Modelling and design of pentice protective structures to resist high-speed projectile impacts

This paper presents the results of a research study carried out to investigate the performance of the pentice structure at an underground mine in NSW during the extension and equipping of a haulage shaft by Macmahon under high-speed impact loading caused by the potential projectiles falling from the surface. This assessment will allow the structure to comply with AS 3785.5 for “Headframes” (Australian Standard, 1998). The pentice structure is installed 1000 m below the surface in a 4.268 m diameter shaft at the considered underground mine. The objective of the pentice is to allow Macmahon construction and shaft sinking crews to work in the shaft without any risk to their safety. The pentice structure includes a number of steel boxes 1-m high that are filled with high yielding foaming grout Tekseal from Minova Australia Pty Ltd. The major aim of this investigation is to evaluate the capacity of the existing pentice to resist high-velocity impacts and to develop a high-performance protective system which is capable of absorbing energy and terminating large projectiles falling from a height of 1000 m. High-fidelity physics based finite element models for the mine pentice were developed to find a satisfactory solution to protect workers 1000 m below the surface from potential falling projectiles. It is established that the existing level 11 pentice structure is not capable of stopping the projectiles dropping from a height of 1000 m. Several high-performance protective solutions for strengthening the pentice against impact loads were proposed and evaluated numerically. As the final design, two-level protection is designed that includes the 9 level and 11 level pentice protective structures. The models of the 9 level pentice and the 11 level pentice are evaluated for the relevant impact loads. It is found that the new 9 level pentice requires an additional layer of the railway concrete sleepers along with the high-strength steel cover plate to provide adequate protection and terminate the falling projectiles. The supporting frames for the 9 level and 11 level pentice structures are designed using the dynamic reaction forces transferred from the pentice boxes. The developed two-level pentice protective system has proven to provide high level of protection against high-speed falling projectiles for workers performing shaft sinking duties in the shaft below

Optimisation of the bolt profile configuration for load transfer enhancement

Both bolt profile shape and profile spacing (rib spacing) have been found to influence the bonding capacity of the grouted rock bolt. The bolt surface profile configuration has greater importance to rock bolt than the steel rebar used in civil engineering construction, because the rock bolt is subjected to greater dynamic loading than the steel rebar. The increased bonding capacity of bolts is important when supported ground is either heavily fractured, faulted or the supported ground is of soft formation, typically that of coal measure rocks. Past laboratory studies have identified the bolt profile spacing as of significant relevance to bolt resin rock bonding increase, however, no attempt has been made to determine the optimum spacing between the bolt profiles spacing. Accordingly, a series of laboratory tests were carried out on 22 core diameter bolts installed in cylindrical steel sleeve. The study was carried out by both push and pull testing. The push testing was carried out in 150 mm long sleeves while the pull testing was made in 115 mm long sleeves. Profile spacing tested include, 12.5, 25.0mm, 37.5 mm and 50 mm lengths. The profile spacing of 37.5 mm wide was found to provide the optimum bearin

Reliability-based conversion of a structural design code for railway prestressed concrete sleepers

Abstract: Ballasted railway track is very suitable for heavy-rail networks because of its many superior advantages in design, construction, short-and long-term maintenance, sustainability, and life cycle cost. An important part of the railway track system, which distributes the wheel load to the formation, is the railway sleeper. Improved knowledge has raised concerns about design techniques for prestressed concrete (PC) sleepers. Most current design codes for these rely on allowable stresses and material strength reductions. However, premature cracking of PC sleepers has been found in railway tracks. The major cause of cracking is the infrequent but high-magnitude wheel loads produced by the small percentage of irregular wheels or rail-head surface defects; both these are crudely accounted for in the allowable stress design method by a single load factor. The current design philosophy, outlined in Australian Standard AS1085.14, is based on the assessment of permissible stresses resulting from quasi-static wheel loads and essentially the static response of PC sleepers. To shift the conventional methodology to a more rational design method that involves a more realistic dynamic response of PC sleepers and performance-based design methodology, comprehensive studies of the loading conditions, the dynamic response, and the dynamic resistance of PC sleepers have been conducted. This collaborative research between several Australian universities has addressed such important issues as the spectrum and the amplitudes of dynamic forces applied to the railway track, evaluation of the reserve capacity of typical PC sleepers designed to AS 1085.14, and the development of a new limit states design concept. This article presents the results of the extensive analytical and experimental investigations aimed at predicting wheel impact loads at different return periods (based on field data from impact detectors), together with an experimental investigation of the ultimate impact resistance of PC sleepers required by the limit states design approach. It highlights the reliability approach and rationales associated with the development of limit states and presents guidelines pertaining to conversion of AS 1085.14 to a limit states design format. The reliability concept provides design flexibility and broadens the design principle, so that any operational condition could be catered for optimally in the design

Static and dynamic behaviours of railway prestressed concrete sleepers with longitudinal through hole

As the crosstie beam in railway track systems, the prestressed concrete sleepers (or railroad ties) are principally designed in order to carry wheel loads from the rails to the ground. Their design takes into account static and dynamic loading conditions. It is evident that prestressed concrete has played a significant role as to maintain the high endurance of the sleepers under low to moderate repeated impact loads. In spite of the most common use of the prestressed concrete sleepers in railway tracks, there have always been many demands from rail engineers to improve serviceability and functionality of concrete sleepers. For example, signalling, fibre optic, equipment cables are often damaged either by ballast corners or by tamping machine. There has been a need to re-design concrete sleeper to cater cables internally so that they would not experience detrimental or harsh environments. Accordingly, this study will investigate the effects of through hole or longitudinal hole on static and dynamic behaviours of concrete sleepers under rail shock loading. The modified compression field theory for ultimate strength design of concrete sleepers will be highlighted in this study. The outcome of this study will enable the new design and calculation methods for prestressed concrete sleepers with holes and web opening that practically benefits civil, track and structural engineers in railway industry

Angle of Incidence Effects on Far-Field Positive and Negative Phase Blast Parameters

The blast overpressure acting on a rigid target is known to vary between the normally reflected overpressure and the incident overpressure as a function of the angle between the target and the direction of travel of the blast wave. Literature guidance for determining the exact effects of angle of incidence are unclear, particularly when considering the negative phase. This paper presents the results from a series of well controlled experiments where pressure transducers are used to record the pressure-time history acting on the face of a large, rigid target at various angles of incidence for varying sizes of hemispherical PE4 charge and stand-off distances. The test data demonstrated remarkable repeatability, and excellent agreement with semi-empirical predictions for normally reflected overpressures. The oblique results show that peak overpressure, impulse and duration are highly dependent on angle of incidence for the positive phase, and are invariant of angle of incidence for the negative phase

Hybrid double-skin tubular members for sustainable mining infrastructure

Hybrid FRP-concrete-steel double-skin tubular members (hybrid DSTMs) are a new form of hybrid structural members. A hybrid DSTM consists of an outer tube made of fibre reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. The two tubes may be concentrically placed to produce a section form more suitable for compression members, or eccentrically placed to produce a section form more suitable for flexural members. In hybrid DSTMs, the three constituent materials are optimally combined to achieve several advantages not available with existing structural members, including their excellent corrosion resistance and energy-dissipation capacity. Hybrid DSTMs are therefore a sustainable alternative to existing structural components, especially for use in structures which are likely to be exposed to a harsh environment. This paper explains the rationale and advantages of this new form of structural members, presents an overview of existing and ongoing research on their structural behaviour and design, and discusses their potential applications in mining infrastructure