ASCE GSP 210-2011 Scour and Erosion.pdf

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1、 GEOTECHNICAL SPECIAL PUBLICATION NO. 210 SCOUR AND EROSIONPROCEEDINGS OF THE FIFTH INTERNATIONAL CONFERENCE ON SCOUR AND EROSION (ICSE-5) November 710, 2010 San Francisco, California, USA SPONSORED BY The Geo-Institute of the American Society of Civil Engineers The Environmental and Water Resources

2、 Institute of the American Society of Civil Engineers International Society for Soil Mechanics and Geotechnical Engineering EDITED BY Susan E. Burns Shobha K. Bhatia Catherine M. C. Avila Beatrice E. Hunt Published by the American Society of Civil Engineers Cataloging-in-Publication Data on file wit

3、h the Library of Congress. American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia, 20191-4400 www.pubs.asce.org Any statements expressed in these materials are those of the individual authors and do not necessarily represent the views of ASCE, which takes no responsibility fo

4、r any statement made herein. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. The materials are for general information only and do not represent a standard of ASCE, nor are

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9、American Society of Civil Engineers. All Rights Reserved. ISBN 978-0-7844-1147-6 Manufactured in the United States of America. Preface Scour and erosion represent some of the most critical threats to maintaining infrastructure and quality of life throughout the world. As the human population expands

10、, the responsibilities of engineers, scientists, and designers continue to increase, and the demands for a creative approach to effective control of scour and erosion also becomes more pressing, requiring cross-disciplinary synthesis of information from the fields of hydraulic and geotechnical engin

11、eering. The papers presented at this conference focus on critical issues in the scour and erosion of hillside, fluvial, estuarine, and coastal environments, at the interface of water, soil, and rock. Following the first four highly successful international conferences on scour and erosion, which wer

12、e held in College Station, Texas, USA (2002), Singapore (2004), Amsterdam, The Netherlands (2006), and Tokyo, Japan (2008), the fifth international conference was commissioned to continue the tradition of presenting state of the art information on the most pressing issues in scour and erosion. The c

13、onference was co-sponsored by the ASCE GeoInstitute and ASCE Environmental and Water Resources Institute, and the resulting conference themes of scour of foundations, erosion of soils, bridge scour, scour and erosion of dams and levees, scour of offshore platforms and underwater pipelines, and rock

14、scour reflect the truly interdisciplinary nature of the subject matter. All papers in the proceedings were reviewed, re-reviewed, and accepted for publication by at least two reviewers who were selected by the proceedings editors. All papers included in these proceedings are eligible for ASCE awards

15、. The editors would like to thank the reviewers listed below who spent many hours and great care reviewing and commenting on the manuscripts. The time they donated to ASCE is greatly appreciated, and it is through their contributions that the quality and integrity of these manuscripts is maintained.

16、 S.E. Burns, S.K. Bhatia, C.M.C. Avila, and B.E. Hunt San Francisco, CA, 07-10 November 2010 iiiAcknowledgments ORGANIZING COMMITTEE Conference Chair: Catherine M.C. Avila, Avila and Associates Consulting Engineers, Inc. Conference Co- Chair: Beatrice E. Hunt, STV Incorporated Proceedings Editors Su

17、san E. Burns, Georgia Institute of Technology Shobha K. Bhatia, Syracuse University Catherine M.C. Avila, Avila and Associates Consulting Engineers, Inc. Beatrice E. Hunt, STV Incorporated Technical Program Giovanna Biscontin, Texas A email: michael.heibaumbaw.de 2Gewatech - Grund- und Wasserbau Gmb

18、H email: infogewatech.de ABSTRACT Partially grouted armourstones combine the high resistance against cur-rents and waves of large elements and their flexibility to adapt to ground deforma-tions and the option of installing comparably thin layers. With partial grouting smaller and such often cheaper

19、armourstones can be used to form conglomerates with the same resistance as large armourstones. Grouting can be done in fresh water and in saltwater. Segregation and erosion of the grout when poured or dumped or when fresh grout is loaded by currents and waves is avoided by using special chemical add

20、itives or by high speed centrifugal mixing (colloidal mortar) common anti-wash-out mortar is not applicable. Stone diameters of 10 to 40 cm and a narrow rock size distribution are best for being grouted. To guarantee a suc-cessful application of partial grouted armours, a number of tests before, dur

21、ing and after installation have been established for quality assurance. INTRODUCTION Scour is a result of the interactions at the boundary of water and soil. In many cases it is considered a hydraulic problem only, looking at it from the water side. So nearly all approaches to assess scour developme

22、nt represent the hydraulic point of view and consider the stability of the top layer, only. But one has also to consider all layers that are influenced by the hydraulic load including the interac-tion of the surface water and the pore water. On the other hand, sublayers need no consideration if the

23、stability of the top layer is 100 % guaranteed! Such stability can be achieved by grouting the top layer to keep the armour material in place even under high hydraulic actions. An important parameter for designing a scour protection is the surface ge-ometry of the protected area. Shallow beaches and

24、 banks are much less suscepti-ble to scouring than steep banks or structures like breakwaters and dikes with steep slopes. The stability of armour elements decreases with an increase ot the inclination of the surface. Since often surface geometry cannot be modified to1the desired shape, solutions ha

25、ve to be sought to stabilize the armour without in-creasing their size to unproportional dimensions, e.g. grouting the top layer. Partial grouting (Fig.1) is a reliable and well established method to meet the requirements for a long lasting scour protection including sufficient permeabil-ity to avoi

26、d excess water pressure below the armour. With this method, the stabil-ity of the traditional armour made of loose elements is increased to a very large extent. Partial grouting means filling the voids of a riprap layer to 35 - 50% with a special mortar, thus creating an armour layer with high resis

27、tance, (still) high permeability and sufficient flexibility. Generally partial grouting of armour stones is used if the stability of armourstones is not sufficient due to the magnitude of the hydrodynamic action (waves, turbulent flow) and because of the weight of the armourstones, thickness of the

28、armour layer and slope inclination. Figure 1. Partially grouted riprap ARMOUR LAYER General An armour layer to prevent scouring may be permeable or impermeable. If impermeable systems are used, the development of excess pore water pressure below the layer should be ruled out. Excess pore water press

29、ure or uplift pressure often is one of the reasons for damage of the armour layer or even the whole sys-tem. A steady excess pore water pressure (uplift pressure) may be caused by a high groundwater table compared to the level of the surface water. An unsteady excess pore water pressure below the co

30、ver layer develops when there is a rapid lowering of the surface water level, for instance due to waves or a ship induced drawdown. But even in the subsoil below permeable cover layers an excess pore water pressure may develop, however to a lesser extent. Since natural water is not an ideal (incompr

31、essible) fluid, the pore water pressure will lag the surface water SCOUR AND EROSION2pressure change. The length of this time delay depends on the permeability and the saturation of the subsoil. At the coast, impermeable protection is used in many places. The top layer in most cases is fully grouted

32、 riprap, using bituminous or concrete grout. Some-times a lining of asphaltic concrete, cast asphalt or cement bonded material is in-stalled. Also geosynthetic mattresses with concrete fill are used. To avoid the de-velopment of excess water pressure, often a permeable cover layer is recom-mended fo

33、r scour countermeasures. At first sight, a permeable cover layer seems to ask for more effort and costs compared to an impermeable one. A filter is al-ways needed and the placement of the top layer material has to be done much more accurately, but in the end such a system is more successful in most

34、cases. Flexibility (serviceability) Besides the problem of excess water pressure below the lining, a second problem occurs with impervious armour layers, namely suberosion of the soil from below the lining. Only in ponds and narrow rivers can the lining be placed from one side of the water to the ot

35、her. So the toe of the armour layer of coastal protection and protection of wider rivers is a critical point. The soil may be washed out from under the layer, initiating regressive erosion and resulting in large voids. This is even intensified by the fact that all the impervious armour lay-ers are r

36、ather rigid even the bituminous systems. Due to these boundary condi-tions, erosion can occur over a long time without being detected, since the strength of the armour is high enough to bridge the voids, but finally a large col-lapse occurs (Fig. 2). Figure 2. Failed concrete mattress lining Scourin

37、g is a dynamic geomorphic process that can be stopped completely only with major effort. Scouring at the borders of scour protection is inevitable but it can be accepted to a certain extent, if the protection system is chosen appropri-ately, hence resulting in a major requirement: a good scour prote

38、ction system has to be flexible. The demand for flexibility holds for all elements of scour repair and SCOUR AND EROSION 3prevention work, i.e. fill, filter and armour. The best measure possible will be im-plemented when all layers adapt to the geometry given and are able to follow a changing geomet

39、ry due to hydrodynamic processes. Secondary scouring at the edges of a scour protection layer is nearly inevitable, since the transition from an artificial to a natural bed always causes local erosion. So only a flexible protection layer guarantees the adaptation to the newly developing geometry unt

40、il an equilib-rium is reached. (Another option would be to bury the toe to a sufficient, i.e. rather great, depth.) Armour elements To meet the above mentioned requirements of permeability and flexibility, the armour for a scour countermeasure is built from single elements that may be placed randoml

41、y or regularly, mutually connected or loose. Armourstones or riprap is maybe the most often used material for protec-tion systems to stop or to reduce scouring or to rehabilitate existing scour holes. Solutions at reasonable costs can be attained if this material is sufficiently avail-able. Sometime

42、s riprap is considered a temporary countermeasure only, an opin-ion which may originate from the use of stones that are too small or from armour layers without a filter beneath. Concrete elements are used when natural material is not available to the necessary extent. The production costs are much h

43、igher but their use is justified if the transportation distance of rock is too large. In certain cases it may be benefi-cial to produce the elements on-site. There is a large variety of concrete elements that are used as an armour layer. They will not be listed here in detail, but there are three gr

44、oups to be mentioned: Elements of many different shapes that are used like riprap, blocks that are placed regularly (“paved“) and elements that are mutu-ally connected. Stability of armour elements The resistance of all single elements against hydrodynamic forces in-creases with the weight of the el

45、ement. But increasing the weight, which is usually linked with an increasing diameter, means increasing the layer thickness, too. And since the voids between larger elements also become larger, the elements of the layer below (filter or cushion layer) have to be larger to ensure their not being erod

46、ed through the cover layer. Maybe even an additional layer is necessary. But an increasing layer thickness may be incompatible with the geometry require-ments. To limit the thickness of the armour layer but to provide a comparable re-sistance against the erosive forces of currents and waves, cover l

47、ayers with mutu-ally connected elements can be fabricated. The general idea is to use smaller and often cheaper elements, but simultaneously to gain high resistance against the hy-draulic load by connecting them to larger elements, to “mattresses“ or to continu-ous layers. Examples of permeable cont

48、inuous layers are open stone asphalt, mu-tually interlocking or cable connected concrete elements, stone mattresses etc. A paved cover layer (natural stones or concrete blocks) also shows an in-creased resistance against hydraulic loads while remaining limited in thickness. SCOUR AND EROSION4But the

49、 high resistance is lost if only one element is missing and the permeability is limited since one element is placed very close to the next. The use of gabions is a well known method to achieve large elements with only small voids. Gabions are made of riprap or even smaller stones filled in a wire mesh basket. They are very versatile elements concerning the shape of the single element as well as the shape of the whole cover layer. Gabions may be pre-fabricated or - in the dry only - filled in place. Sto