EXCAVATION

SHALLOW EXCAVATION

Shallow excavation with unprotected slope

Shallow excavation with sheeting and bracings

Shallow excavation with unsupported slope

Space is available to establish slopesteepness of the slope depends on

the otype of soil, odepth of excavation,otime to which the excavation should stand.

For sandy soil slope of 1 : 1.5 is provided

Shallow excavation with sheeting and bracing

Building site is adjacent to other site or some structure is already existing.

Depth of excavation is less than 2 mVertical planks called sheeting is driven

around the proposed excavationSheeting is kept in position by means of

horizontal beam called wales.Wales in turn are supported by Hz strutsIf the excavation is too wide the wales are

supported by inclined struts called rakes

Shallow excavation with sheeting and bracing

If the excavation is too wide the wales are supported by inclined struts called rakes

Shallow excavation with sheeting and bracing

DEEP EXCAVATION Excavation beyond a depth of 2 m is generally

categorized as deep excavation

Problems encountered:

Collapsing of the sides of the trenchPrevention of water entering the sides of the

trench or from the bottom of the trench

Stay bracingBox sheetingVertical sheetingRunnerSheet piling

Stay bracingSimilar to shallow

excavationUsed in moderately firm

groundDepth of excavation

does not exceed 2mVertical sheets are

placed on opposite sides of the trench and they are held in position by means of struts

Suitable for loose soilDepth of excavation does not exceed

4mPlanks or vertical sheeting are placed

closer Longitudinal rows of wales keep the

sheet in positionStrut hold the wale in position

In very loose soil additional bracings are provided

Box sheeting

Box sheeting

Vertical sheeting Provided in soft ground Upto to a depth of 10 m Work is carried out in satages An offset is provided at each stage (@ 3 – 4 m depth) Width of the offset is 50 -60 cmAt each stage separate sheeting, wales and braces are provided

Runner

Used when immediate support is needed in case of very loose and soft ground Runners are long thick wooden sheet with iron shoes at the ends

Sheet piling

When the depth of excavation exceeds 10 m

Timber sheeting become uneconomical

Flat , arch , z type –sheet piling is used

Excavation tools

Ordinary excavations are carried out with the aid of tools such as spade, pick axe, phaorah, crowbar, rammer, boring rod, basket, pan etc

When rocks are encountered, chisel, jumper, hammer, wedges are used

Components of a buildingSubstructure

Substructure is the lowest portion of a building, which is located below the ground

Provides stability to the ground and distribute the load coming from superstructure

Superstructure Provides adequate stability and safety against

adverse weather conditionProvides aesthetic appearance

Selection of site for a buildingDepending on the type of building, the area and

location has top be decided.Site should have gentle slope, so as to have

natural drainage.Should be fairly level, and should not have ups

and downs.Site should not be a filled up ground.Site should not be water logged or the GWT is

very highSite should have high bearing capacity.Basic amenities such as water supply, electricity,

telephone should be available near the site.Site should be connected by adequate transport

facilities.

Improvement of foundation soilIf unsuitable soil condition are

encountered at the site one of the following three procedures may be adopted

Bypass the unsuitable soil by means of deep foundation.

Remove the poor material and treat it or substitute it by suitable material

Treat the soil in place to improve its properties

Improvement of foundation soilSurface compactionDrainage methodVibration methodPreloading and surcharge fillsVertical drains Grouting and injectionChemical stabilizationSoil reinforcement

SURFACE COMPACTION

One of the widely used and oldest method of soil densification

If the depth to be densified is less surface compaction alone is enough

Most economical method

Drainage methodVibration method

Preloading and surcharge fills

In this process an earth fill or some other material is placed over the site

Soft soil is allowed to consolidateMain requirement are availability of enough space and fill material

Vertical drains

For deep clay soil preloading alone will take long time

Vertical drains are continuous vertical columns of pervious materials installed in clayee soil for the purpose of collecting and discharging the water expelled during consolidation

Grouting and injectionFill the void to prevent excessive settlement

Stabilizing loose soil against liquefaction

strengthening existiting foundationReduction of machine foundation vibration

Chemical stabilization

Lime, cement and fly ash are used in combination

Increases the bearing capacityDecreases settlementReduce permeabilityImprove shear strength

Soil reinforcement

Soil reinforcement is the process of strengthening the weak soil by providing high strength thin horizontal membranes.

Reinforced is analogous to reinforced concrete

Materials such as steel, concrete, glass fibre, rubber, aluminium, and thermoplastic have been used successfully

Spread footingsCommon type of with minimum cost and complexity of construction

Also known as pad footing, isolated footing, square and rectangular footing

Combined footingFootings formed by combining two or more equally or unequally loaded column into one footing

Usually rectangle in shape

Strap is provided to accommodate wide column spacing or column close to property line

Continuous footingFootings carry closely spaced columns or continuous wall , such that the load distribution is uniform and the load intensity is low on the supporting soil

Also called as strip footing or wall footing (L/B > 5)

Mat or Raft footing“N” number of columns

can be accommodated with in the footing.

Used when the soil has low bearing capacity

Settlement decreases with the increase in the width and depth of the footing.

Unit pressure acting on the soil is also reduced

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PILE FOUNDATION

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When the upper soil is soft or weak with low bearing capacity we go for deep foundations.

Piles, caissons and well foundations are the most common types of deep foundations.

GRILLAGE FOUNDATION Steel beams are arranged in layers at right angle to one another and the beams are connected with each other by means of bolts so as to form a rigid unit. The entire assembly is embedded in concrete.

Adopted for structure having concentrated loads (factories, town halls, clock tower..)Depth of foundation is limited to 1 to 1.5mRSJ are fully embedded in concrete, to protect from atmospheric actionsBed of concrete should be 15 cm and no where the depth of concrete should be less than 80mm

This type of footing is provided in sloping ground

Points to be taken care

Over lap should be greater than the depth of foundation

concrete or twice the height of stepsMini mum depth of foundation concrete is 80 cmDepth of foundation concrete should be uniformDistance of sloping surface from lower edge

point should be atleast 100 cm from soils & 60 cm

from rockSlopes should be stable

The horizontal location of a footing is often affected by adjacent structure and property lines.

IS recommendation for footings placed adjacent to a sloping ground or when the footings are at different level

The footing should be founded atleast at a distance ‘S’ from the edge of existiting footing Line from the edge of new footing to the existiting footing should make an angle of 45 ◦New foundation is constructed lower than the old foundation, suitable bracing system should be provided

PrecautionsThe expansive should be completely removedConstruction should be carried out in dry seasonDepth of foundation for small buildings should extend atleast by 30 cm beyond the depth of crackFor important structure raft foundation should be adopted

Under pinning is the method of supporting structure while providing new foundation or carrying out repairs and alterations without disturbing the stability of existiting structure

When a building with deep foundation is to be constructed adjoining a structure with shallow footingProtect an existiting structure from the danger of excessive or differential settlement of foundationIn order t o improve the bearing capacity of a foundation, so as to sustain heavier loadsTo provide a basement for an existiting structure

Pit methodPile methodChemical method

Divide the foundations to be strengthened into various sections (1.2 to 1.5 m)Holes are made at desired height in the existiting wallThrough these hole needle beam with bearing plate are inserted and supported on either side of the wallPit is excavated upto the depth of new foundationOld foundation is extended upto the level of new foundation

RCC pile is installed by making an approach pit under the existiting wallHydraulic jack is arranged to assess the over load and to drive the wedge beam All the material surrounding the pile is removed and the gap is filled with cement concrete to provide firm bearing ground

Steel wedging beams are driven between R.C pile and the steel plate provided under the foundationJacks are removedA complete underpinning Cylinder is formed

The foundation soil is consolidated by employing chemicals

Perforated pipes are driven in the inclined direction beneath the foundationSolution of sodium silicate with water is injected The pipes are withdrawn, at the time of with drawl, calcium or magnesium chloride is injected through the pipeChemical reaction takes place and the soil is consolidated