A review of the tensile strength of rock was conducted to determine the relationship between direct tensile strength (DTS) and Brazilian tensile strength (BTS) and to examine the validity of ...estimating tensile strength from other measured properties, such as the crack initiation (CI) threshold. A data set was gathered from the existing literature where tensile values could be reliably correlated with unconfined compressive strength or CI values. It was determined that the BTS obtained in standard testing is generally greater than the equivalent DTS and that this relationship is rock type dependent. CI yields a reasonable estimate of tensile strength and this correlation is improved when the BTS values are reduced to DTS values by rock type specific correlations. The factor
f
, in DTS =
f
BTS, can be considered to be approximately 0.9 for metamorphic, 0.8 for igneous and 0.7 for sedimentary rocks. The relationships presented demonstrate that there is wide scatter in the available data for estimating tensile strength likely due to both specimen variability and testing configuration, including platen geometry and relative stiffness. Estimates of tensile strength should only be used for preliminary design purposes and measurements should be made to confirm preliminary assumptions for each design.
Empirical estimation of rock mass modulus Hoek, E.; Diederichs, M.S.
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
02/2006, Letnik:
43, Številka:
2
Journal Article
Recenzirano
The deformation modulus of a rock mass is an important input parameter in any analysis of rock mass behaviour that includes deformations. Field tests to determine this parameter directly are time ...consuming, expensive and the reliability of the results of these tests is sometimes questionable. Consequently, several authors have proposed empirical relationships for estimating the value of rock mass deformation modulus on the basis of classification schemes. These relationships are reviewed and their limitations are discussed. Based on data from a large number of in situ measurements from China and Taiwan a new relationship, based upon a sigmoid function, is proposed. The properties of the intact rock as well as the effects of disturbance due to blast damage and/or stress relaxation are also included in this new relationship.
During the construction of an underground excavation, damage occurs in the surrounding rock massdue in large part to stress changes. While the predicted damage extent impacts profile selection ...andsupport design, the depth of damage is a critical aspect for the design of permeability sensitive excavations,such as a deep geological repository （DGR） for nuclear waste. Review of literature regardingthe depth of excavation damage zones （EDZs） indicates three zones are common and typically relatedto stress induced damage. Based on past developments related to brittle damage prediction usingcontinuum modelling, the depth of the EDZs has been examined numerically. One method to capturestress induced damage in conventional engineering software is the damage initiation and spallinglimit （DISL） approach. The variability of depths predicted using the DISL approach has been evaluatedand guidelines are suggested for determining the depth of the EDZs around circular excavations inbrittle rock masses. Of the inputs evaluated, it was found that the tensile strength produces thegreatest variation in the depth of the EDZs. The results were evaluated statistically to determine thebest fit relation between the model inputs and the depth of the EDZs. The best correlation and leastvariation were found for the outer EDZ and the highly damaged zone （HDZ） showed the greatestvariation. Predictive equations for different EDZs have been suggested and the maximum numericalEDZ depths, represented by the 68% prediction interval, agreed well with the empirical evidence. Thissuggests that the numerical limits can be used for preliminary depth prediction of the EDZs in brittlerock for circular excavations.
Abstract Radio-frequency particle accelerators are engines of discovery, powering high-energy physics and photon science, but are also large and expensive due to their limited accelerating fields. ...Plasma-wakefield accelerators (PWFAs) provide orders-of-magnitude stronger fields in the charge-density wave behind a particle bunch travelling in a plasma, promising particle accelerators of greatly reduced size and cost. However, PWFAs can easily degrade the beam quality of the bunches they accelerate. Emittance, which determines how tightly beams can be focused, is a critical beam quality in for instance colliders and free-electron lasers, but is particularly prone to degradation. We demonstrate, for the first time, emittance preservation in a high-gradient and high-efficiency PWFA while simultaneously preserving charge and energy spread. This establishes that PWFAs can accelerate without degradation—an essential step toward energy boosters in photon science and multistage facilities for compact high-energy particle colliders.
Acceleration of positron beams in plasma-based accelerators is a highly challenging task. To realize a plasma-based linear collider, acceleration of a positron bunch with high-efficiency is required, ...while maintaining both a low emittance and a subpercent-level energy spread. Recently, a plasma-based positron acceleration scheme was proposed in which a wake suitable for the acceleration and transport of positrons is produced in a plasma column by means of an electron drive beam Diederichs et al., Phys. Rev. Accel. Beams 22, 081301 (2019). In this article, we present a study of beam loading for a positron beam in this type of wake. We demonstrate via particle-in-cell simulations that acceleration of high-quality positron beams is possible, and we discuss a possible path to achieve collider-relevant parameters.
A finite radius plasma is proposed to generate wakefields that can focus and accelerate positron beams in a plasma wakefield accelerator. The finite radius plasma reduces the restoring force acting ...on the plasma electrons forming the plasma wakefield, resulting in an elongation of the on-axis return point of the electrons and, hence, creating a long, high-density electron filament. This results in a region with accelerating and focusing fields for positrons, allowing for the acceleration and quality-preserving transport of high-charge positron beams.
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