Hailu Ayalew

Graduation Semester and Year

2013

Language

English

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering

Department

Civil Engineering

First Advisor

Laureano Hoyos

Abstract

The main objectives of this work are to determine the effect of net normal stress and volume change on the soil-water retention properties of silty sand and clayey sand soils under stressed/volume-controlled conditions; to model soil-water retention parameters under stress/volume-controlled conditions; and to model the effect of higher net normal stress state on the soil-water retention characteristics of these two test soils based on the experimental test results. In order to investigate the effect of stress/volume change on the soil-water retention characteristics of these two test soils, two test methods were followed: the stress/volume-controlled soil-water characteristic test (Tempe pressure cell test) and the contact filter paper test. The first method uses the concept of axis translation to measure and control matric suction (up to max. of 15 bars). Axis translation is a technique of translating the origin of matric suction from atmospheric air pressure and negative pore water pressure to atmospheric pore water pressure and positive air pressure. This technique avoids the problem of cavitation, which makes the water phase discontinuous during matric suction measurement. The device has two main components: a pressure panel, which is used to apply pore-air pressure and measure water volume change; and a pressure cell assembly with loading frame, which is used both to retain the test specimen and to control loading and volume change during test. Normal stress is applied using a pneumatic loading piston to simulate field condition while volume change is measured using a dial gauge attached to the load shaft. The volume of water expelled out of the soil specimen is measured using a graduated volume-measuring burette attached to the main panel of this device.Due to the limited supply of air pressure in the laboratory (800 kPa), it was not possible to assess the full trend of the soil-water characteristic curves (SWCCs) using the data points obtained from the stress/volume-controlled SWCC cell, except for the silty sand soil. For this reason, the contact filter paper technique was used to obtain additional SWCC points at higher suction values. Finally, the points obtained from both the pressure cell and the contact filter paper tests were combined and the defined trend of the SWCCs was observed for a whole range of suction values.Results obtained from the laboratory tests indicate that the SWCC of clayey sand soil, with volume change consideration, yields a higher air-entry value and a lower desaturation rate when compared with no volume change assumption in the calculation of volumetric water contents. In contrast, the effect of volume change on the air-entry value of the silty sand soil and other key water retention properties is almost negligible due to the fact that relatively coarser soils may not undergo as much volume change during increased vertical loading compared to fine-grained soils. The SWCCs of both soils showed a consistent shift to the right with higher net normal stresses, which is indicative of an increase in the air-entry value with increasing net normal stress, in agreement with results recently reported by different researchers for purely sandy or clayey soils. Finally, it was also observed that the pore-size distribution of a test soil has a marked effect on the nature of the corresponding soil-water characteristic curve. The results obtained from SWCC tests on both soils, for each net normal stresses, were plotted accordingly on the same graph to assess this effect.

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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