By Pat Doig, Deputy Program Manager on the Strategic Tunnel Enhancement Program (STEP) in Abu Dhabi
The Third International Conference on Shaft Design and Construction was held in London on April 24-26, 2012. The first such conference was held in London in 1959, and the second in Harrogate in Yorkshire in 1989. The conference was developed by organizers who felt that the many international conferences on underground works tended to focus on tunnels, with insufficient acknowledgement being given to the importance of shafts. The conference covers shafts for mining and civil engineering projects, and attracts interest from shaft-sinking practitioners from around the world. Over 170 industry professionals attended the event this year, held in the impressive headquarters of the Institute of Materials, Minerals and Mining, just off Pall Mall, London.
Twenty-three years ago, I presented at the Second International Conference on Shaft Design and Construction, and I was honored to be the keynote speaker for civil engineering during this year’s conference, during which I discussed shaft construction for civil engineering projects, including available construction methods, what they involve, and when they are likely to be considered viable.
Civil engineering projects are located predominantly in urban environments because conurbations have tended to develop in areas where there is access to arable land and water, generally associated with a lake or river system. Projects involving subsurface infrastructure, such as sewer collection systems, water distribution networks, or metro lines must frequently deal with potentially unstable sediments and a high water table. Civil infrastructure tends to be relatively shallow, and therefore, there is considerable potential for settlement of adjacent structures, which are usually in close proximity in an urban environment during the excavation phase. The methods, developed by the civil engineering industry to construct shafts, are therefore focused on controlling the ground and water, and limiting the potential for settlement.
Multiple methods exist for excavating and supporting shafts on civil engineering projects. Shafts are an integral part of almost all civil engineering projects involving underground construction, and can generally be categorised according to the support methods and excavation techniques used to construct them. The choices available to the constructor are in turn determined primarily by the prevailing ground conditions and the function of the shaft.
Not surprisingly, constructions of shafts in hard rock generally require very different technologies from those located in soft, saturated soil. As a consequence, it is not unusual to find that constructors tend to specialise in one or the other. Shafts in rock are generally excavated either by drilling and blasting (conventional) or the use of mechanical boring equipment (boring), and are mostly excavated from the top down (sinking). However, if bottom access is available, shafts can be excavated from the bottom up (raising). Conventional mining and boring can be adapted to both sinking and raising. Many, if not most, of the shaft construction methods can be adapted to both circular and rectangular shapes. Deeper shafts tend to be circular to combat the higher loads, and some methods have little limitation in the size of opening they can accommodate. Additionally, different techniques usually have depth limitations.
There are many shaft construction methods in use in civil engineering. For a particular circumstance there will likely be more than one method that would be suitable and the choice will be influenced by many considerations. I have developed a Shaft Construction Matrix (below, click for a larger image) that attempts to compare the methods. Their practical limitations (e.g. size, shape, depth) are highlighted and pros and cons are detailed. This is not intended to infer that larger, deeper shafts could not be constructed, if time and cost allow. The matrix is offered as a broad guide that may be of use to owners, consultants and contractors contemplating a civil engineering project which necessitates shaft construction.
While not all tunneling and conveyance conferences and societies give ample attention to the importance of shaft design and construction, here at CH2M HILL we do. We have extensive shaft design and construction experience, such as the work we are doing on the Strategic Tunnel Enhancement Program in Abu Dhabi that I have the pleasure of working on, or the work we are doing for Thames Water’s London Tideway Improvements. During the conference my colleague Peter Jewell, Engineering Manager on the London Tideway Improvements’ Lee Tunnel project, gave a presentation that detailed the diaphragm wall construction of the deep shafts on the Lee Tunnel. It was great to attend the International Conference on Shaft Design and Construction once again, and to be surrounded by individuals sharing the latest and greatest in technology that ensures every shaft designed meets the needs of specific project and environment, and keeps everyone safe.
During a 40-year career in mining and heavy civil construction, Mr. Doig has gained extensive experience in tunnels and underground structures. Mr. Doig has been involved at a senior level in the construction of over 40 miles of tunnels. He is familiar with all major underground excavation methods, and with a large range of specialist subsurface ground treatment techniques, including dewatering, grouting, freezing, and ground densification. He has knowledge of a wide variety of tunnel support and lining systems, including concrete segments and PVC liners. He has experience working both as the owner’s resident engineer and construction manager, and as the contractor’s project manager. Mr. Doig is currently serving as Deputy Program Manager on the Strategic Tunnel Enhancement Program in Abu Dhabi, a 40-kilometre-long sewer tunnel, new link sewers, and pumping stations for the Abu Dhabi Sewerage Services Company. The 6-year program is needed as population growth is putting increased pressure on the existing system, and the population is expected to double in the next 5 to 7 years.