Letter to the Editor – Updating NZGS (2005)

Published 21 December 2019
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Letter to the Editor – Updating NZGS (2005)

Updating NZGS (2005)

Kevin Hind published a detailed article in the December edition of NZ Geomechanics News (p106 -110) including a call for an update of NZGS (2005). His main thrusts related to properties of fine-grained soils, in particular plasticity, and their classification highlighting inconsistencies between NZGS (2005) and overseas practice documents such as ASTM D2487-17, D2488-17. I support his call for such a review and this note makes a couple of comments on a similar theme, noting that reference made to standards or guideline is by title only.

The primary title of NZGS (2005) is “Field Description of Soil and Rock” followed by “Guideline for the field classification and description of soil and rock for engineering purposes”. If one is to follow the practice outline in Australian standard AS 1726:2017 that ‘soil classification can occur after soil composition has been described’, then it can be argued that the definitions and use of classification (identity) and description (in situ properties) terms in NZGS (2005) should be examined. ASTM D2488 outlines practice for description and identification of soils (visual manual including soil name) while ASTM D2487 outlines practice for classification of soils for engineering practice (unified soil classification system – USCS with Group Symbols) based on laboratory determination of particle size distribution, liquid limit and plasticity index. European practice is similar with separate documents for identification and description of soils (BS EN 14688-1:2018) and classification of material characteristics (BS EN 14688-2:2017). In the cases of BS5930:2015 and AS1726 they are combined into single documents with differing approaches as to the inclusion (or not) of classification Group Symbols for coarse-grained soils and different subdivisions of the (Casagrande) plasticity chart by Group Symbol for fine-grained soils. NZGS (2005) as a single document with its classification and description terminology does not include Group Symbols. 

In most cases fine-grained soils are described in the field through an evaluation of several properties, in particular dry strength, dilatancy and toughness, with plasticity added based on observations made during the toughness test. ASTM D2488, BS EN 14688-1 and AS1327 have tabulations of these behavioural observations in defining a soil as silt or clay by a soil symbol, primary soil fraction or soil description respectively. BS EN 14688-1 defines plasticity as the propensity to undergo permanent deformation when kneaded by hand and plastic behaviour as a similar propensity for fine soil. BS5930 notes that the term for plasticity derived from the tactile (toughness related) plastic limit test with high or low values is different from that in the classification based on the (Casagrande) plasticity chart with use of the A-Line and Liquid Limit. It should also be noted that the USBR Earth Manual states that the Liquid Limit is used to distinguish between clays of high compressibility (H) from those of low compressibility (L), not plasticity as became apparent in earlier versions of BS5390.

Kevin pointed out valid inconsistencies between practice areas and the call to update NZGS (2005) is appropriate. This has been acknowledged by the NZGS committee, with inclusion of the description of rock, for which some aspects, such as the description of weathering, need a similar level of consideration as plasticity. Challenges for the update include consistency in use of terms (e.g. classification) and definition of properties (e.g. plasticity) while providing appropriate guidance without becoming a long and tedious document or erring on the side of brevity and leaving out necessary items. 

Stuart Read



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