Paul J Vardanega

Stiffness of Clays and Silts: Normalizing Shear Modulus and Shear Strain

AbstractAn analysis is presented of a database of 67 tests on 21 clays and silts of undrained shear stress-strain data of fine-grained soils. Normalizations of secant G in terms of initial mean effective stress p′ (i.e., G/p′ versus log γ) or undrained shear strength cu (i.e., G/cu versus log γ) are shown to be much less successful in reducing the scatter between different clays than the approach that uses the maximum shear modulus, Gmax, a technique still not universally adopted by geotechnical researchers and constitutive modelers. Analysis of semiempirical expressions for Gmax is presented and a simple expression that uses only a void-ratio function and a confining-stress function is proposed. This is shown to be superior to a Hardin-style equation, and the void ratio function is demonstrated as an alternative to an overconsolidation ratio (OCR) function. To derive correlations that offer reliable estimates of secant stiffness at any required magnitude of working strain, secant shear modulus G is norma...

Categories of SHM Deployments: Technologies and Capabilities

AbstractThe findings of an extensive literature survey focusing on bridge structural health monitoring (SHM) deployments are presented. Conventional, maturing, and emerging technologies are reviewed as well as deployment considerations for new SHM endeavors. The lack of published calibration studies (and quantification of uncertainty studies) for new sensors is highlighted as a major concern and area for future research. There are currently very few examples of SHM systems that have clearly provided significant value to the owners of monitored structures. The results of the literature survey are used to propose a categorization system to better assess the potential outcomes of bridge SHM deployments. It is shown that SHM studies can be categorized as one (or a combination) of the following: (1) anomaly detection, (2) sensor deployment studies, (3) model validation, (4) threshold check, and (5) damage detection. The new framework aids engineers specifying monitoring systems to determine what should be meas...

Strength mobilization in clays and silts

A large database of 115 triaxial, direct simple shear, and cyclic tests on 19 clays and silts is presented and ana- lysed to develop an empirical framework for the prediction of the mobilization of the undrained shear strength, cu, of natural clays tested from an initially isotropic state of stress. The strain at half the peak undrained strength (gM=2) is used to normal- ize the shear strain data between mobilized strengths of 0.2cu and 0.8cu. A power law with an exponent of 0.6 is found to describe all the normalized data within a strain factor of 1.75 when a representative sample provides a value for gM=2. Multi-linear regression analysis shows that gM=2 is a function of cu, plasticity index Ip, and initial mean effective stress p 0 0. Of the 97 stress-strain curves for which cu, Ip, and p 0 were available, the observed values of gM=2 fell within a factor of three of the regression; this additional uncertainty should be acknowledged if a designer wished to limit immediate founda- tion settlements on the basis of an undrained strength profile and the plasticity index of the clay. The influence of stress his- tory is also discussed. The application of these stress-strain relations to serviceability design calculations is portrayed through a worked example. The implications for geotechnical decision-making and codes of practice are considered.

Analysis of Asphalt Concrete Permeability Data Using Representative Pore Size

The permeability of asphalt concrete has been the subject of much study by pavement engineers over the last decade. The work undertaken has tended to focus on high air voids as the primary indicator of permeable asphalt concrete. This paper presents a simple approach for understanding the parameters that affect permeability. Principles explained by Taylor in 1956 in channel theory work for soils are used to derive a new parameter—representative pore size. Representative pore size is related to the air voids in the compacted mix and the D75 of the asphalt mix grading curve. Collected Superpave permeability data from published literature and data collected by the writers at the Queensland Department of Transport and Main Roads is shown to be better correlated with representative pore size than air voids, reducing the scatter considerably. Using the database of collected field and laboratory permeability values an equation is proposed that pavement engineers can use to estimate the permeability of in-place p...

State of the art: Permeability of asphalt concrete

The findings of an extensive literature review on the permeability of hot-mix asphalt concrete are detailed in a state-of-the-art report on the measurement and interpretation of asphalt concrete permeability data. The permeability of asphalt concrete is affected by a range of factors with various levels of importance, which are reviewed along with their impact on the coefficient of permeability. Many theoretical, empirical, semiempirical, and numerical models have been developed to predict permeability, using a range of indicators. Some of these models are reviewed and their advantages and shortcomings discussed. Recent advances in X-ray tomography studies are also summarized. The review reveals that field permeability measurements are not reported to match numerically well with laboratory measurements, though there is some correlation. The reviewed test methods for permeability all rely on an assumption of laminar flow that is unlikely in more porous mixtures. Attempts to measure connected air voids improve the chances of obtaining more meaningful correlations between permeability and air voids, regardless of the mathematical model used to link the two quantities. The lift-thickness to nominal maximum aggregate size (NMAS) ratio and/or changes in the binder content has a less significant effect on permeability than changes in the porosity and/or mix gradation. Air void gradients and distributions in compacted asphalt concrete mixtures can now be assessed with X-ray techniques: the distribution of air voids appears to be nonuniform in laboratory-prepared specimens.

Analysis of Structural Health Monitoring Data from Hammersmith Flyover

AbstractThere has recently been considerable research published on the applicability of monitoring systems for improving civil infrastructure management decisions. Less research has been published on the challenges in interpreting the collected data to provide useful information for engineering decision makers. This paper describes some installed monitoring systems on the Hammersmith Flyover, a major bridge located in central London (United Kingdom). The original goals of the deployments were to evaluate the performance of systems for monitoring prestressing tendon wire breaks and to assess the performance of the bearings supporting the bridge piers because visual inspections had indicated evidence of deterioration in both. This paper aims to show that value can be derived from detailed analysis of measurements from a number of different sensors, including acoustic emission monitors, strain, temperature and displacement gauges. Two structural monitoring systems are described, a wired system installed by a...

Stiffness of Clays and Silts: Modeling Considerations

A large database has recently been published that details the development of new empirical expressions for the stiffness reduction with strain of clays and silts. In this note, the same database is used to examine two major considerations for engineers using these expressions in numerical analyses: the transformation from secant to tangent stiffness and the effect of stress history.

Design of Geostructural Systems

AbstractThis paper begins with an extensive review of the literature covering the development of design rules for geostructural systems, beginning with traditional global safety factors and developing through partial factors for loads and resistances, and then considering the use of mobilization factors to limit soil strains. The paper then aims to distinguish two possible functions for geotechnical factors: to compensate for the uncertainty regarding soil strength, and to limit soil deformations that could compromise the associated structure before the soil strength can be fully mobilized, whatever it is. At present, design procedures generally conflate and confuse ultimate limit state (ULS) checks and serviceability limit state (SLS) deformation checks. Furthermore, most geotechnical engineers wrongly associate ULS with soil failure rather than with structural failure. The paper addresses this fundamental confusion by advocating mobilizable strength design (MSD), which is based on assumed soil-structure...

Corrigendum: Strength mobilization in clays and silts

The range of settlements implied by the range of M = 2 in the database (shown in Fig. 10) is calculated as 2.2 to 65.1 mm. At the upper end of this range there would be serviceability concerns for most structures. Increasing M to 3.5 reduces this range to 0.9–25.5 mm. The upper end of this range roughly corresponds to an upper limit on tolerable settlements. This further underlines the finding of the paper that a large mobilization factor is needed for settlement control if no knowledge of soil stress–strain behaviour is considered. The authors apologise to the readers of Canadian Geotechnical Journal for these mistakes in the original paper.

Ranking binder creep performance using the arrb elastometer

This paper discusses a laboratory study used to characterize bituminous binders based on their dynamic creep resistance. Laboratory testing using four different loading regimes on asphalt mixes with six different bituminous binders was undertaken. Creep cycles to 2% accumulated strain were used to define the creep resistance of the asphalt mixes with the various binders. Underlying viscosities of the bitumens were derived using the Australian Road Research Board (ARRB) Elastometer. Marshall stability was measured on the specimens that were prepared using gyratory compaction. Regression plots were prepared that link creep resistance, underlying viscosity, and Marshall stability. It was found that the ARRB Elastometer is able to measure underlying viscosity, which is a reasonable predictor of dynamic creep resistance. Marshall stability was also shown to be a good indicator of dynamic creep resistance. Therefore, simpler tests such as Marshall stability and Elastometer can be used to rank bituminous materials for asphalt mix design purposes in the laboratory.