Foundations and underpinning

1. HISTORY
• Up to the latter part of the 19th century, when Portland cement first came into general use for making
concrete, the majority of the buildings were built off the ground.
• Walls of stone or brick were built on a bed of rough stones or brick footings as illustrated below.

2. HISTORY
• Local experiences of the behaviour of soil and rocks under the load buildings gave sufficient
information to choose a foundation of required depth and spread of this method of construction.
• From the beginning of the 20th century concrete was increasingly used as a foundation base
• However, initially it was used as a base for laying stones . This massive and unnecessary form of
construction was accepted practice for some years.
• The move from the practical, common sense approach of the 19th century to the closely regulated
system resulted in some foundations so massive as to exceed the anticipated combined loads onto
the foundations.
• The tendency to over design has been exacerbated by the willingness of the building owners to seek
compensation for damage, caused by claimed negligence of architects, engineers and builders who in
order to avoid paying for such claims tended to over design.

3. Definition
A foundation is that part of a building structure which is in direct
contact with the ground to which the weight of the structure and other
loads of the structure are transmitted. The soil or rock on which the
base directly rests is called a natural foundation while the wall or
column base itself is called an artificial foundation.

4. Functions of Foundations
Following are the main functions of foundations :
1.To transmit and distribute the total load of the structure to a larger area of
underlying support.
2.To prevent differential settlement of the structure.
3.To provide stability to the structure.

5. STRIP FOUNDATION
The commonest type adopted on one or two storey buildings on firm, non-
shrinkage sub‐soils such as gravel. Normally consists of a continuous strip of
concrete which provides a continuous ground bearing for load‐bearing walls.

6. TRADITIONAL STRIP FOUNDATION
The wall footing is a continuous slab strip along the length of the wall.
Stone, brick, reinforced concrete etc. is used for the construction of wall
foundations.

7. WIDE STRIP FOUNDATION
• Wide strip opted for where the soil has a low bearing capacity such
as in marshy, soft
• Clay soils and filled areas.

8. DEEP STRIP FOOTING
This type of strip footing to be used when depth of more than 900mm
has to be reached in clayey or marshy soils.

9. SHALLOW FOUNDATIONS
Shallow foundations are also called spread footings or open footings. The 'open' refers to the fact that the foundations
are made by first excavating all the earth till the bottom of the footing, and then constructing the footing.
RAFT OR MAT FOUNDATIONS
These are large concrete slab laid on the ground to support columns and walls
The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure
compared to the traditionally used strip or trench footings.
Raft Foundations, also called Mat Foundations, are most often used when basements are to be constructed
Mat Foundations are also used where the soil is week, and therefore building loads have to be spread over a large
area, or where columns are closely spaced, which means that if individual footings were used, they would touch each
other.
They are usually used where the strata is unstable with risky of the building sinking
They are also suitable where the are movements of strata for example in mining areas where dynamites are normally
used

10. How raft foundation works
• A raft foundation spread the weight of the building on
the whole ground floor area
• These foundations are placed on the hard rock or firm
strata to ensure minimum movement
• They usually have thick edges for stability
• The foundation may be stiffened by inserting ribs and
beams for extra strength and rigidity

11. Where raft foundations is used
• Heavy buildings such as commercial buildings with large
loads
• Low bearing capacity soils to spread the loads and
improve stability
• When the ratio of footings to the total floor area is high
• When footing have a risk of overlapping due to
closeness of walls to each other

12. Advantages of raft foundation
• The raft can act as a foundation and slab at the same
time saving material and time
• Less excavation is required
• Reduces differential settlement an hence less risky of
cracking
Disadvantage
• Unsuitable for point loads

13. Raft foundation on multistory Cont….
• The ground is excavated up to the firm strata or hard
core for stability
• Reinforcements are laid on the excavated area and
concrete is poured
• Ground beams can be introduced

14. Raft foundations on multistory buildings

15. Materials Used for Raft Foundation Construction
Here are some materials used for the erection of Raft foundation-
• Formwork
• Spacer
• Reinforcement
• Concrete.
Construction Process of Raft foundation
Key steps of Raft foundation construction are noted below.
1. Identify the desired depth at which foundation is to be provided.
2. Excavate soil up to the required depth.
3. Compact the soil.
4. Provide a waterproofing membrane.
5. Pour 3” of plane cement-sand paste.
6. Lay reinforcement maintaining the required spacing using spacers.
7. Pour concrete to the desired depth.
8. Curing.

16. 1Flat Plate Mat
• This is the simplest form of raft foundation. This type of mat is used
when the columns and walls are uniformly spaced at small intervals
and the subjected loads are relatively small. Reinforcement is placed
in both directions and more reinforcement is required at the column
locations and load-bearing walls. The thickness of this types of raft
foundation is generally restricted within 300mm for economic reason.
A thicker slab would not be economical.

17. Piled Raft
• This type of raft foundation is supported on piles. A piled raft is used
when the soil at a shallow depth is highly compressible and the water
table is high. Piles under raft help in reducing settlement and provides
resistance against buoyancy.

18. Rigid Frame Mat/Cellular Raft Foundation
• In this type of raft, the foundation walls act as a deep beam. Rigid
frame mat is referred when columns carry extremely heavy loads and
the connecting beams exceeds 90cm depth. Here two concrete slabs
are placed, one on top of another and connected with foundation
walls in both directions and thus forms a cellular raft foundation. This
type of raft is very rigid and is economical when the required slab
thickness is very high.

19. • Group 1
Building construction

20. Raft foundations
• These are large concrete slab laid on the ground to
support columns and walls
• The slab is spread out under the entire building or at
least a large part of it which lowers the contact pressure
compared to the traditionally used strip or trench
footings.

21. Raft foundations

22. Raft foundation cont..
• They are usually used where the strata is unstable with
risky of the building sinking
• They are also suitable where the are movements of
strata for example in mining areas where dynamites are
normally used

23. How raft foundation works
• A raft foundation spread the weight of the building on
the whole ground floor area
• These foundations are placed on the hard rock or firm
strata to ensure minimum movement
• They usually have thick edges for stability
• The foundation may be stiffened by inserting ribs and
beams for extra strength and rigidity

24. Where raft foundations is used
• Heavy buildings such as commercial buildings with large
loads
• Low bearing capacity soils to spread the loads and
improve stability
• When the ratio of footings to the total floor area is high
• When footing have a risk of overlapping due to
closeness of walls to each other

25. Advantages of raft foundation
• The raft can act as a foundation and slab at the same
time saving material and time
• Less excavation is required
• Reduces differential settlement an hence less risky of
cracking

26. Disadvantage
• Unsuitable for point loads

27. Raft foundations on multistory buildings

28. Raft foundation on multistory Cont….
• The ground is excavated up to the firm strata or hard
core for stability
• Reinforcements are laid on the excavated area and
concrete is poured
• Ground beams can be introduced

29. Pile foundations

30. Piled Foundation
Classification of Piles ~ can be by basic design function or by their method of
construction :-

31. DEFINITION
• Pile foundation is kind of deep foundation, is actually a slender column or long
cylinder made of materials such as concrete or steel which are used to support
the structure and transfer the load at desired depth either by end bearing or skin
friction
• A foundation is described as 'piled' when its depth is more than three times its
breadth.

32. WHEN TO USE
•Heavy and un-uniform loads from superstructure are imposed.
•When the groundwater table is high.
•Other types of foundations are costlier or not feasible.
•When the soil at shallow depth is compressible.
•When there is the possibility of scouring, due to its location near the river bed or
seashore, etc.
•When there is a canal or deep drainage systems near the structure.
• When soil excavation is not possible up to the desired depth due to poor soil condition.
• When it becomes impossible to keep the foundation trenches dry by pumping or by any
other measure due to heavy inflow of seepage.

33. PILE FOUNDATIONS

34. CLASSIFICATION OF PILES

35. FACTORS AFFECTING SELECTION
• Type and loads from the superstructure.
• Properties of soil.
• The depth of the soil layer capable of supporting the piles.
• Variations in length of pile required.
• Availability of materials.
• Durability required.
• Available equipment for pile driving.
• Budget.
• The depth of water level and intensity of underground water flow.
• Types of surrounding structures.

36. CAUSES OF PILE FOUNDATION FAILURE
1.Load implied on the pile is greater than designed load.
2.Defecting workmanship.
3.Dislocation of reinforcement of pile.
4.End bearing pile resting on soft strata.
5.Faulty soil investigation.
6.Selecting the wrong type of pile.
7.Under-reinforcement of the pile.
8.A decay of piles. (like attack of insects, corrosion etc.)
9.Deformation of piles due to lateral loads.
10.Incorrect assessment of pile capacity.
11.Not considering lateral forces for designing of piles.

37. Pile foundations

38. DEFINITION
• Pile foundation is kind of deep foundation, is actually a slender column or long
cylinder made of materials such as concrete or steel which are used to support
the structure and transfer the load at desired depth either by end bearing or skin
friction
• A foundation is described as 'piled' when its depth is more than three times its
breadth.

39. WHEN TO USE
•Heavy and un-uniform loads from superstructure are imposed.
•When the groundwater table is high.
•Other types of foundations are costlier or not feasible.
•When the soil at shallow depth is compressible.
•When there is the possibility of scouring, due to its location near the river bed or
seashore, etc.
•When there is a canal or deep drainage systems near the structure.
• When soil excavation is not possible up to the desired depth due to poor soil
condition.
• When it becomes impossible to keep the foundation trenches dry by
pumping or by any other measure due to heavy inflow of seepage.

40. PILE FOUNDATIONS

41. CLASSIFICATION OF PILES

42. Piled Foundation
Classification of Piles ~ can be by basic design function or by their method of
construction :-

43. Pile Installation Methods
1.Boring methods (non-displacement piles
Continuous flight auger (CFA): If
boring and pouring takes place
simultaneously during pile driving
Negative skin friction

44. Pile Installation
Methods
2.Pile driving methods
• Pile driving using hammer
• Dropping weight: A hammer with approximately the
weight of the pile is raised to a suitable height and is
released to strike the pile head
• Vibration: The amplitude of vibration used here should
be sufficient to break the skin friction on sides of pile.
This is bested suited for sandy or gravelly soils.
• Jacking (only for micro piles): To install jacking piles,
Hydraulic rams are used to push piles into ground
• Jetting: To aid the penetration of piles in to sand or sand
gravel, water jetting can be employed.

45. DIAMETER
• Pile diameter: 600 to 3500 mm depending on soil diameter and
consistency. Pile depth: 60 to 100 m depending on soil diameter and consistency.
• 40 meters deep , in soft ground or in aggressive or contaminated soils.

46. FACTORS AFFECTING SELECTION
• Type and loads from the superstructure.
• Properties of soil.
• The depth of the soil layer capable of supporting the piles.
• Variations in length of pile required.
• Availability of materials.
• Durability required.
• Available equipment for pile driving.
• Budget.
• The depth of water level and intensity of underground water flow.
• Types of surrounding structures.

47. CAUSES OF PILE FOUNDATION FAILURE

48. CAUSES OF PILE FOUNDATION FAILURE
1.Load implied on the pile is greater than designed load.
2.Defecting workmanship.
3.Dislocation of reinforcement of pile.
4.End bearing pile resting on soft strata.
5.Faulty soil investigation.
6.Selecting the wrong type of pile.
7.Under-reinforcement of the pile.
8.A decay of piles. (like attack of insects, corrosion etc.)
9.Deformation of piles due to lateral loads.
10.Incorrect assessment of pile capacity.
11.Not considering lateral forces for designing of piles.

49. UNDER PINNING
• This is a process of strengthening foundations of existing structures.
• The abovementioned process can be used to repair faulty
foundations.
• Also if one desires to increase the storeys of a structure.

50. REASONS FOR UNDER PINNING
• 1. Uneven Settlement - this could be caused by uneven loading of the
building, unequal resistance of the soil action of tree roots or
cohesive soil settlement.
• Increase in Loading -this could be due to the addition of an extra
storey or an increase in imposed loadings such as that which may
occur with a change of use
• Lowering of Adjacent Ground -usually required when constructing a
basement adjacent to existing foundations

51. TYPES OF UNDER PINNING
• Pile method.
• Pier and beam method.
• Cantilever needle beam method.

52. PILE METHOD
• This method is effectively applicable in areas with clay soils and in
water logged areas.
• This method greatly relieves the load previously imposed on the
existing foundation.
• The maximum depth possible is 15m
• A needle penetrates through the wall that is in turn connected to the
piles as illustrated below.

53. PILE METHOD

54. PILE METHOD

55. PROCEDURE
• Piles are driven at regular intervals along both side of wall
• The piles are then connected by concrete or steel needle which
penetrates through the existing wall.

56. ADVANTAGES
• Suitable for sites with restricted access.
• It is faster than the traditional underpinning.
• It can support great loads.

57. DISADVANTAGES.
• The piles should be properly reinforced to withstand handling stresses
during transit.
• Foundations of the adjacent structures can be affected by the
vibrations generated during the process of driving the piles into the
ground.

58. PIER AND BEAM METHOD.
• It is also termed as a base and beam method.
• It came as a successor of mass concrete method which had
limitations.
• RC beams are placed to transfer the load to the base or piers.
• The size and depth of beams is based on the soil’s bearing capacity.
• It is a thrift method for depths shallower than 6m.

59. PIER AND BEAM METHOD.

60. PIER AND BEAM METHOD.

61. PIER AND BEAM METHOD.

62. CANTILEVER NEEDLE BEAM METHOD
• This method is used to repair existing wall foundations either internal
or external.

63. PROCEDURE.
• Construction of piles

64. PROCEDURE.
• Excavation

65. PROCEDURE.
• Backfilling

66. CANTILEVER NEEDLE BEAM METHOD

67. ADVANTAGES
• Access from one side
• Occupants can use the property during construction
• Cantilever beam can be at a higher level if the existing foundation is
too deep
• Suitable for restricted areas

68. DISADVANTAGES
• There is need for specialised equipment
• It requires skilled personnel to be done

69. References
• Building Construction Illustrated 4th edition F. Ching
• Construction Technology by Chudley and Greeno fourth Edition(2)