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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.

In this study, researchers devised a scheme for calibration of earth pressure cells to observe their response to various loading configurations and to recommend a procedure for field installation. As a result of calibration tests, a field installation procedure was developed. Preliminary field data indicate that soil calibration and placement procedure provide reasonably accurate measurements.

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

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

This study deals with the experimental investigation of the effects of moisture and density on the elastic moduli and strength of four subgrade soils generally representing the range of road conditions in Minnesota. The testing approach involved i) reduced-scale simulation of field compaction, ii) field-type testing on prismatic soil volumes, and iii) element testing on cylindrical soil specimens. The field-type testing included: i) the GeoGauge, ii) the PRIMA 100 device, iii) the modified light weight deflectometer (LWD) device, iv) the portable vibratory deflectometer (PVD) and v) the Dynamic Cone Penetrometer (DCP). To compare the Young's modulus values stemming from the field-type and laboratory experiments, cylindrical specimens were extracted from the prismatic soil volumes and tested for the resilient modulus (Mr), small-strain Young's modulus using bender elements. The results reveal that both moisture and density have a measurable effect on the elastic modulus and strength of all four soils. On the element testing side, the small strain estimates from the bender element tests were in good agreement with the resilient modulus values. In the context of field testing, there was significant scatter of the estimated Young's moduli depending upon the particular testing device.

Resilient modulus, shear strength, dielectric permittivity, and shear and compressional wave speed values were determined for 36 soil specimens created from the six soil samples. These values show that the soils had larger stiffnesses at low moisture contents. It was also noted during testing that some non-uniformity was present within the axial displacement measurements; larger levels of non-uniformity were associated with low moisture contents, possibly due to more heterogeneous moisture distributions within these specimens. Lastly, the data collected during this study was used to recommend a relationship between granular materials' small strain modulus and their resilient modulus. This relationship was given in the form of a hyperbolic model that accurately represents the strain-dependent modulus reduction of the base and subgrade materials. This model will enable field instruments that test at small strains to estimate the resilient modulus of soil layers placed during construction.

Earth pressure cells, tiltmeters, strain gages, inclinometer casings, and survey reflectors were installed in fall 2002 during construction of a 26-ft (7.9-m) high Minnesota Department of Transportation (Mn/DOT) reinforced concrete cantilever retaining wall. A data acquisition system with remote access monitored some 60 sensors on a continual basis. Analysis of the data indicated the development of active earth pressure at the end of backfilling, with a resultant at about one-third of the backfill height. Translation of 0.45 in. (11 mm), or about 0.1% of the backfill height, was responsible for development of the active condition. The wall also rotated 0.03° into the backfill as a rigid body, while the top of the stem deflected 0.16 in. (4 mm) away from the backfill. Sensor readings showed the earth pressure distribution to be quite complex during the backfilling process. Evidence was found for residual lateral stresses from compaction. Translation of the wall overnight following the construction workday reduced the compaction-induced lateral stresses. Changes in earth pressure and wall deflection weeks after backfilling were attributed to changes in temperature and rainfall. The data showed that the wall design, while reasonable, could be made more efficient by removing the shear key, which was ineffective.

Currently, MnDOT pavement design recommends granular equivalency, GE = 1.0 for non-stabilized full-depth reclamation (FDR) material, which is equivalent to class 5 material. For stabilized full-depth reclamation (SFDR), there was no guideline for GE at the time this project was initiated (2009). Some local engineers believe that GE of FDR material should be greater than 1.0 (Class 5), especially for SFDR. In addition, very little information is available on seasonal effects on FDR base, especially on SFDR base. Because it is known from laboratory studies that SFDR contains less moisture and has higher stiffness (modulus) than aggregate base, it is assumed that SFDR should be less susceptible to springtime thawing. Falling Weight Deflectometer (FWD) tests were performed on seven selected test sections on county roads in Minnesota over a period of three years. During spring thaw of each year, FWD testing was conducted daily during the first week of thawing in an attempt to capture spring thaw weakening of the aggregate base. After the spring thaw period, FWD testing was conducted monthly to study base recovery and stiffness changes through the seasons. GE of SFDR was estimated using a method established by MnDOT using FWD deflections, and the GE of SFDR is about 1.5. The value varies from project to project as construction and material varies from project to project. All the materials tested showed seasonal effects on stiffness. In general, the stiffness is weaker in spring than that in summer and fall.

The objective of this project was to render the Falling Weight Deflectometer (FWD) and Ground Penetrating Radar (GPR) road assessment methods accessible to field engineers through a software package with a graphical user interface. The software implements both methods more effectively by integrating the complementary nature of GPR and FWD information. For instance, the use of FWD requires prior knowledge of pavement thickness, which is obtained independently from GPR.

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.