Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Figure 10-4: Reduction factor concept in distribution of forces (after Liang and Zeng 2002)
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| Figure 10-4: Reduction factor concept in distribution of forces (after Liang and Zeng 2002). The net resisting force that keeps the drilled shaft in place is the resultant of the driving force on the left of shaft and the resisting force on the right of shaft (Figure 10-4 shown in opposite Figures 10-2 and 10-3 direction of movement, where His the depth to the slip surface at the drilled shaft and K o is the at-rest earth pressure coefficient, as follows πΉπΉ ππβπππππ‘π‘ πππππ‘π‘ = (1 β πΉπΉ ππ )ππ ππ ππ Case I πΉπΉ ππβπππππ‘π‘ πππππ‘π‘ = (1 β πΉπΉ ππ )ππ ππ (ππ β ππ) + οΏ½ππ ππ β πΎπΎ ππ πΎπΎπ»π» 2 2 οΏ½ ππ Case II Equation 10-4) Equation 10-5) The friction angle (Ο), cohesion (C), shaft spacing (S), and shaft diameter (d) are required to determine the percent of residual load (R p ) acting on the soil mass between two adjacent drilled shafts. In Table a) through c, the variation in internal friction angle has a significant influence on the arching effect, particularly for soils with lower cohesion values. Also, soils with a higher friction angle are more likely to interlock and to develop stronger arching, which leads to a reduced R p and thus a higher load on the shafts. It is possible to extrapolate the R p values included in Table a) through c) for shaft diameters larger than 3 feet. However, the design engineer must be cautious in extending the extrapolation too far from the calculated range because the R p increase with shaft diameter trend has not been confirmed. It is recommended that for purely cohesionless soils, the R p values up to d = 3 feet be selected for larger shaft diameters. Interpolation or extrapolation can be exercised to determine cohesion values outside the range in Table a) through c, but it is important to perform numerical simulations to confirm the extrapolated values. Diameter extrapolation can be performed up to d = 6 feet. 137 A step-by-step procedure is described below in accordance with the set of equations presented above 1. Compile available information for the current site conditions, including soil borings, shear strength parameters, and the shape and location of failure surface. Select drilled shaft diameter, spacing, and location within the slope. Assume an initial FS for the slope with drilled shafts using the limit equilibrium method with slices. Select the minimum factor of safety for slope stability. Use Table a) through c) to evaluate the percent of residual pressure acting on the soil mass between adjacent shafts. Find the value of R p corresponding to the soil shear strength parameters and the selected s/d. 5. Calculate the reduction factor, R, using Equation 10-3. 6. By applying the method of slices and Equations 10-1, 10-2 and 10-3 iteratively, calculate interslice forces at each slice and the corresponding FS with drilled shafts. Use the reduction factor (R) calculated in Step 5 in Equation 10-1 only for the slice just behind the shaft on the upslope side. For the rest of the slices, R is set to 1.0. 7. Calculate the net force that is transferred to the drilled shaft by substituting the percent of residual pressure, R p , calculated in Step 4 and the interslice force for the slice just behind the shaft (upslope side) into Equation 10-4 (Case I) or Equation 10-5 (Case II. Confirm the Geotechnical Service Limit State and Strength Limit State resistance for the selected shaft configuration insteps through 7 in accordance with Section 10.3.4. 9. Perform structural design for the drilled shafts in accordance with Section 10.3.5. Download 6.03 Mb. Share with your friends:
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