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

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

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

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

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

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

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

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)

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

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

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

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

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)

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

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

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

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/