| Summary: |
The major damage to bridges at river crossings occurs during floods. Damage is caused for various reasons, the main reason being riverbed scour at bridge foundations, namely piers and abutments. Local scouring is a complex phenomenon induced by the flow field generated around piers and abutments typically inserted in movable bed rivers. In Portugal, the tragic accident of Entre-os-Rios (59 casualties), was partly due to scouring in one of the bridge piers. Piers are frequently composed of a single column. Since the nineteen fifties, hydraulic engineers have made tremendous progresses on the prediction of scour at single column piers. A few scour manuals have been published and are used, at present, in engineering practice. Among the contributors to a better knowledge of scouring, the schools of Auckland, Colorado, and Florida deserve a special reference. Most of the predictors were established on the basis of data obtained in hundreds of experiments run all over the world; still, they are frequently claimed to over-predict the scour depth in natural conditions.
According to recent findings of the school of Florida, over-prediction is due to the incorrect consideration of the effect of the sediment size factor. This effect will, thus, be addressed in the present research project; contacts with the school of Auckland and Florida on this issue will be deepened during the project to share experience and exchange thoughts.
Physical and economic considerations lead, more and more, to bridge foundations composed of a pier column founded on a pile cap, supported by an array of piles. Piers of this configuration are known as complex piers. Examples of the use of complex
piers in Portugal are those Vasco-da-Gama Bridge, Lezirias Bridge or the future bridge over Crestuma-Lever reservoir in Douro River. The cost of these large bridges, amounting to billions of Euro, justifies a rigorous prediction of scour depth, both for economic and safety reasons.
Research on scouring  |
Summary
The major damage to bridges at river crossings occurs during floods. Damage is caused for various reasons, the main reason being riverbed scour at bridge foundations, namely piers and abutments. Local scouring is a complex phenomenon induced by the flow field generated around piers and abutments typically inserted in movable bed rivers. In Portugal, the tragic accident of Entre-os-Rios (59 casualties), was partly due to scouring in one of the bridge piers. Piers are frequently composed of a single column. Since the nineteen fifties, hydraulic engineers have made tremendous progresses on the prediction of scour at single column piers. A few scour manuals have been published and are used, at present, in engineering practice. Among the contributors to a better knowledge of scouring, the schools of Auckland, Colorado, and Florida deserve a special reference. Most of the predictors were established on the basis of data obtained in hundreds of experiments run all over the world; still, they are frequently claimed to over-predict the scour depth in natural conditions.
According to recent findings of the school of Florida, over-prediction is due to the incorrect consideration of the effect of the sediment size factor. This effect will, thus, be addressed in the present research project; contacts with the school of Auckland and Florida on this issue will be deepened during the project to share experience and exchange thoughts.
Physical and economic considerations lead, more and more, to bridge foundations composed of a pier column founded on a pile cap, supported by an array of piles. Piers of this configuration are known as complex piers. Examples of the use of complex
piers in Portugal are those Vasco-da-Gama Bridge, Lezirias Bridge or the future bridge over Crestuma-Lever reservoir in Douro River. The cost of these large bridges, amounting to billions of Euro, justifies a rigorous prediction of scour depth, both for economic and safety reasons.
Research on scouring at complex piers is rather more recent than at single column piers; only four predictors of equilibrium
scour depth at complex piers could be identified in the literature; three of them were formulated in the present millennium. In spite of the complexity of the scour phenomena at complex piers, the four predictors were derived from limited experimental
evidence. Florida's method, being the most comprehensive and using the most robust data basis, relies on the results of only 49 tests, while the possible combinations of column, pile cap and pile group geometries are infinite. It is, thus, with no surprise that the predictions of existing methods lead to erroneous results (Sousa 2007), justifying a strong research commitment.
It is the purpose of the present research team to develop an extensive research programme involving five Portuguese research institutions to systematically map equilibrium scour at plausible configurations of complex piers. The ultimate objective is the production of a pier scour manual covering both single-column and complex piers.
Since FCT research projects last three years, this ultimate mentioned objective had to be scaled down to cover approximately 50% of the long-term envisaged research program. Experiments are organized into four research actions that constitute the basis to 3 MSc and 3 PhD dissertations. One PhD will address i) the sediment size factor at single column piers (action #1) and ii) the equilibrium scour depth at cylindrical-pile groups corresponding to complex piers caped above the water surface (action #2). The other PhD dissertation will address the parcel of the scour depth directly assignable to the column of complex piers with one and two rows of piles and relatively common values of the flow-depth to pile-cap-width ratio as well as the ratio of the pile-cap width to pile-cap length (actions #3 and #4). It is assumed that, within three years, another research project - the continuation of the present one - will concentrate on i) the completion of the mentioned research actions by widening their scope to uncovered ranges of pertinent variables and parameters but, mostly, ii) on the evaluation of the parcels of the scour depth directly assignable to the pile cap and to the pile group; this will be achieved by subtracting the scour depth at submerged piles (to be measured in the future project) and the column scour depth (measured in the present and in the future campaigns) from the scour depth at complete complex piers. A sound method of scour prediction at complex piers will then arise from the six years long research effort.
In the literature review, it will become clear that the research group has a large experience on scour studies. The team believes that it is in a position to successfully carry out the designed research plan; the scientific competence is reflected, e.g, in the
international recognition illustrated by the revision of tens of papers, in recent years, for the JHE and the JHR, on the topic of local scour by members of the research team. |