Mark Talesnick

The elastic deformability and strength of a high porosity, anisotropic chalk

The submission explores the mechanical behavior of a very porous chalk formation, in which a system of ancient caverns was excavated. Incidents of general and localized failure of these ancient caverns initiated a comprehensive laboratory testing program aimed at investigating the anisotropic nature of the stress–strain response and strength of the material. It was felt that these aspects could be of profound importance in the stability of the cavern systems. The effect of water content over a broad range from 1.5% to saturation, on the compressive and tensile strength was also studied. Testing was based on the hollow cylinder methodology and was supplemented with uniaxial compression of solid cylinders and diametric compression of Brazilian disks. Use of the hollow cylinder methodology was extended to failure conditions. Test results illustrate the anisotropic nature of the stress–strain response of the chalk. The material clearly displays transverse isotropy, with horizontal bedding planes corresponding to the plane of material symmetry. The modulus of deformation within the plane of material symmetry is significantly higher than that perpendicular to bedding planes. Torsional shear of hollow cylinder specimens was employed to measure the shear modulus of the chalk. The testing carried out up to failure illustrated the anisotropy of the chalk strength. The compressive strength was found to be 50% higher in compression parallel to bedding than perpendicular to bedding. Increasing water content was found to have a consistent detrimental effect on compressive strength, tensile strength and material stiffness. The most drastic effect was found due to relatively small increases in water content, at initial water contents of less than 5%. Anisotropy of the chalk strength was found to persist over the entire range of water contents considered.

Continuous and discontinuous stability analysis of the bell-shaped caverns at Bet Guvrin, Israel

Abstract The stability of two systems of bell-shaped caverns excavated some 1000 years ago at Bet Guvrin National Park is investigated. The caverns were excavated in a weak, anisotropic, and moderately discontinuous chalk. The cavern stability is considered based on two separate and independent methods: a continuum model framework—FLAC, used for stress analysis, and a discontinuous approach—block theory, used for critical key block analysis. The numerical stress analysis reveals that in the case of very large span openings, tensile fracture of intact rock may be responsible for instabilities, which may lead to global failure. Evidence of tensile rupture at margins of failed caverns is abundant at the Park. The discontinuous block theory analysis reveals that the moderate joint set spacing at Bet Guvrin, up to 45% of the roof area may be comprised of removable blocks. The removable keyblocks in the roof remain in place due to arching stresses, which develop through the roof material. The chalk at the roof can sustain the maximum loads in existing caverns, as predicted by the numerical stress analysis. However, local failures due to exceedingly high compressive stresses at the abutments or by tensile fracture at the roof, may lead to relaxation of arching stresses followed by keyblock displacement. Such a “mixed failure mode” process could eventually lead, over time, to global collapse. Indications that “mixed failure mode” processes are presently active in the studied caverns are substantiated by in-situ measurement of keyblock displacements. It is suggested that in weak and discontinuous rock environments where “mixed failure mode” processes may be active, long term stability evaluation should be based on both continuous and discontinuous stability analyses.

Completing the hollow cylinder methodology for testing of transversely isotropic rocks: torsion testing

Abstract The paper presents the completion of the hollow cylinder methodology in the determination of elastic parameters for the mechanical description of transverse isotropic rocks. The application of a torsional stress to hollow cylinder specimens of transversely isotropic rock allows the direct determination of the shear modulus. Previously presented studies have shown how uniaxial compression and radial compression stress conditions, when applied to hollow cylinder specimens, can be used in the determination of the other four required material elastic parameters. A short description of the design and capabilities of the testing apparatus is given. Testing of an isotropic, aircraft aluminum has illustrated the reliability of the experimental system, procedures and methodology. Tests on several rock types have led to an improved empirical relationship for the approximation of the shear modulus of transverse isotropic rocks when no direct measurements are available. Torsion of a highly micro-cracked granite has illustrated the effect of small normal stress on the development of volumetric strains, and nonlinear stress–strain behavior resulting from small shear stresses.

Analysis of seismically triggered slides off Israel

Evidence of large submarine slides, presumably seismically triggered, is evident on the Israeli continental slope. This article presents a study of the earthquake characteristics which were likely to have caused these slides. Four seismic records with different characteristics were used as input, and propagation and response analyses were carried out. The results of these analyses suggest that the slides were caused by one or more large magnitude events resulting in low frequency tremors propagating through the slope profile. It is likely that these earthquakes were located on the Jordan Rift.

Failure of a flexible pipe with a concrete liner

Abstract This study documents the functional failure of a concrete lined steel sewage pipe. Symptoms of the pipe failure are presented. Failure of the pipe system can be attributed to incompatibility between the mechanical behavior of the pipe and the methodology employed in its design. The underlying cause of the failure may be traced to a lack of sufficient backfill stiffness. In situ testing was used to evaluate the stiffness of the side backfill. The existing pipe–trench system condition was analysed numerically and a criterion developed for the consideration of the structural integrity of the pipeline.

Compatibility of different methodologies for the determination of elastic parameters of intact anisotropic rocks

Abstract The importance of accounting for the mechanical anisotropy of rock in rock engineering has been thoroughly demonstrated. A standard method for determining the required parameters for the constitutive description of anisotropic rocks is less thoroughly accepted. Three different testing methodologies have been carried out on three different materials in an attempt to consider the compatibility of the experimental results. The testing methodologies included uniaxial compression testing, diametrical loading of Brazilian disks and hollow cylinder testing. The material tested included an isotropic aluminum, a relatively stiff bedded sandstone and a soft porous homogeneous nonlinear chalk. The test results and analysis clearly illustrate that the different testing methodologies result in very similar deformation parameters. The testing procedures also illustrate that material nonlinearity can also be anisotropic.

FAILURE OF A FLEXIBLE PIPE WITH A CONCRETE LINER

This study documents the functional failure of a concrete lined steel sewage pipe. Symptoms of the pipe failure are presented. Failure of the pipe system can be attributed to incompatibility between the mechanical behavior of the pipe and the methodology employed in its design. The underlying cause of the failure may be traced to a lack of sufficient backfill stiffness. In situ testing was used to evaluate the stiffness of the side backfill. The existing pipe-trench system condition was analysed numerically and a criterion developed for the consideration of the structural integrity of the pipeline.