Soil and Rock Mechanics Unit 2

Introduction to Geotechnical Engineering and Soil Mechanics

  • Soil mechanics is a major specialty area within civil engineering, often referred to as geotechnical engineering. It focuses on how soil and rock support and affect the performance of structures built on or below the Earth's surface.
  • The field investigates the behavior of soil and rock under the specific influence of four primary factors:
    • Water
    • Load
    • Gravitational forces
    • Temperature

Applications of Soil Mechanics in Engineering

  • Foundations for Structures and Embankments: Soil mechanics is critical for supporting physical structures. It addresses:
    • The effect of static loading on soil, which can lead to the settlement of structures or shear failure of the foundation soil.
    • Stability criteria, which dictate that there should be no shear failure of the foundation soil and that settlement must remain within strictly defined permissible limits.
  • Construction Materials: Soils are utilized as materials for specific engineering purposes, including:
    • Subgrade material for road construction.
    • Building earth dams.
    • Land reclamation projects.
  • Slopes and Landslides: The discipline analyzes the stability of slopes. Moisture variation is a major cause of slope failure, resulting in:
    • A reduction of shear strength.
    • An increase in moisture content.
    • The need for braced excavation when digging trenches for buildings.
  • Earth Retaining Structures: Retaining walls are constructed to retain materials and prevent them from sliding or eroding away.
  • Special Problems: Soil mechanics also deals with the effect of water on soil mass, including specific issues like scouring and land erosion.

Soil Formation and the Rock Cycle

  • Soils are formed through the weathering of rocks. The mineral grains constituting the solid phase of a soil aggregate are direct products of rock weathering.
  • Rock is defined as the solid material forming the outer rocky shell or crust of the Earth.
  • Rocks are classified into three major groups based on their origin:
    • Igneous Rocks: Formed when rock is cooled from a molten state.
    • Sedimentary Rocks: Formed from products of weathering of other rocks deposited from a fluid medium, such as water.
    • Metamorphic Rocks: Formed from pre-existing rocks through the action of intense heat and pressure.
  • The Rock Cycle relates these types to the processes associated with soil grain formation. Grain size, shape, and chemical composition—which dictate soil properties—are determined by the rock type and the specific weathering processes involved.

Weathering and Soil Transportation

  • Weathering: The process of breaking down rocks through mechanical and chemical means into smaller pieces.
    • Mechanical Weathering: The physical disintegration or degradation of rock into smaller pieces without changing its chemical composition. Processes include:
    • Freezing and thawing (also known as frost wedging).
    • Differential expansion and contraction caused by temperature changes, such as in deserts or during forest fires, where different minerals or parts of a rock expand and contract at different rates.
    • Chemical Weathering: The decomposition of minerals where one species is changed into another. Water is a primary agent, providing oxygen and mobility for moving ions. The rate of chemical weathering depends on:
    • Temperature.
    • Available surface area.
    • Availability of water or natural acids.
  • Transportation of Soils: Transported soils are classified by their mode of movement and deposition:
    • Glacial soils: Transported and deposited by glaciers.
    • Alluvial soils: Transported by running water and deposited along streams.
    • Lacustrine soils: Formed by deposition in quiet lakes.
    • Marine soils: Formed by deposition in the sea.
    • Aeolian soils: Transported by wind.
    • Colluvial soils: Formed by the movement of soil from its original location by gravity, often during landslides.

Physical Properties and Weight-Volume Relationships

  • Soils are three-phase systems consisting of soil solids, water, and air.
  • Volume Relationships:
    • The total volume VV is the sum of the volume of solids (VsV_s) and the volume of voids (VvV_v).
    • The volume of voids (VvV_v) is the sum of the volume of water (VwV_w) and the volume of air (VaV_a). Thus, V=Vs+Vv=Vs+Vw+VaV = V_s + V_v = V_s + V_w + V_a.
    • Void ratio (ee): the ratio of the volume of voids to the volume of solids. e=VvVse = \frac{V_v}{V_s}.
    • Porosity (nn): the ratio of the volume of voids to the total volume. n=VvVn = \frac{V_v}{V}.
    • Degree of Saturation (SS): the ratio of the volume of water to the volume of voids. S=VwVvS = \frac{V_w}{V_v}. It is usually expressed as a percentage.
    • Connections: e=n1−ne = \frac{n}{1-n} and n=e1+en = \frac{e}{1+e}.
  • Weight Relationships:
    • Assuming the weight of air is negligible, the total weight WW is the sum of the weight of solids (WsW_s) and the weight of water (WwW_w). W=Ws+WwW = W_s + W_w.
    • Moisture Content (ww) (or Water Content): The ratio of the weight of water to the weight of solids. w=WwWsw = \frac{W_w}{W_s}.
    • Unit Weight (γ\gamma): The weight of soil per unit volume. γ=WV=Ws+WwV=Ws[1+w]V\gamma = \frac{W}{V} = \frac{W_s + W_w}{V} = \frac{W_s[1+w]}{V}.
    • Dry Unit Weight (γd\gamma_d): γd=WsV=γ1+w\gamma_d = \frac{W_s}{V} = \frac{\gamma}{1+w}.
    • Saturated Unit Weight (γsat\gamma_{sat}): The unit weight when the soil is fully saturated (S=1S=1).
  • Key Interrelationships:
    • γs=Gs×γw\gamma_s = G_s \times \gamma_w
    • Vs=1×WsGs×γwV_s = \frac{1 \times W_s}{G_s \times \gamma_w}
    • Ww=w×Ws=w×Gs×γwW_w = w \times W_s = w \times G_s \times \gamma_w
    • γ=Gs×γw[1+w]1+e\gamma = \frac{G_s \times \gamma_w[1+w]}{1+e}
    • γd=Gs×γw1+e\gamma_d = \frac{G_s \times \gamma_w}{1+e}
    • S=w×GseS = \frac{w \times G_s}{e}, or S×e=w×GsS \times e = w \times G_s.
    • For saturated soil (S=1S=1), e=w×Gse = w \times G_s.

Mechanical Analysis and Soil Texture

  • Mechanical analysis describes the grain size distribution of soil particles based on texture, size, shape, and gradation.
  • Major Textural Classes:
    • Gravel: Particle size >4.75 mm> 4.75\,mm.
    • Sand: Particle size between 0.075 mm0.075\,mm and 4.75 mm4.75\,mm.
    • Silt: Particle size between 0.005 mm0.005\,mm and 0.075 mm0.075\,mm.
    • Clay: Particle size <0.005 mm< 0.005\,mm.
  • Engineering Classification:
    • Coarse-grained soils: Gravel and sand. These are generally non-cohesive.
    • Fine-textured soils: Silt and clay. These are cohesive, contain clay minerals, and possess plasticity.
  • Particle Size Determination Techniques:
    • Sieve Analysis: Used for particle sizes >0.075 mm> 0.075\,mm (sands and gravels). It involves shaking soil through a nest of standard test sieves.
    • Hydrometer Test: Used for smaller particles (ϕ<0.075 mm\phi < 0.075\,mm). It is based on Stoke's Law, where velocity is proportional to diameter, used for silts and clays.
    • Combined Analysis: For soils containing both fine and coarse materials, both procedures are utilized.

Sieve Analysis Procedure and Standard Sizes

  • Standard Sieve Sizes:
    • Sieve No. 4: 4.75 mm4.75\,mm
    • Sieve No. 10: 2 mm2\,mm
    • Sieve No. 20: 0.85 mm0.85\,mm
    • Sieve No. 40: 0.425 mm0.425\,mm
    • Sieve No. 60: 0.25 mm0.25\,mm
    • Sieve No. 100: 0.15 mm0.15\,mm
    • Sieve No. 200: 0.075 mm0.075\,mm
  • Procedure:
    1. Determine total mass of soil sample (ΣM\Sigma M).
    2. Sieve the oven-dried sample through a nest of sieves in descending order of mesh size.
    3. Determine the mass of soil retained on each sieve (M1,M2,...MnM_1, M_2, ... M_n) and the pan (MpM_p).
    4. Verify that ΣM=M1+M2+...+Mn+Mp\Sigma M = M_1+M_2+...+M_n+M_p.
    5. Calculate cumulative mass retained above each sieve (ΣMi\Sigma M_i).
    6. Calculate the mass passing the sieve as ΣM−ΣMi\Sigma M - \Sigma M_i.
    7. Calculate Percent Finer (FF) using: F=ΣM−ΣMiΣM×100F = \frac{\Sigma M - \Sigma M_i}{\Sigma M} \times 100.
  • Sample Grading Curve Data:
    • 4.75 mm4.75\,mm opening: 81%81\% finer.
    • 0.85 mm0.85\,mm opening: 55%55\% finer.
    • 0.25 mm0.25\,mm opening: 23%23\% finer.
    • 0.15 mm0.15\,mm opening: 10%10\% finer.
    • 0.075 mm0.075\,mm opening: 3%3\% finer.

Consistency and Atterberg Limits

  • Consistency refers to the physical state of a fine-grained (cohesive) soil at a given moisture content. Plasticity describes how soil responds to changes in moisture content. As water content increases, strength decreases and soils swell.
  • The Four Consistency States:
    • Solid
    • Semi-solid (or semi-plastic solid)
    • Plastic
    • Liquid
  • Atterberg Limits:
    • Liquid Limit (LL): The moisture content where soil ceases to be liquid and becomes plastic.
    • Plastic Limit (PL): The moisture content where soil ceases to be plastic and becomes semi-plastic solid.
    • Shrinkage Limit (SL): The moisture content where drying-shrinkage at constant stress ceases.
  • Indices for Classification:
    • Plasticity Index (PI): The difference between LL and PL. PI=LL−PLPI = LL - PL.
    • Liquidity Index (LI): Relates natural moisture content (mm) to consistency limits. LI=m−PLPILI = \frac{m - PL}{PI}.
  • Degree of Plasticity (British Soil Classification System):
    • Low plasticity: LL<35%LL < 35\%
    • Intermediate plasticity: LL=35%−50%LL = 35\% - 50\%
    • High plasticity: LL=50%−70%LL = 50\% - 70\%
    • Very high plasticity: LL=70%−90%LL = 70\% - 90\%
    • Extremely high plasticity: LL>90%LL > 90\%
  • Plasticity Class (Burmister, 1949):
    • PI 0: Non-plastic
    • PI 1-5: Slightly plastic
    • PI 5-10: Low plasticity
    • PI 10-20: Medium plasticity
    • PI 20-40: High plasticity
    • PI > 40: Very high plasticity

Soil Adhesion, Cohesion, and Stickiness

  • Cohesion: The attraction of one water molecule to another via hydrogen bonding (water-water bond).
  • Adhesion: The attraction of a water molecule to a non-water molecule (water-solid bond).
  • Stickiness: The capacity of soil to adhere to other objects. It is categorized as follows:
    • Non-Sticky: Little or no soil adheres to fingers upon release of pressure.
    • Slightly Sticky: Soil adheres to both fingers but separates with little stretching.
    • Moderately Sticky: Soil adheres to both fingers with some stretching upon separation.
    • Very Sticky: Soil adheres firmly to both fingers and stretches greatly upon separation.

Laboratory Testing for Consistency Limits

  • Liquid Limit (LL) Determination (ASTM D 4318 / BS 1377):
    • Casagrande Method: LL is defined as the water content required to close a 2 mm2\,mm wide groove in a soil sample for a distance of 12 mm12\,mm after exactly 2525 blows in a Casagrande device.
    • Procedure involves taking 150 g150\,g of air-dried soil passing the No. 40 sieve.
    • Water is added (starting at 20%20\%), soil is mixed and placed in the device, and a groove is cut.
    • The number of blows (NN) is counted. The test is run multiple times for varying NN values (typically ranges of 10-20, 20-30, and 35-45 blows).
    • A plot of moisture content vs. log⁡(N)\log(N) determines the LL at N=25N=25.
    • Fall Cone Method (BS 1377): LL is the moisture content at which a standard cone (apex angle 30∘30^{\circ}, weight 0.78 N0.78\,N) penetrates precisely d=20 mmd = 20\,mm in 55 seconds after dropping in contact with the soil surface. A semi-log graph of moisture content vs. penetration dd is used to find the value at 20 mm20\,mm.
  • Plastic Limit (PL) Determination (ASTM D 4318):
    • The PL is the water content at which soil crumbles when rolled into threads of 3.2 mm3.2\,mm (1/8 in1/8\,in) in diameter.
    • Procedure: Take 20 g20\,g of soil passing the 0.475 mm0.475\,mm sieve. Add water, mix, and prepare ellipsoidal masses. Roll the soil on a surface until it reaches the required diameter and crumbles. The moisture content of 6 g6\,g of the crumbled thread is then determined.