The majority of 'insurance projects' are related to claims by home owners whose buildings are affected by subsidence or swelling. Laboratory testing aims to provide information that may determine the cause and allow an informed decision on remediation.
There are some excellent guidance documents, notably by BRE and NHBC, providing advice on actions to identify the reasons for soil swelling or subsidence problems. For example, BRE Digest 412 specifies four indirect methods of assessing desiccation:
a) Comparison of soil water contents with soil index properties
b) Comparisons of soil water content profiles
c) Comparisons of strength profiles
d) Effective stress or suction profiles.
However,it states that (a) and (b) may often give misleading results, and that (c) and (d) - though more expensive - are preferable. It also states that one method of detecting desiccation should not be relied on and it is useful to compare results from a 'control borehole' remote from the effects of nearby trees.
Unfortunately the insurance business is extremely competitive, and for some a low price has overriding priority in competitive tendering situations. As a consequence, there is a temptation to compromise quality in an effort to reduce cost and thus price.
Many large geotechnical testing companies spend a great deal of money annually, ensuring that they remain Namas accredited and their laboratories perform to consistent and repeatable standards. They spend even greater amounts implementing quality assurance systems to retain ISO9000 certification. These firms are inevitably undercut in price by laboratories not operating ISO9000 and that are not Namas accredited, but are still often used by some clients.
Further evidence that quality may be compromised to reduce cost is provided by those who perform insufficient site investigation, or who specify that the liquid limit of soil samples should be assessed in the laboratory using the single point method given in BS1377 (1990), when there is sufficient soil to perform the four point method. The latter example may sound trivial, but may be symptomatic of an attitude whereby an engineer or architect becomes blase, giving principal credence to their own experience and forgetting that even 'uniform' soils are inherently variable. Other examples of cost cutting abound, and include clients who tend to specify suction tests on disturbed soils, regardless of the soil properties. Some guidance documents do suggest that useful supplementary information may be obtained by performing suction tests on disturbed soils, but the key word is supplementary.
This approach probably derives from a section in BRE information paper IP 4/93 suggesting that there is some evidence that suctions can be measured even in disturbed soils. Crucially, however, it states that this approach should be used for comparative purposes only, comparison being made with a nearby borehole unaffected by any external influences that may be causing the structural distress. In addition, the success of assessing matrix suction values in disturbed soils depends on the soil retaining its effective stresses.We have experienced cases where suction tests have been specified on disturbed specimens received two to three weeks after sampling and where no attempt has been made to remove sandy inclusions or to ensure the soil does not dry out. Both will, of course, have a significant effect on the measured suction values.
Another problem is when the person specifying laboratory testing has not actually seen the soil. Immediate visual and tactile examination of samples by a specialist provides invaluable additional information.
So what can be done? I would suggest:
Always attempt to perform a 'control borehole'.
Always take undisturbed samples in the 'control borehole' and in at least one of the boreholes close to the affected building.
Samples should be described immediately on site, taking particular note of soil fabric and the presence and vitality of roots etc.
Steps should be taken to ensure that soil moisture content is unchanged between sampling and testing.
If clay samples include sandy inclusions, these should be removed immediately on site and placed in a separate bag.
Always supplement desiccation assessments based on moisture content and Atterberg Limits profiles, with assessments made by comparing either strength or, preferably, suction profiles.
If possible, suction profiles should be assessed in the laboratory using undisturbed soil specimens. In urgent cases suction can be measured on site using the Imperial College suction probe.
Suction tests on disturbed soil specimens in the laboratory should not be performed if the soil has dried out and if sandy inclusions have not been removed on site.
If there are reasons why the guidelines presented in relevant BRE and NHBC documents have not been followed, these should be highlighted and the increased uncertainty of the results should be noted.
Obviously, not all those involved in the insurance business are cowboys. Most work is performed by competent, experienced professionals.What I am stressing is that the spirit, as well as the wording, of the guidelines should not be compromised. Common sense should always be applied when taking samples and specifying laboratory testing - for we all know the equation: rubbish in = rubbish out.
Nick Ramsey is laboratory technical manager at Fugro (UK).