Alessandro Flora

EVALUATION OF GRAVEL STIFFNESS BY PULSE WAVE TRANSMISSION TESTS

Laboratory techniques for the measurements of very small strain stiffness parameters of coarse-grained materials include: (a) static tests using local strain transducers and (b) wave propagation measurements performed using bender elements, shear plates, and so on. The former method has been employed for most kinds of soil, while the use of wave propagation methods has been restricted to small specimens of sand and clay. At the University of Naples Federico II, a simple device, to generate and monitor compressional and shear waves directly inside a specimen, was designed to be used in large-scale triaxial apparatuses. The testing device was first used on densely compacted gravel specimens in a large triaxial apparatus in Naples and, after some further improvement, also in another large triaxial cell in Tokyo. In the tests carried out in Tokyo, dynamic and static measurements were performed simultaneously on single gravel specimens. In this paper, the details of the new system are presented and the results are discussed. The comparison between stiffness moduli evaluated by dynamic and static measurements shows that density, stress state, and strain history effects are similar, but that the dynamically measured stiffness values are consistently larger, about two times, than the statically determined ones. Since the stiffness of gravels has proven to be strain rate independent, possible reasons for this difference are addressed.

Design of jet grouted excavation bottom plugs

AbstractA methodology for cost-effective design of jet grouted water-sealing bottom plugs is presented in this paper. These massive barriers, made of partially overlapping jet grouted columns, are required to ensure temporary waterproofing to excavation areas and adequate uplift resistance against water loads. Therefore, the proposed calculation procedure simultaneously focuses on the structural performance and continuity of the plug. The design may be optimized by considering the possibility of reducing column length, performing injections only in the lower part of the plug, and leaving the upper part of the plug untreated. The reduction in column length may then result in a very slender slab, and a structural check has to be performed to avoid tension fracturing or overall structural collapse. Additionally, in spite of strict controls, jet grouted columns are never perfectly cylindrical or exactly aligned along their prescribed position, and thus the plug may present imperfections. Statistical evaluatio...

Numerical Analyses of the Effectiveness of Soft Barriers into the Soil for the Mitigation of Seismic Risk

An approach to mitigate the seismic risk of existing structures by means of the creation of a continuous thin layer of grouted soil at a convenient depth is presented. A parametric numerical analysis is reported using different constitutive models with reference to two geometrical schemes. It is shown that if the grouted layer has a stiffness significantly lower than that of the surrounding soil, it may be effective in reducing the seismic demand. In the parametric analyses, the positive role of yielding is also observed, which indicates that the barrier is more effective with larger input amplitudes.

Centrifuge modelling of the seismic performance of soft buried barriers

The paper presents the results of an experimental work carried out in a geotechnical centrifuge at the Schofield Centre of Cambridge University. Two reduced scale models of soft barriers in a sand layer underwent a series of ground shaking. In the first model a thin horizontal layer made of latex balloons filled with a cross-linked gel was created at about mid-height of the sand layer. In the second, the same balloons were deployed to form a V-shaped barrier aimed at isolating a relatively shallow volume of sand. The aim of the study was to get experimental evidence of the capability of such soft barriers to isolate a volume of soil thus reducing amplification of ground motion during severe seismic events. The experimental results were compared with FE numerical analyses of the same models, carried out also in free field to have a benchmark condition. By validating the FE modelling via the comparison with the experimental results, a robust model has been built, aimed at being used for carrying out a wider parametric numerical testing. The experimental results confirm the effectiveness of such soft barriers to reduce amplification in the isolated volumes during seismic events.

Experimental Evidences of the Effect of Fibres in Reinforcing a Sandy Gravel

Studies regarding fibre-reinforcement are restricted to clays, silty and sandy soils. No information is available on gravels. It is worth checking the effect of randomly oriented discrete fibres also on such soils to see if they can be beneficial and to get a better insight into the grain to soil interaction mechanism. In this paper, the effect of a small amount of fibres having high aspect ratio on a sandy gravel was analysed by means of tests carried out in a large triaxial apparatus. Specimens of both natural and fibre-reinforced sandy gravel were prepared by wet tamping at different relative densities, and were tested along monotonic and cyclic stress paths. The results show that the addition of a small amount of fibres causes a slight increase in peak strength and a larger increase in ultimate strength at small confining stress, with an overall more ductile behaviour. The cyclic tests at small confining stress and intermediate strain levels show that, for the lowest applied strain (of the order of 10−2%), the stiffness was larger for the reinforced specimens, with a much sharper decrease at larger strains and final values similar for the reinforced and non-reinforced materials.

Upgrading equipment and procedures for stress path triaxial testing of coarse grained materials

Advanced laboratory testing is often the only way to gain an insight into the mechanical behavior of soils in the whole range from small strains to failure, which in some cases is required for the engineering modeling of complex structures. When such structures are rockfill dams, the use of a large apparatus in-laboratory is virtually unavoidable because of particle size, even though testing becomes far more difficult than usual. This paper presents problems arising from the upgrading of an existing large triaxial cell in order to provide fully automated stress-path control. Because of the large dimensions of the apparatus, implementation of both stress rate and strain rate controls posed a number of unusual problems. Even though the control had to be carefully tuned in order to consider strain-softening behavior, the adopted procedure proved successful. The new cell produced good results for different kinds of stress paths, with satisfactory control in all stages.

Experimental Evidences of the Strengthening of Dredged Sediments by Electroosmotic Consolidation

Nowadays dredged materials are increasingly seen as a resource, and strategies and methodologies for their beneficial reuse are being developed throughout the world. When dealing with fine grained sediments, electro-osmotic treatment is very promising to this aim, being able to speed up dewatering and to remove contaminants. The paper presents the first results of an experimental activity started to get an insight on the ability of electric treatment to strengthen fine grained dredged soils. A special oedometer was adopted to this aim, capable of applying different combinations of mechanical and electric loads to the soil, and large enough to allow the final retrieval of specimens to be tested in a triaxial cell. Both the oedometric and the triaxial test results obtained on a soft dredged material at a very high water content indicate that the application of a low voltage produces a negligible volumetric reduction but a complete change in the mechanical behaviour of the treated soil. Upon electro-osmotic consolidation, the soil—still having a very high void ratio—behaves as being over-consolidated, with a brittle behaviour in the triaxial tests and a large increase of the yielding strength in the oedometric tests. It is argued that such a sharp change in the macro-mechanical response of the treated soil is not related to the amount of water removed but to the change in soil microstructure caused by the removal of part of the electrically bonded water during the electroosmotic process.

Liquefaction Triggering, Consequences, and Mitigation

The importance of predicting ground deformation in loose, saturated granular soils has been widely recognized for a reliable evaluation of liquefaction damage. A procedure is proposed in this paper for the evaluation of post-cyclic consolidation settlements, as a result of volumetric strains induced by the dissipation of excess pore pressure. A stress-based model is first adopted for generating the excess pore water pressure in 1D free-field conditions, allowing for an effective stress analysis according to a loosely coupled approach. Then, the post-cyclic settlement is simply calculated integrating the vertical strains. To this aim, by considering a welldocumented case history in which an extremely small settlement was observed upon seismic excitation, soil stiffness is estimated on the basis of either CPT data or shear stiffness decay curve, to show the effect of modelling hypothesis on the results. Both approaches result into a value of the settlement close to the observed one and much lower than that calculated using a well-established empirical procedure.