Labuz, Joseph F.

There has been a sustained effort in applying fracture mechanics concepts to crack formation and propagation in bituminous pavement materials. Adequate fracture resistance is an essential requirement for asphalt pavements built in the northern part of the US and Canada for which the prevailing failure mode is cracking due to low-temperature shrinkage stresses. The current Superpave specifications address this issue mainly through the use of strength tests on unnotched (smooth boundary) specimens. However, recent studies have shown the limitations of this approach and have suggested that fracture mechanics concepts, based on tests performed on notched samples, should be employed instead. Research in progress at University of Minnesota investigates the use of fracture mechanics principles to determine the low-temperature fracture properties of asphalt mixtures. This paper presents a testing protocol that allows obtaining multiple measurements of fracture toughness as a function of crack propagation based on the compliance method to measure crack length. An increase in fracture toughness with crack length is observed, which is consistent with the behavior displayed by other brittle materials. The plateau of the curves may be representative of the asphalt concrete resistance to fracture because the initial values can be significantly influenced by the presence of the inelastic zone at the crack tip. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 1789, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/1789-21.

An earth pressure cell (EPC) is a device designed to provide an estimate of normal stress in soil. The practice of designing and manufacturing stress measurement devices revolves around the study of the interaction between the measuring device – the earth pressure cell – and the host material. However, distribution of normal stress is not necessarily uniform across a given surface. Consequently, output from an EPC may be different under soil loading conditions than under fluid pressure. In addition, depending upon the design, as the cell deflects, an arching-type phenomenon may develop. The objectives of this study were to devise a scheme for calibration of earth pressure cells and to recommend a procedure for field installation. A new testing device was designed to permit the application of uniaxial soil pressure to the earth pressure cell using various types of soil and load configurations. Sensitivities computed from soil calibrations varied from those determined from fluid calibrations by as much as 30%. A field installation procedure was developed from model tests. In the laboratory, a thin-walled steel cylinder with a geotextile bottom was filled with uniform silica sand in a medium dense state and the earth pressure cell was placed within the sand. The entire apparatus (earth pressure cell, cylinder, and sand) was carried into the field and installed in the desired locations. Once in place, the steel cylinder was pulled up out of the ground, leaving the cell, sand, and geotextile behind. Preliminary field data indicate that the soil calibration and placement procedure provide reasonably accurate measurements of the change in vertical stress. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 1772, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/1772-02.

A pile-supported embankment constructed on TH 241 near St. Michael, MN was instrumented with 48 sensors, including strain gages on the piles and on the geogrid, as well as earth pressure cells and settlement systems near the base of the embankment. Pile supported embankments are relatively novel structures employed largely at bridge approaches and highway expansions where soft soils would otherwise lead to unacceptably large differential settlements. The structure typically consists of a number of capped piles, well-compacted gravel, and one or more layers of geogrid reinforcement above the piles. Analyses of the data suggest that the redistribution of the embankment load to the piles occurs within and above the so-called load transfer platform, a 1 m layer of geogrid reinforced gravel. Arching seemed to take place within the embankment, such that the stress at the top of the platform was concentrated above the piles.

This Technical Summary relates to Report 2007-34, "Implementation of Ground Penetrating Radar," published August 2007.

This report contains the appendices to Report 2007-34, Implementation of Ground Penetrating Radar

Researchers performed laboratory experiments on soil-fabric-aggregate systems to evaluate the effect of a geotextile on unpaved road performance. Direct shear tests performed on gravel indicated a 42 degree friction angle. Similar tests performed on soil-fabric-aggregate systems resulted in an interface friction value for the nonwoven geotextile system similar to that of the gravel alone. The slit film and heavy weight woven systems generated friction angles about 20 percent lower. Observations of model tests showed that in terms of rut depths, the non-woven performed better than the slit film woven geotextile for all gravel thicknesses, most likely because of the nonwoven's higher frictional characteristics. The rut diameters for the slit film and nonwoven reinforced systems tended to be larger than those observed for the unreinforced systems indicating an increased load-spread angle through the gravel. Based on rutting alone, the unreinforced model with 200 mm (8 in.) gravel was equivalent to that of the slit film in reinforced model with 150 mm (6 in.) gravel and the nonwoven model with 100 mm (4 in.) gravel. A so-called bearing capacity factor for the unreinforced models was approximately 50 percent less than the non-woven reinforced models, in reasonable agreement with theory.

This report presents a literature review of instrumentation practices for the measurement of stresses, strains, and deflections in pavement structures. Various types of instruments that are commonly employed in pavement instrumentation projects are discussed, as well as the factors that influence their performance. In a series of laboratory experiments, the performance of three different types of embedment strain gages, two LVDTs, and one soil stress cell are investigated. These experiments are designed to evaluate the accuracy and durability of commercially available transducers. For strain gages, the selection of an appropriate transducer must balance compliance and measurement sensitivity. All of the strain gages tested in concrete gave reasonable results. It was found that hermetically sealed LVDTs should be sufficient enough for robust installations. Experiments with soil stress cells embedded in sand indicate the variability that may be expected in the field due to installation procedures, and emphasize the need for in-soil calibrations. A set of recommendations are provided with respect tot the sensor procurement and installation specifications for Mn/ROAD.

The goal of the project is to establish a non-destructive field testing technique, including a data analysis algorithm, for determining in-place pile lengths by way of seismic waves. The length of each pile supporting a high-mast light tower (HMLT) will be identified through a systematic sensing approach that includes (i) collection and classification of the pertinent foundation designs and soil conditions; (ii) use of ground vibration waveforms captured by a seismic cone penetrometer; (iii) three-dimensional visco-elastodynamic finite element analysis (FEA) used as a tool to relate the sensory data to in situ pile length; (iv) use of machine learning (ML) algorithms, trained with the outputs of FEA simulations, to solve the germane inverse problem; (v) HMLT field testing; and (vi) analysis-driven data interpretation. Several hundred HMLTs throughout Minnesota have foundation systems, typically concrete-filled steel pipe piles or steel H-piles, with no construction documentation (e.g., pile lengths). Reviews of designs within current standards suggest that some of these foundations may have insufficient uplift capacity in the event of peak wind loads. Without knowledge of the in situ pile length, an expensive retrofit or replacement program would need to be conducted. Thus, developing a screening tool to determine in situ pile length — as compared to a bulk retrofit of all towers with unknown foundations — would provide significant cost savings.

A steel pipe-pile section; filled with concrete; was instrumented and tested under axial load. Two types of strain gages; resistive and vibrating wire; were mounted to the steel-pipe pile and checked by determining the known Young's modulus of steel Es. The steel section was filled with concrete and a resistive embedment gage was placed in the concrete during the filling process to measure axial strain of the concrete. The axial load - axial strain responses of the steel (area As) and concrete (area Ac) were evaluated. The stiffening of concrete; related to curing; was also studied. Assuming the boundary condition of uniform axial displacement; i.e.; equal axial strain in the steel and concrete; (Sf (Bzs = (Sf (Bzc = (Sf (Bz; the sum of the forces carried by the two materials; Fs + Fc; where Fs = (Sf (Bz * Es * As and Fc = (Sf (Bz * Ec * Ac; provides a reasonable estimate - within 3% - of the pile force. For the particular specimen studied (12 in. ID; 0.25 in. wall thickness); the stiffness of the composite section of steel and concrete was about three times larger compared to the steel section without concrete. Further; the concrete carried about 70% of the load; but the axial stress in the concrete; at an applied force of 150;000 lb; was less than 20% of the compressive strength of the concrete.

Rock strength and elastic behavior are important for foundations such as spread footings resting on rock and drilled shafts socketed into rock. In addition to traditional rock quality information; stiffness and failure parameters are helpful for design. MnDOT has previously used a low-capacity load frame for routine rock testing but this apparatus does not generate sufficient force for testing hard rock. The report provides a comprehensive suite of results from 134 specimens tested under uniaxial compression and 33 specimens tested under triaxial compression on a wide variety of rock; including hard rock; which frequently is of interest for high-capacity foundation systems. Thus; an economic benefit is realized if the strength of the rock is measured; as opposed to correlated with an index parameter; due to the potential to reduce foundation size and construction time. Information from the testing was used to expand the MnDOT database of rock properties and allow for improved designs based on accurate measurements of Young's modulus; uniaxial compressive strength; and friction angle.