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Soil erosion around bridge supporters
2024-12-23
Assistant Instructor Ahmed Ashour Enad
Upper Euphrates Basin Developing Centre – University of Anbar
Many vital hydraulic installations related to human service are exposed to the flow of rivers and seas, such as bridges that are based on supporters, and that the erosion of the bottom soil around the support may lead to the failure of these facilities and thus lead to large human and material losses, and sacrifice can be defined as erosion of the soil of the bottom of the pillar and poses a serious danger that affects the performance of bridge supporters, in general there are three types of sacrifice at the foundations of bridges, namely: (General sacrifice, waist sacrifice and localized sacrifice). On the other hand, sacrifice was divided into two main types by Cherchenov: (general sacrifice and local sacrifice) they divided the sacrifice according to what is shown in Figure (1).
Illustrates the types of scour (1)
General erosion occurs regardless of the presence of the bridge structure as it leads to the reduction of the bottom of rivers and canals by naturally eroding their floor along the side and longitudinal section. General scour is divided into two types, (long-term and short-term), short-term scour occurs due to one flood or several close floods based on the temporal development of scour, while long-term scour develops over a long period of time that may reach several years, localized sacrifice Unlike general sacrifice, localized scour occurs due to the presence of a bridge or any other river facilities. It is divided into two types, (scour due to waist and localized sacrifice).
As for the reduction of the waist in the rivers, it means small in its width, and it occurs either naturally or because of the presence of bridge supports, which leads to reducing the area of flow and increasing the speed, which in turn leads to the growth of scour forces at the bottom, where the material of the eroded area will move until reaching the state of equilibrium, as shown in Figure (2).
Figure (2) Types of waist in a channel
The depth of scour around the single and successive cylindrical supporters and rectangular supports with semicircular ends has different dimensions and are equivalent to the distance from the beginning of the front support to the end of the rear support. Depending on the variables of the diameter of single or consecutive supports and different discharge and depths for flow under water conditions and subcritical flow with irregular bottom soils with a diameter of (D50) equal to 0.3 mm. The problem of scour around bridge supports has been studied extensively and from different points of view by many researchers and specialists, and most of these studies have documented that the main worrying cause of stability of the foundations of bridges is the scour that occurs around their supports. The pattern of flow near the stent is complex and becomes more complex with the development of the scour pit around the supports and the main mechanisms that cause scour on the supports are the lower runoff at the front face of the stent and the subsequent horseshoe vortices that form at the base of the scour pit that the depth of the scour increases with the increase in the diameter of the supports as well as increases with the increase in the depth of flow and discharge or the speed of flow within a linear relationship whose inclination increases with the increase in the diameter of the support, and the doubling of the diameter of the stent and the doubling of the discharge in At the same time, they double the depth of scour. To reduce scour, synthetic plastic mesh materials can be used that act as reinforcement of the soil under the supporter for cases where the maximum depth of scour has occurred for the purpose of reducing or reducing this depth.
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