Foundations and Underwater Construction

 

Foundations and Underwater Structures Construction Group 2

 

Shallow Foundations

 

Introduction 

  Shallow foundations, sometimes called footings, are usually embedded about a metre or so into soil.

 

Types of Shal Shallow low Foundations     

Wall or Strip Footings Spread or Isolated Footings Combined Footings Strap or Cantilever Foo Footings tings at or !aft Foundations

 

Wall/Stri all/Strip p Footings  

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"rovided for a load#bearing wall "rovided $lso provided for a row of columns which are so closely spaced that their spread footings overlap or nearly touch each other In such case, it is more economical to provide a strip footing than to provide a number of spread footings in one line.

 

Spread or Isolated Fooings  

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"rovided to support an indi "rovided individual vidual column Such footings can be circular, s%uare or rectangular slab of uniform thic&ness Sometimes, It is stepped or haunched to spread the load over a large area.

 

Combined Footings 

Supports two columns and used when the two columns are so close to each other that their individual footings would overlap

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$lso provided when the property line is so close to one column that a spread footing would be eccentrically loaded when &ept entirely within the property line. 'y combining that column to an interior column, load is evenly distributed. It may be rectangular or trape(oidal in plan.

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Strap or Cantilever Footings Consist of two isolated footings connected with a structural strap or lever.  )he strap connects the two footings such

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that they behave as one unit. $ strap footing is more economical that a combined footing when the allowable soil pressure pressur e is relatively high and the distance between columns is large.

 

Mat or Raft Foundations 

$ large slab supporting a number of columns and walls under the entire structure or a large part of the structure stru cture

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$ mat is re%uired when the allowable soil pressure pressur e is low or where the columns and walls are so close that the individual footings would overlap or nearly touch each other.

 

Method of Construction *.

 )he whole area is dug put to the speci+ed depth and - cm more wide than the areas to be covered.

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 )he bed is compacted and sprin&led over with water.  )hen a layer of lime concrete concrete or lean concrete */ 0 / *1 is laid to a suitable thic&ness to act as a bottom cover.

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$fter this, the reinfor reinforcement cement is laid. )he reinforcement consists of closely spaced reinforcement bars placed at right angles to one another another..  )hen the cement concrete */ 2/ 3 3 is laid and compacted to the t he re%uired thic&ness.  )he concrete slab so laid is then properly properly cured. When loads are e6 e6cessive, cessive, thic& concrete beams running under the columns can also be constructed.

 

eep Foundatio oundations ns

 

Introduction 

$ deep foundation is used to transfer the load of a structure down through the upper wea& layer of topsoil tolayer the of stronger subsoill below subsoi below..

 

Types of eep Foundations  

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$ccording to materials used $ccording $ccording $ccor ding to the length and load transfer mechanism $ccording $ccor ding to th the e method of installation

 

!ccording to the materials used

Concr Con crete ete "iles "iles

 

 )imber  )i mber "iles

Steel Steel "i "iles les

 

!ccording to the length and load transfer mechanism

 

!ccording to the method of installation  

'ored "iles 8riven "iles

 

"ored #iles 'ored Cast#in#situ piles are concrete piles.  )hese piles are distinguished from drilled piers as small diameter piles. )hey are constructed by ma&ing holes in the ground to the re%uired depth and then +lling the hole with concrete.

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!einforcement may be used as per the re%uirements.

 

!dvantages of "ored #iles *.

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"iles of any si(e and length may be constructed at the site. 8amage due to driving and handling common in driven piles can be eliminated.  )hese piles are are ideally suitable places where vibrations of any type are re%uired to be avoided to preserve the safety of the ad9oining structure.  )hey are suitable suitable in soils of poor draina drainage ge %ualities since bored piles do not signi+cantly disturb the surr surrounding ounding soil.

 

isadvantages of "ored Installation re%uires re%uires careful supervision and #iles *.

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%uality control of all the materials used in the construction.  )he method is %uite cumbersome. It needs su:cient storage space for all the materials used in the construction.  )he advantage of increased bearing capacity due to compaction in granular soil that could be obtained by a driven pile is not produced by a bored pile. Construction of piles in holes where there is heavy current curr ent of ground water ;ow or artesian pressur p ressure e is very di:cult.

 

Method of Construction

 

riven #iles 

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8riven piles are long, slender column made of preformed preformed materials and having a predetermined predeter mined shape and si(e that c can an be installed by impact hammering, vibrating or pushing it into the ground to a design depth or resistance. 8riven "iles may be timber, steel or concrete.

 

!dvantages of riven #iles *.

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"iles can be precast to the re%uired speci+cations. "iles of any si(e, length and shape shape can be made advance and at the site. $s a result,in the program of used the wor& will be rapid. $ pile driven into granular soil compacts the ad9acent soil mass and as a result the bearing capacity of the pile is increased. 8riven piles may conveniently be used in places where it is advisable not to drill holes for fear of meeting ground water under pressure. pressure.

 

isadvantages of riven "recast "r ecast concrete piles must be properly reinfor reinforced ced #iles *.

to withstand handling stress during transportation and driving. 2.

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Since the e6actin length re%uired at the site cannot be determined advance, the method involving cutting e6tra length or adding more lengths. )his increases the cost of the pro9ect. 8riven piles are not suitable in soils of poor drainage %ualities. 8riven piles should not be used when foundations of ad9acent structures structu res ar are e li&ely to be a