SHEER FAILURE IN A STRIP FOUNDATION

-IF THE THICKNESS OF A CONCRETE STRIP FOUNDATION (WITHOUT STEEL REINFORCEMENT ) WERE LESS THAN ITS PROJECTION EACH SIDE OF A WALL , THE CONCRETE MIGHT FAIL THROUGH THE WEIGHT OF THE WALL CAUSING A 45 DEGREE ANGLE SHEAR CRACK.
-IF THIS OCCURRED , THE BEARING SURFACE OF THE FOUNDATION ON THE GROUND WOULD BE REDUCED TO LESS THAN THAT NECESSARY FOR STABILITY .
-WHERE THE LOAD IS PLACE DIRECTLY ON TOP OF THE CONCRETE FOUNDATION AND COMPRESSED BETWEEN THE GROUND , THE FOUNDATION REMAINS STABLE . HOWEVER , THE CONCRETE OUTSIDE THE 45 DEGREE ANGLE EXPERIENCE SOME TENSION AS THE REACTION OF THE GROUND ATTEMPS TO LEVER THE FOUNDATIONS UPWARDS , THUS THE FOUNDATION WOULD FAIL .

IF THE LENGTH OF THE SIDE OF STRIP FOUNDATION IS GREATER THAN THE DEPTH OF THE REINFORCEMENT CONCRETE , THE FOUNDATION WILL BE FAIL WHERE TENSION IS EXERTED ON THE CONCRETE WILL CAUSE THE 45' DEGREE OF SHEAR , FOLLOWING THE SHEAR FAILURE , THE LOAD IS CONCRENTRATED ON A SMALLER AREA , THE GROUND MAY CONSOLIDATE UNDER THE INCREASED LOAD

DIMESNSION OF A STRIP FOUNDATION

-THE STRIO FOUNDATION MUST AT LEAST 150MM THICK
-THE WIDTH MUST AT LEAST 450MM LONG
-THE HIGH OF FOUNDATION ALSO REQUIRE MINIMUM 450 MM
-THE WALL SHOULD BE CENTRAL OF THE FOUNDATION

sTRIP FOUNDATION CAN BE CONSIDERED UNDER 3 CATEGORIES ;

TRADITIONAL STRIP FOUNDATION

1. THE WIDTH OF A CONCRETE STRIP FOUNDATION DEPENDS ON THE BEARING CAPACITY OF THE SUBSOIL AND THE LOAD ON THE FOUNDATION.
2. THE GREATER THE BEARING CAPACITY OF THE SUBSOIL , THE LESS THE WIDTH OF THE FOUNDATION AND VICE VERSA.
3. THE CONCRETE STRIP WILL GENERALLY BE WIDER THAN THE THICKNESS OF THE WALL

• FOR THE CONVENIENCE OF FILINGS A WIDER TRENCH WITH CONCRETE DUE TO THE WIDTH OF THE EXCAVATION BUCKET;
• FOR THE CONVENIENCE OF LAYING BRICK BELOW GROUND , ALLOWINGS ADEQUATE WORKING SPACE.

4.THE MINIMUM WIDTH OF THE STRIP FOUNDATION IS 450 MM WHICH GIVES A RESONABLEBEARING AREA FOR MOST TWO STOREY HOUSES ON MOST SUBSOIL.

5.THE PRACTICAL MINIMUM DEPTH OF A STRIP FOUNDATION IS USUALLY 450MM TO ALLOW FOR THE REMOVAL OF TOP SOIL AND VARIATIONS IN GROUND LEVEL .

6.THE MINIMUM THICKNESS OF CONCRETE FOR A STRIP FOUNDATION IS GENERALLY 150MM AND THE CONCRETE SHOULD BE AT LEAST AS THICK AS THE PROKECTION OF THE STRIP EACH SIDE OD THE WALL WHERE THE CONCRETE IS NOT REINFOCED .

sTRIP FOUNDATIONS

1. CONSIST OF A CONTINUOUS STRIP OF MASS OR REINFORCED CONCRETE , FORMED CENTRALLY UNDER LOADBEARING WALLS .
2. USED TO TRANSFER LONG CONTINUOUS LOADS SUCH AS WALLS.
3. THE CONTINUOUS STRIP SERVES AS A LEVEL BASE ON WHICH THE WALL IS BUILT.
4. THE MOST ECONOMIC FORM OF FOUNDATIONS FOR SMALL BUILDINGS ON COMPACT SOILS

REINFORCED CONCRETE FOUNDATION

REINFORCEMENT CONCRETE FOUNDATION IS ABOUT 50 TO 75 MM THICK BLINDING LAYER OF WEAK CONCRETE TO PROVIDE ACCURATE STARTING LEVEL

STEEL REINFORCEMENT IS PUT IN THE LOWER OF REINFORCEMENT CONCRETE (ABOUT 50MM FROM THE GROUND )

Mass concrete foundations

-inposed loads induces tensile stresese in excess of concrete own resistance
(FOUNDATION TENDS TO BEND , THE UPPER FIBRES BEING COMPRESSED AND THE LOWER FIBRES BEING STRETCHED AND PUT IN TENSION-REMEDIES INCREASE SIZE OF BASE OR DESIGN AS A REINFORCED CONCRETE FOUNDATION)

AFTER THE SIZE OF FOUNDATION INCREASE , IT TO PROVIDE THE RESISTANCE AGAINST THE INDUCED TENSILE STRESSES -GENERALLY NOT ECONOMIC DUE TO THE EXTRA EXCAVATION AND MATERIAL REQUIRED

Design & Construction of Reinforced Concrete Foundations

1. Foundations are usually made of either mass or reinforced concrete
2. where a structure has olny light loadings , such as a domestic building , it is sufficiet to use a mass concrete foundation
3. Where building is either heavy , or transmit the loadings at a series of points , such as in a framed buildings , it is eneconomic to use mass concrete foundation ..
4. The thickness of a mass concrete foundation for a heavy point load would result in a foundation that is costly and adds unnecessary load to the subsoil
5. Reinforced concrete foundations are generally cheaper and easier to construct than mass concrete foundations but will require a structural engineer to formulate an economic design .
6. The engineer must define the areas in which tension occurs and specify the reinforcement required , as concrete is a material which is weak in tension .
7. Concrete is a mixture of cement , sand , aggregates and water in controlled proportions
8. It is the most suitable material to construct foundation as it is a durable material of edequate strength
9. But concrete is a material which is strong in compression but weak in tension .
10. If its tensile strength is exceeded , cracks will occur and resulting in a weak and unsuitable foundations .
11. One method of providing tensile resistance is to include steel bars as a form of reinforcement in the concrete foundation to resist all the tensile forces induced into the foundation .
12. Steel is amaterial which is readily available and has high tensile strength .

groundwork & FOUNDATION

1. design & construction of reinforced concrete FOundations
2. Strip Foundations
3. Raft Foundations
4. Pad Foundations
5. Pile Foundations
6. Discussion : Blinding for RC Foundations

BUILDING CONSTRUCTION

BRIGHTLY COLOURED PLASTIC TAPE TIED TO METAL STAKES AT 2M MAX , SPACING.
(PROPRIETARY INTERLOCKING RAIL AND POST SYSTEM IS PREFERRED )

PLASTICS HAZARD CONES SECURED BY STAKES

CLOSE BOARDING , SHEETING OR RUNNERS LEFT PROKECTING ABOVE GROUND LEVEL

CONTINUOUS RAIL FIXED TO THE TOP OF PROJECTING POLING BOARDS

CONTINUOUS TUBULAR RAIL

TRIPODS OF SHORT SCAFFOLD TUBES AT 1.8 C/C

SAFETY MEASURES FOR EXCAVATON & TIMBERING

ASSUMING YOU ARE THE SIRE SUPERVISOR OF A CONSTRUCTION SITE , THE CONSTRUCTION MANAGER HAS ASSIGNED YOU TO BRIED THE SITE WORKERS ON THE SAFETY MEASURES FOR EXCAVATION AND TIMBERING:

1. PERSONAL PROTECTIVE EQUIPMENT LIKE SAFETY BOOT , SAFETY HELMET
2. COVER EXCAVATION & TUMBERING WORK
3. CONTRACTOR PROVIDE TO ALL THE WORKERS
4. WHEN HEAVY RAIN THE WORKERS HAVE TO STOP THE WORK
5. ANY ACCIDENT ON THE SIDE OF EXCAVATION MUST REPORTED TO THE SIDE SUPERVISOR
6. AVOID PUTTING THE MATERIAL NEAR TO THE EXCAVATION AREAS

METHOD AFFECTING THE EXCAVATION
-TYPES OF SOIL
-EXCAVATION COST
-WHEATHER CONDITION OF A SIDE
-SURROUNDING AREA
-LOCATION OF A SIDE

FACTORS AFFECTING METHOD OF EXCAVATION & TIMBERING

THE METHOD OF EXCAVATION AND TIMBERING TO BE USED IN ANY PARTICULAR CASE WILL DEPEND UPON A NUMBER OF FACTORS . LIST AT LEAST FIVE OF THESE FACTORS

WITH THE AID OF EXAMPLES , DESCRIBES HOW THESE FACTORS AFFECT THE METHOD OF EXCAVATION AND TIMBERING IN A CONSTRUCTION PROJECT.

Raking struts from formation level

-Face timbering left projecting as safety barrier
-150x150 raking struts at 2.000 c/c
-150x150 puncheons t 2.000 c/c
-150x150 wallings
-225x38 binders to both sides
-225x75 sole piece bolted to slab
-rc basement slab
-225x75 sole pieces

complete excavation with timbered sides

-This method can be used in firm subsoils.
-The centre of the proposed basement can be excavated first to enable the basement slab to be cast thus giving protection to the subsoil at formation level.
-sometimes called complete excavation

ground level
-face timbering left projecting as safety barrier
-150x150 raking struts at 2.000 c/c
-150x150 puncheons at 2.000 c/c
-150x 150 walings
-225x38 binders to both sides
-150x75 sole piece
-formation level
-225x75 sole piece
-firm subsoil

Complete excavation with sloping sides

-Excavation for a basement on an open site can be carried out by cutting the perimeter back to the natural angle of repose of the soil .
-This method required sufficent site space around the intended structure for the over-excavation.
-No timbering is required
-In economic terms , the coasts of plant and manpower to cover the extra excavation , backfilling and consolidating must be offset by the savings made by omitting the temporary support works to the sides of the excavation.
-sometimes called open excavation


battered side excavation to safe angle repose

extra perimeter space required-restricts site space available for circulation , storage etc.

perimeter wedge of earth to be removed and after completon of the basement construction backfilled and consolidated

basement Excavation & Timbering

There are 3 methods that can be used fot excavating a large pit or basement ;- or basement :-
-complete excavation with sloping sides
-complete excavation with timbered sides ;
-Perimeter trench excavation

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In medium-depth trenches, diffferent soil conditions are very often encountered throughout the depth of the excavation , and therefore the method of timbering must be changed to suit the new soil conditions.

Hand trimming is usually required in the trench bottom to form an accurate line and level ; this process is called bottoming of trenches.

Approximately 150mm should be allowed for trimming by hand , and it is advisable to cover the trimmed surface with hardcore to protect the soil at formation level from being disturbed or drying out and shrinking .

trench Excavation & Timbering

LOng, narrow trenches in firm soil may be excavated to the full depth by mechanism excavators , enabling the support timbering to be placed in one continuous operation .

weak and waterlogged ground must be supported before excavation commences by driving timber runners ot steel trench sheeting to a position below the formation level , or by a drive and dig procedure .

In the drive and dig procedure , the runners can be driven to a reasonable depth of approximately 1.5 m followed by an excavation cut of 1.2 m and then the operation repeated until the required level has been reached ; this will make the driving of the runners easier and enable a smeller drving appliance to be used .

struts , poling boards & sheeting

If the sides of the excavation are completely covered with timbering , it is know as close timbering whereas any form of partial covering is called open timbering.

Whichever system of timbering is used , there should be as few struts , that is horizontol members , fixed accross the width of the trench as possible as these obstructs ease of working in the trench.

structs should be cut to fit tightly between poling or walling biards and secured in position so that they are not easily knocked out of place

the type and amount of timbering required will depend upon the depth and anture of the subsoil . Over a short period , many soil may not require any timbering , but wheather condition and duration of the operations must all be taken into account .

Types of Timbering

Timbering is the temporary support to the sides of excavations.
sometimes called planking and strutting.

The purpose of temporary timbering support to sides of excavations is :-
-To uphold the sides of excavation to avoid collapse of the sides
-To protect the operations while in the excavations
-To avoid the wasteful labour of constantly clearing falling earth from the excation bottoms

The material most used for temporary support for the sides of excavations for foundations is rough sawn timber . The timbers used are square section struts , across the width of the trench , supporting open poling voards , close poling voards and walings or poling boards and sheeting .

trench & pit excavation

-diesel power unit on crawler tracks (backactors)
face shovels -hydraulic rams

backactors scope the soil forward
-these machinese are available as cable rigged or hydraulic excavators
-suitable for trench and bulk excavating
-cable rigged backactors are usually available with larger bucket sizes and deeper digging capactities than the hydraulic machines
-but hydraulic machines have a more positive control and digging operation and are also easier to operate.

Face shovels
-These machines are designed to excavate above their own wheel or track evel and are suitable for bulk excavation work
-In basement work , they will require a ramp approach unless they are to be lifted out of the excavation area by means of a crane
-Face shovels are available as cable rigged or hydraulic machines

reduce level excavation

-bulldozer for cut and fill operations
-mechanical shovel and attendant lorries for cut olny operations

Site clearance & removal of topsoil

-top soil-upper level of earth usually not exceeding 300mm deep
-excavation mechanical shovel or bulldozer
roll bar
-dumper to move spoil

mechanical shovel
-take off layers of soil in buckets
-It can be raised , lowered or tilted to deposit the earth into a dumper and removed elsewhere on site .

Bulldozer
-Push the soil layer by layer to one side and pile it up nearby
-It does not dig out or lift out the soil

Dumper
-This truck can perform many task on a buildings site , including the movement of soil over short distances .
-The body of the dumper dips forward and deposits the soil in the required position.

plants & equipment for Excavation

On small contracts such as house extensions , excavation is effectivelu carried out by hand , but on large works it will be more economic to use some form of mechanical excavator .

The choice of actual pieces of plant to be used in any construction activity is a complex matter taking into account many factors such as volume of soil involved , nature of the site and time elements .

At this stage , it is olny necessary to consired basic types for particular operations:

PIT EXCAVATION

-ISOLATED PITS PRIMARILY USED FOR COLOUMS OR PIERS ON PAD FOUNDATIONS

1. SIDES OF EXCAVATION CAN BE BATTERED OR STRAIGHT DEEP PITS MAY HAVE TO BE OVER EXCAVATED IN PLAN TO PROVIDE GOOD ACCESS TO AND GOOD DEGREE FROM THE WORKING AREA FOR BOTH MEN AND MATERIALS

TRENCH EXCAVATION

BATTERED FACES
-WIDTH REQUIRED
-NATURAL ANGLE OF REPOSE OR REST

VERTICAL OR STRAIGHT FACES

TRENCH EXCAVATION

-NARROW EXCAVATION PRIMARLY FOR STRIP FOUNDATIONS AND BURIED SERVICES.
-EXCAVATIONS CAN BE CARRIED OUT BY HAND OR MACHINE.
-SIDES OF EXCAVATION CAN BE BATTERED OR STRAIGHT.

DISAVANTAGE-EXTRA COST OF OVER EXCAVATING AND EXTRA BACKFILLING
ADVANTAGES-NO TEMPORARY SUPPORT REQUIRED TO SIDES OF EXCAVATION.

DISAVANTAGE-SIDES OF EXCAVATION MAY REQUIRE SOME DEGREE OF TEMPORARY SUPPORT.
ADVANTAGE-MINIMUM AMOUNT OF SOIL REMOVED AND THEREFORE MINIMUM AMOUNT OF BACKFILLING .

REDUCE LEVEL EXCAVATION

-CARRIED OUT BELOW OVERSITE LEVEL TO FORM A LEVEL SURFACE ON WHICH TO BUILD.
-CAN CONSIST OF BOTH CUTTING AND FILING OPERATIONS.
-THE LEVEL TO WHICH THE GROUND IS REDUCED IS CALLED THE FORMATION LEVEL.

OVERSITE EXCAVATION

-THE REMOVAL OF TOPSOIL

DEPTH VARIES FROM SITE TO SITE BUT IS USUALLLY IN A 150 TO 300MM RANGE . TOP SOIL CONTAINS PLANT LIFE ANIMAL AND DECAYING MATTER WHICH MAKES THE SOIL COMPRESSUBLE AND THEREFORE UNSUITABLE FOR SUPPORTING BUILDINGS.

groundwork & FOUNDATION

EXCAVATION & TIMBERING
BEFORE A FOUNDATION CAN BE LAID , IT IS NECESSARY TO EXCAVATE A TRENCH OR PIT OF THE REQUIRED DEPTH AND WIDTH .

IF THE TRENCHES OR PITS ARE OF SIGNIFICANT DEPTH , IT MAY BE NECESSARY TO FIX TEMPORARY TMBER SUPPORT TO STOP THE SIDES OF EXCAVATION FROM FALLING IN .


TYPES OF EXCAVATION
EXCAVATION IS CARRIED OUT TO REMOVE EARTH TO FORM A CAVITY IN THE GROUND.

EXCAVATIONS MAY BE CLASSIFIED AS SHALLOW , MEDIUM OR DEEP AS FOLLOWS;-
-SHALLOW-UP TO 1.500M DEEP ;
-MEDIUM-1.500-3.00M DEEP;
-DEEP -OVER 3.00M DEEP

TYPES OF EXCAVATION;
-OVERSITE EXCAVATION
-REDUCE LEVEL EXCAVATION
-TRENCH EXCAVATION
-PIT EXCAVATION