Site Investigation and Soil Exploration

8/10/2019 Site Investigation and Soil Exploration

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Site Investigation and Soil Exploration

Before the engineer is in a position to design a foundation in economic and

intelligent way, he must possess reasonably accurate information about the physical

properties and the arrangement of the underlying materials. The field and laboratory

investigations required to get this essential information are known as soil exploration.

Due to very complex nature of deposits, no one method of exploration is best suited for

all conditions. Thus, the object of soil exploration may be outlined as follows

. Depth, extent, nature and variations of soil strata.

!. "hysical properties of soils encountered.

#. Depth to underlying rock bed, when necessary.

$. %pproximate values of strength and compressibility of soil bed.

&. The seasonal variation in ground water'table.

(. )election of foundation types.

The first step is to inspect the site and study the topographical features for

deciding the future programme of exploration. *any information+s can be gathered from

the local people regarding the type of soil, seasonal variation of ground water'table,

highest flood level, types of cracks and failures in the existing buildings of that area. But

for important, costly and multi'storied buildings, it becomes essential to collect detailed

information+s very accurately, thus necessitating a thorough investigation of the soilavailable at the site.

Bearing Capacity

%fter conducting site reconnaissance and detailed exploration the ivil -ngineer will

possess the following facts about the site

/ "resence of boulders, gravels or other obstructions, if present

!/ The depth of rock strata, if any.

#/ haracter of soil, i.e. clay, silt, sand, gravel, etc.

$/ "hysical properties of soil.

&/ 0ariation in ground water'table throughout the year.

(/ 1ighest flood level of the area.


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2ow, the selection of the suitable type of the foundation, its depth, shape and si3e has to

be decided so that it can safely transmit the load from the building to the ground without

soil failure or detrimental settlement and at the same time it must be economical also.

4ltimately the load of the building or structure will be coming on the soil and hence it is

of utmost importance to know the strength and behavior of the soil. 5or further

considerations, it will be useful to define the bearing capacity as follows

BEARING CAPACITY.

The supporting power of a soil or rock bed is defined as its bearing capacity.

UTI!ATE BEARING CAPACITY.

The maximum pressure intensity at the base of the foundation at which the soil fails in

shear is known as ultimate bearing capacity.

A"#ABE BEARING CAPACITY.

6t is the loading intensity at which neither the soil fails in shear nor there is excessive

settlement detrimental to the structure. 6t is normally obtained by dividing the ultimate

bearing capacity by a suitable factor of safety.

$ETER!INATI"N "% BEARING CAPACITY

The bearing capacity of a soil can be determined by any one of the following methods7

/ 8ocal experience in the construction of similar buildings.

!/ )tudy of the soil type and selecting a corresponding value of bearing capacity

as obtained from the standard building codes.

#/ 5ield tests for determining directly the value of bearing capacity.

$/ 8aboratory tests for engineering properties of soil and hence forth analytical

calculation of bearing capacity of soil.

%"UN$ATI"N

-very structure consists of two parts, i.e. foundation and super structure. 5oundation is

the lowest part of a structure which transmits the weight of the structure, together with

the effect of live loads and wind pressure, to the material on which the structure rests in

such a manner that the underlying material is not stressed beyond its safe bearing

capacity. 9enerally, the foundations re placed below the ground level to increase the

stability of a structure or building. Thus the objects of providing foundations may be

outlined as follows7


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a/ To distribute and transmit the total load coming on the structure or building to

a larger area of underlying support.

b/ To prevent excessive settlement and differential settlement of the structure.

c/ To provide stability to the structures against many disturbing forces, i.e. wind,

rain, earthquake, etc.

6f the underlying material is solid rock, the design of a foundation is greatly simplified,

since the bearing capacity of rock in natural beds is very high and the contact area

between the foundation and the rock can be kept to a minimum. But most of the time the

structures are to be built on ordinary soil.

Different types of materials are used for the construction of the foundations under various

circumstances. )tone, bricks, steel, concrete, reinforced concrete, etc. are some of the

materials used commonly. The type of structure or building and the nature of the

underlying soil are two main governing factors in selection of the material and type of

foundation.

Types o& %o'ndations

5ollowing are the various types of foundations commonly used7

Spread %o'ndations.

The total load coming at the base of the structure is spread over a larger area by such

foundations. The intensity of load transmitted to the supporting soil is less than its

allowable bearing capacity and thus the settlement of the building is prevented to

reasonable extent. The width of the wall is constructed thicker at the base in a stepped

fashion.

Pile &o'ndations.

% pile may be defined as a long vertical load transferring element composed of timber,

steel, concrete or a combination of them. 8oads are transferred to the soil by two

mechanisms, i.e. i/ by the friction between the pile and surrounding soil, and ii/ by the

bearing of pile on a hard rock bed at certain depth. The former is known as friction pile

and the latter is known as end bearing pile. The load, the type of soil encountered, and the

material of pile govern the number of piles to be used under any structure. %ll the pile

tops of a group are connected by :.. beams so that the load may be uniformly


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distributed to the soil. "ile foundation is used with advantage in water'logged area, or in

compressible soil, or in made up soil, or on steep slopes, etc.

Pier &o'ndation.

;hen the structure is situated in sandy soil or soft soil, overlying hard bed at reasonable

depth, this type of foundation used. % hollow well or solid vertical shaft is sunk upto the

hard bed and in case of well, the hollow portion is filled up with some inert material like

sand, in moorum or lean concrete. 6t is rarely, used for buildings.

Spread %o'ndations

The various types of spread foundations are as follows which are constructed to spread

the loads7

i/ ;all footing

iii/ ombined footing

v/ ontinuous footing vii/ 9rillage foundation

ix/ )tepped foundation.

i/ #all &ooting.

;all footings are either simple or stepped.

)imple footings are provided to carry light loads and have only one projection outside

the width of the wall on both sides as illustrated in 5ig. The projection provided in the

footing is & cm. on both side and the depth of the concrete bed is at least twice the

projections.

The base may be of concrete slab or completely build up of bricks or stones. 6n case

no concrete base is provided, the wall is built up in steps to transmit the load safely.

The bricks are projected regularly to one'fourth brick distance beyond the edge of the

wall. :ich mortar is used to join the bricks.

This type of foundation is rarely used these days as it is not economical,

;hen the width of foundation is considerably more than the wall width, it is economical

to make the brick footing stepped over a level concrete bed as shown in 5ig. The footing

should be designed properly otherwise the upward reaction from soil will have a

tendency to break or crack the concrete bed as illustrated in 5ig. #.# and #.$.

The concrete used in footings consists of ordinary "ortland cement, sand, and stone chips

or brick ballast in the proportion 7 # 7 ( or 7 $ 7


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. iv/ antilever footing

!. vi/ 6nverted arch footing viii/ :aft or mat foundation

iii/ ombined footing. % common footing constructed for two or more columns is called

as a combined footing. The construction of combined footing becomes essential when the

external column is situated near the boundary line and it is not possible to project the


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(iv) Cantilever &ooting. antilever footing consists of an eccentric footing of the exterior

column and a concentric footing for the interior column= and they are connected by a

strap or a cantilever beam. )uch footings are used when it is not possible to place afooting directly below a column because of limitation of boundary or eccentric loading

conditions. The load from the exterior column is balanced by the load of the interior

column acting about a fulcrum. )ometimes, when the interior column load is not

available, a huge concrete block is built to act as an anchorage. 6n actual construction,

point shaped fulcrums are not used instead of that a suitable base is constructed for the

exterior column. The connecting strap or beam checks unequal settlement of the external

column. oncrete or steel may be used for the construction of a cantilever footing.

(v) Contin'o's &ooting. % single continuous :.. slab is provided as foundation of two

or three or more columns in a row. This type of construction is more suitable to prevent

the differential settlement in the structure and for the safety against earthquake.

)ometimes, a deeper beam is constructed in between the columns to have better stability

(vi) Inverted arc* &ooting. 6t is used to transmit loads above an opening to the

supporting walls. 6nverted arches are constructed between the two walls at the base. The

loads transmitted to the soil through such footings are distributed over a larger area and

hence even soft soil can bear the pressure safely. 6t is suitable for soil of low bear'

Inverted Arc* %ooting.


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6nverted %rch 5oundation

vii/ 9rillage foundation. 9rill age foundation is used to transfer the heavy structural

loads from steel columns to a soil having low bearing


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apacity. 6t. is light arid economical. The required base area to reduce the pressure

intensity is obtained at a shallow depth arid hence deep excavation is not essential.

9rillage foundation is constructed by rolled steel joists :.)>./ which are placed in single

or double tier. 6n double tier grillage, the top tier is placed at right angles to the bottom

tier. The steel joists of the grillage are kept in the position by !& mm and pipe separators

and nuts. The distance between the flanges of :.).>. should be equal to .& to !.? times

the width of flange or #? cm. whichever is small. The tiers of rolled steel joists are

completely embedded in concrete to protect the steel from corrosion. The bed of concrete

should have a minimum depth of & cm. arid at other points the concrete cover should not

be less than < cm. The concrete filling is not supposed to take any load but it keeps the

steel joists in position and prevents them from corrosion. % grillage foundation may be

constructed for a single column or for more than one columns 5igs. #.$ and #.&/.


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Ra&t or !at &o'ndation.

This type of foundation is very useful when the load coming on the soil is practically

uniform, while the soil is soft clay. made up ground or marshy land with low bearing

capacity. 6f there is any chance of subsidence in the structure due to. its being located in

mining area or due to unpredictable behavior of sub'soil water condition, the use of raft

foundation is recommended. :aft foundation is constructed of reinforced concrete slab

covering the whole area of the bottom of the structure. The slab is provided with steel

reinforcing bars in both the directions and on both the

observed.

Stepped &o'ndation. 6t becomes very expensive to construct foundations at the same

level when the ground has steep inclination. )tepped foundation is provided as illustratedin 5ig. #.@. The overlap between two layers of foundation concrete slab is equal to the

depth of concrete slab or two times the height of the step, whichever is more. The depth

of concrete bed is generally kept in even number of masonry courses.

)tepped 5oundations.

Pile %o'ndationsDefinitions "ile. 6t is an element of construction used as foundation and driven in the

ground vertically or with some inclination to transfer the load of the superstructure to the

soil safely.

Bearing pile.6t transfers the superimposed load to hard strata at considerable depth.

%riction pile.6t transfers the superimposed load to the soft soil by the frictional forces

existing between the ground and the periphery of the pile.

Battered pile.6t is an inclined pile constructed to resist inclined forces.


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S*eet pile. 6t is thin member of steel sheet or timber used as pile.

Under+rea, pile.6t is a pile with one or more bulbs in its vertical shaft. These bulbs are

known as under'reams and it increases the bearing capacity of the soil considerably.

Uses o& Piles - 'nder t*e &olloing circ',stances t*e piles ,ay /e 'sed as

&o'ndation

. The live load and dead load coming from the structure is considerably large.

!. The construction of grillage or raft foundation is not economical.

#. The seasonal variation of ground'water table is considerable.

$. This type of foundation is suitable in the locality where canals, deep sewers,

etc. are to be constructed at a latter date.

&. 6t is used in the construction of marine structures, e.g. docks, piers, etc. This

protects embankment from scouring.

(. 6t may give lateral support to take care of an upward pressure.

8oad bearing 5iles

ast iron, Timber, ;rought iron, ement concrete, )teel, omposite. )and

Under+rea,ed pile.


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Cased Cast+in+sit' concrete piles

;hen casing is left into the ground, it is easier to examine its verticality but the cost of

the pile increases considerably. The following are the examples of such piles

. :aymond pile

!. *onotube pile

#. )wage pile.

$. *ac %rthur pile

&. obi pneumatic mandrel pile

(. Button'bottom pile

@. B)" base'driven pile

Constr'ction 0oints.6t is not possible to place concrete continuously in large construction works. 6n such cases

formwork for construction joints are provided before each stoppage of concreting 5igs.

@.@ to @./. )uch joints are a potential source of weakness, and should be located and

formed with care. 9enerally, joints are either vertical or hori3ontal. 0ertical joints are

formed against a stop board. 1ori3ontal joints are leveled and, whenever possible, so

arranged that the joint lines coincide with the architectural features of the finished work.

Battens may be nailed to the for in work to ensure a hori3ontal line.

6f possible, the position of construction joints should be settled before concreting starts.

%s a general guide joints in columns are made as near as possible to the beam hunching.

>oints in beams and slabs should be made at the centre, or within the middle third of the

span. 1ori3ontal joints in walls are usually provided in positions such as the top of a

plinth or the top or bottom of a window opening. >ust before concreting is resumed, the

roughened joint surface is thoroughly dined and loose matters are removed. 6t is treated

with a thin layer of cement grout, worked well into the surface, or of cement'sand mortar

in which the ratios of sand to cement and water to cement do not exceed those in the new'

concrete. )pecial care should be taken to obtain thorough compaction and to avoid

segregation of the concrete along the joint plane.


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REIN%"RCE$ CE!ENT C"NCRETE

"lain concrete is very strong in compression but at the same time it is very weak in

tension. 1ence plain concrete cannot be used at places where tensile forces may develop.

)teel is equally strong in compression and tension. But a long steel bar can develop its

full tensile strength whereas it cannot carry equal amount of compressive force due to its

buckling which is caused by the slenderness. % combination of concrete and steel is

ideally suited because the two materials are employed to resist the stresses they are most

suitable. The combination of concrete and steel in construction is known as :einforced

concreteC

Properties o& Rein&orced Concrete

. The concrete develops very good bond with the surface of the steel bars. 1ence, the

concrete transmits the excessive stresses which it cannot resist itself to the steel

reinforcement.

!. %s the co'efficient of linear expansion of concrete and steel are nearly same, internal

stresses do not develop in the reinforced concrete due to temperature variations.


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#. The cement grout protects the reinforcement from corrosion and at the same time it

does not react with the reinforcement.

Advantages o& Rein&orced Concrete

. 6ts monolithic character provides more rigidity to the structure.

!. 6t is durable and fire'resisting. 6t is not attacked by vermins, termites, fungus, or any

other insects.

AC"USTICS

The term acousticsC may be defined as the science of sound, and it describes the origin,

propagation and sensation of sound. 6mprovements in construction during recent years

have resulted in buildings with poorer acoustical properties due to more rigid materials

and construction use= while on the other hand radios and radio broadcasting, sound

motion pictures, the noise in the street due to the automobile, and many other factors

have stressed the improvement in acoustical conditions The various acoustical

considerations which enter into the design of buildings are considered briefly in this

chapter so that the problems involved will be appreciated.

So'nd

% sound is produced when the medium enclosing the body is compressed suddenly.

)ound travels in the form of wave through certain medium. 6t cannot travel in vacuum.

0elocity of )ound

The speed at which the sound waves propagate in a medium is known as sound velocity.

The magnitude of the sound velocity depends upon the nature and temperature of the

propagating medium. The under mentioned points should be noted.

. The sound waves take appreciable time in traveling from one place to the other.

!. The sound velocity is not dependent on the frequency or pitch of sound.

#. The sound velocity is greatly affected by the presence of moisture in the air, the

temperature of the air and the intensity of wind.

$. Table .! illustrates some of the representative sound velocities for various materials.


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Re&lection o& So'nd

)ound waves are reflected in the same manner as that of a light ray. ;hen a sound wave

strikes a plane surface, the angle of incidence is equal to the angle of reflection. %

concave surface tends to concentrate the reflected waves at certain points. Thus a concave

surface may be used to work as a reflector of sound waves. En the other hand, a convex

surface spreads the ref sound waves. )ometimes convex surfaces are employed to spree

the sound throughout the room. The laws of reflection of sound help in deciding the

shape of the room and its surfaces.

Rever/eration

The prolongation of the sound after the source ceases is known as reverberation, a

quantitative measure of reverberation was established by ;. )abine who defined the

reverberation time as the time required for a sound to decrease to one millionth of itsoriginal intensity after stopping the source. The sound decreases as the sound energy is

gradually converted in other form of energy by the interior surface of the room, the

furniture, the curtains, the people present in the room the air of the room and other

contents of the room. This transformation is generally brought about by friction between

the sound wave and air particles as well as the sound wave and the surfaces with which it

comes

A/sorption o& So'nd

;hen a sound wave strikes a surface, the total sound energy is distributed in three ways.

% part of its energy is transmitted across the surface, a part of its energy is absorbed by

friction and the remaining of its energy is reflected back by the surface. :everberation

directly depends on the loss of energy of sound wave due to friction and hence this factor

is of greater significance. This property of a surface which converts sound energy into


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other form of energy is known as absorptionC. %bsorption co'efficient of a surface is the

ratio of energy absorbed

A/sor/ent !aterials

The commonly employed building materials absorb sound to a little extent. 1ence for

providing better acoustical properties, suitable materials are placed on the surface of the

room. )uch materials with better absorbing capacities are known as absorbent materials.

The following factors regarding absorbent materials need proper consideration.

. % good absorbent material should be water'proof, fire'proof, strong, better in

appearance and economical and construction and maintenance.

!. %bsorbent materials should considerably decrease the noise level of the hall. )peeches

should be heard clearly and music should be pleasantly enjoyed.

#. The absorbing capacity depends on the thickness of the absorbent materials, its

porosity density and the frequency of sound.

$. The acoustic properties of the absorbent materials varies by their mode of fixing.

&. The covering destroys the absorbent properties of the material.

(. %udience of a big hail contributes considerably to absorbing factor.

)ound %bsorbing *aterials

0arious types of absorbent materials are used. The value of co efficient absorption %)

supplied by the manufacture. The materials generali3ed may be broadly classified to the

following categories7

a/ 1airfelt,

b/ %coustic plaster a plaster which includes granulated insulation material with cement/,

c/ ompressed cane imperforated and perforate,

d/ ;ood particle board,

e/ ompressed wood wool,

f/ *iner all glass wool quilts and mats,

i/ omposite units of perforated board hard board, asbestos board, or

metal sheet/ backed by mineral or glass woo quilt or slab,

/ )pecial absorbers constructed of bard board, teak ply, etc. backed by air.

Conditions &or Good Aco'stics o& an A'ditori',


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. The initial sound should be amplified to adequate intensity such that it can be heard

throughout the hall.

!. The sound should be evenly distributed over the entire area covered by the audience.

#. 6n the hail used for music and dance, the initial sound should reach tke audience with

the same frequency and intensity.

$. 4ndesirable sound originating from either inside or outside of the hall should be

reduced such that it may not interfere with hearing of music or speech.

General Principles o& A'ditori', $esign

a/ )ite selection and planning. The choice ? site for an auditorium is governed by

several factors which may be mutually conflicting, but a compromise has to be done

between the various considerations involved. The problem of noise is an important con

consideration. % noise survey should be made in advance so that noisy locations are

avoided where possible, as otherwise elaborate and costly construction may be required

to provide requisite sound insulation. 6n fact, the quietest possible condition should be

provided so that 6ntel ligibility of speech does not suffer and even soft passages of music

are heard. 6t is particularly necessary to keep the level of extraneous noise low by proper

orientation and site selection in cases where no air'conditioning is provided and doors

and windows are normally kept open during the performance. ;hen air is provided

special care should be taken to attenuate the plant noise and the grill noise. 5or this

purpose plant should be suitably isolated and ducts as well as the plenum should be so

designed that noise gets adequately reduced so as to be within the permissible limits.

g/ *ineralFcompressed glass wool tiles,

h/ omposite units of perforated hard board backed by perforated fibre board,

$e&ects is an A'ditori', and t*eir Re,edies

6n the design of an auditorium the following three objects should be fulfilled7

. The path of sound should be unobstructed and direct. This makes the sound clear and

loud in hearing.

!. 6t should be checked that none of the subsequent reflections has the same strength as

the original sound.

#. The desired degree of reverberation in the should be achieved.

%olloing are t*e co,,on de&ects &o'nd in ,ost o& t*e a'ditori',s.


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. :everberation.

!. 5ormation of echoes.

#. )ound foci.

$. Dead spots.

&. 8oudness.

(. -xternal noise.

Aco'stics o& St'dios

. The noise level in the studio must be in the range of !? to ? dB.

!. There should be no echo formation.

#. 5or finishing the external surfaces of the building, sound reflecting materials should be

used.

$. 5or finishing the internal surfaces of the studio, sound absorbent materials should be

employed. This reduces the time of reverberation.

&. The partition walls and the external walls should be rigid enough to resist vibrations

and to prevent resonance.

(. The provision of window should be minimum.

@. %ir'conditioning plant should be isolated and properly insulated such that the

vibrations of the plant are not carried inside the studio.

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Site Investigation and Soil Exploration

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