FOUNDATIONS

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INTRODUCTION TO FOUNDATIONS

Function of a foundation is to transfer the structural loads from a building safely into the ground.

It should reach the underlying soil that is free of organic matter and unreachable by the winter’s frost.

A larger and heavier building of masonry, steel, or concrete would require its foundations to go deeper into earth such that the soil or the rock on which it is founded is competent to carry its massive loads.

foundation design is a highly specialized field of geotechnical engineering.

Major Building Parts

Superstructure

Substructure

Foundation

Primary Factors Affecting Foundation Choice

Subsurface soil

Ground water conditions

Structural requirements

Lateral earth pressure

Intensity of loading

Secondary Factors Affecting Foundation Choice

Construction access, methods & site conditions

Building Codes & Regulations

Impact on surrounding structures

Construction risks

Environmental factors

SELECTION OF FOUNDATION TYPE

Selection of foundation depends on whether the load is transfer at deeper depths or shallower depths. Following parameters will be considered while selecting a foundation type.

Soil strength Ground conditions Foundation loads Construction methods Impact on adjacent property.

Foundation Loads

Dead Load Live Load Wind Load Earthquake Structural Member Forces

SETTLEMENT IN FOUNDATION

Uniform Differential

Differential settlement must be minimized, depends on site soil conditions and distribution of loads on columns supporting the building

SETTLEMENTS OF FOUNDATIONS

NO SETTLEMENT * TOTAL SETTLEMENT * DIFFERENTIAL SETTLEMENT

Uniform settlement is usually of little consequence in a building, but differential settlement can cause severe structural damage

Main types of foundation

Foundations or ‘footings’ come under two main categories

Shallow foundation. Deep foundation.

Shallow foundationsShallow foundations

Shallow foundations includes spread column footings without or with tie/grade beams, individual or combined wall footing, strip footings, stepped foundation, grillage foundation and rafts. The guidelines for a shallow foundation are They are founded near to the finished ground surface. The depth is generally less than the width of the footing

and less than 3m. They are used when the surface soils are strong enough to

support the load imposed upon it.

SPREAD FOOTINGS Made from reinforced concrete

– Square (B x B)-Usually one column– Rectangular (B x L)-When large M is needed– Circular (D/B<3, Rounded)-Flagpoles, transmission lines – Continuous (Strip)-Support of bearing walls– Combined (Cantilever)-Provides necessary M to prevent

failure. Desirable when load is eccentric and construction close to property line.

Weight Distribution in footing

Sizes of Footings

The width of the wall is the height of the footing

The width of the footing is twice the size at the wall

W, W, 2W is the rule

Footing Specifications Footings should extend down at least 6” into

undisturbed earth…no fillings under footing Footing must be at least 6” below the frost

line If soil is bad, increase the size of footing

Sizes of Foundation Walls

Depends on lateral earth pressure and vertical load to be supported– Typically walls are 8” thick

Basement walls must extend 8” above the finished grade. Min height is 7’5”.

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Combination Spread & Strip Footing Spread Footing

Spread FootingReinforcement in Spread Footing

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Types of Spread Footing

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Grillage Foundation

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Steped Foundation

Strip foundations These are use to support

a line of loads such as a load bearing wall.

They could also be used where the line of column positions are so close that individual pad foundations would be pointless.

Raft Foundations These are used to spread

the load from a structure over a large area.

This would normally be the entire area of the structure.

Raft foundations are often needed on soft or loose soils which have a low load bearing capacity.

ADVANTAGES OF RAFT FOUNDATIONS

– Spread the load in a larger area-Increase bearing pressure– Provides more structural rigidity– Reduce settlement– Heavier-More resistant to uplift– Distributes loads more evenly

Deep foundationsDeep foundations

Shallow foundations are unsuitable in weak or highly compressible soils so, deep foundation is provided in such areas where soil bearing capacity is very low and heavy loads are expected. Deep foundations are usually at depths deeper than 3m and use to transmit the loading to a deeper, more competent strata.

Caissons with or without sockets, end bearing or friction piles, pile walls and piers.

Made of concrete (precast or cast in situ) or steel or wood

Deep Foundations - Purposetransfer building loads deep into the earth

Basic types– Drilled (& poured)

– Driven

(End bearing piles for point load and friction piles will transfer load by friction resistance between the pile and the earth)

Pile foundations

Piles are used to support buildings in poor soil conditions.

A basic pile foundation is a series of stilts which rest on a solid load bearing layer.

Precast Concrete Plies

Benched Excavation

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Backhoe

DOZERS

Solder Beam & Lagging

Bank Requiring a Retention System

Retention System Depends On:

Proximity to Buildings Type of Soil Water Table Level Temporary or Permanent Cost - KEY Consideration

Dewatering

A process of removing Water and/or lowering the Water Table within a construction site

Purpose: To Provide a Dry working platform - (typically required by Code and Specification)

If the Water Table is above the working platform;Options:

» Keep water out» Let water in & remove it» Combination

SOIL TYPES

The make up of the soil has a major influence on the choice of foundation.

A good soil type needs to be able to cope with loadings.

A good soil needs to drain water well. A poor soil type will shrink, swell or move

depending on the loads or conditions placed upon it.

SILT Silty soil is found in flood plains or around

lakes. Silt holds water well and is soft when wet. Silty soil is not a very good foundation

material unless it has been compressed and hardened, or has been dried out.

SANDY SOIL

Sand is usually considered favourable from the standpoint of foundation support.

It can be a problem though usually due to water.

Water raising through a sand deposit can create an unstable condition.

Sandy soils can hold water.

CLAY

Clay is composed rock particles ground extremely fine or reduced by weathering.

Clay soils normally contain water. Clay drains slowly and compresses when

foundations are placed upon them. Clay has a tendency to absorb water and

swell.

ClaysPorous(sandy)

GRAVEL

Gravel can be well compacted and allows water to drain freely.

Gravel soils do not hold water. The variety in particle sizes in gravel means

that even when closely packed it still contains voids and drains well.

Gravel is least likely to be affected by drying out.

Waterproofing

Structures Below Ground subject to penetration of ground water

More extreme, if below H2O table Two basic approaches to Waterproofing

– Waterproof Membranes, or– Drainage– Generally - both used in tandem

Waterproofing Membranes

Materials– Liquid or Sheet (Plastic, asphaltic, synthetic rubber)– Coatings (asphaltic)– Cementitious Plasters & admixtures

Accessories– Protection Board– Waterstop

Unit of Measure - SF, Mils (thickness)

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