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Evaluating Mechanical Properties of Earth Material During Intelligent Compaction (2020)

Chapter: Chapter 8 - Framework of IC Specification

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Suggested Citation:"Chapter 8 - Framework of IC Specification." National Academies of Sciences, Engineering, and Medicine. 2020. Evaluating Mechanical Properties of Earth Material During Intelligent Compaction. Washington, DC: The National Academies Press. doi: 10.17226/25777.
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Suggested Citation:"Chapter 8 - Framework of IC Specification." National Academies of Sciences, Engineering, and Medicine. 2020. Evaluating Mechanical Properties of Earth Material During Intelligent Compaction. Washington, DC: The National Academies Press. doi: 10.17226/25777.
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Page 101
Page 102
Suggested Citation:"Chapter 8 - Framework of IC Specification." National Academies of Sciences, Engineering, and Medicine. 2020. Evaluating Mechanical Properties of Earth Material During Intelligent Compaction. Washington, DC: The National Academies Press. doi: 10.17226/25777.
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Page 102
Page 103
Suggested Citation:"Chapter 8 - Framework of IC Specification." National Academies of Sciences, Engineering, and Medicine. 2020. Evaluating Mechanical Properties of Earth Material During Intelligent Compaction. Washington, DC: The National Academies Press. doi: 10.17226/25777.
×
Page 103
Page 104
Suggested Citation:"Chapter 8 - Framework of IC Specification." National Academies of Sciences, Engineering, and Medicine. 2020. Evaluating Mechanical Properties of Earth Material During Intelligent Compaction. Washington, DC: The National Academies Press. doi: 10.17226/25777.
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Page 104

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100 Framework of IC Specification The goal of this project was to develop and propose an expanded IC specification that goes further than any existing specification. For this project, success involved ensuring balance and harmony among the technical rigors of the specification, the feasibility and level of effort to obtain the necessary input parameters, the sophistication of the forward model, and the robust- ness of the backcalculation process. All these elements would have been moot, however, if the roller manufacturers could not provide the necessary input parameters. Appendix A contains two proposed specifications: 1. The “Proposed Standard Specification for Extracting Modulus of Compacted Geomaterials Using Intelligent Compaction (IC),” and 2. The “Proposed Standard Specification for Quality Management and Design Verification of Earthwork and Unbound Aggregates Using Intelligent Compaction (IC).” The research team perceives the two specifications as complementary. The use of the second (stiffness-based) specification is adequate and practical if the goal of the SHA is routine quality management, because this approach is robust, almost real-time, and provides mechanistic-based field-target values. If the goal is to extract the moduli of the layers, however, the modulus-based approach will be more desirable. For the implementation of the modulus-based specification, the SHA should be prepared to conduct some laboratory testing up front and institute more rigorous process controls during the compaction process. Appendix A also presents two proposed test methods that complement the proposed speci- fications and provide device-specific protocols. One test method addresses determining the mechanical properties of geomaterials using IC Technology, and the other addresses determining the modulus of geomaterials using LWD. Development of the proposed specifications and test methods included the following constraints: • The specification(s) were to be based on field measurement of the mechanical ICMVs of compacted geomaterials; • Acceptance criteria were to be correlated with design moduli; • The specification(s) were to provide a practical, coherent, user-friendly and well-defined method for determining mechanical properties that would be compatible with a variety of compacted geomaterials; • Variations in the modulus of compacted geomaterials with different levels of moisture content needed to be accounted for in establishing the specification criteria and limits for compaction; and • Available models, devices, and methods were to be incorporated in the proposed specifications. C H A P T E R 8

Framework of IC Specification 101 AASHTO PP 81-14 was used as a baseline to provide continuity in the development of the specifications. The research team envisions that the SHAs may implement both earthwork specifications, following these major steps: Step 1: Estimating Properties of All Layers. Geomaterials can be either in place or imported from quarries. The main properties of the geomaterials required in the proposed IC specifica- tions are: • Geometrical properties, such as thickness of layers, including base/subgrade/embankment; • Index properties, including gradation parameters and Atterberg limits; • Mechanical properties that may include the materials’ resilient/elastic properties; and • Moisture-density characteristics. Parameters k ′1 through k ′3 should preferably be determined from laboratory tests on the geomaterial sampled from the site. Understanding the constraints that this activity may bring to the operations of highway agencies, the proposed specifications also provide an option for estimating these parameters from index properties of the geomaterial. Different resilient modulus laboratory test protocols (e.g., AASHTO T-307 and NCHRP 1-28A) may yield different nonlinear parameters k ′1 through k ′3. The relationships provided in this report are based on the AASHTO T-307. The proposed relationships in the proposed specifications and test methods should be recalibrated by highway agencies that use other test protocols. Equation 8-1 presents the MEPDG nonlinear material model from Ooi et al. (2004): 1 . (8-1)1 2 3 MR k P P P a a k oct a k = θ   τ +   Equation 8-2 modifies the MEPDG nonlinear material model in a way that seems to yield more representative responses of the modulus-based devices: 1 1 . (8-2)1 2 3 MR k P P P a a k oct a k = ′ θ +   τ +   ′ ′ Understanding the practical problems that this change may cause for highway agencies that utilize the MEPDG material model, relationships developed in NCHRP Project 10-84, “Modulus-Based Construction Specification for Compaction of Earthwork and Unbound Aggregate,” have been provided to convert parameters k1 through k3 (as recommended by the MEPDG) to k ′1 through k ′3 (as used in this study) in the test procedure for determining LWD modulus. Step 2: Simulating Roller Measurements. The structural response algorithms are described in Chapter 3 and evaluated in Chapter 5 of this report. As discussed under the heading “Evaluation and Calibration of Forward Models,” the nonlinear FE algorithm seems more appropriate for estimating the behavior of compacted geomaterials under roller-induced loads. However, the traditional FE model for simulating roller compaction of soil systems requires intensive computational resources and long simulation times, making it prohibitive for the estimation of target field measurement values during field operations. A simplified model to predict pavement responses/target measurement values with minimal computational effort and reasonable accuracy was proposed and developed using soft computing techniques. For this purpose, a comprehensive database generated by simulating a wide range of pavement structures subjected to roller compaction using a nonlinear structural model was used to develop the model

102 Evaluating Mechanical Properties of Earth Material During Intelligent Compaction to predict pavement responses/measurement values. These responses have been calibrated using field measurements. The database, development, and calibration of these models are discussed in Chapter 3 under “Evaluation of Approaches for Developing Forward Models” and Chapter 5 under “Evaluation and Calibration of Forward Models.” The roller parameters significantly affect both the roller measurements and the behavior of geomaterials during compaction. The following roller parameters should be defined prior to the numerical simulations and field evaluations: • Geomaterial nonlinear properties, • Layer thickness (for two-layer systems), and • Weight and dimensions of the drum. Once the material properties (Step 1) and roller parameters (Step 2) are defined, the pro- posed simulation tool is employed to estimate a stiffness value representative of the composite response of the comprising layers down to the roller’s depth of influence that will be used as a target field-measurement value. This task can be integrated with the pavement design, if desired. The process might be accompanied by estimation of discrete NDT device target values to further ensure the credibility of the established target values. Step 3: Pre-Mapping of Layer of Interest. Given the depths of influence of the IC rollers, the variability in the stiffness of the existing layers propagates to the next layer. IC measurements can be performed on an existing layer to extract information about the uniformity of that layer before placing the next layer. This process is called pre-mapping. The vibration frequency and amplitude, as well as the roller direction and speed, should be nominally identical to the values specified for the mapping of the layer after the completion of compaction. The statistical infor- mation of the collected ICMVs will be determined in terms of the distribution of the ICMVs to identify the range, mean, and standard deviation. If the variability of the existing layer is significant according to either the engineer or the specification, it may be prudent to rework the existing ground to improve its uniformity before placing the next layer. The results from the pre-mapping can also be used in the subsequent backcalculation scheme. Without pre-mapping, only one input data point is available per location. As such, only one stiffness parameter (the global stiffness) can be extracted from the IC measurement. Pre-mapping provides a second piece of information as an input to the backcalculation. Spot testing during pre-mapping is necessary for the implementation of a more robust inverse algorithm for the extraction of the layer moduli. Step 4: Performing Compaction to Achieve Target Stiffness. The optimal goal of the compaction process is to achieve a uniform pavement that provides mechanical parameters that meet the design moduli. This step is conceptually straightforward but practically complex. For the contractors, one major source of frustration with the implementation of the IC is this step. For earthwork, the roller type and roller setting that are needed to achieve optimal compaction can differ considerably from the roller type and roller setting necessary for proper mapping. For example, on clayey soils, the padfoot rollers are by far more effective than the smooth drum rollers. Most current specifications require a smooth drum roller set at a low vibration setting and a slow roller speed for IC mapping. These settings are quite reasonable for mapping, but they are not always reasonable for compaction of the layer. The contractor may also need to utilize several different compactors to expedite compaction. Currently, it is marginally possible to utilize the IC data to establish the number of line passes from several rollers simultaneously; however, it is not yet feasible to extract and harmonize the ICMVs. Even when the technology becomes available, the integration of the ICMVs from more than one IC roller may need regular

Framework of IC Specification 103 harmonization (i.e., the contractors must be able to ensure that the different rollers yield similar ICMVs on the same section). The opinion of the research team is that the use of the IC technology will accelerate when the contractors are convinced that using the IC measurements during compaction is essentially a process control that benefits their production rate and the uniformity of the final project. A contractor representative and/or the roller operator can review the real-time map of the collected IC data during compaction as a process control tool to ensure uniformity. The pro- posed process also can positively affect the data management issues experienced by many DOTs. DOT staff can focus on the results of the mapping, as discussed in the next step for quality control and eventually perhaps for quality assurance. Step 5: Mapping Compacted Layer. After compaction is completed, the mapping of the compacted layer is performed with an IC roller. The vibration parameters, in terms of the frequency, amplitude, roller speed, and roller direction, should be identified for the mapping process. The statistical parameters of the collected ICMVs, along with the differences between the mapping and pre-mapping values, should be identified. Mapping and pre-mapping imply the use of geospatial coordinates extracted from a Global Navigation Satellite System (GNSS). Ideally, the planar coordinates can be used to locate the position of the roller and the altitude coordinates can be used to extract the thickness of the layer being mapped. In practice, the accuracy and precision of GNSS readouts and the frequency of IC measurements will dictate the certainty of these values. In addition to the typical mapping of ICMVs, maps allow the identification of areas that lack uniformity. As was discussed in Chapter 4, a color-coded map showing the COV of the ICMV can be used for that purpose. Maps of other operating features also can be provided as a quick check of the appropriateness of the mapping process. Step 6: Post-Processing to Extract Layer Mechanical Properties. Once the mapping of the compacted layer has been completed, the post-processing is performed to extract the modulus of the compacted layer. This process has been documented as part of a second proposed specification. Ideally, one should be able to extract the layer-specific mechanical properties of the last layer and the combination of the previous layers if both pre-mapping and mapping data are available. For this purpose, soft computing techniques were used to develop inverse models to determine the layer modulus. The two approaches to developing the inverse models are documented in Chapter 6. The inputs used for feeding the models strongly affect the level of accuracy and sophistication of selected forward and inverse models, as well as the accepted tolerance in prediction of modulus. The use of more robust inverse algorithms to extract the layer properties would require additional modulus/stiffness-based nondestructive spot tests. The unsaturated soil mechanics concepts in the extraction of stiffness parameters of the geomaterials is conceptually desirable and beneficial. To consider this concept properly in the analyses, almost-continuous measurement of the suction of the geomaterial is required. Given the current state of instrumentation, near-continuous measurement may not be possible. Alternatively, the continuous estimation of the variation in the degree of saturation (perhaps through precise measurements of the moisture content and density) is desirable. Currently, this is possible only through the use of NDG in the field. As developed in NCHRP Project 10-84, moisture content-based relationships can be used as a surrogate for the degree of saturation. The moisture content of the compacted layer can be ideally determined using either a well-calibrated NDG or a microwave oven in the field or using the oven-dry approach in the laboratory.

104 Evaluating Mechanical Properties of Earth Material During Intelligent Compaction The research team believes that a balanced approach must be devised to develop a practical tool. Until a device that can continuously measure the moisture content and density (or, better yet, the suction) of the material, it might be more feasible and practical to use less-advanced param- eters for this purpose. The uncertainties associated with the extracted mechanical properties relate directly to the uncertainties in the measured ICMVs and to the accuracy and precision of the geospatial coordinates. The uncertainties in the ICMV measurements relate not only to the characteristics and installation and capture of the sensors but also to the analysis technique.

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Satisfactory pavement performance can only be assured with appropriate process controls to ensure compacted materials meet proper density and stiffness requirements.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 933: Evaluating Mechanical Properties of Earth Material During Intelligent Compaction details the development of procedures to estimate the mechanical properties of geomaterials using intelligent compaction (IC) technology in a robust manner so that departments of transportation can incorporate it in their specifications.

Appendix A, containing the proposed specifications and test methods, is included in the report. Appendices B through H appear in a supplementary file.

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