Chapter 1 - Soil Mechanics Review Part A

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1 Chapter 1 - Soil Mechanics Review Part A 1.1 Introduction Geotechnical Engineer is concerned with predicting / controlling Failure/Stability Deformations Influence of water (Seepage etc.) Soil behavour is complex: - Anisotropic - Non-homogeneous - Non-linear - Stress and stress history dependant Complexity gives rise to importance of: - Theory - Lab tests - Field tests - Empirical relations - Computer applications - Experience, Judgement, FOS Unlike steel, soil properties can t be easily determined with a single grade#: - the best we can do: Soil Texture Phase relationships Atterberg Limits Clay mineralogy Fabric and structure Genetic Factors Classification 1.2 Soil Texture Particle size, shape and size distribution - Coarse-textured (Gravel, Sand) - Fine-textured (Silt, Clay) - visibility by the naked eye (0.05mm is the approx limit) CE Class Notes I.R. Fleming Page 1

2 There are several Classification systems with some differences: we use USCS: Particle size distribution - Sieve/Mechanical analysis or Gradation Test - Hydrometer analysis for smaller than.05 to.075 mm (#200 US Standard sieve) Particle size distribution curves - Well graded - Poorly graded D C u = D Cu & Cc together provides a better indication of if a soil is well graded Particle Shape: Roundness vs Angularity Sphericity Higher angularity tends to give higher strength for gravels and sands. Modulus: Not certain Table 1.1 Effect of Particle size: Gravels, Sands Silt Clay generally High Strength High modulus High permeability Granular Cohesionless Effect of water unimportant (except for cyclic loading) generally Lower strength Lower modulus Lower permeability Granular Cohesionless Effect of water important generally Lowest strength Lowest modulus Lowest permeability Non-granular Cohesive Effect of water v. important CE Class Notes I.R. Fleming Page 2

3 Particle size distribution: Important for gravels and sands - well graded material gives higher strength and higher modulus For some sands D 10 can be related to Permeability, but D 10 relates poorly to K for silts and clays Soil is Particulate in Nature Properties of interest strength (stability) and compressibility (settlement) Phases solid, liquid, vapour and others? 1.3 Basic Volume/Mass Relationships The three main physical phases of soil can be represented in graphical form: Figure 1.1 Volume/Mass Relationships. CE Class Notes I.R. Fleming Page 3

4 Based on the volume/mass relationships the following relationships can be defined: Void ratio, e = V V v s Vv Porosity, n(%) = x100% V Vw Degree of saturation, S(%) = x100% V M w Water content, ω(%) = x100% M t Vw Volumetric water content, θ(%) = x100% V M Density, ρ = V t t s v Dry density, ρ = dry M V t s Specific gravity of solids, Ms G s = ρ V water s Based on the above definitions the following relationships may be derived: ρ d ρ ρ s = 1 + e =ρd( 1 w) B + e n = 1+ e ρ B 1+ Se / G = 1+ e Se = wg n e = 1 n s s CE Class Notes I.R. Fleming Page 4

5 When solving problems with phase relations: if possible, choose one quantity to be UNITY and determine the remaining quantities similar to solving a crossword puzzle (i.e. filling the remaining blanks on the phase diagram) Void Ratio: Generally (for the same soil) - modulus increases with decreasing void ratio - strength increases with decreasing void ratio - provides an indication of stress history Moisture content: Saturated moisture content is directly proportional to void ratio. Given that G s is typically between 2.6 to 2.8, all other phase relations depend on w and e. Generally, water content or void ratio alone may not tell much about soil properties when comparing two soils. 1.4 Atterberg Limits Atterberg limits are defined as limits of engineering behavior based on water contents Liquid limit (LL) the water content, in percent, at which the soil changes from a liquid to a plastic state Plastic limit (PL) the water content, in percent, at which the soil changes from a plastic to a semisolid state Shrinkage limit (SL) the water content, in percent, at which the soil changes from a semisolid to a solid state Plasticity index (PI) the difference between the liquid limit and plastic limit of a soil, PI = LL PL CE Class Notes I.R. Fleming Page 5

6 Figure 1.2 Definition of Atterberg Limits. 1.5 Clay mineralogy Clay fraction, clay size particles Particle size < 2 µm (.002 mm) Clay minerals Kaolinite, Illite, Montmorillonite (Smectite) - negatively charged, large surface areas Non-clay minerals - e.g. finely ground quartz, feldspar or mica of "clay" size Implication of the clay particle surface being negatively charged - double layer Exchangeable ions - Li + <Na + <H + <K + <NH + 4 <<Mg ++ <Ca ++ <<Al Valance, Size of Hydrated cation, Concentration Thickness of double layer decreases when replaced by higher valence cation - higher potential to have flocculated structure When double layer is larger swelling and shrinking potential is larger CE Class Notes I.R. Fleming Page 6

7 Soils containing clay minerals tend to be cohesive and plastic. Given the existence of a double layer, clay minerals have an affinity for water and hence has a potential for swelling (e.g. during wet season) and shrinking (e.g. during dry season). Smectites such as Montmorillonite have the highest potential, Kaolinite has the lowest. Generally, a flocculated soil has higher strength, lower compressibility and higher permeability compared to a non-flocculated soil. Sands and gravels (cohesionless ) : Relative density can be used to compare the same soil. However, the fabric may be different for a given relative density and hence the behaviour. Effect of moisture content on strength of Cohesive Soils Lower water content more elastic, more brittle, stiffer and of higher strength (compared to the same soil with higher water content) Higher water content (same material) more plastic, less stiff and of lower strength (compared to the same soil with lower water content) Plasticity - material can be molded to various shapes without breaking it. CE Class Notes I.R. Fleming Page 7

8 Atterberg Limits are water contents, simple and inexpensive Plastic Limit (PL, w P ) Liquid Limit (LL, w L ) Plasticity index (PI, I P ) = LL - PL Water content itself does not tell the whole story about two soils Engineering behaviour expressed by relative position of water content Liquidity Index (LI, I L ) = (w-pl)/pi I L <0 = w<pl - brittle, stiff, high strength 0<I L <1 = PL<w<LL - plastic, lower stiffness, lower strength I L >1 = w>ll - viscous liquid or quick clay Ultra sensitive (Quick) w > LL or LI > 1 Plasticity (PI = LL - PL) increases with %clay - depends on clay mineral Activity = slope of the PI vs % Clay plot Depends on - type of clay mineral present (not just % clay) Sodium smectites A > 4 Calcium smectites A 1.5 Illite A = Kaolinite A = Quartz A = 0 Swell Potential depends on: Type of Clay mineral and % clay A soil that would develop to a thicker double layer: Higher LL and Higher PI Higher PL - not certain There is a relationship between PI and LL indicated the type of clay mineral. Knowing index properties (PI, LL, PL, LI) we have a better idea about the 3 main properties e.g. LI - Strength and modulus, Swell Potential Some empirical relations use PI etc to estimate Strength or modulus CE Class Notes I.R. Fleming Page 8

9 1.6 Soil Classification Systems Classification may be based on grain size, genesis, Atterberg Limits, behavior, etc. In Engineering, descriptive or behaviourbased classification is more useful than genetic classification. American Assoc of State Highway & Transportation Officials (AASHTO) Originally proposed in 1945 Classification system based on eight major groups (A-1 to A-8) and a group index Based on grain size distribution, liquid limit and plasticity indices Mainly used for highway subgrades in USA Unified Soil Classification System (UCS) Originally proposed in 1942 by A. Casagrande Classification system pursuant to ASTM Designation D-2487 Classification system based on group symbols and group names The USCS is used in most geotechnical work in Canada Figure 1.3 Plasticity Chart for Clays and Silts. Group symbols G (gravel), S (sand), M (silt), C (clay), O (organic silts and clay), Pt (peat and highly organic soils), H (high plasticity), L (low plasticity), W (well graded) and P (poorly graded) Group names several descriptions CE Class Notes I.R. Fleming Page 9

10 Table 1.2 Group Symbols According to the UCS. CE Class Notes I.R. Fleming Page 10

11 Table 1.3 Group Names for Course Grained Soils. CE Class Notes I.R. Fleming Page 11

12 Table 1.4 Group Names for Inorganic Fine Grained Soils. CE Class Notes I.R. Fleming Page 12

13 Table 1.4 (cont) Group Names for Organic Fine Grained Soils. CE Class Notes I.R. Fleming Page 13

14 Table 1.5 Flow Cart for USCS (from Codutto, 2001) CE Class Notes I.R. Fleming Page 14

15 1.7 Permeability & Seepage Flow through soils affect several material properties such as shear strength and compressibility If there were no water in soil, there would be no geotechnical engineering Darcy s Law Developed in 1856 h Unit flow, q = K L Where: K = hydraulic conductivity h =difference in piezometric or total head L = length along the drainage path Figure 1.4 Definition of Darcy s Law. Example: For a sandy sample with K=3E-03 cm/s, if h =0.1 m and L = 1 m, what is Q? q = Q/A = k h / L = 3E-05 m/s x 0.1 = 3E-06 m/s (i.e. m 3 /s/m 2 ) CE Class Notes I.R. Fleming Page 15

16 1-D Seepage Q = k i A i = hydraulic gradient = h / L h = change in TOTAL head Downward seepage increases effective stress Upward seepage decreases effective stress 2-D Seepage (flow nets) Reservoir elevation = 168 masl 140 m 40 metres Tailwater Elevation 139 ASL A B Bedrock Elevation variable, i t l 95 ASL CE Class Notes I.R. Fleming Page 16

17 1.7 Effective Stress Effective stress is defined as the effective pressure that occurs at a specific point within a soil profile The total stress is carried partially by the pore water and partially by the soil solids, the effective stress, σ, is defined as the total stress, σ t, minus the pore water pressure, u, σ' = σ u Changes in effective stress is responsible for volume change The effective stress is responsible for producing frictional resistance between the soil solids Therefore, effective stress is an important concept in geotechnical engineering t Overconsolidation ratio, σ OCR = σ ' c ' z Where: σ c = preconsolidation pressure Critical hydraulic gradient σ =0 when i = (γb-γw) /γw σ = 0 CE Class Notes I.R. Fleming Page 17

18 Example: Determine the effective stress distribution with depth if the head in the gravel layer is a) 2 m below ground surface b) 4 m below ground surface; and c) at the ground surface. Steps in solving seepage and effective stress problems: 1. set a datum 2. evaluate distribution of total head with depth 3. subtract elevation head from total head to yield pressure head 4. calculate distribution with depth of vertical total stress subtract pore pressure (=pressure head x γ w ) from total stress 2m Sandy silt till γ B =19 kn/m 3 8 m Sandy Gravel γ B =21.5 kn/m 3 2m CE Class Notes I.R. Fleming Page 18

Geotechnical Properties of Soil

Geotechnical Properties of Soil 1 Soil Texture Particle size, shape and size distribution Coarse-textured (Gravel, Sand) Fine-textured (Silt, Clay) Visibility by the naked eye (0.05 mm is the approximate