Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities


Figure 6-4: Illustration of graphical p-y analysis results (from Hannigan et al. 2016, modified from



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Figure 6-4: Illustration of graphical p-y analysis results (from Hannigan et al. 2016, modified from
Reese 1986).
The p-y method is a method that has been developed based on the extension of the elastic solution and the subgrade reaction concept. The deep foundation element is treated as a beam-column with lateral support, following the approach described previously of an elastic beam rotated on an elastic foundation. The general behavior of the foundation element under lateral and axial loading can be obtained by solving the fourth-order differential equation (Hetenyi 1946): Equation 6-1) Where
P
x
= Axial load in the pile/shaft.
y = Lateral deflection of the pile/shaft.
x = Depth along the pile/shaft.
E
p
= Elastic modulus of the pile/shaft.
I
p
= Moment of inertia of the pile/shaft.
p = Lateral soil reaction per unit length of the pile/shaft.
w = Distributed load along the length of the shaft, if applicable. It can be seen from Equation 6-1 that the axial load influences the bending moments and lateral deflections of a laterally loaded deep foundation. Other beam formulae in the analyses include
𝐸𝐸
𝑝𝑝
𝐼𝐼
𝑝𝑝
𝑑𝑑
3
𝑦𝑦
𝑑𝑑π‘₯π‘₯
3
= 𝑉𝑉 Equation 6-2)
𝐸𝐸
𝑝𝑝
𝐼𝐼
𝑝𝑝
𝑑𝑑
2
𝑦𝑦
𝑑𝑑π‘₯π‘₯
2
= 𝑀𝑀 Equation 6-3)
4 2
4 2
0
p p
x
d y
d y
E I
P
p w
dx
dx
+
βˆ’ βˆ’ =


70 Equation 6-4)
𝑑𝑑𝑦𝑦
𝑑𝑑π‘₯π‘₯ = Where
V
= Transverse shear in the pile/shaft.
M
= Bending moment in the pile/shaft.
S
= Slope of the deflection diagram. The soil reaction, p, is a function of the deflection, y. The relationship between p and y is nonlinear, and is referred to as the p-y curve. The soil-pile interaction is modeled as a nonlinear elastic beam and the soil resistance replaced by a series of discrete, nonlinear springs, in which Equation 6-5) Where E
py
is the soil modulus or reaction modulus, also referred to as the spring constant for the soil spring model. The soil modulus is a function of deflection, y, and depth, x. The physical model of the p-y method is presented in Figure 6-5. This figure illustrates how the soil around the deep foundation is represented by nonlinear springs, and how the p-y relationship varies with displacement, y, and depth, x.

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