Unit 2 Foundations

2.1. Foundation and its types

Foundation is the lowest part of a structure, also known as the substructure, which transmits loads from the
superstructure to the soil below. It ensures stability, prevents settlement, and resists lateral forces like wind or
earthquakes.

2.1.1 Types of Foundations:

2.1.1.1 Shallow Foundation:

Shallow foundations are defined as foundations where the depth of the foundation is less than or equal
to its width.
They are also known as open foundations and are placed directly beneath the lowest part of the
superstructure.
Shallow foundations are typically practical for depths up to about 5 meters and are usually convenient
to construct above the water table
Types of Shallow foundations

A. Strip Footing Foundation

 Provides continuous longitudinal bearing.

 Suitable for load-bearing walls in low to medium-rise buildings.
Types:
1. Wall footing (stepped or without steps).
2. Inverted arch footing.

B. Spread/Isolated Footing Foundation

 Supports individual columns or piers.

 The base is wider than the top to distribute loads over a larger area.
Types:
1. Isolated footing: For single columns.
2. Combined footing: Supports two or more columns when they are close
together.
3. Strap/Cantilever footing: Used when one column is near a property line.
4. Eccentrically loaded footing: For columns subjected to eccentric loads.

C. Raft/Mat Foundation

 Covers the entire area beneath a structure.

 Used when soil bearing capacity is poor or column spacing is small.
Types:
1. Slab raft (solid, up to 30 cm thick).
2. Slab and beam raft (for thicker slabs).
3. Cellular raft (for very thick slabs, over 90
cm).

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D. Grillage Foundation

 Consists of layers of steel or timber joists arranged at

right angles to distribute loads over a large area.
 Used where deep excavation is impractical or soil
bearing capacity is low.
Types:
1. Steel grillage: Uses rolled steel joists (RSJ).
2. Timber grillage: Used in waterlogged areas where steel may corrode.

2.1.1.2 Deep Foundation:

Deep foundations are structural elements that transfer loads from a building or structure to deeper, more
stable soil or rock layers when the near-surface soil lacks sufficient bearing capacity or is otherwise unsuitable.
These foundations are used when shallow foundations are inadequate due to heavy loads, poor soil conditions,
high water tables, or the need to resist uplift or lateral forces.
Types of deep foundations

1. Pile Foundations

Piles are long, slender structural members (typically made of concrete,

steel, or timber) driven or drilled deep into the ground to transfer loads
to stronger soil or rock layers.

Classification Based on Function:

End-Bearing Piles: Transfer loads directly to a hard stratum (e.g., rock or dense sand).
Friction Piles: Resist loads through skin friction along the pile’s surface.
Sheet Piles: Thin, interlocking piles used for retaining soil or
water (common in cofferdams and seawalls).
Tension/Uplift Piles: Resist upward forces (e.g., in
transmission towers or tall chimneys).
Anchor Piles: Provide anchorage against horizontal forces
(e.g., for retaining walls).
Batter Piles: Driven at an angle to resist lateral loads (e.g.,
in bridges and tall structures).
Compaction Piles: Compact loose soil to improve bearing
capacity (not load-bearing themselves).
Fender Piles: Protect structures from impact (e.g., ship
docks).

Classification Based on Material & Construction:

Concrete Piles:
Precast Piles: Manufactured off-site and driven into the ground.
Cast-in-Situ Piles: Concrete poured into drilled holes (e.g., bored piles).
Under-Reamed Piles: Have enlarged bases for extra bearing capacity.
Steel Piles:
H-piles, Pipe piles, Screw piles (used in weak soils).
Timber Piles: Used in temporary structures or waterlogged areas.
Composite Piles: Combine materials (e.g., steel and concrete).

2. Pier Foundations

Large-diameter cylindrical columns (wider than piles) constructed by drilling or

excavating.
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Transfer loads through end-bearing rather than friction.
Used where driving piles is difficult (e.g., in decomposed rock or stiff clay).

3. Caisson (Well) Foundations

Large, prefabricated hollow structures sunk into the ground and filled with
concrete.
Used for bridges, dams, and offshore structures.
Types:
Box Caissons: Precast and floated into position.
Open Caissons: Top and bottom open during sinking.
Pneumatic Caissons: Sealed and pressurized to keep water out during
construction.

2.2 Soil exploration and methods to improve bearing capacity of soil

Soil exploration (or site investigation) is the process of studying subsurface conditions to determine soil
properties, stratigraphy, and groundwater levels. It helps in selecting the appropriate foundation type and
ensuring structural stability.

Objectives of Soil Exploration:

1. Determine safe bearing capacity of soil.
2. Identify soil type (clay, sand, rock, etc.).
3. Assess groundwater conditions.
4. Predict settlement behavior.
5. Detect potential construction challenges (e.g., loose soil, expansive clay).
6. Select the best foundation type (shallow or deep).

Methods of Soil Exploration

1. Test Pits
Excavating small pits (1.5m × 1.5m) to visually inspect soil layers.
Suitable for shallow foundations (up to 3m depth).

2. Probing
Driving a steel rod into the ground to estimate soil resistance.
Quick but less accurate.
3. Boring
Auger Boring: Hand or machine-driven augers extract soil samples (up to 15m depth).
Wash Boring: Water jet washes soil out of a casing pipe (for cohesive soils).
Percussion Boring: Hammering a chisel to break hard strata (for rocky soils).
Rotary Drilling: Core drilling for undisturbed rock/soil samples.

4. Standard Penetration Test (SPT)

A split-spoon sampler is driven into the soil using a 65
kg hammer.
Measures N-value (blows per 30 cm penetration),
indicating soil density.

5. Geophysical Methods
Seismic Refraction: Measures wave velocities to identify
soil layers.
Electrical Resistivity: Detects soil composition based on
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conductivity.

6. Plate Load Test

A steel plate is loaded incrementally to measure settlement and determine bearing capacity.

Methods to Improve Bearing Capacity of Soil

1. Compaction
Mechanical Compaction: Using rollers, rammers, or vibrators to densify loose soil.
Dynamic Compaction: Heavy weights dropped from height to compact deep layers.
2. Drainage
Lowering the water table to reduce pore pressure and increase soil strength.
Installing perforated pipes or sand drains for faster water removal.
3. Soil Stabilization
Cement Stabilization: Mixing cement with soil to increase strength.
Lime Stabilization: Adding lime to reduce plasticity in clayey soils.
Bitumen/Geotextiles: Binding soil particles to prevent erosion.
4. Grouting
Injecting cement grout or chemical solutions into soil to fill voids and strengthen weak zones.
5. Use of Geosynthetics
Geogrids/Geotextiles: Reinforce soil to distribute loads evenly.
6. Piling & Deep Foundations
Transferring loads to deeper, stronger strata using piles, piers, or caissons.
7. Vibroflotation
Vibrating probes densify loose sandy soils by rearranging particles.

2.3 Some common problem with existing foundation

Causes of Foundation Failures

1. Unequal settlement of subsoil:

Due to Non-uniform nature of soil Unequal soil distribution on soil strata Consolidation of soil particles General
earth movement etc.

2. Unequal settlement of masonry

Portion of masonry between GL and concrete footing has mortar joints which may either shrink or compress

3. Action of ground water level

The fluctuating ground water table directly changes the properties of soils

4. Action of tree roots

Absorbs the moisture of soil, also roots may spread inside the wall

5. New building construction

With deep foundation near to the existing building Affects during pile driving, concrete pouring, vibration
effects 6. Water leakage from water supply lines or sewage pipeline Due to increase in pressure either inside
the pipeline (liquid pressure) or outside the pipeline (soil pressure), the w/s or sewage pipeline may .

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6. Sliding cause by earthquake

Due to EQ, lateral movement or vertical movement or sliding of the foundation

Common problems found in Foundation Failures

A. Bulge and Bowing

Bulges:an outward bump in the foundation

Bowing: an inward curve.
Both can be caused by the expanding and contracting caused by temperature changes

B. Cracking

 Cracking can be caused by temperature change or inadequate framing.

 Cracking may also be caused by settling of the soil beneath the foundation or even by nearby construction,
depending on the quality of the foundation and the amount of vibrations caused by the construction.
 Cracks can be fixed with some mortar, filling and then sealing the hole. While minor cracks do not pose much of a
threat to the actual structure of the home, they can lead to another foundation problems.

C. Leaks

 Leaking is caused by water coming in through cracks or holes.

 A leaking foundation allows water into the basement creating a damp and unpleasant environment, and at worst,
ruining part of the living space of your home

D. Settlement

Settlement of foundation occurs due to :

 Reduction of moisture content (due to drying evaporation)
 Consolidation of soil particles (due to super imposed loads, compaction)
 Heaving of soil (soil moves up and down with large regular movements)
 Subsidence : It is the ground settlement resulting from loss of underground support. This is occurred due to
withdrawal of groundwater causing the earth above to settle and fill the remaining voids.
 General earth movement (mining operations, earthquakes etc)

Effects of unequal settlements:

 Distortion of the structure
 Failure of the structure

Prevention of unequal settlement

 Proper foundation design
 Proper soil investigation

Suitability of different types of foundation

A. Foundation in Black Cotton Soil

Precautions for foundations in black cotton soil

1. Foundation depth be enough to hard strata.
2. Measures to be applied to avoid water reaching to bottom of foundation.
3. Prevent foundation from direct contact with black cotton soil.
4. If thickness of black cotton soil is high, foundation is to be laid on piles.
5. Raft foundation is the choice in this condition.
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6. Tie-beam in plinth is important.

2.4 Retaining properties and waterproofing of basements

2.4.1. Retaining Walls in Basements

Basements require retaining walls to resist lateral
earth pressure and hydrostatic pressure from
surrounding soil and groundwater.
Forces Acting on Retaining Walls:
Active Earth Pressure: Pushes the wall outward
due to retained soil and water.
Passive Earth Pressure: Resists wall movement
by compressing soil in front.
Hydrostatic Pressure: Water pressure increases
with depth, requiring drainage.

2.4.2. Types of Retaining Walls Used in Basements

1. Gravity Walls
Thick masonry or concrete walls relying on their weight for stability.
Suitable for low heights.
2. Cantilever Walls
Reinforced concrete (RCC) walls with stem, heel, and toe.
Economical for moderate heights (up to 6–7m).
3. Counterfort Walls
Vertical brackets (counterforts) behind the wall for extra support.
Used for heights exceeding 7m.
4. Buttressed Walls
Similar to counterfort but with supports in front (buttresses).

Counterfort Retaining wall

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2.4.3. Waterproofing Methods for Basements
Basements are prone to moisture infiltration. Common waterproofing techniques:
Membrane Waterproofing
Bitumen Felts: Applied in layers (2–3 for high moisture).
Polymer Membranes: Flexible and durable against cracks.
Monolithic Waterproofing
Uses high-density concrete (M20 mix, 1:1.5:3) with low water-cement ratio (<0.54).
Integral waterproofing agents (chemical additives) reduce porosity.
Foundation Drainage System
Perforated Pipes: Installed around the basement to divert groundwater.
Weep Holes: Allow trapped water to escape.
Damp Proof Course (DPC)
Asphalt Layer: Applied beneath the basement floor and walls.
Proper Lapping: Ensures no gaps at joints.

2.5. Sealing Of cracks in basement

2.5.1. Causes of Cracks

Structural Movement: Settlement, expansion, or shrinkage.

Hydrostatic Pressure: Water forcing cracks open.
Poor Construction: Weak concrete, inadequate curing.

2.5.2. Methods to Seal Cracks

1. Epoxy/Polyurethane Injectionyy
Used for structural cracks.
Injected under pressure to fill and bond cracks.
2. Hydraulic Cement
Fast-setting, used for active leaks.
Expands to seal gaps.
3. Flexible Sealants (Polymer-Based)
For non-structural cracks.
Accommodates minor movements.
4. Waterproof Coatings
Applied after sealing to prevent future leaks.

2.5.3. Preventive Measures

Proper Drainage: Keep water away from foundations.

Control Joints: Allow controlled cracking in concrete.
Regular Inspections: Detect and repair early-stage cracks.