基础工程英文复习课件

1、12:36:481基础工程英文复习 基础工程基础工程 研究下部结构物与岩土研究下部结构物与岩土相互作用相互作用共同承担上部结构物共同承担上部结构物 所产生各种所产生各种强度强度、变形变形与与稳定稳定问题。问题。 桩、墩基、箱基、支挡结构、围护结构、水泥搅拌体，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，都可视为砂石桩体，锚固，土钉、砂井等等，都可视为结构物结构物 与岩土相互作用问题与岩土相互作用问题。 基础工程英文复习 Depends on Required reliability Consequences of a failure Uncertainti

2、es in soil properties and applied loads Construction tolerances Ignorance of the true behavior of foundations Cost-benefit ratio of additional conservatism in the design. 基础工程英文复习 12:36:484基础工程英文复习 0.75 a0.75 a 0.75 a 0.75 a The design load is the most critical combination of the various load source

3、s ,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+(W or E)(2.4 ) 12:36:485基础工程英文复习 n i QikciQikQQGkG SSSS 2 11 组合值系数c 12:36:486基础工程英文复习 n i QikciQiGkG SSS 1 组合值系数c 12:36:487基础工程英文复习 n i QikciQikQQGkG SSSS 2 11 n i QikciQiGkG SSS 1 1 QiG 组合值系数

4、c 12:36:488基础工程英文复习 Intended to produce foundations that perform well when subjected to the service loads. The Requirements include: Settlement Heave抬升 Tilt Lateral movement Vibration Durability 12:36:499基础工程英文复习 Adjust the foundation design Use a more elaborate foundation Improve the properties of

5、the soil Redesign the structure so it is more tolerant of settlements 12:36:4910基础工程英文复习 Definition The differential settlement is the difference in total settlement between two foundation or between two points on a single foundation. Reasons for Differential settlements: The soil pro not be uniform

6、 across the site The ratio between the actual load and the design load may be different for each column. The ratio of dead load to live load may be different for each column. The as-built foundation dimensions may differ from the plan dimensions. 12:36:4911基础工程英文复习 12:36:4912基础工程英文复习 d/b=2.5 Shallow

7、 foundations Those transmit structural loads to the near-surface soil. Types: Spread footing foundations (扩展基础) Mat foundations （筏板基础） 12:3613 d b d b 基础工程英文复习 基础工程英文复习 基础工程英文复习 基础工程英文复习 1. Shallow foundations are those that transmit structural loads to the near-surface soils. There are two kinds: s

8、pread footing foundations and mat foundations. 2. Although 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.

9、Mats are most often used on larger structures, especially 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. 12:3617基础工程英文复习 5. A floa

10、ting foundation is one where the weight of the foundation is 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 p

11、ressure distribution also will be eccentric. In such cases, the 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

12、 base of the foundation has compressive bearing pressures, and thus avoids problems with uplift. 12:3618基础工程英文复习 12:36:4919基础工程英文复习 General failure case Relatively incompressible and reasonably strong soil Rock, dense sands Saturated undrained clay Failure occurs quite suddenly 12:36:4920基础工程英文复习 Pu

13、nching failure case Very loose sand A thin crust of strong soil underlain by a very weak soil Weak clays under drained conditions Failure develops gradually. 12:36:5021基础工程英文复习 Local failure case-the intermediate case Loose to medium sands Shear surface are well defined under the foundation, and the

14、n become vague near the ground surface. Failure develops gradually. 12:36:5022基础工程英文复习 A 30-m by 50-m mat foundation is to be built as shown in Figure 6.10. Compute the ultimate bearing capacity. 12:36:5023基础工程英文复习 12:36:5024基础工程英文复习 Assumptions: DB No sliding between the foundation and the soil. Se

15、mi-infinite mass and uniform MC strength law General shear case No consolidation Rigid foundation Soil above the foundation bottom is No shear strength Central loading 12:36:5025基础工程英文复习 12:36:5026基础工程英文复习 27 wb zDB 1 wbb DzDB 2 wb zD 3 +z Db B B ob DB对和均无影响 ob DB对和均有影响 B仅对有影响，计算时，取厚度加权平均值。 12:36:50

16、基础工程英文复习 Given: A square footing as Figure 6.6. Dw=50 ft. Find: the ultimate bearing capacity the column load required to produce a bearing capacity failure. 12:36:5128基础工程英文复习 12:36:5129基础工程英文复习 Methods: Evaluate the bearing capacity using the lowest values of c, f and in the zone between the botto

17、m of the foundation and a depth B below the bottom. Evaluate the bearing capacity using the weighted average values of c, f and 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. 12:36:5130基础工程英文复习

18、 12:36:5131基础工程英文复习 The application of structural loads The 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 excavations 基础工程英文复习 基础工程英文复习 基础工程

19、英文复习 基础工程英文复习 The 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. 基础工程英文复习 见课本224 基础工程英文复习 基础工程英文复习 基础工程英文复习 Es from Standard Penetration Test (SPT) results Soil Type0(kP

20、a)1(kPa) Clean sands 5,0001,200 Silty sands and clayey sands2,500600 基础工程英文复习 12:36:5241基础工程英文复习 qMost of the Bearing Capacity will be lost . qeasy to be undercut by scour(冲刷冲刷), especially on bridge piers(桥墩桥墩) qThe top soil usually is poor in strength. qUnable to resist horizontal load, or very po

21、or if any. qEasy to be impacted by the weather. q Frost heave（冻胀） tan (1)cot (Prandtl,1920) 2(1)tan qp cq q NK e NN NN f f f 12:3642基础工程英文复习 q Make D shallower, if possible (both bearing capacity and settlement are satisfied.). q Save time, money and material q 宽基浅埋 q Make D above the groundwater le

22、vel in construction stage, if possible. 12:3643基础工程英文复习 When a new footing is carried out near a old ones what will happen? 挖墙脚挖墙脚 12:3644基础工程英文复习 Bearing capacity D the smallest one Dw The shallowest one F Safety factor against a bearing failure( Per Fig. 6.11) qa The smallest applied normal load.

23、Bearing.xls, or qult_tzg or qult_vsc Settlement d da and d dDa Allowable settlement and allowable differential settlement Ch2 d dD/d d Table 7.5 or local experience d da =Min(d da ,d dDa/(d dD/d d) )由由 differential settlement算出来的算出来的 允许沉降和由允许沉降的最小允许沉降和由允许沉降的最小 值。值。 Settlement analysis The largest ap

24、plied normal load Settlement.xls or S4chmertmann.xls. keep d20 For steel and concrete pile ,D/B35 12:36105基础工程英文复习 Assume EI is infinite. Not accurate as the nonrigid method Used primarily for lightweight short lateral loaded foundation Streetlights Small highway signs 12:36106基础工程英文复习 1. A lateral

25、load is any load that acts perpendicular 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

26、 batter piles to resist horizontal loads. 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,

27、 and possibly more reliable. 4. The analysis of laterally-loaded deep foundations is a soil structure interaction problem that requires simultaneous consideration fo structural and geotechnical aspects. 12:36:56107基础工程英文复习 5. When conducting lateral load analyses, engineers usually assume one of the

28、 following boundary conditions at the top of the foundation: 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-struc

29、ture interaction analyses. Load tests are sometimes used 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 emp