BSCE 2020, magna cum laude, Undergraduate Research Project Title: Soil-Structure Interaction of Mechanically Stabilized Earth (MSE) Retaining Walls Subjected to Construction Sequencing and Seismic Loading

Abstract:

Recent innovations in geosynthetics have led to numerous ground improvement techniques leading to the creation of Mechanically Stabilized Earth (MSE) wall systems that perform well under seismic loads. MSE wall systems are a type of retaining wall system consisting of facing element, soil reinforcement, and backfill material.  In order to accurately model this soil-structure, soil structure elements or interface layers are significant to model since the load transfer from geosynthetics to backfill material is affected by backfill properties, geogrid length, geogrid spacing, and material properties of reinforcement. By utilizing 3D finite element software, simulation of general construction sequencing and seismic loads are conducted to determine the complex nature of 5 m and 10 m MSE walls. Through a parametric study by variation of backfill and geogrid spacing, limiting fine content of backfill was found to be 24.60%, 47.61%, 53.83% for 5 m wall height while 12.78%, 14.50%, and 34.33% for 10 m wall height by linear regression relating fine content to maximum lateral displacement. Moreover, the only criteria more sensitive to geogrid spacing than fine content is effective vertical stress that peaked up to 70.57% compared to a 16.58% for 40% fine content. On the other hand, settlements are within the limit states considered and there is only an increase of up to 33.82% with fine content. Finally, for 10 m walls, the geogrid tensile stress exceeds allowable tensile stress at about fine content of 20% for geogrid spacing of 0.375 m. It can be inferred that using the assumptions provided by the study, the limiting fine content (15%) by the NSCP 2015 and geogrid spacing range of 0.375 m to 0.75 m will yield a MSE wall that conforms with the set limit states by the study. As for the determination of dynamic characteristics i.e. approximate natural frequency range through frequency sweep was approximated to be about 4.00 Hz - 4.60 Hz for 5 m MSE wall for peak response in velocity and acceleration respectively. On the other hand, the 10 m wall obtained a peak response for both velocity and acceleration at 3.0 Hz.  The dynamic analysis confirmed that the MSE wall cases considered performed well under a waveform with peak ground acceleration (PGA) of approximately 0.5g. Since, the residual top wall deflections relative to post construction were found to be 0.173 mm for 5 m  wall and 1.382 mm for 10 m wall which is apparently minimal. Because of the clayey soil characteristic of the unreinforced area it is recommended that ground improvement techniques be applied before construction of MSE wall.

Keywords: Geosynthetics, Mechanically Stabilized Earth (MSE) Walls, Backfill, 3D Finite Element Dynamic Analysis, Construction Sequencing

*This research project was co-advised with Asst. Prof. Giancarlo P. Ventura of Geotechnical Engineering Group (GEG)