ORCID Identifier(s)

0000-0001-9152-9556

Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Sahadat MD Hossain

Abstract

Globally, slope failures cause substantial death tolls and economic loss. Embankments constructed on high plasticity clay are vulnerable to cyclic swelling and shrinkage when subjected to climate. Thus, over time, soil softens and reduces effective shear strength, and shrinkage cracks on slopes can act as pathways for rainfall infiltration, which increases pore water pressure and lowers the shear strength below critical levels, resulting in slope failure. This study aims to develop early warning criteria for rainfall-induced slope failure. Study areas were Tarrant, Dallas, Johnson, and Ellis, where soil types and a humid subtropical climate result in frequent slope failures. The field study was conducted on a 260 ft section of a highway slope (3H:1V) located over US 287 near Midlothian, TX. The test slope sections were instrumented using moisture sensors, temperature sensors, water potential sensors, and a rain gauge. According to the geotechnical investigation, there are two distinct soil strata: top 22 feet high plasticity clay (CH), followed by Eagle Ford Shale. Hydraulic conductivity and shear strength of soils fluctuated seasonally. Field tests using the Guelph Permeameter and Mini-Disk Infiltrometer showed 100 times higher permeability at the surface than at two feet below ground due to loosen and porous soil at the surface. As measured by Dynamic Cone Penetration (DPC) tests, soil shear strength was higher in the dry season and lower in the wet season. According to field instrumentation, soil moisture content and soil matric suction change with depth throughout the year: surface soil experiences more changes than deeper soils. Water content fluctuates most at the top sensors (10%) than at the deeper levels (3%). During the dry season, soil matric suction increased, but decreased after each rainfall. A slope failure inventory map for the DFW area was developed based on previous literature, thesis, and Google Earth analyses. Seventy percent of failure events were observed on highways built in high plasticity clay soils in Tarrant and Dallas counties, where the Eagle Ford Shale predominates. A susceptibility map was developed for the study area based on the slope failure inventory, geology, and topography data. Based on the frequency ratio (FR) method, slope, soil, elevation, aspect, curvature, profile curvature, plan curvature, normalized vegetation index, and soil moisture index were weighted. According to the map, there are five levels of susceptibility: very low, low, moderate, high, and very high. The slope failure susceptibility model demonstrated a success rate of 71.54% and a prediction rate of 70.12%. Slope failures tend to increase with increased rainfall. Empirical thresholds such as Intensity-duration (ID), Event rainfall-duration (ED), Event rainfall-intensity (EI), daily rainfall-antecedent rainfall (3, 5, 10, and 30 days) were established. The numerical study (seepage and slope stability) demonstrated that rainfall duration has a significant impact on rainfall-induced slope failure. A high-intensity rainfall can lead to a drastic drop in pore-water pressure in a short period of time, resulting in slope failure much faster than a low-intensity rain. The slope becomes more vulnerable to failure if it cracks and faces a higher rainfall intensity. Based on a numerical simulation of a field slope section, site-specific thresholds of rainfall intensity and duration were developed and can be used to prevent slope failure caused by rainfall. The numerical model successfully replicated the failed field section at the edge of the study area which failed in 2020. Empirical thresholds predicted the failure as well. Therefore, the established thresholds may be a useful tool for regional slope failure warning systems to predict rainfall-induced slope failure.

Keywords

Rainfall, Slope failure, Landslides, Susceptibility map, Slope failure inventory, GIS, Field instrumentation, Unsaturated soil, High plasticity clay, Early warning system, Early warning criteria, Thresholds

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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