For the power plant project, SRT worked closely with the owner’s geotechnical engi- neering firm to create a design that would repair the levee and raise the factor of safety to 1.5. The pertinent design parameters include plate pile spacing and dimensions (e.g., steel section, length of pile shaft, length of plate). The shear and bending capacity of the plate piles are dependent on the pile dimensions and the subsurface soil profile. SRT Design Approach The first step in the SRT design approach is evaluating the stability of the unreinforced slope. Since the power plant levees had already failed, material property values of the subsurface soils were back-calculated based on a factor of safety of 1.0. The maximum depth to the back-calculated failure plane was approximately 8 ft (2.4 m), which corresponded to the conditions observed in the field. Plate piles are typically installed 4 ft (1.2 m) on center in the horizontal direc- tion (i.e., parallel to the slope) in order to mobilize arching between the elements. The vertical spacing of the plate piles is dependent upon soil conditions, the predicted or actual depth of sliding and the slope inclination. Initial plate pile spacings of 6 ft (1.8 m) on-center in the up-slope direction and lengths of 10, 12 and 14 ft (3, 3.7 and 4.3 m) were chosen, with pile lengths increasing in the up-slope direction. These initial estimates were then confirmed in the following design steps. Preparation of slope for remediation LPILE computes deflection, shear, Conceptual illustration of LPILE model 62 • DEEP FOUNDATIONS • MAR/APR 2015 bending moment and soil response with depth of laterally loaded piles in nonlinear soils. Soil behavior is modeled with internally-built p-y curves. The plate piles are loaded in LPILE by lateral soil movements over the depth of the slide plane until a limiting state is reached. The displacement value is transitioned to zero over a distance of 12 in (305 mm) at the piles are represented as micropiles. The plate pile input parameters include up- slope spacing and the mobilized resistance, which is set equal to the shear force at the slide plane. The analysis of the reinforced slope should result in the desired stabilized factor of safety. If an adequate stabilized factor of safety is not achieved, the length and/or the spacing of the plate piles must be adjusted. For the New Madrid levee project, Illustration of plate piles in section view; plate piles in plan view The shear and bending capacity of the piles were evaluated using the finite differ- ence software program LPILE by ENSOFT, Inc. A 3 x 3 x 3/8 in (76 x 76 x 9 mm) steel angle was chosen as the plate pile section. This size steel angle has an area equal to inertia equal to 1.75 in (72.8 cm ) and section modulus in the x-direction equal to 0.825 cu in (13,500 cu mm); the elastic modulus value is 29,000 ksi (200,000 MPa) and yield strength is 50 ksi (344 MPa). 2.11 sq in (1,361 sq mm), moment of 4 4 approximate location of the failure surface. This approach is empirical but has been verified by instrumented case histories and numerical analyses, and is supported by Loehr & Brown (2007), White et al. (2008), and by Kourkoulis et al. (2012). The limit state of the plate pile may be equal to the ultimate bending moment or the a l lowabl e l a t e r a l soi l movement. The shear force at the sliding depth, when the first limit state is reached, is considered to be the mobilized resistance for that sliding depth. Using the same slope geometry, subsurface profile and loading conditions used in the analysis of the unreinforced levee, the factor of safety of the reinforced levee is evaluated next. Depending on the slope stability software, plate piles are modeled as structural elements in a variety of ways. For example, in the stability software, Slide (Rocscience 2013), plate
