In-service structural health monitoring (SHM) of engineering structures has assumed a significant role in assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, in situ, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field. In this article, the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed, highlighting the areas where further work is needed. A few packaging schemes for FBG strain sensors are also discussed. Finally a few limitations and market barriers associated with the use of these sensors have been addressed.

Fibre Bragg gratings in structural health monitoring—Present status and applications

The properties of clean sands pertaining to shear strength and stiffness have been studied extensively. However, natural sands generally contain significant amounts of silt and/or clay. The mechanical response of such soils is different from that of clean sands. This paper addresses the effects of nonplastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5–20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.

Shear strength and stiffness of silty sand

This paper describes recent advances in stability analysis that combine the limit theorems of classical plasticity with finite elements to give rigorous upper and lower bounds on the failure load. These methods, known as finite-element limit analysis, do not require assumptions to be made about the mode of failure, and use only simple strength parameters that are familiar to geotechnical engineers. The bounding properties of the solutions are invaluable in practice, and enable accurate limit loads to be obtained through the use of an exact error estimate and automatic adaptive meshing procedures. The methods are very general, and can deal with heterogeneous soil profiles, anisotropic strength characteristics, fissured soils, discontinuities, complicated boundary conditions, and complex loading in both two and three dimensions. A new development, which incorporates pore water pressures in finite-element limit analysis, is also described. Following a brief outline of the new techniques, stability solutions ...

Geotechnical stability analysis

Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon empirical correlations. Improvements have been made in identifying the processes that occur within the critical zone of soil immediately surrounding the pile, but quantification of the changes in stress and fabric is not straightforward. This paper addresses the degree of confidence we can now place (a) on the conceptual and analytical frameworks for estimating pile capacity, and (b) on the quantitative parameters required to achieve a design. The discussion is restricted to driven piles in clays and siliceous sands, with particular attention given to extrapolating from design approaches derived for closed-ended piles of relatively small diameter to the large-diameter open-ended piles that are used routinely in the offshore industry. From a practical viewpoint, we need design approaches that minimise sensitivity...

Science and empiricism in pile foundation design

Tire shreds and tire shred  soil mixtures can be used as alternative backfill material in many geotechnical applications. The reuse of tire shreds may not only address growing environmental and ec...

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Behaviour of tire shred - sand mixtures

The growing interest in utilizing waste materials in civil engineering applications has opened the possibility of constructing reinforced soil structures with unconventional backfills. Scrap tires are a high-profile waste material for which several uses have been studied, including the use of shredded tires as backfill. A triaxial testing program was conducted to investigate the stress-strain relationship and strength of tire chips and a mixture of sand and tire chips. The test results and additional information from the literature were used in the numerical modeling of wall backfills, both unreinforced and reinforced with geosynthetics. The numerical modeling results suggest tire shreds, particularly when mixed with sand, may be effectively used as backfill.

Shredded Tires and Rubber-Sand as Lightweight Backfill

Class F fly ash and bottom ash are the solid residue byproducts produced by coal-burning electric utilities. They are usually disposed of together as a waste in utility disposal sites with a typical disposal rate of 80% fly ash and 20% bottom ash. Direct use of these materials in construction projects consuming large volumes of materials, such as highway embankment construction, not only provides a promising solution to the disposal problem, but also an economic alternative to the use of traditional materials. Representative samples of class F fly and bottom ash were collected from two utility power plants in Indiana and tested for their mechanical properties (compaction, permeability, strength, stiffness, and compressibility). Three mixtures of fly and bottom ash with different mixture ratios (i.e., 50, 75, and 100% fly ash content by weight) were prepared for testing. Test results indicated that ash mixtures compare favorably with conventional granular materials.

Geotechnical Properties of Fly and Bottom Ash Mixtures for Use in Highway Embankments

A theory, based on cavity expansion and stress rotation analyses, is developed for computing the cone penetration resistance of sand. The sand is modeled as a nonlinear elastic-plastic material. The theory can be used for analysis of calibration chamber tests, as it takes full account of chamber size and boundary conditions. The theory was used to calculate values of cone penetration resistance for the conditions corresponding to each of about 400 cone penetration tests performed in different calibration chambers. The chamber diameters range from 0.76 to 1.20 m and the cone penetrometer diameters from 1 to 3.57 cm. The samples were prepared of different sands at different densities and stresses. The agreement between theoretical and experimental values was better than ±30% in most cases.

Cavity Expansion and Penetration Resistance in Sand

This paper compares conventional limit-equilibrium results with rigorous upper and lower bound solutions for the stability of simple earth slopes. The bounding solutions presented in this paper are obtained by using two newly developed numerical procedures that are based on finite-element formulations of the bound theorems of limit analysis and linear programming techniques. Although limit-equilibrium analysis is used widely in practice for estimating the stability of slopes, its use may sometimes lead to significant errors as both kinematic and static admissibility are violated in the method. Because there are no exact solutions available against which the results of limit-equilibrium analysis can be checked, the present bounding solutions can be used to benchmark the results of limit-equilibrium analysis. In addition, the lower bound solutions obtained also can be applied directly in practice because they are inherently conservative. purely cohesive and cohesive-frictional soils. These numerical procedures are based on finite-element formulations of the bound theorems of limit analysis and the best lower and upper bound solutions are obtained, respectively, by optimizing stat­ ically admissible stress fields and kinematically admissible ve­ locity fields using linear programming techniques. Over the years, many limit-equilibrium computer codes have been developed to locate the most critical failure surface by using various search strategies. In this study, the limit-equi­ librium results are obtained from the well-known slope stabil­ ity computer code STABL with the option of Bishop's sim­ plified method (Kim et al. 1997).

Rodrigo Salgado

Papers

Shear strength and stiffness of silty sand

Shredded Tires and Rubber-Sand as Lightweight Backfill

Geotechnical Properties of Fly and Bottom Ash Mixtures for Use in Highway Embankments

Cavity Expansion and Penetration Resistance in Sand

Limit Analysis versus Limit Equilibrium for Slope Stability