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



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10.3 DRILLED SHAFT ANALYSIS
10.3.1 Background
Numerous examples have demonstrated the successful utilization of drilled shafts to stabilize a slope, e.g., Fukumoto (1972 and 1973), Sommer (1977), Ito et.al. (1981 and 1982), Nethero (1982),
Morgenstern (1982), Gudehus and Schwarz (1985), Reese et.al. (1992), Rollins and Rollins (1992), and
Poulos (1995 and 1999). Past research relevant to the analysis of drilled shaft stabilized slopes include work by Reese (1992), Ito and Matsui (1975), Hassiotis et.al. (1997) Poulos (1999), and Liang and Zeng
(2002). The Liang and Zeng (2002) method presented herein was selected because it relies on a detailed formulation based on limit equilibrium theory, incorporates soil arching effects, generates a general slip surface, handles complex slope geometry and soil layers, and allows optimizing the shaft location and spacing. The general principle of soil arching as used by the Liang and Zeng (2002) method is depicted by Figure 10-1, and detailed procedures for its application are presented in the following sections of this manual.


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Figure 10-1: Direction of movement and soil arching (after Liang and Zeng 2002).
Whereas the Liang and Zeng (2002) method is based upon calculation of a lateral earth force to be resisted by the drilled shafts, Brown, et al. (2010) presents alternative approaches where a) the shear resistance and bending moments in the shafts are evaluated as a function of lateral movement above the slip plane, and b) shafts are laterally analyzed relative to soil masses subject to liquefaction-induced instability.
10.3.2 LRFD Analysis for Slope Stabilization
Using the method of slices, as depicted in Figure 10-3, the Liang and Zeng (2002) method develops a resultant net force (F
net-shaft
) that can be distributed in an equivalent loading diagram along the shaft length above the slip surface, as shown by Figure 10-5, to compute shear, moment, and lateral deflections with a laterally loaded pile p-y analysis software. The force analysis is based on the target geotechnical resistance factors as discussed in Section 10.3.3 for stabilization of the slope. Upon determination of the net resisting force, F
net-shaft
, required to stabilize the slope, the distributed resultant can be used in a p-y analysis to assess the Geotechnical Service Limit State for the drilled shafts. Thereafter, F
net-shaft is factored to confirm adequate geotechnical resistance at the Strength Limit State, as discussed in Section
10.3.4 and to design the shaft reinforcement, as discussed in Section 10.3.5.

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