I recently gave an evening talk entitled 'Did the earth move for you? - prediction of vibration characteristics from tunnel blasts'. The tabloidesque title hides the serious nature of the talk about
a subject rarely covered in standard texts or university courses in the UK. Perhaps it should be, because it emerged that few practitioners have any concept of how damaging blast vibration can be, or to phrase it more appropriately, how unlikely it is that blast vibrations from civil engineering projects will damage local structures. In the discussions afterwards one participant said that in his latest
tunnel project the contractual vibration limit set was 3mm/s measured at the nearest structure. This is extremely low - a door slamming induces vibrations of 10-15mm/s - and it is clear that such levels are now common practice.
Why do we set these low vibration limits? A possible explanation is that practitioners writing contracts and setting vibration limits do not really understand the subject and are scared. So they look at the last contract their firm wrote and halve the blast vibration limits, in spite of the mounting evidence on lack of damage to structures from blast vibrations.
Blasting seems to induce a state of panic in all involved. Interestingly, such panic is not induced in people whose properties are subjected to piling vibrations, even though such vibrations, speed for speed, are more likely to be damaging because of their lower frequency. Blast vibrations tend to be of high frequency and therefore less damaging. Frequency is a key factor in damage as incoming vibrations are greatly amplified by structures if the incoming vibration approaches that of the natural resonant frequency, which tends to be high. Conservatism in limit setting is, inevitably, leading to unnecessary increases in cost or avoidance of blasting in tunnel contracts altogether. This is a shame
because blasting can be an extremely efficient excavation technique and when compared with machine-driven tunnelling methods has a low mobilisation cost and a far greater flexibility to deal with varying conditions.
Now I must justify my sweeping statements and provide some advice on choosing blast limits for tunnelling contracts. Well, adequate guidance is given in British Standards for both structural damage and human response. Usually the human response standard dictates the vibration limit, as it is more stringent. The maximum suggested level for human response is 12.7mm/s, whereas the limits given for cosmetic damage to structures are between 15mm/s (worst case) and 50mm/s (common case) depending on the vibration frequency and the nature of the local buildings and structures. The only justification for lowering these levels might be in the vicinity of extremely sensitive equipment or hospitals where the stress caused by disturbance may conceivably cause harm. As civil engineering projects are of short duration when compared with quarrying operations, negotiation with the environmental health officer may result in increasing the limit for human response.
Of course, whatever the limit set in the contract it is likely you will get some complaints. A few residents may complain at very low levels of vibration and this may be independent of the actual vibration level experienced. This is because the human body is very sensitive to vibrations and it is often assumed that if a vibration can be felt, it must be damaging property.
The way to minimise these complaints is to have a positive PR campaign and to win the confidence of locals. Pre-project blast trials help gain confidence of all involved and pre-construction damage surveys are essential. Most important, however, is to employ a resident engineer with a talent for PR and a sympathetic ear.
Such a person can save considerable costs and may make a blasting project viable. This was the case for a tunnel I was recently involved with: blasting had been ruled out by the PR sensitive client but ruled back in when alternative tenders for a blasted tunnel came back at 60% of the budget. Despite blasting only 15m below properties with a maximum limit of 19.7mm/s the project was completed successfully and within the tender amount. The tunnel was in an affluent residential area with many properties housing retired people who were in for many of our working hours. Their confidence was won by good management and the maintenance of open relationships.
The moral of the story? He who sets reasonable limits based on sound evidence will enable like-for-like cost comparisons of tunnel methods and will allow tendering for contracts on a fair basis. This may also save client organisations significant amounts of money.
Lucy Philip is a lecturer in engineering geology at the School of Earth Sciences, University of Leeds.