In the April issue of GE, the photo of the sinkhole in Guatemala City, accompanying the news item 'Houses swallowed up by huge hole', looked remarkably similar to that of a sinkhole in Chryssoskalitissa, south west Crete.
The phenomenon was investigated by a team at my company, EDAFOS, along with a consultant, Professor Marinos. We concluded that the sinkhole formation should be attributed mainly to internal erosion towards underlying karstic cavities. These surfaced above sea level due to the tectonic uplift of western Crete. The subsidence took the form of an almost perfect cylinder with a 10m diameter.
Spyros Cavounidis, civil engineer, EDAFOS
In a hard place
Forgetting stones is one of the standard geotechnical blunders ('Stoned', GE April 2007) and can occur in the field, in the laboratory and in the ofce.
This is not conned to studies of contamination. I've seen particle size analysis results reported as a percentage of whatever remained after larger particles had been removed.
While it ought not be necessary to write a standard specifying that actions shall be recorded and records shall not be overlooked, section 9.2 of BS ISO 10381-1:
2002 does state: 'Where the sampling of soil involves the separation of oversized material. . . the material removed shall be weighed or estimated and recorded and described to enable the analytical results to be given with reference to the composition of the original sample.'
Perhaps, as far as contamination is concerned, regulators could specify compositions within particle size fractions, for example, no more than x% of X shall be permitted in material ner than 2mm.
Alternative rules would be required for large particles with a long-term potential to decay and release harmful substances.
Peter Smart, Glasgow University
It was with great interest that I read the article 'Stoned' by Mark Perrin (GE April 2007).
Soil sampling and the effect it has on contamination test results is an underdiscussed topic and of massive importance. However, Perrin misses out the impact that sampling has on waste management implications of contaminated soils and how these may be at odds with human health risk assessments.
I work for an environmental and ground work construction company in south east England. About 80% of our work deals with development on brownfield sites. About half of these have levels of contamination that could lead to human health risks.
But on almost all sites this is irrelevant. The reason is simple - basements. With land prices so high, developers look to maximise the use of their land. This is often done by making the building as large as practical and putting car parking underground. The upshot is that contaminated land stops becoming a human health issue (because it will be removed) and becomes a waste management issue.
On a typical medium-sized ofce or at development within the M25 corridor, a basement construction will Produce between 2000m 3 and 4000m 3 of soils that need to be disposed of. In cost terms the impact of whether a material is classied as inert or hazardous is huge, with the price difference in disposal being upwards of £180/m 3. This can add thousands of pounds to the cost of a project.
As Perrin pointed out, stones are of no great interest to consultants carrying out human health risk assessments as they are unlikely to be a contamination source and are less likely to be ingested by humans or absorbed by plants. Stones, however, are of great interest for waste classication because landll operators focus on contamination levels as a proportion of total weight, that is including stones, which can reduce apparent contamination levels. Conversely, excluding stones may cause the removal of tarmac fragments from the sample and tarmac is one of the most common sources of hydrocarbon contamination in made ground.
I recently visited a project where the environmental consultant, sampling some made ground on site for hydrocarbons, correctly insisted on using glass bottles. He produced a bottle with a top to it of no more than 30mm in diameter that excluded the inclusion of any fragment of tarmac in the sample.
Following testing, the made ground was classied as inert, sent to the appropriately licensed inert landll, where it was rejected due to its high tarmac content.
I accept waste classication and disposal is not as sexy to consultants as soil guideline values, CLEA, and human health risk assessments. But by carrying out site investigation work and contamination testing, consultants may be undertaking irrelevant work that could cost their clients money through incorrect waste classication and higher disposal charges.
Peter Hewitt, Mann Environmental