Chapter 32 - Deep Foundations - Index of - Free

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DowndragFor piles in soft soil, another deformation-related issue should be noted. When the soil surroundingthe pile settles relative to a pile, the side friction, also called the negative skin friction, should beconsidered when there exists underlying compressible clayey soil layers and liquefiable loose sandlayers. Downdrag can also happen when ground settles because of poor construction of caissons insand. On the other hand, updrag should also be considered in cases where heave occurs around thepiles for uplift loading condition, especially during installation of piles and in expansive soils.32.4.4 Settlement of Individual Pile, t–z, Q–z CurvesBesides bearing capacity, the allowable settlement is another controlling factor in determining theallowable capacity of a pile foundation, especially if layers of highly compressible soil are close toor below the tip of a pile.Settlement of a small pile (diameter less than 350 mm) is usually kept within an acceptable range(usually less than 10 mm) when a factor of safety of 2 to 3 is applied to the ultimate capacity toobtain the allowable capacity. However, in the design of large-diameter piles or caissons, a separatesettlement analysis should always be performed.The total settlement at the top of a pile consists of immediate settlement and long-term settlement.The immediate settlement occurs during or shortly after the loads are applied, which includes elasticcompression of the pile and deformation of the soil surrounding the pile under undrained loadingconditions. The long-term settlement takes place during the period after the loads are applied, whichincludes creep deformation and consolidation deformation of the soil under drained loading conditions.Consolidation settlement is usually significant in soft to medium stiff clayey soils. Creep settlementoccurs most significantly in overconsolidated (OC) clays under large sustained loads, and canbe estimated by using the method developed by Booker and Poulos (1976). In principle, however,long-term settlement can be included in the calculation of ultimate settlement if the design parametersof soil used in the calculation reflect the long-term behavior.Presented in the following sections are three methods that are often used:• Method of solving ultimate settlement by using special solutions from the theory of elasticity[50,85]. Settlement is estimated based on equivalent elasticity in which all deformation ofsoil is assumed to be linear elastic.• Empirical method [79].• Method using localized springs, or the so called t–z and Q–z method [52a].Method from Elasticity SolutionsThe total elastic settlement S can be separated into three components:S= Sb + Ss + Ssh(32.19)where S bis part of the settlement at the tip or bottom of a pile caused by compression of soil layersbelow the pile under a point load at the pile tip, and is expressed asSb=pDIbEb bbs(32.20)S sis part of the settlement at the tip of a pile caused by compression of soil layers below the pileunder the loading of the distributed side friction along the shaft of the pile, and can be expressed asSs( fl∆z) I= ∑ Eisi i i bss(32.21)© 2000 by CRC Press LLC
andS shis the shortening of the pile itself, and can be expressed asSshflsi izi + pAb bzi= ∑ ( ∆ ) ( ∆ )E ( A)ici(32.22)wherep b= averaged loading pressure at pile tipA b= cross section area of a pile at pile tip; Ap b bis the total load at the tipD b= diameter of pile at the pile tipi = subscript for ith segment of the pilel = perimeter of a segment of the pile∆z = axial length of a segment of the pile; L= ∑ ∆zis the total length of the pile.f s= unit friction along side of shaft; fl is the side frictional force for segment of the pilesi i∆ziiE s= Young’s modulus of uniform and isotropic soilE c= Young’s modulus of the pileI bb= base settlement influence factor, from load at the pile tip (Figure 32.4)I bs= base settlement influence factor, from load along the pile shaft (Figure 32.4)Because of the assumptions of linear elasticity, uniformity, and isotropy for soil, this method isusually used for preliminary estimate purposes.Method by Vesic [79]The settlement S at the top of a pile can be broken down into three components, i.e.,iiS= Sb + Ss + Ssh(32.23)Settlement due to shortening of a pile isS Q Q Lsh= (p+ αs s)AEc(32.24)whereQ pQ sα sLAE c= point load transmitted to the pile tip in the working stress range= shaft friction load transmitted by the pile in the working stress range (in force units)= 0.5 for parabolic or uniform distribution of shaft friction, 0.67 for triangular distribution ofshaft friction starting from zero friction at pile head to a maximum value at pile tip, 0.33 fortriangular distribution of shaft friction starting from a maximum at pile head to zero at thepile tip= pile length= pile cross-sectional area= modulus of elasticity of the pileSettlement of the pile tip caused by load transmitted at the pile tip isCQpSb=Dqop(32.25)© 2000 by CRC Press LLC
Page 1 and 2: Deng, N., Ma, Y. "Deep Foundations"
Page 3 and 4: The merit of a deep foundation over
Page 5 and 6: TABLE 32.1Range of Maximum Capacity
Page 7 and 8: piles can also be used as casings t
Page 9 and 10: FIGURE 32.3Resistances of an indivi
Page 11 and 12: Inclined or battered piles should n
Page 13 and 14: TABLE 32.4Summary of Design Methods
Page 15 and 16: • Rupture of active fault and she
Page 17 and 18: TABLE 32.5Typical Values of Bearing
Page 19 and 20: whereq c1= averaged cone tip resist
Page 21: TABLE 32.10Typical Value of Pile-So
Page 25 and 26: whereC p= empirical coefficient dep
Page 27 and 28: FIGURE 32.6 Load transfer for side
Page 29 and 30: ⎧ L′ 20 - 2L′L 0 ′ + 0.5L
Page 31 and 32: p-y Curves for Soft ClayMatlock [35
Page 33 and 34: TABLE 32.13Relative toWater TableFr
Page 35 and 36: where p is the lateral force per un
Page 37 and 38: FIGURE 32.14Block failure model for
Page 39 and 40: 90°< θ < 180°β β β β β θ
Page 41 and 42: NGx ,= ∑ Khh,ii=1KNGy ,= ∑ Khh,
Page 43 and 44: 16. Dunnavant, T. W. and M. W. O’
Page 45 and 46: 62. Rowe, R. K. and H. H. Armitage,

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