SCGEO3110 - Engineering Geology

1. SCGEO3110 - Engineering Geology soil mechanics stuff: - compaction - stress & strain - consolidation

2. What is compaction? A simple ground improvement technique, where the soil is densified through external compactive effort. Compactive effort + water =

4. Increases strength • Decreases permeability • Reduces settlement • Reduces shrinkage Applications: Roads Foundations Embankments Dams Aircraft runways Parking areas Paving Retaining walls Rammed earth structures Etc. etc. Why compact soils?

5. Laboratory tests Mould (standard dimensions) Hammer (standard cross-section area, weight, drop) Method (standard number of layers and number of drops for each layer) Mositure content Theory: • Ralph R. Proctor (circa 1933) related compaction to four variables: • Dry density • Moisture content • Compactive effort • Soil type

6. Dry density (d) d, max optimum water content Water content Compaction Curve Soil grains densely packed - good strength and stiffness - low permeability

7. air water soil Dry density (d) difficult to expel all air lowest void ratio and highest dry density at optimum w Water content Compaction Curve What happens to the relative quantities of the three phases with addition of water?

8. Dry density (d) Zero air void curve (S=100%) Water content Zero Air Void Curve - corresponds to 100% saturation S<100% S>100% (impossible) All compaction points should lie to the left of ZAV curve

9. Dry density (d) E2 (>E1) E1 Water content Effect of Compactive Effort Increasing compactive effort results in: • Lower optimum water content • Higher maximum dry density

10. Dry density (d) more dispersed fabric more dispersed fabric Water content Compaction and Clay Fabric Higher water content or higher compactive effort gives more dispersed fabric.

11. Dry density (d) Compaction curves for different efforts Line of optimum Water content Line of Optimum

12. hammer 1000 ml compaction mould Laboratory Compaction Test • to obtain the compaction curve and define the optimum water content and maximum dry density for aspecific compactive effort. Modified Proctor: Standard Proctor: • 5 layers • 25 blows per layer • 3 layers • 25 blows per layer • 4.9 kg hammer • 450 mm drop • 2.7 kg hammer • 300 mm drop

13. Compaction Control • a systematic exercise where you check at regular intervals whether the compaction was done to specifications. e.g., 1 test per 1000 m3 of compacted soil • Minimum dry density • Range of water content • Field measurements (of d) obtained using • sand cone • nuclear density meter

21. Stress & Strain

22. simple stress (Axial stress) Force (Newton) Stress (Pascal) = Area (square metre) Mass x gravity Area Reaction force

23. Stress due to the weight of soil above sv = gh s2 s3 s3 Horizontal stresses sv= vertical stress (kPa) s2 g = unit weight of soil (kN/m3) h= depth (m) Soil mechanics: stress Stress ellipsiod

24. Stress due to the weight of soil above sv = gh s2 s3 s3 Horizontal stresses sv= vertical stress (kPa) s2 g = unit weight of soil (kN/m3) h= depth (m) Soil mechanics: stress

25. Soil mechanics: stress & strain Circular failure surface due to shearing of the soil

26. Barham River valley Apollo Bay 1987 Moorabool River valley Gheringhap 2001

27. shear stress Normal stress (s) Shear stress (t) Charles-Augustin de Coulomb 1736 - 1806 t = c + stanf Coulomb Equation t = shear stress c = cohesion s = normal stress f = angle of shearing resistance

28. shear stress The groundwater in the pore spaces creates an uplift pressure – the pore water pressure – to the shear plane. The pore water pressure relates to the pressure head caused by the weight of water and rock above Water table The normal stress (s )is countered by the pore water pressure (u) and the result (s – u) is called the effective stress (s’) s Mohr - Coulomb Equation t = c’ + s’tanf’ u t = shear stress c’ = effective cohesion s’ = effective stress f’ = effective angle of shearing resistance

29. Slope mechanics: rainfall as a trigger of instability Water table Rainfall event Raising the watertable increases the pore-water pressure and reduces the effective stress, which in turn lowers the soil’s shear strength and causes a shear failure Pore water pressure time

30. Stress due to the weight of soil above sv = gh s2 s3 s3 Horizontal stresses sv = vertical stress (kPa) s2 g = unit weight of soil (kN/m3) h= distance (m) Soil mechanics: stress Stress ellipsiod

31. Soil mechanics: strain Strain is the change in shape caused by the application of stress

32. Strain ellipsoid Oblate (s1 > s2 = s3) Prolate (s1 = s2 > s3) Triaxial (s1 > s2 > s3) Kinds of strain

33. GL saturated clay Introduction to Consolidation When a saturated clay is loaded externally, the water is squeezed out of the clay over a long time (due to low permeability of the clay).

34. settlement time Soil Consolidation This leads to settlements occurring over a long time, which could be several years.

35. settlement time In granular soils… Granular soils are freely drained, and thus the settlement is instantaneous.

36. Laboratory testing consolidation