Table 10-1 (c Percent of residual load on soil mass between shafts d = 3 feet

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10.3.4 Geotechnical Resistance of Drilled Shafts
In a computer-based p-y analysis, the net shaft force calculated in Step 7, F
net-shaft
, is distributed along the shaft length from the top to the slip surface with an equivalent triangular loading diagram as shown in Figure 10-5. Adaptation of the Liang and Zeng (2002) method in Geotechnical Bulletin GB (2014) by the Ohio Department (ODOT) allows that despite the complexity of loading, the triangular distribution is a close enough approximation of the actual condition to develop a realistic calculation of distributed shear, moment, and displacement of the drilled shaft. More conservatively, the triangular distribution of load can be converted to an equivalent trapezoidal load diagram in units of pounds per inch (lb/in) of shaft length above the slip surface to determine the lateral deflections, shear forces, and bending moments along the shaft length. Boundary conditions at the shaft head within the p-y analysis should beset based upon the appropriate freedom to move both laterally and rotationally, with a value of zero (0) input for both the shear and the moment at the head.
10.3.4.1 Service Limit State
Even though the primary objective of slope stabilization is to achieve a minimum factor of safety against shear failure, the stiffness of the shaft elements must be adequate to inhibit movement from occurring along with the moving soil mass. In the process of checking the Service Limit State, apply an (unfactored) vehicular live load surcharge (LS) equal to two feet of soil with a unit weight of 125 pcf to the computation of F
net-shaft
, as per AASHTO (2014) 3.11.6.4. to assess the shaft deflection if the traffic surcharge is within the failure zone above the drilled shafts, or the horizontal distance between the drilled shafts and traffic loading is less than or equal to half the depth to the shear surface at the location of the drilled shafts.
Geotechnical Service Limit State design is further discussed in Chapter 6.
10.3.4.2 Strength Limit State
The computed net force (F
net-shaft
) must be factored to assess the Geotechnical Strength Limit State for determining shaft penetration beneath the slip surface and to verify adequate soil resistance below the slip surface. For the Strength Limit State analysis, use a load factor of LS
= 1.75 for the vehicular live load surcharge (LS) and a load factor of EH
= 1.50 for the horizontal earth pressure (EH, in accordance with Section 3.4.1 of AASHTO (2014). Geotechnical Strength Limit State design is further discussed in Chapter 6.

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