Soil Types

1. Soil Types • Soil – all unconsolidated material in the earth’s crust • Soil includes – • Mineral particles – sand and clay • Organic Materials – found in topsoil and marsh • Air • Water

2. Mineral Soils • Result from weathering of rock that forms the solid crust of the earth • Physical weathering – due to the action of frost, water, wind, glaciers, landslides, plant and animal life, and other weathering agents – that break particles away from the bedrock • Particles are often transported by wind, water , or ice • Rounds them and further reduces their size • Soils created this way are referred granular soils • Grains or particles are similar to the original bedrock

3. Mineral Soils • Chemical weathering – occurs when water flows through rocks and leaches out some mineral components • New soil particles are formed from these mineral • Called clays • Clay particles are mineral crystals that have very different properties from those of the original bedrock

4. Types of Mineral Soils • Gravel • Sand • Silt • Clay • Course grained soils – gravel and sand • Fine grained – silt and clay • Cobbles – over 75mm or 3 in • Boulders – over 200mm or 8 in • Clays are cohesive soils – bonded to each other

5. Gravels and Sand • Composed mainly of rounded or cubical grains that are supported by adjacent grains • Can carry significant loads • Loads are spread across many particles through friction • Fairly easy to compact • Excellent soils for construction

6. Clays and Silts • Clays are softer • Do not carry loads very well • Clay grains are small size and flat plate like shape • The mass of the grain as a force is negligible when compared to the forces resulting from the surface properties of the grain • Clays have charges on surface – figure 1-1 page 3 • Result of these charges is clay can hold a lot of water • Surface charges attract water molecules • Clays absorb or hold water – permanently unless conditions change • May dry out due to evaporation – or squeeze water out when load is applied – • Will absorb moisture quickly when re applied • The plates of clay are attracted to opposites charges

7. Field Test to Identify Soil types • Large grains (sand and gravel) are easily to identified • Organics are also easy • Silts and clays – are not as easy cause grains are not visible • Page 5 table 1-2 differences between silt and clay

8. Mass-Volume Relations • Soil sample contains • Mineral – possibly organic particles • Water • Air • Mass and volume of each phase is usually calculated • Va = Volume of Air • Vw = Volume of water • Vv = Volume of voids (=Va +Vw) • Vs = Volume of dry soil solids • V = Total volume (=Va +Vw +Vs) • Ma = Mass of air (=0 by definition) • Mw = Mass of water • Ms = Mass of dry soil solids • M = Total mass (=Mw +Ms)

9. Mass and Volume • Soil sample consisting of 10cm3 of air, 25cm3 of water (mass = 25g) and 65cm3 of soil solids (mass = 175g) • Vv = 35cm3 • Va = 10cm3 • Vw = 25cm3 • Vs =65cm3 • V=100cm3 • Mw =25g • Ms=175g • M=200g

10. Mass and Volume • Relationship between the mass and volume • Water • Pw = Mw/Vw • Where Pw = density of water • Density of water is 1g/cm3 or 1000kg/m3 • Example 1-1 Pw = 25g/25cm3 = 1g/cm3 • Soil solids • Psoil solids = Ms/Vs • Where Psoil solids = density fo the dry soil solids • Ratio between soil solids and density of water is the relative density of the solids or specific gravity Gs • RD (relative density) = Psoil solids/Pw = Ms/(Vs*Pw) • Or RD =Ms/(Vs x Pw) • Example 1-1 RD=175g/(65cm3 x 1g/cm3) = 2.69 • Most soils – RD is between 2.6 and 2.8

11. Properties Calculations • Density (P) P=m/v • Dry Density (Pd) Pp=Ms/V • Water content (W) W=Mw/Ms • Void ratio (E) e = Vv/Vs • Degree of saturation (S) S=Vw/Vv • Porosity(n) n=Vv/V • Problem 1-1 on page 8

12. Classification Tests • Two Tests • Grain size – to measure grain sizes • Sieve analysis used for sands and gravels • Grain size distribution graph • Example 1-9 page 17 • Hydrometer used for silts and clays • Sedimentation test • Rate at which particles settle • Strokes law states –that particles in a suspension settle out at a rate that varies with their size • Plasticity – to measure grain types

13. Grain Size Distribution Curve • Used to help describe and classify a soil • Shape – • Uniform soil –curve a on page 19 • Well graded – curve b on page 19 • Effective size • 10% size is considered effective size – page 19 – sample b .09mm • Uniformity coefficient – • Value gives some indication of the shape of the curve • Cu=D60/D10 • Coefficient of curvature • Cc=(d30)2/(d60xD10)

14. Textural Classification • American Society for Testing and Materials • Gravel – larger than 4.75 (no. 4) • Sand 4.75mm to 0.075mm (no. 4 to No. 200) • Silt .075mm to .005mm (No. 200 to .005mm) • Clay – smaller than .005mm

15. Plasticity Test • Measures the amount of water that a soil adsorbs • Plastic limit – soil is roll into a thread(Wp) • Liquid limit (WL) • Index of plasticity – range of water contents over which this soil is plastic • Ip=wl-wp • Atterberg Limits test

16. Soil Water • Type of water found in soil • Free water or gravitational water – found below groundwater – free to flow under the laws of gravity • Capillary water – brought up through the soil pores – above the groundwater table • Attached water or held water – moisture film around soil grains • The rate of water flow – or permeability of the soil • Darcy’s law • Q=k (h/l)a – page 33 • Problem page 34

17. Soil Strength and Settlement • Two type of soil failure • Failures due to shear – grains slide with respect to other grains • Settlement failures – where a layer of soil is compressed and becomes thinner under loading • Forces on soil • Forces acting perpendicular to the plane are normal forces • Forces acting parallel are shear • Shear strength in most clays is due to cohesion • T=c were t is sheering pressure and c = cohesion • Shear strength in granular soils is due to friction • T=o tan o/