基础工程英文复习112课件

1、基础工程基础工程研究下部结构物与岩土相互作用共同承担上部结构物所产生各种强度、变形与稳定问题。桩、墩基、箱基、支挡结构、围护结构、水泥搅拌体，砂石桩体，锚固，土钉、砂井等等，都可视为结构物与岩土相互作用问题。 Factors of SafetyDepends onRequired reliabilityConsequences of a failureUncertainties in soil properties and applied loadsConstruction tolerancesIgnorance of the true behavior of foundationsCost-

2、benefit ratio of additional conservatism in the design.第二章20:12:5830.75 a0.75 a Allowable stress design(use in this book)The design load is the most critical combination of the various load sources ,as defined by codes.ANSI/ASCE，D(2.1)D+L+F+H+T+(Lr or S or R)(2.2)D+L+(Lr or S or R)+(W or E)(2.3 )D+(

3、W or E)(2.4 )20:12:584（1）由可变荷载控制时：组合值系数c20:12:585（2）由永久荷载控制时：组合值系数c20:12:586（四）计算挡土墙、地基或斜坡稳定基滑坡推力时，荷载效应，按承载能力极限状态下荷载效应的基本组合，但其分项系数均为1：组合值系数c20:12:587Serviceability requirementsIntended to produce foundations that perform well when subjected to the service loads.The Requirements include:SettlementHea

4、ve抬升TiltLateral movementVibrationDurability20:12:588Measures for a large settlementAdjust the foundation designUse a more elaborate foundationImprove the properties of the soilRedesign the structure so it is more tolerant of settlements20:12:589Differential settlementsDefinitionThe differential sett

5、lement is the difference in total settlement between two foundation or between two points on a single foundation.Reasons for Differential settlements:The soil profile may not be uniform across the siteThe ratio between the actual load and the design load may be different for each column.The ratio of

6、 dead load to live load may be different for each column.The as-built foundation dimensions may differ from the plan dimensions.20:12:5810第五章20:12:5811What is shallow foundations?d/b=2.5Shallow foundationsThose transmit structural loads to the near-surface soil.Types:Spread footing foundations (扩展基础

7、)Mat foundations （筏板基础）20:1212dbdbFoundations with eccentric or moment loadsOne-way loadingTwo-way eccentric or moment loadingsummary1. Shallow foundations are those that transmit structural loads to the near-surface soils. There are two kinds: spread footing foundations and mat foundations.2. Altho

8、ugh other materials have been used in the past, today virtually all shallow foundations are made of reinforced concrete.3. Spread footings are most often used in small- to medium-size structures on sites with moderate to good soil conditions. Mats are most often used on larger structures, especially

9、 those with differential settlement problems and those with foundations below the groundwater table.4. The bearing pressure is the contact pressure between the bottom of a shallow foundation and the underlying soils.20:1216summary5. A floating foundation is one where the weight of the foundation is

10、substantially less than the weight of the excavated soils. This occurs in buildings with basements and other similar structures.6. If the loads applied to a foundation are eccentric, or if moment loads are applied, the resulting bearing pressure distribution also will be eccentric. In such cases, th

11、e foundation needs to be designed so the resultant of the bearing pressure is within the middle third of the foundation (for one-way eccentricity) or in a diamond-shaped kern (for two-way eccentricity). This requirement ensures the entire base of the foundation has compressive bearing pressures, and

12、 thus avoids problems with uplift.20:1217第六章20:12:5818Bearing capacity failureGeneral failure caseRelatively incompressible and reasonably strong soilRock,dense sandsSaturated undrained clayFailure occurs quite suddenly 20:12:5819Bearing capacity failurePunching failure caseVery loose sandA thin cru

13、st of strong soil underlain by a very weak soilWeak clays under drained conditionsFailure develops gradually.20:12:5820Bearing capacity failureLocal failure case-the intermediate caseLoose to medium sandsShear surface are well defined under the foundation, and then become vague near the ground surfa

14、ce.Failure develops gradually.20:12:5821Example 6.3A 30-m by 50-m mat foundation is to be built as shown in Figure 6.10. Compute the ultimate bearing capacity.20:12:582220:12:5823Terzaghis bearing capacity formulasAssumptions:DBNo sliding between the foundation and the soil.Semi-infinite mass and un

15、iformMC strength lawGeneral shear caseNo consolidationRigid foundationSoil above the foundation bottom is No shear strength Central loading20:12:5824Terzaghis bearing capacity formulas20:12:5825Groundwater effects26123+zDbBB20:12:58Example 6.1Given:A square footing as Figure 6.6. Dw=50 ft. Find:the

16、ultimate bearing capacity the column load required to produce a bearing capacity failure. 20:12:582720:12:5828Bearing capacity on layer soilsMethods:Evaluate the bearing capacity using the lowest values of c, f and g in the zone between the bottom of the foundation and a depth B below the bottom.Eva

17、luate the bearing capacity using the weighted average values of c, f and g in the zone between the bottom of the foundation and a depth B below the bottom.Rigorous analysis use limit method, similar to slope stability analysis.20:12:5829第7章20:12:5830The causes of settlementThe application of structu

18、ral loadsThe weight of a recently placed fill A falling groundwater table Underground mining or tunneling The formation of sinkholes Secondary compression of the underlying soils Lateral movements resulting from nearby excavationsConsolidation settlement(NC)Consolidation settlement(OC-I)Consolidatio

19、n settlement(OC-II)EXAMPLE 7.4The allowable settlement for the proposed continuous footing in Figure 7.9 is 25 mm. Using the classical method. compute its settlement and determine if it satisfies this criterion. 见课本224Settlement analyses based on in-situ testsSchmertmanns MethodEs from Standard Pene

20、tration Test (SPT) resultsSoil Type0(kPa)1(kPa)Clean sands 5,0001,200Silty sands and clayey sands2,500600第八章20:12:5840Question: can D be 0? Most of the Bearing Capacity will be lost .easy to be undercut by scour(冲刷), especially on bridge piers(桥墩)The top soil usually is poor in strength.Unable to re

21、sist horizontal load, or very poor if any.Easy to be impacted by the weather.Frost heave（冻胀）20:1241Make D shallower, if possible (both bearing capacity and settlement are satisfied.).Save time, money and material宽基浅埋Make D above the groundwater level in construction stage, if possible.Other general

22、principles to determine D20:1242The influence of the adjacent buildingsWhen a new footing is carried out near a old ones what will happen?挖墙脚20:1243The procedure to develop qABearing capacityD the smallest oneDw The shallowest oneF Safety factor against a bearing failure( Per Fig. 6.11) qaThe smalle

23、st applied normal load.Bearing.xls, or qult_tzg or qult_vscSettlementda and dDaAllowable settlement and allowable differential settlementCh2dD/dTable 7.5 or local experienceda =Min(da ,dDa/(dD/d)由differential settlement算出来的允许沉降和由允许沉降的最小值。Settlement analysisThe largest applied normal loadSettlement.x

24、ls or S4chmertmann.xls.keep d20For steel and concrete pile ,D/B35 20:12104Rigid analysisAssume EI is infinite.Not accurate as the nonrigid methodUsed primarily for lightweight short lateral loaded foundationStreetlightsSmall highway signs20:121051. A lateral load is any load that acts perpendicular

25、to the foundation axis. Thus, shear moment are lateral loads, but axial compression or tension or torsional loads are not. 2. Until the middle of the twentieth century, engineers assumed that deep foundations were only able to resist axial loads, so they used batter piles to resist horizontal loads.

26、 More recently, we have reconsidered that assumption and now rely on both axial and lateral capacities.3. The utilization of lateral capacities in design often produces foundations that are more economical to build, more efficient in resisting seismic loads, and possibly more reliable.4. The analysi

27、s of laterally-loaded deep foundations is a soil structure interaction problem that requires simultaneous consideration fo structural and geotechnical aspects.20:12:581065. When conducting lateral load analyses, engineers usually assume one of the following boundary conditions at the top of the foun

28、dation: The free-head condition, the restrained0head condition, or the pure moment condition.6. lateral load capacity may be evaluated using full-scale load tests, model load tests, rigid analyses, depth to fixity analyses, or nonrigid soil-structure interaction analyses. Load tests are sometimes us

29、ed on larger projects, and rigid analyses may be appropriate for some lightly-loaded foundations, but most lateral load problems are analyzed using the p-y method, which is a type of nonrigid soil structure interaction analysis.7. The p-y method uses nonlinear p-y curves to describe the lateral soil pressure acting on the foundation and a finite difference analysis to compute the deflections, shears, and moments .Design p-y method curves are based on empirical