In the case of large dia meter bored piles (drilled shafts), the ultimate base resistance is fully mobilized at displacements not less than 200 mm (see §4.2) and often much larger; the shaft resistance, on the contrary, is mobilized at much smaller displace- ments, around 20 mm. In dense or stiff soils the shaft resistance often exhibits unsta- ble beha vi our, in the sense that it de velops a peak followed by a decrease towards a residual value. At displacements large enough to fully mobilize the base resistance, the shaft resistance can attain a value much smaller than the peak; accordingly, the ultimate pile capa city cannot be computed as the sum of the ultimate base and shaft capacities. Even if the shaft resistance increased mono tonically up to the ultimate value and then kept constant, the displacements needed would be so high that a ser- vice load obtained by applying the usual safety factor would cor res pond usually to displacements that cannot be sustained by the structure.
To exemplify these con sidera tions, let us refer to Figure 4.7, where schematic dia- grams of the de velopment of the resistances for different piles embedded in a homo- gen eous granular soil are reported.
For a medium dia meter pile, either displacement or replacement (Figures 4.7a, 4.7b), the base resistance is usually a minor fraction of the ultimate pile resistance.
Table 4.4 Suggested values of k and µ, Eq. 4.5 (after Viggiani 1993)
Pile type Values of k for relative density Values of µ
Loose Dense
Displacement: steel H section closed end pipe precast concrete cast in place concrete
0.7 1.0 1.0 1.0 1.0 2.0 2.0 3.0 tg20° = 0.36 tg3φ / 4 tgφ
Intermediate presso drill 0.7 0.9 tgφ
Replacement drilled shaft
CFA 0.50.6 0.40.6 tgφtgφ
Table 4.5 Suggestions for the evaluation of the coefficient k, Eq. 4.5
Pile type Soil type k Reference
Replacement Sand
z = depth below surface; l = pile length; kP = passive pressure coefficient;
kO = at rest pressure coefficient;
α = 0.2 (typically)
Yasufuku et al. (1997)
0.5 + 0.02 NSPT Go and Olsen (1993)
Displacement Clay k = (1 – sinφ′)OCR0.5; φ′ = effective angle of
friction; OCR = overconsolidation ratio; also δ = φ′
Burland (1973) Meyerhof (1976)
68 Present practice: vertical loads
At the ser vice load, defined as Qlim/FS, the fraction of the load taken by the base is very small, but the safety of the pile is not significantly affected.
For a large dia meter bored pile (Figure 4.7c), the base resistance can be a major portion of the bearing capa city; in fact, it may be even larger than in Figure 4.7c, because the pile base is kept usually to better soils. It is evid ent that at ser vice load the shaft resistance is fully mobilized and there is a small safety margin against very large displacements.
For these reasons the bearing capa city of large dia meter bored piles is usually intended as a ser viceability, rather than ultimate, limit state.
Berezantsev (1965) claims that the onset of plastic deformations around the pile base occurs at a displacement in the range 0.06d to 0.1d, and suggested to as sume the cor res ponding value of the base load in the evalu ation of the bearing capa city. Such a load may be expressed as:
(4.6) Eq. 4.6 is equi val ent to Eq. 4.3, referring to the ultimate base load, but the values of N*q (Figure 4.8) are much smaller than the cor res ponding values of Nq.
Of course, shaft resistance may be evalu ated by the same methods suggested for medium dia meter bored piles (§4.3.2).
Concrete driven pile L = 16m; D = 0.4m shaft base total Bored pile L = 16m; D = 0.4m Bored pile L = 16m; D = 1.6m a b c . . .
4.3.4 Micropiles
Due to their peculiarities, the bearing capa city of micropiles is still more de pend ent on the installation pro ced ures than that of the other replacement piles.
Some authors (Salgado 2008) suggest evaluating the bearing capa city of micro- piles in accordance with the suggestions given above for replacement piles of the CFA type. In the writers’ ex peri ence, this cri terion is overcon ser vat ive because it neg- lects that micropiles are pressure injected. A merely empirical approach, such as that proposed by Bustamante and Doix (1985) may be pre fer able; it is widely adopted in France and other Euro pean countries.
First of all, a distinction is made between root piles (§2.2.5), that are called IGU (injection globale unique, i.e. single global injection) and the pile injected by a tube with valves, called IRS (injection repetitive et select ive, i.e. repeated and select ive injections). The soil characterization is made preferably by Ménard pressuremeter, but also by SPT; accordingly, the values of the lateral resistance s are given as a func- tion of the limit pressure pL of Ménard pressuremeter or SPT blowcount NSPT. It is further as sumed that the shaft grouting pressure pg is in the fol low ing ranges:
• pg≥ pL for the IRS micropiles
• 0.5pL≤ pg≤ pL for the IGU micropiles
and that the injection is carried out at a rate in the range 0.3 to 0.6 m3/h in cohesive soils, and 0.8 to 1.2 m3/h in cohesionless soils.
Nq*
Figure 4.8 Values of the bearing capacity coefficient Nq* corresponding to the onset of plastic deformations (source: Berezantzev 1965).
70 Present practice: vertical loads
The shaft resistance of a micropile is expressed as:
where ds is the expanded dia meter, Ls the length of the injected portion of the pile
and s is the shear resistance at the interface between the soil and the injected portion of the pile (Figure 4.9). The expanded dia meter is expressed as ds = αd; the values of
the factor α are given in Table 4.6.
The shear resistance s at the interface between the soil and the injected portion of the pile is given as a function of pL or NSPT by the fol low ing expressions:
(4.7) (4.8) where the values of the para meters a, b, α and β are given in Table 4.7.
If the injected portion of the micropile extends until the soil surface, it is re com- mended that the upper 5 m are con sidered in any case as if they were of the IGU type. It is also re com mended that the injected length Ls, not con sidering the upper
5 m, should be not less than 4 m; this means that the total length of a micropile of the IRS type should be at least 9 m.
The point resistance is usually as sumed equal to 15% of the shaft resistance; therefore:
Ls
d d
ds = �d
ds = �d