chapter 2 foundation1

8/12/2019 Chapter 2 Foundation1

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Prepared By: Er. Arvind Kumar Jha

CHAPTER 2 FOUNDATIONS AND BASEMENTS

Definition of foundation:

It is the lowest part of the structure. It provides base for super structures. It transmits loads to

soil below. It is the part that is below the ground level.

Or, it is the lowest part of the structure which provides a base for super-structure proper.

Function of foundation:

1. To transmit all superimposed loads (wind, vibration, dead and live loads)

2. To withstand against all kinds of settlements (against failure of underlying soil)

3. To give stability to structure by resisting in firm base.

4. To prevent lateral movement of supporting materials

Characteristics

The foundation that has following characteristics is preferred.

1. Wide enough section to distribute weight over larger base area within safe bearing

capacity

2. Evenly loaded condition that prevents unequal settlement.

3. Deep enough preventing overturning and increasing stability.

Types of foundation

1. Shallow and

2. Deep foundation

Shallow foundation

The depth of the foundation is less than or equal to its width.

It is placed immediately below the lowest part of the superstructure.

Distribute the structural load over a wide horizontal area at shallow depth below ground

level.

Types of shallow foundations1) Spread footings 2) Grillage foundation 3) Eccentrically loaded footing 4) Combined footings

5) Mat or Raft foundation.

1) Spread footing




I. Wall footings: - consists of several courses of bricks, the lowest course being usually

twice the breadth of the wall above.

II. Reinforced concrete footings: - places where walls are subjected to relatively heavy

loading and bearing capacity of soil on which wall footing is to rest is low.

III. Inverted arch footings: - to be provided for multistoried buildings in olden times.

Generally used in soft soils this reduces depth of foundation.

IV. Column footings (Independent footings): is provided under a column or other similar

member for distributing the concentrated loads in uniformly distributed load on the soil

below. It may be

a. Footing for brick pillars

b. R.C.C. column footings

c. Stone pillar footings

Fig: Wall / strip Footing Fig: Spread or Isolated Footing




Fig: wall and column foundations

2) Grillage foundation

When heavy structural loads from columns, piers or stanchions are required to be transferred to a

soil of low bearing capacity, grillage foundation is often found to be lighter and more

economical. Depending upon material used, it can be classified as:

Timber grillage

Steel grillage

Fig: Plan and section of a typical Grillage footing

3) Eccentrically loaded footing

When walls or columns are to be placed close to property lines, the required supporting areas of

the base cannot be placed eccentrically with imposed loads without overlapping the property

lines. Hence following methods are adopted to ensure stability of wall or column without

encroaching the area outside property line of the building.

i) Offsetting the footingsii) By providing strap footing

4) Combined footings: is so proportioned that the centre of gravity of the supporting area is in

line with c.g. of the two column loads. It may be rectangular or trapezoidal shape.




Fig: Combined Footing

5) Mat or Raft foundation: When the bearing capacity of ground is low or uncertain behavior

of sub-soil water condition, raft or mat should be preferred.

Slab (solid) – up to 30 cm

Slab and beam – slab> 30cm

Cellular – slab > 90 cm

Fig : Plan and section of a mat foundation.




Deep foundation

Deep foundations are those founding too deeply below the finished ground surface for their base

bearing capacity to be affected by surface conditions, this is usually at depths >3 m below

finished ground level. They include piles, piers and caissons or compensated foundations using

deep basements and also deep pad or strip foundations. Deep foundations can be used to transferthe loading to deeper, more competent strata at depth if unsuitable soils are present near the

surface.

Types:

Pile foundation

Well foundation

Piles:

Pile is the pillar like structure driven deep into the ground strengthens the strength of soil below.

It acts as support to the spread footing

It is used individually or in cluster throughout wall.

Fig: Piles

Uses of piles

1. It’s uses is pronounced

2. In very poor soil condition

3.

In waterlogged soil (high water table)4. In filling areas

5. In areas with heavy loads

6. In compressible soil

7. In the areas where the mat or grillage foundations are not possible




Types of piles

According to the uses

1. Bearing pile: End bearing piles are those which terminate in hard, relatively

impenetrable material such as rock or very dense sand and gravel. They derive

most of their carrying capacity from the resistance of the stratum at the toe of the pile.

Fig: Bearing pile

2. Friction piles: Friction piles obtain a greater part of their carrying capacity by skin

friction or adhesion. This tends to occur when piles do not reach an impenetrable

stratum but are driven for some distance into a penetrable soil. Their carrying

capacity is derived partly from end bearing and partly from skin friction between

the embedded surface of the soil and the surrounding soil.

Fig: Friction Piles

3. Sheet piles: Sheet piling is a form of driven piling using thin interlocking sheetsof steel to obtain a continuous barrier in the ground. The main application of sheet

piles is in retaining walls andcofferdams erected to enable permanent works to

proceed. Normally, vibrating hammer, t-crane and crawle drilling are used to

establish sheet piles.

4. Anchor piles




5. Batter piles

6. Compaction piles

7. Fender piles

According to materials use:

1. Steel piles – H-beam, box piles, pipe piles, screw piles and disc piles

2. Cement concrete piles

Cast in situ piles

1. Cased Raymond, Mcarthor, Monotube, Buttom- buttom, BSP base driven,

Swage etc.

2. Uncased simplex, Franki, Vibro, Vibro-expanded, Pedestral, Pressure etc.

Pre-cast piles

Pre-stressed piles

3. Timber piles

4. Composite piles5. Sand piles

Methods of pile driving

Drop hammer : commonly used method of insertion of displacement piles

Stem hammer

Water get

Boring

Selection of types of pile

Nature of structure

Loading in structure

Ground water table

Length of pile required

Availability of materials and equipment

Factors causing deterioration of piles

Cost of piles

Well foundation

Well foundation is the water tight box structure of wood/RCC/Steel and mostly used in the

foundation of the bridges. The purpose of well foundation is to develop an enclosure below for

plumb and provide access shaft to reach a deep tunnel transmitting the loads to hard bearing

strata.




Caissons

1. Box caissons

2. Well foundations and open caissons- single, double and cylindrical

3. Pneumatic caissons

Soil and its properties

Soil as uncemented geological deposits is basically of two types: cohesive and cohesion less. It

deserves high importance in foundation engineering. Soil investigation is must before

undertaking construction works.

Sub-soil exploration

Very important phase in construction and determines the characteristics of underlying

soil.

Acquires general picture of geology of area

Objectives of soil exploration

1. To determine the value of safe bearing capacity of soil

2. To select economic types of foundation

3. To determine the depth of proposed foundation

4. To predict likely settlement and make allowance for that in design

5. To know underground water level and its problem

Methods of soil exploration

1. Inspection

2. Naked eye observation

3. Test pits : helps to know type of soil at small depth, pit size : 1.5 * 1.5 m2 and depth

1.5m

4. Probing: hollow tube of 35-50mm is driven to ground at about 30cm at a time

5. Boring

Auger

Deep percussion and rotating boring

Wash: case tube is driven along with this a wash jet is inserted, this washes the

soil below and bring it to the surface.

6. Test piles: wood steel piles are driven under hammer blows.

7. Geo-physical methods

Electrical methods: electric current is passed through cathode and anode in soil

and flow of current through cathode to anode is the measure of soil below.




Seismic methods: vibrations are caused by artificial explosions and the movement

of these vibration waves measures the soil characteristics.

Bearing capacity of soil

It is the ability of soil to support the load coming over it

It is the strength of soil to resist minimum load coming to its unit area causing no failure

Maximum bearing capacity of soil = W/area of sole plate (foundation)

Safe bearing capacity = W/ (A*f)

Where,

W is total load including self weight

A is area of sole plate and

f is factor of safety

Methods of improving bearing capacity of soil

Mechanical stabilization

1. Mixing different graded soil (soil, gravel, sand etc.)

2. Change of grading of soil

3. Driving sand piles

4. Compaction

Cement stabilization

1. Mixing soil, cement and water

Lime stabilization

1. Mixing soil, lime and water

Bituminous stabilization

1. Mixing of bituminous with soil

Chemical stabilization

1. Calcium chloride

2. Sodium chloride

3. Sodium silicate

4. Polymers

Thermal stabilization1. Heating- decreases water content, decreases electric repulsion between clay

particles and this increases soil strength

2. Freezing- pore water freezes thus sol stabilizes

Electrical stabilization

1. Electrical osmosis process




Electrode inserted in the soil, water flows from anodes (+ve) towards cathode (-

ve) and pumping water deposited.

Grouting under pressure

Cement, clay, chemicals, chrome lignin, polymers, bitumen etc.

Geo-textile and fabric stabilization

Synthetics, polyethylene, polyesters etc.

Sheet piling

Sub-soil drainage

Suitability of different types of foundation

The choice and design of foundations mainly depends on

Total loads of building

Nature and bearing capacity of sub-soil.

A good foundation is judged by:

Location

Stability

Settlements

Choice of deep foundation depends on

Heavy loads

Poor bearing capacity of sub-soil

High water table and marshland etc

Cost consideration

Foundations in black cotton soil

Black cotton soil:

1. Good for agriculture and bad for structure

2. High shrinkage value due to change in moisture content

3. Volume varies as 20-30% of original volume

4. Develops very wide and deep cracks due to excessive shrinkage (may be upto

20cm wide and 2-4 m deep)

5. Very weak in saturation

6. Problematic for foundation

Precautions for foundations in black cotton soil




1. Foundation depth be enough below from cracks 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 the black cotton soil is high, foundation is to be laid on piles

5. Raft foundation is the choice in this condition

6. Tie-beam in plinth is important

2.1 Some common problems with existing foundations

Settlement of foundation

Causes of foundation settlement

1. Consolidation of soil particles

2. Reduction of moisture content

3. Heaving of soil due to pressure

4.

General earth movement Effects of unequal settlements

1. Stresses in the structures

2. Distortion of the structure fabrics

3. Failure of structure

Prevention of undue unequal settlements

1. Proper foundation design

2. Proper soil investigation

Causes of foundation failure

1. Unequal settlement of sub-soil

2. Unequal load distribution

3. Horizontal movement of soil adjoining structure

4. Lateral pressure tending overturn

5. Shrinkage due to withdrawal of moisture from soil

6. Atmospheric action

7. Lateral escape of soil below foundation

8. Nearby building construction

9. Trees etc.

2.2 underpinning of foundations of existing building

Definition: It is an excavation process under existing foundation. It is a process of improving

and strengthening existing foundation. It facilitates to support structure to better soil strata. It

assists in transferring loads to new bearing strata. It is very sensitive work.




Necessity

Occurrence of excessive settlement

1. Uneven loading

2. Unequal resistance of sub soil

3. Action of sub soil water

4. Action of tree roots

Increasing load bearing capacity of foundation

1. Change of functional use

2. Addition in loading pattern

Permitting to lower adjacent ground below existing foundation

1. Construction of new basement nearby

Operation to be carried out before underpinning

1. Survey of the structure

2. Settlement if any

3. Noticing neighbors (adjacent building)

4. Set indicators to identify probable cracks while underpinning

5. Carry out corrective measures for cracks etc.

6. Investigate sub-soil

Precautions

Excavation in one time done for less than one fourth of length

For weak soil it is done for less than one fifth to one seventh of length normally length of

one bay is taken as 1.5m

To be carried out slowly in stages and not at a time

Sequences of operation




Suitable holes driven through the wall and a needle beam are inserted. This needle beam

is placed on wooden block in one side and the other side is supported on the jack. This is

preciously checked before starting excavation.

Excavation is started below foundation and footing of the foundation is reached.

Excavation of trench should be slightly wide so that the trench goes under foundation.These sides are held in place by adequate timbering.

The offset of the foundation is cutoff and removed

Excavation is reached to be defined depth

New foundation is laid in the desired depth upto the underside of the existing foundation

- This process is repeated in stages.

- To give continuity to the concrete foundation. Dowel bars of 25mm dia. Are inserted by

the end of each bay to connect to the next bay.

- Brick underpinning is toothed to enable continuous bonding

- Final layer of pinning work just underside of existing foundation should be done with

the mortar from rapid hardening cement.

Methods of underpinning

Pit method

1. Ordinary

2. Cantilever

Pile method

Improving of foundation by grouting and chemical consolidation




Fig: Sketch of a cast in situ RC cantilever needle Fig: Sketch of a standard needle beam on

beam on micro piers or piles. Access to micro piers or piles. Inside access needed

inside not needed.

Fig: Sketch showing the traditional method of needling a wall to reduce the weight on the foundations during underpinning work




Basement

Basement is a space, a storey or a floor immediately below the adjacent ground level while

taking about the basement we have to deal with the retaining wall.

Wall

It is a structural element and primary function of this is to enclose or divide space and the

element may even have to provide support

Types of wall

1. Load wall

2. Non load bearing wall

3. Retaining wall

Retaining wall

This is the wall, the primary function of which is to resist the lateral thrust of a mass of earth on

one side and sometimes the pressure of sub-soil water and in many cases, the wall may also be

required to be support vertical loads from a structural above. Structural walls in basement are in

fact retaining walls.

Function of retaining wall

1. Strength stability and durability

2. Resistance to overturn

3. Resistance to horizontal slide

4. Resistance to overstress in the materials of construction

5. Resistance to overstress in the soil in which the wall rest

Forces acting on the retaining walls

Active earth pressure

It is lateral pressure which tends to move or overturn the wall at times and this is the

result of the earth wedge being retained together with any hydrostatic pressure caused by

the presence of ground water.

Passive earth pressure




It is the reactionary pressure that builds up to resist any forward movement of the wall,

because any forward movement will compress the soil in front and reaction to counteract

this movement builds up.

Angle of repose

It is natural slope taken by any soil and it is given in terms of the angle to the horizontal

baseline. This varies from 450 to 0

0 angle for wet clay, but for most soils,this angle of

repose is 30o

Wedge of soil

This is the mass of the soil resting on the upper plane of the angle of repose.

Surcharge

This the additional mass of the soil above the top surface of wall

Factors affecting strength, stability and durability of retaining wall

Effect of ground water

The sub-soil water has adverse effects upon the strength and stability of retaining walls

by reducing the soil shear strength which reduces the bearing capacity of the base and

the soil and the possible passive pressure acting in the front of wall.

Solution:

It is important to drain out the water behind the retaining wall.

Effect of inadequate passive earth resistance

The passive earth resistance at the front of the wall in conjunction with friction on the

underside of the base resists sliding. When the frictional resistance is insufficient,

passive earth resistance must be used to increase the total resistance to sliding to the

required level, or else the wall will slide.

Solution:

In case of inadequate passive earth resistance, it is important to increase this by provision

of ribs under the head and toe.

Types of retaining walls

Gravity or mass retaining wall




Cantilever or L-Shaped retaining wall

1. Base entirely in front of stem.

2. Base partly in front and partly behind the stem

3. Base wholly behind the stem

4. Counterfort retaining wall

Gravity or mass retaining wall

It may be brick, stone or mass concrete, where, the mass of this type of the wall gives the

desired strength and stability. The width of this type of the wall is such that the resultant of

the lateral earth pressure and weight of the wall falls in such a position that the maximum

compressive stress at the toe does not exceed the maximum safe bearing capacity. Width of

the base is usually H/4 or H/2, where H is the height of the wall. For efficiency the wall issloped in the front face of rectangular size. The front face may be reinforced to avoid

cracking. The height is usually limited to 1.8 to 2.0 meters.

Cantilever or L-Shaped retaining wall

1. Base entirely in front of stem

It is the most common used form, where it is not possible to excavate behind the stem.

Maximum height of this type of wall is 6.0 meters.

2. Base partly in front and partly behind the stem

In this wall, the base is projected partly in the front and at the behind. This is possible

where excavation can be carried out behind the wall. The weight of soil heel assists in

counterbalancing the overturning tendency. The most economical arrangement is if the

length of the heel is twice the length of toe. Maximum height of this type of wall is 6.0

meters.




3. Base wholly behind the stem

In this type of the wall, the base is projected wholly at the behind. It gives greater

stability; therefore the base can be shorter. It can only be used in places, where

excavation behind the stem is possible. Cost decrease due to short base may be offset by

increased cost of excavation. It is not suitable, if it carries superimposed loads fromstructure above. Maximum height of this tyoe of the wall is 6.0 meters.

4. Counterfort retaining wall

This is a cantilever wall with vertical ribs angled at right angle. Vertical ribs may be at

the front or back of stem. Wall with ribs (counterforts) at the front is called buttressed

retaining wall. The action of counterforts is to tie the vertical wall slab with the base of

the wall. This wall provides better stability and strength. This wall is efficient and

economic for greater height, usually greater than 8.0 meters.

Design principle of retaining wall

Retaining wall must ensure that:

1. Overturning doesnot occur

2. Sliding doesnot occur

3. The beneath the wall is not overloaded

4. The materials in the wall are not overstressed




Factors to be considered during the design

1. Nature and type of soil

2. Height of water table

3. Sub-soil water movements

4. Types of wall

5. Material used in the wall

2.3 Shoring of existing building during foundation strengthening

Shoring is the temporary structure required supporting an unsafe structure. It may be used in all

cases of strengthening any parts of the building and to give support to the building at risk.

Causes of structure to be unsafe

1. Unequal settlement of the foundation

2. Dismantling adjacent structure

3. Bulging out of the wall

4. Addition and alteration of different part of the building

The shoring may be of the timber, steel or both.

Objectives of the shoring (necessity)

1. To give support to walls, which are at risk (bulging or leaning outwards etc.)

2. To avoid failure of the boundary wall caused by the removal of adjacent support like

basement bear to bound wall

3. To give the support to the adjacent building during demolition works.

4. To support the upper part of the wall during the formation of the large opening.

5. To give support to the floor or roof to enable a support wall to be removed and replaced

by a beam.

Types of shoring

I. Raking shoring (slant or sloped roof)




This is the inclined support to the unsafe wall. It consists of the wall plate, inclined

member (shore), bracing, cleat, needle, iron hoop etc. the shore is rest on the firm ground.

The section of the wall, plate and the shore should be such that it can be withstand the

probable loads of the wall safeguarding from any risks. The rakers may be one or more

depending on the distribution of pressure over large area. In places, where more rakers

are provided, they are bounded together by the means of iron hoops and bracing. The

inclination of raker may be between 600 to 75

0

Procedure

Site investigation

Marking out of the location

Fixing of the wall plate with needle and cleats inserted to the holes in external

walls

Setting a firm ground level and if necessary the base is made with grillage

platform as sole plate.

Cutting the rakers to appropriate length and fixed to the cleats on the wall plate

Rakers are braced and tightened with the help of the wedges

Near the sole plate, rakers are tied together with iron hoop.




Fig: Detail of Head of the raker




Fig: Raking shore for multistoried Building where inclination of the rakers has to be

limited due to short land width available

II. Dead shoring ( vertical shore)

This type of the shoring is used to render vertical support to the walls, floors and roofs

etc. when the lower part of the wall has to be removed for the purpose of providing

opening in the wall or to repair or rebuild the defective bearing wall in a structure. It

supports dead loads that act vertically downwards. The dead shore consists of an

arrangement of beams and posts to support the weight of structure and transfer it to the

ground below.

Procedure

Site investigation

Marking out of the location




Holes are cut in appropriate position in the external wall

Needle beam is inserted through the hole in the wall

Needle beam is then supported with vertical posts at the end of offsetting from the

wall

Vertical posts are fixed on the firm ground surface above soleplate and tightened

with wedge

Before dismantling defected area of wall, all doors, windows, floors, wall above

etc. are properly strutted

The loads of the floors and the doors/windows may be independently supported so

far it is possible

Fig: Dead Shore




III. Flying shoring (horizontal shore)

These kinds of the temporary support has the same function as raking shore with an

advantage of providing clear working space under the shoring. In this shore, the wall at

the risk is supported in front of it. The horizontal beam is provided as a support to the

wall. It is then fixed with struts, cleats, needles, and straining pieces. Depending upon thespan shore may be double or single. If two beams are fixed, it is double and it is single

with single beam. It is obvious that beam is fixed on the wall plate.

Fig: Flying Shore Fig: Flying shore when the distance

between two walls is considerable.

Procedure

Refer raking shore

2.4 Retaining properties and water proofing of basements

Damp proof course for basement

Procedure

provided on outside surface of wall and underside of floor of basements

DPC must withstand the water pressure from underside

Basement must have sufficient dimension

Adequate dewatering arrangement should be applied to keep water table below

basement

Suitable shuttering to be provided to prevent collapse of the sides




Base concrete (PCC) of sufficient thickness to be provided with minimum projection

of 15 cm beyond outer wall as a protective before DPC

RCC wall and slab be provided after DPC course

Asphalt layer is best DPC in basement and it should be continuous

In case of existing basement, damp mapping has to be made with chemical with

pressure

There must be proper lapping of DPC joints and cracks

2.5 Sealing of cracks in basement

There is always a danger of leakage and seepage in the basement if the damp proofing treatment

is not properly done. Moreover cracks may result due to reaction of the forces in the structure.

The cracks have to be properly sealed. Sometimes the cracks may be invisible and have to be

precisely checked. In this case proper damp mapping has to be undertaken.

chapter 2 foundation1

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