Many of the criticisms AH Marsh levels at current practice in site investigation (GE January) are justified and recognised by the AGS. So it is disappointing that he does not feel able to support the AGS initiative to establish a code of conduct in order to improve the quality and the perception of the site investigation industry.
One of the primary requirements of the Code of Practice is the need to appoint a geotechnical advisor who should give support and input to all stages of planning, investigation, design and construction. It also stresses the need for proper supervision of fieldwork and the need to modify the scope of the field/laboratory work in the light of conditions actually revealed.
The 'AGS Guide to good practice in site investigation' makes it clear that this supervision should be undertaken by a geotechnical consultant or a ground investigation specialist (and the qualifications and experience underpinning these terms are clearly defined).
Although the guide does not specify that the same consultant or GI specialist should take responsibility for each phase, this is likely to happen in many small projects - and at worst the responsibility is only likely to be split between the consultant and contractor - both of which should be appropriately qualified within the definitions laid down by the Code of Practice for Site Investigations and the AGS, and both of whom should be working under the direction of a geotechnical adviser. Mr Marsh himself, illustrates well the possible consequences if the linkages are too close.
He also draws attention to the problems faced through the involvement of nonspecialists in the procurement process. This is of continuing concern to the AGS which feels that some of the shortcomings in the SI industry relate to the responsibility taken by inappropriately qualified non specialist personnel - whose experience in commissioning SI is likely to be of the type that the AGS is seeking to eliminate.
Informal questioning reveals that even some of the largest consultancies do not routinely pass SI enquiries to their geotechnical departments.
It is a pity that Mr Marsh feels the need to belittle the AGS initiative. A generation or more of geotechnical engineers have known that SI practice has often been inadequate. Only concerted action by the major players is likely to have any impact and the AGS invites Mr Marsh - and anyone else with the best interests of the geotechnical, geoenvironmental or ground investigation industry at heart - to join us and strengthen our hand against the cowboys.
Association of Geotechnical & Geoenvironmental Specialists
Pumping up heat
Further to your short article on the Integer Millennium House in GE February, your readers might be interested to know of another geotechnical innovation incorporated within the project.
Last summer TechnoDrill was invited to install an 'earth energy borehole' at the site which was also featured in the first episode of the BBC's Dream House television series.
The concept is to take heat from the ground and transfer it into the house through a heat pump. The system is much cheaper to run and reduces green house gases by some 40% compared to a conventional heating system. This is a renewable energy source that is one of the most environmentally friendly.
The concept is simple. Ground temperatures at depth in the UK are stable all year round, typically at about 11degreesC-13degreesC, and this is sufficient to increase the temperature of the cold water being pumped into the bore by 3degreesC. The heat pump further increases the temperature to 50degreesC, which is used to run the house's central heating.
On a practical level the borehole was drilled to 70m depth using a TechnoDrill mini rig and two connected plastic pipes fed into the hole giving flow and return. The installation was completed by filling the bore around the pipes with a high-density bentonite grout, which protects the pipes and helps to conduct the heat from the ground into the heating system.
It would be quite feasible to incorporate energy boreholes into new and existing houses. Installation costs are low, as the job is run with two drillers and the site set-up costs are low. The rig and tooling used weighed in at 3t, which means transportation can be as simple as pick- up and trailer. Positioning the track mounted rig takes only about five minutes, and because the rig is so small, with care, damage to pathways and gardens can be avoided. The borehole can also be drilled very close to an existing building, which reduces the amount of ducting needed to connect the bore to the heat pump.
Surely this represents a great opportunity for the UK geotechnical and drilling industry. Will the plumbing and heating industry get ahead of us? If our drillers take this opportunity by the scruff of the neck, it could well provide for a lucrative future.
Simon Duck, Managing director
Wiltshire SP5 3HU
Ralph Peck and Alan Powderham each have made distinguished use of the observational method 'to ensure safety, and save time or money' in works under their influence or control. They have shown that the method can 'forewarn of problems by measuring deviations from expected behaviour', and that it adds value on a range of major projects' by 'progressively introducing beneficial design modifications and efficiency'. Their assessment in Ground Engineering February, is that the method is 'under-used' because it has become linked in some minds 'with low factors of safety' or high risks of 'additional cost and delay in contingency measure'.
They urge that 'we should aim to produce designs that are right the first time, but we should always leave the door open for improvement', and add that use of the observational method 'stimulates innovation and promotes teamwork and motivation'.
What do the writers think of my suggestion in my Ground Engineering, August 1998 paper that the observational method can be combined with centrifuge tests? Can cost and time spent in learning from models reduce spending on observations on major projects? If an array of 'too much instrumentation' is studied in a centrifuge model will it clarify and define the purpose of instruments of 'the right type in the right place' in the field? Will studies of models with various strength parameters in a laboratory define the range of behaviour to be expected in the field?
Will use of centrifuge model tests stimulate innovation in geotechnical teaching and motivate young engineers to use the observational method as their 'key to opportunity and progress' when they in their turn follow in the writers footsteps? What other change is needed in soil mechanics? Do Peck and Powderham have views on 'Mohr Coulomb Error Correction'?
I amplify these suggestions in two papers copied on to my home page at www2.eng.cam. ac.uk/~ans/habibx.html, and www2.eng.cam.ac.uk/~ans/gc.html.
Andrew Schofield raises some interesting questions and emphasises the importance of innovation and teaching. He asks whether the observational method could be combined with centrifuge tests to save cost and time and reduce spending on observations on major projects. (This point is made in his paper in Vol II of Centrifuge 98, the Tokyo Conference of TC2, September 1998).
There are two kinds of technical uncertainties in sub-surface engineering. One is in the modelling of the mechanism or mode of failure. The other is in the geological character of the deposit involved. The centrifuge is useful in the first kind, but it cannot forecast behaviour in the second kind. The observational method discloses the effects of undiscovered features in the deposit before they compromise the stability or integrity of the deposit and its supported structures.
A centrifuge would allow a series of 'what if' scenarios to be investigated with testing to failure. Time and the 'window of opportunity' would present major constraints in using the centrifuge directly with the observational method. This would generally confine opportunities to major projects with long lead times or a series of similar projects where a feedback loop for continuous improvement could be established.
In this context, centrifuge testing has recently been used to assess the stability of railway embankments. This is being progressed in parallel with the actual construction on a number of sites. While it was not used as direct input to an observational approach it did involve material obtained insitu from the sites to assess potential failure mechanisms.
The results from such centrifuge testing should enable better risk management for the long term maintenance of these facilities. In turn, the improved understanding of uncertainty could assist in future applications of the observational method where appropriate.
Ralph Peck, University of Illinois
Alan Powderham, Mott MacDonald, Croydon
Editor's note: Ground Engineering welcomes and encourages letters. I hope that the geotechnical community will use this forum to develop and contribute to the wider issues raised by the Mohr-Coulomb error.