This issue of Ground Improvement contains a selection of revised papers from the May 2009 Okinawa Symposium on Deep Mixing and Admixture Stabilisation. The deep mixing method, an in situ soil stabilisation technique employing cement- and/or lime-based binders, was first practised in Japan and the Nordic countries independently in the mid 1970s. In the past two decades, the latest information on equipment, material properties, case studies, design procedures and quality assurance/quality control (QA/QC) has been updated and shared by the international deep mixing community through a series of international conferences/symposia. The first was co-organised by the Japanese Geotechnical Society and the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE TC17) in Tokyo, 1996. This was followed by events in Stockholm (1999), Helsinki (2000), Tokyo (2002), New Orleans (2003) and Stockholm (2005). The International Symposium on Deep Mixing and Admixture Stabilisation, Okinawa 2009, Japan, was a continuation of this tradition. The next international gathering takes place in New Orleans in February 2012.
Along with these international forums, CEN TC288/WG10, with delegates from nine European countries and invited international experts from the USA and Japan, produced the European Standard for the Execution of Special Geotechnical Works – Deep mixing (CEN, 2006). Deep mixing technology is increasingly gaining popularity throughout the world and is practised in an increasing number of countries. In order to maintain the high quality of deep mixing work in the global market, it is essential to share common concepts of QA/QC and correlate the output of different procedures. The behaviour of ground improved by deep mixing and admixture stabilisation in general is characterised by the complicated time-dependent interaction between the manufactured element (treated soil column, wall, block and slab) and the native soft soil. The design of improved ground is an iterative process both in geotechnical and process designs. The success of construction projects involving deep mixing and admixture stabilisation depends on the harmony of both designs and is guaranteed by carefully performed QA/QC, which is regularly performed as an essential part of the construction process. Current practices of QA/QC comprise laboratory mix tests, construction control, coring and testing of treated soils, and various in situ tests. However, differences exist in the QA/QC procedures in different countries and areas owing to differences in soils, climate, execution equipment, preferred in situ test methods, major applications and target strength level.
This special issue includes eight papers representing the wide range of themes covered by the conference including laboratory testing, design aspects, QA/QC and different applications. The first paper (Åhnberg and Holmén, 2011) investigates the use of bender element and resonant column free-free tests in assessing the compressive and shear wave velocities in stabilised soils, developing correlations with their unconfined compressive strengths. Next Filz et al. (2011) present and apply a simplified design analysis method for stability of an earthen levee in New Orleans supported on deep-mixed continuous shear walls which considered multiple modes of failure and produced conservative factors of safety.
The paper by Al-Tabbaa et al. (2011) presents an overview of recent innovations in the application of soil mix technology in land remediation covering: equipment developments and applications, including systems for rectangular panels and trenching systems; treatments, such as chemical oxidation; and additives, such as modified clays, zeolites and reactive magnesia. The authors also provide a number of case studies of such applications. The paper concludes with an overview of the on-going research and development project SMiRT (Soil Mix Remediation Technology) which involve large-scale remediation trials on a contaminated site in West Yorkshire where a number of those innovations are being employed.
Havukainen et al. (2011) present a case study of the successful application of mass stabilisation of a large area of contaminated sediments in the Vuosaari harbour in Helsinki brought about through co-operation between the project owner, environmental authorities, binder industry, the contractor and numerous experts in environmental and geotechnical engineering. In the next paper Takahashi et al. (2011) present details and results of a series of dynamic shaking model tests in the centrifuge, to investigate the seismic resistance of cement-treated soils for quay walls. The model tests revealed the failure mode of the improved ground and the effect on the deformation behaviour of a crack and the dimensions of the cement-treated soil.
Terashi and Kitazume (2011) present and discuss the similarities and differences in the QA/QC procedures of the deep mixing method as employed in different parts of the world and propose future research needs, incorporating the results of an international survey. Given the large number of factors that influence the quality and performance of deep-mixed soils and practice differences across the globe, the authors highlight the importance of clarity and co-ordination between project partners as well as standardising procedures for QA/QC.
The paper by Asaka and Abe (2011) presents details of the non-destructive technique for inspecting the strength of cement-treated ground in the field using bender elements and leading to unique correlations between the unconfined compressive strength and shear wave velocity. Finally Watabe et al. (2011) present the results of a 10 year follow-up study of airfoam-treated lightweight soil placed as backfill at the seawall in Kobe Port Island and Tokyo International airport. The paper concludes that the lightweight soil demonstrated sufficient durability and long-term compliance with the required performance criteria.
