Research

The 2017 Rangamati landslide was one of the most devastating landslides in the history of Bangladesh. This massive landslide had drawn the attention of geotechnical engineers countrywide. As a part of this response, the civil engineering team of BUET performed a field visit to the affected site and collected soil samples for detailed geotechnical investigation. Therefore, the primary purpose of this research work is to investigate the geoenvironmental characteristics of the landslide. We performed various laboratory testing such as the Atterberg limit test, grain size analysis, unconfined compression test, permeability test, etc., to get the soil parameters and classified the soil using the USCS classification system. Moreover, we performed a numerical analysis using PLAXIS 2D, a Finite Element Method (FEM) based software for analyzing the hill slopes. With the help of both experimental and numerical analysis, we ended up with some significant findings. The study indicated that the soil type played an important role behind the landslide. Besides, Excessive precipitation, hill cutting, and unplanned human settlement added to the problem. The numerical modeling revealed that the slopes were vulnerable to failure. We found that the increase in moisture content is associated with the decrease in shear strength. Heavy rainfall during the monsoon also added to it by increasing the weight of the soil mass. The study concluded that the landslide occurred due to heavy precipitation for which rainwater infiltrated into the soil leading to a markable increase in the degree of saturation. Due to the soil nature and infiltration of water, the shear strength reduced gradually. Moreover, the FEM analysis validated the instability of the slope surface. The Academic Research Grant of BUET funded the study.

Among many retaining structures, the Mechanically Stabilized Earth (MSE) Walls are prevalent in many countries. The application of an MSE wall encompasses from highway or hill slopes to any vulnerable soil mass. This study is about an MSE wall situated in Abilene, Texas. The construction of the MSE wall has been analyzed numerically. The backfill soil of the wall is reinforced with geogrids. We used PLAXIS to analyze the gradual construction of the MSE wall. All the phases of construction have been simulated using the Finite Element Method through PLAXIS. The principal purpose of the study is to investigate the effects of various geogrid parameters on the overall stability and performance of the wall. We have used two design parameters for validating the numerical models - Factor of Safety and Wall displacement. We checked the interrelationships between various geogrid parameters graphically. Initially, we found that a particular area's geological parameters play an important role in the overall construction phase of an MSE wall incorporating geogrids. Since commercially available geogrid parameters vary, we need to understand which type of geogrid is best suitable for a particular area. Therefore, we considered all the essential geogrid parameters such as geogrid length, geogrid spacing, geogrid strength, etc. We also tried to find the most optimum combination considering two design criteria (safety and deflection) which will provide both stability and sustainability. Consequently, two design charts have been proposed, one for Factor of Safety and the other for Wall Deflection. Due to the presence of clay soils, we also analyzed the differential settlement the wall might experience graphically. Considering all the factors and associated issues, we finally suggested three optimum geogrid combinations to ensure both structural and economic feasibility.

Hillslope failures, specifically landslides, are very common in hilly regions of Bangladesh. Especially during the rainy season, the vulnerable hill slopes experience landslides leading to loss of lives and property damage. Therefore, stabilizing hill slopes are mandatory in Bangladesh. There are many methods of stabilizing hill slopes. However, many of them are neither environment-friendly nor sustainable. Recycled Plastic Pin (RPP) is a material that can be produced from industrial plastic wastes. It can be used as structural material and needs no maintenance after installation on the slopes. Moreover, they are resistant to several kinds of biochemical attacks, including moisture, corrosion, etc. In this study, we analyzed the effectiveness of using RPP for stabilizing hill slopes from Bangladesh's perspective. Finite Element Model (FEM) based program PLAXIS 2D is used to analyze the stability of slopes that are usually encountered in the hilly regions of our country. Two slope surfaces of 10m and 15m height, each with an angle of 50⁰, have been simulated in PLAXIS. Four different RPP spacings (1 m, 2 m, 3 m, and 4 m) have been analyzed to understand the change in the Factor of Safety of the slopes. The slopes were modeled both in their bare form and RPP installed form. In the case of bare slopes, the Factors of Safety (FoS) were below 1.3, which demonstrates their unstable nature. However, the Factor of Safety increased up to 1.7 by installing the RPP in slopes which validate the stabilization using RPP. Moreover, the relation between the vertical spacing of RPP and the Factor of Safety is also investigated. Hence, an optimum vertical spacing within 1-2 m is suggested at the end.