Kimberly A. Warren

Deciphering the role of solar-induced thermal stresses in rock weathering

A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study’s data. Most observed cracking coincides with the timing of calculated maximum, insolation-driven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weather-actuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar-induced tensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcritical crack growth in the boulder. Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth’s surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock’s stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.

Foil Strain Gage Attachment Techniques for Geotextile and Geogrid

Seventeen full-scale test sections were constructed on a low-volume, flexible pavement. The test sections were reinforced with geotextiles and geogrid, instrumented with Micro-Measurement Group foil strain gages. While strain gage attachment to rigid materials is well understood, methods used to attach foil strain gages to geotextiles, specifically, are more challenging due to the matrix of fibers in the material, and the method used to waterproof and protect these gages in a field environment is critical. The survivability of a method used to attach and waterproof the foil strain gages was successfully demonstrated in the field and is described herein.

Three-Year Evaluation of Thermally Induced Strain and Corresponding Lateral End Pressures for a GRS IBS in Ohio

Geosynthetic-reinforced soil (GRS) integrated bridge systems (IBS) integrate conventional bridge superstructures with a GRS abutment foundation and GRS approach for a cost-effective, rapid construction alternative. A 42.7 m long single span GRS IBS was constructed and instrumented to monitor the thermally induced behaviors and better understand the interaction between the superstructure and substructure within the limits of this system. Strain gauges were attached to the steel girders, and lateral end pressures were monitored using earth pressure cells to determine the level of stress thermally induced in the GRS approach over a 3.5 year monitoring period and evaluate the rigidity of the boundary conditions that exist at the interface. During this 3.5-year monitoring period, the data show that the GRS approach is engaged with the superstructure and experiences both active and passive lateral pressures during each thermal cycle without displaying an increase in passive pressure with time. The stress-strain data acquired during this project indicate that the GRS IBS is behaving significantly more like a system with unrestrained boundaries due to the flexibility of the GRS approach at each end. The tightly spaced reinforcements create a composite material at the ends of the superstructure that enable the approach fill to move successfully with thermally induced superstructure deformations without creating a failure within the soil or at the surface of the roadway (interface included).

Liquid Extraction Using Prefabricated Vertical Wells (PVWs) Under Vacuum in Clay

A large-scale clay specimen was prepared in the laboratory using select soil to simulate Well Injection Depth Extraction (WIDE) technology using a geosynthetic wick drain. While a full-scale field test is necessary to quantify system flow and contaminant removal rates using WIDE technology, laboratory work is necessary to investigate flow phenomenon local to each PVW. Two methods were developed to measure the consistency of the water content and unit weight distribution for quality control purposes. Specimen preparation techniques, flow rates, settlement, piezometer, and tracer test concentration data were measured. The radius of influence, maximum extraction depth, and potential effects of long term PVW performance were discussed. The radius of influence ranged from 0.3 to 0.4 m. Due to the band shape of the PVW, the radius of influence and extraction depth were maximized near the center of the 100-mm-wide dimension, and flow efficiency was reduced near the comers of the PVW due to the decrease in PVW surface area near the 4-mm dimension. Subsequent to laboratory testing, portions of the geotextile filter jacket were utilized to determine the decrease, if any, in permeability. While there appeared to be a 14 % reduction in geotextile permeability during the test interval, soil particles also existed within the flow channels of the PVW core, indicating that long-term PVW efficiency may be a concern and needs further consideration.

Investigation of thin flexible pavement response between traffic and the falling weight deflectometer (FWD)

Abstract The Falling Weight Deflectometer (FWD) is commonly used to evaluate structural characteristics of pavements. The FWD simulates vehicular loads at typical highway speeds and the data is used to back-calculate layer moduli and design overlays (among other functions). In routine testing, the only data obtained is on the surface of the pavement, which leaves potentially significant behaviors within the pavement undetected. The focus of this paper is to present the analysis of a heavily instrumented, full-scale flexible pavement that was subjected to vehicular traffic and FWD loads under controlled testing conditions. Statistical analysis and finite element modeling separately demonstrated that the responses produced by the FWD were, in general, lower than those from corresponding vehicular traffic, and this behavior was more pronounced with depth. Finite element modeling resulted in traffic asphalt strain, base pressure, and subgrade pressure responses exceeding FWD responses by factors of 1.27, 1.76, and 2.43, respectively. Similarly, a statistical analysis resulted in factors of 1.36, 1.66, and 2.39, respectively. Additional analysis showed the deflection from FWD loading would have to be increased between 1.3 to 1.6 times to produce corresponding vehicular traffic stress states.

Statistical Analysis of Unbound Subgrade Pressure Measurements

The objective of this paper is to present statistical analysis of unbound subgrade pressure measurements. A heavily instrumented, flexible pavement was constructed and received an uncharacteristically low amount of rainfall for several months thereafter. The large amount of data acquired under the relatively constant moisture conditions during this period served as an excellent tool to analyze variability form a full scale pavement test, especially in the context of measurement redundancy and appropriate analysis techniques. The analysis presented in this paper focuses on measured subgrade pressures and is part of a more comprehensive effort to assess all pavement layers. It was shown that sensor redundancy alone may lead to erroneous conclusions if the redundancy is taken as a substitute for variability analysis. Future work aims to demonstrate optimal techniques useful for generating meaningful conclusions by incorporating sensor redundancy, measurement variability, and construction variability.

IMPROVED DATA ACQUISITION FOR INSTRUMENTED FLEXIBLE PAVEMENTS REINFORCED WITH GEOSYNTHETICS

Flexible pavement design is currently evolving from a qualitative design procedure, based on road test data, to a “mechanistic-empirical” design procedure. To support the upcoming standard of design, a data acquisition plan has been developed for a full-scale field test to be performed on a heavily instrumented flexible pavement with 13 test sections. The data will be utilized during a finite element analysis to generate design charts for flexible pavements reinforced with geosynthetics. The data acquisition plan developed during this study is designed to minimize the collection of unnecessary data and post-processing effort by using independent trigger mechanisms in each test section. Prior to full-scale construction, a pilot-scale study was employed to determine and verify the critical program acquisition parameters and test the trigger mechanism. Additionally, the data acquisition hardware and software code generated for this application were evaluated. The objective of this paper is to describe the results of the pilot-scale study, outline the overall data acquisition methodology, and highlight the key programming challenges.

Finite Element Analysis of Instrumented Thin Flexible Pavement to Quantify Variability

Variability analysis is used for an instrumented pavement consisting of a thin asphalt surface, granular base, geosynthetics, and a fine grained subgrade. The pavement was modeled with the finite element method using Plaxis software where stationary transient loading and stress dependent material models were incorporated. The results show how significant variability can occur within a pavement built to acceptable standards and that without methods to account for variability, instrumented measurements can be misleading in some instances. Realizing that variability is present is far removed from accounting for it effectively. Asphalt strain changes from 8% to 141% were calculated due to the effects of variability, and vertical sensor positioning within customary installation tolerances was shown to vary strain by + 31%. The use of asphalt strain gauges in thin flexible pavements was shown to be highly prone to error, with variability easily dominating the measurement. Subgrade stress changes from 17% to 45% were calculated from the effects of variability, and vertical sensor positioning with customary installation tolerances was shown to vary pressure by +3%. Subgrade stress variability was less relative to asphalt strain, though it was too high to neglect in analysis.

Preliminary Results for a GRS Integrated Bridge System Supporting a Large Single Span Bridge

In order to refine Geosynthetic Reinforced Soil Integrated Bridge System technology (developed by the FHWA), girders from a 42.7 m single span bridge were instrumented with strain gages, end pressures were measured using horizontal pressure cells, and the abutments were instrumented with vertical pressure cells and survey targets to measure girder footing and abutment wall movements. After a six month monitoring period, vertical deflections are within tolerable limits (ranging from 1.1 to 4.6 cm) and there are no visible cracks at the bridge-approach interface. The strain gages and earth pressure cells continue to collect meaningful data in terms of magnitude and trend. A change in ambient temperature causes a temperature induced strain in the steel, which affects the lateral pressure measured behind the steel girders as expected. Vertical pressures in the abutment are also affected by the thermal cycle. This paper will display the preliminary results from this project.

Preliminary Analysis Comparing Measured Responses of FWD and Traffic Data

The objective of this paper is to present findings where measured subgrade stresses from vehicular traffic and the Falling Weight Deflectometer (FWD) are compared. Both loading techniques were employed on a heavily instrumented pavement under identical conditions during a period immediately after construction. The analysis presented clearly showed that the difference in pressure response in the compacted subgrade of a thin flexible pavement was statistically significant with vehicular traffic producing higher values in every instance. FWD readings, on average, would need to be increased by a factors of 3.73 and 2.39 for the front and back axles, respectively, to prevent statistical differences from being detected. Such a large difference in pressure response is significant since the FWD is routinely used to, among many other functions, back calculate layer moduli of stress softening soils.