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REPORT
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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 9. SPECIFICATIONS, PLANS, AND CONTRACTOR SUBMITTALS

Regardless of the contracting approach selected, the quality of the project documents, including specifications, plans, and contractor submittals, is critical to promoting understanding and aligned expectations among the contracted parties. The contract specifications and plans must include sufficient detail in order for the contractor to understand the design intent to allow for fair and complete bidding. Contractor submittals must thoroughly and clearly explain the procedures to be used to produce deep mixed foundations that meet the design intent.

9.1 Specifications

Use of a combination of performance and method specifications is recommended for DMM projects, as outlined in chapter 8. Guide specifications are provided appendix C of this report. Clear guidance is provided on the parameters that should be specified by the engineer and those that should be selected by the contractor such that responsibility is appropriately allocated at the start of the project and contractual exposure and potential for miscommunication for all parties as the project progresses are reduced. A qualified contractor must be selected for DMM projects (contractor qualifications are discussed in chapter 8).

The guide specifications outline the intended content of the specifications for a DMM project. However, it is essential that specific project requirements be considered to convert these guide specifications to contract documents that are appropriate for a particular project.

Part 1 of the specifications outlines the scope, references, definitions of terms, project description and requirements, contractor qualifications, available information, site survey, submittals, and preconstruction meeting. Terminology in chapter 2 of this manual is used throughout the specification to avoid confusion and encourage consistency of understanding between all contracting parties.

The quality of the information provided is critical to the success of any foundation project. All subsurface data, utility plans, and structure requirements must be outlined clearly to promote understanding. Deep mixing design criteria are typically outlined in a separate design report that is included with the contract documents. The subsurface conditions expected can significantly impact the contractor's choice of equipment, methods, materials, bidding process, and contract administration. The owner must include with the contract documents all subsurface information available and results of all preconstruction testing conducted by the owner to support the design. Bench-scale and field validation testing are discussed in chapter 10.

Table 16 summarizes the geometric parameters that the owner/engineer should specify. By specifying only the minimum and/or maximum allowed values for certain geometric parameters, the owner/engineer is affording the contractor flexibility in construction while still assuring that the final design will satisfy the requirements for performance.

Layouts and sizes of deep mixing elements that adhere to the minimum and maximum values listed in table 16 will be deemed acceptable to meet the requirements of the owner's design, and additional design calculations will not be required. If layouts and element sizes do not adhere to these requirements, design calculations must be prepared and stamped by a registered professional engineer retained by the contractor and submitted to the owner/engineer for approval after the contract is awarded. However, the owner/engineer is under no obligation to accept alternate designs in a design-bid-build contract.

Part 2 of the specifications includes provisions for materials and equipment, including requirements for binder material handling and storage and mixing equipment. These provisions generally allow the contractor flexibility in selecting appropriate techniques and materials. However, the owner/engineer may specify the minimum capacity of the binder handling and deep mixing equipment and the minimum requirements of the monitoring equipment and materials handling and mixing procedures. By specifying such minimums, the contractor is not relieved from satisfying the specified requirements for the project schedule or the end product in a hybrid specification. In addition, the owner/engineer may outline methods and materials that are not permitted, if any.

Part 3 of the specifications outlines provisions for testing, preparing binders, locating DMM elements, mixing, and QC/QA sampling, testing, acceptance, and documentation. A suggested level of detail of execution provisions is provided, although the engineer is reminded to consider project-specific requirements when developing the specifications. Detailed QC/QA requirements are discussed in chapter 12.

Procedures to be used if obstructions are encountered and standby time is incurred must be clearly detailed. It is not always practical for the contractor to maintain production by moving to a different section of the site and continuing mixing while an obstruction is being investigated.

Specifications for accepting DMM materials are often based on unconfined compressive strength. On several past projects, the minimum required strength has driven a very conservative mix design such that the average strength of the treated ground is many times greater than the design strength. This situation can produce higher than necessary construction costs, which may lead to collateral problems and contract disputes. Acceptance criteria that do not include a minimum strength requirement are preferred, as outlined in chapter 12. It is recommended that criteria allow a specified percentage of test results to be below specified strength values. Strength values that properly reflect material variability can provide the owner/engineer with sufficient assurance that the design intent is being met, provided that such variability is incorporated in the design process, as described in chapter 6.

Specifications must accurately detail consequences of failure to meet the requirements.

9.2 PLANS

Project plans must be detailed to illustrate the layout of the proposed embankment or structure and the limits of the DMM foundation to be constructed. The engineer is encouraged to specify only the minimum and/or maximum allowed values for certain geometric parameters, as outlined in chapter 6, to permit the DMM contractor flexibility in construction while still assuring that the final design will satisfy the requirements for settlement and stability.

Plans should be drawn to scale and depict the following items, at a minimum:

If a specific arrangement of the deep mixed elements (i.e., diameter and spacing of isolated columns and similar details of shear walls) has been specified, then typical plans and elevations of the deep mixing improvement should be provided. However, if minimums and maximums like those in table 16 are provided instead, then example plans and elevations should be provided. They should be clearly labeled as examples, and reference should be made to the controlling minimum and maximum values. In either case, any required details and special arrangements at transitions, terminations, and connections should also be shown in the plans.

9.3 CONTRACTOR SUBMITTALS

9.3.1 Contractor Experience Profile

Experience of the contractor and specific project personnel must meet the requirements outlined in chapter 8. Any changes in approved site personnel must be approved in writing by the owner/engineer prior to substitution.

9.3.2 Bench-Scale Testing Results

The contractor must submit results from all bench-scale tests conducted. The report should provide all data collected, including (at a minimum) descriptions of sampling techniques used, boring logs, classifications of all major soil strata to be mixed, site groundwater conditions, binder materials used, mixed design proportions, laboratory mixing techniques used, and curing curves for unconfined compressive strength versus time for each major soil type. Discussion of the test results should be provided, including proposed mix designs for use in the field.

9.3.3 Field Validation Program Plan

This plan contains descriptions of the construction procedures, equipment, and ancillary equipment to be used for mixing and binder proportioning and injection, mix design parameters and associated soil strata to be evaluated, operational and material parameters to be monitored during field validation, layout of the DMM elements to be constructed, and summary of QC/QA samples to be collected and tested. Examples of the forms that will be used to document the work are also provided.

9.3.4 Deep Mixing Work Plan

Based on the results of the bench-scale and field validation programs, at least 30 days prior to the start of deep mixing work, the contractor must submit a deep mixing work plan for review and approval. The deep mixing work plan must include the following items:

9.3.5 Material Certifications

Certificates of compliance must be submitted as proof of conformance to materials standards and requirements for each truck load of binder, admixtures, and steel, as needed.

9.3.6 Production Records

By the end of the next business day following each deep mixing shift, the contractor should submit a daily production report in the approved format. The report should be completed and signed by the contractor's project superintendent and include the following information:

Binder factor (weight of dry binder per cubic meter of untreated soil) of each column (single axial mixing tool) or element (multiple axial mixing tool) may be calculated from the measured and recorded values. These reduced data could be required in the daily production report as additional construction parameters.

The comprehensiveness of information submitted by the contractor on a daily basis is critical to ensuring accurate tracking of production and quality. Prior to production, the owner should review the list of information to be provided daily by the contractor to ensure adequate information will be required to assure the quality of the foundation construction. Generating past data may be impossible or can lead to errors or omissions.

9.3.7 QC/QA Records

Data from calibrated instruments must be submitted for all measurement devices used for binder production, deep mixing operational monitoring, and laboratory testing. Within 3 business days of completing any QC/QA monitoring and testing, the contractor should submit the results, including original data sheets from the laboratory and an evaluation of the compliance of the test results with project acceptance criteria. Instruments used for monitoring and testing should be calibrated at the beginning of the project and repeated every 3 months. Access to monitoring equipment should be made available to the owner/engineer.

9.3.8 As-Built Field Measurement Data

After completing the project (or a phase of the project, depending on project size and layout, but not less than every 2 weeks), the contractor must submit as-built field measurement data indicating surveyed as-built plan locations of each DMM element, including element center per site-specific coordinates), column dimensions and verticality, and top and bottom elevation of each element to the accuracy required by the project specifications.

 

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