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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-13-046    Date:  October 2013
Publication Number: FHWA-HRT-13-046
Date: October 2013

 

Federal Highway Administration Design Manual: Deep Mixing for Embankment and Foundation Support

CHAPTER 1. INTRODUCTION TO THE DEEP MIXING METHOD IN THE UNITED STATES

1.1 Introduction

The deep mixing method (DMM) is an in situ soil treatment in which native soils or fills are blended with cementitious and/or other materials, typically referred to as binders. Compared to native soils or fills, the soil-binder composite material that is created has enhanced engineering properties such as increased strength, lower permeability, and reduced compressibility. Soils best suited to DMM include cohesive soils with high moisture contents and loose, saturated, fine granular soils. DMM has also been used successfully in a wide range of less cohesive soils and fills, but it is typically not feasible in very dense or stiff materials or in ground with obstructions such as cobbles or boulders. The treated soil properties obtained by DMM reflect the characteristics of the native soil, binder characteristics, construction variables, operational parameters, curing time, and loading conditions.

Two types of DMMs are used in the United States: wet mixing and dry mixing. Wet mixing involves injecting binders in slurry (wet) form to blend with the soil. Primarily single-auger, multi-auger, or cutter-based mixing processes are used with cement-based slurries to create isolated elements, continuous walls or blocks for large-scale foundation improvement, earth retaining systems, hydraulic barriers, and contaminant/fixation systems. Dry mixing uses binders in powder (dry) form that react with the water already present in the soil. Primarily single-auger dry mixing processes are used with lime and lime-cement mixtures to create isolated columns, panels, or blocks for soil stabilization as well as reinforcement of cohesive soils.

The generic term DMM is recommended and used within this manual. This term is inclusive of other terms such as deep soil mixing (DSM) and cement deep soil mixing (CDSM).

1.2 Scope of Manual

In 2000 and 2001, the Federal Highway Administration (FHWA) produced a three-volume research report outlining the use of DMM for geotechnical applications.(1-3) The study focused on the applications, equipment, market conditions, and properties of treated soils produced using DMM. This study was followed by the research contributions of the National Deep Mixing Program, a Transportation Pooled Fund (TPF) Program study, TPF-5(001). TPF is a funding mechanism for State transportation departments and FHWA to pool financial and personnel resources to plan and conduct research projects of mutual interest. To increase the benefit of these efforts, FHWA commissioned the development of an unpublished literature review report as well as this design manual. The purpose of the literature review was to compile the information relevant to this report that was available from FHWA research efforts and national and international technical literature, provide background information on U.S. deep mixing for transportation projects, and identify additional sources information on design and construction methods.

The purpose of this report is to provide user-oriented DMM design and construction guidelines for the support of embankments and typical transportation-oriented foundations. The use of DMM for liquefaction mitigation and excavation support is also discussed in general terms since these applications are often associated with DMM projects for embankments and foundations. Detailed liquefaction mitigation guidance is presented by Siddharthan and Suthahar.(4) Design guidance for excavation support is presented by Rutherford et al.(5) DMM is also frequently and successfully used to create hydraulic (seepage) cutoff walls or to remediate and/or contain environmentally hazardous materials. Seepage cutoffs and environmental applications are not addressed within the scope of this manual.

The embankment and foundation applications addressed in this manual are described in table 1 and depicted schematically in figure 1 through figure 6.

Table 1. Use of DMM for embankment and foundation applications.


Application

Description

Embankment support (both new embankments and embankment widening)

Embankment supported on isolated DMM elements (single columns, multi-auger overlapping columns, or barrettes), continuous shear walls formed by overlapping elements, or fully treated blocks formed by overlapping elements

Culvert through an embankment on DMM

Cut and cover culvert supported on DMM through an embankment supported on DMM

Bridge abutment support

Abutment supported on deep foundations and embankment on DMM

Abutment and embankment supported on DMM

Retaining wall foundations

Retaining wall supported on DMM without retained soil supported on DMM

Bridge pier support

Bridge pier supported on DMM; generally, DMM would be used for this application if DMM is also being used to support the bridge approaches

This line drawing shows an embankment supported on a deep mixing method (DMM) application. A description of this application can be found in table 1 of the report.
Figure 1. Illustration. Embankment supported on DMM.

This line drawing shows a cut and cover culvert supported on a deep mixing method (DMM) application through an embankment.
Figure 2. Illustration. Cut and cover culvert supported on DMM through embankment.

This line drawing shows an abutment supported on deep foundations and an adjacent embankment supported on a deep mixing method (DMM) application.
Figure 3. Illustration. Abutment supported on deep foundations and adjacent embankment supported on DMM.

This line drawing shows an abutment and embankment supported on a deep mixing method (DMM) application.
Figure 4. Illustration. Abutment and embankment supported on DMM.

This line drawing shows a retaining wall supported on a deep mixing method (DMM) application without retained soil supported on DMM.
Figure 5. Illustration. Retaining wall supported on DMM without retained soil supported on DMM.

This line drawing shows a bridge pier supported on a deep mixing method (DMM) application. A description of this application can be found in table 1 of the report.
Figure 6. Illustration. Bridge pier supported on DMM.

The design portion of this manual focuses on deep mixing support of embankments. In most circumstances, for DMM to be considered for structure support, DMM would also have to be used for support of the embankment. Equipment mobilization costs and other start-up costs are often too high to justify the use of DMM at structure locations alone. Design recommendations for DMM support of structures are provided in this manual, but because embankment support is generally the deciding factor, this report mainly describes embankment support.

The chapters and associated content of this report are organized as follows:

1.3 Study and USE OF DMM BY U.S. transportation agencies

Although DMM was invented in the United States in 1954, current methods mainly reflect developments made in Japan and Nordic countries over the past 40 years.(1) Until 1996, the international technical literature included many papers on the development and use of DMM. However, most of the papers were published in Japanese and Swedish. In 1996, an international conference on grouting and deep mixing was held in Tokyo, Japan, and the official conference language was English. This conference represented the first major exposure of U.S. engineers to the extent and potential use of the various techniques included in DMM. Some types of modern DMM had been in use in the United States since 1986.(6)

In 1997, FHWA began a concerted effort to research and study DMM and its applications.The first studies focused on gathering and summarizing information on DMM applications, equipment, and operations.(1,2) A later study focused on compiling and defining engineering characteristics of ground treated using DMM.(3)

In 1999, FHWA developed a ground improvement course and an accompanying reference manual.(7) This course covered traditional ground improvement techniques, included a module on DMM, and was offered and taught nationwide to State transportation departments. This was the first FHWA teaching vehicle that systematically introduced DMM to U.S. transportation engineers. The DMM module included applications, advantages, and potential limitations and introduced contemporary design techniques. Sufficient information was provided to allow engineers to assess the feasibility of using DMM as compared to alternative technologies. Comprehensive design and construction details were not included. In 2003, this demonstration course was developed into a 3-day National Highway Institute course, “Ground Improvement Methods,” which is offered nationwide and internationally. The accompanying reference manuals, Ground Improvement Methods Volume I and Ground Improvement Methods Volume II, represent an update to the 2001 reference manual.(8,9)

Recognizing the need for more detailed engineering tools for practicing engineers, FHWA developed the National Deep Mixing Research Program in 2001. The program was created as a TPF Program study and was established to facilitate the advancement and implementation of DMM technology through partnered research and dissemination of international experience.(10) Researchers from the National Deep Mixing Research Program administered and developed a series of practice-oriented reports on a range of DMM topics that address various design and laboratory issues.

In addition to these efforts, numerous researchers and practitioners have contributed to the published technical literature on DMM in the United States. Many papers on U.S. DMM advances have been presented at national and international conferences. These presentations have addressed case histories, analysis of project data, equipment advancements, laboratory testing results, characterization of engineering behavior of treated soils, and development of methods for analysis and design of foundation systems incorporating DMM. Short courses and seminars have been developed and offered by U.S. professional societies and universities to promote DMM as a viable alternative to other ground improvement technologies.

Since 1991, DMM has been used successfully on over 20 large transportation projects in the United States. Project details are summarized briefly in chapter 3. However, DMM usage in the United States has been limited for a number of reasons, most notably the absence of readily accessible and user friendly design guidelines and the lack of widely accepted and effective QC/QA practices. This situation contrasts with that in the huge, active and largely transportation-driven markets in Japan and Nordic countries.

 

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