Engineering Properties of Soils Soil Definition Engineering refers

Engineering Properties of Soils

Soil Definition (Engineering) – “refers to all unconsolidated material in the earth’s crust, all material above bedrock” • mineral particles (sands, silts, clays) • organic material (topsoil, marshes) + air + water

• Engineering Properties of Soils – foundation for the project – construction material (road embankments, earth dams)

Engg Properties • Specific gravity : Determined by pycnomter (refer to practical class) • Void Ratio • Porosity • Moisture • Grading (refer to practical class) • Clay • Consistency limits • Thixotropy • Permeability • Capillarity • Frost Heave • compressibility

Soils by Size • Gravel (Bigger than 2 mm 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)

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

Engineering Properties of Soils • Granular and Cohesive soil types – difference in engineering properties result from the large variation in size and shape of the grains • Cohesive soil type (clays) – grains are extremely small and flat • the mass of a grain as a force is negligible when compared to the forces resulting from the surface properties of the grain

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

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

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

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

Engineering Properties of Soils • Water Holding Capacity of Clays – Shrinkage • evaporation of exposed clays • loading – Expansion • dry side may absorb moisture • Structure of Clays – deposited by settling out in lakes

Engineering Properties of Soils • Structure of Clays – surface charges forces grains to edge to side pattern – clay structure as opposed to granular soils which are deposited in a denser configuration because the force of gravity on the mass of these grains is more important

Engineering Properties of Soils • Clays have surface charges due to the very large surface area per gram of material • Chemical composition results in: – negative charges along the sides of a grain – positive charges at the ends of a grain clay grain • Results of these surface properties – water holding capacity of clays surface charges attract water – structure of clay deposits

Engineering Properties of Soils • Clay Soils – Small flat shape – Negative/positive surface charges – Bound water on the surface – Different clay minerals are different in size – Swelling clays absorb water into the crystal lattice – Shrinkage due to evaporation or loading

Thixotropy • Thixotropy is the property of a material which allows it to undergo an isothermal gel-to-sol-to-gel transformation upon agitation and subsequent rest. • The softening and subsequent recovery of thixotropic soil appears to be due to first destruction and then to rehabilitation of the molecular structure of the adsorbed layers of the clay particles • When the material is at rest, the ions and water molecules tend to reorient themselves and strength is thereby recovered

Thixotropic clay • A thixotropic clay affected by vibration turns into liquid, becomes sols, without addition of water. • Thus the soil losses its shearing strength, since liquid posses little or no strength. • After a period of rest, however, sols again becomes gels and the clay rehardens. • A thixotropic clay may become fluid during an earthquake

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 Law”: – 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.

Capillarity • • The rise of water in a small-diameter tube Cause: – – • Capillarity in soil – • Capillarity tube in soils are the void spaces among soil particles. Height of capillary rise – – – • cohesion of the water’s molecules adhesion of the water to the tube’s wall 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

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 1. increase water content of the upper soil 2. decrease its strength 3. subsequent settlement of structures

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.

Engineering Properties of Soils Mass-Volume Relationships

Mass-volume relations Soils ENCI 579 23

Mass volume relation Soils ENCI 579 24

Engineering Properties of Soils • Grain Size – grain size distribution curve / Grading Curve (refer to Practical class) • Sieve analysis gravel and sand • Hydrometer test for silt and clay

Hydrometer Test – Used to find the size of smaller grains to plot a grain size distribution curve – Stokes Law • particles in suspension settle out at a rate which varies with their size • hydrometer measures the density of a soil-water mix at various times as the grain settles • The size of particle to the center of the bulb can be calculated and density of the solution indicates the percentage of the sample still in solution

Procedure for grain size determination • Sieving - used for particles > 75 mm • Hydrometer test - used for smaller particles – Analysis based on Stoke’s Law, velocity proportional to diameter Schematic diagram of hydrometer test

Engineering Properties of Soils Sieve Analysis

Curve A - Uniform Soil Curve B - Well Graded Soil

Grading curves F U W Well graded U Uniform P Poorly graded C Well graded with some clay F Well graded with an excess of fines C P W

Engineering Properties of Soils • Grain Size Distribution Curve – Shape • Uniform soil is composed of mainly one size grain • Well graded soil contains a wide range of grain sizes – Effective Size • Effective size is the grain size that only 10% of the grain sizes are finer than. • The amount and type of fine grains in a soil are important in assessing the properties of that soil

Engineering Properties of Soils • Grain Size Distribution Curve – Uniformity Coefficient Cu • indication of the shape of the curve and range of particle sizes that the soil contains • Cu = D 60 / D 10 – Coefficient of Curvature Cc • indication of the shape of the curve. • Cc = (D 30)2 / (D 60 x D 10)