6.3.3.1 Pile Capacity Estimation from Standard Penetration Test Results
Meyerhoff (1976) proposed a relationship (Equation (6.18)) to determine the point capacity of a pile in coarse sand and gravel, in kPa, using standard penetration test (SPT) data:
(6.18) whereN is the weighted SPT average in an influence zone between 8Bbelow and 3Babove the pile tip,qpuis in kPa (Bowles, 2002 suggests the use ofN55forNin this relationship), Lbis the pile penetration in the bearing layer, andDis the pile diameter (or the equivalent
diameter).
As pointed out inSection 6.3.1, the point resistance reaches a limiting value at a critical Lb/D. For the above outlinedqpuvs. N relationship (Equation (6.18)), the suggested critical Lb/Dis about 90. Meyerhoff (1976) also proposed the following alternative relationship for nonplastic silt:
qpu=300N
(6.19)
FIGURE 6.7
Dependence of the λ factor on pile penetration.
On the other hand, for the ultimate unit skin friction in sands, the following relationships were proposed by Meyerhoff (1976):
(6.20a) for moderate to large displacement piles and
(6.20b) for small displacement piles such as steel H piles, where is the weighted average SPT of soil layers within the embedded length.
6.3.3.2 Pile Capacity Estimation from Cone Penetration Test Results AASHTO (1996) recommends the following technique proposed by Nottingham and Schmertmann (1975) to determine the point bearing capacity in clay based on cone penetration data:
(6.21) whereqc1 andqc2are minimum averages (excluding sudden peaks and troughs) ofqc values in the influence zones below the pile tip and above the pile tip, respectively. These influence zones are shown in Figure 6.8.R1is a reduction factor evaluated fromTable 6.3.R2is 1.0 for the electrical cone and 0.5 for the mechanical cone.
A similar expression is available for the evaluation of point bearing resistance of sands (DeRuiter and Beringen, 1979):
(6.22) whereqc1 andqc2are minimum averages (excluding sudden peaks and troughs) ofqc values in the influence zones below the pile tip and above the pile tip, respectively. for normally consolidated sand and 0.67 for overconsolidated sand.
FIGURE 6.8 Tip influence zone.
TABLE 6.3 Cuvs.R1
Cu(kPa) R1
<50 1
75 0.64
100 0.53
125 0.42
150 0.36
175 0.33
200 0.30
Nottingham and Schmertmann (1975) also developed a correlation between skin friction and the sleeve resistance obtained from cone penetration test (CPT) as expressed in Equation (6.23):
fsu=α'fs
(6.23) In the case of electrical cone penetrometers,a',the frictional resistance modification factor can be evaluated from Table 6.4based on the depth of embedment, Z/B.
Tomlinson (1994) advocates the use of the cone resistance in evaluating the skin friction developed in piles since the former is found to be more sensitive to variations in soil density than the latter. Tomlinson (1994) provides the empirical data inTable 6.5for this evaluation.
Example 6.2
The SPT profile of a site is shown inFigure 6.9. Estimate the depth to which a HP 360×
108 pile must be driven at this site if it is to carry a load of 1500 kN. Assume that the SPT test was performed in silty clay in the absence of water and the unit weights of peat, silty clay (dry), saturated silty clay, and saturated medium-dense sand are 10.5, 16.0, 17. 5, 17.2 kN/m3, respectively. Use Meyerhoffs method for estimating point bearing and theαmethod for estimating skin-friction capacity.
TABLE 6.4
Frictional Resistance Modification Factors Applied to CPT Results(α')
Z/B Timber Concrete Steel
5 2.5 1.4 2.0
10 1.7 1.1 1.25
15 1.25 0.85 0.9
20 1.0 0.8 0.82
25 0.85 0.7 0.8
30 0.8 0.7 0.75
35 0.8 0.7 0.75
40 0.8 0.7 0.75
TABLE 6.5
Relationships between Pile Shaft Friction and Cone Resistance
Pile Type Ultimate Unit Shaft Friction
Timber 0.012qc
Precast concrete 0.012qc
Precast concrete with enlarged base 0.018qc
Steel displacement 0.012qc
Open-ended steel tube 0.0008q
Open-ended steel tube driven into fine to medium sand 0.0033qc
Source: From Tomlinson, M.J., 1994, Pile Design and Construction Practices, 4th ed., E & FN Spon, London.
With permission.
Dimensions of HP 360×108 pile
Depth=346 mm, width=371 mm, flange thickness=12.8 mm, web thickness
=12.8mm
Plugged area=0.371m×0.346m=0.128m2=Ap
Pile perimeter (assuming no plugging for skin friction)=371(2)+12.8(4)+346(2) + (371–12.8)(2)=2.2m=p
Minimum pile dimension=0.346 m
Limiting skin-friction depth in sand=Ls,lm=15(0.346)=5.19m (assumed to apply from the clay-sand interface)
Critical end-bearing penetration=Lp,cr(Figure 6.5)=3(0.346) for clays=1 m
=10(0.346) for sand=3.46m
The following soil strength properties can be obtained based on the SPT values (Table 6.6):
value are obtained fromFigure 6.4 δ=2/3 N
K=1.4K0tanδ=1.4(1−sin N) tanδ
From Equations (6.7) and (6.14), the maximum total ultimate resistance produced by the clayey layers=point bearing+skin friction=9(0.128)(25)+(2.2)[1.03(19)(1) +
0.92(50)(4)+(1.0)(25)(2)]=28.8+556.6–585.4 kN.
Hence, the pile has to be driven into sand (say up to a depth of Lm).
TABLE 6.6
Soil Parameters Related to the Pile Design inExample 6.2
Depth (m) Cu(kPa) a δ(°) K Ls,lm=5.2Applies? Lp,cr
0–1 19 9 1.03 1
1–5 50 9 0.92 1
5–7 25 9 1.0 1
7–10.1 34 100 23 0.26 No 3.46
10.1–12.3 36 150 24 0.26 No 3.46
12.3− 38 200 25 0.25 Yes 3.46
FIGURE 6.9
Illustration forExample 6.2.
Since the critical embedment is 3.46 m, one can assume that the pile needs to be driven passing a 12.3 m depth for complete mobilization of point capacity and skin friction.
Assume that for depths greater than 16.8 m the soil properties are similar to those from 12.3 to 16.8m.
Effective clay overburden=(10.5)(1)+16(4)+(17.5–9.8)(2)=89.9 kPa Effective sand overburden=(L−7)(17.2–9.8)=7.4L−51.8
From Equation (6.5) for net ultimate point resistance Ppu=(200–1)(0.128)[89.9+7.4L−51.8]=(188.5L+970.5) kN Psu=(in sand)=2.2{(0.26)(3.1)(1/2)[89.9+89.9+(3.1)(17.2–9.8)] + (0.26)(2.2)(1/2)[89.9+(3.1)(17.2–9.8)+89.9+(3.1+2.2) (17.2–
9.8)]+0.25(L−12.3)[89.9+(5.3)(17.2–9.8)]}
=2.2{81.7+69+32.3(L−12.3)}=2.2(32.3L−246.6) kN
Total ultimate resistance=556.6+188.5L+970.5+2.2(32.3L−246.6) =259.6L+984.6 Applying Equation (6.3),
1500=(259.6L+984.6)/2.5 L=10.7m
Hence, it must be driven to only 13.5 m below the ground.
Example 6.3
A cased concrete pile is required to carry a safe working load of 900 kN in compression at a site where the CPT results are given inFigure 6.10. Recommend a suitable pile size and a depth of penetration.
Page 252
FIGURE 6.10
CPT results forExample 6.3.
FromFigure 6.10(a), it is seen clearly that the immediate subsurface consists of a loose sand layer up to a depth of 11.0 m underlain by a denser sand layer.
Based on the cone resistance (Figure 6.10), and Equation (6.22), the maximum endbearing resistance that can be obtained from the loose fine sand layer is
Assume that a 400 mm diameter pile is employed in order not to overstress the concrete as shown later in the solution. The tip area of this pile=0.126m2and the pile perimeter= 1.26m.
Then, the maximum working load that can be carried at the tip is computed as 4.5(1000)(0.126)/2.5=226 kN.
Hence, it is advisable to set this pile in the dense sand with an embedment of 13.0 m as shown inFigure 6.10(b).
By applying Equation (6.22) again,
TABLE 6.7
Computational Aid forExample 6.4
Depth Interval (m) fs(MPa) αfs(kPa) Psu(kN), Equation (6.11)
0–2.0 0.1 0.07 176
2.0–4.0 0.11 0.077 194
4.0–6.0 0.12 0.084 211
6.0–8.0 0.14 0.098 246
8.0–10.0 0.16 0.112 282
12.0–13.0 0.22 0.154 194
Page 253 For a depth of embedment(z/B)ratio of 13.5/0.4=33.75, fromTable 6.4,α=0.7. As shown in Table 6.7, Equation (6.23) can be applied on incremental basis,
Total ultimate skin friction=1654 kN
Static pile capacity=(1260+1654)/2.5=1165.6kN
Hence, the load can be carried safely at a pile embedment of 13.0 m. The same design is repeated under LRFD guidelines in Section 6.9.