FOUNDATION - Building Materials short notes and previous year questions
Foundations
• IS 1924
• Foundations ( sub structure) is the lower portion the building , usually located below ground level, which transmits the load of the super structure to the sub soil.
TERMS
• Gross pressure intensity (q)
• Net pressure intensity (qn )
• Ultimate bearing capacity (qf)
• Net ultimate bearing capacity (qnf)
• Net safe bearing capacity (qns)
• Safe bearing capacity (qs)
• Allowable bearing capacity (qa)
• ULTIMATE BEARING CAPACITY (QF ) : It is the maximum pressure that a foundation soil can withstand without undergoing shear failure.
• NET ULTIMATE BEARING CAPACITY (QNF) :
• It is the maximum extra pressure (in addition to initial overburden pressure) that a foundation soil can withstand without undergoing shear failure.
• qnf = qf – qo
• Here, qo represents the overburden pressure at foundation level and is equal to үD for level ground without surcharge where ү is the unit weight of soil and D is the depth to foundation bottom from Ground Level.
• SAFE BEARING CAPACITY (QS ) :
• It is the safe extra load the foundation soil is subjected to in addition to initial overburden pressure.
• Ratio of net ultimate bearing capacity to FOS ( 2.5 - 3 )
• ALLOWABLE BEARING PRESSURE (QA ) : It is the maximum pressure the foundation soil is subjected to considering both shear failure and settlement.
• NET ALLOWABLE BEARING CAPACITY : qna = qns + үD
PLATE LOAD TEST
• Bf/Bp = Df/Dp (breadth, depth of plate and foundation )
• Test plate :
• Clayey : 60 cm x 60 cm
• Sandy : 30 cm x 30 cm
• Pit width Bf = 5 x Bp
• Least count of dial gauge : 0.02 mm
• Load values : multiple of 250
• ULTIMATE LOAD : point where further load increment does not result in settlement
• BEARING CAPACITY OF SOIL = ultimate load / area of test plate
• SAFE BEARING CAPACITY = bearing capacity / FOS
SETTLEMENT
• The vertical downward movement of the base of a structure is called settlement
1. NORMAL SETTLEMENT : settlement value same in all direction
2. DIFFERENTIAL SETTLEMENT : settlement value different in different direction
• Permissible differential settlement :
• Clay soil : 40 mm
• Sandy soil : 25 mm
SUBSOIL INVESTIGATION
• Exploration done in soil to determine diff properties of soil
1. OPEN EXCAVATION METHOD
• min depth and width 1.5 m
• Depth measured by bonning rod
2. BORING
• Auger bore is used
• For residential building : central boring, for multistoried : ventral and 4 corners
3. SUBSURFACE SOUNDING : expensive
4. GEOPHYSICAL METHOD : expensive
LOADS OF DESIGNING FOUNDATION
1. DEAD LOAD
2. LIVE LOAD : FURNITURE , PEOPLE
3. WIND LOAD
DESIGN LOAD
Brick masonry 18.8 KN/m3
Stone masonry 20.4 - 26.5 KN/m3
Timber 5 - 8 KN/m3
RCC - 25 KN/m3
PCC - 24 KN/m3
CLASSIFICATION OF FOUNDATION
SHALLOW FOUNDATION
• If breadth is larger than depth
1. WALL / STRIP FOOTING
• Normal footing adopted in building
2. ISOLATED FOOTING
• Provided for isolated columns
• If load transmitted is less pcc is used , if high rcc is used
• Permissible settlement for clay : 75 mm, sand : 50 mm
3. COMBINED FOOTING
• If 2 columns near to each other
• If one column near boundary
1. Rectangular combined : load from both columns are same
2. Trapezoidal combined : load different
4. CANTILEVER / STRAP FOOTING
• When impossible to provide a footing below the Column due to any existing structure
• A strap is provided below the column
5. CONTINUOUS FOOTING
• Used if 2 or more column on same row
6. INVERTED ARCH FOOTING
• Used when bearing capacity of soil is less
7. GRILLAGE FOOTING
• To transfer load from heavy steel or column or heavy equipments
• Maximum depth is 1.5 m
• c/c distance between 2 RSJ = 2 x flange width
8. RAFT OR MAT FOUNDATION
• Most economical and safe in case of multi storied building
• when area of footing required is more than 50 % of plot area
• Resists differential settlement
• Max settlement
• Clay : 100 mm
• Sand : 75 m
9. STEPPED FOOTING
• Constructed for architectural buildings in which steps or footing is provided along the slope of land
• Min lap of 2 steps : 80 cm
DEEP FOUNDATION
PILE FOUNDATION
• most economical and safe in water logged areas
• Classification based on load carrying capacity
1. LOAD BEARING PILE
2. NON LOAD BEARING PILE
• Classification based on load transmission
1. END BEARING PILE
2. FRICTION PILE
• Min no of non-load bearing pile to support a wall : 3
• Min c/c distance between two friction piles : 3 - 4 m
CASED PILES
1. RAYMOND PILES
• Inserted with help of steel mandarin
• Top diameter : 40 - 60 cm
• Bottom diameter : 20 - 28 cm
2. MAC AUTHOR PILE
• Inserted with help of steel core
• Bottom enlargement takes place
UNCASED PILES
1. SIMPLEX PILE
• Caste iron shoe is present at the bottom of pile
• Rate of bulging is less
• Used as support pile to increase bearing capacity of main pile
2. FRANKIE PILE
• Compacted by dropping hammer
• Bottom enlargement takes place
• Permissible bulb diameter 50 - 60 cm
3. VIBRO PILE
• Caste iron shoe is provided
• Compacted with vibrating device
• 80 blows / min
4. PEDESTAL PILE
• compacted with help of steel core
5. PRESSURE PILE
• diameter : 40 cm
• Auger bore is used to drill hole
• Concrete us filed under pressure
6. UNDER REAMED PILES
• Developed at IIT Rourke under CBRI
• Bulb is located in the stem
• For lighter load transmission single bulb is used
• For heavy load transmission double bulb is used
• Dia of bulb : 2.5 d
• Spacing between bulb : 1.5 d
• Improves bearing capacity of soil by 50 %
SPECIAL PILES
1. BATTER PILES
• Placed in an inclined position
• Carries vertical and horizontal load at the same time
2. SHEET PILES
• Used to block soil flow from bottom of water storing structure
3. FENDER PILES
• Used to protect the coastal area
• Dolphins : Dolphins are also used to
protect structures from
possible impact by ships
4. COMPACTION PILES
• Used to improve bearing capacity of soil
WELL FOUNDATION
• Used to transfer load below water level
• Used in tajmahal
• Circular shape is used due to low surface area
1. CAISSONS
• Initially acts as a dewatering structure later becomes a load bearing part
i. Box caissons : top open , bottom closed
ii. Pneumatic caisson : top closed , bottom open
iii. Well caissons : top and bottom open
• Single Well caissons• Double Well caissons• Open cylinder
COFFER DAMS
• Temporary dewatering structure
• Not suitable for large area
• Upto a depth of 20 - 30 m
TYPES
1. Dikes :
• commonly used
• Earth fill, stone filled, sand bag
2. Single wall : single row of pile used
3. Double wall
4. Cellular : most efficient
5. Rock fill crib : alternate layers of timber and rock used
6. Concrete
7. Suspended : recyclable