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Blog: The benefits of well managed CPT investigations

Monday, March 24th, 2014

Lankelma Operations Director Ben Magee explains how well-managed CPT investigations, using experienced engineers, can bring great benefits to projects.

An obvious benefit of CPT is that it is fast, producing almost instantaneous results. The broad range of equipment and techniques also means it can be used in difficult ground conditions and on sites with access constraints.

In fact, a well-designed and controlled CPT programme, combined with engineering experience, has been proven to deliver an accurate, high resolution primary dataset (1,000 data points per 10m push) which can be used with confidence in ground modelling and geotechnical design.

It is, however, critical to maintain quality control in the field, to prevent errors creeping into the raw data, which can affect interpretation. To reduce data uncertainty and improve confidence, one of the main aspects is to ensure equipment is properly maintained and appropriate for the investigation being carried out.

First of all, it is important to calibrate cones – checking the certificate matches the cone – before work begins. Lankelma maintains all of its equipment itself and carries out calibration testing of cones, data loggers and other instrumentation in-house. As well as having control over the entire process, engineers gain a deeper understanding of the instrumentation – how the instrument works, for which conditions it is best suited, and how to get the best out of it.

Cone wear observations, plus tip and sleeve measurements, should also be made by operators routinely, as worn cone tips, sleeves, data cables and faulty load cells are all likely to cause issues. After each test, cone geometry, damage and data outputs should be checked before the next test is carried out. And, when obtaining pore pressure data, the piezocone should be re-saturated and filters swapped for fresh, fully saturated filters between every test.

In extreme environments, such as on very cold mornings, care should also be taken to allow cones to reach the ambient temperature before use and should even be preloaded before the first test in some cases.

Of course, once data is collected, it needs to be interpreted. Nothing beats an experienced engineer, who can validate characteristic data points, layer boundaries and soil behaviour type.

Gone are the days where CPT was an ‘add-on’ to conventional ground investigation. The method should, in fact, play a key role in any integrated soil investigation programme. Well-maintained and calibrated equipment, combined with expert operators and engineers, can ensure that data gathered is of the highest quality and interpretation is accurate and useful.