Geotechnical site investigations
AS 172 6:2 0 1 7This Australian Standard® was prepared by Committee CE-015, Site Investigations. It was approved on behalf of the Council of Standards Australia on 7 April 2017.
This Standard was published on 2 May 2017.
The following are represented on Committee CE-015: Australasian Tunnelling Society
Australian Drilling Industry Association Australian Geomechanics Society Austroads
Cement Concrete and Aggregates Association Consult Australia
CSIRO
International Association of Hydrogeologists Australia New Zealand Geotechnical Society
University of Newcastle University of Wollongong
This Standard was issued in draft form for comment as DR2 AS 1726:2016.
Standards Australia wishes to acknowledge the participation of the expert individuals that contributed to the development of this Standard through their representation on the Committee and through the public comment period.
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Australian Standard
®
Geotechnical site investigations
Originated as AS 1726—1978. Previous edition 1993. Fourth edition AS 1726:2017.
COPYRIGHT
© Standards Australia Limited
All rights are reserved. No part of this work may be reproduced or copied in any form or by any means, electronic or mechanical, including photocopying, without the written permission of the publisher, unless otherwise permitted under the Copyright Act 1968. Published by SAI Global Limited under licence from Standards Australia Limited, GPO Box 476, Sydney, NSW 2001, Australia
ISBN 978 1 76035 743 6
PREFACE
This Standard was prepared by the members of the joint Standards Australia/Standards New Zealand Committee CE-015, Site Investigations, to supersede AS 1726—1993.
After consultation with stakeholders in both countries, Standards Australia and Standards New Zealand decided to develop this Standard as an Australian Standard only, at this time, rather than a joint Australian/New Zealand Standard. This document may become a joint stand in future revisions.
The objective of this Standard is to establish the requirements for the execution of effective geotechnical site investigations and to provide a standardized system for the description and classification of soils and rocks. It addresses spatial and physical characteristics of soil, rock and groundwater, but does not cover the chemical, biological or other environmental aspects of the investigation of contaminated ground.
Commentary on the changes from the 1993 edition is set out in Appendix F.
Statements expressed in mandatory terms in Notes to Tables are deemed to be requirements of this Standard. Figures provided in this Standard are informative.
The term ‘informative’ has also been used in this Standard to define the application of the appendices to which it applies. An ‘informative’ appendix is only for information and guidance.
CONTENTS
Page 1 SCOPE ... 4 2 EXCLUSIONS ... 4 3 NORMATIVE REFERENCES ... 4 4 DEFINITIONS ... 45 OVERVIEW OF GEOTECHNICAL SITE INVESTIGATIONS ... 7
5.1 Process ... 7
5.2 Geotechnical model ... 9
5.3 Execution of geotechnical site investigation ... 10
5.4 Initial assessment of site conditions... 11
5.5 Fieldwork ... 11
5.6 Reporting and interpretation ... 14
5.7 Review of geotechnical site investigation ... 15
6 SOIL, ROCK AND GROUNDWATER ... 16
6.1 Soil description and classification ... 16
6.2 Rock identification, description and classification... 36
6.3 Surface water and groundwater observations ... 58
6.4 Gases ... 58
APPENDICES A GEOTECHNICAL SITE INVESTIGATION TECHNIQUES ... 59
B LABORATORY EXAMINATION AND TESTING ... 61
C GROUNDWATER CONSIDERATIONS ... 64
D PROBLEMATIC MATERIALS ... 66
E SYMBOLS ... 69
F COMMENTARY ... 72
BIBLIOGRAPHY ... 74
STANDARDS AUSTRALIA
Australian Standard
Geotechnical site investigations
1 SCOPEThis Standard specifies requirements for the execution of geotechnical site investigations and provides a standardized system for the identification, description and classification of soils and rocks.
This Standard applies to geotechnical site investigation of natural or filled ground for— (a) new construction;
(b) maintenance of existing facilities;
(c) the evaluation of post construction performance; (d) the assessment of failure; and
(e) broad geotechnical studies.
NOTE: Commentary on the changes from the 1993 edition is set out in Appendix F.
2 EXCLUSIONS
This Standard does not cover the following:
(a) The application of geotechnical site investigation outcomes for geotechnical design. (b) The chemical, biological or environmental aspects of the investigation of
contaminated ground.
3 NORMATIVE REFERENCES
The following normative documents are referenced in this Standard:
NOTE: Documents referenced for informative purposes are listed in the Bibliography. AS
4133 Methods of testing rocks for engineering purposes
4133.4.1 Method 4.1: Rock strength tests—Determination of point load strength index 4133.4.2.1 Method 4.2.1: Rock strength tests—Determination of uniaxial compressive
strength of 50 MPa and greater
4 DEFINITIONS
For the purpose of this Standard, the definitions below apply.
4.1 Acid sulfate soil
Naturally occurring soils, sediments or organic substrates (e.g. peat) that contain sulfide minerals (predominantly pyrite) or their oxidation products. In an undisturbed state where soil is saturated, acid sulfate soils are generally benign. However, if the soils are excavated or exposed to air by a lowering of the groundwater level, the sulfides react with oxygen to
4.3 Carbonate soil
A soil containing more than 50% by weight of carbonate compounds (such as calcium carbonate).
4.4 Cemented soil
A soil bound with a cementing substance, such that if remoulded and recompacted to its original density and moisture content, exhibits a significantly lower strength than in its undisturbed condition.
4.5 Classification
A system which places a material into one of a limited number of groups on the basis of a defined characteristic or set of characteristics. For example, a soil classification may be based on the grading and plasticity of disturbed samples.
4.6 Cohesive and non-cohesive soils
Soils are conveniently divided into two groups based on the ability of a soil mass to hold together. Those capable of holding together are termed ‘cohesive’ and those having no ability, or strength, to hold together by themselves at very low applied total stress levels are termed ‘non-cohesive’.
4.7 Competent person
A person who has, through a combination of training, education and experience, acquired knowledge and skills enabling that person to correctly perform a specified task.
4.8 Consistency
The ability of the soil, at specific moisture contents, to resist mechanical stress or manipulation (remoulding).
4.9 Contamination
The condition of land or water where any chemical substance or waste has been added as a direct or indirect result of human activity above background level and represents, or potentially represents, an adverse health or environmental impact. Contamination may have an impact on human health during construction or the service life of a structure erected on the site or may have detrimental effects on the environment.
NOTE: While this Standard does not address investigation of the presence of contamination or management of such contamination, the possibility of the presence of contamination should be considered during the planning and conduct of geotechnical site investigations.
4.10 Controlled fill
Any fill for which engineering properties are controlled during placement. Sometimes referred to as structural or engineered fill.
4.11 Description, soil or rock
A statement of the physical characteristics of a sample of soil or rock.
4.12 Desk study
A study to collate and review the existing information relevant to the site.
4.13 Dispersive soils
Those soils, which by nature of their mineralogy and pore water chemistry, are susceptible to separation in water of individual clay particles.
NOTE: Refer to Appendix D.
4.14 Duricrust
A cemented zone occurring in weathered rock or soil formed by the mobilization and deposition of minerals.
4.15 Engineered fill
Refer to ‘Controlled fill’.
4.16 Fill
A volume of material that has been placed by anthropogenic processes.
4.17 Geotechnical
Pertaining to the nature, condition and physical properties of the earth’s crust (whether soil or rock and including water and gases therein), which affect its engineering performance.
4.18 Geotechnical model
The interpretation of ground conditions in a form useful for engineering design or assessment. It may contain a surface and subsurface model detailing the geological and engineering characteristics of the various materials and groundwater.
4.19 Geotechnical site investigation
The process of assessing and evaluating the geotechnical characteristics of a site.
4.20 Groundwater
Water located beneath the earth’s surface in pore spaces, fractures and voids in soil and rock.
4.21 In situ
In the place and condition in which it exists naturally.
4.22 Liquid limit (wL)
Moisture content at which the soil passes from the plastic to the liquid state as determined by the liquid limit test.
4.23 May
Indicates that a statement is an option.
4.24 Monitoring
Recording observations and/or measurements over a period of time.
4.25 Mottled
Having areas of two or more colours or shades in a spotted or blotched, irregular configuration.
4.26 Plastic limit (wP)
Moisture content at which the soil becomes too dry to be in a plastic condition as determined by the plastic limit test.
4.27 Plasticity index (IP)
4.30 Should
Indicates that a statement is a recommendation.
4.31 Soil
Particulate materials that occur in the ground and can be disaggregated or remoulded by hand in air or water without prior soaking.
4.32 Rock
Any aggregate of minerals and/or organic materials that cannot be disaggregated by hand in air or water without prior soaking.
4.33 Uncontrolled fill
Materials that have been deposited by anthropogenic processes, which do not meet the definition of ‘controlled fill’.
4.34 Undisturbed sample
A term applied to samples obtained using techniques designed to minimize changes in the properties of the sample.
5 OVERVIEW OF GEOTECHNICAL SITE INVESTIGATIONS 5.1 Process
5.1.1 General
The delivery of geotechnical site investigations should follow an iterative process in which the outcomes of the investigations are reviewed against the purpose for which the investigation is being carried out and further investigations are planned as required. This process is illustrated in Figure 1. Quality assurance and work health and safety programs should be in place during this entire process.
FIGURE 1 GEOTECHNICAL INVESTIGATION—OVERVIEW
5.1.2 Project description
Geotechnical site investigations are usually carried out in service of a wider activity, such D e f i n e t h e p u r p o s e o f t h e i nve s t i g a t i o n a n d i d e n t i f y t h e s c o p e a n d o b j e c t i ve s A s s e m b l e i n f o r m a t i o n r e l a t i n g t o t h e p r o j e c t D eve l o p t h e G e o t e c h n i c a l M o d e l b a s e d o n g e o l o g i c a l c o n c e p t s , g e o t e c h n i c a l i n f o r m a t i o n a n d p r o j e c t i n f o r m a t i o n a n d a n t i c i p a t e w h a t m i g h t b e e n c o u n t e r e d o n s i t e P l a n t h e i nve s t i g a t i o n t o a d d r e s s t h e o b j e c t i ve s R e f i n e t h e G e o t e c h n i c a l M o d e l a n d r ev i e w t h e i nve s t i g a t i o n o u t c o m e s a g a i n s t t h e s c o p e a n d o b j e c t i ve s C a r r y o u t t h e g e o t e c h n i c a l i nve s t i g a t i o n H ave t h e o b j e c t i ve s b e e n m e t a n d t h e s c o p e a c h i eve d? Ye s No C o n c l u d e i nve s t i g a t i o n R ev i s e o b j e c t i ve s a n d /o r s c o p e a n d /o r investigation methods
5.1.3 Purpose of investigation
The general purpose of the geotechnical site investigation and its specific investigation objectives within the context of the project shall be clearly defined and documented to the satisfaction of all parties involved.
5.1.4 Scope of work
The proposed scope of work shall reflect the investigation objectives and shall be clearly documented.
5.1.5 Staging
Geotechnical site investigations are usually carried out in stages, with the number of stages dependent upon the geotechnical complexity of the site.
A program of staged investigation should be developed to implement the selected investigation activities. This program should recognize that, during the investigation, conditions may be revealed which were not anticipated or which trigger a need for review of the remainder of the program. For larger or more complex investigations it may be appropriate to include one or more reporting and review stages in order to refine or revise the latter stages of the investigation.
The stages may include the following: (a) Literature review.
(b) Site walkover.
(c) Field investigation staged in order to retrieve data most economically (e.g. carry out cone penetrometer testing first before deciding where to position boreholes with
in situ testing and sampling).
(d) Review after each stage to assess the need to carry out further stages.
(e) Laboratory testing, staged and optimized for the development of geotechnical design parameters and the soil-groundwater chemistry (e.g. testing to identify acid sulfate soils).
Investigations should be executed in such a way that adequate data are obtained to refine the geotechnical model at the end of each stage, to allow advancement of wider studies or to guide subsequent investigations. Each stage may involve interpretation, analysis and reporting.
5.2 Geotechnical model
A geotechnical model shall be developed for every geotechnical site investigation.
The level of refinement and model detail will depend on the complexity of the project. In its most basic form the geotechnical model may consist of a simple description of the local geology derived from existing data together with some of the engineering characteristics of the project area. More usually, the geotechnical model would include a geological map and cross-section depicting the strata likely to be encountered and information on the engineering characteristics of the soils and rocks and the groundwater levels.
On some large projects, a more sophisticated geotechnical model, based on a large data set and presented as a 3D visualization, may be developed to present the interaction between a large complex structure and a variable soil/rock/groundwater system.
The geotechnical model should be based on factors such as the following: (a) The nature of the project.
(b) The regional geological and hydrogeological setting.
(c) The stratigraphic succession, including the presence of significant aquifers.
(d) Expected groundwater levels. (e) The geological structure.
(f) Geomorphology and surface processes.
(g) Engineering properties of the soils and rocks encountered.
The geotechnical model shall provide a consistent explanation of the concepts, observations and interpretations associated with the geology, geomorphology, hydrogeology, hydrology and engineering characteristics of the project area that are relevant to the project.
The geotechnical model should be used to communicate geotechnical information about the site to all interested parties involved in the project. This information may include:
(i) Past and present surface processes/activities. (ii) Types of soil or rock units and their distribution. (iii) Groundwater levels and groundwater flow directions.
(iv) Preliminary geotechnical characteristics of soil and rock units. (v) The types of geological structure and their orientation.
(vi) Seismic risk.
(vii) Potential occurrence of contaminated ground or groundwater that might be hazardous to people, the environment or the durability of construction materials.
(viii) Potential occurrence of hazardous gases.
5.3 Execution of geotechnical site investigation 5.3.1 Health and safety
Relevant work health and safety legislation applies to all geotechnical site investigations. Site-specific risk assessment and work method statements should be established. All significant risks associated with the geotechnical site investigation should be assessed, and control measures implemented.
5.3.2 Competency
All personnel involved in geotechnical site investigations shall have geotechnical experience, training and qualifications appropriate for their role in the investigation.
5.3.3 Literature review
A literature review shall be carried out as part of the geotechnical site investigation. This review should include the assembly of reports, maps and other information pertaining to the site. In selecting relevant information, the site should be considered in terms of its position in the overall landscape and the broader geological setting, prior to the investigation focusing exclusively on the immediate area of the site.
Information that can be used to gain an understanding of the site prior to fieldwork includes, but is not limited to, the following:
(a) Geological maps and memoirs of the site.
(g) Geohazard maps. (h) Published soils maps. (i) Acid sulfate soil risk maps. (j) Maps of vegetation.
(k) Previous local and anecdotal experience from the area. (l) Historical records such as newspaper articles.
(m) Mine working maps.
(n) Construction records for the site or nearby projects.
5.4 Initial assessment of site conditions
An initial assessment shall be made of the conditions in the area to be investigated, which could be expected to influence the investigation. This may be drawn from site reconnaissance, earlier studies, experience, background reports and published information. This assessment of site conditions influencing the design of the field investigation should include, but not be limited to—
(a) accessibility;
(b) hazards to health, safety and environment; (c) proposed geotechnical model;
(d) structures or services which could affect or be affected by the investigation; (e) groundwater conditions;
(f) geographical, geomorphological and geological features; (g) seismic setting;
(h) potential for encountering contamination and hazardous gases; and (i) regulatory approvals required.
The initial assessment of the site should include an appraisal of the area and volume of ground that would affect or be affected by the project.
5.5 Fieldwork 5.5.1 General
Fieldwork should typically include the following:
(a) Mapping of the topography, geology, geomorphology and other relevant features. (b) Logging of cuttings or other exposures.
(c) A program of sub-surface works (such as boreholes, test pits and probe tests such as cone penetration tests).
(d) Measurements, e.g. recording of groundwater levels and in situ testing. (e) Collection of soil, rock and groundwater samples for subsequent testing.
Fieldwork may also involve use of indirect methods, e.g. seismic or resistivity surveys and use of satellite and airborne sensing.
All observation locations (especially pits, boreholes and probe tests) shall be surveyed to the required level of accuracy.
5.5.2 Selection of investigation methods
The methods to be used for investigation should be selected taking account of— (a) objectives of the investigation;
(b) site conditions; (c) available equipment; (d) cost and time constraints;
(e) health, safety and environmental considerations; and (f) regulatory requirements.
NOTES:
1 A list of some of the available field investigation methods is provided in Appendix A. Laboratory studies may also be required to measure the properties of materials.
2 A list of some of the available laboratory investigation methods is provided in Appendix B. 3 Notes on groundwater considerations are provided in Appendix C.
5.5.3 Data collection and record keeping
Results from routine field tasks shall be recorded in the field (either electronically or on paper). In addition to the results of the field task, records should reference the project, the date, the location and the person carrying out the work.
The records of the results of fieldwork and laboratory testing should be maintained in a form suitable for archival and information transfer. Ideally, this should be in a digital form consistent with standards expected by the client and other professionals contributing to the project.
5.5.4 Sample handling and management 5.5.4.1 Soil
Geotechnical site investigation often involves taking soil samples, which may be either disturbed or undisturbed. Project specific procedures for sample handling and labelling, transport and storage, and chain of custody, shall be developed in order to reduce deterioration in sample quality and the potential for errors.
All samples shall be clearly labelled and logged with a unique reference number immediately after being taken. Samples suspected of contamination shall be identified on the label. Samples taken for geotechnical purposes should be maintained in a temperature range to avoid freezing or heat damage and wide temperature variations.
As soon as practicable after sampling, samples should be stored in airtight bags or containers. Where the intention is to limit disturbance, samples should be coated with wax (preferably microcrystalline) or retained in sealed sample tubes in order to reduce moisture changes during transit to the laboratory. Excess moisture associated with sample collection, such as from drilling fluids, should be removed prior to storage. Soil should be removed from the ends of tube samples to a depth of about 25 mm, and the air gap filled by custom made plugs or molten wax, followed by end caps and adhesive tape.
Undisturbed block samples should be cut and trimmed by hand and wrapped in cloth and coated with molten wax. At least three layers of cloth and wax should be applied on each
Samples taken for the purpose of testing of the soil chemistry (such as for acid sulfate soil analysis) require special handling, and should be managed in accordance with regulatory guidelines pertaining to the relevant state or territory.
5.5.4.2 Rock
Project-specific procedures for core handling and labelling, transport and storage, and chain of custody shall be developed in order to reduce deterioration in core quality and the potential for errors. The same storage temperature control criteria used for soil should be adopted for rock.
Measures should be taken to mitigate moisture loss of rock core while working with it in the field. Rock core shall be placed as soon as practicable into a core box in order of increasing depth, left to right, and top to bottom. The core box shall be uniquely identified and labelled with borehole number and core run depths. In order to provide a permanent visual record, all rock core shall be photographed moist under uniform lighting conditions as soon as practicable after placement in core boxes, and prior to sampling or disturbance during logging. Each photograph should include a reference colour chart.
When core is prone to degradation on drying, the core should be wrapped in plastic film after field logging to reduce changes in moisture content. When core is fragile and may break up in the core box, it should be placed into PVC splits located within the core box. When rock core specimens are sub-sampled from the core tray for laboratory testing, a process for sample handling and management should be developed to reduce the likelihood of damage of the samples during handling and transport. Intervals of core loss or where core is extracted for testing should be marked as such, and in-filled with polystyrene or similar.
Where core is observed to have degraded during handling or storage, this shall be noted on logs.
5.5.4.3 Groundwater
Groundwater samples may contain dissolved or suspended materials. The method of collection, storage and treatment of samples can affect the results obtained from subsequent laboratory testing. The field procedure used to collect samples shall be recorded and reported, indicating—
(a) time of sampling;
(b) purging prior to sample collection;
(c) whether field filtering of the groundwater sample was carried out and if so, the type of filter;
(d) preservation methods employed after sampling and prior to delivery to a laboratory for testing; and
(e) quality control and assurance methods employed.
5.5.4.4 Identification and labelling of samples
Sample identification shall be shown on sample bags, labels or tags and shall be secure and legible. Where bottles or containers are used to store samples, the identification should be placed on the vessel and also on the lid or cap as required.
Each sample retained during the geotechnical site investigation shall be labelled, including a unique identifier. The following details should be included:
(a) Project or job number. (b) Date sampled.
(c) Test pit, borehole, or hand auger hole number.
(d) Sample location and depth. (e) Sample reference number.
(f) Any other relevant information such as requirements for special handling.
Samples suspected of containing hazardous materials should be labelled and packaged such that those handling the samples from accidental exposure are protected.
5.5.4.5 Storage of samples
Prior to and during testing, samples should be stored in designated areas within the laboratory. Samples should be stored in a manner that provides protection from damage, corrosion or contamination that may invalidate test results. All samples should be stored away from direct sunlight and rain.
Careless handling of undisturbed samples after they have been received by the laboratory may cause disturbance that could influence test results, potentially leading to serious design and construction consequences. All tube or block samples should therefore be handled by a competent person and stored in an upright position until required for testing, and in a location where they are not likely to be knocked over or dropped.
The potential for disturbance, moisture migration and corrosion within tube samples increases with time. It is therefore important that samples are prepared and tested in a timely manner. When samples are tested more than 30 days after their retrieval, this should be noted on the laboratory data and test results sheet.
5.6 Reporting and interpretation 5.6.1 General
A report (or reports) shall be produced that presents the information obtained from the geotechnical site investigations. The report content may include—
(a) factual information and observations; (b) interpretations; and
(c) opinions.
The type of report is dependent on the requirements of the project objectives and shall be as agreed during the planning stage. The various types of geotechnical engineering reports are further explained in Clauses 5.6.2 and 5.6.3.
NOTE: A list of graphical symbols that may be useful for reporting purposes is contained in Appendix E.
5.6.2 Geotechnical data report
This report documents the procedures employed and the data collected, and despite the fact that soil and rock logging has an interpretive nature attached to it, a geotechnical data report is considered predominantly factual and may also be referred to as a factual report. A geotechnical data report should include but may not be limited to the following information:
(a) Objectives and agreed scope.
5.6.3 Geotechnical interpretive report
Interpretation is a continuous process, which should begin in the preliminary stages of data collection and should proceed as information from the ground investigation becomes available.
The interpretive report should include but is not limited to the following components: (a) Reference to the data upon which the interpretation has been made.
(b) An interpretation of the site geology and the development of the geotechnical model. (c) A summary of the geotechnical properties of the ground applicable to the project. (d) An engineering interpretation of the implications of the ground conditions for the
project.
(e) An assessment of potential geotechnical risks to the project. (f) Recommendations for further work, if relevant.
In developing geotechnical interpretive reports, there are important aspects to be mindful of, which include the following:
(i) The nature and constraints of the project and proposed development These define
how and where structures or facilities will interact with the ground, including the type, degree and period of loading and any site constraints. Possible impacts on the nearby built or natural environment and potential future uses of the site should be considered.
(ii) The nature and limitations of the geotechnical model The geotechnical model is an interpretation that will change both during the course of the investigations and the development of the overall project, as more information becomes available. Reporting of the geotechnical model should clearly indicate the information on which it is based, the varying reliability of the interpretation and the process whereby an acceptable level of reliability will be achieved.
(iii) The nature and limitation of data Consistency and reliability of data should be assessed (previous work may have been done by different consultants to varying standards of work and assumptions). Cross-checking and verification to the degree practicable should be undertaken. The limitations of the data collected during the investigation program should be highlighted.
A geotechnical interpretive report may also contain expressions of professional opinion. A professional opinion is dependent on conclusions derived from consideration of relevant available facts, interpretations and analysis and judgement. Since the process involves interpretation and judgement, opinions of professionals may differ, although substantial agreement is expected.
5.7 Review of geotechnical site investigation
On completion of a geotechnical site investigation, the findings of the investigation should be reviewed against the objectives of the investigation. Where critical objectives are not adequately achieved, the consequence of this inadequacy should be considered, the risks to the project assessed and recommendations for further investigation developed.
Geotechnical project risks identified in the geotechnical interpretive report may be mitigated by review of geotechnical conditions exposed during construction or by monitoring of performance (such as ground movement monitoring or groundwater level monitoring). Recommendations for such review and monitoring should form part of the geotechnical interpretive report. This may include the recommendation for development of a formal risk assessment and risk management plan.
6 SOIL, ROCK AND GROUNDWATER 6.1 Soil description and classification 6.1.1 General
The classification system adopted in this Standard differs in a number of important respects from the Unified Soil Classification System (USCS) and AS 1726—1993 and may result in some soils being named and classified differently from USCS and AS 1726—1993. A major difference is the criteria used to distinguish between coarse and fine-grained soils. A behavioural approach has been adopted in this Standard when identifying, naming and classifying soil. However, the boundary defining the change in behaviour between coarse and fine-grained soils is not a precise one. Nevertheless, this boundary has been defined in Clause 6.1.4.2, Table 1 and Figure 2.
6.1.2 Basic approach
Description of soil is the process of identifying its components and assessing their relative proportions and behavioural characteristics, observing the condition and structure of the soil and interpreting its origin. Classification of soils involves allocating the soil into different soil groups on the basis of different observable characteristics.
Soil description and classification requires, as a minimum, the identification of the engineering characteristics and properties of the soil through a visual and tactile assessment. Observation and identification of a soil should be carried out in a series of logical steps where the components of the soil are considered and assessed individually. The visual–tactile assessment process may be augmented by laboratory testing. Where laboratory tests are carried out subsequent to visual–tactile assessment, and where these indicate that the visual–tactile assessment was inaccurate, logs and other records of the assessment shall be adjusted if required, and any adjustments made to the logs and other records shall be documented.
Soils may be disturbed or undisturbed but if the soil is disturbed there are limits to what can be described.
The approach described in this Standard is equally applicable to both natural and artificial soil materials.
Although systematic description of the soil composition must be completed before classification can occur, the soil group is usually reported at the beginning of the full description and classification.
6.1.3 Systematic description
A soil description should be presented as a series or list of specific terms, separated by semi-colons, generally without these being formed into sentences. A systematic and standardized order of description shall be used. When it is possible, the following characteristics shall be described:
(a) Composition of soil (disturbed or undisturbed state) The description shall include
the following:
(i) Soil name (use BLOCK LETTERS).
(ii) Plasticity, behavioural or particle characteristics of the primary soil component. (iii) Colour of the soil.
(b) Condition of soil The description shall include the following: (i) Moisture condition (disturbed or undisturbed state). (ii) Consistency of fine-grained soils (undisturbed state only).
(iii) Relative density of coarse-grained soils (determined by in situ tests).
(c) Structure of soil In the undisturbed state, the description shall include the following:
(i) Zoning. (ii) Defects. (iii) Cementing. (d) Origin of soil.
(e) Additional observations.
NOTE: The order of descriptions given above is recommended.
6.1.4 Composition of soils 6.1.4.1 General
Observations of the primary, secondary and minor soil components are used to construct the soil name, which describes the composition of the soil.
6.1.4.2 Soil components
Soils are composed of solid particles, water and gas, sometimes with the inclusion of organic substances. Soil particles are differentiated on the basis of size, according to the definitions in Table 1.
TABLE 1
PARTICLE SIZE DEFINITIONS
Fraction Components Subdivision Size* mm
Oversize BOULDERS >200 COBBLES 63–200 Coarse grained soil GRAVEL Coarse 19–63 Medium 6.7–19 Fine 2.36–6.7 SAND Coarse 0.6–2.36 Medium 0.21–0.6 Fine 0.075–0.21 Fine grained soil SILT 0.002–0.075 CLAY <0.002
* These sizes correspond approximately to standard sieve sizes.
As differentiation of grain sizes of fine particles between clay and silt is difficult, and as the grain size of fine particles is less important than their engineering behaviour, the sizes in Table 1 for silts and clays are taken as indicative only, and instead, fine soils shall be described as silts or clays on the basis of their behaviour as assessed by visual tactile techniques. Additional guidance for field assessment of fine grained soils is provided in Tables 7 and 8.
Soils that contain a significant organic content or a significant carbonate content shall be identified.
6.1.4.3 Identification of components
NOTES:
1 Gravel, sand, silt and clay are the major components of a soil. They are defined in Table 1. 2 Assessment of component proportions is by dry mass.
3 Dilatancy is assessed on the reaction of wet soil to shaking. Table 7 provides a method of assessment of dilatancy as well as other diagnostic characteristics of silt and clay.
FIGURE 2 PROCESS FOR THE FIELD IDENTIFICATION OF SOIL PRIMARY COMPONENTS
C a n a ny o f t h e m a t e r i a l b e d i s a g g r e g a t e d b y h a n d i n w a t e r t o p a r t i c l e s s m a l l e r t h a n c o b b l e s i ze (< 6 3 m m)? Y ES N O N O Y ES N O N O Y ES N O Y ES U s e s o i l d e s c r i p t i ve t e r m s f o r t h e m a t e r i a l l e s s t h a n 6 3 m m i n s i ze D o e s t h e s o i l r e a c t (f i z z) w i t h d i l u t e hyd r o c h l o r i c a c i d? D o v i s i b l e s o i l p a r t i c l e s (s a n d a n d g r ave l ) d o m i n a t e — m a ke u p m o r e t h a n 6 5% o f t h e s o i l? D e s c r i b e p a r t i c l e s l a r g e r t h a n g r ave l s (> 6 3 m m) a s c o b b l e s / b o u l d e r s o r u s e r o c k d e s c r i p t i o n t e r m s a s a p p r o p r i a t e O r g a n i c s o i l a d d t h e p r e f i x “ O r g a n i c ” t o t h e s o i l n a m e C a l c a r e o u s o r c a r b o n a t e s o i l a d d t h e a p p r o p r i a t e p r e f i x f r o m Ta b l e 5 Y ES Y ES G R AV EL N O SA N D PE AT S I LT C L AY Y ES N O F i n e g r a i n e d s o i l I s t h e s o i l d a r k c o l o u r e d , w i t h a n o r g a n i c o d o u r a n d s o m e v i s i b l e o r g a n i c m a t t e r ? D o e s t h e f i n e g r a i n e d s o i l b e h ave l i ke a s i l t — i s t h e s o i l d i l a t a n t ? D o e s t h e s o i l h ave a s p o n g y f e e l o r f i b r o u s t ex t u r e, w i t h s i g n i f i c a n t v i s i b l e o r g a n i c m a t t e r a n d a n o r g a n i c o d o u r ? C o a r s e g r a i n e d s o i l: a m i x t u r e o f s a n d a n d g r ave l
Is more than 50% of the coarse g r a i n e d s o i l g r ave l > 2 m m?
6.1.4.4 Soil name
The soil name shall be based on the identified components of a soil and their behavioural characteristics. The components of a soil are assessed to be primary, secondary, or minor, on the basis of their significance for the probable engineering characteristics of the soil at any particular moisture content.
The primary component of a soil is that component which dominates its engineering behaviour. A secondary component of a soil is any component of a soil which is not the primary component, but which is significant to the engineering properties of the soil. A minor component is present in the soil but is not significant to its engineering properties.
6.1.4.5 Assessment of primary component
In the field, the size of grains in the coarse fraction is assessed from a representative portion of the soil, from which any boulders and cobbles have been removed. It may be helpful to examine the soil under air-dried and/or submerged conditions in order to achieve a clear separation between the coarse and fine fractions. In the laboratory, coarse and fine fractions can be separated by wet sieve analysis.
The primary component is chosen to reflect whether the soil is either fundamentally fine or coarse grained. This assessment is made according to whether the total dry mass of coarse fractions exceeds 65% (a coarse soil) or the total dry mass of fine fractions exceeds 35% (a fine soil).
If it is a coarse grained soil, the relative proportions of the sand and gravel can be estimated or measured and the primary component identified as SAND if sand exceeds gravel, or GRAVEL if gravel exceeds sand.
If it is identified as a fine grained soil, the behaviour shall be assessed to decide if the soil behaves as a silt or a clay and this will indicate primary component and soil name. Where assessed by visual tactile techniques (refer to Tables 7 and 8), a silt is indicated by low dry strength, low wet toughness and dilatancy, whereas a clay is indicated by high dry strength, high wet toughness and plasticity. Where assessed by laboratory testing, clay plots above the A-line and silt below the A-line on the Casagrande chart (refer to Clause 6.1.6 and Figure 5). In borderline cases, the terms silty CLAY or clayey SILT may be used at the discretion of the classifier, noting that there is no specifically defined criterion for their use, and that these descriptions imply only that the materials are borderline between behaving as silts or clays.
The presence of cobbles and boulders shall be specifically noted by beginning the description with ‘MIXTURE OF SOIL AND COBBLES/BOULDERS’ with the word order indicating the dominant proportion first and the proportions of cobbles and or boulders described together with an indication of whether they are supported by a matrix of soil or supported by themselves. The remaining soil shall then be described.
6.1.4.6 Assessment of secondary and minor components
Any accessory soil components (i.e. those other than the primary component) are deemed to be either secondary or minor components, on the basis of their type and their amount, as determined by their presence on a percentage dry mass basis, according to Table 2.
TABLE 2
DESCRIPTIVE TERMS FOR ACCESSORY (SECONDARY AND MINOR) SOIL COMPONENTS
Designation of components
In coarse grained soils In fine grained soils
% Fines Terminology % Accessory coarse fraction Terminology % Sand/ gravel Terminology
Minor 5 Add ‘trace clay/silt’ to description, as applicable 15 Add ‘trace sand/gravel’ to description, as applicable 15 Use ‘trace’
>5, 12 Add ‘with clay/silt’ to description, as applicable >15, 30 Add ‘with sand/gravel’ to description, as applicable >15, 30 Add ‘with sand/gravel’ to description, as applicable Secondary >12 Prefix soil name as
‘silty’ or ‘clayey’, as applicable
>30 Prefix soil name with ‘sandy’ or ‘gravelly’, as applicable >30 Prefix soil name with ‘sandy’ or ‘gravelly’, as applicable
FIGURE 3 DIAGRAM OF VARIOUS PERCENTAGES OF GRAINS
6.1.4.7 Assignment of a name
The name of a soil is made up of the primary and secondary components. The primary component is included as a noun, written in BLOCK LETTERS on logs, qualified by the secondary components (if present), included as adjectives.
Minor fractions are not included in the name, but are included in the description using phrases which include the words ‘trace’ or ‘with’, according to their relative importance, as indicated in Table 2.
As an example, consider a soil with 30% plastic fines and 70% coarse fractions, which comprise 10% gravel and 60% sand. The soil is coarse and sand is the primary component. Gravel is a minor component and does not appear in the name. The plastic fines exceed 12% and are a secondary component, so ‘clayey’ is added as an adjective to the word ‘sand’. Hence it would be named ‘Clayey SAND’.
5% 12 % 3 5%
6.1.4.8 Peat soils
Organic content of soils can have a significant effect on their geotechnical properties. Colour and odour are the key properties for field identification of organic soils. Organic soils are usually either dark grey or black. Fresh, moist organic soils generally have a mouldy odour which can be intensified by heating. Putrefying, rotten organic components in soil can be recognized by their odour typical of hydrogen sulfide, which can be intensified by pouring dilute hydrochloric acid on the sample. Dry inorganic clays have an earthy odour after being moistened. Organic soils should be named with ‘Organic’ preceding the primary soil name. For example, ‘Organic CLAY’ or ‘Organic SILT’.
Peat can be identified in the field by its spongy feel and fibrous texture.
Where laboratory tests are available, organic soils and peat may be identified using Table 3. Table 4 provides terms that may be used to describe the degree of decomposition of Peat.
TABLE 3
IDENTIFICATION OF ORGANIC SOILS USING LABORATORY TESTS
Material Organic content – % of dry mass
Inorganic soil <2
Organic soil 2 to 25
Peat >25
TABLE 4
DESCRIPTIVE TERMS FOR THE DEGREE OF DECOMPOSITION OF PEAT
Term Decomposition Remains Squeeze
Fibrous Little or none Clearly recognizable Only water No solids Pseudo-fibrous Moderate Mixture of fibres and
amorphous paste
Turbid water <50% solids Amorphous Full Not recognizable Paste
>50% solids 6.1.4.9 Carbonate soils
The carbonate content of soils can have a significant effect on their geotechnical properties as soil particles composed of calcium carbonate can have high porosity and low crushing strengths. The carbonate content should be assessed by the application of droplets of dilute hydrochloric acid (10% HCl, see Note to Table 5). Table 5 gives an approximate carbonate content based on the reaction to acid, and provides descriptive terms for carbonate content.
TABLE 5
ASSESSMENT OF CARBONATE CONTENT
Term Reaction to acid carbonate content Approximate
Non-calcareous HCl produces no effervescence Negligible Calcareous HCl produces weak or
sporadic effervescence
<50%
Carbonate HCl produces clear sustained effervescence
>50%
NOTE: 10% hydrochloric acid is made by taking 10 mL of concentrated HCl acid solution (36% HCl) and making it up to 100 mL. This gives 3.6% HCl by mass which is about 1.2 molar.
6.1.4.10 Plasticity and behaviour (fine grained soils)
When laboratory tests are available, clay and silt, both alone and in mixtures with coarser material, shall be described according to their plasticity as defined in Table 6.
TABLE 6
DESCRIPTIVE TERMS FOR PLASTICITY
Descriptive term Range of liquid limit for silt Range of liquid limit for clay
Non-plastic Not applicable Not applicable
Low plasticity 50 35
Medium plasticity Not applicable >35 and 50
High plasticity >50 >50
When laboratory tests are not available, plasticity and soil behaviour are assessed in the field using the visual–tactile techniques described in Table 7. These procedures are to be performed on particles less than 0.2 mm in size. For field classification purposes, screening is not intended, simply remove by hand the coarse particles that interfere with the tests. Table 8 gives a guide to typical soil names and plasticity characteristics.
Standar ds Australia 24 AS 1726:20 17 TABLE 7
FIELD ASSESSMENT OF FINE GRAINED SOILS
Dry strength Dilatancy (reaction to shaking) Toughness (consistency near plastic limit)
Mould a pat of soil to the consistency of putty, adding water if necessary. Allow the pat to dry completely by oven, sun or air drying, and then test its strength by breaking and crumbling between the fingers. This strength is a measure of the character and quantity of the colloidal fraction contained in the soil. The dry strength increases with increasing plasticity. High dry strength is characteristic for clays of the CH group. A typical inorganic silt possesses only very low dry strength.
Silty fine sands and silts have about the same dry strength, but can be distinguished by feel when powdering the dried specimen. Fine sand feels gritty whereas a typical silt has the smooth feel of flour.
Prepare a pat of moist soil with a volume of about 10 cm3.
Add enough water, if necessary, to make the soil soft but not sticky.
Shake the pat horizontally in the palm of the hand, striking vigorously against the other hand several times. A positive reaction consists of the appearance of water on the surface of the pat which changes to a livery consistency and becomes glossy. When the sample is squeezed between the fingers, the water and gloss disappear from the surface. The pat stiffens, and finally it cracks or crumbles. The rapidity of appearance of water during shaking and its disappearance during squeezing assist in identifying the character of the fines in the soil. Very fine clean sands give the quickest and most distinct reaction whereas a plastic clay has no reaction. Inorganic silt, such as a typical rock flour, shows a relatively rapid reaction.
Mould a pat of soil to the consistency of putty. If too dry, add water, and if sticky, the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation. Then, roll a thread of the soil by hand on a smooth surface or between the palms until it is about 3 mm in diameter. The thread is then folded and re-rolled
repeatedly. During this manipulation the moisture content is gradually reduced, the specimen stiffens, finally loses its plasticity, and crumbles. When the thread crumbles, the pieces should be lumped together with a kneading action. The plastic limit has been reached, when the soil crumbles at about 3 mm thickness. The tougher the thread near the plastic limit and the stiffer the lump when it finally crumbles, the more potent is the colloidal clay fraction in the soil.
Weakness of the thread at the plastic limit and rapid loss of coherence of the lump below the plastic limit indicate either inorganic clay of low plasticity, or materials such as kaolin-type clays and organic clays which plot below the A-line. Highly organic clays have a very weak and spongy feel at the plastic limit.
Criteria for describing dry strength Criteria for describing dilatancy Criteria for describing toughness
None The dry specimen crumbles into powder with mere pressure of handling.
None No visible change in the specimen. Low Only slight pressure is required to roll the thread near the plastic limit. The thread and the lump are weak and soft.
Low The dry specimen crumbles into powder with some finger pressure.
Slow Water appears slowly on the surface of the specimen during shaking and does not
disappear or disappears slowly upon squeezing. Medium The dry specimen breaks into pieces or
crumbles with considerable finger pressure.
Medium Medium pressure is required to roll the thread to near the plastic limit. The thread and the lump have medium stiffness.
TABLE 8
IDENTIFICATION OF FINE GRAINED SOILS BY VISUAL—TACTILE METHODS
Soil description Identification of inorganic fine-grained soils
Dry strength Dilatancy Toughness and plasticity
SILT None to low Slow to rapid Low or thread cannot be formed
Clayey SILT—Clay/silt mixtures of low plasticity
Low to medium None to slow Low to medium
Silty CLAY—Silt/clay mixtures of medium plasticity
Medium to high None to slow Medium
High plasticity CLAY High to very high
None High
6.1.4.11 Particle characteristics (coarse grained soil)
Particle size shall be reported in millimetres or by use of the subdivisions in Table 1.
The spread of sizes represented should be described using one of the following terms (refer to Table 9):
(a) ‘Well graded’—having good representation of all particle sizes from the largest to the smallest.
(b) ‘Poorly graded’—with one or more intermediate sizes poorly represented. (c) ‘Gap graded’—with one or more intermediate sizes absent.
(d) ‘Uniform’—essentially of one size.
The assignment of one of the above terms may be made solely on the basis of visual–tactile examination, or on the basis of laboratory measurements. Where laboratory data are available, a well graded soil is one for which the coefficient of uniformity Cu > 4 and the
coefficient of curvature 1 < Cc < 3. Otherwise, the soil is poorly graded. These coefficients
are given by 60 u 10 D C D and
2 30 c 10 60 D C D D where D10, D30 and D60 are those grain sizes for
which 10%, 30% and 60% of the soil grains are smaller.
Where significant, particle shape should be reported as follows:
(i) Equi-dimensional particles: ‘rounded’, ‘sub-rounded’, ‘sub-angular’, or ‘angular’, as shown in Figure 4.
(ii) Essentially two-dimensional particles with the third dimension small by comparison: ‘flaky’ or ‘platy’.
(iii) Essentially one-dimensional particles with the other two dimensions small by comparison: ‘elongated’.
Where significant, particle composition should be described using the rock or mineral name (e.g. quartz sand or carbonate sand).
FIGURE 4 PARTICLE SHAPES
6.1.5 Colour
The colour of a soil shall be described in the moist condition, using simple terms such as black, white, grey, red, brown, orange, yellow, purple, green, blue, etc.
These may be modified as necessary, e.g. by ‘pale’, ‘dark’, or ‘mottled’. Borderline colours may be described as a combination of these colours, e.g. red-brown. Where a soil colour consists of a primary colour with a secondary mottling it shall be described as follows:
(Primary colour) mottled (secondary colour), e.g. grey mottled red-brown clay.
Where a soil consists of two colours present in roughly equal proportions the colour shall be described as mottled (first colour) and (second colour), e.g. mottled brown and red-brown. A mixture of distinct colours may be described as, for example, mottled red/grey.
6.1.6 Soil classification
Soil classification can occur after the soil composition has been described.
Soils shall be classified into one of a number of soil groups designated by a two character group symbol. Classification is based on the grading of the coarse particles, and the behaviour and plasticity of the fraction of the material passing the 0.425 mm sieve. This may be assessed by visual–tactile methods or from laboratory tests.
Where the classification derived from laboratory tests conflicts with that derived from visual–tactile methods the conflict shall be reported and some or all of the visual–tactile classifications may be modified and, where modified, this shall be documented.
The group symbol classifications are given in Tables 9 and 10.
Soils are classified to reflect their primary component and significant secondary components. The first classification symbol shall be G, S, M, or C, where the primary
R o u n d e d A n g u l a r
TABLE 9
CLASSIFICATION OF COARSE GRAINED SOILS
Major divisions symbol Group Typical names Field classification of sand and gravel Laboratory classification
Coarse grained soil (more than 65% of soil excluding oversize fraction is greater than 0.075 mm) GRAVEL (more than half of coarse fraction is larger than 2.36 mm) GW Gravel and gravel-sand mixtures, little or no fines
Wide range in grain size and substantial amounts of all intermediate sizes, not enough fines to bind coarse grains, no dry strength 5% fines Cu > 4 1 < Cc < 3 GP Gravel and gravel-sand mixtures, little or no fines, uniform gravels Predominantly one size or range of sizes with some
intermediate sizes missing, not enough fines to bind coarse grains, no dry strength 5% fines Fails to comply with above GM Gravel-silt mixtures and gravel-sand-silt mixtures
‘Dirty’ materials with excess of non-plastic fines, zero to medium dry strength 12% fines, fines are silty Fines behave as silt GC Gravel-clay mixtures and gravel-sand-clay mixtures
‘Dirty’ materials with excess of plastic fines, medium to high dry strength 12% fines, fines are clayey Fines behave as clay SAND (more than half of coarse fraction is smaller than 2.36 mm) SW Sand and gravel-sand mixtures, little or no fines
Wide range in grain size and substantial amounts of all intermediate sizes, not enough fines to bind coarse grains, no dry strength 5% fines Cu > 6 1 < Cc < 3 SP Sand and gravel-sand mixtures, little or no fines Predominantly one size or range of sizes with some
intermediate sizes missing, not enough fines to bind coarse grains, no dry strength 5% fines Fails to comply with above SM Sand-silt mixtures
‘Dirty’ materials with excess of non-plastic fines, zero to medium dry strength 12% fines, fines are silty NA SC Sand-clay mixtures
‘Dirty’ materials with excess of plastic fines, medium to high dry strength
12% fines, fines are clayey
NOTE: Where the grading is determined from laboratory tests, it is defined by coefficients of curvature Cc and
uniformity Cu derived from the particle size distribution curve, as specified in Clause 6.1.4.11.
For fines contents between 5% and 12%, the soil shall be given a dual classification comprising the two group symbols separated by a dash, e.g. for a gravel with between 5% and 12% silt fines, the classification is GP-GM.
Soils that are dominated by boulders, cobbles or peat (Pt) are described separately and are not classified.
TABLE 10
CLASSIFICATION OF FINE GRAINED SOILS
Major divisions symbol Group Typical names
Field classification of silt and clay classification Laboratory Dry
strength Dilatancy Toughness % < 0.075 mm
Fine grained soils (more than 35% of soil excluding oversize fraction is less than 0.075 mm) SILT and CLAY (low to medium plasticity, %) ML Inorganic silt and very fine sand, rock flour, silty or clayey fine sand or silt with low plasticity None to low Slow to rapid
Low Below A line
CL, CI Inorganic clay of low to medium plasticity, gravelly clay, sandy clay Medium to high None to slow
Medium Above A line
OL Organic silt Low to medium
Slow Low Below A line
SILT and CLAY (high plasticity)
MH Inorganic silt Low to medium None to slow Low to medium Below A line CH Inorganic clay of high plasticity High to very high
None High Above A line
OH Organic clay of medium to high plasticity, organic silt Medium to high None to very slow Low to medium Below A line Highly organic soil Pt Peat, highly organic soil — — — —
NOTE: The U line is an approximate upper bound for most natural soils. Data which plot above the U line may represent unusual/problem soil behaviour, or unreliable data and should be considered carefully.
FIGURE 5 MODIFIED CASAGRANDE CHART FOR CLASSIFYING SILTS AND CLAYS ACCORDING TO THEIR BEHAVIOUR
6.1.7 Condition of soil
The condition of a soil shall be described. The following terms are used to describe the soil condition:
(a) Moisture condition Where the moisture condition of a coarse grained soil is
estimated in the field it should be described by the appearance and feel of the soil using one of the following terms:
(i) Dry (D) – Non-cohesive and free-running. (ii) Moist (M) – Soil feels cool, darkened in colour.
– Soil tends to stick together.
(iii) Wet (W) – Soil feels cool, darkened in colour.
– Soil tends to stick together, free water forms when handling. The moisture condition of fine grained soils shall be described based on a judgement of the soil’s moisture condition relative to the plastic limit (or liquid limit for soils with high moisture contents), as follows:
(A) ‘Moist, dry of plastic limit’ (hard and friable or powdery) (or ‘w < PL’).
(B) ‘Moist, near plastic limit’ (soils can be moulded at a moisture content approximately equal to the plastic limit) (or ‘w ≈ PL’).
(C) ‘Moist, wet of plastic limit’ (soils usually weakened and free water forms on hands when handling) (or ‘w > PL’).
(D) ‘Wet, near liquid limit’ (or ‘w ≈ LL’). (E) ‘Wet, wet of liquid limit’; (or ‘w > LL’).
0 0 10 2 0 3 0 4 0 5 0 6 0 10 2 0 3 0 4 0 5 0 6 0 70 8 0 9 0 10 0 L I Q U I D L I M I T WL, % P L A STI CIT Y IN D E X IP , % the A lin e IP = 0.7 3 (W L - 20) the A line IP = 0. 73 (W L - 2 0) the U li ne IP = 0.9 (WL - 8 ) the U line IP = 0. 9 (W L - 8 ) C L - M L C L - M L C L o r O L C L o r O L C I o r O I C I o r O I C H o r O H C H o r O H M H o r O H M H o r O H M L o r O L M L o r O L
(b) Consistency The consistency of cohesive soils describes the ease with which the soil can be remoulded. Consistency shall be described using the terms in Table 11.
Cohesive soils include fine-grained soils, and coarse grained soils with sufficient fine-grained components to induce cohesive behaviour.
In the field, the consistency of the soil may be assessed either by tactile means, or by measuring the undrained shear strength by mechanical testing (refer to Table 11). Mechanical determination methods should be carried out in accordance with AS 1289 series. Methods not covered by AS 1289 may also be used provided the method is suitably calibrated.
Values of undrained shear strength assessed by field tests for classification purposes may not necessarily be appropriate for use in design.
TABLE 11
CONSISTENCY TERMS FOR COHESIVE SOILS
Consistency Field guide to consistency
Indicative undrained shear strength
kPa
Very Soft (VS) Exudes between the fingers when squeezed in hand 12 Soft (S) Can be moulded by light finger pressure >12 and 25 Firm (F) Can be moulded by strong finger pressure >25 and 50 Stiff (St) Cannot be moulded by fingers >50 and 100 Very Stiff (VSt) Can be indented by thumb nail >100 and 200 Hard (H) Can be indented with difficulty by thumb nail >200 Friable (Fr) Can be easily crumbled or broken into small pieces
by hand
—
NOTE: Consistency is affected by the moisture content of the soil at the time of measurement.
(c) Relative density of non-cohesive, coarse grained soils The density of
non-cohesive soils is described in terms of density index, as defined in AS 1289.5.6.1.
The relative density of coarse grained soils is inherently difficult to assess by visual or tactile means and these techniques should not be used. Relative density assessment should be carried out using a combination of penetration test procedures (standard penetration test, dynamic penetrometer or static cone penetration test, as specified in AS 1289, Methods 6.3.1, 6.3.2, 6.3.3 or 6.5.1) in conjunction with well-established correlations. Alternatively, in situ density tests may be conducted in association with minimum and maximum density tests performed in the laboratory. Table 12 lists descriptive terms applicable to these soils.
TABLE 12 RELATIVE DENSITY OF NON-COHESIVE SOILS
Term Density index %
Very Loose (VL) 15 Loose (L) >15 and 35 Medium Dense (MD) >35 and 65 Dense (D) >65 and 85 Very Dense (VD) >85
NOTE: The moisture content may influence the inferred relative density.
(d) Cementation Soils or defects within soils may be cemented together by various substances. The following terms should be used to describe cemented soils:
(i) ‘Weakly cemented’—the soil may be easily disaggregated by hand in air or water.
(ii) ‘Moderately cemented’—effort is required to disaggregate the soil by hand in air or water.
Where consistent cementation throughout a soil mass is identified as a duricrust, it shall be described in accordance with Clause 6.2 and Table 18.
The nature of the cementing agent shall be identified if possible from its appearance, strength, and reaction to acid.
6.1.8 Mass properties of soil
If present, the structure of the soil shall be described. The following terms should be used. If alternative descriptions are used, the terms shall be defined:
(a) Zoning Soil in situ or in samples may consist of separate zones differing in colour,
grain size or other properties. The patterns of these zones shall be described using the following terms:
(i) ‘Layer’, i.e. the zone is continuous across the exposure or sample.
(ii) ‘Lens’, i.e. a discontinuous layer of different material, with lenticular shape. (iii) ‘Pocket’, i.e. an irregular inclusion of different material.
The thickness, orientation and any distinguishing features of the zones shall be described. The boundaries between zones shall be described as gradational or distinct. ‘Interbedded’ or ‘interlaminated’, shall be used if layers of alternating soil types are too thin to describe individually.
NOTE: The maximum/mean/minimum thickness of the beds/laminations should be given. (b) Defects Defects in soil shall be described using the terms defined in Table 13.
NOTE: The approximate dimensions, orientation and spacing of defects should be given. (c) Mixed soils ‘Intermixed’ may be used to describe two or more soil types arranged in
an irregular manner.
