SCGEO3110 - Engineering Geology

1. SCGEO3110 - Engineering Geology Introduction to engineering soil mechanics

2. 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

3. When is a soil a rock? When is a rock a 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

4. 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

6. Engineering soil description and classification 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

7. Engineering soil description and classification Coarse grained Fine grained

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

9. Engineering soil classification Coarse grained soils Sieve analysis testing gear

10. Engineering soil classification Fine grained soils Hydrometer Laser particle sizer

11. Grain Size Distribution Curve • can find % of gravels, sands, fines • define D10, D30, D60.. as above.

12. Engineering soil classification Sand, poorly graded (uniform) Gravel, well graded Sand, gap graded Clay Silt Sand Gravel

13. Engineering soil classification • 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 Behaviour: Cohesive = sticks together. Usually contains clay minerals. Non-cohesive = friable, flows like sand. Can stick together when wet (apparent cohesion)

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

15. Engineering soil classification Consistency: Cohesive soils UCS = Unconfined Compressive Strength measured in kiloPascals (kPa) – dealt with in later lectures

16. Engineering soil classification Consistency: Non-cohesive soils Relative density = relative to maximum dry density achieved in a standard soil compaction test – dealt with in later lectures

17. Engineering soil classification Thixotropy: Looses strength under vibration. Sensitivity:

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

19. 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

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

21. Engineering soil classification Liquid limit testing devices

22. 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. • The primary division is made between: • Coarse grained soils >50% of the soil is >60 mm • Fine grained soils >50% of the soil is <60 mm

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

24. Engineering soil calculations Va air Vv Mw Vw water Vt Mt solids Vs Ms Density: Bulk density, Wet density & unit weight Density: Dry density expressed in g/ml or t/m3 expressed in g/ml or t/m3

25. Engineering soil calculations 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 Particle density: Specific Gravity (Ms & Vs) usually expressed as g/ml

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