Numerical investigation of the effect of geobags on the scour control of bridge piers

Master's Thesis in the field

Road, Building and Environmental Engineering-Hydraulic Structures

In this research, the effect of geobags on bridge abutment scour control has been studied using Computational Fluid Dynamics (CFD) method. Geobags are geotextile bags that are filled with materials such as sand, concrete or materials from river dredging. Due to the availability, low cost and no need for skilled labor, the use of these materials is very economical compared to traditional materials. In this regard, using FLOW-3D software, the flow and erosion of sediments around the bridge piers and the effect of geobags and geomets (large geobags) on reducing scour depth were modeled. This software is used to solve three-dimensional Neuer-Stokes equations by finite difference method. The RNG turbulence model has been used to model the flow field around the bag, the place where horseshoe vortices are formed and the turbulent flow is dominant. Confirmation of the correctness and accuracy of the software was investigated using the results of the laboratory model of flow and erosion around the bag without a protective layer with a vertical wall. In the modeling process, first the flow around the bag was expanded until it reached a stable state, and then the model was allowed to erode. The simulation results are in good agreement with the laboratory model in terms of quantity and quality. Based on the simulations, the geobag and geomet layers protect the bridge abutment well against erosion, but they cause the erosion to be transferred to the downstream of the abutment. Also, the effect of the geometry of the geomet layer on reducing the maximum scour depth, as well as the efficiency of the geomet layer in the water depth, speed and size of different sediment particles were studied. Key words: scour water - bridge bag - computational fluid dynamics Chapter 1 Introduction

1-1- Types of bunds, location and construction

Although the morphology of river waterways has fundamental differences from one place to another, but bunds have a single general characteristic that can be used to define their type to predict the flow field in the geometry of different waterways. The general characteristics of bridges can be defined in the form of the type of bridge, the general location of the access embankment and the condition of the construction of the bridge. Each of these characteristics, along with the geometry of the waterway and the type of bed sediment, will have a great impact on the flow field around the bridge and as a result of scouring.

1-1-1- Types of bridge abutments

In general, bridge abutments can be divided into three main types:

1) a sloped wall abutment [1] (the most common type)

2) an abutment Wings[2]

3) Bag with a straight wall

In bags with a sloping wall, the sides are inclined like the front face (usually with an angle lower than the angle of placement[3] of the materials used in the embankment); And the corners connecting the faces and sides are rounded like a part of a cone (Figure 1-1). The side faces of the embankment are also sloped in the wing bags, but the front face is vertical. The angle between the front face and the fin is usually 45?; Although other angles are also used. Due to the sudden connection of the fin to the opposite face, a sharp corner is formed, which makes the flow less turbulent compared to bags with sloping walls [4] (Figure 1-1). In a bag with a straight wall, both the side faces and the front face are vertical. The angle of the side and front faces is 90°, so the flow is less turbulent than the flow around the bridges.

Location of bridge bridges

The location of bridge bridges located on rivers can be expressed with the parameters of the length of the bridge (L), the width of the flood plain [5] (Bf), and the half width of the waterway (B). Usually, the following locations are common (Morales & Ettema, 2011):

1) The tank is placed in the flood plain of the compound waterway in such a way that it is This location is common for the slopes with sloping walls.

2) The entire flood plain covers the main waterway so that it is This positioning is suitable for wing bags in small channels.

3) The bag should be placed in a rectangular channel. This placement is not common, and may be considered a short bridge in a wide floodplain..

1-1-3- Bag dimensions and construction method

American bridges usually have at least two 12-foot lanes (3.66 m), which gives a width of 40 feet (12.2 m) for a full road width of 24 feet (7.32 m) plus two 8-foot shoulders (2.44 m) on each side. The side embankment is also implemented with slopes of 2H:1V to 3H:1V, although the most common side slope is 2H:1V.

Piles are usually placed on a concrete retaining wall, or a column located on a pile cap maintained by piles, or a wide foundation, and connected to the access embankment.

1-2- Flow field

The flow passing through the waterway of a bridge, which is constricted due to the presence of bridge piers and embankments, is similar to the flow around a bottleneck, with the characteristics of the flow shown in Figure (1-2): the width of the flow is narrowed and the flow accelerates in the constricted section, causing the creation of highly turbulent structures [6] (different eddies and vortices that rotate around the boundaries of the bottleneck) that are dispersed and then inside the flow they disappear Stream narrowing and turbulence in many bridge waterways are more complicated due to the shape and roughness of the channel in which the waterway is located. Normally, waterways pass through a deeper main channel located next to flood plains. Figure (1-3) shows the flow near a weir with a sloping wall, which is located in the floodplain of a mixed waterway. The flow accelerates from upstream to the narrowest section, and just downstream of Kole, the flow continues with decreasing acceleration. A separation occurs in the flow field immediately downstream of the culvert before the flow re-establishes along the channel. On the upstream side of the dam, small eddies may be formed whose size depends on the length and direction of the dam (Morales & Ettema, 2011).

When the foundation of the dam is rigidly connected to the floodplain or bed, the curvature of the flow around the constriction can cause turbulent structures. Therefore, the flow may cause erosion and create a pit in its path by creating a local spiral movement (vortex-like) that has a high scouring power. Also, the resulting eddy creates a series of more complicated secondary eddies. Wong (1982), Tey (1984), Kawn (1988), Kouchakzadeh (1996) have provided more information about the eddy system. is applied Two types of scour may occur at the bridge site: 1) General scour 2) Bag scour In the following, these two types of scour will be described briefly. it is not General scour can be divided into two terms: short-term scour, due to floods, and long-term scour, related to geomorphic changes related to the imbalance of sediment storage, water flow, and the slope of the waterway. The waterways are opened. Therefore, the main characteristics of scour can be defined as follows (Morales & Ettema, 2011):

1) scour is very affected by the distribution of flow passing through a short bottleneck and turbulent structures that are created and spread by the flow entering the bottleneck.

2) for most bridge scours, the presence of scour constricts the flow uniformly along the bridge channel. However, in the case of a short embankment connected to a wide channel, scour is reduced under the following two conditions:

a) If the width of the channel is constant and the length of the embankment is reduced, the depth of the scour at the weir reaches zero.

b) For a full shape of the weir with a constant length in a waterway with increasing width, the scour depth at the weir reaches a limited value related to the scour around a weir in a very wide waterway. It reaches wide. This washout depth can be approximately estimated by the local narrowing term of the flow around the bag itself.

The modeling of the second condition by hydraulic models is difficult, because most of the laboratory flumes are not wide enough.