Revision: Soil = function (climate, organisms, relief (landscape), parent material, time) Hans Jenny (1941) Regolith = “all surficial materials above fresh bedrock” (Ollier & Pain 1996) Definition of a “soil” varies according to the science Pedology - soil profile… A horizon, B horizon, C horizon… soil physics, soil chemistry, soil biology Agriculture – soil as a resource… soil fertility, soil health… topsoil, subsoil Geomorphology – soil series, transported soil, residual soil

Engineering soil: “…a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. Rock on the other hand, is a natural aggregation of minerals connected by strong and permanent cohesive forces… the boundary between rock and soil is necessarily an arbitrary one.” (Karl Terzaghi) Engineering soil: Loose, aggregated, low strength, inelastic. Rock: Compact, solid, high strength, some elasticity When is a soil a rock? When is a rock a soil?

Mechanical properties of soils = function (soil mineralogy, particle size, particle shape, moisture, fabric, organic content) Soil mineralogy: clay minerals have the most influence. Kaolin clays - 1:1 clays (1 silica : 1 gibbsite) - Low CEC - Low water absorption - Low plasticity Illite clays - 2:1 sheets - Intermediate CEC - Intermediate water absorption - Intermediate plasticity Montmorillonite clays - 2:1 sheets - High CEC - High water absorption - High plasticity

Origins: Residual soil = product of in situ weathering of the underlying parent rock Transported soil = transported from their place of origin to where they are now Engineering soil description and classification

ParticleSize Boulder>200 mm Cobble60 mm – 200 mm Gravel2 mm – 60 mm Coarse20 mm – 60 mm Medium6 mm – 20 mm Fine2 mm – 6 mm Sand 2 mm – 60  m Coarse 2 mm – 600  m Medium 600  m – 200  m Fine 200  m – 60  m Silt 60  m – 2  m Clay < 2  m Fine grained Coarse grained

Particle Size Distribution In coarse grain soils …... By sieve analysis Determination of particle size distribution: In fine grain soils …... By hydrometer analysis or particle sizer or sedimentation (pipette) Sieve Analysis Hydrometer Analysis soil/water suspension hydrometer stack of sieves sieve shaker

Sieve analysis testing gear Coarse grained soils Engineering soil classification

Hydrometer Laser particle sizer Fine grained soils Engineering soil classification

Grain Size Distribution Curve can find % of gravels, sands, fines define D 10, D 30, D 60.. as above.

Engineering soil classification GravelSandSiltClay Gravel, well graded Sand, poorly graded (uniform) Sand, gap graded

Engineering soil classification Behaviour: Cohesive = sticks together. Usually contains clay minerals. Non-cohesive = friable, flows like sand. Can stick together when wet (apparent cohesion) Colour: - Relatively unimportant as it is not an mechanical property - Stick to primary colours (red-brown clay, pale yellow sand, grey silt, etc.) - Can be useful to indicate: Organic matter Saturation Filled ground (imported soils) Deleterious materials

“card house” fabric Soil structure Soil fabric Engineering soil classification

Consistency: Cohesive soils TermAbbreviationUCS (kPa)Tactile test Very softVS<25Easily penetrated 5 cm by fist SoftS25 – 50Easily penetrated 5 cm by thumb FirmF50 – 100Can be penetrated 5 cm by thumb with moderate effort StiffS100 – 200Readily indented by thumb but penetrated only with great effort Very stiffVS200 – 400Readily indented by thumb nail HardH≥ 400Indented with difficulty by thumb nail UCS = Unconfined Compressive Strength measured in kiloPascals (kPa) – dealt with in later lectures

Engineering soil classification Consistency: Non-cohesive soils TermAbbreviationRelative density (%)Tactile test Very looseVL< 15Ravelling LooseL15 – 35Easy shovelling Medium denseMD Hard shovelling DenseD Picking Very denseVD> 85Hard picking Relative density = relative to maximum dry density achieved in a standard soil compaction test – dealt with in later lectures

Engineering soil classification Sensitivity: TermUCS in situ : UCS remoulded Insensitive< 2 Medium sensitive2 – 4 Sensitive4 – 8 Very sensitive8 – 16 Quick> 16 Thixotropy: Looses strength under vibration.

Slaking soils: Dispersive soils: Engineering soil classification Behaviour when saturated:

TermAbbreviationTactile DryDLooks and feels dry MoistMFeels and looks moist, but no fee water on hands when remoulding WetWFeels and looks wet, free water on hands when remoulding Engineering soil classification Moisture: Moisture content is given by: Where:  % is the percentage moisture Wt is the initial weight of the soil Wd is the dry weight of the soil

Engineering soil classification Consistency indices: Atterberg Limits Liquid limit – the  above which the soil behaves as a liquid Plastic limit – the  below which the soil ceases behaving as plastic Shrinkage limit – the  below which no further volume change occurs Sticky limit – the  below which the soil looses its adhesive quality Cohesion limit – the  below which the soil particles no longer stick together Liquid Limit (LL) - Plastic Limit (PL) = Plasticity index (PI) Linear shrinkage Free swell

Liquid limit testing devices Engineering soil classification

An international unified soil classification (USC) was developed to for use in airfield construction (Casagrande 1948) and dam construction (1952). The USC provides a common terminology for soils based on their mechanical behaviour. Engineering soil classification ComponentSymbolQualifierSymbol GravelGWell gradedW SandSPoorly gradedP SiltMSiltyM ClayCClayeyC OrganicOLow plasticityL PeatPtHigh plasticityH The primary division is made between: Coarse grained soils >50% of the soil is >60  m Fine grained soils >50% of the soil is <60  m

Engineering soil classification For coarse grained soils: If ≤5% is <60  m, then the soil is either GW, GP or SW, SP If 5% < 60  m ≤ 12% then it carries a dual classification If >12% <60  m then it’s GM, GC or SM, SC For fine grained soils: Use the plasticity chart

Engineering soil calculations solids air Va Vt VsVv Vw Mw Mt Ms water Density: Bulk density, Wet density & unit weight expressed in g/ml or t/m 3 Density: Dry density expressed in g/ml or t/m 3

Particle density: Specific Gravity (Ms & Vs) MaterialParticle density (g/ml) quartz2.65 kaolinite2.4 basalt2.7 – 3.0 silt2.67 silty clay2.75 sandy clay2.70 peat1.50 – 2.15 usually expressed as g/ml Soil particle density is equivalent to the specific gravity of the solids, which is the ratio of the unit weight of solids to the unit weight of water Engineering soil calculations

Water (moisture) content expressed as a percentage Porosity expressed as a percentage Void Ratio Degree of Saturation expressed as a percentage Engineering soil calculations