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