As indicated in SECTION 2.0 GEOTECHNICAL INVESTIGATION, an accurate description of the soil profile forms the basis of the geotechnical investigation for any engineering development. It is important that each layer is described in a consistent way to ensure accurate interpretation of the soil profile by those involved in the geotechnical design and construction process.
The description of the soil in profile, based on the work of Jennings, Brink and Williams (1973), is related to the following:
Designation Heading Example
M Moisture Moist
C Colour Reddish Brown C Consistency Stiff
S Structure Intact S Soil Tvpe Clay
O Origin Residual shale
Moisture
The moisture content is assessed as: DRY, SLIGHTLY MOIST, MOIST, VERY MOIST and WET. The assessment at the moisture content is dependant on the soil type. With a moisture content of say 20%, sand will probably be described as wet, whilst clay will probably be described as slightly moist.
Colour
Colour is important for description and for correlation. Colour is described from the soil in profile and also from a small sample of soil made into a creamy paste with water. A profile is MOTTLED when small exposures of different colours occur. A profile is BLOTCHED when larger exposures (say 75 mm and larger) of different colour occur. Colour charts obtainable from the South African Institution of Civil Engineers illustrate the main colours as well as variations in hue and lightness of each colour. These charts illustrate the following colours.
Blue: Dusky Red: Dusky, Dark Pale Pale, Light. Green: Dusky. Grey: Dark
Pale Light
Olive: Dark Orange: Dark reddish Light Light reddish Brown: Dark Dark yellowish
Light Light yellowish Dark reddish Yellow: Dark
Light reddish Light
Consistency
Consistency is a measure of the strength or density of the soil. Observations are based on the effort required to dig into the soil or to mould it with the fingers. The consistency of cohesive soils is based on the undrained shear strength and described as VERY SOFT, SOFT, FIRM, STIFF AND VERY STIFF. Consistency vs. Undrained shear strength guidelines are set out in SECTION 3.3. Non-cohesive soil consistency is based on the angle of shearing resistance of the soil and described as VERY LOOSE, LOOSE, MEDIUM DENSE, DENSE AND VERY DENSE. Consistency vs angle of shearing resistance guidelines are given in SECTION 3.3.
Structure
The presence and type of discontinuities in the soil mass define the structure. Structural characteristics are generally related to cohesive soils in the following terms:
INTACT Absence of fissures and joints, though tension cracks may occur in firm samples when broken with a pick.
FISSURED Presence of closed joints.
SLICKENSIDED Highly polished fissures, usually indicative of expansive soils.
SHATTERED Indicates fissures which have opened up and allowed entry of air, often associated with expansive soils.
MICRO-
SHATTERED Shattering on a small scale with shattered fragments the size of sandgrains. If well developed, the soil appears granular when cut, but the grains break down into clay and/or silt when wetted and rubbed. Indicates the presence of a highly expansive soil.
Soil Type
The soil type is described on the basis of the grain size of the individual particles. The basic grain size classes are given below. Most natural soils occur as a combination of these classes e.g. Silty clay or gravelly sand.
BOULDERS Fragments of rock > 200 mm GRAVEL COBBLES 60mm -200mm
COARSE 20mm -60mm MEDIUM 6mm -20mm FINE 2.0mm -6mm
The range of size of boulders and gravel, the shape, the proportion by volume of the matrix and the description of the matrix are important.
SAND COARSE 0.6mm -2.0mm MEDIUM 0.2mm -0.6mm FINE 0.06mm -O.2mm
Sand particles are visible to the naked eye. SILTS 0.002mm -0.060mm
Silts are barely gritty between fingers and thumb when wet, but are gritty on tongue against teeth. Silts are not easily rolled into threads when moist. Silts exhibit dilatancy when moulded with water into a pat, (i.e. it increases its volume when shearing occurs which is illustrated by the film of water on the surface being absorbed if the pat is distorted.) Silts dry moderately quickly and can be dusted off the fingers. Dry lumps possess cohesion but powder easily in the fingers.
CLAY Particles less than 0.002 mm
Clay particles are flaky (not powdery) when broken and will soften with the addition of water. They have a soapy or greasy feel when wetted and rubbed on the palm of the hand. Clay sticks to fingers and dries slowly. There is no dilatancy or grittiness on tongue against teeth.
Origin
In any soil profile there are four basic categories of origin: • Rock
• Residual soil • Pedogenic material • Transported soil
In the South African context, the demarcation between residual soils and overlying transported soils is often defined by the "pebble marker". This horizon is generally characterised by a gravel layer overlying the residual soil.
• Rock
Materials described as rock comprise igneous, metamorphic or sedimentary (not pedogenic) horizons with unconfined compressive strengths of the intact or unjointed material in excess of 1000 KPa.
• Residual Soil
A residual soil is formed from in-situ decomposition of rock. Decomposition can be caused by chemical weathering or mechanical disintegration which is a function of potential evaporation (temperature, humidity, wind) and average annual precipitation.
• Pedogenic Material
Pedogenic material is residual or transported soil that has become strongly cemented or partially replaced by one of the cementing agencies.
Description Cementing Agency
Ferricrete Iron oxide Calcrete Calcium carbonate
• Transported Soil
This is soil which has been transported by a natural agency (water, wind, gravity) during relatively recent geological times (Pleistocene or Tertiary) and which has not undergone lithification into a sedimentary rock or cementation into a pedogenic material.
Type Agency Source Resulting Soil Talus (scree and
coarse colluvium) Gravity Rock outcrops Unsorted gravel and bouldersangular Hillwash (fine
colluvium) Run-off Acid crystallineBasic crystalline Arinaceous sediment Argillaceous sediment Clayey sand Clay Sand Clay or silt Alluvium (gully wash)
Rivers, streams and gullies
Various rocks and soils Boulders Gravels Sands Silts Clays Lacustrine Deposits Streams terminating
in lake, pan or pool
Various rocks and soils
Sand Silt Clay Estuarine Deposits Tidal rivers and
waters
Mixed Sand Silt Clay Littoral Deposits Waves Mixed Beach sand
Aeolian Deposits Wind Mixed Sand and clayey sand
Subsurface Water Condition
The water table is that level or those levels in the soil where the water in the pores of the soil occurs at atmospheric pressure, i.e. the level to which the water finds its own way in a borehole. The perched water table is a table which is only present in the soil temporarily. It will disappear and sometimes re-appear depending upon seasons or drainage conditions of the site. The permanent water table is the water table which persists throughout the seasons of the year with only minor seasonal fluctuations of level.
A typical soil profile and a tabulation of the various soil symbols are given in Figures 3.1.1 and 3.1.2 respectively.