3 Terms and definitions
For the purposes of this British Standard, the definitions given in Table 1a), Table 1b), Table 1c), 3.2 and 3.3 apply, together with those given in BS 1377-1, BS EN 1997-1:2004 and BS EN 1997-2:2007.
Within the Eurocode family of documents, BS EN ISO 14688-1:2002 and BS EN ISO 14688-2:2004 establish the basic principles for the identification and classification of soils for engineering purposes. Identification and description of a soil is initially undertaken on the basis of visual techniques in accordance with BS EN ISO 14688-1:2002. BS EN 1997-2:2007, Table 2.1 states that the main soil groups are according to BS EN ISO 14688-1:2002 which defines soils as being:
- Made Ground;
- organic soil;
- volcanic soil;
- fine soils [see Table 1a)];
- coarse soil [see Table 1a)]; or
- very coarse soil [see Table 1a)].
BS EN ISO 14688-1:2002, Figure 1 is a "flow chart for the identification and description of soils" for general (preliminary) characterization of soils after the very coarse fraction (cobbles and boulders) has been screened out. A general test is used to classify the soil type on the basis of "does the soil stick together when wet?"
- Yes (i.e. particles stick together) then = fine soil.
- No (i.e. particles don't stick together) then = coarse soil.
Basic soils are defined as soils with uniform grading that consist of particles of only one size range, the definition of particle names and particle size ranges are set out at Table 1 of BS EN ISO 14688-1:2002, which are illustrated in Table 1a).
In reality almost all uniformly graded soils have some proportion of other particle sizes. The Eurocode system does not set an upper bound on the proportion of secondary fraction required before a soil is defined as a composite soil; this is likely to depend on the soil type. However, in earthworks less than 10% of secondary fraction might be considered as a general guide below which the soil could be considered as a basic soil (rather than a composite soil). The most likely exception would be sand where a fines content of less than 10% can significantly influence the engineering behaviour in earthworks and in that case would be considered a composite soil.
BS EN ISO 14688-2:2004 provides the principles for a more detailed classification of a soil where laboratory test data are available to allow grading, plasticity and organic content to be taken into account more fully. With regard to grading, BS EN ISO 14688-2:2004, Table 2 sets criteria for the designation of coarse fractions based on the shape of the grading curve (both uniformity coefficient and coefficient of curvature CC). In earthworks particular consideration should be given to the uniformity coefficient (CU, previously designated UC, defined as particle size at which 60% of the material is finer/particle size at which 10% is finer). Soils that have a uniformity coefficient of less than 6 (and CC < 1) are distinctly uniform in grading and described as "evenly-graded". The phrase "uniformly graded" is a general descriptive term in BS EN ISO 14688-2:2004 (unlike "evenly-graded"), but, the SHW  system sets a uniformity coefficient upper limit of 10 for classification of uniformly graded granular material which remains a valid consideration for earthworks.
The grouping of soils by particle size for testing purposes [see Table 1b)] is significantly different to that used for soil particle size classification [see Table 1a)]. BS 1377-1 divides soil groups based on the soil sizes that are suitable for different forms of test.
Most soils consist of a principal and secondary fractions and are defined as "composite soil", the identification of which is described at BS EN ISO 14688-1:2002, 4.3. A composite fine soil is one where the fines fraction determines the engineering properties of the soil (this can include relatiμvely coarse soil where the fine matrix is sufficient to result in the soil being matrix dominated); where the fines fraction is insufficient to determine the engineering properties the soil is a composite coarse soil. To fully classify the soil consideration is given to other factors including plasticity of the fines fraction (test carried out on < 425 μm material). However, the principles for classification of soils set out at
BS EN ISO 14688-2:2004 is useful in assessing likely earthworks behaviour.
In the field of earthworks the assessment of whether the fines fraction (i.e. passing a 63 μm sieve) is sufficient to determine the engineering properties of the soil will differ between two major cases:
- where a soil is considered as an engineered fill, where all fills with > 15% fines are classified as cohesive; and
- soil assessed for geotechnical design (e.g. slope stability, settlement or bearing capacity) where generally fine (cohesive) soils are likely to include > 35% fines.
Therefore, in accordance with industry practice, the soils described in this standard as "coarse/granular", "intermediate" or "fine/cohesive" contain different percentages of fines depending on the context in which the descriptions are used [see Table 1c)]. Table 1c) also provides a simplified summary of the BS EN ISO 14688-1:2002 approach to illustrate that both approaches fit within that framework.
In composite soils the assessment of the "dominant soil fraction" which will determine the engineering properties of the soil requires consideration of a variety of soil characteristics (particularly plasticity, grading and soil fabric). Within the intermediate zones illustrated in Table 1c) experience shows that seemingly similar soils can behave differently with relatively small differences in these soil characteristics. These variations are very notable in glacial till as described in CIRIA C504 .
Note that Table 1a) and Table 1b) are presented based on particle sizes, while Table 1c) shows percentage passing the 63 μm sieve.
|Particle Sizes mm||
|Soil fraction||Fine soil||Coarse soil||Very coarse soil|
|Soil Group||Fine-grained soil A)||Medium-grained soil B)||Coarse-grained soils C)||Soil too coarse to be covered by BS 1377 D)
(Zone X Material)
A) Soils containing not more than 10% retained on a 2 mm test sieve.
B) Soils containing more than 10% retained on a 2 mm test sieve but not more than 10% retained on a 20 mm test sieve.
C) Soils containing more than 10% retained on a 20 mm test sieve but not more than 10% retained on a 37,5 mm test sieve.
D) Soils with more than 10% of material retained on a 37,5 mm test sieve are not covered by the laboratory tests detailed in BS 1377-1: except for particle size analysis, and the moisture content and plasticity tests of the finer fraction if present. However, for earthworks control purposes the designer may still choose to utilize the test methods on a modified sample in order to generate test data to consider potential performance (see 7.6.4).
|% passing a 63 μm sieve||
|UK standard approach to earthworks material classification by grading (after SHWA), B))||fill behaviour||granular fill||intermediate
fill C), classified as cohesive fill
|UK traditional approach to classification for geotechnical design (after BS 5930:1999+A1 D))||soil parameters||coarse/granular||intermediate zoneC)||cohesive/fine grained|
|BS EN 1997-1:2004 geotechnical design approach, (after
BS EN ISO 14688-1:2002 E))
|simplified interpretation for comparison purposes||coarse soil||composite coarse soil||composite fine soil||fine soil|
|BS EN 1997-1:2004 approach does not set any fixed boundary but generally > 10% of the secondary fraction is likely to be needed in most soil types to constitute a composite soil.|
|A) SHW  sets the granular/intermediate divide at 15% in recognition of pore water pressure in dynamic action of compaction.
B) The terms "granular" and "cohesive" are included here with regard to behaviour, the soil description terms in accordance with BS 5930:1999+A1 are "coarse" and "fine".
C) The designer has to use judgement of how a soil will behave within the intermediate zone, which is not considered in BS 5930:1999+A1.
D) Most fills in the UK that are in the intermediate range are classified as class 2C. Alternatively, the designer can create a new site-specific class e.g. "class 2F, clayey sand".
E) The BS EN 1997-2:2007 approach for identification and description of soils is set out within BS EN ISO 14688-1:2002, 4.3, by this system many soils are classified as composite soils and the distinction between soil terms can be summarized as follows:
• "composite fine soil" is a soil where the fines content is sufficient to determine the engineering properties;
• "composite coarse soil" is a soil where the fines content is not sufficient to determine the engineering properties (BS EN ISO 14688-1:2002 should be referred to for the full determination procedures).
3.2 Terms used in Section 2
relatively narrow bench or shelf which is provided to break the continuity of a long slope, or as a trap to contain loose material rolling down a slope
3.2.2 colluvial deposits
weathered material transported by gravity, e.g. scree, talus, and landslip debris
work of excavating, or the raising or sloping of ground
NOTE 1 From BS 6100-1:2004.
NOTE 2 The work can be a stand-alone activity or part of a larger project.
- 1) structures formed by the excavating, raising or sloping of ground, e.g. embankments, cuttings or remediated natural slopes
- 2) civil engineering process that includes extraction, loading, transport, transformation/improvement, placement and compaction of natural materials (soils, rocks), and/or secondary or recycled materials, in order to obtain stable and durable cuttings, embankments or engineered fills
readily perceptible down-slope movement of a soil or rock mass, occurring primarily through shear failure on discrete surfaces at the boundaries of the moving mass
open or closed tube or other device installed downward from the ground surface and used to measure the ground water pressure in the region where the piezometer tip is situated
accumulated rock debris at the foot of a cliff
NOTE See colluvial deposits.
3.2.8 slope face angle
angle of any slope expressed either in degrees to the horizontal or as the tangent of the angle to the horizontal (e.g. a slope of 1 in 3 makes an angle to the horizontal whose tangent is 1/3, i.e., 18,5°)
slow downhill movement of soil or scree cover as a result of the alternate freezing and thawing of the contained water
soil, rock or other excavated material which is not required for filling in embankments or as backfill of excavations, and is surplus material removed from the site
downward movement, predominantly vertical in direction, due to removal, consolidation, or displacement of the underlying strata
3.3 Terms used in Section 3
excavation whose length greatly exceeds its width
3.3.2 shallow trench
trench up to 1,5 m in depth
3.3.3 medium trench
trench between 1,5 m and 6,0 m in depth
3.3.4 deep trench
trench exceeding 6.0 m in depth
3.3.5 narrow trenches
class of excavation too narrow to allow the entry of personnel
excavation ranging from that required to receive the foundation base for a pier or column to that required to receive the basement and foundations of a building, including trial pits excavated for site investigation purposes
3.3.7 shallow pit
pit up to 1,5 m in depth
3.3.8 medium pit
pit between 1,5 m and 6,0 m in depth
3.3.9 deep pit
pit exceeding 6.0 m in depth
excavation, which may be either vertical or inclined, constructed to give access to underground works
NOTE Shallow, medium and deep shafts are defined in the same way as shallow, medium and deep pits.