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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 10. BENCH-SCALE TESTING AND FIELD VALIDATION PROGRAM

Preconstruction testing programs, including both bench-scale and field validation (full-scale testing), are critical to the successful design and construction of DMM projects. The engineering properties of soils treated with DMM are dependent on a variety of factors, such as the original soil characteristics (type, water content, organic content, etc.), non-uniformity of the soil deposit, mix design, curing conditions, loading conditions, and mixing energy. Bench-scale testing involves laboratory preparation and testing of treated soil (soil-binder mixtures) to study the influence of these various factors on measured engineering properties. The results of bench-scale tests are used to define a range of mix designs and installation procedures that are likely to produce treated soils in production that meet the specified design parameters for the project.

Full-scale field work involves installing test elements of the size, arrangement, and depth required in production. The contractor uses production mixing equipment to install the elements and assess the suitability and workability of the materials and the installation parameters to produce the treated soil that meets the specified performance requirements.

10.1 Timing of PreConstruction Testing Activities

Bench-scale and field validation programs can be implemented in different phases of projects for different purposes. The owner is often responsible for specifying the target strength, uniformity, and permeability requirements for the deep mixed material. Bench-scale tests should typically be conducted by the owner/engineer during the design phase to establish the reasonably attainable treated soil properties for use in design. However, if the owner/engineer has prior experience in similar soils in the same nearby geology, bench-scale testing may not be necessary. All results from bench-scale tests conducted by the owner should be provided to all bidders.

For bench-scale testing results to be understood and used with confidence by the owner/engineer and the bidders, standard procedures for sample preparation, handling, and testing must be used. Recommended procedures are outlined in appendix A. If standard procedures are used, DMM contractors with adequate experience may be able to use the owner's bench-scale testing results to select the mixing procedures and binder slurry injection process required to meet the project requirements. However, the DMM contractor may want to conduct additional bench-scale tests to confirm results or optimize binder quantities and mix designs, especially if standard procedures were not used by the owner or cannot be confirmed. The DMM contractor may also want to perform bench-scale testing using modified procedures that simulate the specific mixing conditions for a proprietary DMM system or that may be correlated to the field operation of his/her DMM method.

A field trial program may also be conducted during design to help establish that DMM can be accomplished successfully at a given site and to determine a reasonable range of strengths that can be considered for use in the design process. Field trials during design are completed under a separate contract that is executed prior to completing the design. Field trials conducted during design are expensive for a design-bid-build project. However, for design-build or ECI in design projects, field trials during construction are more likely to be cost effective. Field trial programs have been conducted during the design stage on several large and complicated projects, including the Woodrow Wilson Bridge project for VDOT, the New Orleans Levee Stabilization test project for USACE, and the San Pablo Dam seismic upgrades for the East Bay Municipal Utility District.(22,59,125)

Although field trials during design are uncommon and only used on large or complex projects, field validation programs during construction but prior to production mixing are very common and are used on virtually all projects. Field validation programs conducted by the contractor are used to determine and demonstrate the means and methods for producing the DMM elements that satisfy the requirements of the project specifications and for performing the specified QC/QA activities.

10.2 BENCH-SCALE TESTING

10.2.1 Goals of Bench-Scale Testing Program

A bench-scale testing program conducted by the owner during the design stage should be developed to achieve the following goals:

10.2.2 Bench-Scale Testing Program

Conventional soil testing methods are typically used for testing treated soil samples. Sample size and collection considerations for preparing, curing, and testing treated samples in unconfined compression are included in appendix A. The method of calculation used in the laboratory procedure is included in appendix B. Standard laboratory procedures should be used for testing to provide reliable and understandable data.

It is important to recognize that measured test values from laboratory-prepared specimens will differ from those of field-produced samples. Laboratory mixing equipment imparts greater mixing energy, which promotes greater treated soil uniformity than can be achieved in production by full-scale equipment. Published results indicate that the strength obtained from bench-scale testing is 1 to 5 times the strength obtained in the field using the same mix design.(46) Laboratory curing conditions and loading conditions also often differ from in situ conditions. Laboratory testing allows treated soil strength gain with time to be studied.

Bench-scale testing should consider parameters that are practical to use for full-scale production, considering that some DMM techniques can impart more mixing energy than others. The binder factors that can be introduced by DMM equipment generally range from 170 to 840 lb/yd3 (100 to 500 kg/m3) of in situ soil. Certain mix designs that can be used to produce treated soil samples in the laboratory may not be applicable for full-scale production due to workability restraints. High mixing energies and low water-to-cement ratio binder slurries may be used successfully to mix relatively small laboratory soil samples, but they may be unsuccessful for use in full-scale production.

Bench-scale testing may include a wide range of tests that provide various strength and compressibility data. However, it is necessary to correlate these data with a field value that will be measured during production QC/QA activities. The unconfined compressive strength of the treated soil is the parameter most often measured for QC/QA purposes. Published correlations are discussed in chapter 5. For example, specifications would generally be based on unconfined compression tests. Additionally, specifications would not require that certain modulus values or tensile strengths be achieved. Instead, the engineer would correlate other properties used in design with unconfined compressive strength, which would be the value that the specifications are based on.

Binder costs often account for 20 to more than 30 percent of DMM construction costs. To optimize the construction cost, binders that are available in the local area should be considered for bench-scale testing. Water from local sources must also be used to prepare the binder slurry for the bench-scale study. The mix design should include a matrix of at least three binder factors and three water-to-binder ratios for each main soil type that will be encountered at the site. If multiple binder types are considered, a matrix of mix designs for each binder is developed.

Bench-scale testing results should be clearly reported regardless of whether the testing is conducted by an owner or a contractor. Bench-scale testing reports should include, 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, curing conditions, and plots of unconfined compression versus time for each soil type and mix design.

10.3 FIELD VALIDATION PROGRAM (FULL-SCALE FIELD TESTING)

After the contract is awarded, the contractor should conduct a full-scale field validation program to demonstrate that the contractor's DMM equipment, mix design, and installation procedures can produce treated soil with material and geometric parameters that meet the specification requirements.

10.3.1 Goals of Full-Scale Field Testing

The goals of full-scale field testing include the following:

This testing provides evidence that the expected design parameters will be achieved with the means, methods, and materials proposed by the contractor. Results of full-scale field testing are also used to assess the influence of DMM operations on the overall construction sequence of the project. The DMM contractor may experiment with mixing parameters during field testing to identify suitable mix designs and installation procedures that can provide the necessary quality while also achieving schedule and cost objectives. The DMM contractor may also conduct test sections to develop and support an alternative cost effective design that may be part of a value engineering proposal.

10.3.2 Full-Scale Field Testing Program

The full-scale field testing program involves the installation of trial production elements using the means, methods, and materials proposed by the DMM contractor and defined based on information from bench-scale testing.

At least 30 days before the start of the field validation program, the contractor must submit a field validation program plan that describes the construction procedures; equipment and ancillary equipment used for mixing, 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 a summary of QC/QA samples to be collected and tested. Examples of the forms that will be used to document the work as outlined in section 9.3.6 should also be provided.

During the full-scale field validation program, the contractor evaluates the installation processes to optimize mixing, binder slurry or binder injection quantities, and operational procedures. The trial elements must be installed in ground conditions representative of the project conditions. Ideally, the test elements are installed near borings so that results can be correlated with known ground conditions. Various geometric configurations or column layouts may be evaluated during the field testing program (e.g., column type, wall type, cell type, and block type). Geometric overlap and verticality should also be evaluated. Uniformity may be evaluated by coring or by exposing DMM elements through extracting or excavating around the elements. Exposure and extraction are time-consuming and costly processes, and extraction may not be possible for particularly deep elements.

The same QC/QA methods proposed for the production columns should be used to assess the quality of the field test program elements. Generally, the testing frequency for test columns is very high compared to the testing frequency for production columns. The test panels or columns may be used as production elements if properties and configurations meet specified requirements. Elements not meeting project requirements may be abandoned in place if they are not acceptable to the owner. If elements cannot be abandoned in place, test sections should be installed outside the production area in a part of the site with similar soil conditions. The contractor takes on a risk by installing test elements at production element locations, and it is often preferable to install test elements at locations different from production element locations unless the test elements in question are installed using conservative mix parameters. Sampling and testing requirements for test sections are outlined in chapter 12.

The contractor uses the field validation program results to develop the deep mixing work plan, as outlined in section 9.3.4. This plan must be submitted to the owner for approval at least 30 days prior to the start of deep mixing.

The results of testing from field test columns can be used to estimate the ratio between the laboratory strength values and the field strength values. By modifying the mixing tool, increasing the mixing energy, and adjusting the installation procedures during full-scale testing, the difference in strength between samples produced by the full-scale DMM equipment and the laboratory mixing may be reduced. Lower laboratory-to-field strength ratios generally indicate improved uniformity of the treated soil.

 

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