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105 Conclusions and Recommendations Summary of Activities This study started with a thorough literature review of national and international states of practice and implementation of quality control with IC technology. Reviews of the different approaches to simulate the IC roller compaction process during the mapping operation and of machine learning algorithms to extract mechanical properties of compacted geomaterials also were synthesized. A realistic 3D nonlinear FE model to simulate the mapping process of single-layer and two-layer geosystems was developed for predicting the representative responses of the geo- materials. The FE model considered a contact model to account for the complex soil-drum interaction, permitting the loss of contact between the drum and the soil. A comprehensive database of cases with different input parameters was assembled for single-layer and two-layer geosystems and various drum dimensions with different operating conditions. Various levels of complexity in the model were evaluated to assess the impact of the vibratory conditions and to consider both linear and nonlinear geomaterial constitutive models on the pavement responses. Relationships were developed among the responses of the models with different levels of complexity to simplify the modeling. Laboratory and field-testing activities were conducted to validate the responses from the models. To that end, the research team developed a system to evaluate the vibration character- istics of the IC rollers as well as the responses of ground layers during the mapping process. The IC data collected were partitioned into virtual sublots equal to the width of the roller and lengths equal the minimum length of the compacted section that was practical to rework. For mapping purposes, all ICMV measurements falling inside a sublot were averaged to obtain representative ICMVs. This approach allowed the researchers to accommodate the inherent uncertainty related to the accuracy of the GPS devices and the precise position of the moving roller. For practical purposes, a three color-coded scheme was used to map the representative ICMVs. To ensure uniformity throughout the site, another color-coded map was developed to assess the variability of compaction. Mapping the COV of the ICMVs within each sublot allowed the identification of sublots where the representative ICMVs were no longer reliable due to construction- or equipment-related issues. The researchers also developed a series of inverse algorithms to provide reliable, layer- specific ICMVs for construction quality control. To achieve this, the research team assembled a comprehensive database consisting of pavement response data for single-layer and two-layer geosystems with widespread layer properties and base thicknesses. To select the parameters that had a more significant impact on the pavement responses, a sensitivity analysis was conducted to identify those variables best suited as inputs into the proposed inverse solvers. Various inverse C H A P T E R 9
106 Evaluating Mechanical Properties of Earth Material During Intelligent Compaction solvers with differing levels of complexity were proposed with the expectation that greater complexity would improve precision but also would require additional laboratory effort. The inverse solvers were evaluated, and those best suited for predicting layer moduli for both single-layer and two-layer systems were selected for validation purposes. The predictive power of the inverse solvers improved when local adjustment factors were used. Based on the activities discussed, draft specifications were proposed and evaluated using field tests. These field tests were conducted in four tests cells constructed at the MnROAD test track facility with different subgrade and unbound aggregate base materials. Additional NDT testing, conducted along the test section at each sublot in all four cells, measured modulus-based properties of the compacted materials. The implementation of the spot tests along the test section allowed the research team to address the variability of the material in the field and its impact on the mapped ICMVs. Samples of the materials used in the construction of the test sections were collected and transported to the laboratory to measure their in-place moisture content, index properties, and perform resilient modulus tests. The collected dataset, made up of ICMVs, roller operating settings, field test measurements using NDT, and the properties obtained from the laboratory tests, was used to calibrate the numerical models and to develop machine-learning algorithms to extract the mechanical properties of compacted geomaterials. Based on those results, the specifications were modified. To identify any practical restrictions, fine-tune the proposed models and approaches for the extraction of the mechanical properties, and improve the proposed quality control process, the modified specifications were applied and evaluated at three new sites involving actual construction projects. The results of this evaluation were also used to improve the specificationâs framework. General Conclusions The general conclusions based on the evaluation of the proposed specification for the extraction of mechanical properties of compacted geomaterials using IC are the following: ⢠The adoption of the specification needs to be approached in the context of the levels unifor- mity of the compaction. ⢠The most consistent results are obtained when proof mapping using IC is carried out in conjunction with the modulus-based measurements, and when variability in the ICMVs is kept within (less than or equal to) 25%. ⢠Given the large diversity in construction practices and material types, the implementation of the draft specification requires more localized field studies by DOTs to adopt it to their local materials and construction practices. Based on this study and interaction with the highway agencies, the following comments and suggestions can be made: ⢠This research study provides a critical review of the strengths and concerns about the implementation of a specification to extract the mechanical properties of compacted materials using IC technology. The research team attempted to highlight the complexities that could arise and made an effort to address them in a comprehensive manner. ⢠Even though this report emphasizes both the strengths and concerns with the proposed specification, the proposed specification is a large step toward higher-quality highway construction.
Conclusions and Recommendations 107 Recommendations Related to the Proposed Specifications Two specifications were developed: one for stiffness-based acceptance, titled âProposed Standard Specification for Quality Management and Design Verification of Earthwork and Unbound Aggregates Using Intelligent Compaction (IC),â and a second for extraction of modulus of compacted layers, titled âProposed Standard Specification for Extracting Modulus of Compacted Geomaterials Using Intelligent Compaction (IC).â The proposed specifications and test methods are presented in Appendix A of this report, and contain suggestions about the areas that the state highway agencies (SHAs) should modify to match them to their needs and institutional cultures. The use of the proposed stiffness-based specification could serve as an almost real-time approach for determining mechanistic-based field-target values for routine quality management purposes. On the other hand, the proposed modulus-based specification would be preferred if the goal of the highway agencies is to extract the moduli of the layers. For the implementation of the modulus-based specification, highway agencies should be prepared to conduct laboratory testing up front and institute more rigorous process control during the compaction process. The proposed specifications make use of an approach for the mapping of ICMVs that consists of partitioning the lot into virtual sublots, each of them with dimensions equal to the width of the roller and the length equal to the minimum length of the compacted section that is practical to rework as set at the discretion of the engineer. ICMVs falling inside a sublot are averaged to obtain representative ICMVs. This approach is proposed to accommodate the inherent uncertainties related to the accuracy of the GPS devices and the precise position of the moving roller. The approach also facilitates identifying the less stiff areas. The proposed modulus-based specification requires using IC to identify the more uniform sublots in terms of ICMVs for conducting additional spot tests. Future Activities This study demonstrates the technical benefits and challenges related to the implementation of the proposed specification for the extraction of the mechanical properties of compacted geomaterials using IC. Even though all aspects of the development of the protocols were evaluated, the number of test sections and compacted geomaterials where the algorithms and specification were evaluated, fine-tuned, and validated was limited. The current speci- fications for quality management of compaction of geomaterials vary significantly among the different SHAs. Because the testing conducted for this study involved a selected set of protocols and procedures that were uniformly implemented, the research team recommends a follow-on implementation project to assist SHAs as they implement and adjust the pro- posed protocols and specifications to local practices. The additional data gathered from such implementation projects also could contribute to a better understanding of the limitations of the process. The findings of this research should be disseminated in a balanced way to the transportation community. The presentations should not only emphasize the benefits of the specifications but should enumerate the changes in day-to-day operations of the SHAs and means of adapting them in their operations in a gradual manner.
