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:

• Ground improvement: Discrete elements or continuous panels used as reinforcement to improve the overall performance of weak and/or large compressible soil masses. Ground improvement is the main application addressed in this design manual.
• Ground treatment: Block treatment used to uniformly strengthen large volumes of foundation soil for deep excavations and structural foundations to support heavy loads with tight settlement tolerances.
• Liquefaction mitigation: Interlocking box or cellular DMM structures used to reduce the tendency for mass liquefaction and lateral spreading during seismic events.
• Excavation support walls: Walls typically containing reinforcing steel elements used to resist lateral earth pressures in deep excavations.
• Hydraulic cutoff walls: Walls used to prevent water movement through or under retaining structures and into excavations below the water table.
• Environmental remediation: Walls used to contain or block treatment to remediate environmentally hazardous materials through solidification and/or stabilization.

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.

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:

• The ground is neither very stiff nor very dense and does not contain large cobbles, boulders, or other obstructions.
• Treatment depths of less than about 130 ft (40 m) are required.
• There is relatively unrestricted overhead clearance.
• A constant and adequate supply of binder can be ensured.
• A significant amount of spoil can be tolerated for the wet methods, but negligible spoil is generated for the dry method.
• Relatively vibration‑free technology is required.
• Treated or improved ground volumes are large.
• Performance specifications are applicable.

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

• The project schedule requires more rapid construction than would be possible using staged construction and prefabricated vertical drains.
• Borrow material is expensive, environmentally destructive to obtain, or not readily available for some other reason. DMM support generally permits steeper embankment side slopes and use of spoils in the embankment fill, both of which reduce the need for imported borrow materials.
• Ground movements induced by embankment construction would impact adjacent structures. For example, when widening an existing embankment on soft ground, DMM can prevent settlement of the existing embankment and pavement due to the load from the new embankment.
• Adjacent land use, property ownership, or environmental impacts dictate a narrow footprint for the embankment.
• Construction involves contaminated materials whereby DMM can reduce excavation spoils requiring offsite disposal and/or provide fixation of contaminates to
  reduce leachability.

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

Table 4. Alternative technologies to DMM.

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.

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⁾

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.

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.

Figure 9. Illustration. External stability mode of failure for overturning and sliding.⁽³⁵⁾

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⁾

Figure 11. Illustration. Internal stability mode of failure for circular sliding surface.⁽³⁵⁾

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.