Browsing by Author "Ozer, A. Tolga"

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Citation Count: 11
Application of fibrous streambank protection against groundwater seepage erosion
(Elsevier Science Bv, 2018) Akay, Onur; Ozer, A. Tolga; Fox, Garey A.; Wilson, Glenn V.
Groundwater flow is one of the main driving factors in the erosion of streambanks, particularly during return flow of bank storage as sediment particles on the bank face may be entrapped or liquefied by seepage flow into the stream, and when acting in concert with fluvial forces. Previous research has mainly focused on seepage erosion mechanisms, whereas in this study, a remedial solution using randomly distributed 6-mm-long polypropylene fibers mimicking the behavior of plant roots in slopes was investigated by laboratory physical streambank model experiments. Reduced-scale sandy (14 kN/m(3) unit weight) streambank models (45 degrees bank slope) with dimensions of 195 cm long, 100 cm wide and 110 cm high were constructed in an erosion flume. Two different seepage gradients were generated within the streambanks by maintaining piezometric heads of 50 cm H2O and 100 cm-H2O in the upstream section of the erosion flume during experiments. Models were equipped with vibrating wire piezometers to measure the pore-water pressures within the streambank. In addition, tensiometers measured pore-water pressures near the wall section. Erosion of sediment from the streambank initiated concurrently as seepage flow emerged on the bare streambank surface. Erosion volumes were computed by three-dimensional laser scanning. Triaxial compression tests on sand samples (fiber gravimetric content ranged from 0% to 1.0%) indicated an increase in cohesion by fiber content. Fibrous streambank protection with 0.3% fiber content inclusion reduced the total amount of seepage erosion by 35%, and 47% under 50-, and 100 cm-H2O piezometric head boundary condition (BC), respectively. Seepage erosion rate and seepage discharge demonstrated a power-law relationship. Due to the increased cohesion, fibrous streambank protection with 1.0% fiber content effectively prevented seepage erosion during streambank experiments under the same BCs.
Citation Count: 7
Behavior of Fiber-Reinforced Sandy Slopes under Seepage
(Amer Soc Civil Engineers, 2016) Akay, Onur; Ozer, A. Tolga; Fox, Garey A.; Wilson, Glenn V.
Seepage flow is a major contributor to instability of natural hill slopes, river banks and engineered embankments. In order to increase the factor of safety, an emerging technology involves the inclusion of synthetic fibers in the soil. The addition of tension resisting fibers has a favorable effect on strength properties of sandy soils. In this study, laboratory lysimeter experiments were conducted on fiber reinforced slopes with two different values of constant pressure head boundary condition (25 and 50 cm) in the water reservoir. Fiber reinforced sand was compacted in the soil compartment of the lysimeter to obtain a slope with dimensions of 55 cm height, 20 cm width, and 100 cm base length. The gravimetric fiber content (percentage of dry weight of sand) was selected as 1% after reviewing the results of comprehensive triaxial compression tests on fiber reinforced sand specimens with varying fibrillated polypropylene fiber (12 mm long) contents from 0.1 to 1%. This study included slope stability modeling in order to quantify the global factor of safety. The triaxial compression tests indicated the increase in peak deviatoric stress with increase in fiber content. The fiber reinforced sand slope was stable against seepage conditions which would otherwise cause a shallow-seated failure of the nonremediated slope under 25 cm water pressure head. In addition, fiber reinforced sand slope maintained its global stability under 50 cm water pressure head which caused a deep-seated failure of the unreinforced slope. However, sloughing at the toe occurred under 50 and 55 cm water pressure head.
Citation Count: 32
Behavior of sandy slopes remediated by EPS-block geofoam under seepage flow
(Elsevier Sci Ltd, 2013) Akay, Onur; Ozer, A. Tolga; Fox, Garey A.; Bartlett, Steven F.; Arellano, David
Expanded polystyrene (EPS) geofoam (geofoam block) is commonly used as a lightweight fill for many civil engineering applications. However, when used for slope remediation, the behavior of geofoam block for slope systems undergoing seepage flow is not well known. In this study, a total of 36 laboratory lysimeter experiments (dimensions of 60 cm height, 20 cm width, and 200 cm length) were conducted to investigate the behavior of sandy slopes containing geofoam blocks as a lightweight fill material. These experiments were conducted with three different values of constant water pressure in the water reservoir located at one end of the lysimeter. In addition, three different configurations of geofoam block were tested with geofoam blocks placed on the face of the packed sandy slope. The dimensions of the geofoam blocks were 2.5 cm high, 5 cm wide, and 15 cm long to achieve a 1:20 scale corresponding to actual block size that is commonly manufactured. Laboratory physical test results were quantified by coupled seepage flow and slope stability models showing the adverse effect of seepage on the factor of safety (FS). Geofoam block configurations were found to be stable against seepage conditions which would cause a shallow-seated failure of the slope in the absence of the geofoam blocks. This is due to the fact that the geofoam blocks could completely fill the mass of the existing slope material subjected to failure. However, the geofoam block configurations were unstable against seepage conditions that resulted in deep-seated failures of marginally stable, sand slopes. (C) 2013 Elsevier Ltd. All rights reserved.
Citation Count: 1
Comparison of Methodologies for Establishing Design Properties of Horizontal Drainage in Soft Cohesive Soils
(Natl Acad Sciences, 2015) Farnsworth, Clifton B.; Ozer, A. Tolga; Bartlett, Steven F.; Lawton, Evert C.
Estimating the rate of settlement for foundation soils treated with vertical drains requires an understanding of the horizontal drainage behavior of the soil, because the time of consolidation settlement may be critical to the overall construction schedule and sequencing. This paper provides a case study comparison of the results of methodologies associated with obtaining design parameters for horizontal drainage for use with vertical drain design, including backcalculation of field settlement data, cone penetrometer testing for pore pressure dissipation, and laboratory Rowe cell testing, by means of the soft, cohesive Lake Bonneville soil deposits in Salt Lake City, Utah. Each of these methodologies has an inherent set of strengths and limitations that should be considered when vertical drains are being designed or time of consolidation settlement is being estimated. Backcalculation of field performance data is effective in identifying true in situ settlement behavior but is not always feasible. Rowe cell testing tends to provide values that more closely correspond with those obtained from backcalculation but is not often performed. Testing for pore pressure dissipation is the most used technique, but it can provide drainage values much higher than the other two methodologies.
Citation Count: 6
Fiber Reinforced Sandy Slopes under Groundwater Return Flow
(Asce-amer Soc Civil Engineers, 2018) Akay, Onur; Ozer, A. Tolga; Fox, Garey A.; Wilson, Glenn V.
The instability of earthen embankments caused by subsurface flow draining out of the banks has been a major concern. In an effort to prevent embankment failures, tension resisting synthetic fibers may be an effective additive to increase their mechanical properties such that drainage does not cause failure. In this study, triaxial compression tests measured the increase in peak deviatoric stress with increase in fiber content and length. In addition, laboratory lysimeter experiments (total of eight experiments) were conducted on sandy slopes reinforced with polypropylene (PP) fiber using two different fiber lengths (6 and 12mm) and gravimetric fiber contents (0.3 and 1.0%) under two different constant piezometric head boundary conditions (25 and 50cm) maintained in the water reservoir of the lysimeter. Fiber-reinforced sand was compacted in the lysimeter to obtain a 45 degrees slope with dimensions of 55cm height, 20cm width, and 100cm base length. The only experiment that experienced seepage erosion by particle mobilization under 25-cm water pressure head boundary condition was the sand slope reinforced with fiber length and content of 6mm and 0.3%, respectively. The increase in the water pressure head boundary condition to 50cm resulted in small-scale sapping of slopes reinforced with 0.3% fiber content, independent of the fiber length. For slopes reinforced with 1.0% fiber content, sapping did not occur, only erosion of sand particles caused by seepage was observed. A slope stability analysis reflected the favorable effect of fiber inclusion, as the increased effective cohesion increased the factor of safety of slopes.
Citation Count: 18
Interface Shear Strength Characteristics of Interlocked EPS-Block Geofoam
(Asce-amer Soc Civil Engineers, 2016) Ozer, A. Tolga; Akay, Onur
Lightweight fills constructed by the use of expanded polystyrene block (EPS-block) geofoam (geofoam block) is known to be susceptible to horizontal loading since the vertical pressure which enhances the shear resistance along the interface of individual blocks is low. Therefore, research has focused on the methods to increase the interface shear resistance of geofoam blocks. This study investigated the use of interlocking mechanisms by conducting direct shear tests on four different interlock configurations. An interlock configuration utilized a number of ledges and notches to interrupt the continuous horizontal shearing plane in-between geofoam blocks. Two different densities of geofoam blocks (EPS19 with a minimum density of 18.4kg/m3 and EPS29 with a minimum density of 28.8kg/m3) were used in order to investigate the effect of material hardness on stress-strain behavior and interface friction properties. The results of this study showed that the interlocking mechanism improved the interface shearing resistance of geofoam blocks. The number of ledges and notches used in an interlocked configuration had an effect on the interface shear strength properties, and higher numbers raised the interface failure envelope to the level of internal shear strength of monoblock EPS. In conclusion, implementing an interlocked configuration changed the EPS/EPS interface friction mechanism from purely frictional to frictional-adhesional.
Citation Count: 23
A new method for remediation of sandy slopes susceptible to seepage flow using EPS-block geofoam
(Elsevier Sci Ltd, 2014) Ozer, A. Tolga; Akay, Onur; Fox, Garey A.; Bartlett, Steven F.; Arellano, David
Using expanded polystyrene (EPS) geofoam (geofoam block) in slope remediation projects has drawn interest from the civil engineering sector for its ease of application and budget saving features. According to design precedence, all slope remediation applications that use geofoam blocks should incorporate permanent drainage systems to prevent instability of the lightweight geofoam blocks due to hydrostatic and seepage pressures. In this study, a new method for slope remediation using geofoam blocks was tested through physical laboratory experiments. For this purpose, a total of 24 lysimeter (dimensions of 60 cm height, 20 cm width, and 200 cm length) experiments (including duplicates) were conducted in which seepage through a geofoam block slope system were generated with three different constant water levels in the water reservoir of the lysimeter. Geofoam blocks (dimensions of 2.5 cm height, 5 cm width, and 15 cm length) were assembled to form embankment type configuration at the toe section of the sandy slopes. This study also included coupled numerical model simulations that were comprised of variably saturated flow modeling and slope stability modeling which could be implemented successfully for the global static failure analysis of the geofoam block slope system comprised of two mediums with different geotechnical characteristics. In addition to global static stability failure analysis, which involved conventional limit equilibrium analysis for the geofoam block slope system, hydrostatic sliding mechanism was investigated which provided insight into using an overburden concept to increase the resistance against horizontal driving forces. Experimental and numerical modeling results showed that the geofoam block slope system was stable even though the phreatic surface was above the bottom of the geofoam block assemblage. For this reason, the embankment type configuration tested in this study can be considered a viable remediation technique where seepage induced deep-seated global stability and hydrostatic sliding failures are a concern. (C) 2014 Elsevier Ltd. All rights reserved.
Citation Count: 17
New method to determine proper strain rate for constant rate-of-strain consolidation tests
(Canadian Science Publishing, Nrc Research Press, 2012) Ozer, A. Tolga; Lawton, Evert C.; Bartlett, Steven F.
The development of a new semiempirical method to predict the proper strain rate for constant rate-of-strain (CRS) consolidation tests is described herein. The validity of the proposed method is analyzed using experimental results from CRS and incremental loading tests on four types of soil: Lake Bonneville clay, Massena clay, kaolinite, and montmorillonite. It is found that the maximum allowable strain rate depends on the initial void ratio of the soil and thus is related to the compressibility of the soil. The effect of the strain rate on the distribution of the pore pressure within the sample is investigated by comparing values of effective vertical stress calculated using a linear equation published by Wissa et al. in 1971 with values of effective stress at the base of the specimen determined from measured values of pore pressure. Overall, the proposed method predicts the maximum allowable strain rate very well for three of the four soils and moderately well for the other soil.
Citation Count: 44
Stability assessment of an earth dam on silt/clay tailings foundation: A case study
(Elsevier, 2012) Ozer, A. Tolga; Bromwell, Leslie G.
This case study describes the design, and construction of an earth dam built on recently sedimented tailings consisting of low strength, low plasticity silt/clay, and variably silty sand. The tailings were encountered within the construction limits of Cell 7 at the 4 km(2) Loxahatchee (L-8) Reservoir in western Palm Beach County, Florida. Fully coupled steady state and transient seepage models with slope stability analyses were performed for operational and rapid drawdown conditions, respectively, using both limit equilibrium methods (LEM) and finite element stress-based modeling (FESBM) techniques. In addition, stability calculations utilizing fully coupled stress/pore pressure analysis were performed to analyze rapid drawdown stability during drawdown. (C) 2012 Elsevier B.V. All rights reserved.