عنوان مقاله [English]
نویسندگان [English]چکیده [English]
One of the most common reasons for the bridge failures is local scour around bridge piers. Finding an accurate method to predict the formation, development and final shape of the scour hole is of great importance in designing hydraulic structures. Most of previous researches have focused on finding a method to estimate the depth of local scouring. Despite the fact that the importance of scour depth is inevitable, the length and width of the holes are also of a great important. One of the reasons for its importance is the possibility of overlapping of the scour holes of two piers, which increases the scour depth, and the need to determine the extent to create a reinforce layer around the piers. Regardless of the scour hole’s dimension importance, the length and the width of the hole at a complex pier have not been investigated. In this research the effect of pile cap's shape on the scour hole dimensions was investigated using physical simulation.
To provide safe design and minimize scouring problem and economic and geotechnical reasons, bridge piers on deep alluvial streambed are commonly constructed with complex geometries. A typical composite bridge pier has consisted of the column, pile cap, and pile group. After the construction of a bridge, pile cap is typically embedded in the streambed. Over time, the pile cap location in respect to the undisturbed streambed changes due to a combination of long term bed degradation, local scour, lateral shifting of the stream, and contraction scour. These issues change the pile cap role in scouring at composite piers through a flood. Since the number of effective parameter on local scour around bridge piers is considerably high, the implication of all the parameters in the estimation of scouring seems to be impossible. Therefore, the method of dimensional analysis using non-dimensional variables was implemented to evaluate how the scour hole formed. The data of the complex bridge piers in clear water conditions, extracted from the National Hydrological Research Institute of Malaysia, NAHRIM. Experiments were performed in a laboratory flume with a length of 46 m, a width of 1.52 m and a depth of 1.9 m that made of block and glass walls. The flume’s length of 15 m was filled with uniform sediment of 0.55 m thickness at a distance of 10.3 m downstream of the entrance. To conduct experiments in this research, three different models with different shapes were made from wood and PVC. In choosing models, various shapes and dimensions were sought. Three square, rectangular and chamfered shapes were selected in models 1, 2 and 3, respectively. In the next stage, the dimensions of the scour hole (depth, length, and width) were measured by a point gage in a Vernier scale with a precision of 0.1 mm. These dimensions were measured relative to the initial level of the streambed and the position of the hole in the three directions of X, Y and Z was extracted. Data analysis was performed using the Civil3D software. The 3D coordinates of the points can be imported to the software by various formats. The data was inputted into the software in PENZD format where P is number, E is value of x, N is value of y, Z is value of height and D is code or point description. In this format, the coordinates of the different points are separated, and the software distinct between the dimensions, longitudinal, transverse, or depth of the coordinates by a whitespace character. Changes in the scour dimensions of hole were investigated in the models by drawing diagrams for the models. In these diagrams, changes in the length and width of the scour hole were studied in relation to the elevation of the pile cap.
In this research, the physical simulation of the scour hole dimension changes in the complex bridge pier was performed in the laboratory and effective variables were identified and their effect was investigated. Three models were tested in different shapes and at different heights of the initial bed. The results of this research can be summarized as follows. Among the entire complex pier components (pile cap, column and pile), the geometric shape of the pile cap plays an important role in the shape of the scour holes. Similar to the depth, the process of changing the length and width of the scour hole depends on the height of the cavity relative to the initial bed. The greatest length and width of the scour hole occurs at a negative height (above the top of the bed). The maximum length and width of the hole in a model with a chamfered shape pile cap occurs at the time of the pile cap undercutting, in the rectangular and square pile cape’s shape models, after the pile cap is undercut. The largest dimensions of scour hole’s length and width were respectively 11Lpc and 8.5bpc, respectively, and the depth of scour hole was equal to 9T.