Showing papers by "Rodrigo Salgado published in 2006"

Assessment of Variable Uncertainties for Reliability-Based Design of Foundations

Abstract: LRFD shows promise as a viable alternative to the present working stress design (WSD) approach to shallow foundation design. The key improvements of LRFD over the traditional WSD are the ability to provide a more consistent level of reliability and the possibility of accounting for load and resistance uncertainties separately. For LRFD to gain acceptance in geotechnical engineering, a framework for the objective assessment of resistance factors is needed. Such a framework, based on reliability analysis, is proposed in this paper. Probability density functions (PDFs), representing design variable uncertainties, are required for analysis. A systematic approach to the selection of PDFs is presented. A procedure such as that proposed provides a rational probabilistic basis for the development of LRFD methods in geotechnical engineering.

Resistance Factors for Use in Shallow Foundation LRFD

Abstract: In shallow foundation design, the key improvements offered by LRFD over the traditional working stress design (WSD) are the ability to provide a more consistent level of reliability between different designs and the possibility of accounting for load and resistance uncertainties separately. In the development of LRFD, a framework for the objective, logical assessment of resistance factors is needed. Additionally, in order for LRFD to fulfill its promise for designs with more consistent reliability, the methods used to execute a design must be consistent with the methods assumed in the development of the LRFD factors. In this paper, a methodology for the estimation of soil parameters for use in design equations is proposed that should allow for more statistical consistency in design inputs than is possible in traditional methods. Resistance factors for ultimate bearing capacity are computed using reliability analysis for shallow foundations both in sand and in clay, with input parameters obtained from both the cone penetration test and the standard penetration test, and for both ASCE-7 2000 and AASHTO 1998 load factors. Resistance factor values are dependent upon the values of load factors used. Thus, a method to adjust the resistance factors to account for code-specified load factors is also presented.

Determination of Ash Mixture Properties and Construction of Test Embankment - Part A

Abstract: Class F fly ash and bottom ash are solid residue byproducts produced by coal-burning plants. They are usually disposed off with a typical disposal rate of 80 % fly ash and 20 % bottom ash. To maximize the use of the coal ash, and thus significantly reduce the disposal problem that electric utility companies and our society in general face, the direct use of ponded or landfilled ash that is composed of high proportions of fly ash would be desirable. However, a general understanding of the behavior of high volume fly ash mixtures is needed. Although there have been investigations into the properties of separated single types of ash, the studies of the fly/bottom ash mixtures, especially with high fly ash contents, are very limited. Representative samples of class F fly ash and bottom ash were collected from two utility plants in Indiana and a series of extensive laboratory tests were conducted to obtain the mechanical properties of the mixture. Since the mechanical properties of ash mixtures are dependent on the mixture proportions, the investigations evaluate fly/bottom ash mixtures with different mixture ratios (with Fly ash contents of 50%, 75%, and 100%). The results obtained are merged with other considerations relevant to embankment design and construction and used to develop guidelines on coal ash utilization in highway embankments. In order to examine suitable fly/bottom ash mixture compositions and embankment geometries, slope stability analyses were performed on ash embankments with different geometries using the different properties of the ash mixtures with different mixture ratios. The limit equilibrium method was used for the stability analyses. Additionally, the corrosion potential to metal structures, which are commonly included in highway construction, is examined by performing corrosivity tests on the ash mixtures.

Interpretation of Cone Penetration Tests in Cohesive Soils

Abstract: This report focuses on the evaluation of the factors affecting cone resistance measurement during cone penetration in saturated clayey soils and the application of the result to pile shaft capacity analysis. In particular, effects of drainage conditions around the cone tip were studied. Rate effects related to both drainage and shear strength dependence on loading rate were studied. In order to investigate the effects of drainage during penetration, penetration tests were performed with various velocities in the field and in a calibration chamber, and the obtained data were analyzed. For the field tests, two sites which have homogeneous clayey soil layers below the groundwater table were selected, and CPTs were performed with various penetration rates. Penetration tests in the calibration chamber were performed to investigate the transition points between undrained and partially drained, partially drained and fully drained conditions based on cone penetration rate and the coefficient of consolidation. A series of flexible-wall permeameter tests were conducted for various mixing ratios of clays and sands to obtain values of the coefficient of consolidation, which is a key variable in determining the drainage state during cone penetration. Nine piezocone penetration tests were conducted at different rates in calibration chamber specimen P1 (mixture of 25 % clay and 75 % sand) and eight penetration tests were carried out in calibration chamber specimen P2 (mixture of 18 % clay and 82 % sand). From the results of the penetration tests in the calibration chamber, a cone resistance backbone curve, with qc plotted versus normalized penetration rate, was established. Guidelines were proposed for when to interpret CPT tests, whether in estimating soil properties or in estimating pile resistances, in soils for which penetration takes place under conditions that cannot be established as either drained or undrained a priori.

Closure to “Estimation of Bearing Capacity of Circular Footings on Sands Based on Cone Penetration Test” by Junhwan Lee and Rodrigo Salgado

Abstract: The authors acknowledge the observations of discussers Decourt and Rodriguez-Roa regarding the adequacy of the bearing capacity equations used in the research. The authors cite the differing views of the discussers, pointing to the concern raised by discusser Rodriguez-Roa that the wide variance in results might serve to entirely discredit the usefulness of the bearing capacity equation; while noting the point of discusser Decourt, who, citing others in agreement, believes that discrediting the bearing capacity equation is precisely the point. However, the authors, themselves, state the belief that the bearing capacity equation, as seen in the forms emerging from current research, should be embraced. The authors add that the purpose of the bearing capacity equation is to calculate the limit or plunging load of the footing. They claim to have not used it beyond its designed purpose. As for discusser Decourt's point that the bearing capacity equation (the so-called triple N formula) is wrong, the authors explain that the equation, initially proposed for the indentation of metals, has evolved over time to include more realistic geotechnical and foundation engineering problems (i.e. shape and depth, along with base, ground, and load inclinations). In its present form, the authors note the bearing capacity equation has high empirical content, despite being derived from rigorous analysis at the beginning. Addressing discusser Rodriguez-Roa's point about using experimental data to determine the validity of the bearing capacity equation, the authors believe that the final solution to the question must first come from establishing a proper analytical basis, as explored by the authors (e.g., Salgado et al. 2004, Lyamin et al. 2006, Martin 2005). These examples rigorously calculate limit loads and then follow this with proper validation against well-performed, well-instrumented load tests on properly and completely characterized soils. The authors note that the bearing capacity equation, even its traditional forms, was never designed to compute ultimate loads defined on a basis of a specific settlement value. The discussers agree with discusser Decourt about the bearing capacity formula having limitations in its present form. At the same time, the authors also agree with discusser Rodriguez-Roa about retaining the concept of the bearing capacity equation as a tool for geotechnical and foundation engineers. The authors predict that practitioners in the near future will be provided with both an extremely useful and nearly exact version of the bearing capacity formula; one that will have a small, precisely known level of uncertainty.