The New Zealand Geotechnical Society’s “Guideline for the field classification and description of soil and rock for engineering purposes” (NZGS, 2005) has for 13 years been the standard by which soil and rock logging has been undertaken in New Zealand. NZGS (2005) does not go into exhaustive detail on technical matters regarding soils, but instead refers the reader to the Unified Soil Classification System (USCS) and recommends that its general principles be followed. The limited extent of both in-depth narration in NZGS (2005) and understanding of the USCS in New Zealand has created uncertainty as to which approach should be adopted on a number of important technical matters. This in turn has resulted in an inconsistency in output and interpretation. Using examples of where change would be beneficial, a case is made herein that NZGS (2005) should be updated to present clearer guidance on a range of subjects. Furthermore, it is argued that NZGS (2005) and the USCS are sufficiently different on fundamental issues that the NZGS should develop its own standalone soil and rock logging guide and that reference to the USCS be consigned to history.
The New Zealand Geotechnical Society’s Guideline for the field classification and description of soil and rock for engineering purposes (NZGS, 2005) is the standard New Zealand reference when it comes to geotechnical logging procedures. Its approach is broadly similar to those used in long-established foreign standards such as BS 5930: 2015 Code of Practice for Site investigations, AS 1726: 2017 Geotechnical investigations and ASTM D2487 Unified Soil Classification System (ASTM).
NZGS (2005) refers to the Unified Soil Classification System (USCS) as “the basis for systematic [soil] classification” and states that although it is not intended for the USCS to be “followed to the letter”, its general principles should be adopted. NZGS (2005) is a short document, intended apparently to build on the USCS by providing definitions, brief insights into some technical matters and highlighting where New Zealand practice differs from the USCS, rather than being a standalone New Zealand classification system. In practical terms, the reader of NZGS (2005) needs to be somewhat familiar with the USCS in order to understand the complete classification process. The limited extent of in-depth narration within NZGS (2005), accompanied by a generally limited understanding of the USCS in New Zealand has, in the authors view, resulted in widespread uncertainty within the geotechnical industry as to how to approach the logging of soils. This in turn has resulted in inconsistent interpretations across the New Zealand Geotechnical industry.
In this article the author sets out a case for the NZGS to update NZGS (2005). It is argued that a substantially expanded document is needed, firstly to provide greater clarity on some technical matters and secondly to allow the New Zealand guide to become fully independent of the USCS. Examples of why this update is required are presented.
The case for an expanded document
The fundamental purpose of a classification system is to place something into one of a finite number of categories based on defined criteria. When a classification method is neither clearly defined nor widely understood, confusion and misclassification are likely to result. One area of geotechnical engineering that suffers from a lack of clarity is the classification of soil plasticity.
Despite plasticity being the single most important characteristic of a fine-grained soil, Section 126.96.36.199 “Plasticity” of NZGS (2005) dedicates a total of only seven lines to the subject. The terms “low plasticity” and “high plasticity” are introduced at this point, however no complete plasticity classification system is described. Example soil descriptions use the additional terms “slightly plastic” and “some plasticity”. Despite being widely used in New Zealand practice, the category of “medium plasticity” is not mentioned. A list of acceptable plasticity categories is not presented.
With little clear guidance on what terminology to use, readers of NZGS (2005) are required to either accept the categories included in the document “as is”; to interpret the USCS for themselves; or to follow what they were taught at university or by senior colleagues in the expectation that this will be adequate. People who learnt their profession within a British or Australian context (the author included) may choose to fall back on the general principals of either BS5930: 2015 or AS1726: 2017, potentially unware of the sometimes significant differences between these systems and the USCS/NZGS (2005).
Many geotechnical practitioners believe that only low plasticity and high plasticity should apply in New Zealand, either because these are the only two terms used in Section 188.8.131.52. and/or because these are the only terms supposedly allowed by the USCS plasticity chart. The absence of any mention of “medium plasticity” in NZGS (2005) does nothing to dispel this impression.
If readers of NZGS (2005) are seeking guidance on soil plasticity from the USCS, they should refer to ASTM D2488 Description and identification of soils (visual-manual method) and not the actual definition of USCS, the laboratory-based ASTM D2487 Classification of soils for engineering purposes (Unified Soil Classification System). ASTM D2488 defines four states of plasticity: nonplastic, low plasticity, medium plasticity and high plasticity (Table 1). Given that NZGS (2005) is a field-based system that refers specifically to ASTM D2488-00, these same four plasticity categories should apply in New Zealand. Most people however are not familiar with ASTM D2488 and assume that the applicable plasticity terms are defined by the USCS plasticity chart (ASTM D2487), which is typically interpreted to have just two plasticity categories: low and high.
ASTM D2487 has four non-organic soil groups: lean clay (CH), fat clay (CH), silt (ML) and elastic silt (MH) as defined by the plasticity chart. These are almost universally referred to in New Zealand as low plasticity clay, high plasticity clay, low plasticity silt, and high plasticity silt respectively i.e. there are only low and high categories of plasticity and these are denoted within USCS by the letters L and H respectively. A recent survey undertaken by the author amongst nearly 70 colleagues returned a 98% response along these same lines. This interpretation of the USCS is however incorrect.
Table 1: Criteria for Describing Plasticity (ASTM D2488)
From the very beginning of the USCS, the soil group codes L and H referred to liquid limit and not plasticity. This was clearly stated by USACE (1953) – “The symbols L and H represent low and high liquid limits, respectively”. The initial version of the USCS (USACE, 1953; USBR, 1953) identified the following five plasticity categories: nonplastic, slightly plastic, low plasticity, medium plasticity and high plasticity. These were associated with the relevant soil groups as shown in Table 2. This same information appeared in one form or another in the USCS through multiple editions of the USACE Technical Memorandum No. 3-357, the USBR “Earth Manual”, as well as the first 20 years of ASTM D2487.
It needs to be acknowledged here that Casagrande (1948) could get a bit loose with terminology, for instance describing “the entire range of plasticity is at present is divided into two main groups designated with the letters L and H.” Here Casagrande was referring to the possibility of introducing an intermediate liquid limit range of 35 to 50 to what was then simply a low and high liquid limit system. We know that Casagrande was not referring to two classes of plasticity because he describes the four plasticity classes presented in Table 2. Basically he is acknowledging that the two liquid limit classes cover the entire suite of four plasticity categories. This language was clarified in the USCS (USACE, 1953; USBR, 1953) with the statement given earlier. The USCS never adopted an intermediate category, although BS5930 and AS1726 subsequently did.
The reasons for the misconception that only low and high plasticity grades are allowed under the USCS are probably threefold. The first and most obvious reason is that many geotechnical textbooks straight-out state that the USCS uses L and H to define plasticity. This appears to be most common amongst British and European authors who may be conflating their own systems with the USCS. Wikipedia, everyone’s first port of call to plug an information gap, continues to spread this fallacy (https://en.wikipedia.org/wiki/Unified_Soil_Classification_System).
Secondly, the USCS (ASTM D2487) does not actually discuss plasticity grades, not even with respect to the plasticity chart. All that it provides is a soil group name and group code. The only time plasticity is mentioned in ASTM D2487 is to state that plasticity is a putty-like property of clay and that silt is “nonplastic to very slightly plastic”. The reader of ASTM D2487 is left to their own devices when it comes to assigning a plasticity class to a cohesive soil, which usually means interpreting that L and H in the soil group code refer to the only two valid plasticity categories. Prior to a major revision of ASTM D2487 in 1983, the information on the various plasticity categories presented in Table 2 was included in the USCS to provide the reader with guidance on the matter of plasticity. The revision committee apparently considered plasticity to be a descriptor and not a classifier of soils and it was best left to ASTM D2488. ASTM D2487-83 bears a lot of responsibility for the loss of knowledge in this area.
The third likely reason is the long-standing British influence on geotechnical engineering in New Zealand. Under BS5930, L and H do actually refer to low and high plasticity respectively (as they do in AS1726). It would be natural to assume that this also applies to the USCS, especially when many available references tells us that it does. BS5930 is free to define soil plasticity differently to USCS if it likes, however given that the British Code of Practice CP2001 (1957) included what they called the “modified form of this [“Casagrande scheme”]” in which “fine-grained soils are subdivided into soils of low, medium and high plasticity (suffices L, I and H respectively”, the author suspects that rather than deliberately going its own way with respect to using the Casagrande plasticity chart, the authors of CP2001 (1957) misinterpreted the USCS when they directly correlated plasticity to the liquid limit. This typically does not affect soils that plot above the a-line or are parallel to it, as plasticity index (and plasticity) will increase with liquid limit in such cases. Those soils that plot below the a-line however may exhibit very different liquid limits for the same plasticity index, meaning plasticity and plasticity index are poorly correlated, which does not seem right. Casagrande never assigned high plasticity to MH soils, choosing to call them “elastic” instead. High liquid limits are not always associated with high plasticity or even a high plasticity index.
It is apparent from the above assessment that NZGS (2005) should, as a minimum, define the categories of plasticity that are allowed, which in the authors opinion should be nonplastic, low plasticity, medium plasticity and high plasticity. Slightly plastic is also a possible additional term, given its historical use in the USCS, however low plasticity is probably sufficient on its own. If indeed this was not the intent of NZGS (2005), then the required classification should have been clearly presented.
As an aside, the evaluation of plasticity set out in Table 1 essentially describes the Plastic Limit test (NZS 4402 Test 2.3). It would therefore be possible for a soil’s “field” plasticity to be determined in the lab by an experienced technician. This would serve as a useful means of evaluating the accuracy of the field determinations presented on the logs. Currently only some laboratories provide descriptions of plasticity as part of their Atterberg Limit reporting.
Table 2: USCS classification of fine-grained soils (partial reproduction of USACE, 1953)
Notes: 1: No plasticity is assigned to MH soils in the USACE table
The case for creating a standalone system
The primary purpose of the USCS is to assign soils to one of 15 soil groups, each of which has a unique name and code e.g. CL – lean clay. NZGS (2005) states that “the use of these [i.e. soil group codes] is not encouraged in this guideline as this tends to force rather narrow, artificial limits to the classification process. In this respect NZGS (2005) is probably referring to the fact that the New Zealand approach to soil classification allows for the use of transitional soil types such Silty CLAY and Clayey SILT, not just SILT and CLAY to which the USCS is limited.
Despite this advice, it is not uncommon to see the USCS soil group codes used on logs in New Zealand without apparent recognition that the two systems are incompatible at a rather fundamental level. According to NZGS (2005) it takes a fines content of only 35% for a soil to be considered fine-grained, whereas under the USCS it requires a minimum of 50%. Soils with fines contents of between 35% and 50% will therefore be considered fine-grained under NZGS but coarse-grained under the USCS. The potential impact of this will vary according to geological environment, however it is worth investigating what the outcomes might be if we consider the database of Auckland soils previously reported by Hind (2017).
All of the soils in the database are fine-grained according to NZGS (2005) and each has returned a valid Atterberg Limit test result. Of these demonstrably cohesive soils, 16% classify as coarse-grained (Silty SAND) according to USCS. Yet if we consider the Atterberg Limit data for these Silty SAND’s, the most common classification according to the USCS plasticity chart is CH or high liquid limit CLAY (Figure 1). Clay is the third most abundant material in these soils after sand and silt, however the quantity and mineralogy of the clay is sufficient for most of them to be classified as CLAY according to the USCS. If one was using the USCS system correctly, such soils would not have even been submitted to the laboratory for the Atterberg Limit testing as they must be classified as SM based purely on their particle size distribution alone. Using the USCS and NZGS (2005) systems together in this manner results in the geotechnical log describing a fine-grained soil (probably Sandy CLAY) together with a USCS group code for Silty SAND (SM).
Figure 1: Cohesive Auckland soils that classify as coarse-grained (Silty SAND) according to USCS (data from Hind, 2017)
The example given above is without question an undesirable outcome, not only because of the clear contradictions it presents but also the potential impact on geotechnical assessments such as susceptibility to liquefaction, depending on which classification one chooses to believe. Some may argue that the two systems should not be used together in this manner and the author agrees. However until a NZGS (2005) update mandates that USCS codes not be used, then this practice will continue to occur. Furthermore, it is inevitable that laboratory results are compared to the field data when assessing geotechnical properties and performance of the soils, so we are inevitably forced to consider NZGS (2005) and USCS classifications together.
In an analysis of soil classifications undertaken for the entirely of Auckland soils, Hind (2017) presented evidence of a wide gulf between field and laboratory-derived classifications. The data indicated that field classifications were on the whole much more reflective of the actual composition of the soil (i.e. field classifications were silt-dominated, as were the soils) than the behavioural characteristics supposedly reflected in the plasticity chart (Figure 2). The latter classified the vast majority of Auckland soils as CLAY, and a large majority as CH.
Figure 2: Auckland soils classified according to USCS (left) and NZGS (2005) (middle). The percentage of clay in the soils is summarised in the chart on the right. The field classifications (centre) are much more closely aligned to the proportion of clay in the sample (right) than the USCS basis of classification
A recent reassessment of the same database has indicated a relationship between field classifications and the actual clay content of the soil (Figure 2). It appears that as the clay content of a soil reduces, it is far more likely to be classified in the field as silt-dominated (e.g. Clayey SILT) than a CLAY. There are very few, if any, soils in Auckland that get classified in the field as low plasticity CLAY. This appears to be on account of a lower plasticity being seen as the product of an elevated silt content rather than a less active clay mineralogy.
The evidence presented by Hind (2017) and Figure 2 below suggests that the field logger is able to determine that the non-clay component is dominant (in a volumetric sense) and hence gives the soil a name such as Clayey SILT, yet the soil will generally be sufficiently plastic to plot above the a-line and be classified as a CLAY according to the USCS. In an environment such as Auckland, where the cohesive soils often have a subordinate clay fraction but one that is formed from high activity (swelling) clays, this may well be what is being observed. It needs to be remembered that many of the “clays” that Casagrande (1948) used to define the a-line actually had a significant non-clay component (e.g. Boston Blue Clay, London Clay), so the fact that many of the silt and sand-rich Auckland soils plot above the a-line should not at all be unexpected.
With field classifications and the plasticity chart sometimes resulting in different classifications for the same soil, it can be difficult to use both the field logs and laboratory data in a single coherent geotechnical assessment. There is often pressure to change logs to better reflect the results of the Atterberg Limit test results, without recognising that the two do not really tell the same story and are not measuring the same thing. The fact that NZGS (2005) and the USCS don’t use the same soil names or even the same definition of what a fine-grained soil is, it must be acknowledged that the two systems really don’t have that much in common. Using both systems together is bound to result in widespread inconsistencies and contradictions.
The issue here goes well beyond the USCS constraining the classification process. The author maintains that the USCS is incompatible with NZGS (2005) to such an extent that all links to it, actual or purported, should be removed in favour of a complete and standalone New Zealand system.
A case has been made for the New Zealand Geotechnical Society’s Guideline for the field classification and description of soil and rock for engineering purposes to undergo an extensive update and expansion. It is based on the author’s opinion that reliance on the USCS has resulted in the existing document being insufficiently detailed to adequately inform the reader of the necessary approach to take in classifying soil in New Zealand. NZGS (2005) attempts to remain close to the USCS even though there are clear incompatibilities between the two.
It has been shown that some commonly held beliefs with respect to the classification of soil plasticity in New Zealand are incorrect. This could be addressed by the NZGS guidelines clearly stating what plasticity categories are allowed, together with presenting Table 1 (above) which will be the definitive means of determining plasticity in the field. Although NZGS (2005) was specifically developed for fieldwork, an update should also address the sometimes problematic issue of comparing field and laboratory classifications. Although this is potentially a bit of scope-creep for the document, it would nevertheless be very beneficial to the geotechnical practitioner.
The recommended approach going forward is to acknowledge that the USCS is the origin of the various soil classification systems that we are familiar with, but that the modifications NZGS (2005) has introduced has made the two incompatible. The update should be expansive enough to not only provide sufficient explanatory text but also that it becomes entirely self-contained, with no need for the reader to refer to other documents, be it the USCS or anything else. Relying on outside documents simply results in misinterpretation and contradictions. At this point there seems to be no benefit at all of NZGS (2005) being aligned with the USCS. If parts of the USCS were to remain valid for the NZGS update (such as the plasticity chart?) then these should be adopted fully into the document.
This article has touched on only a couple of issues in and around soil classifications in arguing for an update to NZGS (2005). There are a number of additional areas of contention, such as whether the recording of RQD should continue or not. These and many other topics will need to be addressed in any update which the author hopes the NZGS will commission.
- ASTM D2487-83. 1983. Classification of soils for engineering purposes (Unified Soil Classification System). American Society for Materials and Testing
- ASTM D2487-00. 2000. Classification of soils for engineering purposes (Unified Soil Classification System). American Society for Materials and Testing.
- ASTM D2487-17. 2017. Classification of soils for engineering purposes (Unified Soil Classification System). American Society for Materials and Testing.
- ASTM D2488. 2017. Description of identification of soils (visual-manual procedure). American Society for Materials and Testing.
- AS 1736. 2017. Geotechnical site investigations. Standards Australia.
- BS 5930. 2015. Code of practice for site investigations. British Standards Institute.
- CP2001. 1957. Site Investigations. British Code of Practice. British Standards Institution.
- Casagrande, A. 1948. Classification and identification of soil. Transactions of the American Society of Civil Engineers, Vol 113, Issue 1, 901-930
- Hind, K.J. 2017. The Casagrande plasticity chart – does it help or hinder the NZGS soil classification process?. Proc. 20th NZS Geotechnical Symphosium. Eds. GJ Alexander & CY Chin, Napier.
- USACE. 1953. Unified Soil Classification System (1953a). Technical Memorandum No. 3-357. U.S. Army Corps of Engineers. Office of the Chief of Engineers, Waterways Experiment Station, Vicksburg, Mississippi, Vol 1 to 3.
- USBR. 1953. Unified Soil Classification System. A supplemental to the Earth Manual. U.S. Department of the Interior, Bureau of Reclamation. Denver, Colorado.