Peter J. Bosscher

DEM simulation of granular media—structure interface: effects of surface roughness and particle shape

An enhanced discrete element method for the numerical modelling of particulate media is presented. This method models a particle of general shape by combining several smaller particles of simpler shape, such as a circle, into clusters that act as a single larger particle. The clusters more accurately model the geometry- dependent behaviour of the particles, such as particle interlock and resistance to rolling. The method is implemented within the framework of an existing DEM program without the introduction of new contact or force algorithms. An extensive set of numerical ‘experiments’ is performed which demonstrate the methods effectiveness. The granular media–structure interface shear test simulations are performed using both clustered and non-clustered particles. The results indicate that the clustered particles undergo less rolling and provide for increased shear resistance of the medium. Copyright

Modulus-suction-moisture relationship for compacted soils

The ultimate parameter of interest in engineering design of compacted subgrades and support fills for highways, railroads, airfields, parking lots, and mat foundations is often the soil modulus. Modulus of compacted soils depends not only on dry unit weight and moisture but also on matric suction and soil structure (or fabric) resulting from the compaction process. However, these relationships in the as-compacted state (i.e., immediately after compaction) have not yet been extensively explored. This paper presents an experimental laboratory study of the shear modulus – matric suction – moisture content-dry unit weight relationship using three compacted subgrade soils. Compacted subgrade specimens were prepared over a range of molding water contents from dry to wet of optimum using enhanced, standard, and reduced Proctor efforts. A nondestructive elastic wave propagation technique, known as bender elements, was used to assess the shear wave velocity and corresponding small-strain shear modulus (Go) of the ...

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus ( Go ) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil...

Relationship Between Pavement Temperature and Weather Data: Wisconsin Field Study to Verify Superpave Algorithm

Six test sections were constructed on US-53 in Trempealeau County by using different performance-graded asphalt binders to validate the Superpave pavement temperature algorithm and the binder specification limits. Field instrumentation was installed in two of the test sections to monitor the thermal behavior of the pavement as affected by weather. The instrumentation was used specifically to monitor the temperature of the test sections as a function of time and depth from the pavement surface. A meteorological station was assembled at the test site to monitor weather conditions, including air temperature. Details of the instrumentation systems used and analysis of the data collected during the first 22 months of the project are presented. The analysis was focused on development of a statistical model for estimation of low and high pavement temperatures from meteorological data. The model was compared to the Superpave recommended model and to the more recent model recommended by the Long-Term Pavement Performance (LTPP) program. The temperature data analysis indicates a strong agreement between the new model and the LTPP model for the estimation of low pavement design temperature. However, the analysis indicates that the LTPP and Superpave models underestimate the high pavement design temperature at air temperatures higher than 30°C. The temperature data analyses also indicate that there are significant differences between the standard deviation of air temperatures and the standard deviation of the pavement temperatures. These differences raise some questions about the accuracy of the reliability estimates used in the current Superpave recommendations.

DEVICE FOR MEASURING SHEAR RESISTANCE OF HOT-MIX ASPHALT IN GYRATORY COMPACTOR

The development of a gyratory load-cell and plate assembly (GLPA) to measure the shear resistance of hot-mix asphalt mixtures is described. The GLPA is a simple tool that allows the measurement of the eccentricity of the resultant load applied by the gyratory compactor in real time during compaction. The GLPA is inserted on top of the mixture specimen in the compaction mold, requiring no changes in the compaction procedure. The results from the GLPA give a continuous measure of the resistance of asphalt mixtures to shearing under gyratory loading at a fixed angle. On the basis of a simplified analysis, it is hypothesized that the bulk shear resistance estimated from the GLPA is a good indicator of the compactibility of asphalt mixtures and their potential resistance to rutting under traffic. The shear resistance and volumetric characteristics of a number of trial mixtures fabricated in the laboratory were tested to show the utility of the GLPA. The results show that the shear resistance is highly sensitive to gradation, asphalt content, and temperature. They also indicate that there are interactive effects of these factors that are independent of the volumetric properties. Although the relationship between the results of the GLPA and the field performance of mixtures is yet to be determined, this device has the potential to be a low-cost and effective tool to complement the current volumetric mixture design procedure. It provides a tool to measure an important mechanical property that is a good indicator of bulk shear resistance of asphalt mixtures.

FIELD ASSESSMENT OF CHANGES IN PAVEMENT MODULI CAUSED BY FREEZING AND THAWING

Three secondary highways with flexible pavements were instrumented for 18 months to monitor changes in pavement moduli caused by seasonal meteorological changes. Temperatures, water contents, and water phase changes in the subsurface as well as meteorological conditions were recorded every 2 hours throughout the study. Pavement moduli were regularly determined by conducting surveys with a falling weight deflectometer and by performing backcalculations using the program Modulus. The moduli of the base and subgrade typically increased 12 to 4 times, respectively, when frost penetrated the subsurface. When the base and subgrade began to thaw, the moduli decreased substantially. The base moduli decreased the most, typically being about 35 percent of the prefreezing values by the end of thaw. Smaller decreases occurred in the subgrade. The subgrade moduli were about 65 percent of their prefreezing values by the end of thaw. The moduli continued to decrease until thaw was complete and then gradually recovered back to the original prefreezing values. Complete recovery required approximately 4 months. Similar changes in moduli were observed regardless of site or year.

RELATIONSHIP BETWEEN SOIL STIFFNESS GAUGE MODULUS AND OTHER TEST MODULI FOR GRANULAR SOILS

Recently, there has been a concerted effort to develop methods for direct measurement of soil stiffness, modulus, or both. A new field test device called the soil stiffness gauge (SSG), which is currently marketed as GeoGauge, shows potential to assess near-surface stiffness. A comparison is presented of moduli obtained from the SSG with moduli obtained from other tests on granular soils. The maximum singleamplitude dynamic force produced during the SSG measurement is determined to be 17.3 N. On this basis, an estimate of the shear strain amplitude produced from the SSG is made by finite element analysis. A plot of shear modulus versus shear strain amplitude on a medium sand obtained from different laboratory tests, including the SSG, is presented. The comparison of the SSG modulus with the moduli from other laboratory tests indicates that the SSG outputs a dynamic modulus corresponding to a strain amplitude approximately 20 times higher than the expected range and with a magnitude lower than it should be on the basis of the induced strain. Nevertheless, the SSG modulus is still higher than that from the resilient modulus test typically used for pavement design.

LABORATORY EVALUATION OF THE SOIL STIFFNESS GAUGE

A new alternative geotechnical field testing device called the soil stiffness gauge (SSG), also known as GeoGauge™, exhibits particular promise for monitoring in situ soil stiffness during construction quality control. However, there has been only limited research on this device regarding its characteristics and limitations. The results of laboratory testing and a finite-element analysis (FEA) of the SSG are presented. Based on the FEA and the SSG measurement in the test box, the radius of measurement influence extends to 300 mm. For two-layer materials with different stiffness, the SSG starts to register the stiffness of an upper-layer material of 125 mm or thicker. The effect of the lower layer, however, may continue to be present even at an upper-layer material thickness of 275 mm, depending on the relative stiffness of the layer materials. Caution needs to be exercised in interpreting the results from the SSG when it is used on multilayer systems, especially those with geosynthetic separators. The presence of a geosynthetic separator between the layers may cause a stiffness decoupling of the layers.

Development and validation of a low-cost electrical resistivity tomographer for soil process monitoring.

This paper presents the development and validation of a low-cost electrical resistivity tomography (ERT) system for the monitoring of process in soils. The ERT technique is capable of evaluating the distribution of electrical properties within a cross section of soil specimens by measuring the applied electrical current and monitoring the voltage distribution along boundary electrodes. This new ERT system consists of (i) a two-dimensional (2D) acrylic testing cell with strip copper electrodes, (ii) an acquisition system and control software, and (iii) a third-party reconstruction software. The tomographer is developed to collect low-noise data at moderate acquisition speeds. To validate the quality of the proposed ERT system, noise levels were evaluated with and without the testing cell and imaging tests were conducted to calibrate the system, evaluate the sensitivity of the instrument to locate a low-resistivity inclusion, and to monitor chemical diffusion in a saturated Ottawa sand specimen. Results sho...

Dimensionless Limits for the Collection and Interpretation of Wave Propagation Data in Soils

The use of bender elements to generate and receive shear waves in soils has become a very popular technique in geotechnical engineering studies. However as with any other wave propagation technique, the interpretation of bender element-collected data is controlled by wave characteristics, boundary conditions, and properties of the medium. This paper presents experimental data and simple closed-form solutions in order to investigate and to evaluate the effects due to the near field and boundary conditions in different types of specimen geometries and boundary conditions. Results yield dimensionless limits that must be taken into account to properly monitor soil parameters and to avoid misleading results in the interpretation of wave propagation data from the bender elements.