Abstract

Engineering for design and construction of safe and reliable high pressure water tunnel systems requires good knowledge of in situ stress conditions of the rock mass to evaluate confinement and potential for hydraulic jacking to occur. Without adequate confinement, an impermeable lining system would be required, typically consisting of steel-lining. Considerable importance is placed during investigations in obtaining reliable rock stress information for selection of an appropriate and safe lining system. Various methods are used, including standardized hydraulic fracturing, hydraulic jacking testing, and over-coring. With each of these there are limitations in providing reliable data in a cost effective and efficient way. A modified approach to hydrojacking testing is described that has been used successfully on hydropower projects in the Himalayas and Andes where other more traditional methods have been unproductive. It involves use of modern water pressure test and grouting equipment, often available in-country or already under contract on a construction project. Lower costs and rapidity of testing permit more data to be collected than would otherwise be achieved, resulting in significant benefits. This approach is becoming increasingly common but which possibly suffers from not being sufficiently recognized in the profession or properly standardized so as to gain universal acceptance.

1. INTRODUCTION

In the planning and design of a pressurized water tunnel system, it is imperative to have knowledge of the ability of a rock mass to withstand leakage of high-pressure water from waterways that are not equipped with totally impervious liners. It is well understood in our profession that all shotcrete, plain concrete, or reinforced concrete linings should be considered as pervious since they will crack due to shrinkage and the effects of internal pressure. Reinforcement added to shotcrete and concrete will assist in the control and distribution of cracks but will not eliminate them. Schleiss points out that when a lining is pervious, seepage will flow into the surrounding rock mass through cracks, joints, or fractures, with the result that the internal water pressure will exert a load not only on the inner surface of the lining but also within the rock mass itself [1]. As long as the natural stresses in the rock mass exceed the seepage pressures contributing to the hydraulic forces within the rock mass, the load-bearing capacity of the rock will not be exceeded. In this condition, the rock mass is said to provide adequate confinement. However, if the water pressure in a given rock fracture exceeds the existing normal stress, the fracture will dilate and the water pressure will be transmitted further into the rock mass. This failure mode, known as hydrojacking, can result in an unacceptable and uncontrolled water loss and potential for serious damage to surrounding structures, undesirable changes in the water table, or possible surface slope stability issues.

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