2014/04/04
University of Pretoria
GEOLOGICAL PROCESSES AND
MATERIALS
SGM210
23 April 2014
1
SOIL PROFILING
According to Jennings, Brink & Williams (1973)
Very important part of a
geotechnical site investigation.
Description of soil profile is the
first tool for a design engineer.
A consistent approach to
profiling enables profile
descriptions from different
people to be comparable.
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MATERIALS IN A SOIL PROFILE
(Jennings, Brink & Williams, 1973)
R
P
T
B
B
B
R
P
T
T
R
B
B
P
Bedrock Residual soils
Transported soils
Pedogenic material (present, absent or weakly developed) Pebble marker (present, absent or weakly developed)
PEDOGENIC MATERIALS
Soils that became cemented or replaced by:
•
•
•
•
Iron oxide (ferricrete)
Calcium carbonate (calcrete)
Silica (silcrete)
Gypsum (gypcrete )
Can have very significant thicknesses. Can be extremely hard.
Can be good founding layer.
Can be useful as gravel for structural fills when well developed.
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SOIL PROFILING – tools of the trade
(A TLB – Tractor-Loader-Backhoe)
SOIL PROFILING – tools of the trade
(an excavator)
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SOIL PROFILING – tools of the trade
(an auger drill)
SOIL PROFILING – tools of the trade
(a geological pick)
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SOIL PROFILING – what to desribe?
Profile description according to six criteria:
M C C S S O
•
•
•
•
•
•
Moisture
Colour
Consistency
Structure
Soil Type
Origin
Moisture
• Dry
• Slightly moist
• Moist
• Very moist
• Wet
Does not refer to a specific water content %.
Moist is near optimum moisture content for compaction.
Wet is generally from below the water table. Never say very wet!!
Dry is bone dry!!
Clays below water table may sometime feel relatively dry, near optimum moisture content.
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Colour
Describe dominant colour (limit to two):
• e.g. reddish brown, yellowish brown.
Secondary colours described as follows:
Speckled
Very small patches of colour.
Mottled
Irregular patches of colour (20 – 60mm)
Blotched
Large irregular patches of colour (60 – 200mm).
Banded
Approx. parallel bands of varying colour.
Streaked
Randomly orientated streaks of colour.
Stained
Local colour variations. Associated with discontinuity
surfaces.
Consistency (non‐cohesive soils)
Crumbles very easily when scraped with geological pick.
Small resistance to penetration by sharp end Loose
of geological pick.
Medium dense Considerable resistance to penetration by sharp end of geological pick.
Very loose
Dense
Very Dense
Very high resistance to penetration by sharp
end of geological pick. Requires many blows
of full size pick for excavation.
Requires power tools for excavation.
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Consistency (cohesive soils)
Very soft
Sharp end of geological pick can be pushed in up to shaft. Easily moulded by fingers.
Soft
Sharp end of geological pick can be pushed in 30
– 40mm. Easily penetrated by thumb.
Sharp end of geological pick can be pushed in 10mm. Indented by thumb with effort.
Slight indentation by push with sharp end of geological pick. Penetrated by thumb nail.
Firm
Stiff
Very stiff
Slight indentation by blow of geological pick. Indented by thumb nail with difficulty.
Structure
• Indicates the presence (or absence) of discontinuities and their nature.
Intact
No discontinuities. (typically in sands)
Fissured
Jointed. (closed joints)
Slickensided
Polished discontinuity surfaces.
Shattered Micro‐shattered
Closely spaced discontinuities resulting in gravel sized soil fragments. (usually in clays)
Stratified, layered,
laminated (very thin
Sedimentary layering.
(clays)
layering)
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“Intact” sand
Polished surfaces are called “slickensided”
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Bottom layer is “sub‐horizontally bedded or layered”
Soil type
Soil texture is described on the basis of grain size
– gravel (fine gravel, medium gravel, coarse gravel etc.)
– sand (fine sand, medium sand, coarse sand)
– silt
– clay
– eg. sandy clay, clayey sand etc. (use soil triangle)
• Boulders
‐
> 200mm
• Gravel
‐
2 ‐ 200 mm
• Sand
‐
0.06 ‐ 2mm
• Silt
‐
0.002 ‐ 0.06mm
• Clay
‐
< 0.002mm
We use MIT system, not
sizes given in Jennings et
al. (1973)
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2014/04/04
Origin
• Geological origin
Check your map
– Residual
– Transported
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Types of transported soil
Aeolian
Wind
Alluvium
Streams
Talus (coarse colluvium)
Gravity (hillside)
Hillwash (fine colluvium)
Water (hillside)
Littoral
Waves, current and tides
Estuarine
Tidal river depositing in saline water
Lacustrine
Stream depositing in lake, pan or vlei
Typically fine sands or silts
Anything from very fine to ve ry
coarse.
Talus (mistake or not??)
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2014/04/04
Addition information mentioned
in profile description
• Depth of water table (if encountered) – perched or permanent.
• Depth of refusal (or not).
• Other inclusions in the soil profile (e.g. roots, termites etc.)
• Samples taken.
• Stability of side walls.
• Type of trial hole (e.g. back‐actor pit or auger hole)
Soil profile
example
ABC Clever Engineers
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Soil profile
example
Residual basalt
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Residual siltstone
Residual granite
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Transported
SOIL PROFILING
SAFETY??
Deep
unsupported
test pits
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SOIL PROFILING SAFETY
National Health and Safety Act:
No person shall enter an unbraced excavation if there is a risk of side wall
collapse. A person shall only enter an excavation when the excavation has
been sufficiently braced, or when authorised in writing by a competent
person who has assessed the risk of side wall collapse and found it to be
negligible.
Standing near an excavator.
Side wall stability: (generally deteriorates with time)
Watch out for test pits in:
• Moist sands.
• Slickensided clays.
• Wet conditions (low strength below water table).
Testpit could collapse very rapidly without any warning!!
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