In order to separate their behavioural responses to applied pile load, soils are classified as either granular/noncohesive or clays/cohesive. The generic formulae used to predict soil resistance to pile load include empirical modifying factors which can be adjusted according to previous engineering experience of the influence on the accuracy of predictions of changes in soil type and other factors such as the time delay before load testing.
The load settlement response is composed of two separate components, the linear elastic shaft friction Rs and non-linear base resistance Rb. The concept of the separate evaluation of shaft friction and base resistance forms the bases of "static or soil mechanics" calculation of pile carrying capacity. The basic equations to be used for this are written as:
Q = Qb + Qs - Wp or
Rc = Rb + Rs - Wp
Rt = Rs + Wp
Where: Q = Rc = the ultimate compression resistance of the pile
Qb = Rb = base resistance
Qs = Rs = shaft resistance
Wp = weight of the pile
In terms of soil mechanics theory, the ultimate skin friction on the pile shaft is related to the horizontal effective stress acting on the shaft and the effective remoulded angle of friction between the pile and the clay and the ultimate shaft resistance Rs can be evaluated by integration of the pile-soil shear strength t a over the surface area of the shaft:
t a = Ca + s n tanf a
Where: s n = Ks s v
\ t a = Ca + KS s v tanf a
where: p = pile perimeter
L = pile length
f = angle of friction between pile and soil
Ks = coefficient of lateral pressure
the ultimate bearing capacity, Rb, of the base is evaluated from the bearing capacity theory:
Ab = area of pile base
C = undrained strength of soil at base of pile
NC = bearing capacity factor
……………………………………………4.1
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Nevertheless, in practise, for a given pile at a given site, the undrained shear strength Ca varies considerably with many factors, including, pile type, soil type, and methods of installations.
Ideally, Ca should be determined from a pile-load test, but since this is not always possible, Ca is correlated with the undrained cohesion Cu by empirical adhesion factor a so that the general expression in e.q. (4-1) could be simplified to the following expression:
……………………………………………4.2
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Where: Ws = weight of soil replaced by the pile
1 The undrained load capacity (total stress approach)
For piles in clay, the undrained load capacity is generally taken to be the critical value unless the clay is highly over consolidated. If the undrained or short-term ultimate load capacity is to be computed, the soil parameters C, q ,a , gshould be appropriate to undrained conditions and s v and s vb should be the total stresses. If the clay is saturated , the undrained angle of friction f u is zero, and f a (angle of friction between pile and soil) may also be taken as zero. In addition, Nq = 1, Ng = 1, so that the eq in(4-1) reduces to:
……………………………………………4.3
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Where: Nc, Nq, Ng ,= bearing capacity factors and are functions of the internal angle of friction f of the soil, the relative compressibility of the soil and the pile geometry.
2 Drained load capacity (effective stress approach)
For piles installed in stiff, over consolidated clays, the drained load capacity is taken as design criterion. If the simplified assumption is made that the drained pile-soil adhesion Ca is zero and that the term in eq (4-1)…involving Nc, Ngignoring the drained ultimate bearing capacity of the pile may be expressed as :
……………………………………………4.4
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Where: s v, and s vb = effective vertical stress at depth z respective at pile base
f a,= effective angle of friction between pile/soil and implied can be taken as f ,
Nq which is dependant up on the values of f may be taken to be the same as for piles in sand, and can be decided
3 Pile in sand
If the pile soil adhesion Ca and term Nc are taken as zero in e.q (4-1)… and the terms 0.5g d Ng is neglected as being small in relation to the term involving Ng , the ultimate load capacity of a single pile in sand may be expressed as follows:
……………………………………………4.5
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Where: s v, and s vb = effective vertical stress at depth z respective at pile base
Fw = correction factor for tapered pile ( = 1 for uniform diameter)
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