Chapter 32 - Deep Foundations - Index of - Free

TABLE 32.5Typical Values of Bearing Capacity FactorN qϕ a (degrees) 26 28 30 31 32 33 34 35 36 37 38 39 40N q(driven pile displacement) 10 15 21 24 29 35 42 50 62 77 86 120 145b(drilled piers) 5 8 10 12 14 17 21 25 30 38 43 60 72aLimit ϕ to 28° if jetting is used.b1. In case a bailer of grab bucket is used below the groundwater table, calculate end bearing based on ϕ notexceeding 28°.2. For piers greater than 24-in. diameter, settlement rather than bearing capacity usually controls the design. Forestimating settlement, take 50% of the settlement for an equivalent footing resting on the surface of comparablegranular soils (Chapter 5, DM-7.01).Source: NAVFAC [42].N qQ end_maxcN c A pile32.4.2 End BearingEnd bearing is part of the axial compressive resistance provided at the bottom of a pile by theunderlying soil or rock. The resistance depends on the type and strength of the soil or rock and onthe stress conditions near the tip. Piles deriving their capacity mostly from end bearing are calledend bearing piles. End bearing in rock and certain types of soil such as dense sand and gravel isusually large enough to support the designed loads. However, these types of soil or rock cannot beeasily penetrated through driving. No or limited uplift resistance is provided from the pile tips;therefore, end-bearing piles have low resistance against uplift loading.The end bearing of a pile can be expressed as:=⎧⎪⎪σ v ′ N q A pile⎨U ⎪----- c⎪ 2 N k A pile⎩for clayfor sandfor rock(32.6)whereQ end_max= the maximum end bearing of a pileA pile= the area of the pile tip or baseN c, N q, N k= the bearing capacity factors for clay, sand, and rockc = the cohesion of clayσ′v= the effective overburden pressureUcU c= the unconfined compressive strength of rock and Su, the equivalent shearstrength of rock2 =ClayThe bearing capacity factorN cfor clay can be expressed asLN = c60 . ⎛1⎝+ 02 . ⎞D⎠ ≤ 9(32.7)where L is the embedment depth of the pile tip and D is the diameter of the pile.SandThe bearing capacity factor N qgenerally depends on the friction angle φ of the sand and can beestimated by using Table 32.5 or the Meyerhof equation below.© 2000 by CRC Press LLC