Projects where our pressuremeters have been used

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The range of projects and environments where the pressuremeter test can be used is extensive. It is widely used off-shore as well as on land, in deserts, in mountains and in tropical conditions.

Every project has its own set of problems and difficulties that have to be overcome. This might mean man-handling equipment in remote locations, such as Tanzania or The Gambia, or using helicopters to deposit equipment on a rock glacier in Switzerland. It is not usual for us to run a completely self-contained operation. Most of the time we have to work with a local drilling contractor and operatives who will be unacquainted with our equipment and unused to what is required for a successful pressuremeter test. This is not a major difficulty, and our engineers are accustomed to looking after the on-site training involved.

Our pressuremeters have been used on some of the world’s major civil engineering projects, such as Crossrail in London or the proposed crossing of the Padma river in Bangladesh. What follows is a selection of some of the more unusual projects.

The underground research facility at Mol, Belgium

Horizontal self boring in Boom Clay using air from a modified drill rigWe have at intervals over the last 15 years made visits to the SCK-CEN facility at Mol, Belgium, to carry out pressuremeter testing in the underground research facility HADES. This is a system of shafts and tunnels some 224 metres below ground level in a zone of Boom Clay in a highly plastic condition. The clay has interesting self healing properties when fractured, displays extremely low levels of permeability and offers a possible solution to the problem of the disposal of high level nuclear waste. Since 2000 the facility has been run by an expert group called EURIDICE and pressuremeter testing has been used during the construction of the facility and after to examine the engineering properties of the clay. We ourselves began work there in 1999 with a self boring pressuremeter. We were not allowed to introduce water into the formation and so drilled using air from a modified drill rig to implement the self boring process. Special casing and drilling parts were designed by us with some help from the drilling contractor to give the ideal flow path for delivering the air and returning the cuttings. The bulk of the testing has been horizontal. Speed is important in this material – it must be bored and tested as rapidly as possible because after one hour the material will close onto the probe with sufficient force to make extracting the equipment almost impossible.

Successful pre-bored tests have also been carried out with a 95mm HPD. This allowed a larger pressure to be applied and a greater cavity expansion achieved than is possible with a self bored probe.

Testing waste and investigating barrier walls

A view across a municipal waste site where pressuremeter testing was carried outWe were approached by Dr Neil Dixon of Loughborough University (now Professor of Engineering) about the possibility of using a pressuremeter to investigate the mechanical properties of municipal solid waste. Most of the work took place at a landfill site in Calvert, Buckinghamshire. The waste was a mixture of residential and commercial residue, not well-sorted, in various stages of degradation and depending on its age could be lightly to heavily compacted.

The primary purpose of the testing was to obtain engineering parameters that would permit the interaction between the body of the waste and the components of the protective barriers to be modelled and quantified.

Self-boring, pushing and pre-boring were all attempted. If metallic materials were encountered then the damage caused to equipment could be spectacular. The most satisfactory results were obtained with a 95mm HPD, where the large expansion capability proved to be helpful. The pockets for this were cut dry, using a modified bit resembling a large hole cutter. The waste is heterogeneous, may be partially saturated and of no particular particle size so the results were variable and the analytical processes were not necessarily appropriate. However shear stiffness from unload/reload cycles proved to be a plausible and repeatable parameter, and it was possible to relate the stiffness values to stress level.

Partly as a result of this work we became interested in the properties of the barriers themselves, and have (in conjunction with Cambridge University) carried out research work on the mechanical properties of man-made and natural barriers, with special attention being paid to permeability.

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Kolkata Metro East - West Project

Twenty four self bored tests were carried out at four critical locations along the alignment of the proposed metro in Kolkata, India. The pressuremeter testing component of the site investigation had been specified by W S Atkins. For the most part the tests were conventional in material that behaved either as a clay or sand. What was different about this project was the technical and practical difficulties that had to be overcome to achieve success - see figure 1.

figure 1 self boring rig in Kolkata
Fig.1 Self boring in Kolkata (click for full-size)

The work was conducted on a 24 hour basis at pavement locations in the heart of the city. The only rig available turned out to be a small, light and rather old quill drive system where most of the controls had long broken down. Rate of rotation and advance was down to the skill of the driller, who knew his rig and how to coax results from it. On more than one occasion the rig rotation system broke down whilst driving the pressuremeter, and the boring was completed by rotating rods by hand.

In some ways a worse problem was an inadequate water pump, as no boring is possible if the pump is not delivering a sufficient flow. However as our report noted at the time, these issues were a problem for the rate of progress of the fieldwork rather than the tests themselves, which were of reasonable quality.

Some of the expansions in the more clay-like material were turned into consolidation tests. The tests typically took two hours to complete, and were popular with the drilling crew. There is an increasing need for complex transport infrastructure in such locations and this project is typical of the kind of testing we are asked to do.