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New meter for vibration levels

Following on from the theme of instrumentation in the January issue of Ground Engineering, I would like to outline developments in the trend towards monitoring vibration dose values during construction.

The need for environmental protection brings with it a demand for monitoring instrumentation, as it is of course necessary to take measurements in order to be able to quantify levels.

Continuous developments are being made and just as we become used to one way of doing things another is introduced. Ground vibration monitoring was a relatively novel idea as recently as the mid-sixties. If measurements were taken, simple mechanical instruments were used.

A typical unit used a suspended weight via a series of levers to move a stylus pressed against an acetate ribbon advanced by clockwork.

Only one axis at a time could be monitored and the trace, showing displacement, was scratched onto the acetate. This had to be measured on the acetate and converted to peak particle velocity (ppv) with a multiplication factor.

Over the following years, electronic instruments became available. These automatically triggered and recorded three axes, providing an instant printout of ppv, acceleration, displacement, frequency and resultant.

The latest instruments are able to store large amounts of data for uploading to a computer for archiving, printing, plotting or in-depth analysis using post processing software.

Thirty years ago, blasting techniques were relatively unrefined and high levels of ground vibration were accepted as normal. Sometimes levels of ppv generated were close to the levels at which minor cosmetic damage might be caused.

Bearing in mind that damaging levels of ppv are closely related to the frequencies at which they occur, initially the idea of ground vibration monitoring was to prevent damage to structures, often people's houses.

Once the means were available to measure levels of ground vibration accurately, it was not long before planning conditions imposed limits for new developments.

Over the years acceptable levels were steadily reduced and now the UK enforces some of the strictest legislation in the world. Damage to property, due to ground vibration is a very rare occurrence in the UK.

Current ideology is to set limits designed to prevent levels of vibration that may be regarded merely as a nuisance to residents. Perception, while frequency dependent, roughly begins at ppv levels between 0.5 and 1mm/s. New methods of classification are beginning to emerge.

The latest trend in the field of ground vibration monitoring is to measure acceleration with regard to human perception, using daytime (7am-11pm) and night-time (11pm-7am) values.

These time-weighted levels are known as vibration dose values (vdvs) and there are three axes to consider: the z axis is from head to toe, the y axis is from front to back and the x axis is from side to side.

In daytime, the body is assumed to be upright (sitting or standing) and hence the z axis is at 90 to the ground, whereas at night-time the body is assumed to be in recumbent and hence the z axis will be parallel to the ground.

Until recently, the only practical way of measuring vdvs was to use a sound level meter and triaxial accelerometers, a fairly time consuming and tricky exercise. While British Standards which expressly mention vdvs first appeared in 1984, they were rarely written into planning conditions or consents, largely because they were little known about and even less understood.

If measurements of vdv were ever asked for, seismographs such as Vibrock's V401 were often used to measure ppv in mm/s. A fiddle factor was then used to convert ppv to vdv.

Recently there has been an increase in the occurrence of specifications demanding that vdvs be measured. This trend seems likely to continue as an understanding of the theory of vdvs becomes more widely known.

Vibrock's design engineers have produced what is thought to be the world's first vibration dose meter, which monitors vibration in compliance with BS:6472.

The brief was to keep the instrument extremely simple so that the technology would be completely transparent to the user. After all, the major concern of the operator in the field is to establish the vdvs, in three axes, to ensure compliance with set levels.

Looking at the practicalities of such a monitoring device, it is necessary to measure acceleration in three planes, so accelerometers are used as sensors in a triaxial arrangement.

During the day, the z axis is monitored using the accelerometer which is at 90 to the ground, while at night z axis data is taken from the y axis accelerometer. Electronic filters apply weightings to the readings to produce the vdv for each axis.

A vibration dose meter may be used to detect levels of vibration which, while not of a damaging nature, are of sufficient magnitude to be perceived by the human frame and therefore to cause psychological distress.

Typically, such levels of vibration may emanate from railways, generators, engines, ventilation fans, drop hammers and many other sources. If perceived levels are continuous or very frequent, a high degree of stress may result. Hence the need to be able to measure and quantify this type of disturbance.

It is envisaged that the main users of the meter will be civil engineering contractors.

Indeed, the recently awarded Channel Tunnel Rail Link - the construction project which will link St Pancras Railway Station with the Channel Tunnel rail head at Folkestone - specified that vdvs must be monitored during the construction of the project.

Andy Maslin

Vibrock

Heanor

Derbyshire

What about Sir Harold Harding?

I was surprised when reading your '50 years of BGS' volume (Ground Engineering December 1998) that no mention was made of the late HJB (later Sir Harold) Harding.

There can be no doubt that, along with Glossop and Golder, he was the moving force in establishing geotechnics in the UK and was one of the few outstanding engineers of his generation.

EH Steger

consulting engineer

London

Dr Paul Nathanail looks at the major changes in the new draft code of practice for the investigation of potentially contaminated sites, and, despite its shortcomings, urges its wider use.

Last year the British Standards Institute released a revised edition of the draft Code of Practice for the Investigation of Potentially Contaminated Sites, 10 years after the release of its predecessor DD175 (1988).

The old and much misused draft has now been withdrawn by BSI, and investigation of contaminated sites should be undertaken to the new draft code DC 98/564053.

Publication of BSI's draft coincides with the release of other guidance on the investigation of potentially contaminated land, with new publications available now or due shortly from DETR, CIRIA, the Environment Agency and others.

The BSI drafting committee has recognised this work and pointed to it rather than trying to summarise or duplicate it - for which they should be praised.

Surprisingly, though, they do not seem to acknowledge the significant changes in the draft BS 5930: Code of Practice for Site Investigations (particularly with respect to the description of contaminated materials) and cite only the 1980 version.

DC 98/564053 was recently the subject of a workshop held at the Society of Chemical Industries, where the most burning issue was that of the number of samples to be taken. In particular, the absence of specific numbers of samples for sites of a given size prompted wide criticism.

On a positive note, the draft's recognition of the importance of the conceptual site model is most welcome. The draft code details what level of information such models should include at the end of the different stages of investigation, from desk study and walkover stage to the end of the intrusive investigation.

The requirement for a conceptual model may go a long way to resolving the 'how many samples' debate, for the answer becomes 'enough to reduce unacceptable uncertainty in the conceptual model'. That number may be determined by professional judgement, using a site assessor following the advice in CLR 4.

The draft code also covers the integration of geotechnical and contamination investigations, which is a worthy concept provided the warning is heeded that one is not compromised at the expense of the other. This may mean deploying teams comprising ground specialists and contamination specialists, ensuring that logging is detailed enough for both purposes and of course that the investigation creates no new pathways.

The advice on gas sampling includes a warning about the limitation of spiker surveys. There are also nuggets of great value in the brief section on groundwater sampling. The section on preparation and analysis of soil samples should be required reading for all those conducting risk assessments using the results of laboratory analyses of soil. Adoption and awareness of the procedures described would certainly help to raise the quality of reporting required and accepted from analytical laboratories.

Health and safety only merits inclusion as an appendix, and even then in a cursory manner, which is shame and a missed opportunity. However, overall the new draft Code of Practice for Investigation of Potentially Contaminated Sites is a marked improvement on its predecessor.

Tightening up of definitions, better integration with existing and forthcoming guidance and an expansion on the health and safety aspects are needed before the final British Standard is issued. But do not wait until then to tear up your 1988 vintage DD 175: acquire and use the 1998 version now!

Dr Paul Nathanail is senior lecturer at the School of Chemical Environmental and Mining Engineering at the University of Nottingham and a director of Land Quality Management .

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