More Than 25 Years of Research, Transforming a Black Art into a Science
Pipe jacking has been practiced in the United Kingdom for approximately 60 years and in the United States for much longer — more than 100 years. For many years, pipe jacking was practiced as “black art” with contractors using their tunneling skills to install pipes with miners at the face. It mostly worked but failures were frequent.
Research and mechanization has transformed the method into an engineering technique that is now used with confidence by engineers around the world. In the United Kingdom, the Pipe Jacking Association (PJA) has contributed significantly to the science through a program of research at some of the U.K.’s leading universities that have included the civil engineering faculties at both Oxford and Cambridge.
The high pressures required during the jacking process were not fully appreciated by the concrete pipe industry and some of the early concrete jacking pipes were basically open-cut spigot and socket pipes manufactured with in-line joints adapted to enable them to be pushed through the ground. Manufacturers subsequently developed purpose-designed pipes, with steel reinforcement cages and designed to withstand jacking loads. These were generally satisfactory but failures still occurred. Following a catastrophic failure of a pipe string on a pipe jacking contract for London’s Thames Water Authority, the PJA, backed by the pipe manufacturers and a number of U.K. water companies, undertook a program of fundamental research at Oxford University. The object of the research was to develop standard tests that could be used as a control on the quality of pipe manufacture, in particular pipe joints and packings where much of the load transfer occurs. This work, carried out in late 1980s and early 1990s has ensured that pipes manufactured to today’s exacting U.K. and international standards can be used with confidence and pipe failures are a rare occurrence, when pipes are correctly handled and installed.
Jacking pipes, although a key component of the pipe jacking system, are only a part of the jacking process. The interface between the pipe and the ground, and how it is efficiently excavated and processed are also key elements that have formed part of a continuing research program. Oxford and Cambridge universities have worked on a number of projects that evaluated various methods of conditioning soils and their effect on jacking loads and mechanized excavation systems.
Having invested in research to maximize the engineering efficiency of the system the PJA moved its focus to environmental issues, in particular the efficient use and processing of slurries that are a by-product of many pipe jacking contracts. Slurries usually contain additives and under European regulations generally have to be pre-treated and dried prior to disposal. Research has looked at ways of undertaking this efficiently and also ways of optimizing the slurry separation process. This has also included developing an understanding of how various ground types break down during the slurry process, a project completed recently at London’s City University.
As an extension of its environmental program the PJA has developed a carbon calculator that is able to evaluate the carbon savings of pipe jacking compared to open-cut construction. Based on a concept originally conceived by the BC chapter of NASTT, the PJA invested in the calculator that was developed by the U.K.’s Transport Research Centre and verified by the country’s prestigious Water Research Centre. Carbon savings of up to 75 percent have been demonstrated when comparing pipe jacking to open-cut. The calculator can be accessed via the PJA website.
Mechanization has also been a significant driver for the pipe jacking industry. The technique was used extensively in Japan in the 1970s and 1980s when new sewerage infrastructure was installed throughout many major towns and cities in that country and when smaller diameter microtunneling machines were developed. Computer technology and laser surveying techniques are now standard operating methods, not only for microtunneling but also in the larger diameters. This ensures accuracy of drives, the opportunity to jack longer lengths to fine tolerances and the ability to navigate around curves with confidence. In the United Kingdom, around 98 percent of all pipe jacking is now fully mechanized.
The PJA continues to drive forward its desire for a professionally engineered approach to all components of the pipe jacking process. It is currently evaluating the requirements for temporary works for entry and exit eyes at shafts. It is also currently updating its design guide, internationally recognized as the leading publication for anyone involved with pipe jacking and microtunneling design, and which reflects the state-of–the-art research undertaken by the association.
Graeme Monteith is chairman of the U.K. Pipe Jacking Association and tunnel engineering manager at Barhale Construction.