Master of Engineering Science
Civil and Environmental Engineering
Najafi, Mohammad Reza
Overtopping flow can undermine the stability of hydraulic structures including embankments and spillways through the shear stress and the subsequent soil erosion. Increased flow velocity can cause scour, which can result in the failure of dams, bridges, and overall open channel hydraulic structures. Sediment transport is another crucial issue in open channels that can cause severe socioeconomic and environmental consequences. Two general approaches are commonly considered for scour prevention. The first approach involves flow modifications to minimize the corresponding effects on the structures. The second approach is associated with bed armoring and placing physical barriers on the natural bedding such as the Articulated Concrete Block system (ACBs). The Articulated Concrete Block (ACB) system includes a series of concrete blocks or slabs that are interconnected using steel or synthetic cables, which are commonly employed for the bed and bank protection of open channels. Two flow-induced failure mechanisms can undermine the performance of ACBs: Direct failure, which is attributed to the direct impact of free-surface flow on concrete blocks leading to the overturning or rolling-up of the edge of the ACB. Indirect failure is associated with the scouring of the underlying bedding that can result in the deformation of the ACB structure and its failure. The main objective of this study is to analyze the ACBs protection layer, understand the factors that can influence scouring, and develop measures to reduce the failure risks and minimize the amount of sediment transport of the bedding and the subgrade. The indirect failure mechanisms of ACBs laid over the downstream face of an embankment are explored through a suite of laboratory experiments (∼ 40 tests). The effects of substrate structure, soil particle size, the slope of the embankment, and different types of permeable filters (i.e., geotextile, geogrid, and filter cloth mats) on the incipient motion of the bedding and the amount of soil loss are assessed. The stability of the embankment is investigated through various configurations with different water depths and velocities. Results show that overtopping flow with high velocity causes soil erosion and sediment transport in a very short period of time with the absence of a protection layer.
ACB can noticeably improve channel stability and reduce sediment transport due to its perfect contact with subgrade and its flexibility. However, the risk of ACB failure increases if the subgrade is not designed and implemented properly. ACBs are to be in close contact with the subgrade and the fabrics and spaces between the blocks should be optimized otherwise, the overtopping of the blocks and sediment transport happen. The fabrics placed in between the layers can prevent the suction of the finer particles and reduce the risk of failure. Coarser and well-grade particles underneath the ACB structure can drain more water due to their higher hydraulic conductivity and improve the stability of the embankment. Further, risks of failure are higher near the downstream toe, which can be partly controlled by sufficient flow drainage and anchoring the downstream segments. The subgrade beneath the ACB is to be uniformly compacted so that the water flow passes through the blocks and over the subgrade steadily. The median diameter and the slope of the embankment played a key role in mitigating the amount of soil loss.
Summary for Lay Audience
As rainfall washes away bare soil or a stream erodes a muddy bank, sediment makes its way into the waterways. Fine suspended particles cause turbidity in waterways, making the water less transparent by blocking sunlight. The decreased light will deteriorate the growth of aquatic plants, which provide essential habitat for many aquatic animals, including young fish. In addition to its effects on aquatic plants and animals, sediment can fill streams, lakes, and ponds, obstruct waterways and clog storm sewers and ditches. Sediment deposits in rivers can alter the flow of water and reduce water depth making navigation and recreational use more difficult. Morphological changes (physical changes over a large area) to large aquatic systems can also result in major changes in natural sediment erosion and sedimentation patterns. As an example, the change in the size and shape of a water body will result in new water flow patterns leading to erosion or sediment removal from sensitive areas. Many marine environmental indicators show that the sea in many parts of the world, especially in coastal areas, is at high risk. Scouring occurs when fast-moving overtopping flow of water over an embankment or the water flow around a bridge or embankments removes sediment from the streambed, leaving behind scour holes below the foundations. These holes can seriously damage the structure and affect the functionality. Some of the fundamental elements of these hydraulic structures can hardly be seen such as abutments and footings submerged under the waterline. Hydraulic open channels structures failures cost millions of dollars each year in direct expenditure for replacement and restoration in addition to the indirect expenditure related to the disruption of transportation facilities. A series of failures due to soil wash and soil erosion, as reported during floods, has rekindled interest in our understanding of the scouring process and for developing improved ways of protecting the structures against scour. As such, attention is being given to the scour preventive measures to minimize the amount of soil loss and sediment transport.
There are some countermeasure techniques that could be used for preventing or minimizing local scour on embankments, culverts, spillways, etc., including ripraps and Articulated Concrete Blocks. Articulated Concrete Blocks consist of an interconnected matrix of concrete block units (hard-armor products) with hydraulic performance characteristics placed across the channel that is commonly used for scour prevention. ACBs systems are used to provide erosion protection to underlying soil from the hydraulic forces of moving water. The stability of the protection layer depends on different configurations, subgrade characteristics, the thickness of the layers, flow depth, etc. In this study, I conducted a large set of experiments to assess the underlying bed strength, which has been mostly neglected in the ACB design, and to find the best configuration to reach the desired protection strength.
Marandi, MohammadOmid, "Erosion Control of Steep Open Channels Using Articulated Concrete Blocks" (2022). Electronic Thesis and Dissertation Repository. 8379.