Geotechnology in Construction

The problem of Civil Engineering has generally been seen either to relate to "Design" or to "Construction". Research in Civil Engineering Design has often been taken to mean the study of some static scenario so as to eliminate unwanted behaviour. Research in Civil Engineering Construction has often been taken to mean the study of management techniques for the control of construction activities.

The new challenge is to relate Design to Construction, which requires a study of the Technology used to create some facility. Spin-off should include the optimisation of construction processes, and the development of new technology, including:

The construction of Civil Engineering facilities is transient in space and time, by definition. Furthermore, the ground is sensitive to the whole history of its deformation: its fundamental properties can be transformed by the construction process itself. It follows that the study of Geotechnology - the technologies and processes used in ground and foundation engineering to make insertions, extractions, injections, abstractions, compactions, excavations and other necessary modifications - must encompass change as a central issue.

The simulation of transient processes such as tunnel construction, both in centrifuge models and in finite element computations, has led to strong involvement with leaders in the industry. Analyses of the successful New Austrian Tunnelling Method (NATM) trial at Heathrow, and the subsequent collapse of the Heathrow Express tunnels in the Central Terminal Area, were carried out for W.S. Atkins by Dr Ganesh Dasari.

Compensation grouting - the injection of grout during tunnel construction, at an intermediate level between ground surface and tunnel crown, to eliminate subsidence and collateral surface damage - has been under investigation. The basic mechanism of hydro-fracture, widely used for compensation grouting, is under ongoing study, together with the global plastic mechanisms which control subsidence at the ground surface. This research has now moved into the field following a BRITE Euram project, COSMUS, inspired by Bachy-Soletanche Ltd with involvement from Tractabel, CEA-LETI, EPFL, and Glötzl. This allowed us to study real-time data analysis which is the essential pre-cursor to the automatic control of any construction process. The Cambridge team was led by Dr Kenichi Soga and myself.

Other aspects of tunnel construction include the influences of soil grouting on newly constructed tunnel linings: this was studied by Dr Siew Wei Lee in conjunction with Nishimatsu Construction. London Underground Ltd are also interested in tunnel linings. We recently completed an EPSRC project supported by LUL, to measure the actual earth and water pressures being exerted on the lining of a 75 year old tunnel in London, and to consider how construction activities followed by ground-water changes might have brought these about. In addition to myself, the work was steered by Prof Robert Mair and Dr Kenichi Soga, and conducted by Dr Susan Gourvenec.

Tunnel machines fall into a much wider class of technologies for soil extraction, including drilling, boring and straightforward digging. "Diggability" is notoriously difficult to predict.

Since, in every case, these technologies depend on the creation of a dis-continuum it seems essential to undertake a proper study of fracture, the cracking and disintegration of a soil body. Our understanding of the micro-mechanics of plastic deformation and flow must also be improved.

Insertion is just as open to future research as is extraction. Dr David White and I are collaborating with Giken Seisakusho Ltd on the hydraulic pressing-in of piles, as contrasted with driving by hammer for example. The degree to which there is soil disturbance, or the transmission of damaging vibrations, is obviously of interest. Construction in the urban environment demands the consideration of neighbours as a prime concern. Equally, the prediction of the bearing capacity and settlement of piled foundations has very much been a matter of creating an empirical database. There is now an opportunity to establish databases of piles inserted by other means, and to describe properly the micro-mechanisms of pile penetration and subsequent service behaviour. Boring and driving are found to result in completely different end-bearing behaviour, and it may be anticipated that variations within these contrasting techniques also lead to appreciable differences, especially in settlement.

Geotechnical construction off-shore has been researched mainly in the context of pile-driving, but other technologies are also of interest. Coflexip Stena Offshore, Fugro, Boreas and BP have been supporting work on the modelling of shallow deformation mechanisms in soft muds, such as those involved in laying sub-sea pipelines. Pipelines may be buried partly to provide thermal insulation, and partly to isolate them from possible damage from shipping. One concern is the possible upheaval buckling of such pipelines under the influence of internal pressure and high temperature, and the principal uncertainty lies in the mobilisation of uplift resistance of the soil above: centrifuge tests have already helped clarify this. Much more work is needed on the characterisation of shallow sea-bed soils, the possible entrainement of water during ploughing and back-filling, and the behaviour of such back-fills after construction and in service. Other mechanisms such as pipeline snaking when laid on the sea bed at 2 km depth in the Gulf of Mexico, are also under investigation. This work forms part of a collaboration with Professor Andrew Palmer.

Those with an interest in the research and development of construction technology and processes in geotechnical engineering are encouraged to contact me. You may

Let me know, by e-mailing me with the subject geotechnology.