neglected. The analyses assumed that the colluvial layer would move 1 in (25 mm) due to soil relaxation and would engage resistance of the underlying layers. This deformation was assumed to occur from the ground surface down to the failure plane determined from the global stability analyses. The analyses considered a drilled shaft spacing of 3Dp along each row to maximize passive resistance from soil arching. The drilled shafts were spaced about 7.5 ft (2.3 m) or 3Dp apart and the rows were spaced 15 ft (4.6 m) or 6Dp apart and were staggered approximately 4 ft (1.2 m) to avoid “shadowing” and group effects that could potentially reduce the shaft resistance. A W18x106 (W460x158) section for the drilled shaft reinforcement was selected based on structural capacity checks. The maximum moment and total stress on the piles was checked against the plastic moment and yield strength of different section sizes to adequately size the reinforcement. The section was sized so that the moment and stress from the output did not exceed 0.67 times the plastic moment and yield strength of the section (FS of 1.5). Two separate FS were used in the computations: FS=1.5 for required resisting force from the slope stability analyses and FS=1.5 to reduce the ultimate strength of the reinforcing elements. The structural pile was checked against shear, but bending was found to control the design. The depth of the piles was extended in the model until fixity was achieved, which was defined as the second point of zero deflection, as outlined in AASHTO LRFD Manual for Pile Foundations. The global stability analyses followed a FS approach, however, the LPILE analyses considered LRFD methodology (i.e., resistance factor = 1/FS). Construction The drilled shafts were constructed using vertically placed W18x106 (W460x158), 50 ksi (345 MPa) steel beams and concrete with a 28-day compressive strength of 3,000 psi (20.7 MPa). The drilled shafts were installed approximately 10 ft (3 m) into competent bedrock with the beam major axis perpendicular to the proposed pipeline, resulting in an overall length between 30 and 40 ft (9.1 and 12.2 m). The LPILE with initiation of movement of colluvial layer and pile response Installation of pipeline along embankment construction bench construction sequence for the slope maximized overall stability during the temporary condition, such that the first row of drilled shafts was installed prior to grading of the const ruct ion pad. Reconstruction of the slope including the construction bench and installation of the pipeline was successfully completed within four weeks. No significant observations (i.e., tension cracks, differing subsurface conditions, etc.) were encountered during construction that required deviations from the proposed design. Conclusions The design procedures for stabilizing slopes with deep foundations, such as drilled shafts, are still in their infancy. Although several publications exist for using this stabilization technique, none are widely accepted nor considered design standards. A simplified design procedure based on global stability and lateral deformation analyses, as suggested in the literature, was used for the case study presented. A.G.E.S. was able to observe the construction of this design alternative and did not find any indications of instability. This suggests that the implemented design procedure provides an adequate factor of safety. As projects like these begin to gain momentum, particularly in the public sector, the procedures for design will become clearer. Sebastian Lobo-Guerrero, Ph.D., P.E., is a geotech- nical project manager/AAP laboratory manager at American Geotechnical & Environmental Services (A.G.E.S.) in the Pittsburgh, Pa. headquarters. He has more than 18 years of experience in geotechnical engineering, specializing in the design of deep/ shallow foundations, earth retaining structures and landslide stabilization. He is a member of the DFI Tiebacks and Soil Nailing Technical Committee. Todd DeMico, P.E., is a geotechnical project engineer at A.G.E.S. in Pittsburgh, Pa. He has more than six years of geotechnical experience in the design of bridge foundations, earth retaining structures and soil/rock slide stabilization. Vishal B. Patel, M.S.C.E., P.E., is a geotechnical project engineer at A.G.E.S. in Pittsburgh, Pa. He has more than nine years of geotechnical experience in the design of bridge and building foundations, earth retaining structures and soil/rock slide stabilization. DEEP FOUNDATIONS • NOV/DEC 2018 • 87
