Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

TALKING POINT

Use of low tech site investigation methods should be confined to low risk sites, argues Phil West.

Talking point copy The past 20 years have seen major advances in geotechnical engineering, with increased computer power and more sophisticated programming leading the way.

Technological advances have enhanced the capability and accuracy of instrumentation and computer monitored laboratory tests, while the range of insitu tests that can be carried out by static cone testing has also increased.

But although data acquisition and accuracy of testing and analysis have improved, the quality of many routine site investigations has declined over the same period.

Reasons for the decline include:

A lack of commitment to training of shell and auger drillers

Prolonged periods of below inflationary rises, or even reductions, in drilling and testing rates

A reduction in the skills of graduate engineers and a significant reduction in the number of civil engineering graduates entering the industry

An increasing reliance on contractors rather than fully trained in-house staff.

Many of these factors are market driven and beyond the control of most consulting engineers and site investigation contractors.

However, the decline in standards has been exacerbated by increasing reliance on inappropriate techniques and the apparent inability of engineers to question and challenge solutions.

In recent years driven window and windowless sampler boreholes have often been used in place of cable percussion boreholes. Twenty years ago cable percussion drillers served a four- or five-year apprenticeship.

Many driven window sampler operatives have little or no training and do not always understand what they are drilling through.

So it is not surprising that these owner operators do not know when the technique is appropriate. Why are geotechnical engineers not challenging the inappropriate use of these low-tech techniques?

Driven window sampler and, generally, windowless sampler techniques are only suitable for recovering disturbed samples.

At depth, for windowless samplers and in window sampler boreholes where small diameter tubes are employed, the large area ratio creates significant disturbance at the perimeter of the sample tube.

While 'undisturbed' descriptions can be obtained from recovered samples there is often 'short recovery'.

This is a function of the geometry of the window sampling equipment.

Driven window sampler techniques are often accompanied by dynamic probe heavy (DPH) and probe super heavy (DPSH) testing.

With some equipment, such as the Competitor rig, both techniques can be carried out with the same equipment and in the same borehole, in the same way that U100 samples and SPT tests are carried out in cable percussion boreholes.

Many operators of probing rigs refer to the tests carried out as SPT tests but, while the DPSH method adopts the same drop weight, drop height and cone area, the mechanism through which it transfers the potential energy at the top of its free fall to kinetic energy at the cone is subtly different.

There can be significant differences in energy and variations in 'SPT N values'.

The SPT test in cable tool boreholes is notoriously prone to errors and analysis is based on empirical methods, so any variance in the test method only compounds inaccuracies. Where dynamic probes are used they should be considered as measures of comparative density and ideally correlated to SPT tests in cable percussion boreholes.

If the geotechnical industry is to provide a valuable contribution to construction, engineers must challenge investigation proposals.

They must ensure techniques are appropriate to the acquisition of samples of a quality compatible with the analytical methods and relevant to the geotechnical solutions.

The use of driven window sampler boreholes and dynamic probes should ideally be confined to low risk sites or sites with restricted access, and even here their limitations should be borne in mind.

Phil West is manager of Bureau Veritas' Geotechnical Group.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.