Shotcrete

Engineering Developments

E.S. Bernard

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000151107
• Topic: Technology & Engineering
• Subtopic: Industrial Design
• Index: Technology & Engineering

A reference for shotcrete technologists and practitioners on this method of concrete placement and its great scope for adaptability, optimization, and error. The text assesses laboratory research projects and also focusses on innovative developments in this field.

Other papers

Dynamically loaded young shotcrete linings

A. Ansell & J. Holmgren

Royal Institute of Technology (KTH), Stockholm, Sweden

ABSTRACT: A criteria for how close, in time and distance, to young shotcrete blasting can take place would be an important tool in planning for safe and economical tunnelling projects. As a first step, field tests with shotcrete at ages of 1 to 25 hours, sprayed on tunnel walls, have been conducted in a Swedish mine. The shotcreted areas were subjected to vibrations from explosive charges detonated inside the rock. The acceleration measurements showed that the shotcrete had withstood high particle velocity vibrations without being seriously damaged. Drumminess of shotcrete appeared, indicating that the major failure mechanism was sudden loss of adhesion to the rock. Elastic wave propagation theory has been implemented numerically to further study the failure mechanism. Due to the changes in shotcrete-rock impedance ratio as the shotcrete cures, the stresses at the interface between shotcrete and rock experience a maximum during a short period within the first 24 hours of shotcrete age.

1 BACKGROUND

The ability to project shotcrete on a rock surface at an early stage after blasting is vital to safety during the construction and function of a tunnel. A complication arises when the need for further blasting affects curing of the newly applied shotcrete. If concrete, cast or sprayed, is exposed to vibrations during early age while it is still in the process of curing, damage that threatens the function of the cured concrete may occur.

In tunnelling, the search for a more time-efficient construction process naturally focuses on the possibility of reducing the periods of waiting between stages of construction. As an example, the driving of two parallel tunnels requires coordination between the two excavations so that blasting in one tunnel does not, through vibrations, damage temporary support systems in the other tunnel prior to placing a more sturdy, permanent support. There also arises similar problems in mining. To be able to excavate as much ore volume as possible, the grid of drifts in a modern mine is dense. This means that supporting systems in one drift are likely to be affected by vibrations in a neighbouring drift. A criteria for how close, in time and distance, to the young shotcrete blasting can take place would be an important tool in planning for safe and economical tunnelling projects.

In the past there has been little, or no, research carried out on the behaviour of young shotcrete subjected to vibrations. Some interesting work has however been carried out on fully matured shotcrete. Wood & Tannant (1994), Tannant & McDowell (1993) and McCreath et al. (1994) presented results from tests in a Canadian mine, where steel fibrereinforced and steel mesh-reinforced shotcrete linings have been subjected to vibrations from explosions. During the tests, it was found that steel fibrereinforced shotcrete can maintain its functionality even though exposed to vibration levels of 1500-2000 mmls. It was also seen that mesh-reinforced shotcrete performs better than steel fibre-reinforced shotcrete under very severe dynamic loading conditions. This is due to its ability to retain broken rock even when extensively cracked, which is not the case with most fibre reinforced shotcrete.

As a first step towards a damage criterion for young shotcrete, field tests were conducted at the LKAB mine in Kiruna, Sweden (Ansell 1999). Shotcrete, without mesh reinforcement and fibers, was sprayed on tunnel walls. Unreinforced shotcrete was used as it was expected to have lower resistance to vibrations than reinforced shotcrete. At ages of 1 to 25 hours, the areas were subjected to vibrations from explosive charges detonated inside the rock. Accelerations were recorded and later numerically integrated to give particle velocities. The maturing shotcrete linings were also tested to determine the development of compressive strength.

Figure 1. Schematic view of a test site. Explosive charge in rock behind shotcrete areas (in section AA).

2 FIELD TESTS

2.1 Test set-up

The performance of young shotcrete with respect to blast induced vibrations was tested during May 1998. The tests were carried out in cooperation with the research department of the mining company LKAB and the Swedish Rock Engineering Research (SveBeFo). A series of four tests, at four closely situated sites in a tunnel, were carried out. Each test was performed with a unique type of explosive charge, not repeated in any of the other tests. Two shotcrete areas of varying age, one fresh and one young, were subjected to vibrations in each test. A lining thickness of 50 mm was expected but variations between 40-80 mm were later measured. No shotcrete area was subjected to vibrations more than once. The geometry of each test site is described in Figure 1. For each test site, the following procedure was repeated:

1. Projection of first shotcrete area, approximately 1.5×10 m².

2. Hardening of first shotcrete area, between 9 hrs. 35 min to 25 hrs.

3. Continuous testing of young shotcrete properties until detonation.

4. Projection of second shotcrete area, approximately 1.5 ×10 m².

5. Mounting and connection of accelerometers.

6. Charging of explosives.

7. Detonation of explosive charge, measurement of accelerations.

8. Time delay due to ventilation.

9. Damage detection on shotcrete and surrounding rock.

The testing of young shotcrete properties (Step 3, above) was intended to determine the compressive strength development of the young shotcrete. At each application of shotcrete (Steps 1 and 4, above), moulds were filled for 28 day compression tests on 150 mm cubes. At a later stage, adhesion tests were also carried out. The shotcrete used during the test was of the standard quality, without fibres, used by LKAB. The water-cement ratio was 0.45. On the four test sites, charge holes were drilled at an angle of approximately 40⁰ relative to the tunnel surface. Within each of the test sites, measurement points for placement of accelerometers were located in the same horizontal plane as the charge holes. The shot-crete was sprayed on rock consisting of iron-ore, with a density of 4800 kg/m³ and a compressive strength of 115 MPa. The rock contained a zone of partially crushed material close to the test sites.

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Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgements
• Keynote papers
• Other papers
• Author index