Ground investigation is critical to success of geotechnical design but what are the risks when the design is based on inaccurate or poor quality soil characteristics?
With use of finite element analysis becoming increasingly common, ever greater emphasis is being placed on having high quality ground investigation results to produce realistic design parameters. If consistency and accuracy of the soil testing on which this design work is based is called into question then how much additional risk is the ground engineering industry being exposed to?
Many of the UK’s soil testing laboratories take part in a bi-annual Proficiency and Interlaboratory Comparison Testing Scheme that allows labs to check their accuracy and benchmark themselves against others. The report for the 2017 scheme has just been published and, while some labs have performed well, a number of engineers have voiced concern over the variability of the results.
Lack of consistency between the results has also led calls for such testing schemes to be formally linked to accreditation.
In total 51 laboratories from across the UK took part in the scheme managed by Geolabs and undertook a suite of tests commonly specified following ground investigation work so should be standard tests that are done on a regular basis.
There were 16 standard tests and laboratories were invited to propose additional ones, which resulted in five other tests being added. The initial list of tests included determination of liquid and plastic limits determined by both four point and one point penetrometer; specific gravity by gas jar method; particle density by small pyconometer; particle size distribution by wet and dry sieve analysis; mechanical analysis by both pipette and hydrometer sedimentation; organic content by potassium dichromate method; water soluble sulphate and total sulphate content; pH value; compaction testing; California bearing ratio; and the moisture condition value of soil at a single moisture content.
Tests added by demand from participants included determination of bulk density by immersion in fluid; state of desiccation in clay soil by the filter paper method; one dimensional consolidation properties in the oedometer using five effective pressures; and undrained shear strength in triaxial compression without measurement of pore pressure on a single 100mm diameter soil specimen.
Samples of the same material were supplied for the testing so results reported by participants should be similar. However, the report shows wide variation which has led to industry concern.
The report suggests that the variability could be the result of a number of factors including not following test procedures or criteria, faulty equipment or poor calibration and not following the specified report procedures.
The report states: “The aim of the scheme was not to determine the reason for the differences and variability as only factual reporting of submitted test data was requested but clearly there should be significant concerns with the performance and reporting of geotechnical testing in the UK.”
The report calls for further quality assurance procedures to be considered by laboratory management including more rigorous auditing of test procedures and a better understanding of test and reporting requirements.
The need for more staff training was also highlighted. The report calls for laboratory management and staff should make themselves more aware of the requirements of the test procedures and test reporting requirements and implement them into their internal auditing arrangements.
“There are many tests in the scheme where it is apparent that the procedures adopted and reporting do not comply with the test standards or what was required by the scheme”.
While the report names the laboratories that took part in the scheme, the result reported by each is only identified by number which is known to the laboratory, so it is effectively anonymous.
GE contacted UKAS, which accredits soil laboratories in the UK, for comment on the results but it has declined to comment.
Association of Geotechnical and Geoenvironmental Specialists chairman Neil Parry commended laboratories for participating in the scheme. “However, the fact that there were several tests where outliers had to be removed to enable a meaningful statistical examination of the results is of some concern,” he added.
“Where this is the case we would hope that these laboratories are able to establish the reasons for these variations and change their testing protocols accordingly. Although it is very difficult to replicate the ‘real life’ scenario for geotechnical testing, the results of the scheme reinforce the need for designers to thoroughly examine the data provided by laboratories, questioning them where necessary.
“The possibility of variations highlights the need for training and the rigorous application of standards within laboratories to ensure results are reliable and provide robust parameters for design.
“On a more positive note there appears to be consensus amongst the best parts of the industry that comparison of testing should continue, with 51 laboratories submitting results this year. Ideally it would be best to have an officially recognised scheme that could lead to future registration.”
Results in focus
GE has picked three out of the 21 tests carried out under the 2017 Laboratory Proficiency Testing scheme to highlight the wide range of results reported by participating laboratories and the potential issues that could result for designers relying on these parameters.
Liquid limit by four point cone penetrometer
Only half of the participants submitted results for this test and the report describes the results as “disappointingly inconsistent”. The reported values for the liquid limit vary from 48% to 57%, with greater variation than seen in previous Laboratory Proficiency Testing schemes.
A number of laboratories reported penetration values outside of the BS1377 requirements (a range of penetration values of approximately 15mm to 25mm) with 13mm penetration recorded at the lower end and 27.4mm penetration recorded at the upper end.
The report also questions the line of best fit drawn by some laboratories for the cone penetration versus moisture plots. The report states: “The best straight lines for some participants show a wide discrepancy and can suggest possible equipment malfunctions, efficacy of the equipment or equipment requiring re-calibration and/or operator error or inappropriate interpretation.”
Particle size distribution by wet sieve analysis
The 34 laboratories that took part in this test generally showed good agreement between results. However, there are some significant differences in percentages passing in the fine gravel to medium sand sized results with laboratories reporting between 57% and 72% passing a 6.3mm sieve and between 41% and 55% passing a 1.18mm sieve.
The report states that the range of results for material passing the 6.3mm sieve is “disappointingly wide and probably results in either inadequate washing to remove the clay and silt material or test sieve not being cleaned out before use”.
Undrained triaxial compression test
This test was added to the scheme at the request of participants and 12 laboratories submitted results.
This test was also included in the 2012 and, similar to previous years, this test showed the largest scatter in test results. The range of the twelve corrected deviator stress results submitted was 190 to 417kPa, while the results for strain at failure varied from 8.6% to 16.2%.
The report states that figures reported by three laboratories for strain were removed from statistical analysis because the results were too “spurious”. The remaining results vary from 12.6% to 16.2%.
The report states: “As the remoulding densities and moisture contents recorded are reasonably consistent, and assuming all of the equipment was fit-for-purpose and calibrated, the most likely cause of the wide range and differences in both compressive stress and strain at failure is probably due to uneven and inadequate remoulding of test specimens.”