 85% of total weight of soil Silty sand Content of grains with its diameter larger than 0.075mm is exceeding the 50% of total weight of soil Note: When classifying the sandy soil shall accord to the content of grains set column from top to bottom, it is determined by the first conformable one. 3.1.5 The density of sandy soil may be classified into loose, slightly dense, medium dense and dense, according Table 3.1.5. Table 3.1.5 Density of Sandy Soil Blow count from standard penetration test N Density N≤10 Loose 10 Note: When the density of sandy soil is judged by the obstructive force of static probing head, it may be determine according to the locality experience. 3.1.6 The cohesive soil, which is the soil with the plasticity index Ip larger than 10, may be classified into clay, silty clay, according to Table 3.1.6. Table 3.1.6 Classification of Cohesive Soils Plasticity index Ip Name of soil Ip>17 Clay 10 Note: When the blow count from the obstructive force of static probing head or the standard penetration test is used to judge the states of cohesive soil, it may be determined according to with the locality experience. 3.1.8 The mud is formed by the deposition in the stagnant water or the slowly flowing water environment, and through the biochemical reaction, its natural moisture content is larger than the liquid limit, and the natural void ratio is larger or equal to 1.5 of the cohesive soil. When the soil with the natural void ratio is less than 1.5, but it is larger than or equal to 1.0, hence, it belongs to muddy soil. 3.1.9 The laterite belongs to high plasticity clay, which is formed from carbonate series rock through the action of laterization, its liquid limit is usually larger than 50. The basic features of laterite will be reserved after transporting repeatedly, when the liquid limit of laterite is larger than 45, hence, it belongs to secondary laterite. 3.1.10 Silty soil shall be the soil with plasticity index less than or equal to 10. Their properties are between sandy soil and clay soil. 3.1.11 The artificial earth-fill may be classified into plain earth-fill, miscellaneous earth-fill and flushing earth-fill according to their composition and the cause of formation. The plain earth-fill is the fill composed by broken stone soil, sandy soil, silty soil, cohesive soil etc. The miscellaneous earth-fill is the fill contained the impurity substances including the building refuse, the industrial waste materials, the living garbage etc. The flushing earth-fill is the fill formed by hydraulic flushing filled silts. 3.2 Engineering Property Indexes 3.2.1 When standard value of ground bearing capacity determined by load test, the area of press plate should be0.25~0.50m2, the load test shall meet the requirements specified in Annex 4 of this code. 3.2.2 Standard value for the bearing capacity of ground soil and rock determined by indoor test, standard penetration test, portable penetration sounding or field identification method, the methods and procedures shall comply with the provisions of Annex 5 of this code, in statistical data of not less than six. Standard penetration and portable penetration tests shall comply with the requirements of Annex 6 of this code. 3.2.3 When the standard value or other soil indexes of ground bearing capacity are determined by static sounding, pressuremeter and other in-situ tests, the results shall be determined after comparison with the direct test results of load tests or corresponding soil indexes. 3.2.4 The shearing strength index of soil may be determined by the test methods including: the laboratory shear test of undisturbed soil specimen, the compression strength without lateral pressure test, the shear test in situ, the vane shear test etc., and shall meet the following requirements: 1 For Class I buildings, when indoor shearing test is adopted, earth-borrowing holes shall not be less than six. For uniform soil, drilling holes at the same soil layer along the depth test shall not be less than three sets; when the multilayer and thin soil, the test shall not be less than one set, the unconsolidated, undrained test in the triaxial compression test shall be selected; for other buildings, direct shear test may be adopted for castable clay soil and silty soil of saturation not greater than 0.5. 2 If consolidated shearing tests are taken, the extent to which the foundation may be consolidated under the action of building load and preloading load shall be taken into account. The standard value of shearing strength indexes may be determined according to Appendix 7 of this code. 3.2.5 The design value of bearing capacity of rock foundation, may be determined according to the test methods for batholite load specified in Annex 8 of this code. The design value of bearing capacity of rock foundation with slight weathering and moderate weathering, can also be calculated according to the saturated uniaxial compressive strength by using the formula below: Main Symbols 1 General Provisions 2 Basic Requirement 3 Classification of Geotechnical Rocks, Soils and Their Engineering Property Indexes 3.1 Classification of Rocks, Soils 3.2 Engineering Property Indexes 4 Embedded Depth of Foundation 4.1 General provisions 4.2 Embedded Depth of Foundation and Treatment of Frozen Soil Ground 5 Calculation of Ground 5.1 Calculation of Load-bearing Capacity 5.2 Calculation of Deformation 5.3 Calculation of Stability 6 Ground in Mountainous Region 6.1 General Requirement 6.2 Soil-rock Composite Ground 6.3 Compacted Fill Ground 6.4 Side Slope and Retaining Wall 6.5 Landslide Prevention 6.6 Karst and Earth Cave 7 Feeble Ground 7.1 General Requirement 7.2 Utilization and Treatment 7.3 Architectural Measures 7.4 Structural Measures 7.5 Large Area Ground Surface Load 8 Foundations 8.1 Rigid Foundation 8.2 Spread Foundation 8.3 Strip Footing under Columns 8.4 Raft Foundation under Wall 8.5 Shell Foundation 8.6 Pile Foundation 8.7 Rock Anchored Bolt Foundation Annex 1 Main Points for Settlement Observation Annex 2 Division of Rock Annex 3 Field-distinction of Broken Stone Soil Annex 4 Main Points of Ground Soil Load Test Annex 5 Standard value for the Bearing Capacity of Soil (Rock) Annex 6 Main Points of Standard Penetration and Light Penetration Tests Annex 7 Standard value of Shear Strength Indexes c, φ Annex 8 Main Points for Loading Test of Rock Ground Annex 9 Main Points for Uniaxial Compression Strength Test of Rock Annex 10 Coefficient of Additional Stress α and Coefficient of Average Additional Stress Annex 11 Coefficient of Active Earth Pressure ka for Retaining Wall Annex 12 Calculation for Additional Settlement Amount of Ground under the Action of Large Area Ground Surface Load Annex 13 Internal Force Formulae of Film Theory of Shell Foundation Annex 14 Main Points for Vertical Static Loading Test of Single Pile Annex 15 Standard value of Vertical Bearing Capacity of Precast Pile Annex 16 Explanation of Wording in This Code 第一章 总 则 第1.0.1条 为了在地基基础设计中贯彻执行国家的技术经济政策,做到技术先进、经济合理、安全适用、确保质量,特制定本规范。 第1.0.2条 地基基础设计,必须坚持因地制宜、就地取材的原则;根据地质勘察资料,综合考虑结构类型、材料情况与施工条件等因素,精心设计。 第1.0.3条 本规范适用于工业与民用建筑(包括构筑物)的地基基础设计。对于湿陷性黄土、多年冻土、膨胀土、地下采空区以及在地震和机械振动荷载作用下的地基基础设计,尚应符合现行有关标准、规范的规定。 第1.0.4条 本规范系根据国家标准《建筑结构设计统一标准》GBJ 68—84的基本原则,并按国家标准《建筑结构设计通用符号、计量单位和基本术语》GBJ 83—85的规定制定的。 第1.0.5条 采用本规范设计时,荷载取值应符合国家标准《建筑结构荷载规范》GBJ 9—87的规定;基础的计算尚应符合国家标准《混凝土结构设计规范》GBJ 10—89和《砌体结构设计规范》GBJ 3—88的规定。当基础处于侵蚀性环境或受温度影响时,尚应符合专门规范的规定,采取相应的防护措施。 第二章 基本规定 第2.0.1条 根据地基损坏造成建筑物破坏后果(危及人的生命、造成经济损失和社会影响及修复的可能性)的严重性,将建筑物分为三个安全等级,设计时应根据具体情况,按表2.0.1选用。 建筑物安全等级 表2.0.1 安全等级 破坏后果 建 筑 类 型 一 级 很严重 重要的工业与民用建筑物;20层以上的高层建筑;体型复杂的14层以上高层建筑;对地基变形有特殊要求的建筑物;单桩承受的荷载在4000kN以上的建筑物 二 级 严重 一般的工业与民用建筑 三 级 不严重 次要的建筑物 第2.0.2条 根据建筑物安全等级及长期荷载作用下地基变形对上部结构的影响程度,地基设计应符合下列规定: 一、一级建筑物及表2.0.2所列范围以外的二级建规物,均应按地基变形计算,计算时应同时满足本规范第6.2.1条及5.1.1条的规定; 二、表2.0.2所列范围内的二级建筑物如有下列情况之一时,仍应作变形验算: 1.地基承载力标准值小于130kPa,且体型复杂的建筑; 2.在基础上及其附近有地面堆载或相邻基础荷载差异较大,引起地基产生过大的不均匀沉降时; 3.软弱地基上的相邻建筑如距离过近,可能发生倾斜时; 4.地基内有厚度较大或厚薄不均的填土,其白重固结未完成时。 其他情况下的二级建筑物和三级建筑物,在符合本规范第五章第一节的规定时,可不做变形验算; 三、对经常受水平荷载作用的高层建筑和高耸结构,以及建造在斜坡上的建筑物和构筑物,尚应验算其稳定性。 可不作地基变形计算的二级建筑物范围 表2.0.2 地基主要受力 地基承载力标准值 fk(kPa) 60≤fk<80 80≤fk<100 100≤fk<130 130≤fk<160 160≤fk<200 200≤fk<300 层情况 各土层坡度(%) ≤5 ≤5 ≤10 ≤10 ≤10 建 筑 类 型 砌体承重结构、框 架结构(层数) ≤5 ≤5 ≤5 46 46 ≤7 单层排架结构 (櫦櫐 柱距) 单 吊车额定起重量(t) 5~10 10~15 15~20 20~30 30~50 50~100 跨 厂房跨度 (m) ≤12 ≤18 ≤24 ≤30 ≤30 ≤30 多 吊车额定起重量(t) 3~5 5~10 10~15 15~20 20~30 30~75 跨 厂房跨度 (m) ≤12 ≤18 ≤24 ≤30 ≤30 ≤30 烟囱 高度(m) ≤30 ≤40 ≤50 ≤75 ≤100 水塔 高度(m) ≤15 ≤20 ≤30 ≤30 ≤30 容积(m3) ≤50 50~100 100~200 200~300 300~500 500~1000 注:①地基主要受力层系指条形基础底面F深度为3b(6为基础底面宽度),独立基础F为1.5b,且厚度均不小于5m的范围(二层以下一般的民用建筑除外); ②地基主要受力层中如有承载力标准值小于130kPa的土层时,表中砌体承重结构的设计,应符合本规范第七章的有关要求; ③表中砌体承重结构和框架结构均指民用建筑,对于工业建筑可按厂房高度、荷载情况折合成与其相当的民用建筑层数; ④表中吊车额定起重量、烟囱高度和水塔容积的数值系指最大值。 第2.0.3条 按地基承载力确定基础底面积及埋深时,传至基础底面上的荷载应按基本组合、土体白重分项系数为1.0,按实际的重力密度计算。 计算地基变形时,传至基础底面上的荷载应按长期效应组合,不应计入风荷载和地震作用。 计算挡土墙的土压力、地基稳定及滑坡推力时,荷载应按基本组合,但其分项系数均为1.0。 第2.0.4条 对一级建筑物应在施工期间及使用期间进行沉降观测,并应以实测资料作为建筑物地基基础工程质量检查的依据之一。沉降观测的方法及要求,可按本规范附录一执行。 第三章 地基土(岩)的分类 及工程特性指标 第一节 土(岩)的分类 第3.1.1条 作为建筑地基的土(岩),可分为岩石、碎石土、砂土、粉土、粘性土和人工填土等。 第3.1.2条 岩石应为颗粒间牢固联结,呈整体或具有节理裂隙的岩体。岩石根据其坚固性可分为硬质和软质;根据其风化程度可分为微风化、中等风化和强风化。岩石的划分,可按本规范附录二执行。 第3.1.3条 碎石土应为粒径大于2mm的颗粒含量超过全重50%的土。碎石土可按表3.1.3分为漂石、块石、卵石、碎石、圆砾和角砾;其密实度可按本规范附录三确定。 碎石土的分类 表3.1.3 土的名称 颗粒形状 粒组含量 漂 石 块 石 圆形及亚圆形为主 棱角形为主 粒径大于200mm的颗粒超过全重50% 卵 石 碎 石 圆形及亚圆形为主 棱角形为主 粒径大于200mm的颗粒超过全重50% 圆 砾 角 砾 圆形及亚圆形为主 棱角形为主 粒径大于2mm的颗粒超过全重50% 注:分类时应根据粒组含量由大到小以最先符合者确定。 第3.1.4条 砂土应为粒径大于2mm的颗粒含量不超过全重50%、粒径大于0.075mm的颗粒超过全重50%的土。砂土可按表3.1.4分为砾砂、粗砂、中砂、细砂和粉砂。 砂土的分类 表3.1.4 土的名称 粒 组 含 量 砾 砂 粗 砂 中 砂 细 砂 粉 砂 粒径大于2mm的颗粒占全重25~50% 粒径大于0.5mm的颗粒超过全重50% 粒径大于0.25mm的颗粒超过全重50% 粒径大于0.075mm的颗粒超过全重85% 粒径大于0.075mm的颗粒超过全重50% 注:分类时应根据粒组含量由大到小以最先符合者确定。 第3.1.5条 砂土的密实度,可按表3.1.5分为松散、稍密、中密、密实。 砂土的密实度 表3.1.5 标准贯入试验锤击数N 密 实 度 N≤10 松 散 10 第3.1.6条 粘性土应为塑性指数Ip大于10的土,可按表3.1.6分为粘土、粉质粘土。 粘性土的分类 表3.1.6 塑性指数IP 土的名称 IP>17 粘 土 10 硬 塑 可 塑 软 塑 流 塑 第3.1.8条 淤泥应为在静水或缓慢的流水环境中沉积,并经生物化学作用形成,其天然含水量大于液限、天然孔隙比大于或等于1.5的粘性土。当天然孔隙比小于1.5但大于或等于1.0的土应为淤泥质土。 第3.1.9条 红粘土应为碳酸盐岩系的岩石经红土化作用形成的高塑性粘土。其液限一般大于50。经再搬运后仍保留红粘土基本特征,液限大于45的土应为次生红粘土。 第3.1.10条 粉土应为塑性指数小于或等于10的土。其性质介于砂土与粘性土之间。 第3.1.11条 人工填土根据其组成和成因,可分为素填土、杂填土、冲填土。 素填土应为由碎石土、砂土、粉土、粘性土等组成的填土。杂填土应为含有建筑垃圾、工业废料、生活垃圾等杂物的填土。冲填土应为由水力冲填泥沙形成的填土。 第二节 工程特性指标 第3.2.1条 以载荷试验确定地基承载力标准值时,压板面积宜为0.25~0.50m2,载荷试验应符合本规范附录四的规定。 第3.2.2条 以室内试验、标准贯入、轻便触探或野外鉴别等方法确定地基土(岩)承载力标准值时,其方法和步骤应符合本规范附录五的规定,参加统计的数据不宜小于六个。标准贯入和轻便触探试验,应符合本规范附录六的要求。 第3.2.3条 以静力触探、旁压仪及其他原位测定试验确定地基承载力标准值或其他土性指标时,应与载荷试验或相应土性指标的直接试验结果进行对比后确定。 第3.2.4条 土的抗剪强度指标,可选用原状土室内剪切试验、现场剪切试验、十字板剪切试验等方法确定,并应符合下列要求: 一、对于一级建筑物当采用室内剪切试验时,取土钻孔不得少于六个。当土层均匀时每钻孔同一层土沿深度试验不得低于三组;当为多层且土层较薄时,试验不得少于一组,并应采用不固结不排水三轴压缩试验,对于其他等级建筑物如为可塑状粘性土与饱和度不大于0.5的粉土时,可采用直接剪切试验; 二、如采用固结剪切试验,则应考虑在建筑物荷载及预压荷载作用下地基可能固结的程度。 抗剪强度指标的标准值,可按本规范附录七确定。 第3.2.5条 岩石地基承载力设计值,可按本规范附录八用岩基载荷试验方法确定。对微风化及中等风化的岩石地基承载力设计值,也可根据室内饱和单轴抗压强度按下式计算: (3.2.5) 式中 f——岩石地基承载力设计值(kPa); frk——岩石饱和单轴抗压强度标准值(kPa),可按本规范附录九确定; ψ——折减系数。微风化岩宜为0.20~0.33;中等风化岩宜为0.17~0.25。取值时,对于硬质岩石着重考虑岩体中结构面间距、产状及其组合,软质岩石着重考虑其水稳性。 注:①上述折减系数值未考虑施工因素及建筑物使用后风化作用的继续; ②对于粘土质岩,在确保施工期及使用期不致遭水浸泡时,也可采用天然湿度的试样,不进行饱和处理。 第3.2.6条 土的压缩性指标,应由原状土的压缩试验确定,并应符合下列规定: 一、压缩系数和压缩模量,应按下列公式计算: (3.2.6—1) (3.2.6—2) 式中 α——压缩系数(MPa-1); Es——压缩模量(MPa); p1、p2——固结压力(kPa); e1、e2——对应于p1、p2时的孔隙比; e0——土的天然孔隙比。 二、地基压缩性可按p1为100kPa,p2为200kPa时相对应的压缩系数值a1-2划分为低、中、高压缩性,并应按以下规定进行评价: 1.当a1-2<0.1时,为低压缩性; 2.当0.1≤a1-2<0.5时,为中压缩性; 3.当a1-2≥0.5时,为高压缩性。 第3.2.7条 工程地质勘察报告,应按地基土(岩)的类别提供分层的土工试验总表,对于一、二级建筑物,尚应根据需要提供相应的强度试验、压缩试验以及原位试验等曲线,以及其他专门要求的测试结果。 第四章 基础埋置深度 第一节 一般规定 第4.1.1条 基础的埋置深度,应按下列条件确定: 一、建筑物的用途,有无地下室、设备基础和地下设施,基础的型式和构造; 二、作用在地基上的荷载大小和性质; 三、工程地质和水文地质条件; 四、相邻建筑物的基础埋深; 五、地基土冻胀和融陷的影响。 第4.1.2条 在满足地基稳定和变形要求前提下,基础应尽量浅埋,当上层地基的承载力大于下层土时,宜利用上层土作持力层。除岩石地基外,基础埋深不宜小于0.5m。 第4.1.3条 位于土质地基上的高层建筑,其基础埋深应满足稳定要求。位于岩石地基上的高层建筑,其基础埋深应满足抗滑要求。 第4.1.4条 基础宜埋置在地下水位以上,当必须埋在地下水位以下时,应采取地基土在施工时不受扰动的措施。 当基础埋置在易风化的软质岩层上,施工时应在基坑挖好后立即铺筑垫层。 第4.1.5条 当存在相邻建筑物时,新建建筑物的基础埋深不宜大于原有建筑基础。当埋深大于原有建筑基础时,两基础间应保持一定净距,其数值应根据荷载大小和土质情况而定,一般取相邻两基础底面高差的1~2倍。如上述要求不能满足时,应采取分段施工、设临时加固支撑、打板桩、地下连续墙等施工措施,或加固原有建筑物地基。 第二节 冻土地基的基础埋深及处理 第4.2.1条 地基土的冻胀性类别,应按表4.2.1分为不冻胀、弱冻胀、冻胀和强冻胀。 地基土的冻胀性分类 表4.2.1 土的名称 天然含水量 w(%) 冻结期间地下水位低于 冻深的最小距离(m) 冻胀性类别 岩石、碎石土、砾砂、粗砂、中砂、细砂 不考虑 不考虑 不冻胀 粉 砂 w<14 >1.5 不冻胀 ≤1.5 弱冻胀 14≤w<19 >1.5 ≤1.5 冻胀 w≥19 >1.5 ≤1.5 强冻胀 粉 土 w≤19 >2.0 不冻胀 ≤2.0 弱冻胀 19 ≤2.0 冻胀 22 ≤2.0 强冻胀 w>26 不考虑 粘性土 w≤wp+2 >2.0 不冻胀 ≤2.0 弱冻胀 wp+2 ≤2.0 冻胀 wp+5 ≤2.0 强冻胀 W>wp+9 不考虑 注:①表中碎石土仅指充填物为砂土或硬塑、坚硬状态的粘性土,如充填物为粉土或其他状态的粘性土时,其冻胀性应按粉土或粘性土确定; ②表中细砂仅指粒径大于0.075mm的颗粒超过全重90%的细砂,其他细砂的冻胀性应按粉砂确定; ③wp为土的塑限。 第4.2.2条 基础的最小埋深和基底下允许残留冻土层厚度,应符合下列规定: 一、对于埋置在不冻胀土中的基础,其埋深可不考虑冻深的影响;对于埋置在弱冻胀、冻胀和强冻胀土中的基础,其最小埋深可按下式计算: dmin=z0·ψt-dfr (4.2.2—1) 式中 dmin——基础最小埋深; z0——标准冻深,系采用在地表无积雪和草皮等覆盖条件下多年实测最大冻深的平均值。在无实测资料时,除山区外,可按图4.2.2采用; ψt——采暖对冻深的影响系数,可按本规范第4.2.3条规定采用; dfr——基底下允许残留冻土层的厚度。 二、基底下允许残留冻土层的厚度,应根据土的冻胀性类别按下列公式计算: 对弱冻胀土 dfr=0.17z0ψt+0.26 (4.2.2—2) 冻胀土 dfr=0.15z0ψt (4.2.2—3) 强冻胀土 dfr=0 (4.2.2—4) 当冻深范围内地基由不同冻胀性土层组成时,基础最小埋深可按下层土确定,但不宜浅于下层土的顶面。 注:当有充分依据时,允许残留冻土层厚度也可根据当地经验确定。 第4.2.3条 当室内地面直接建在有冻胀性土层上时,采暖对冻深的影响系数可按表4.2.3确定,对在采暖期间室内月平均温度小于10℃的建筑物可取1.00,不采暖的建筑物可取1.10。 采暖对冻深的影响系数ψ1 表4.2.3 室内外地面高差(mm) 外墙中段 外墙角段 ≤300 0.70 0.85 ≥750 1.00 1.00 注:①外墙角段系指从外墙阳角顶点起两边各4m范围以内的外墙,其余部分为中段; ②采暖建筑物中的不采暖房间(门斗、过道和楼梯间等),其外墙基础处的采暖对冻深的影响系数值,取与外墙角段的相同。 第4.2.4条 在有冻胀性土的地区,宜采用下列防冻害措施: 一、应尽量选择地势高、地下水位低、地表排水良好和土冻胀性小的建筑场地。对低洼场地,宜在沿建筑物四周向外一倍冻深距离范围内,使室外地坪至少高出自然地面300~500mm; 二、为了防止施工和使用期间的雨水、地表水、生产废水和生活污水浸入地基,应做好排水设施。在山区必须做好截水沟或在建筑物下设置暗沟,以排走地表水和潜水流,避免因基础堵水而造成冻害; 三、在冻深和土冻胀性均较大的地基上,宜采用独立基础、桩基础、白锚式基础(冻层下有扩大板或扩底短桩)。当采用条基时,宜设置非冻胀性垫层,其底面深度应满足基础最小埋深的要求; 四、对标准冻深大于2m、基底以上为强冻胀土的采暖建筑及标准冻深大于1.5m、基底以上为冻胀土和强冻胀土的非采暖建筑,为防止冻切力对基础侧面的作用,可在基础侧面回填粗砂、中砂、炉渣等非冻胀性散粒材料或采取其它有效措施; 五、在冻胀和强冻胀性地基上,宜设置钢筋混凝土圈梁和基础联系梁,并控制建筑物的长高比,增强房屋的整体刚度; 六、当基础联系梁下有冻胀性土时,应在梁下填以炉渣等松散材料,根据土的冻胀性大小可预留50~150mm空隙,以防止因土冻胀将基础联系梁拱裂; 七、外门斗、室外台阶和散水坡等宜与主体结构断开。散水坡分段不宜过长,坡度不宜过小,其下宜填以非冻胀性材料; 八、按采暖设计的建筑物,如冻前不能交付正常使用,或使用中因故冬季不能采暖时,应对地基采取相应的过冬保温措施;对非采暖建筑的跨年度工程,入冬前基坑应及时回填。 第五章 地基计算 第一节 承载力计算 第5.1.1条 基础底面压力的确定,应符合下式要求: 当轴心荷载作用时 p≤f (5.1.1—1) 式中 p——基础底面处的平均压力设计值; f——地基承载力设计值。 当偏心荷载作用时,除符合式(5.1.1-1)要求外,尚应符合下式要求: pmax≤1.2f (5.1.1—2) 式中pmax——基础底面边缘的最大压力设计值。 第5.1.2条 确定地基承载力时,应结合当地经验按下列规定综合考虑: 一、对一级建筑物采用载荷试验、理论公式计算及其他原位试验等方法综合确定; 二、对表2.0.2所列的二级建筑物,可按本规范第3.2.2条或其他原位试验确定。其余的二级建筑物,尚应结合式(5.1.4)计算确定; 注:当由本规范第3.2.2条确定的数值与当地经验有明显差异时,仍应由载荷试验、理论公式计算等综合确定。 三、对三级建筑物可根据邻近建筑物的经验确定。 第5.1.3条 地基承载力设计值,应符合下列规定: 一、当基础宽度大于3m或埋置深度大于0.5m时,除岩石地基外,其地基承载力设计值应按下式计算: f=fk+ηb (b-3)+ηd o (d-0.5) (5.1.3) 式中 f——地基承载力设计值; fk——地基承载力标准值,按本规范第3.2.1条至3.2.3条确定; ηb、ηd——基础宽度和埋深的地基承载力修正系数,按基底下土类查表5.1.3; ——土的重度,为基底以下土的天然质量密度ρ与重力加速度g的乘积,地下水位以下取有效重度; b——基础底面宽度(m),当基宽小于3m按3m考虑,大于6m按6m考虑; o——基础底面以上土的加权平均重度,地下水位以下取有效重度; d——基础埋置深度(m),一般白室外地面标高算起。在填方整平地区,可自填土地面标高算起,但填土在上部结构施工后完成时,应从天然地面标高算起。对于地下室,如采用箱形基础或筏基时,基础埋置深度自室外地面标高算起,在其他情况下,应从室内地面标高算起。 当计算所得设计值f<1.1fk时,可取f=1.1fk; 二、当不满足按(5.1.3)式计算的条件时,可按f=1.1fk直接确定地基承载力设计值。 承载力修正系数 表5.1.3 土 的 类 别 ηbP ηd 淤泥和淤泥质土 fk<50kPa fk≥50kPa 0 0 1.0 1.1 人工填土 e或IL大于等于0.85的粘性土 e≥0.85或Sr>0.5的粉土 0 1.1 红 粘 土 含水比αw>0.8 含水比αw≤0.8 0 0.15 1.2 1.4 e及IL均小于0.85的粘性土 e(0.85及Sr≤0.5的粉土 粉砂、细砂(不包括很湿与饱和时的稍密状态) 中砂、粗砂、砾砂和碎石土 0.3 0.5 2.0 3.0 1.6 2.2 3.0 4.4 注:①强风化的岩石,可参照所风化成的相应土类取值; ②Sr为土的饱和度,Sr≤0.5,稍湿;0.5 第5.1.4条 当偏心距e小于或等于0.033倍基础底面宽度时,根据土的抗剪强度指标确定地基承载力可按下式计算: fv=Mb b+Md od+Mcck (5.1.4) 式中 fv——由土的抗剪强度指标确定的地基承载力设计值; Mb、Md、Mc——承载力系数,按表5.1.4确定; b——基础底面宽度,大于6m时按6m考虑,对于砂土小于3m时按3m考虑; ck——基底下一倍基宽深度内土的粘聚力标准值。 承载力系数Mb、Md、Mc 表5.1.4 土的内摩擦角标准值 ψk(°) Mb Md Mc 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 0 0.03 0.06 0.10 0.14 0.18 0.23 0.29 0.36 0.43 0.51 0.61 0.80 1.10 1.40 1.90 2.60 3.40 4.20 5.00 5.80 1.00 1.12 1.25 1.39 1.55 1.73 1.94 2.17 2.43 2.72 3.06 3.44 3.87 4.37 4.93 5.59 6.35 7.21 8.25 9.44 10.84 3.14 3.32 3.51 3.71 3.93 4.17 4.42 4.69 5.00 5.31 5.66 6.04 6.45 6.90 7.40 7.95 8.55 9.22 9.97 10.80 11.73 第5.1.5条 基础底面的压力,可按下列公式确定: 一、当轴心荷载作用时 (5.1.5-1) 式中 F——上部结构传至基础顶面的竖向力设计值; G——基础白重设计值和基础上的土重标准值; A——基础底面面积。 二、当偏心荷载作用时 (5.1.5—2) (5.1.5—3) 式中 M——作用于基础底面的力矩设计值; W——基础底面的抵抗矩; pmin——基础底面边缘的最小压力设计值; 图5.1.5偏心荷载 下基底压力计算示意 b—力矩作用方向基础底面边长 当偏心距e>b/6时(图5.1.5),pmin应按下式计算: (5.1.5—4) 式中 l——垂直于力矩作用方向的基础底面边长; α——合力作用点至基础底面最大压力边缘的距离。 第5.1.6条 当地基受力层范围内有软弱下卧层时,应按下式验算: pz+pcz=fz (5.1.6) 式中 pz——软弱下卧层顶面处的附加压力设计值; pcz——软弱下卧层顶面处土的白重压力标准值; fz——软弱下卧层顶面处经深度修正后地基承载力设计值。 第5.1.7条 当上层土与下卧软弱土层的压缩模量比值大于或等于3时,对条形基础和矩形基础,式(5.1.6)中的pz值可按下列公式简化计算: 条形基础 (5.1.7—1) 矩形基础 (5.1.7—2) 式中 b——矩形基础和条形基础底边的宽度; l——矩形基础底边的长度; pc——基础底面处土的白重压力标准值; z——基础底面至软弱下卧层顶面的距离; θ——地基压力扩散线与垂直线的夹角,可按表5.1.7采用。 地基压力扩散角θ 表5.1.7 Es1/Es2 z/b 0.25 0.50 3 5 10 6° 10° 20° 23° 25° 30° 注:①Es1为上层土压缩模量:Es2为下层土压缩模量; ②z<0.25b时一般取θ=0°,必要时,宜由试验确定;z>0.50b时θ值不变。 第5.1.8条 对于沉降已经稳定的建筑或经过预压的地基,可适当提高地基承载力。 第二节 变形计算 第5.2.1条 建筑物的地基变形计算值,不应大于地基变形允许值。 第5.2.2条 地基变形特征可分为沉降量、沉降差、倾斜、局部倾斜。 第5.2.3条 在计算地基变形时,应符合下列规定: 一、由于建筑地基不均匀、荷载差异很大、体型复杂等因素引起的地基变形,对于砌体承重结构应由局部倾斜控制;对于框架结构和单层排架结构应由相邻柱基的沉降差控制;对于多层或高层建筑和高耸结构应由倾斜值控制; 二、在必要情况下,需要分别预估建筑物在施工期间和使用期间的地基变形值,以便预留建筑物有关部分之间的净空,考虑连接方法和施工顺序。此时,一般建筑物在施工期间完成的沉降量,对于砂土可认为其最终沉降量已基本完成,对于低压缩粘性土可认为已完成最终沉降量的50~80%,对于中压缩粘性土可认为已完成20~50%,对于高压缩粘性土可认为已完成5~20%。 第5.2.4条 建筑物的地基变形允许值,可按表5.2.4规定采用。对表中未包括的其他建筑物的地基变形允许值,可根据上部结构对地基变形的适应能力和使用上的要求确定。 |
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