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. 1
MATERIALS IN A SOIL PROFILE (Jennings, Brink & Williams, 1973) B R B T B P R T B P T R 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. 2
SOIL PROFILING tools of the trade (A TLB Tractor-Loader-Backhoe) SOIL PROFILING tools of the trade (an excavator) 3
SOIL PROFILING tools of the trade (an auger drill) SOIL PROFILING tools of the trade (a geological pick) 4
SOIL PROFILING what to desribe? Profile description according to six criteria: MCCSSO Moisture Colour Consistency Structure Soil Type Origin Dry Slightly moist Moist Very moist Wet Moisture 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. 5
Colour Describe dominant colour (limit to two): e.g. reddish brown, yellowish brown. Secondary colours described as follows: Speckled Mottled Blotched Banded Streaked Stained Very small patches of colour. Irregular patches of colour (20 60mm) Large irregular patches of colour (60 200mm). Approx. parallel bands of varying colour. Randomly orientated streaks of colour. Local colour variations. Associated with discontinuity surfaces. Consistency (non cohesive soils) Very loose Loose Medium dense Dense Very Dense Crumbles very easily when scraped with geological pick. Small resistance to penetration by sharp end of geological pick. Considerable resistance to penetration by sharp end of geological pick. 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. 6
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. Firm Stiff Very stiff 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. 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 Fissured Slickensided Shattered Micro shattered Stratified, layered, laminated (very thin layering) No discontinuities. (typically in sands) Jointed. (closed joints) Polished discontinuity surfaces. (clays) Closely spaced discontinuities resulting in gravel sized soil fragments. (usually in clays) Sedimentary layering. 7
Intact sand Polished surfaces are called slickensided 8
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 sizesgiveninjenningset al. (1973) 9
Origin Geological origin Residual Transported Check your map 14 18" 26" JJJ' 22 4 (.) 5I cr w o(aigcegroup, tic.) 65 DKelahal1Group Urtenhage ond Zulullllld Gtoopt Dml<6nlbergend lebombo M*no,Eliot end a..-fomebonl Karoo SIMPLIFIED GEOLOGY OF SOUTH AFRICA LESOTHO AND SWAZILAND llofswana t,. 0?' Uḻ 0 crn ze Q. 250 I Cepe lleuor1gtajp Dwpca end EoceGtoupe Sope<vroup, Notal Group Cape GraniteSuite, etc. 5<15 DVnnrhyntdolp end NarneG!ouJ.e NA 11BlA D Alkaline cortlliox" 2050 l!ultmiid c:ornpex (folllc) '> /'!.!.. t- N JfJ" \ 0 80 160 240 320km 34' l'o'iipiulir\ ' U:Ysta (dp"tkicj)ft ('1X,'0ltt*CHO 'I('f 1e -------- ------ ------------ ---------------- 22' ----------- 10
Types of transported soil Aeolian Wind Typically fine sands or silts Alluvium Talus (coarse colluvium) Hillwash (fine colluvium) Littoral Estuarine Lacustrine Streams Gravity (hillside) Water (hillside) Anything from very fine to ve coarse. Waves, current and tides Tidal river depositing in saline water Stream depositing in lake, pan or vlei ry Talus (mistake or not??) 11
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 12
Soil profile example Residual basalt 13
Residual siltstone Residual granite 14
Transported SOIL PROFILING SAFETY?? Deep unsupported test pits 15
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!! 16