This guidance note on standard penetration test (SPT) procedure and reporting covers some issues that have recently arisen, specifically:

• Procedure for tests terminated early, including a possible conflict between BS EN ISO 22476-3 and guidance from the British Drilling Association (BDA).
• Reporting for tests terminated early, on logs and in AGS data.
• Extending SPTs to 100 blows in soft rock and other materials.
• Reporting of seating blows, on logs and in AGS data including discussion of BS EN ISO 22476-3 requirements.

Procedure for tests terminated early due to bouncing or similar

For the purposes of this guidance ‘terminated early’ refers to a test that has been stopped before any of the normal criteria for completion have been achieved, i.e. 300mm penetration or 50 blows for the main drive.

The BDA document Guidance For The Operation Of Cable Percussion Rigs And Equipment (Issue 2.7, 2024) includes the following:

“Where double hit or bouncing occurs or there is no penetration for 25 blows the test should be aborted and the total penetration reported, and test ‘refusal’ noted.”

This guidance can be perceived as being in conflict with the requirements of the test standard (BS EN ISO 22476-3 :2005+A1:2011 current at time of writing) which is silent on provision for early termination prior to 50 blows for the main test drive.

It is recommended that a common-sense approach is adopted when double hit or bouncing or similar occurs. If, based on understanding of the prevailing ground conditions and observations during the test, there is good reason to believe that continuing the test will not lead to further penetration and/or there is a risk of equipment damage, then it is considered reasonable and correct to terminate the test early. Reporting of this is covered in the next section.

The Investigation Supervisor should be informed at the earliest opportunity and the matter discussed, including agreement on action to be taken should the same situation arise again.

In making this recommendation it has been recognised that the most important information obtained in such cases is the observation of bouncing (or similar), i.e. something resistant to penetration has been encountered in the ground. This may be indicative of an obstruction that the design will need to consider, or that constructors need to be aware of.

In situations such as this, the SPT N value that may be interpreted based on actual penetration for a particular number of blows will rarely be useful for design. Such data points would normally be disregarded when interpreting a design line. Therefore, there is no merit in risking equipment damage merely to satisfy a very strict interpretation of the standard.

Requiring a contractor to redrill a borehole, despite that contractor following BDA guidance, would not be a common-sense approach.

Reporting for tests terminated early

Where a test has been terminated early in accordance with the above, this fact should be clearly indicated on borehole logs and/or test reports.

BS EN ISO 22476-3 reporting requirements include:

“all unusual events or observations during the operation (e.g. low blow count, penetration without blows, temporary obstructions, malfunction of the equipment);”

AGS does not offer any specific recommendation on the format or wording of such observations other than it should be clear and concise. This should be provided in addition to the usual reporting of blow count and penetration information.

For digital data supplied in the AGS data format*, it is recommended that the ISPT_REM field is used for reporting additional comments or observations.

However, data producers must ensure that this information is also shown on the logs produced from their data management software. If the log report forms do not show ISPT_REM data then there may be merit in duplicating this data in a field that will be seen on the logs, e.g. DREM_REM. Duplication is far from ideal but this is better than having this important information omitted from the logs.

Extending SPTs to 100 blows

BS EN ISO 22476-3 states:

“If a total of 50 blows for the test drive is reached, the test may be finished (N = 50); in soft rocks it can be increased to 100 blows (N = 100).”

Normal practice in the UK is to finish the test at 50 blows in all cases, unless extending to 100 blows has been specified, or there is a specific prior agreement to continue.

Specifiers should only consider extending to 100 blows when this will clearly and justifiably benefit the design. As suggested by the standard, one such case is weak rocks.

Another example, witnessed by the author on several projects, is extending to 100 blows in Thanet Sand, a very dense over-consolidated silty sand. Here the SPT results were used alongside other more precise (but expensive) tests to help identify design critical changes in clay/silt content with depth.

Blow counts > 50 may be encountered in very stiff clays. However, SPT are not normally extended in such conditions because extrapolation of the N value to account for early termination is usually found to be sufficient. Such extrapolation should only be undertaken by the designer, who will need to be mindful of its limitations.

Reporting of seating blows

BS EN ISO 22476-3 requires the incremental blow counts for the main drive to be individually reported. Either two or four increments may be used. Traditionally, in the UK we have reported four x 75mm increments, in accordance with the original BS 1377 Part 9.

For the seating blows BS EN ISO 22476-3 requires 150mm penetration or maximum of 25 blows. However, it also requires the seating blow count/penetration to be reported as one increment, not two as per past (and current) UK practice.

Therefore, a BS EN ISO 22476-3 compliant result reported on a log may look like this, using five increments:

“N=16  (9/4,3,4,5)”

whereas a traditional UK log may report the same in this format, using six increments:

“N=16  (4,5/4,3,4,5)” or “N=16  (4,5,4,3,4,5)”

Despite BS EN ISO 22476-3 having been published for many years now, it appears that the specific requirement for reporting seating blows has not been commonly adopted. However, it also appears that few have noticed. It is also fair to say that it is a relatively minor issue in the grand scheme of things.

It is considered that forcing a change in SPT reporting format across the industry is likely to lead to disruption and confusion, with no obvious benefit. Furthermore, it is possible that a future revision or clarification of the standard may permit reporting in six increments.

The recommendation is that both the five and six increment methods are acceptable, but one of these should always be applied consistently within a given report.

It is recommended that “/” is used as the separator between seating and main blows, with “,” separating the main blows, as shown above. This should provide a helpful visual cue for UK users used to interpreting the first increment(s) as seating.

In AGS format data, by rule the main blows/penetrations are reported in increments 3, 4, 5 and 6 as required, with increments 1 and 2 reserved for seating. If the BS EN ISO 22476-3 five increment system is used then seating should all be in increment 1 with increment 2 unused (blank/null entries preferred to make it clear), i.e. the total seating blows in ISPT_INC1 and the total seating penetration (normally 150mm) in ISPT_PEN1. Main blows then start at increment 3 as normal.

* In accordance with Association of Geotechnical & Geoenvironmental Specialists publication Electronic Transfer of Geotechnical and Geoenvironmental Data, AGS4, Edition 4.1.1 – March 2022 , or earlier editions thereof.

Article by Neil Chadwick, Director at Digital Geotechnical Ltd