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Papers by Mahmoud al-Riffai
In the field of geotechnical engineering, the degree of saturation is a critical parameter since ... more In the field of geotechnical engineering, the degree of saturation is a critical parameter since its effect on pore pressures can significantly influence the slope stability of an earthen dam. In the event of a high intensity storm, quasi-saturated soil conditions in the downstream slope will result in an increase in the local soil coefficient of permeability. In addition, it is likely other activities such as piping may also have further adverse affects. This paper investigates the different methods for achieving unsaturated conditions and maintaining them in earth retaining structures. Furthermore, constitutive relationships that exist between the unsaturated coefficient of permeability and other soil parameters such as soil material, soil layer thickness, poresize and water content are investigated using an integrated approach taking into account hydrologic and geotechnical events. RÉSUMÉ
This paper presents results from an extensive experimental program on dam breaching mechanics and... more This paper presents results from an extensive experimental program on dam breaching mechanics and focuses on the erosion and side-slope instability of the breach channel that occurs on the downstream face of an embankment dam. The experimental work was conducted on homogeneous (non-cohesive) embankment models (30 to 60cm in height). Eight dam models were breached: four embankments with different soil compaction efforts, two embankments with different downstream toe drainage systems (inverted horizontal toe-drain filter vs. no drain) and two embankment dams with different soil compaction using the selected drainage system. It was found that a lower degree of saturation and a higher compaction can delay the breaching process significantly. Results for the different toe-drainage systems reinforce their importance as a model component in laboratory-scale testing for dam breach modeling and must therefore be incorporated in any physical model in order to obtain a realistic simulation of ...
A comprehensive experimental program dealing with three-dimensional overtopping and breach develo... more A comprehensive experimental program dealing with three-dimensional overtopping and breach development as well as two-dimensional overtopping physical tests of noncohesive earth embankments has been conducted on scale models in the Hydraulic Laboratory at the Department of Civil Engineering at the University of Ottawa. The experimental program which consisted of three phases focused on geotechnical and hydraulic aspects of the embankment breach mechanism. The first two phases focused on two test series for the three-dimensional breach overtopping tests: drainage and compaction. The test series were designed to determine the embankment breach characteristics using test parameters which have not been adequately identified or controlled in past noncohesive physical models: initial soil-water state and optimum dry unit weight. Both parameters were controlled in laboratory tests by means of compaction effort and seepage through the embankment body, respectively. The dynamic compaction te...
As part of a larger experimental program on dam breaching mechanics, this paper presents experime... more As part of a larger experimental program on dam breaching mechanics, this paper presents experimental results from breach tests conducted on non-cohesive earth embankment dams under several breach initiation conditions. The initial breach conditions include V-notch breach channels, as well as a more realistic breach initiation over a flat, centrally-tapered crest. A third instantaneous breach initiation was simulated through a crater emulating an explosive effect at the crest of the dam. The relative failure rates of each of the three initial breach mechanisms were compared against each other, as well as against previous experimental studies. Surface velocities through the breach were estimated by large-scale particle image velocimetry using GIS software. Special attention was paid to the geotechnical characteristics, such as moisture content and compaction effort for the embankments in these tests to more accurately scale the laboratory results.
The influence and importance of geotechnical factors such as the unsaturated zone and dry density... more The influence and importance of geotechnical factors such as the unsaturated zone and dry density in overtopped earth dams, as well as other geotechnical processes, has often been neglected or improperly considered in both experimental and numerical dam breaching models. As part of an extensive experimental program on breaching mechanics of homogeneous non-cohesive earth dams and dikes, both the erosion and lateral slope failures within the channel forming on the downstream face of a breached dam was assessed in small-scale homogeneous and non-cohesive dam models. In this particular study, two of the constructed test series are discussed. The first used two different drainage conditions (saturated vs. unsaturated) on the downstream face of the dam models, where two modes of side-slope failures (sliding vs. collapsing) were observed. The same analogy between the failure modes exists when the dry unit weight (second test series) of the dam material was increased (higher vs. lower comp...
This study presents a novel experimental technique for measuring the transient response of pore-w... more This study presents a novel experimental technique for measuring the transient response of pore-water-pressures (PWP) through a sand bed on a steep slope, typical to homogeneous non-cohesive earthfill dams or dikes, under supercritical flow using planar overtopping tests. The use of seepage-control elements in 3-D breach overtopping simulations of embankment breach failures carried out in laboratory flumes has been previously investigated by the authors. In this study, the real-time PWPs were recorded using inexpensive tensiometer-transducer probe assemblies (TTPAs), designed, assembled and tested for planar overtopping tests in hydraulic flumes. The TTPAs, comprised of a 2 mm diameter and 2 cm long ceramic probe, were installed at several locations below the downstream embankment slope face to capture the transient PWP response throughout the saturated-unsaturated zone with minimal intrusion in the soil and water mediums. This paper also describes the installation process, calibrat...
As part of a comprehensive experimental program on the failure mechanics of overtopped and subseq... more As part of a comprehensive experimental program on the failure mechanics of overtopped and subsequently breached earth embankments, this study presents the results of a novel experimental technique for measuring pore-water-pressure and erosion rate of a sand bed on a steep slope of an embankment under supercritical flow conditions and a falling head. The influence of seepage induced by the transient drainage conditions occurring on steep slopes on breaching mechanics, such as those of earthfill dams or dikes, is investigated and discussed. The erosion and subsequent slope retreat of the sand bed was captured using high-definition video camcorders and digital photo cameras and analyzed using a photogrammetric technique. Using the record of time-series of bed elevations, z b , the evolution of the bed slope profile was converted into sediment flux, ∂q b /∂s, and an associated erosion rate, ∂z b /∂t. Measurements of real-time matric suction in the unsaturated zone and excess pore-water...
… of the 34th World Congress of the …, 2011
Journal of Hydraulic Engineering, 2011
Canadian Journal of Civil Engineering, 2013
In the field of geotechnical engineering, the degree of saturation is a critical parameter since ... more In the field of geotechnical engineering, the degree of saturation is a critical parameter since its effect on pore pressures can significantly influence the slope stability of an earthen dam. In the event of a high intensity storm, quasi-saturated soil conditions in the downstream slope will result in an increase in the local soil coefficient of permeability. In addition, it is likely other activities such as piping may also have further adverse affects. This paper investigates the different methods for achieving unsaturated conditions and maintaining them in earth retaining structures. Furthermore, constitutive relationships that exist between the unsaturated coefficient of permeability and other soil parameters such as soil material, soil layer thickness, poresize and water content are investigated using an integrated approach taking into account hydrologic and geotechnical events. RÉSUMÉ
This paper presents results from an extensive experimental program on dam breaching mechanics and... more This paper presents results from an extensive experimental program on dam breaching mechanics and focuses on the erosion and side-slope instability of the breach channel that occurs on the downstream face of an embankment dam. The experimental work was conducted on homogeneous (non-cohesive) embankment models (30 to 60cm in height). Eight dam models were breached: four embankments with different soil compaction efforts, two embankments with different downstream toe drainage systems (inverted horizontal toe-drain filter vs. no drain) and two embankment dams with different soil compaction using the selected drainage system. It was found that a lower degree of saturation and a higher compaction can delay the breaching process significantly. Results for the different toe-drainage systems reinforce their importance as a model component in laboratory-scale testing for dam breach modeling and must therefore be incorporated in any physical model in order to obtain a realistic simulation of ...
A comprehensive experimental program dealing with three-dimensional overtopping and breach develo... more A comprehensive experimental program dealing with three-dimensional overtopping and breach development as well as two-dimensional overtopping physical tests of noncohesive earth embankments has been conducted on scale models in the Hydraulic Laboratory at the Department of Civil Engineering at the University of Ottawa. The experimental program which consisted of three phases focused on geotechnical and hydraulic aspects of the embankment breach mechanism. The first two phases focused on two test series for the three-dimensional breach overtopping tests: drainage and compaction. The test series were designed to determine the embankment breach characteristics using test parameters which have not been adequately identified or controlled in past noncohesive physical models: initial soil-water state and optimum dry unit weight. Both parameters were controlled in laboratory tests by means of compaction effort and seepage through the embankment body, respectively. The dynamic compaction te...
As part of a larger experimental program on dam breaching mechanics, this paper presents experime... more As part of a larger experimental program on dam breaching mechanics, this paper presents experimental results from breach tests conducted on non-cohesive earth embankment dams under several breach initiation conditions. The initial breach conditions include V-notch breach channels, as well as a more realistic breach initiation over a flat, centrally-tapered crest. A third instantaneous breach initiation was simulated through a crater emulating an explosive effect at the crest of the dam. The relative failure rates of each of the three initial breach mechanisms were compared against each other, as well as against previous experimental studies. Surface velocities through the breach were estimated by large-scale particle image velocimetry using GIS software. Special attention was paid to the geotechnical characteristics, such as moisture content and compaction effort for the embankments in these tests to more accurately scale the laboratory results.
The influence and importance of geotechnical factors such as the unsaturated zone and dry density... more The influence and importance of geotechnical factors such as the unsaturated zone and dry density in overtopped earth dams, as well as other geotechnical processes, has often been neglected or improperly considered in both experimental and numerical dam breaching models. As part of an extensive experimental program on breaching mechanics of homogeneous non-cohesive earth dams and dikes, both the erosion and lateral slope failures within the channel forming on the downstream face of a breached dam was assessed in small-scale homogeneous and non-cohesive dam models. In this particular study, two of the constructed test series are discussed. The first used two different drainage conditions (saturated vs. unsaturated) on the downstream face of the dam models, where two modes of side-slope failures (sliding vs. collapsing) were observed. The same analogy between the failure modes exists when the dry unit weight (second test series) of the dam material was increased (higher vs. lower comp...
This study presents a novel experimental technique for measuring the transient response of pore-w... more This study presents a novel experimental technique for measuring the transient response of pore-water-pressures (PWP) through a sand bed on a steep slope, typical to homogeneous non-cohesive earthfill dams or dikes, under supercritical flow using planar overtopping tests. The use of seepage-control elements in 3-D breach overtopping simulations of embankment breach failures carried out in laboratory flumes has been previously investigated by the authors. In this study, the real-time PWPs were recorded using inexpensive tensiometer-transducer probe assemblies (TTPAs), designed, assembled and tested for planar overtopping tests in hydraulic flumes. The TTPAs, comprised of a 2 mm diameter and 2 cm long ceramic probe, were installed at several locations below the downstream embankment slope face to capture the transient PWP response throughout the saturated-unsaturated zone with minimal intrusion in the soil and water mediums. This paper also describes the installation process, calibrat...
As part of a comprehensive experimental program on the failure mechanics of overtopped and subseq... more As part of a comprehensive experimental program on the failure mechanics of overtopped and subsequently breached earth embankments, this study presents the results of a novel experimental technique for measuring pore-water-pressure and erosion rate of a sand bed on a steep slope of an embankment under supercritical flow conditions and a falling head. The influence of seepage induced by the transient drainage conditions occurring on steep slopes on breaching mechanics, such as those of earthfill dams or dikes, is investigated and discussed. The erosion and subsequent slope retreat of the sand bed was captured using high-definition video camcorders and digital photo cameras and analyzed using a photogrammetric technique. Using the record of time-series of bed elevations, z b , the evolution of the bed slope profile was converted into sediment flux, ∂q b /∂s, and an associated erosion rate, ∂z b /∂t. Measurements of real-time matric suction in the unsaturated zone and excess pore-water...
… of the 34th World Congress of the …, 2011
Journal of Hydraulic Engineering, 2011
Canadian Journal of Civil Engineering, 2013