Soil Physical Properties

1. Soil Physical Properties COSC 323: Soils in Construction

2. Soils by Size • Gravel (Bigger than 2mm in particle size) • Sand (0.1 mm – 2 mm) • Gravel and sand can be classified according to particle size by sieve analysis. • Silt (0.005 mm – 0.1 mm) • Clay (Smaller than 0.005 mm in particle size) • Particle size may be determined by observing settling velocities of the particles in a water mixture. • Coarse-grained soils • Coarser than 0.075 mm or a No. 200 sieve size • Fine-grained soils • Finer than 0.075 mm (silt and clay)

3. Soils by Properties • Granular ( or cohesionless) Soils • Soil particles do not tend to stick together • Gravel • Sand • Silt • Cohesive Soils • Soil particles tend to stick together. • Surface chemical effects • Water-particle interaction and attractive forces between particles • Clay • Organic Soils • Spongy, crumbly, and compressible • Undesirable for use in supporting structures

4. Granular Soils • High shear strength - Large bearing capacity • Small lateral pressure; High permeability (easily drained) • Good backfill materials for retaining walls • Relatively small settlements • Good embankment material • Good foundation materials for supporting roads and structures • Engineering properties of granular soils are affected by • Grain sizes • Shapes • Grain-size distribution • Compactness

5. Cohesive Soils • Sticky, plastic, and compressible • Expand when wetted; Shrink when dried • Creep (deform plastically) over time under “constant” load (when the shear stress is approaching its shear strength) • Develop large lateral pressure • No good for retaining wall backfills • Low permeability or Impervious • Good core materials for earthen dams and dikes • Lower shear strength • Generally undesirable engineering properties

6. Silty Soil • On the border between clayey and sandy soils • Result of mechanical weathering • Clay: result of chemical weathering • Fine-grained, but cohesionless • High capillarity and susceptibility to frost action • Low permeability, Low relative densities

7. Organic Soil • Soil containing a sufficient amount of organic matter to affect its engineering properties • Property: spongy, crumbly, compressible • Low shear strength • May contain harmful material • Unacceptable for supporting foundations

8. Scientific Ways to Name Soils • American Association of State Highway and Transportation Officials (AASHTO) system • Unified Soil Classification System (USCS)

9. Properties of soil • Behavior of sands and gravels is inferred from • Shape • Size • Density of packing of the constituent particles • Behavior of silts and clays is controlled by • Surface activity of the particles • Interaction of the particles with water

10. Permeability • The movement of water within soil • Water moves through the voids • Large void – more permeable • Profound effects on soil properties and characteristics • Rate of consolidation of soil • Related settlement of structures • Amount of leakage through and under dams • Infiltration into excavations • Stability of slopes and embankments • The flow of water through soil is governed by “Darcy’s Low”: • Flow rate through the soil in the conduit varied directly with both the hydraulic head difference and the cross-sectional area of the soil, and inversely with the length over which the hydraulic head difference occurred.

11. Capillarity • The rise of water in a small-diameter tube • Cause: • cohesion of the water’s molecules • adhesion of the water to the tube’s wall • Capillarity in soil • Capillarity tube in soils are the void spaces among soil particles. • Height of capillary rise • Calculation is virtually impossible • Inversely proportional to the tube’s diameter • Associated with the mean diameter of a soil’s voids • The smaller the grain size, the greater will be the capillary rise • Largest capillary rise occurs in soils of medium grain size (such as silts and very fine sands) • Where it occurs? • at the water table

12. Frost Heave • Vertical expansion of soil caused by freezing water within the soil • Serious damage may result from frost heave when structures are lifted • The amount of frost heave is not uniform in a horizontal direction • Develop cracks • When frozen soil thaws, the melted water can not drain through underlying frozen soil • increase water content of the upper soil • decrease its strength • subsequent settlement of structures

13. Compressibility • If soil is compressed • Its volume is decreased • Why? - Reduction in voids within the soil • Result? - Extruding of water from the soil • Building settlement: • Cohesionless soil (sand, gravel) • Compress relatively quickly • Most of the settlement will take place during the construction phase • Compression of cohesionless soils can be induced by vibration. • Cohesive soils (clays) • Compressibility is more pronounced • Lower permeability – expulsion of water from the soil is slow • Compress much slowly • Settlement of a structure built on this soil may not occur until some time after the structure is loaded.

14. Two-phases of settlement • Immediate settlement • Occurs very rapidly • Consolidation settlement • Occurs over an extended period of time (months or years) • Characteristic of cohesive soils • Primary consolidation • Faster and generally larger • Easier to predict • Secondary consolidation (creep) • Occurs subsequent to primary consolidation • Due to plastic deformation of the soil

15. Shear Strength of Soil • Ability to resist shear stress • Cause of shear stress • sloping hillside, filled land, weight of footing • If Shear Strength < Shear Stress • Landslide, footing failure • Where does shear strength come from? • Frictional resistance to sliding; • Interlocking between adjacent solid particles in the soil • Cohesion and adhesion between adjacent soil particles • Important in • Foundation design • Lateral earth pressure calculation • Slope stability analysis, pile design…

16. Shear Strength of Soil Coulomb equation s = shear strength c = cohesion s = effective intergranular normal (perpendicular to the shear plane) pressure f = angle of internal friction tan f = coefficient of friction Cohesionless Soil s = s tanf sand Shear Strength, s s = c + s tanf Cohesive Soil, s = c clay f Cohesion, c Effective Intergranular Normal Pressure, s

17. Angle of Internal Friction • A measurement of the ability of the coarse grained fraction of a soil to resist shear through intergranular friction. • Depends on • Size, shape, and distribution of the grains • Moisture content and the degree of compaction • Larger angle of internal friction when: • Grains are angular rather than rounded • Wide range of grain size rather than a uniformity of size • Fine grained soil does not possess angle of internal friction. • Measured by Direct Shear Test, Triaxial Compression Test

18. Compactness – Relative Density • If soil is in densest condition • Lowest void ratio • Highest shear strength • Greatest resistance to compression • If soil is in loosest condition • Highest void ratio • Lowest shear strength • Lowest resistance to compression • Compactness is the relative condition of a given soil between two extremes

19. Compactness – Relative Density • Relative Density (Dr) • emax : Highest void ratio possible for a given soil (void ratio of the soil in its loosest condition) • e0 : Void ratio of the soil in-place • emin : Lowest void ratio possible for the soil (void ratio of the soil in its densest condition) • Alternative expression using dry unit weights

20. Compactness – Relative Density • Varies between 0 and 100%