I. FOUNDATIONS
A. Introduction & Requirements
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Definition: Foundation is the lower part of a structure that transmits all loads (dead, live, environmental) from the superstructure to the underlying soil or rock mass.
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Purpose:
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Distribute structural loads over a large area to reduce bearing pressure.
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Prevent excessive or differential settlement.
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Provide stability against overturning, sliding, and seismic forces.
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Basic Requirements:
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Must rest on soil/rock with adequate bearing capacity.
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Depth should be below the frost line to avoid frost heave.
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Should be rigid enough to distribute loads uniformly.
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Must withstand all imposed loads, moments, and lateral forces.
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Constructed on undisturbed or properly compacted soil.
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Causes of Foundation Failure:
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Uneven settlement due to variable soil strata or inconsistent compaction.
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Overloading beyond the soil's ultimate bearing capacity.
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Soil erosion or scour around foundations due to water flow.
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Frost heave in cold climates causing upward movement.
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Lateral pressure from adjacent excavations or slope failure.
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Poor construction like inadequate compaction or wrong materials.
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[!TIP] Common exam question: "Describe any three causes of foundation failure." Focus on settlement, overloading, and scour. Always relate causes to soil behavior.
B. Shallow Foundations
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Definition: Foundations with depth less than their width, transferring loads to near-surface soils (typically depth < width).
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Types:
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Spread/Isolated Footing: Supports a single column. Usually square or rectangular. May be stepped or tapered.
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Strip/Continuous Footing: Supports a load-bearing wall or a row of columns. Length much greater than width.
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Raft/Mat Foundation: A large slab covering the entire footprint, used when soil bearing capacity is low or loads are heavy. May be thickened under columns.
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Combined Footing: Supports two or more columns, often when property lines restrict isolated footings. Shapes: rectangular, trapezoidal, or L-shaped.
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Design Considerations:
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Size determined from column load and allowable bearing pressure: $$\displaystyle A = \frac{P}{q_a} $$.
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Thickness based on shear capacity (one-way or two-way shear).
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Reinforcement for bending moments and temperature/shrinkage.
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Must have adequate depth to develop reinforcement and resist punching shear.
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Applications:
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Isolated: Most common for framed structures.
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Strip: For masonry walls.
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Raft: For soft soils, basements, or heavy structures.
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Combined: When columns are closely spaced or near boundaries.
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| Foundation Type | Typical Shape | Primary Use |
|---|---|---|
| Isolated | Square/Rectangular | Single column |
| Strip | Long strip | Load-bearing walls |
| Raft | Full area slab | Weak soil, heavy loads |
| Combined | Rectangular/Trapezoidal | Multiple columns |
[!TIP] Sketches showing plan and section of each type are frequently asked. Practice drawing with dimensions, reinforcement details, and pressure distribution.
C. Deep Foundations
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Purpose: To transfer loads through weak surface soils to deeper, competent strata when shallow foundations are inadequate.
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Classification:
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By Material: Concrete (precast/cast-in-situ), Steel (H-piles, pipe piles), Timber (temporary).
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By Function:
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End-bearing: Load carried by tip on hard stratum.
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Friction: Load carried by skin friction along shaft.
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Combined: Both end-bearing and friction.
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By Installation:
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Driven: Piles prefabricated and driven by hammer, vibratory, or drop.
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Bored: Drilled hole, then cast-in-situ. Includes continuous flight auger (CFA).
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Screwed: Helical piles screwed into ground.
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Pile Types:
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Driven Precast Piles: Precast concrete or steel. Advantages: quick, quality control. Disadvantages: noise, vibration, soil displacement.
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Cast-in-situ Piles:
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Bored Piles: Drilled with casing or bentonite, then concrete poured. Suitable for most soils.
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Driven Cast-in-situ: Tube driven, filled with concrete, withdrawn. Forms concrete pile in place.
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Under-reamed Piles: Special bored piles with enlarged bulbs (under-reams) at intervals. Used in expansive soils to resist uplift and provide anchorage.
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Construction: Boring to required depth, forming bulbs with under-reaming tool, placing reinforcement, casting concrete.
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Advantages: Good for swelling soils, high uplift capacity.
DiagramSEARCH: under-reamed pile construction diagram -
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Precautions for Piles on Rock:
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Driven Piles: Use driving shoes, pre-drill sockets, control hammer energy to avoid rock damage.
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Cast-in-situ: Clean rock surface, roughen for bond, use bonding agent, ensure concrete placement without segregation.
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[!TIP] "Purpose and classification of pile foundation" (Jun 2025) and "under-reamed piles" (Jun 2022) are frequent. Know differences between driven and bored piles, and their suitability.
D. Special Foundations
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Caisson Foundations: Large-diameter, watertight retaining structures sunk into position.
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Types:
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Open Caisson: Open at top and bottom, excavated inside, sunk by self-weight.
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Pneumatic Caisson: Working chamber under compressed air, used underwater.
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Floating Caisson: Built on shore, floated to site, then sunk and filled.
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Applications: Bridge piers, deep-water docks.
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Well Foundations: Cylindrical wells made of masonry or concrete, used for bridges and heavy structures.
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Components:
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Cutting Edge: Steel angle at bottom for cutting soil.
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Well Curb: Bottom frame supporting cutting edge.
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Well Steining: Curved masonry above curb, provides weight for sinking.
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Well Cap: Top slab distributing load.
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Sand Filling: Inside well for stability.
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Forces: Self-weight, soil pressure, water pressure, scour.
DiagramSEARCH: well foundation components diagram -
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Grillage Foundations: Arrangement of steel beams (I-sections) in one or more layers, with concrete filling, to spread load over large area.
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Types: Single-layer, double-layer, or more. Beams placed perpendicular and parallel.
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Uses: For heavy columns, stanchions, machinery foundations where soil is weak.
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Advantages: Reduces pressure on soil, economical for heavy loads.
DiagramSEARCH: grillage foundation arrangement diagram -
[!TIP] "Caisson and Well Foundation" (Jun 2022) and "Grillage Foundation" (Jun 2023) are common. Distinguish based on construction, materials, and typical applications.
E. Foundation Design & Analysis
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Site Investigation and Soil Analysis:
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Importance: Determines soil profile, bearing capacity, settlement potential, groundwater, and presence of weak zones. Essential for safe and economical design.
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Methods:
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Exploratory: Test pits, trenches.
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Boreholes: With sampling (disturbed/undisturbed).
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In-situ Tests: Standard Penetration Test (SPT), Cone Penetration Test (CPT), Vane shear.
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Lab Tests: Grain size distribution, Atterberg limits, shear strength (triaxial, direct shear), consolidation, permeability.
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Forces Acting on Foundations:
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Vertical Loads: Dead load, live load, wind load (vertical component).
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Lateral Loads: Wind, earthquake, soil pressure.
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Moments: Due to eccentric loading or lateral forces.
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Sketch: Show foundation with column, load $P$, possible eccentricity $e$, resultant force $R$, and soil pressure distribution (uniform or trapezoidal).
DiagramSEARCH: forces acting on foundation diagram -
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Bearing Capacity:
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Ultimate Bearing Capacity ($$\displaystyle q_u $$): Maximum pressure before failure.
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Allowable Bearing Capacity ($$\displaystyle q_a $$): Safe pressure considering factor of safety (FOS).
\boxed{q_a = \frac{q_u}{FOS}}
where FOS typically 2.5–3 for dead + live loads.
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Factors Affecting: Soil cohesion ($c$), effective surcharge ($$\displaystyle q = \gamma D_f $$), soil friction angle ($\phi$), foundation width ($B$), depth ($$\displaystyle D_f $$), shape, load inclination.
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Terzaghi's Equation (for strip footing):
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$$q_u = cN_c + qN_q + 0.5\gamma BN_\gamma$$
where $$\displaystyle N_c, N_q, N_\gamma $$ are bearing capacity factors from $\phi$.
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For square footing: $$\displaystyle q_u = 1.3cN_c + qN_q + 0.4\gamma BN_\gamma $$.
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Settlement Considerations:
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Total Settlement ($S$) = Immediate ($$\displaystyle S_i $$) + Consolidation ($$\displaystyle S_c $$) + Secondary compression.
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Immediate Settlement (elastic):
\boxed{S_i = \frac{qB(1-\nu^2)}{E_s} I_s}
where $q$ = net pressure, $B$ = width, $\nu$ = Poisson's ratio, $$\displaystyle E_s $$ = modulus of elasticity, $$\displaystyle I_s $$ = shape factor (1.0 for square, 1.2 for strip).
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Consolidation Settlement:
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$$S_c = \frac{C_c}{1+e_0} H \log \frac{\sigma'_0 + \Delta\sigma}{\sigma'_0}$$
where $$\displaystyle C_c $$ = compression index, $$\displaystyle e_0 $$ = initial void ratio, $H$ = thickness of compressible layer, $$\displaystyle \sigma'_0 $$ = initial effective stress, $\Delta\sigma$ = increase in vertical stress.
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Differential Settlement: Difference in settlement between adjacent foundations. More critical than total settlement; can cause structural damage, cracking, and functional issues.
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Allowable Settlement: Depends on structure type: residential (25–40 mm), industrial (50–75 mm), but must limit differential settlement to $L/400$ for walls, $L/1000$ for frames.
[!TIP] "Discuss importance of site investigation" (Jun 2024) and "forces on foundations with sketch" (Nov 2023) are key. Know basic bearing capacity and settlement equations. Settlement types (immediate vs consolidation) are often asked in differentiation.