Roy Borden

The North Carolina Department of Transportation (NCDOT) has funded a research project (will be referred to as Phase I study) to develop criteria for situations where soft soils need to be undercut and replaced and/or stabilized with mechanical or chemical measures (undercut refers to the removal of soft subgrade during the construction or reconstruction of new pavement sections). In this funded study, a large scale laboratory testing program was conducted to evaluate the performance of undercut subgrade stabilization measures under construction traffic loading, prior to final paving. Twenty-two simulated undercut sections, with four different stabilization configurations, were built in a large-scale test pit. Undercut areas backfilled with aggregate base course (ABC) and reinforced with geosynthetics showed improvement over unreinforced sections, but only when reinforcement was placed at depth approximately equal to the loaded area diameter and after initial displacements mobilized the strength of the geosynthetic. The soft nature of the subgrade and its consequences on the ability to compact the ABC layer showed the importance of carefully analyzing the results when viewed on a comparative basis, and the need for documented field performance. In this case, a trend of an accelerated deformation rate was observed during the first two hundred load cycles, with a steady state deformation rate emerging after approximately 1000-2000 loading cycles. It is not clear, however, whether this is a situation specific to the results from the laboratory testing, due to the limitation of the laboratory-sized equipment, or it is a behavior representative of field performance, and is occurring due to the limited ability to compact backfill over the soft subgrade layer. The main objective of the proposed project is to validate the findings from the Phase I laboratory study at a construction site in the Piedmont geologic area of North Carolina. The proposed work will seek to investigate the applicability of the proposed undercut criteria in the Piedmont Physiographic region and validate approaches to improving soil bearing properties investigated in the laboratory. The proposed plan includes the field implementation of four instrumented test pads for performance monitoring. In addition to a control pad, one pad will implement undercutting and replacement with select fill, a second will include undercutting in conjunction with ABC and the use of geosynthetics, and a third will include chemical stabilization. The research work will address the following objectives: i. Identify test sites in the Piedmont Physiographic region for implementation of alternative or supplemental approaches to undercut, including the use of geosynthetics and/or chemical stabilization. ii. Instrument test pads at the identified site and monitor performance in terms of induced rut depth, maximum curvature, tension cracks development, and stress attenuation with depth under repeated truck loading. iii. Perform Dynamic Cone Penetrometer (DCP) testing to validate proposed undercut criteria for site conditions. In addition, perform FWD testing to supplement the DCP data for comprehensive subgrade characterization. iv. Use field data to verify performance of alternative or supplemental approaches to undercut to limit volume change and improve soil properties and workability. Accordingly, update and verify the undercut criteria and comparative cost analyses developed during Phase I. Provide a recommendation of the relative cost of each measure and the most suitable stabilization measure(s).

The main objective of the proposed project is the more economical design of temporary slopes and retaining structures in North Carolina (NC) residual soils. In general, the current design methods and procedures for temporary slopes and temporary excavation support systems do not consider the short-term characteristics of NC residual soils, and therefore may result in overly conservative designs and unnecessary construction costs. Even though the geotechnical engineers are aware of the over conservatism of the current design methods and procedures, they do not have rational means by which to improve the design cost effectiveness. It is the development of these rational design procedures that is the heart of the proposed research.

The North Carolina Department of Transportation (NCDOT) is progressing toward developing quantitative and systematic criteria that address the implementation of undercutting as a subgrade stabilization measure. As a part of this effort, a laboratory study and numerical analyses were performed over the past two years with the results providing proposed criteria for undercutting and alternative stabilization measures under various roadway site conditions. These criteria provide provisions for discerning possible rutting and pumping of the subgrade under construction loading, and provide response and subgrade stiffness under repeated loading of 10,000 cycles. The objective of work proposed herein is to perform testing in the field on instrumented roadway sections to collect data for the validation of the guidelines developed from the laboratory and modeling study. The proposed plan encompasses field instrumentation of four test pads; one with the implementation of undercutting and replacement with quality fill and stabilization fabric, a second includes undercutting in conjunction with the use of geotextile, a third stabilized with geogrids, and a fourth includes chemical stabilization.

The North Carolina Department of Transportation (NCDOT) is currently seeking to develop systematic criteria that address the implementation of undercutting as a subgrade stabilization measure. As a part of this effort, a laboratory study is proposed to develop criteria for undercutting under various roadway site conditions; these criteria will include systematic short-term criteria for expected construction loading and long-term criteria to establish the subgrade stiffness for the design of pavement layers under traffic loading. The objective of work proposed in this project is to perform testing in the field on instrumented pavement sections to collect data for the validation of the guidelines to be developed from the laboratory study. The proposed plan encompasses field instrumentation of three test pads; one with the implementation of undercutting of poor soils and replacement with quality fill, a second includes undercutting in conjunction with the use of geosynthetics, and a third includes chemical stabilization. Each test location will include a control section in addition to the test pad with the specified stabilization measure.

The main objective of the proposed project is to develop criteria for undercutting under various roadway site conditions; these criteria should include systematic short-term criteria for expected construction loading and long-term criteria to establish the subgrade stiffness for the design of pavement layers under traffic loading. The research work will provide tools for identifying the required depth of the undercut as well as alternative or supplemental approaches to improve soil-bearing properties and workability. The approach to be developed will be supplemented with the use of expedient in situ techniques, such as the Dynamic Cone Penetrometer (DCP). The proposed research plan encompasses laboratory work as well as the modeling and analysis of data.

With the goal of accurate pavement performance evaluation, the PI and his co-workers at NCSU have been developing advanced models for hot-mix asphalt (HMA) mixtures under complex loading conditons. Over the past decade, they have been successful in developing material models that can accurately capture various critical phenomena such as: microcrack induced damage this is critical for fatigue modeling; strain rate-temperature interdependence; and viscoplastic flow that is critical for rutting evaluation The resulting model is termed the viscoelastoplastic continuum damage (VEPCD) model.

The proposed research is designed to determine the optimum asphalt content of the Lsub mixture for the Charlotte lime test sections using performance testing. The performance tests to be used in this study include: Dynamic modulus tests at varying frequencies and temperatues; Direct tension tests at 5 degrees and 40 degrees C for the viscoelastoplastic continuum damage (VEPCD) model; and Triaxial repeated load permanent deformation tests at 55 degrees C

The primary objectives of this proposed research is to evaluate the differences in performance among lime-modified asphalt mixes with different mix designs. The performance characteristics to be evaluated in this study include the time-and-temperature-dependent stiffness characteristics, rutting performance, and fatigue cracking performance.