Productivity and working costs of modern trench-cutters for the construction of concrete diaphragms in an urban environment

In the excavation of shallow underground works in an urban environment using the ‘cut and cover' method, the choice is often made to use concrete diaphragms in order to support the side walls, before proceeding with the excavation of the ground. When these diaphragms exceed a depth of about 20 m, trench-cutters are generally used to excavate the panels, using a supply of bentonite mud. A remarkable development of trench-cutters has taken place over the last 30 years and these machines today allow panels to be excavated in any type of ground whatsoever, even when it is highly cemented. The experience that has been gained in Turin (Italy) in recent years can be considered interesting, because of the huge number of diaphragms that have been completed and the varying characteristics of the ground in the urban area, which ranges from loose sand and gravel to highly cemented ones. On the basis of detailed analysis of the in situ behaviour of trench-cutters in Turin and of laboratory investigations on the effects of wear on the tools, it has been possible to make a preliminary estimation of the construction costs and the productive times of the concrete diaphragms for the different types of geolog

Technical-operational comparison between trench-cutters and clam excavators for concrete diaphragm construction in underground works at shallow depths

The most common excavation methods for the construction of tunnels in urban areas are the ‘cut and cover' method and the traditional method of excavation at the tunnel face. The first method is usually used for works at shallow depths, even though it frequently involves burdensome deviations and shifting of the existing underground plants and networks. It has been used to realise the Underground Railway Link in Turin (Italy), where excavation was made using trench-cutters and clam excavators. The results of a technical-operational comparison between the two techniques are presented in this work, from which the advantages concerning the use of trench-cutters compared to the use of clamshell bucket excavators have been determine

Origin of the vibrations induced by tunnels’ excavation

In rock excavation (especially with explosives, but also in mechanical excava-tion), the vibrations of the medium where the excavation develops (rock mass) and those of the surrounding air (sound) are of great importance. After a synthetic description of the most common expressions of the “site laws” (charge - distance - vibration intensity correlations), attention is paid to the different types of technical problems where such correlations can be employed, on the basis of experimental cases and of literature data. Examples are presented con-sisting of suitable modifications of the excavation procedure to reduce the problem of vibra-tions. These modifications may concern, even jointly, the reduction of the pull, the increase of the drilling density (with corresponding reduction of the charges of the single holes), the increase of the delay numbers, the modification of the excavation system or, also, the isolation of the volume to be excavated with mechanical pre-cuts. The conclusions provide a straightforward approach to the assessment of charge per delay limits and recommendations are provided

A review of the benefits of electronic detonators

Computerized drilling and the electronic timing of detonations are two technological breakthroughs which have had an important role in updating drilling and blasting excavation methods, although the electronic timing of detonators is still a comparatively infrequent technical solution to precision blasting problems. On the basis of an extensive collection of published cases, this paper reviews the successes achieved and the main expected advantages from the electronic ignition devices. After describing the primary characteristics of these detonators, some elements will be considered, in order to better understand their applications in different conditions, both in open pit and underground sites: extension of the time delay number, freedom in the choice of time intervals between detonations, timing accuracy, reduction of vibrations, control of back-break and fragmentation. The results are compared to those obtained by pyrotechnical timing devices, and summarized in the concluding remarks

ISRM Suggested Method for Determining the Abrasivity of Rock by the CERCHAR Abrasivity Test

Rock abrasivity plays an important role in characterizing a rock material for excavation purposes. Abrasion can be defined as the wearing or tearing away of particles from the surface, i.e. it is a process causing removal or displacement of material at a solid surface, which will lead to wear, especially on tools that are used in mining, drilling, and tunneling applications. The CERCHAR Abrasivity Test is a method to determine an index called CERCHAR Abrasivity Index (CAI) for the rock's abrasivity. The test was originally developed by the Laboratoire du Centre d'Etudes et Recherches des Charbonnages (CERCHAR) de France for coal mining applications (Cerchar 1986). Two standards exist for this test method: the French standard AFNOR NF P 94-430-1 (2000) and ASTM D7625-10 (2010). The test is widely used in research and practice. There are essentially two designs of testing apparatus: the original design as developed at the CERCHAR Centre (Valantin 1973) and a modified design as reported by West (1989). While the designs are similar there are some important differences as well as ambiguities in test conditions that include equipment actuation, material properties of the stylus and sample preparation as summarized by Plinninger et al. (2003

Bentonite in two-component grout applications

Two-component grout is a cement-based material, currently the most used technology for backfilling in tunnelling applications. Despite its intensive, knowledge on this material is quite limited, especially as concern the role of ingredients and their effect on the properties of fresh mortar and hardened grout. In this work, an accurate and innovative test campaign focused on the role of the bentonite was performed. Three different bentonites were used. The activation of the bentonite and its effect on both mortar stability and grout strength was investigated with the purpose to recognise the bentonite parameters useful to select, at the design stage, the best bentonite according to the designers’ requirements. Swell index and Atterberg's liquid limit were recognised as useful parameters for predicting results in terms of suitable bleeding and surface compression strength

Experiences from Experimental Mining in Brazil

The Experimental Mine (EM) of the Research Center of Responsible Mining of the University of São Paulo became the subject of investigation in a few years after its development ; it is an open-pit quarry currently exploiting marble and gneiss , used to produce industrial limestone and construction aggregates. It is a developing enterprise, dealing with the challenges of a technological upgrade from a small -scale operation to the characteristics of a medium -sized company. The Experimental Mine Project (EMP) was born to attend a double demand: to provide research and development (R&D ) support to a growing company and to provide experimental opportunity for a field of knowledge such as mining engineering that requires a large scale for its experiments. The main challenges of the EMP are related with the remaining small -scale mining features, such as large variety of equipment available, high level of operational flexibility, scarcity or absence of mine planning, being focused on daily operations. In such an environment, the first role of the EMP was to evaluate in a quantitative way the effects of unit operations over the whole mining process. The current excavation technique is by drilling and blasting. Many experimental campaigns have been conducted on site, with different purposes. One of the main research lines was to increase the productivity of the quarry by lowering production costs and improving the quality of the product, then optimizing the entire production cycle; the relationship between the unit costs of drilling and explosives were evaluated, as well as the link between the blast design and some factors affecting the downstream processing of the product. The paper describes the methods employed to conduct the research and the improvements to be pursued, with the due consideration to the influence and interference of the many parameters involved, from the rock-mass characteristics to the final products

Analysis of predictor equations for determining the blast-induced vibration in rock blasting

The paper proposes a new empirical correlation designed to complement the "site laws" currently used to evaluate the attenuation in the rock masses of vibrations induced by rock blasting. The formula contains a deformed exponential known as the K-exponential, which seems to well represent a large number of both natural and artificial phenomena ranging from astrophysics to quantum mechanics, with some extension in the field of economics and finance. Experimental validation of the formula was performed via the analysis of vibration data covering a number of case studies, which differed in terms of both operation and rock type. A total of 12 experimental cases were analysed and the proposed formulation exhibited a good performance in 11 of them. In particular, the proposed law, which was built using blast test data, produced very good approximations of the points representing the vibration measurements and would thus be useful in organising production blasts. However, the developed formula was found to work less well when a correlation obtained for a given site was applied to another presenting similar types of rocks and operations, and thus should not be employed in the absence of measurements from test data. Keywords: Rock blasting, Vibrations, Predictor equation, Site law, K-statistics, K-exponentia

Performance analysis of parallel hole-cuts based on dimensionless ratios

For over 150 years, a wide array of parallel hole-cut patterns for tunnel driving has been designed, tested, and (successfully) used. The geometrical aspect of the parallel cut problem is crucial, and the paper, in its first part, meticulously analyses the correlations linking dimensionless ratios (characterising the patterns) to performance indicators. This rigorous and comprehensive procedure is the most logical way to derive practical general-purpose design rules from the vast array of successfully used patterns. The statistical basis for this analysis is derived from literature and directly observed cases, further enhancing the reliability of the proposed rules. The second part of the paper delves into the sensitivity of pattern performance to drilling precision, a critical factor in pattern selection. Drawing from literature and observed cases, the paper proposes tolerance standards for drilling machinery related to drilling pattern features and highlights their immediate practical relevance. These standards, when implemented, can significantly improve the efficiency and effectiveness of tunnel excavation, demonstrating the practical relevance of this research. The paper succinctly summarizes the investigation’s key findings in the concluding remarks. It highlights the general guidelines and correlations that have emerged from the analysis. These insights guide future tunnel excavation projects, providing a direction for further research and practical application

The Music of Blasting

A common adage in the Explosives Industry goes by saying that "Blasting is not bombing". One of the key differences between the two employs of explosive energy lays in the same gap existing between the application of acoustic energy that differentiates noise from music: timing and the distribution of energization in time. While timing in blasting is widely accepted to influence blast induced vibrations, it is still not completely investigated when related to rock fragmentation and downstream benefits. This paper shows a research about this topic, developedon two phases: 1) test blasts at the Experimental Mine of the Research Center of Responsible Mining of the University of São Paulo; 2) development project for large-scale production blasts in an open-cast mine. The first phase of the research was performed attempting to increase the productivity of the experimental mine, by lowering production costs and improving the quality of the product. Some Key Performance Indicators (KPIs) were established to monitor the results. A new blast design method and a more appropriate initiation sequence were designed according to the principles of: i) decomposition of the blast; ii) taking advantage of the free surfaces to favor the movement of the blasted material; iii) simultaneous holes firing as far away as possible, to avoid undesired cooperation of charges that may induce the explosive energy to work with shear effect instead of producing fragmentation. The results show that the proper selection of delay timing leads to significant benefits for rock fragmentation, downstream processes and the quality of final walls. The second phase of the study was a research and development (R&D) project in an open-cast mine with the goal to achieve an average P80 of 300 mm (11,8”) in the run-of-mine (ROM) product without altering the existing budget. The project included several variables in the blast design that were not previously taken into account, such as the orientation of natural joint sets in the rock mass, specific energy of the explosive and firing sequence. The new blast design method considered the directions of natural joint sets and determined the drilling pattern and the firing sequence accordingly to favor the movement of the blasted rock along its preferential direction, to reduce its confinement. At the end of the project, it was achieved an average P80 of 304 mm (12”) in the ROM, 50% lower than the one at the beginning of the project. The final blast of the project showed a reduction of 3% of the drill and blast cost, employing the same powder factor and the same drill pattern size used at the beginning of the project. The results of this study show how blast performance is related to variables that are not contemplated in the most common design methods or fragmentation models: the firing sequence, the degree of freedom and the direction of movement in the blast