Foundation Engineering (CE-802 (B)) - Unit 1 Short Notes

UNIT 1: SUBSURFACE INVESTIGATION & SOIL SAMPLING

I. Planning & Execution of Site Exploration

Objectives: To determine soil profile, groundwater, and engineering properties for safe, economical foundation design.

IS 1892 (Part 1) Criteria for Borehole Depth & Spacing:

• Depth: Should penetrate to a significant depth where stress increase from foundation ≤ 10-20% of effective overburden. For preliminary studies, depth ≈ width of largest planned foundation.

• Spacing: Governed by soil variability.

  • Homogeneous soils: 30-50 m.

  • Variable soils: 15-30 m.

  • Critical sites/structures: Closer spacing (5-15 m).

Boring Methods:

Geophysical Methods (Indirect Exploration):

• Seismic Refraction: Measures wave velocity to infer soil/rock layers & depth to bedrock.

• Electrical Resistivity: Measures soil resistivity to map stratigraphy & groundwater.

• Ground Penetrating Radar (GPR): High-resolution imaging of shallow layers (limited by conductivity).

[!TIP] Exam Focus: Rotary drilling is frequently asked for its advantages (speed, depth, stability). Remember IS 1892 criteria link depth to stress distribution (Boussinesq) and spacing to soil variability.

II. In-Situ Testing

Standard Penetration Test (SPT):

• Procedure: Drive a split spoon sampler (50 mm ID) 450 mm into soil at borehole bottom using a 63.5 kg hammer falling 760 mm. Count blows for each 150 mm penetration. Last 300 mm blows = N-value.

• Significance of N-value: Empirical index for:

  • Relative density of sands.

  • Consistency of clays.

  • Bearing capacity & settlement estimation.

  • Liquefaction potential assessment.

Corrections to Field N-value (N_field):

1. Overburden Pressure Correction (N₁): For cohesionless soils.

$$N_1 = N_{field} \times \left( \frac{\bar{\sigma}_v'}{100 \text{ kPa}} \right)^{0.5} \quad \text{(for } \bar{\sigma}_v' \text{ in kPa)}$$

2. Dilatancy Correction (N₂): For dense, saturated fine sands/silts (N₁ > 15). Corrects for negative pore pressure.

$$N_2 = 15 + 0.5(N_1 - 15) \quad \text{(if } N_1 > 15\text{)}$$

3. Energy Correction (N₆₀): Corrects to 60% standard energy (E_B = 2.68 N-m). Most crucial for design.

$$N_{60} = N_{field} \times \frac{E_m}{E_B}$$

where $$\displaystyle E_m $$ = actual hammer energy ratio (often 50-90%).

Final Corrected N-value (N₆₀,corr): Apply corrections sequentially: N_field → N₁ → N₂ → N₆₀. Use N₆₀,corr for design correlations.

Cone Penetration Test (CPT/CPTu):

• Procedure: Push a cone (10 cm² area) into soil at 20 mm/s, measuring cone resistance (q_c) and sleeve friction (f_s) continuously.

• Output: Continuous soil profile, identifies layers, estimates strength/compressibility.

• CPTu: Adds pore pressure (u) measurement for better soil typing and consolidation parameters.

• SPT vs. SCPT:

  | Feature | SPT | SCPT | | :--- | :--- | :--- | | Sample | Disturbed (split spoon) | No sample (continuous profiling) | | Disruption | High (dynamic) | Low (static) | | Data | Discrete (every 1.5 m) | Continuous | | Cost | Low | High |

Vane Shear Test: For soft clays (< 25 kPa). Inserts a four-blade vane, measures peak & residual torque to calculate undrained shear strength ($$\displaystyle c_u $$).

Pressuremeter Test (PMT): Inflates a membrane in a borehole, measures pressure-deformation to derive in-situ modulus (E_M) and limit pressure (p_L) for bearing capacity.

III. Soil Sampling

Disturbed vs. Undisturbed Samples:

Sampling Tools & Quality Parameters:

• Open Drive Sampler (Shelby Tube): Thin-walled, pushed/driven. Inside Clearance (C_i) & Outside Clearance (C_o) critical.

$$C_i = \frac{D_i - D_c}{D_c} \times 100\% \quad ; \quad C_o = \frac{D_c - D_o}{D_o} \times 100\%$$

*   **C_i (1-3%):** Reduces friction, aids sample entry.

*   **C_o (0-2%):** Reduces soil compression on sample.

• Area Ratio (A_r): Ratio of cutting edge area to tube area.

$$A_r = \frac{(D_o^2 - D_i^2)}{D_i^2} \times 100\%$$

*   **A_r < 10%** for undisturbed clays. Lower = less disturbance.

• Length-Diameter Ratio (L/D): Should be > 2 for good quality.

CNS Layer (Cavity, No Strain): The idealized sampling disturbance zone. A thin layer around the sample where soil is strained but not sheared. Minimizing CNS layer is key for undisturbed sampling.

Bore-log Report Components:

1. Project details, borehole location & elevation.

2. Soil description (color, consistency, stratification).

3. Depth of water table & strata changes.

4. SPT N-values (with corrections noted).

5. Sample type & recovery.

6. Laboratory test results (if any).

7. Graphical log (soil symbols, N-value plot).

[!TIP] Common Pitfall: Confusing Area Ratio (A_r) formula. Remember it's based on annular area of cutting edge vs. inner area of tube. For Shelby tube, $$\displaystyle D_i $$ is inner diameter.

UNIT 1: SHALLOW FOUNDATIONS - BEARING CAPACITY & SETTLEMENT

I. Types & Selection

Types: Isolated, Combined, Strip, Raft/Mat, Floating. Selection Factors:

• Soil: Strength, compressibility, depth to bedrock.

• Load: Magnitude, type (axial, moment), distribution.

• Structure: Sensitivity to settlement, rigidity.

• Cost: Excavation vs. material.

• Basic Criteria: Adequate bearing capacity & acceptable total & differential settlement.

II. Bearing Capacity

Key Definitions:

• Gross Pressure (q): Total load / area.

• Net Pressure (q_net): $$\displaystyle q_{net} = q - \gamma D_f $$ (D_f = depth).

• Ultimate Bearing Capacity (q_u): Max gross pressure before shear failure.

• Net Ultimate (q_nu): $$\displaystyle q_{nu} = q_u - \gamma D_f $$.

• Net Safe (q_ns): $$\displaystyle q_{ns} = \frac{q_{nu}}{FOS} $$.

• Allowable Bearing Pressure (q_all): Usually = q_ns or settlement-controlled value.

Modes of Shear Failure (Sketch Essential):

1. General Shear: Deep foundations, dense soils. Continuous failure surface to surface, large settlements, distinct peak.

2. Local Shear: Medium dense/medium stiff soils. Failure surfaces limited, moderate settlements.

3. Punching Shear: Very loose soils/very deep foundations. Soil punches under footing, minimal surface heave, large settlements.

Terzaghi's Bearing Capacity Theory (1943):

• Assumptions: Strip footing, rough base, soil above base ignored (γD_f term added later), Rankine's active earth pressure at sides, Mohr-Coulomb failure.

• Equation (for strip footing):

$$q_u = c N_c + \gamma D_f N_q + 0.5 \gamma B N_\gamma$$

*   $$\displaystyle N_c, N_q, N_\gamma $$: Bearing capacity factors (function of φ).

*   For **square footing:** $$\displaystyle q_u = 1.3c N_c + \gamma D_f N_q + 0.4 \gamma B N_\gamma $$.

*   For **circular footing:** $$\displaystyle q_u = 1.3c N_c + \gamma D_f N_q + 0.3 \gamma B N_\gamma $$.

*   For **rectangular footing:** Use shape factors (s_c, s_q, s_γ).

IS Code Method (BIS: IS 6403):

• Generalized Equation:

$$\boxed{q_{nu} = c N_c s_c d_c i_c + \gamma D_f N_q s_q d_q i_q + 0.5 \gamma B N_\gamma s_\gamma d_\gamma i_\gamma}$$

*   **Shape Factors (s):** Account for footing shape (e.g., $$\displaystyle s_c = 1 + 0.2 \frac{B}{L} $$ for rectangular).

*   **Depth Factors (d):** For deep foundations ($$\displaystyle D_f/B > 1 $$).

*   **Inclination Factors (i):** For inclined loads.

*   **Note:** For most shallow foundations, $$\displaystyle d_c = d_q = d_\gamma = 1 $$, $$\displaystyle i_c = i_q = i_\gamma = 1 $$ if load vertical.

Water Table Correction:

• If water table at/above foundation base, use submerged unit weight (γ') for the γB N_γ term.

• For water table within depth D_f, use effective overburden at base for γD_f N_q term.

• General Rule: Use effective stresses in the bearing capacity equation.

Numerical Approach:

1. Determine soil parameters (c, φ, γ).

2. Find N_c, N_q, N_γ from tables (Terzaghi or IS).

3. Apply shape, depth, load inclination factors.

4. Apply water table correction (use γ' where appropriate).

5. Compute q_nu, then q_ns with FOS (typically 2.5-3.0).

III. Settlement of Foundations

Components:

1. Immediate (Elastic) Settlement (S_i): Occurs during/just after construction in cohesive soils (undrained) & cohesionless soils (due to shear distortion).

2. Primary Consolidation Settlement (S_c): Due to expulsion of pore water from cohesive soils under sustained load. Time-dependent.

3. Secondary Compression (S_s): Post-primary consolidation due to soil structure rearrangement.

Immediate Settlement (S_i) for Cohesive Soils (Elastic Half-Space):

$$\boxed{S_i = \frac{q B (1 - \mu^2)}{E_s} I_f}$$

• q = net pressure.

• B = footing width.

• μ = Poisson's ratio.

• E_s = Secant modulus from undrained triaxial test (E_s ≈ 2-3 E_u for normally consolidated clays).

• I_f = Influence factor (from charts like 2:1 distribution or 3D elastic charts). For flexible square footing on clay, I_f ≈ 1.0 - 1.2.

Consolidation Settlement (S_c) - One Dimensional:

$$S_c = \frac{C_c}{1 + e_0} H \log_{10} \left( \frac{\bar{\sigma}_0' + \Delta \bar{\sigma}'}{\bar{\sigma}_0'} \right)$$

• C_c = compression index.

• e_0 = initial void ratio.

• H = thickness of compressible layer.

• $$\displaystyle \bar{\sigma}_0' $$ = initial effective stress.

• $\Delta \bar{\sigma}'$ = increase in effective stress (from 2:1 or other distribution).

Plate Load Test:

• Procedure: Load a rigid plate (usually 0.3 m sq.) at ground level, measure settlement.

• Load-Settlement Curve: Ultimate load (q_u,plate) from curve (usually at settlement = 20% plate width).

• Extrapolation to Field Footing:

  • For Clay: Settlement is proportional to width (B).

$$S_{field} = S_{plate} \times \frac{B_{field}}{B_{plate}}$$

*   **For Sand:** Bearing capacity is proportional to width.

$$q_{u,field} = q_{u,plate} \times \frac{B_{field}}{B_{plate}}$$

*   **Note:** Assumes similar stress distribution & soil homogeneity.

[!TIP] Critical Distinction: In clay, settlement ∝ B (same pressure). In sand, bearing capacity ∝ B (same settlement). This is a classic exam question.

UNIT 1: DEEP FOUNDATIONS - PILES

I. Introduction & Classification

Necessity: When shallow foundations inadequate (low bearing capacity, large settlement, scour, etc.). Classification:

• By Material: Timber, Concrete (precast/cast-in-situ), Steel, Composite.

• By Function/Action:

  • End-bearing: Transfers load to hard stratum.

  • Friction: Load by skin friction along shaft.

  • Combined: Both end-bearing & friction.

  • Tension/Anchor: Resists uplift.

  • Sheet: Retains soil (cofferdams, bulkheads).

• By Construction:

  • Driven: Precast, displacement (crowding soil).

  • Bored: Cast-in-situ, non-displacement (less disturbance).

  • Screwed/Under-reamed: Special types.

II. Pile Load Capacity

Static Load Carrying Capacity (Q_u):

$$Q_u = Q_b + Q_s$$

• End Bearing (Q_b): $$\displaystyle Q_b = q_b \times A_p $$

  • Clay: $$\displaystyle q_b = N_c c $$ (N_c ≈ 9 for deep piles).

  • Sand: $$\displaystyle q_b = \gamma D_p N_q $$ (N_q from bearing capacity factors).

• Shaft Friction (Q_s): $$\displaystyle Q_s = f_s \times A_s $$

  • α-method (Clays): $$\displaystyle f_s = \alpha \cdot c_u $$ (α = adhesion factor, 0.4-1.0, decreases with depth/softness).

  • β-method (Sands): $$\displaystyle f_s = \beta \cdot \bar{\sigma}_v' $$ (β = friction factor, ≈ K tanδ, K ≈ 1-2, δ ≈ 0.75φ).

Dynamic Formulae (Impact Driving):

• Engineering News Formula (ENF):

$$Q_{safe} = \frac{W h}{S + C} \times \frac{W + n W'}{W + W'}$$

*   W = hammer weight, h = fall, S = set (penetration/blow), C = constant (2.5 cm), n = efficiency (0.6-0.8), W' = pile weight.

*   **Limitations:** Ignores elastic compression, restitution.

• Hiley's Formula (Improved ENF):

$$\boxed{Q_{safe} = \frac{\eta W h}{S + \frac{C}{2}} \times \frac{W + n W'}{W + W'}}$$

*   η = **coefficient of restitution** (0.25-0.5 for concrete-steel).

*   **Includes** average elastic compression (C/2).

*   **Most commonly used** in practice.

Pile Capacity from SPT/CPT:

• SPT: $$\displaystyle Q_b = N \cdot A_p \cdot q_b $$ (q_b from correlations), $$\displaystyle Q_s = \sum (f_s \cdot \Delta A_s) $$ where $$\displaystyle f_s $$ from N-value correlations.

• CPT: Directly uses $$\displaystyle q_c $$ and $$\displaystyle f_s $$ with empirical correlations (e.g., $$\displaystyle f_s = \alpha \cdot q_c $$ for clays).

III. Pile Groups & Negative Skin Friction

Pile Group Efficiency (η_g):

$$\eta_g = \frac{Q_{ug}}{n \cdot Q_{up}}$$

• η_g < 1 for most groups due to group effect (overlap of stress bulbs).

• Block Failure: Occurs in soft clays at close spacing (< 3D). Entire soil block fails.

$$Q_{ug(block)} = c \cdot (B_g \cdot L_g) \cdot N_c + \gamma D_f (B_g L_g) N_q$$

*   B_g, L_g = group dimensions.

• Individual Failure: Piles fail individually, group capacity < nQ_up.

Geometric Properties for Spacing:

• Center-to-center spacing (s): Minimum 2-3 times pile diameter (D) for clays, 3-4D for sands to minimize group effect.

• Group Shape: Square, rectangular, triangular. Efficiency depends on arrangement.

Negative Skin Friction (NSF):

• Cause: Downward movement of soil relative to pile (e.g., soft clay consolidation, fill placement, water table drop).

• Effect: Increases load on pile (reduces capacity), causes additional settlement.

• Calculation for Single Pile in Cohesive Soil:

$$\boxed{Q_{nsf} = \bar{f}_{nsf} \times A_s = \left( \gamma \cdot \Delta z \cdot K \cdot \tan \delta \right) \times (\pi D L_{nsf})}$$

*   $$\displaystyle \bar{f}_{nsf} $$ = average NSF stress.

*   Δz = thickness of compressible layer.

*   K = lateral earth pressure coefficient (≈ 1.0 for NC clays).

*   δ = interface friction angle (≈ φ for rough concrete).

*   L_nsf = length through compressible layer.

• Design: Net allowable load = Q_u - Q_nsf.

IV. Special Pile Types

Under-reamed Piles:

• Concept: Single/multiple bulbs (under-reams) on shaft in expansive soils.

• Suitability: Expansive soils (high shrink-swell), loess, collapsible soils. Bulbs provide tension anchorage to resist uplift.

• Ultimate Tensile Capacity:

$$Q_{tu} = Q_{s(adhesion)} + Q_{b(under-ream)}$$

*   $$\displaystyle Q_{s(adhesion)} = \alpha \cdot c_u \cdot A_s $$ (up to bulbs).

*   $$\displaystyle Q_{b(under-ream)} = A_b \cdot q_b $$ (q_b = 9c_u for clay, bearing on bulb base).

*   **Neglect suction** (as per problem statement).

Well Foundations (Caissons):

• Components (Neat Sketch Required):

  1. Well curb: Bottom cutting edge (steel/iron).

  2. Well steining: Masonry/concrete above curb (provides weight for sinking).

  3. Cutting edge: Beveled edge for penetration.

  4. Lining/Wall: Brick/stone/concrete rings.

  5. Bottom plug: Concrete plug at bottom (after sinking).

  6. Top plug: Concrete plug at top (after dewatering).

  7. Well cap: RCC beam to distribute load from pier.

• Types: Open (dry), Pneumatic (compressed air), Box (prefabricated).

• Sinking Methods: By gravity, with kentledge, with water jetting (in sand), with pneumatic pressure.

V. Numerical Problems (Comprehensive)

• Single Pile Capacity: Always check Q_b and Q_s separately. For clay, $$\displaystyle Q_b = 9c_u A_p $$ (deep pile). For sand, $$\displaystyle Q_b = \gamma D_p N_q A_p $$.

• Pile Group Capacity (Neglecting End Bearing):

$$Q_{ug} = \alpha \cdot c_u \cdot (A_s)_{group}$$

where $$\displaystyle (A_s)_{group} $$ = total surface area of all piles in group. Apply group efficiency if spacing < 3D.

• Layered Soils: Calculate Q_b from bearing stratum. Calculate Q_s by summing contributions from each layer: $$\displaystyle Q_s = \sum (\alpha_i c_{ui} \cdot \Delta A_s) $$.

• Dynamic Formula (Hiley's): Must compute elastic compression (C) of pile + cap:

$$C = \frac{Q_u L}{A E} \quad \text{(in cm)}$$

where Q_u = ultimate load, L = length, A = area, E = modulus.

[!TIP] Exam Trap: In pile group problems, if spacing is given as 90 cm for 300 mm dia piles, s/D = 3.0. For soft clay, this is critical spacing—may need to consider block failure or reduced efficiency. Always check: if s < 3D, group efficiency < 1.

UNIT 1: EARTH PRESSURE & RETAINING STRUCTURES

I. Types of Lateral Earth Pressure

• At-rest (K₀): Wall does not move (e.g., basement walls before backfill). $$\displaystyle K₀ = 1 - \sin \phi' $$ (for NC soils).

• Active (Kₐ): Wall moves away from soil (unloading). Minimum pressure.

• Passive (Kₚ): Wall moves into soil (loading). Maximum pressure.

• Relationship: $$\displaystyle K_a K_p = 1 $$ (for $$\displaystyle \phi' > 0 $$).

II. Classical Theories

Rankine's Theory (1875):

• Assumptions: Wall smooth (δ=0), horizontal backfill, no wall friction, infinite wall.

• For Cohesionless Soil (φ'):

$$K_a = \tan^2 \left(45^\circ - \frac{\phi'}{2}\right) \quad ; \quad K_p = \tan^2 \left(45^\circ + \frac{\phi'}{2}\right)$$

• For Cohesive Soil (c', φ'):

  • Active: $$\displaystyle \sigma_h = K_a \gamma z - 2c \sqrt{K_a} $$ (tension crack if $$\displaystyle \sigma_h < 0 $$).

  • Passive: $$\displaystyle \sigma_h = K_p \gamma z + 2c \sqrt{K_p} $$.

• Pressure Diagram: Triangular for c-φ soil with c'=0. For c'>0, active diagram has negative zone near top.

Coulomb's Theory (1776):

• Assumptions: Inclined failure plane, wall friction (δ) considered, inclined backfill (β) possible.

• Derivation: Wedge analysis, minimize active thrust by varying failure plane angle.

• Active Earth Pressure Coefficient:

$$K_a = \frac{\cos^2(\phi' - \delta)}{\cos^2 \delta \cos(\delta + \beta) \left[1 + \sqrt{\frac{\sin(\phi' + \delta) \sin(\phi' - \beta)}{\cos(\delta + \beta)}} \right]^2}$$

*   For δ=0, β=0 → reduces to Rankine's K_a.

• Merits over Rankine:

  1. Considers wall friction (δ) → more realistic.

  2. Allows inclined backfill (β).

  3. Applicable for both cohesionless & cohesive soils.

  4. Passive pressure from Coulomb is more reliable (Rankine overestimates).

Comparison:

III. Graphical Methods

• Culmann's Graphical Method: For active pressure with irregular, inclined backfill and surcharge. Construct failure wedges from wall, draw pressure lines.

• Rebhann's Graphical Method: For passive pressure.

IV. Earth Pressure on Retaining Walls

Numerical Calculation Steps:

1. Draw pressure diagram for each layer (consider water table, surcharge).

2. For cohesionless soil: $$\displaystyle \sigma_h = K \gamma z $$ (above WT: γ_dry; below: γ_sat).

3. For cohesive soil (active): $$\displaystyle \sigma_h = K \gamma z - 2c \sqrt{K} $$. Check for tension crack depth $$\displaystyle z_c = \frac{2c}{\gamma \sqrt{K}} $$.

4. With water table/seepage: Use submerged unit weight (γ') below WT. Add hydrostatic pressure (u = γ_w z) on total stress diagram.

5. Resultant Thrust (P): Area of pressure diagram. Point of application = centroid.

   • Triangular: at h/3 from base.

   • Trapezoidal: divide into triangle+rectangle.

Example (Stratified Backfill): Calculate K for each layer (use φ of that layer). Sum pressures at interfaces. Plot diagram, find total P and its point of action.

V. Retaining Wall Design & Stability

Modes of Failure:

1. Overturning: Wall rotates about toe. Check FOS = Resisting Moment / Overturning Moment ≥ 1.5.

2. Sliding: Wall slides along base. Check FOS = (μ W + P_p) / P_a ≥ 1.5 (μ = friction coeff., P_p = passive force at toe).

3. Bearing Capacity Failure: Excessive pressure on soil. Check $$\displaystyle q_{max} \leq q_{all} $$.

4. Excessive Settlement/Differential Settlement.

Sheet Piles vs. Retaining Walls:

Uses of Sheet Piles:

1. Cofferdams for foundations in water.

2. Retaining walls for excavations.

3. Bulkheads for waterfronts.

4. Slope stabilization.

5. Noise barriers.

[!TIP] Earth Pressure Calculation: Always draw the pressure diagram. For cohesive soil with water table, remember: active pressure = (K γ' z) - 2c√K + (γ_w z) (hydrostatic on top of effective stress diagram). Tension crack depth is critical for cohesive backfills.

UNIT 1: SPECIAL TOPICS & SOIL IMPROVEMENT

I. Problematic Soils

Expansive Soils (Black Cotton Soils):

• Characteristics:

  • High montmorillonite clay content.

  • High shrinkage/swelling potential (low liquid limit? No, high LL > 50%).

  • Low strength when wet, hard when dry.

  • High compressibility, low permeability.

  • Cracks on drying.

• Problems:

  • Heave (swelling) → lifts foundation.

  • Shrinkage → settlement & cracks in structure.

  • Volume change → differential movement.

• Preventive Measures:

  1. Moisture Control: Maintain constant water content (impermeable blanket, landscaping).

  2. Replacement: Remove & replace with non-expansive fill.

  3. Stabilization: Lime/cement treatment.

  4. Under-reamed Piles: Best solution for heavy structures. Bulbs anchor in non-swelling zone.

  5. Raft Foundations: Spread load, reduce pressure.

Collapsible Soils:

• Characteristics:

  • Loose, porous, cemented (e.g., loess, wind-blown silt).

  • Dry has moderate strength.

  • Sudden collapse upon wetting (loss of cementation).

  • Low density, high void ratio.

• Problems: Sudden, large settlement upon wetting (rainfall, leakage).

• Preventive Measures:

  1. Pre-wetting: Saturate soil before construction to induce collapse.

  2. Compaction: Dynamic compaction, heavy tamping.

  3. Piles: Transfer load through collapsible zone.

  4. Chemical Stabilization: Lime/cement to break cementation bonds before wetting.

II. Soil Stabilization & Improvement

Need: To improve strength, reduce compressibility/permeability, control swell. Situations: Weak subgrade, expansive soils, fills, slope stabilization.

Methods:

1. Mechanical: Compaction (increases density, strength).

2. Chemical: Lime (for clay, reduces plasticity, pozzolanic), Cement (for sand/clay, binds particles), Bitumen (for waterproofing, base course).

3. Electrical: Electro-osmosis.

   • Mechanism: Apply DC electric field → water migrates from anode to cathode in clay (electro-osmotic flow).

   • Application: Consolidate very soft, saturated clays (e.g., for slope stabilization, preloading). Often combined with chemical injection (electro-chemical).

III. Geosynthetics

Types & Functions:

Uses in Foundation Engineering:

1. Separation: Prevent mixing of fine subgrade with coarse ballast/aggregate (railways, roads).

2. Reinforcement: Increase bearing capacity, reduce settlement in weak soils (geogrids/geotextiles in embankments, rafts).

3. Filtration: Allow water flow but retain soil particles (drainage layers, behind retaining walls).

4. Containment: Liners for ponds, landfills (geomembranes).

5. Drainage: Geocomposite drains for vertical/horizontal drainage (prefabricated).

IV. Compaction

Objectives: Increase density → increase strength, decrease compressibility/permeability, control swelling.

Field Equipment:

• Sheepsfoot: Best for cohesive soils (kneading action).

• Smooth-wheel (Tamping): Granular soils.

• Pneumatic-tired: Flexible mat, good for both.

• Vibratory: Best for clean sands/gravels.

Proctor Tests:

[!TIP] Key Difference: Modified Proctor has higher energy → higher MDD, lower OMC. Used for highway/airfield fills. Standard Proctor for general earthworks.

Final Note: This compilation strictly follows the RGPV CE-802(B) blueprint, prioritizing repeated exam topics (SPT corrections, IS bearing capacity, pile group/NSF, earth pressure theories, problematic soils). All definitions, formulas, and procedures are boxed for quick revision. Practice numerical problems from past papers using the step-by-step methods outlined.