Roy H. Borden

Fatigue Cracking Resistance of Fiber-Reinforced Asphalt Concrete

The influence of fibers on the fatigue cracking resistance of asphalt concrete is investigated using fracture energy. Nylon, a popular facing yarn of carpets, is used for the actual recycled carpet fibers in asphalt pavement. The experimental program is designed with two phases: the single fiber pull-out test and the indirect tension strength test. Through pull-out tests of 15-denier single nylon fibers, the critical fiber embedded length is determined to be 9.2 mm. As for indirect tension strength tests, samples of asphalt concrete mixed with nylon fibers of two lengths, 6 and 12 mm, based on results of the pull-out tests (critical embedded length) and three volume fractions, 0.25, 0.5, and 1%, are prepared and tested. Asphalt concrete samples fabricated with fibers of 1% and 12 mm results in 85% higher fracture energy than non-reinforced specimens, showing improved fatigue cracking resistance. Although an optimized asphalt mix design with fibers has not been developed for this study, the increased fracture energy represents a potential for improving asphalt fatigue life, which may be facilitated through the use of recycled carpet fibers.

LATERALLY LOADED DRILLED SHAFTS EMBEDDED IN SOFT ROCK

The current design criterion for laterally loaded drilled shafts embedded in weathered Piedmont rock profiles requires a challenging effort on the part of the engineer. A substantial cost saving could be realized, while maintaining an acceptable and safe performance, if a rational method were developed for the analysis and design of drilled shafts in such a profile. In a current research project, the primary objective is to develop and validate a procedure for design and analysis of laterally loaded drilled shafts embedded in the Piedmont weathered rock profiles. A major component of this research is a field-testing program. Presented are the results of the first in a series of several lateral load tests performed on two drilled shafts 0.762 m (30 in.) in diameter embedded in Piedmont weathered rock. These shafts were instrumented with inclinometers and strain gauges. Field data obtained from the instrumented shafts were used to develop P-y curves. Field testing also encompassed the use of a borehole dilatometer to establish correlations between the rock strength and deformation parameters and potential P-y curves. A comparison is made between backcalculated P-y curves, P-y curves predicted by using Reese’s method, and P-y curves from the rock dilatometer. Loaddeformation results are presented and discussed for all methods used.

Numerical Simulation of MSE Wall Behavior Induced by Surface-Water Infiltration

AbstractA series of numerical simulations, including transient seepage analyses and stress deformation analyses, was performed to predict the behavior of a mechanically stabilized earth (MSE) wall subjected to surface-water infiltration. In this research, two mechanisms to cause the deformation because of wetting were considered: (1) the deformation induced by shear strength decreases and (2) the volumetric deformation (swell or collapse) because of wetting. The effects of low as-compacted water content and a low-quality compaction zone behind the wall face on the wall behavior were investigated. As result of the simulations, the wall deformations (face deflections and reinforced-soil settlements) and reinforcement tensions (maximum tensions) are presented at the end of construction and after periods of surface-water infiltration.

Inverse Analysis of Plate Load Tests to Assess Subgrade Resilient Modulus

Cyclic plate load testing is commonly used to investigate subgrade response under repetitive loads. Two frameworks for performing inverse analysis are described for backcalculating resilient moduli on the basis of measured key outputs. In the first approach, an elastic modulus is back-calculated in each selected domain; in the second, selected parameters in the resilient modulus model are estimated. The axisymmetric finite element model analysis results suggest that the second approach is more robust because it allows the modulus to be distributed in the selected domain. A series of sensitivity analyses was conducted with the second approach to illustrate how the assumed properties or model geometry affects the backcalculated parameters. Discrepancies between the back-calculated parameters and their known values were observed when the distance to the boundary–-that is, the radial distance from centerline to sidewall–-was not properly assigned. When backcalculating only selected parameters in the resilient modulus equation, it is necessary to assign the other parameters carefully (i.e., from laboratory tests or references). An example analysis shows the application of the proposed approach to an actual plate load test.

Laboratory Performance Comparison of Stabilized Undercut Subgrade Under Cyclic Loading

This study evaluated the performance of undercut subgrade stabilization measures during construction traffic loading prior to final paving. Twenty-two simulated undercut sections with different stabilization configurations over a typically undercut Coastal Plain clay subgrade were built in a large-scale test pit. The subgrade was placed at a California Bearing Ratio of ∼2–3 % and stabilized with granular layers, granular layers reinforced with geosynthetics, and lime. Granular layers consisted of either aggregate base course (ABC), sandy select fill, or a multi-layer system with both soil types. The four geosynthetics tested were a woven reinforcement geotextile, a woven separation geotextile, and two biaxial polypropylene geogrids. The soft nature of the subgrade and its consequences on the ability to compact the ABC layer show the importance of carefully analyzing the results when viewed on a comparative basis. Cyclic plate loading simulating construction traffic showed that thicker granular layers produced less surface displacement, barring subgrade strength differences from remolding effects. Tests with lime stabilized subgrade showed the least magnitude of deformation over initial and post-rut repair cycles. ABC tests with geotextile showed improvement over unreinforced sections but only when placed at depths approximately equal to the loading plate diameter and after initial displacements mobilized the geosynthetic strength.

Testing techniques for evaluating the shear strength of lime/fly ash slurry stabilized soil

Lime slurry pressure injections have been successfully used to stabilize and control swelling and expansive soils and low-strength clay soils to depths of up to 40 ft (12 m). When the activity of soils is somewhat lower, or when soils have large void ratios, lime/fly ash (L/FA) slurry mixtures have been injected to fill voids. It has been postulated that this action creates stronger seams, helps to mend possible failure surfaces, and at the same time increases the overall stability of a slope by decreasing water infiltration. The investigation reported herein was undertaken to develop laboratory testing techniques suitable for evaluating the strength improvement realized by introducing L/FA into a soil mass in the form of seams. To accomplish this goal, six specimen configurations were conceived, and a total of 45 direct shear specimens, including 35 containing vertical and/or horizontal seams of L/FA, were tested. In addition to demonstrating the viability of testing techniques developed, the testing program resulted in a small data base on one soil stabilized with an L/FA slurry having a lime-to-fly ash weight ratio of 1:3.75 and a solids-to-water ratio of 7.6 lb to 1 gal (0.9 kg to 1 L).

Subgrade Undercut Criteria Based on Modeling of Rutting and Pumping Response

AbstractThe stability of subgrade soils is a major concern during roadway construction with inappropriately soft layers often undercut and replaced by competent or stabilized materials. Systematic undercut criteria are established using numerical modeling with varying the strength and stiffness parameters of the subgrade and representing the mechanistic behavior as an elastic-perfectly plastic medium. Two modes of domain configurations were considered: the plane strain and axisymmetric conditions. The plane strain mode is assumed to simulate proof roller loading with four parallel tires and mainly provides information about excessive pumping response as materials at deeper layers are affected. The axisymmetric mode provides information related to excessive rutting and is used to simulate the effect of single or dual tires representing construction traffic, rather than a series of closely spaced axle loads. Undercut criteria are proposed for meeting a deformation limit state of 25 mm for both pumping and r...

Energy Efficiency and Rod Length Effect in Standard Penetration Test Hammers

Twenty-eight standard penetration test (SPT) hammers owned by the North Carolina Department of Transportation and private consultants were used to investigate the average energy efficiency and variability of manual versus automatic hammers, as well as the effect of SPT rod length on hammer efficiency. The results agree with published data in several regards. Automatic hammers in the study were found to have an average transferred efficiency of 80.9%. This finding agrees very well with the 80% efficiency assumed in geotechnical engineering practice for automatic hammers. Manual hammers in the study averaged 63.9%, close to the 60% efficiency assumed for manual hammers. Manual hammers were found to be twice as variable as automatic hammers in transferred energy from blow to blow within an SPT blow count. The study demonstrated that the measured transferred energy appeared to be affected by rod length. Lengths shorter than approximately 40 ft caused reduced energy to be transferred into the rod. An empirical formula is presented for correcting short rod length energy losses. The data did not demonstrate a strong dependence on SPT N-value, although the data set lacked observations where the N-value was less than 6 blows per ft.

Radial Strain Measurements in Resonant Column and Torsional Shear Tests

During resonant column and torsional shear tests using a Stokoe device, conventionally only the change in height of the specimen is measured. To obtain the change in diameter of the specimen, a relationship between radial strain and longitudinal strain is assumed. In this study, the Stokoe device was modified to enable the measurement of change in diameter of the specimen along with the change in its height during resonant column and torsional shear tests. The diameter change measurement system comprises three proximity probes, coaxial cables, proximitor conditioners, and DC power supply. The probes (7.9 mm diameter) are capable of making noncontact displacement measurements with a resolution of 0.025 mm using a metallic target. These probes are fixed along the circumference of the outer chamber at equal 120 degree spacing. The vertical location of these probes is such that they correspond to the midheight of the specimen. Each probe is mounted on a micrometer to enable accurate adjustment of the probe. T...

Correlation of dynamic cone penetrometer index to proof roller test to assess subgrade soils stabilization criterion

The proof roller test has been traditionally carried out as a technique for subgrade quality assessment in road construction and the induced pumping and permanent deformation are used as the basis for subgrade’s pass/fail criteria. The objective of the study presented herein is to assess the feasibility of using the dynamic cone penetrometer index, DCPI, to discern the need for undercut and the quality of subgrade. A series of 3D FEM analysis was performed to develop a subgrade stabilization recommendation chart, based on the predicted properties from DCPI and deformation response under cyclic proof roll loading. The ‘not passing’ criterion (will be referred to herein as ‘failure’ criterion) was determined as a 25 mm permanent deformation under proof roll test. It was determined that the failure deformation occurs at DCPI value of 38 mm/blow for single layer subgrade soil with the Mr/E50 of 6, ratio of resilient modulus to the secant modulus at 50% shear failure. These results are consistent with NCDOT 38 mm/blow undercut criteria. The numerical results indicated that the subgrade with 300 mm stiff layer (DCPI < 20 mm/blow) on top does not require any subgrade soil stabilization given the assumed properties of the underlying layers. On the other hand, the results indicated that subgrade stabilization is required where top 300 mm of the subgrade is a very soft soil layer (DCPI > 60 mm/blow), unless the sublayers underneath are composed of stiff soil (DCPI < 20 mm/blow).