CHAPTER 2 SUBSTRUCTURE CHAPTER 2 SUBSTRUCTURE

2.1 Introduction2.2 Building foundation - types and functions2.3 Deep Foundation

spun pile, micro pile, bakau pile, bore pile and pile cap

2.4 Shallow Foundationpad footing, raft foundation, strip foundation

2.5 Column, stump, ground beam

What is Substructure?

• The lowest portion of the building structure.

• Usually located below the ground level.

• A foundation is a part of the structure which is in direct contact with the ground to which the loads are transmitted.

Main functions of the foundations

• To distribute the load of the structure over a larger area.

• To transmit the load uniformly under the structure. structure.

• To provide a firm, level and strong base over which the superstructure may be constructed.

Main functions of the foundations

• To avoid any settlement or other movement that can cause damage to any part of the building (a stable foundation should bear the loads without sinking or should bear the loads without sinking or settling more than an inch at the most).

• To increase the stability of the structure by preventing its tilting or overturning against winds, earthquakes and uneven distribution of live load (Lateral Stability).

Load Distribution

Ground Level

Load

45o

Type of foundation is selected based on

• Loading of the building, big load need big foundation such as raft foundation or piling

• Types of soil such as peat soil prefer piling or deep foundation or deep foundation

• Economical / financial for number of building or story – (pad footing or pilling)

Type of foundation is selected based on

• The loads that must be transferred from the structure to the soil strata supporting it. This also should evaluate the ability of the soil to support the ultimate loads.soil to support the ultimate loads.

• The capability of the structure that will safely transfer the loads from the superstructure to the foundation bed.

Type of foundation is selected based on

• The possibility and extent of settlement of the soil due to the presence of mines and quarries in the vicinity.

• The ability for engineers to fix the depth of • The ability for engineers to fix the depth of the foundation.

• The ability to determine if the underground water has sulfates or other salts that can degrade the foundation materials.

Factors That Need To Be Considered in the Foundation Design

• For more safety precaution use factor of safety FOS 3

• Increase number of bore hole or suffient number of borehole so that the result of number of borehole so that the result of the report is more accurate.– Choose the critical point load for borehole– Every end of the building– Supervise the S.I properly make sure no

mistake

Factors That Need To Be Considered in the Foundation Design

• For the safety of the foundation design use the lowest of bearing capacity value

• The engineer must have good enough data for the S.I such as previous soil data for the S.I such as previous soil report, cutting or filling area.

• Engineer also must make sure the original ground level and purposed level or formation level while designing the foundation.

Factors That Need To Be Considered in the Foundation Design

• The correct parameter is important to prevent from foundation failure that may occur causing building collapse. It will cause a big loss of material and even cause a big loss of material and even peoples life.

Highland Tower Collapse

Types of Foundation

Shallow foundation :-• Spread Footings.• Square Pad Footing.• Raft Foundation. Deep foundation :-

• Pile.• Bored pile.• Micro pile.

Strip Foundation

• This type of foundation is also known as wall foundation or continues spread footing foundation.

• It uses is to support load bearing wall. • It uses is to support load bearing wall.

Continuous Spread Footing

• This type of footing is commonly used to support the walls of above-above-ground circular storage tanks

Foundations

Definition:• A foundation is a part of the structure which is in direct contact with

the ground to which the loads are transmitted.

• Every structure, whether big or small, needs a foundation.

• The foundation carries the load of the building and provides it stability.

• The foundation transmits the loads imposed on the structure to the soil below it and therefore the type of soil used is very important.

• Depending upon whether the soil is stable or loose and unstable and the type and size of the structure, the foundation is designed as either shallow or deep. either shallow or deep.

• A stable foundation should bear the loads without sinking or settling more than an inch at the most. Even this settlement should be uniform under the entire building.

Construction Terminology

• The foundation is known as the substructure (under the soil structure) and the building itself is known as the superstructure (above ground structure).

• We can say that the main function of the foundation is to support • We can say that the main function of the foundation is to support the load of the superstructure and transmit the load evenly to the soil

Main Functions• To distribute the load of the structure over a larger area.

• To transmit the load uniformly under the structure.

• To provide a firm, level and strong base over which the superstructure may be constructed.superstructure may be constructed.

• To avoid any settlement or other movement that can cause damage to any part of the building.

• To increase the stability of the structure by preventing its tilting or overturning against winds, earthquakes and uneven distribution of live load. (Lateral Stability)

Load

Ground Level

Figure 1.1 Foundation

450

Forces pushing down must equal the forces pushing up -EQUILIBRIUMEQUILIBRIUM

Heave

If the forces pushing up is greater than the forces pushing down the building will be pushed upwards –HEAVE

Subside

If forces pushing down is greater than the forces pushing up the building will sink – SUBSIDENCE

Problems if the rules are not followed

The load spreads at about 400 through the foundation

Loads Acting on the Foundation• The foundation has to bear more than just the load of the

superstructure.

• A load can be defined as anything, which exerts pressure or thrust on a structure.

• The following are the different types of loads that act on the building foundation:

• Live Load (Q k) - A live load or imposed load is a movable, temporary or transferable load. This can include moving vehicles, people walking or children jumping.

Types of loads

• Dead Load (G k)- This load is permanent and immovable. It is the non- transferable load of the structure itself.

• Wind Load (W k) - This load is applicable when the structure is • Wind Load (W k) - This load is applicable when the structure is tall.

• Snow Load - This load is considered when the structure is situated in snowy, hilly areas.

Rock or soil Typical bearing value (kN/m2)

Massive igneous bedrockSandstoneShales and mudstoneGravel, sand and gravel, compact

10,0002,000 to 4,000600 to 2,000600

compactMedium dense sandLoose fine sandHard clayMedium claySoft Clay

100 to 300Less than 100300 to 600100 to 300Less than 75

Typical allowable bearing values

Types of Soils

The following are the different types of soils on which foundations are constructed:

• Soft soils - This soil is compressible and • Soft soils - This soil is compressible and yields when loaded. Examples are clayey soil and loam. Small buildings or ordinary structures can be built on these types of soils.

• Spreading soils - These are non-cohesive soils. Examples of this type of soil are sand and gravel.

• Hard or rocky soils - These are incompressible and strong soils. They can incompressible and strong soils. They can withstand heavy loads without yielding. Multistoried buildings and water reservoirs are designed on such soils.

Shallow Foundation Strip Footing

Types of Foundation

Pad Footing

Raft Foundation

Bore Pile

Bakau Pile

Types of Foundation

Deep FoundationMicro Pile

Spun Pile

• The type of foundation used is selected based on the type of the structure that has to be built, the type of soil and the type of material used.

• They are classified into shallow and deep foundations.

Shallow FoundationsShallow Foundations – for low-rise construction

• When the foundation is placed just below the superstructure, it is known as shallow foundation.

• The purpose of these is to transmit the • The purpose of these is to transmit the loads of the superstructure over a wider area.

• These foundations are suitable for small buildings.

A deep foundation is a type of foundation.

-distinguished from shallow foundations by the depth they are embedded into the ground.

-the common reasons are because of large design loads, a poor soil at shallow depth, or site constraints (like property lines). constraints (like property lines).

-deep foundations including piles, drilled shafts, caissons, and piers. The naming conventions may vary between engineering disciplines and firms.

-Deep foundations can be made out of timber, steel, reinforced concrete and pre-tensioned concrete.

-Deep foundations can be installed by either driving them into the ground or drilling a shaft and filling it with concrete, mass or reinforced

Spread/Strip Footings

• Also known as footer or simply a footing.• An enlargement at the bottom of a column or

bearing wall that spreads the applied structural loads over a sufficiently large soil area.loads over a sufficiently large soil area.

• Typically, each column and each bearing wall has its own spread footing.

Characteristic of Spread Footings

• Low Cost• Ease of construction• For small-medium size structures with

moderate-good soil.moderate-good soil.• For large structures with exceptionally

good soil or shallow bedrock.

• Spread footing may be built in different shapes and sizes to accommodate individual needs.individual needs.

Types of spread footings based on size and shape

No Types of Spread Footings Applicable

1 Square for a single centrally-located column

2 Rectangular when large moment load are present

3 Circular for light standards, flagpoles etc3 Circular for light standards, flagpoles etc

4 Continuous for bearing walls

(wall/strip footings)

5 Combined when columns are close together

6 Ring for walls of above-ground circular storage tanks

7 Strap (cantilever footing) when very close to a property line/other structure

CONSTRUCTION ACTIVITY ON PAD FOOTING OR STRIP FOOTING

• PEGGING• EXCAVATION WORKS• PROVIDING A MATERIALS

– REBAR– FORMWORK– FORMWORK– CEMENT– SAND– AGGREGATE

• FORMWORK– EXCAVATE A WORKING SPACES FOR

BETTER CONSTRUCTION

• REINFORCEMENT INSTALLATION• CONCRETING

– MIXING– MIXING– POURING– CURING– TESTING

Stepped Foundation (Spread Footings)

• This type of foundation is one of the pad foundation types.

• When the structure is to be constructed on the hill slopes, this type of foundation is used.

• The foundation trenches are excavated in • The foundation trenches are excavated in steps and each step is filled with some concrete.

• Each block of concrete is overlapped by the next block, and so on.

• Square footings usually support a single centrally-located column.

Step no greater than 450 mm

Overlap to be equal to or greater than the depth of the concrete foundation

Square Footings

Combined Footing Foundation

• In this type, the two walls or columns of a superstructure are provided with a single combined footing.

• This is designed so that the center of gravity of • This is designed so that the center of gravity of the supporting area is in proportion to the center of gravity of the tow column loads.

• These can be rectangular or trapezoidal in shape.

These are usefull when columns are when columns are located too close together for each to have its own footing.

Rectangular Spread Footings

• It have plan dimension of B x L, where L is the longest dimension.

• These are useful when obstructions prevent construction of a square footing prevent construction of a square footing with a sufficiently large base area and when large moment loads are present.

Circular Spread Footings

• This foundation are round in plan view. • These are more frequently used as

foundation for light standard, flagpoles, and power transmission line. and power transmission line.

• If these foundation extend to a large depth, they may be have more like a deep foundation.

Continuous Spread Footings

• This type of foundation is also known as wall foundation or strip foundation.

• It uses is to support bearing wall.

Ring Spread Footings

• This footing are continuous footing that been wrapped into a circle.

• This type of footing is commonly used to support the walls of above-ground circular support the walls of above-ground circular storage tanks.

Ring Spread Footing

Raft Foundation

• Also known as Mat Foundation or Floating Foundation.

• Used where heavily constructed loads are to be distributed over a large surface area. to be distributed over a large surface area.

• It is used where the soil is marshy, clayey or soft, with weak bearing capacity.

• This consists of reinforced concrete slabs covering the entire area of construction, like a floor.

• Always made of reinforced concrete.• Always made of reinforced concrete.

Consideration of using Raft Foundation

• The structural loads are so high.• The soil condition is so poor.• The bottom of structure is located below

groundwater table.groundwater table.• Raft foundation are more easier to

waterproof.

Conditions for Mat Foundations-Structural loads require large area to spread the load-Soil is erratic and prone to differential settlements-Structural loads are erratic-Unevenly distributed lateral loads-Unevenly distributed lateral loads-Uplift loads are larger than spread footings can accommodate; weight of the mat is a factor here-Mat foundations are easier to waterproof

Deep Foundation• In cases where the soil stratum is not

stable or strong, it is necessary to take the foundation deeper to reach the stable stratum. the stable stratum.

• For instance, in cases of river bridges, it is important to reach soil stratum below the scour depth of the rivers to prevent settlement.

Deep Foundation

• Spun pile• Micro pile• Bakau pile• Bakau pile• Bore pile

Pile Foundation

• Pile foundations are the part of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface.

• The main components of the foundation are the pile cap and the piles.

• Piles are long and slender members which transfer the load to • Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soil of low bearing capacity

• The main types of materials used for piles are wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps.

• Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.

Function of pilesAs with other types of foundations, the purpose of

a pile foundations is:• to transmit a foundation load to a solid ground• to resist vertical, lateral and uplift load• A structure can be founded on piles if the soil

immediately beneath its base does not have immediately beneath its base does not have adequate bearing capacity. If the results of site investigation show that the shallow soil is unstable and weak or if the magnitude of the estimated settlement is not acceptable a pile foundation may become considered. Further, a cost estimate may indicate that a pile foundation may be cheaper than any other compared ground improvement costs.

Function of piles

As with other types of foundations, the purpose of a pile foundations is:

• In the cases of heavy constructions, it is likely that the bearing capacity of the shallow soil will not be satisfactory, and the construction should be built on pile foundations. Piles can also be not be satisfactory, and the construction should be built on pile foundations. Piles can also be used in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.

Classification of pile with respect to load transmission and functional behaviour

• End bearing piles (point bearing piles)• Friction piles (cohesion piles )• Combination of friction and cohesion piles• Combination of friction and cohesion piles

End bearing piles

• These piles transfer their load on to a firm stratum located at a considerable depth below the base of the structure and they derive most of their carrying capacity from the penetration resistance of the soil at the toe of the pile.

• The pile behaves as an ordinary column and should be designed as such. Even in weak soil a pile will not fail by buckling and this effect need only be considered if part of the pile is unsupported, i.e. if it is in either air or water. Load is transmitted to the soil through friction or cohesion.

End bearing piles

• But sometimes, the soil surrounding the pile may adhere to the surface of the pile and causes "Negative Skin Friction" on the pile. This, sometimes have considerable effect on the capacity of the pile. Negative skin friction is caused by the drainage of the ground water and caused by the drainage of the ground water and consolidation of the soil. The founding depth of the pile is influenced by the results of the site investigate on and soil test.

End bearing piles

Friction piles

• These piles also transfer their load to the ground through skin friction.

• The process of driving such piles • The process of driving such piles does not compact the soil appreciably.

• These types of pile foundations are commonly known as floating pile foundations.

Spun Pile• Size : 250mm to 1000mm• Lengths : 6m, 9m and 12m (Typical)• Structural Capacity : 45Ton to 520Ton• Material : Grade 60MPa & 80MPa • Material : Grade 60MPa & 80MPa

Concrete• Joints: Welded• Installation Method :

–Drop Hammer–Jack-In

Spun Piles vs RC Square PilesSpun Piles have …• •Better Bending Resistance• •Higher Axial Capacity• •Better Manufacturing Quality• •Able to Sustain Higher Driving Stresses• •Able to Sustain Higher Driving Stresses• •Higher Tensile Capacity• •Easier to Check Integrity of Pile• •Similar cost as RC Square Piles

Bored Piles• Size : 450mm to 2m• Lengths : Varies• Structural Capacity : 80Ton to 2,300Tons• Concrete Grade : 20MPa to 30MPa • Concrete Grade : 20MPa to 30MPa

(Tremie)• Joints : None• Installation Method : Drill then Cast-In-Situ

Bored piles

• Bored piles (Replacement piles) are generally considered to be non-displacement piles a void is formed by boring or excavation before piles is produced.

• Piles can be produced by casting concrete in the void. Some soils such as stiff clays are particularly amenable to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to to the formation of piles in this way, since the bore hole walls do not requires temporary support except cloth to the ground surface.

• In unstable ground, such as gravel the ground requires temporary support from casing or bentonite slurry. Alternatively the casing may be permanent, but driven into a hole which is bored as casing is advanced. A different technique, which is still essentially non-displacement, is to intrude, a grout or a concrete from an auger which is rotated into the granular soil, and hence produced a grouted column of soil.

Borepile Cosiderations…• •Borepile Base Difficult to Clean• •Bulging / Necking• •Collapse of Sidewall• •Collapse of Sidewall• •Dispute on Level of Weathered Rock

Micropiles• Size : 100mm to 350mm Diameter• Lengths : Varies• Structural Capacity : 20Ton to 250Ton• Material : Grade 25MPa to 35MPa Grout• N80 API Pipe as Reinforcement• N80 API Pipe as Reinforcement• Joints: None• Installation Method :

–Drill then Cast-In-Situ–Percussion Then Cast-In-Situ

Protecting timber piles from decay: a) by pre-cast concrete upper section above water level. b) by extending pile cap below water level

Factors to be considered in selecting a pile type

The applied loads

The required diameter

The required length(limit: 18m)

Factors to be considered in selecting pile type

selecting a pile type(timber, steel, concrete or composite)

The anticipated driving conditions

The durability of pile material in specific

environment

The local availabilityof each pile type

Advantages & Disadvantagesbakau pile

No Advantages Disadvantages

1 Low construction cost Medium axial loads (100 - 400 kN)

2 Used as waterfront structures Susceptible to decay

3 For light driving conditions

Susceptible to damage when

driving

(in loose sands and soft to medium clays)

• Damage during driving can be controlled by:–Using lightweight hammers–Using steel bands near butt–Using a steel shoe on the toe–Pre-drilling

Advantages & Disadvantages Spun ile

No Advantages Disadvantages

1 Best suited for use as friction piles that Expensive to splice and cut

don't meet refusal during driving

(refusal: pile can't be driven any further, so(refusal: pile can't be driven any further, so

it becomes necessary to cut off the portion)

2 Best suited for toe-bearing piles where the Difficult to cut

required length is uniform and predictable

3 Less expensive than steel piles Succeptible to damage during handling

or driving

4 Have a large load capacity Not suited for hard driving conditions

Advantages & DisadvantagesBore Pile

No Advantages Disadvantages

1 Less costs of mobilizing and demobilizing a drill rig Dependent on contractor's skills

2 Less noise and vibration Lower unit end bearing capacity

3 Soils excavated can be observed and classified Expensive for full-scale load test3 Soils excavated can be observed and classified Expensive for full-scale load test

during drilling

4 Size of shafts can easily be changed during const.

5 Can penetrate soils with cobbles, boulders and

many types of bedrock

6 Possible to support each column with one large

shaft (no pile cap)

Drilled Equipments

• Drilling Rigs– Truck-mounted drilling rig

• For usual shaft, d=500 – 1200mm and H=6.24m

– Specialized rigs– Specialized rigs– A-Shaped Frame Rigs

• Drilling Tools� The helix-shaped flight auger (most common

used)– Effective in most sols and soft rocks

� Augers with hardened teeth and pilot stingers� Augers with hardened teeth and pilot stingers– Effective in hardpan or moderately hard

rock� Spiral-shaped rooting tools

– Help loosen cobbles and boulders

� Bucket augers– To collect cuttings in a cylindrical bucket– Used in running sands

� Belling buckets– To enlarge the bottom of the shaft (bells or

under reams)� Core barrels� Core barrels

– To cut a circular slot,creating a removablecore

– Used in hard rock� Multi-roller percussion bits

– To cut through hard rock� Cleanout buckets

– To remove final cuttings from hole

Drilled Techniques

• Drilling in Firm Soils– Using dry method (open-hole method)– Most common used: simple, economy and

good reabilitygood reability

• Steps:– Holes usually advance using conventional

flight auger– Holes remain open without any special

supportsupport– Check the open hole for cleanliness and

alignment– Insert steel reinforcing cage– Pour the concrete

• Drilling in Caving (Cave-in) or Squeezing Soils– Caving:

• The side of a hole which is collapse before • The side of a hole which is collapse before or during concrete placement.

• Usually in clean sands below the groundwater table.

– Squeezing:• The sides of hole bulging inward during or after

drilling• Usually in soft clays and silts or highly organic

soils.soils.

– Most common techniques:• Using casing• Drilling fluid (slurry method) using bentonite clay or

attapulgite clay.

Pile Cap In the British Standard Code of Practice BS 8004, a pile cap is defined as a concrete block cast on the head of a pile, or a group of piles, to transmit the load from the structure to the pile or group of piles.

Generally, pile cap transfers the load form the structures to a pile / pile group, then the load further transfers to from soil

External pressures on a pile are likely to be greatest near the ground surface. Ground stability increases with depth and pressure. The top of the pile therefore, is more vulnerable to movement and stress than the base of the pile. Pile caps are thus incorporated in order to tie the pile heads together so that individual pile movement and settlement is greatly reduced. Thus stability of the pile group is greatly increased.

Foundations relying on driven piles often have groups of piles connected by a pile cap (a large concrete block into which the heads of the piles are embedded) to distribute loads which are larger than one pile can bear.

Pile caps and isolated piles are typically connected with grade beams to tie the foundation elements together; lighter structural elements bear on the grade beams while heavier elements bear directly on the pile cap.

Pile cap

• Function:– To distribute the structural loads to the piles.– To tie the piles together so they can act as a

unit.unit.– To laterally stabilise individual piles thus

increasing overall stability of the group– To provide the necessary combined

resistance to stresses set up by the superstructure and/or ground movement

SUMMARY• Importance of Preliminary Study• Understanding the Site Geology• Carry out Proper Subsurface Investigation

that Suits the Terrain & Subsoilthat Suits the Terrain & Subsoil• Selection of Suitable Pile• Pile Design Concepts

SUMMARY• Importance of Piling Supervision• Typical Piling Problems Encountered• Present Some Case Histories• Present Some Case Histories