University of South Australia
Faculty of Engineering and the Environment
School of Geoscience, Minerals & Civil Engineering
10818 Mining Industry Project: Execution
Blast Design & Optimisation at Carroll Cave, Missouri
Student: Ryan Freeman
ID Number: 9805104X
Supervisor: Brian Roberts
Due Date: 04/11/02
Carroll Cave is a limestone karst feature located in Camden County, Missouri, USA. The only known natural entrance to the cave falls on private land, onto which entrance could not be obtained. The Carroll Cave Conservancy was established to conserve and protect the cave's geological and biological features and wished to gain access to Carroll Cave in order to survey the cave system. The artificial entrance took the form of a 76cm (30in) shaft sunk from the surface using blasting methods to excavate the shaft. This optimisation of the blast pattern aims to sink the shaft into Carroll Cave without disturbing the limestone structures found within the cave.
The shaft was to be sunk through interbedded dolomites and cherts with occasional clay seams thought to form the contact between the Gasconade Dolomite and the basal sandstone member of the Roubidoux Formation. Field testing was carried out that suggests the mean uniaxial compressive strength of the blasted rock was approximately 100MPa with a tensile strength of 12.7 MPa for cleanly blasted rock. A 23cm (9in) pilot hole had previously been sunk in an attempt to excavate the shaft by mechanical means and was used as the basis for the blast design. A number of parameters have been established to measure the performance of the blast design including design simplicity, safety, cost, ground vibrations (related to damage to cave structures) and pull.
An initial 6 hole blast design was used based on the method suggested by Langefors and Kihlstrom (1978). Poor pull and fragmentation saw the introduction of an 8 hole design with lower burdens and spacing. Excessive drilling times saw much material infilling these holes before loading, reducing the hole depth and hence the pull. The optimised blast design was a 6 hole design with reduced burdens. Only three holes were blasted at a time, allowing additional holes to be blasted where required to open the shaft up to the desired size and shape. The optimised design increased pull significantly and reduced the number of hangups as less material passed through the pilot hole at once. Holes were drilled to 107cm (3.5ft) and loaded with four sticks of dynamite, each fired on a separate delay.
Geometry, rather than powder factor was seen to be the most important parameter in the blast design. Though increasing the powder factor increased the fracturing of the material, the burdens were too small to fragment the material, releasing it from the surrounding rock. No limestone formations within the cave were damaged by the blasting process even though many blasts exceeded the destruction limit for limestone structures of 15.2mm/s suggested by Kirkbride, Worsey & Rupert (1995). The values suggested by Langefors & Kihlstrom (1978) for the design of a burn cut should be revised to reflect the rock type as they do not accurately reflect those required for the predominately limestone Ozark Aquifer.
I declare the following to be my own work, unless otherwise referenced, according to the University's policy on plagiarism.
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Ryan Freeman
The author would like to give to Paul Worsey, John Bowles, Mark Schmidt, Soek Bin Lim and the rest of the team at the University of Missouri Rollas Rock Mechanics and Explosive Research Centre (RMERC) for their agency and assistance in undertaking this project. A special thanks must also be forwarded to Rick Hines and Carroll Cave Conservancy for providing the project as well as Brian Roberts and Tony Meyers at the University of South Australia for their assistance.
Executive Summary I
Disclaimer III
Acknowledgements IV
Table of contents V
Figures VIII
Photographs XI
Tables XII
Symbols used XIV
1 Introduction 1
1.1 Carroll Cave 1
1.2 Carroll Cave Conservancy (CCC) 2
1.3 Aim 3
1.4 Performance parameters 4
2 Geology 5
2.1 Regional geology 5
2.2 Karst formation in the Ozark Plateau 6
2.3 Carroll Cave geology 7
2.4 Blast site geology 9
3 Initial blast design 11
3.1 Method selection 11
3.2 Development of initial blast design 13
3.3 Void ratio 16
4 Background information 18
4.1 Explosives selection 18
4.1.1 Explosive parameters 18
4.1.2 Available explosives 20
4.2 Effect of blast design on fragmentation size 22
4.2.1 Burden and spacing 22
4.2.2 Stemming 23
4.2.3 Delay sequencing 23
4.2.4 Drillhole diameter 24
4.2.5 Explosives selection 25
4.2.6 Fragmentation models 25
4.3 Effect of geology on blast performance 26
4.3.1 Water 26
4.3.2 Discontinuities and zones of weakness 28
4.3.2.1 Joint density 28
4.3.2.2 Orientation of joint and discontinuities 28
4.3.2.3 Mechanical properties 29
4.3.2.4 Energy absorption 30
4.3.2.5 Weak layers 30
4.3.3 Plastic rock 30
4.4 Ground vibrations 31
4.4.1 Introduction 31
4.4.2 Damage due to ground vibrations 31
4.4.2.1 Amplitude 31
4.4.2.2 Frequency 33
4.4.3 Ground vibration attenuation 34
4.4.4 Ground vibration analysis 36
5 Field testing 39
5.1 Rock strength testing 39
5.1.1 Uniaxial compressive strength 39
5.1.2 Tensile strength 40
5.2 Detonator testing 41
5.3 Vibration testing 42
5.3.1 Powder factor 43
5.3.2 Stemming 44
5.3.3 Decking 46
5.3.4 Hole diameter 47
5.3.5 Burden and spacing 48
6 Blast optimisation 50
6.1 Six hole design 50
6.2 Eight hole design 53
6.3 Optimised blast design 54
7 Blast reconciliation 56
7.1 Breakthrough 56
7.2 Vibrations due to blasting 57
7.3 Reconciliation 59
8 Conclusion 61
9 References 63