An investigation into the use of vibro-impact techniques in the design of a ground moling system

• Main Author: Stevenson, Russell G.
• Published: University of Aberdeen 1992
• Subjects: 624.15
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.593509

The steady increase in demand on underground utilities experienced over the last decade, combined with a greater appreciation of the indirect costs of open trench techniques and a heightened environmental awareness, has resulted in the utilisation of trenchless techniques becoming increasingly financially and politically attractive. Inadequancies in existing percussive moling techniques employed in the installation of small diameter (<300 mm) pipes and cables over short, straight distances (<35 m), have shown that a need exists for a device with improved penetrative abilities through difficult ground conditions whilst reducing the risk of damage to existing services. The vibro-impact principle, originally developed in the USSR c.1940, employs a combination of vibration and impact to provide the driving energy to a driven element. A resilient connection between the vibrator unit and the driven element permits a vibro-impact device to efectively self-adust the relative level of transmitted vibration and impact. This self-adjustment capability has been shown to increase pentration depths through difficult ground conditions over those possible with pure vibratory or pure impact techniques, and reduce disturbance to the surrounding soil. This thesis considers the application of the vibro-impact principle in the design of a ground moling system. Both the theoretical and practical development of the vibro-impact principle to date has been concerned with vertical application of such a device. The existing theoretical models and design practice have been reviewed and developed to consider the horizontal application of a vibro-impact machine. The development of a laboratory based model vibro-impact moling system is presented. Through a variable parameter investigation on this model system a range of optimum parameters were identified for horizontal penetration of a driven element into a sand test bed. Furthermore the model investigation identified three distinct zones of horizontal vibro-impact penetration, different in form from those previously identified for vertical penetration. Based on the results of both the theoretical development of the vibro-impact principle and the model investigation, a design process was established for a full scale vibro-impact moling system. A theoretical, computer based model was developed to optimise the parameters of the proposed design. The design of the full scale prototype was finalised utilising computer aided design software. Following manufacture the prototype system was subject to a series of laboratory based commissioning trials. Further developments to the pneumatic actuation system based on the results of these laboratory tests led to the development of a fully operational vibro-impact ground moling system. The prototype moling system was then subject to a series of full scale field trials. The results of these trials confirmed the operational response indentified by the model investigation. Furthermore the optimum operating parameters derived from the results of the field trials correlated well with the results of the computer based theoretical model. The field trials thus validated the theoretical analysis and validated the design procedure established by the author. The penetration rates achieved with the full scale prototype vibro-impact moling system indicated an improvement in performance over that which could be expected with conventional impact moles. Successful trials were also conducted in conjunction with a coring tube end fitting, identifying the possibility of developing the system into a portable ground investigation system. The work described in this thesis thus represents a development of existing vibro-impact theory and practice, and highlights the commercial potential of this line of research.