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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 3. APPLICATIONS, FEASIBILITY, AND FLOW OF DESIGN AND CONSTRUCTION FOR DMM PROJECTS

3.1 Typical DMM Transportation applications

Generic applications of DMM include the following:

The generic applications encompass the specific embankment foundation applications listed in table 1 in chapter 1. Frequently, DMM has more than one function on any particular project (e.g., excavation support in combination with retaining wall support).

DMM has been used on at least 21 U.S. transportation projects since 1991, as shown in table 2. Projects are listed in chronological order, and the primary and secondary applications of DMM for each project are identified. The primary application reflects the main function of DMM as designed and constructed. The secondary application reflects associated functions and benefits provided by DMM.

Table 2. Summary of DMM usage for U.S. transportation projects.


Application of DMMa

Year

Project and Location

Owner

Quantity
of
DMM

Cost of DMMb

Ground Improvement

Ground Treatment

Liquefaction Mitigation

Excavation Support Walls

Hydraulic Cutoff Walls

Environmental Remediation

 

 

 

 

 

P

1991

Allegheny County Jail, Pittsburgh, PA(11)

Pennsylvania Department of Transportation (PennDOT)

1,800 m3

$147,600

P

S

 

S

 

 

1992-1995

Bird Island Flats, Central Artery Tunnel, Boston, MA (12-14)

Massachusetts Turnpike Authority and Massachusetts Department of Transportation (MassDOT)

3.,180 m2

$35.8 million

 

 

 

S

P

 

1996-1997

7th Street seal slab, San Francisco, CA

California Department of Transportation (Caltrans)

N/A

N/A

 

 

 

P

 

 

1997

Lake Parkway, Milwaukee, WI(15,16)

Wisconsin Department of Transportation (WisDOT)

20,900 m2

$4 million

P

 

 

 

 

 

1997

I-15, Salt Lake City, UT(17,18)

Utah Department of Transportation

3.,500 m3

$2.2 million

P

 

 

 

 

 

1997-2002

Fort Point Channel, Boston, MA(14)

Massachusetts Turnpike Authority and MassDOT

420,000 m3

$72 million

P

 

S

 

 

 

1998

San Francisco Bay Area Rapid Transit (BART) Culvert,
San Francisco, CA(19)

Caltrans

170,500 m3

$2.6 million

 

 

 

P

 

 

1999

Danville, PA(20)

PennDOT

4,000 m2

$700,000

P

 

 

 

 

 

2000

Woodrow Wilson Bridge, Alexandria, VA(18,21,22)

Virginia Department of Transportation (VDOT)

124,300 m3

$11.4 million

 

 

 

 

P

 

2000

Doolittle Drive and Airport Drive Interchange, Oakland Airport, Oakland, CA(23)

Caltrans

N/A

N/A

P

 

 

S

 

 

2000

Hackensack Meadows, Hoboken, NJ(24)

New Jersey Department of Transportation

9,500 m3

$364,000

P

 

 

 

 

 

2001-2002

Airport Drive overcrossing Air Cargo Road, Oakland Airport, Oakland, CA(23)

Caltrans

15,000 m3

$1.2 million

S

 

 

P

 

 

2001-2002

Taxiway B and Air Cargo Road Grade Separation, Oakland Airport, Oakland, CA(23,25)

Caltrans

30,500 m3

$1.83 million

P

 

 

 

 

 

2001-2003

Glen Road interchange,
Newport, MN(26,27)

Minnesota Department of Transportation (MnDOT)

22,770 m3

$2 million

P

 

S

 

 

 

2002-2003

I-5 expansion,
San Diego, CA

Caltrans

21,300 m3

$1.7 million

 

P

 

S

S

 

2003

Tukwila, WA(23)

Washington State Department of Transportation (WSDOT)

44,000 m3

$5 million

 

P

 

 

 

 

2004-2005

4th Street exit of I‑80, San Francisco, CA(28,29)

Caltrans

N/A

$16 million

P

 

 

 

 

 

2005-2006

Route 1 over Jewfish Creek(30)

Florida Department of Transportation (FDOT)

275,400 m3

$11.9 million

 

 

 

P

 

 

2007

North Shore connector,
Pittsburgh, PA(31)

Port Authority of Allegheny County

10,200 m2

$4 million

 

 

 

P

S

 

2010

Warm Springs
BART extension,
Fremont, CA(32)

Bay Area Rapid Transit Authority

24,600 m3

$5.5 millionc

P

 

P

 

 

 

2011

1-5 high-occupancy vehicle extension, Tacoma, WA(33)

WSDOT

11,095 m3

$1.35 million

N/A = Not available.
a P indicates primary application, S indicates secondary application, and blank cells indicate that DMM application was not used.
b Cost does not include mobilization.
c Cost includes jet grouting at utility crossings and setting of soldier beams.
1 ft3 = 0.028 m3
1 ft2 = 0.093 m2

3.2 Feasibility of using DMM

The feasibility of using DMM for any given project is dependent on a number of diverse factors, including practical project considerations (i.e., cost, schedule, performance, geotechnical, logistical, accessibility, and environmental) and conventional considerations (i.e., regional and historical practices and preferences and the degree of influence of local contractors, consultants, and owners).

3.2.1 Advantages and Potential Limitations of DMM

The relative advantages and potential limitations of using DMM for embankment and foundation support are listed in table 3. DMM, similar to any geotechnical construction technique, is not a solution for all soft ground treatment, improvement, retention, and containment problems. However, for certain applications, it can be more practical, more economic, faster, or otherwise preferable to competing technologies. Typical alternative technologies are listed in table 4.

In the most general terms, DMM may be attractive for the following project conditions:

DMM is especially useful for embankment support under the following conditions:

Table 3. Relative advantages and disadvantages of DMM for general transportation project applications (adapted from FHWA).(1)


Item

Application

Ground Treatment and Improvement

Liquefaction Mitigation

Excavation Support Walls

Relative advantages/benefits
of DMM

  • Low relative cost per unit volume to depths of 130 ft (40 m).
  • Strength of treated soil ranges from 75 to 600 psi (0.5 to 4 MPa).
  • Layout may be varied based on diameter and spacing of columns or thickness and spacing of panels.
  • Dry mixing methods provide very low spoil volumes.
  • The spoil from wet mixing methods may serve as excellent site fill material.
  • Little vibration and medium-low noise (equipment can be muffled).
  • High production capacity in certain conditions.
  • Quickly verifiable in situ performance.
  • Can be used for marine projects.
  • Generally good lateral and vertical levels of treatment.
  • Can be used in most types of soils and fills (without obstructions).
  • Execution is relatively constant and straightforward.
  • Excellent theoretical, laboratory, and field experimental data to supplement advanced design theory.
  • Economical for large projects in very soft, compressible soils.
  • Spacing and composition of individual columns infinitely variable.
  • Some types (e.g., lime cement columns) have low mobilization costs.
  • Typical design strengths are about 145 psi (1 MPa) for ground improvement projects.
  • Excellent proven performance record in Japan.
  • Economical on large projects.
  • Engineering properties of treated soil can be designed up to about 600 psi (4 MPa).
  • Construction quality highly verifiable (wet and dry).
  • There are minimal lateral or vertical stresses that could potentially damage adjacent structures.
  • No recurrent post-construction expenses.
  • Lower relative cost per unit area, especially in the range of 50 to 130 ft (15 to 40 m) in depth relative to slurry walls and secant pile walls.
  • No need for other types of lagging.
  • Relatively low permeability; therefore, no need for additional sealing.
  • Spoil from the wet method can be used as excellent site fill material.
  • Little vibration and medium-low noise (equipment can be muffled).
  • In fluid state, allows structural elements to be introduced.
  • Can provide good lateral continuity.
  • High production in certain conditions (up to 2,150 ft2
    (200 m2) per shift).
  • Can uniformly treat layered heterogeneous soils.

Potential
disadvantages
of DMM

  • Depth limitations (130 ft (40 m) practical).
  • Need large working space for large powerful equipment and no overhead restrictions.
  • Not applicable in soils that are very dense, very stiff, or that may have boulders.*
  • Can only be installed vertically.
  • The wet method produces a significant volume of spoils.
  • Underground utilities may pose problems.
  • Limited ability to treat isolated strata at depth.
  • High mobilization cost.
  • Weight of equipment may be problematic for very weak soils.
  • Significant variability in treated soil strength may occur, and this may be important in certain applications.
  • Cannot be installed in close proximity to existing structures.
  • Limited geometric flexibility of drilling and treatment.
  • Depth limitations (130 ft (40 m) practical).
  • Need large working space for large powerful equipment and no overhead restrictions.
  • Not applicable in soils that are very dense, very stiff or that may have boulders.*
  • Can only be installed vertically.
  • Underground utilities may pose problems.
  • Limited ability to treat isolated strata at depth.
  • High mobilization cost.
  • Not applicable for remediations directly through or under existing concrete structures.
  • Freeze/thaw degradation may occur.
  • Depth limitations (130 ft (40 m) practical).
  • Need large working space for large powerful equipment and no overhead restrictions.
  • Not applicable in soils that are very dense, very stiff, or which may have boulders.*
  • Can only be installed vertically.
  • Other methods may provide no spoils (e.g., sheet piles).
  • Significant variability in treated soil strength may occur, and this may be important in certain applications.
  • Underground utilities may pose problems.
  • Limited ability to treat isolated strata at depth.
  • High mobilization costs.

*DMM techniques designed to produce walls may be capable of penetrating denser or stiffer materials or strata with cobbles. (See technical data in appendix D.)

 

Table 4. Alternative technologies to DMM.


Application

Alternative Technology to DMM

Ground treatment

  • Permeation grouting
  • Jet grouting

Ground improvement

  • Various pile types (e.g., auger cast, bored, driven, and micropiles)
  • Stone columns
  • Lightweight fills
  • Compacted stone columns
  • Vibro-concrete columns

Liquefaction mitigation

  • Vibro-densification
  • Vibro-replacement
  • Deep dynamic compaction
  • Compaction grouting
  • Dewatering and drainage

Excavation support walls/cutoff walls

  • Secant piles
  • Sheet piles
  • Soldier beams and lagging
  • Soil nailing
  • Structural diaphragm walls

 

3.2.2 Feasibility Evaluation for Using Deep Mixing for Transportation Projects

The factors listed in table 5 should be considered when assessing the feasibility of using DMM for a project.

Table 5. Factors to consider in feasibility assessment for using DMM.


Factor

Question

Commentary

Geologic applicability

Are soils suitable for mixing?

  • DMM is suitable in locations with soils that can be stabilized with cement, lime, slag, or other binders (typically cohesive soils with high moisture contents
    and loose saturated sandy soils without cobbles
    or boulders).
  • DMM is not suitable in soils that are very stiff or very dense or in geologic conditions with large cobbles
    or boulders.

Geometric applicability

Are site conditions conducive to using DMM?

  • Treatment depths should be less than about 130 ft (40 m).
  • Relatively unrestricted overhead clearance should be available.
  • The project area should be large enough to accommodate large and heavy mixing rigs and binder plants for wet mixing.
  • Treated or improved ground volumes are large enough to warrant mobilization/demobilization costs.
  • Adjacent land use, property ownership, or environmental impacts dictate a narrow footprint for the embankment.
  • Adjacent facilities (e.g., an existing embankment and pavement in an embankment widening application) could be damaged unless the loads from the new embankment are transferred to a competent bearing layer.

Project constraints

Are construction materials readily available?

  • Constant and adequate supply of binder can be ensured.
  • Borrow material for staged loading is expensive, environmentally destructive to obtain, or not readily available.

Are there environmental constraints?

  • A significant amount of spoil from wet mixing can be tolerated or used productively on the project.
  • Relatively vibration‑ or noise-free technology is required.

Contractual vehicles

Are performance specifications applicable?

  • Being a contractor-driven and method-dependent process, DMM is well suited for the use of performance specifications.

Cost considerations

Is project cost a driving factor?

  • DMM can be less expensive than excavation and replacement since the in situ soil is used.
  • DMM is generally more expensive than staged construction.
  • DMM columns can be used in column-supported embankment foundations and can be used in conjunction with lightweight fills for embankment construction.

Schedule considerations

Is the project schedule a driving factor?

  • Construction can proceed more rapidly when using DMM than when using preloading with or without prefabricated vertical drains or staged construction on soft compressible soil.

Design constraints

Must treated ground strengths be closely engineered?

  • DMM treated ground strengths range typically from
    75 to 600 psi (0.5 to 4 MPa). Most design strengths for ground improvement are in the range of 96 to 145 psi (0.66 to 1 MPa).
  • Ground movements induced by embankment construction would impact adjacent structures or embankments.

 

3.3 design and construction flow for DMM projects

A flow chart depicting the overall process of design and construction for DMM projects is shown in figure 7. The flowchart includes four main project phases: (1) data collection (yellow), (2) design (green), (3) procurement (blue), and (4) construction with continuous QC/QA (red).(34)

This flowchart shows the stages of project flow for deep mixing method (DMM) projects from data collection (yellow) design (green), procurement (blue), and construction with continuous quality control/quality assurance (red). The data collection stage begins with loads and performance, site characterization studies, bench-scale mix design testing, and published data and prior experience. These four elements lead to the question, “Need for field trial during design?” If the answer is “Yes,” a field trial is conducted before entering the design phase. If a field trial is not needed, the design phase is entered immediately. The design phase begins with establish design strength, then treatment geometry, then analyses, which lead to the question, “Design requirements satisfied?” If the answer is no, the design phase begins again with establish design strength. If the requirements are satisfied, the question “Need for field demo by contractor?” is asked. The design phase ends with prepare plans and specs either with or without a field demo, depending on the answer to the preceding question. In both cases, the procurement stage consists of bidding, and then the construction phase begins with bench-scale testing. If a field demo was required, it is performed following the bench-scale testing. The final step in all cases is construction with on-going contractor quality control (QC) and owner/engineer quality assurance (QA).
Figure 7. Flowchart. Design and construction for DMM projects.(34)

3.3.1 Data Collection

The information collection process includes defining structure performance expectations, gathering site-specific soil and groundwater information, and reviewing available information related to local DMM experience. Specific considerations for site investigations for DMM projects are discussed in chapter 4. Prior experience for other DMM projects may be obtained from published literature and from discussions with experienced engineers and contractors.

Laboratory bench-scale testing and possible field trials are also part of the data collection phase. If prior experience is not available, laboratory mix design studies (i.e., bench-scale testing) should generally be conducted prior to or at the same time as analysis and design to establish a range of strengths that can reasonably be achieved in the field. A preliminary field trial including installation of full-scale DMM elements may also be performed during the design phase to ensure treated soil properties may be achieved as required. Although expensive, full-scale field trials can provide valuable data for large or complex projects. Bench-scale testing and field trial programs are discussed in chapter 10.

3.3.2 Analysis and Design

The analysis and design phase includes engineering evaluations of the DMM configuration being proposed for the project, laboratory bench-scale testing, possible field trials of DMM techniques to be used, and preparation of specifications for construction. Design procedures are discussed in chapter 6, field trial and validation programs are outlined chapter 10, and a guide to writing effective specifications is provided in chapter 9.

Isolated columns and continuous shear walls, as illustrated in figure 8, are the most common DMM configurations for transportation embankments. Typically, isolated columns are constructed beneath the central portion of the embankment to control settlement, and continuous shear walls are constructed beneath the side slopes (and oriented perpendicularly to the embankment centerline) to prevent embankment stability failure.

This illustration shows a plan and elevation view of a cross section of embankment with deep mixing method (DMM) treatment. Isolated columns are shown on a square grid pattern below a crest, and parallel lines of overlapping columns are shown below side slopes.
Figure 8. Illustration. Typical configuration of DMM columns for transportation applications.

The design process includes an evaluation of external (global) stability and internal stability under a variety of potential failure modes to ensure that the stresses induced within and adjacent to the treated ground do not exceed the material capacities and that settlements are limited to acceptable levels. Initial trial values of DMM properties are assumed, analyses are performed, and the results are compared with performance criteria. Analytical procedures are discussed in detail in chapter 6.

For global stability, the treated ground beneath the embankment is modeled as a rigid body, and its stability is evaluated under various modes of failures, including lateral sliding, overturning, bearing capacity, and rotation/sliding along potential sliding surfaces that pass entirely beneath or entirely above the deep mixed shear walls. Figure 9 and figure 10 illustrate overturning and sliding as well as bearing capacity modes, respectively.

This illustration shows an embankment cross section of external failures modes overturning and sliding. It shows passive and active earth pressure distributions and sliding resistance force on a deep mix shear wall block under an embankment slope.
Figure 9. Illustration. External stability mode of failure for overturning and sliding.(35)

This illustration shows an embankment cross section of bearing pressures acting upward below a deep mixed shear wall block under an embankment slope.
Figure 10. Illustration. External stability mode of failure for bearing capacity.(35)

Settlement of the embankment is calculated based on the assumption of equal strains in the treated ground and the adjacent untreated soil within the deep mixed zone underlying the central portion of the embankment. This approach is equivalent to using a composite modulus of the deep mixed ground and the adjacent soil. Compression of soil below the deep mixed ground can be evaluated using a load spread method similar to that used for pile groups. Compliance of the embankment above the deep mixed columns can be evaluated using methods for column-supported embankments.

Two internal stability modes of failure are illustrated in figure 11 and figure 12—a sliding surface passing through the deep mixed zone and vertical shearing along column overlaps within the deep mixed zone. If the deep mixed ground overlies a hard bearing stratum, lateral loads could produce toe pressures that exceed the capacity of the deep mixed ground. Analyses should check for crushing of the shear walls at the outside toe of the panels. The overlap of the columns must be sufficient to prevent shearing along vertical planes within the shear walls produced by lateral loading, which could produce a racking-type failure mode. This analysis involves comparing the vertical shear stress on the critical vertical plane with the design strength value. Internal stability analyses also address potential sliding surfaces that pass through the deep mixed shear walls and adjacent soil. The geometry of the shear walls must be checked to ensure that soil does not extrude between the panels due to unbalanced forces caused by active and passive earth pressures acting on the deep mixed zone.(36)

This illustration shows an embankment cross section of internal stability modes of failure for a circular sliding surface shown through a deep mixed shear wall block embankment slope. An area of isolated columns under an embankment crest is also visible.
Figure 11. Illustration. Internal stability mode of failure for circular sliding surface.(35)

This illustration shows vertical shearing (racking) failure in a deep mixed shear wall block. There are overlapped columns tilting toward the toe of the embankment.
Figure 12. Illustration. Internal stability mode of failure for vertical shearing.(35)

For internal stability analyses, composite shear strength and unit weight values are used to model the deep mixed zone beneath the embankment based on the configuration of the columns (i.e., area replacement ratio and spacing of the shear walls). Separate composite shear strengths for vertical and non-vertical planes are estimated for the deep mixed shear walls beneath the side slope.

Construction specifications should clearly communicate the required geometry, continuity, and strength of the deep mixed ground. The specifications should also detail the acceptance criteria to assure performance. Guide specifications are discussed in chapter 9.

3.3.3 Contractor Procurement

Traditional design-bid-build procurement practices involve bidding on the project after the contract documents (including plans and technical specifications) are prepared. Innovative procurement vehicles involving design-build methods and early contractor involvement (ECI) have merit on DMM projects due to quality of DMM being influenced by various contractor-controlled variables. Procurement vehicles for DMM projects are discussed in chapter 8.

3.3.4 Construction and QC/QA

The contractor is responsible for controlling the geometry of the deep mixed elements by using certain tooling dimensions and installation procedures. The contractor documents the quality of the operations and provides reports of the QC data on a daily basis. The owner conducts QA activities, including sampling and testing to assure the quality of the deep mixed ground. Means and methods for construction and QC/QA are discussed in chapters 11 and 12, respectively.

A field validation program involves installing full-scale DMM elements to demonstrate that the contractor's materials and methods can satisfy the project specifications, including the strength and continuity of the deep mixed ground. Validation tests are conducted after the contract is awarded but before production mixing. Such field validation programs are much more common than trial columns installed during the design phase.

 

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