Inﬂuence of Shear Rate on Undrained Vane Shear Strength of Organic Harbor Mud Benjamin Friedrich Schlue, Ph.D. 1 ; Tobias Moerz, Ph.D. 2 ; and Stefan Kreiter, Ph.D. 3 Abstract: Dredging operations in European harbors for maintenance of navigable water depth produce vast amounts of harbor mud. Between 2005 and 2007, the second largest harbor construction project in Germany was designed as a pilot study to use dredged harbor mud as backﬁll material to avoid expensive disposal or ex situ treatment. During this project, a partial collapse of the backﬁll highlighted the need for an improved assessment of undrained shear strength of naturally occurring liquid harbor mud. Using vane shear testing, this study evaluates the effect of shear rate on the undrained shear strength of harbor mud. It is shown that measured values for both peak and residual shear strength are signiﬁcantly inﬂuenced by shear rate effects. Furthermore, the inﬂuence of shear rate on the peak shear strength is found to be independent of water content while the inﬂuence of the shear rate on the residual shear strength strongly depends on water content. New shear rate dependent correction factors  are proposed using the test results and the observed time to failure in the harbor basin. The proposed correction leads to signiﬁcant lower design undrained shear strengths than the classical Bjerrum correction and would have predicted the failure during the construction. DOI: 10.1061/ASCEGT.1943-5606.0000356 CE Database subject headings: Mud; Clays; Shear tests; Organic matter; Shear strength; Dredging; Harbors; Europe. Author keywords: Dredged mud; Soft clay; Vane shear test; Time effects; Correction factor; Shear rate effects. Introduction The major North Sea tidal estuaries of the Ems, Weser, and Elbe Rivers are characterized by vast accumulations of organic-rich mud derived from marine and river suspension transport. High rates of mud accumulation in these dynamic systems are driven by a complex interaction of variable river discharge and tide and wind induced processes. Increased mud accumulation is promoted by tidal asymmetry and particle ﬂocculation in the freshwater/ saltwater mixing zone Schrottke et al. 2006. The highly engi- neered estuaries of the Ems, Weser, and Elbe Rivers belong to the most frequented waterways worldwide and host a number of im- portant ports. Deepening the navigation channel in these rivers causes an artiﬁcial rise in the tidal range and an increase in the suspension load. The small-grained material is carried through the locks into the basins of the harbors in northern Germany where it settles as organic harbor mud. This mud must be regularly dredged out of the harbor basins in order to maintain the naviga- tional depth. To avoid costly disposal of this often contaminated material, the dredged harbor mud is increasingly reused on-site as backﬁll and construction material, a procedure used during the backﬁlling of the harbor basin in the East Harbor of the interna- tional port area of the Free Hanseatic City of Bremen in Bremer- haven, Germany Metzen et al. 2006. During this excavation and construction project, a total of about 180, 000 m 3 of soft to liquid harbor mud was relocated and used as backﬁll behind a heavy sheet pile structure that will serve as a new wharf Fig. 1, creat- ing 14 acres of new harbor area Schlue et al. 2007, 2009. To accelerate the consolidation of the mud, a layer of geotextile and several thin sand layers were placed on top of the mud backﬁll. During the placement of the sand layers, larger parts of the back- ﬁlling area underwent vertical displacements indicating a partial collapse of the mud layer; nevertheless, the project was ﬁnished successfully in 2007. The described difﬁculties over the course of this project raised several questions regarding the determination of the undrained shear strength in harbor mud, which this study would like to address. Vane shear testing played an important role during the plan- ning and construction phase of the East Harbor extension project. It is widely used for its simplicity, speed and relative cost, and it is the only method commonly used in both laboratory and ﬁeld settings in contrast to other soft soil test methods such as fall cone penetrometers or full-ﬂow penetrometers Zreik et al. 1995; Stew- art and Randolph 1994. However, the results of vane shear tests are affected by many factors such as shear rate, strength aniso- tropy or rod friction effects Aas 1965; Flaate 1966; Wiesel 1973; Menzies and Mailey 1976; Torstensson 1977; Biscontin and Pestana 2001; Schlue et al. 2007. Shear rate or peripheral veloc- ity is among the most important factors affecting vane shear test results but there is currently no common international standard for peripheral velocity or rotation rate in vane shear tests Leroueil and Marques 1996. The problems in deriving the actual soil resistance from labo- 1 Project Manager, ARCADIS Deutschland GmbH, Europaplatz 3, 64293 Darmstadt, Germany corresponding author. E-mail: b.schlue@ arcadis.de 2 Professor, Dept. of Marine Engineering Geology, MARUM-Center for Marine Environmental Sciences, Univ. of Bremen, Leobener Str., MARUM-Building, 28359 Bremen, Germany. 3 Postdoc, Dept. of Marine Engineering Geology, MARUM-Center for Marine Environmental Sciences, Univ. of Bremen, Leobener Str., MARUM-Building, 28359 Bremen, Germany. Note. This manuscript was submitted on February 28, 2008; approved on March 17, 2010; published online on March 19, 2010. Discussion period open until March 1, 2011; separate discussions must be submitted for individual papers. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 136, No. 10, October 1, 2010. ©ASCE, ISSN 1090-0241/2010/10-1437–1447/$25.00. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2010 / 1437 J. Geotech. Geoenviron. Eng. 2010.136:1437-1447. Downloaded from ascelibrary.org by USP - Universidade De Sao Paulo on 06/18/14. Copyright ASCE. For personal use only; all rights reserved.