Geotechnical Properties of Soil

Size: px

Start display at page:

Download "Geotechnical Properties of Soil"

Leo Burke
6 years ago
Views:

1 Geotechnical Properties of Soil 1

2 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 limit) 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 D60 Poorly graded Cu D 10 C c D 2 30 D D

3 Effect of Particle Size 3

4 Basic Volume/Mass Relationships 4

5 Additional Phase Relationships Typical Values of Parameters: 5

6 Atterberg Limits 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. 6

7 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 7

8 Clay Mineralogy 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 nonflocculated 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. 8

9 Soil Classification Systems Classification may be based on grain size, genesis, Atterberg Limits, behaviour, etc. In Engineering, descriptive or behaviour based 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 9

10 Soil Classification Systems 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 P - poorly graded Group names: several descriptions Plasticity Chart 10

11 Grain Size Distribution Curve Gravel: Sand: 11

Permeability 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

12 Permeability 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 Unit flow, h q k L Definition of Darcy s Law where: K = hydraulic conductivity h = difference in piezometric or total head L = length along the drainage path Darcy s law is valid for laminar flow Reynolds Number: Re < 1 for ground water flow 12

13 13

14 Permeability of Stratified Soil 14

15 Seepage Analysis 1-D Seepage: Q = k i A where, 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) 15

16 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 16

17 Effective Stress 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 Overconsolidation Ratio, where: σ c = preconsolidation pressure Critical hydraulic gradient σ = 0 when i = (γ b -γ w ) /γ w σ = 0 17

18 Effective Stress Profile in Soil Deposit 18

Vertical Stress Increase with Depth Allowable settlement, usually set by building codes, may control the allowable bearing capacity The vertical stress increase with depth must be determined to

19 Vertical Stress Increase with Depth Allowable settlement, usually set by building codes, may control the allowable bearing capacity The vertical stress increase with depth must be determined to calculate the amount of settlement that a foundation may undergo Stress due to a Point Load In 1885, Boussinesq developed a mathematical relationship for vertical stress increase with depth inside a homogenous, elastic and isotropic material from point loads as follows: 19

20 Vertical Stress Increase with Depth For the previous solution, material properties such as Poisson s ratio and modulus of elasticity do not influence the stress increase with depth, i.e. stress increase with depth is a function of geometry only. Boussinesq s Solution for point load- 20

21 Stress due to a Circular Load The Boussinesq Equation as stated above may be used to derive a relationship for stress increase below the center of the footing from a flexible circular loaded area: 21

22 Stress due to a Circular Load 22

the footing from a flexible rectangular loaded area: Concept of

23 Stress due to Rectangular Load The Boussinesq Equation may also be used to derive a relationship for stress increase below the corner of the footing from a flexible rectangular loaded area: Concept of superposition may also be employed to find the stresses at various locations. 23

24 Newmark s Influence Chart The Newmark s Influence Chart method consists of concentric circles drawn to scale, each square contributes a fraction of the stress In most charts each square contributes 1/200 (or 0.005) units of stress (Influence Value, IV) Follow the 5 steps to determine the stress increase: 1. Determine the depth, z, where you wish to calculate the stress increase 2. Adopt a scale of z = AB 3. Draw the footing to scale and place the point of interest over the center of the chart 4. Count the number of elements that fall inside the footing, N 5. Calculate the stress increase as: 24

25 Simplified Methods The 2:1 method is an approximate method of calculating the apparent dissipation of stress with depth by averaging the stress increment onto an increasingly bigger loaded area based on 2V:1H. This method assumes that the stress increment is constant across the area (B+z) (L+z) and equals zero outside this area. The method employs simple geometry of an increase in stress proportional to a slope of 2 vertical to 1 horizontal According to the method, the increase in stress is calculated as follows: 25

26 Consolidation Settlement total amount of settlement Consolidation time dependent settlement Consolidation occurs during the drainage of pore water caused by excess pore water pressure 26

27 Settlement Calculations Settlement is calculated using the change in void ratio 27

28 Settlement Calculations 28

29 Consolidation Calculations Consolidation is calculated using Terzaghi s one-dimensional consolidation theory Need to determine the rate of dissipation of excess pore water pressures 29

30 Consolidation Calculations 30

31 Shear Strength Soil strength is measured in terms of shear resistance Shear resistance is developed on the soil particle contacts Failure occurs in a material when the normal stress and the shear stress reach some limiting combination 31

32 Direct Shear Test Simple, inexpensive, limited configurations 32

33 Triaxial Shear Test May be complex, expensive, several configurations. Consolidated Drained Test 33

34 Triaxial Shear Test Undrained Loading (f = 0 Concept) Total stress change is the same as the pore water pressure increase in undrained loading, i.e. no change in effective stress Changes in total stress do not change the shear strength in undrained loading 34

35 Stress-Strain Relationships 35

36 Failure Envelope for Clays 36

Unconfined Compression Test A special type of unconsolidated-undrained triaxial test in which the confining pressure, σ 3, is set to zero The axial stress

37 Unconfined Compression Test A special type of unconsolidated-undrained triaxial test in which the confining pressure, σ 3, is set to zero The axial stress at failure is referred to the unconfined compressive strength, q u (not to be confused with Q u ) The unconfined shear strength, c u, may be defined as, 37

Chapter 1 - Soil Mechanics Review Part A

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