BRIDGE FOUNDATIONBRIDGE FOUNDATION

PRESENTATION ONPRESENTATION ON

Contents of this presentation –• Different Types of Bridge Foundation• Selection of Foundation Type• Open Foundation

– Construction Method, Design and Detailing• Pile Foundation

– Construction Method, Design and Detailing• Well Foundation

– Construction Method, Design and Detailing

Contents

Different Types of Bridge Foundation

Types of Bridge FoundationTwo Major Types –• Shallow Foundation

• Deep Foundation

Shallow Foundation – Open Foundation:• When top soil strata is strong / Rock level close to ground level• For River Bridges when flow velocity is less, i.e. less scour• Required excavation depth is less

Deep Foundation – Pile / Well Foundation:• When firm strata / Rock level is much below ground level• For River Bridges when discharge is high, i.e. more scouring of bed• Load on the structure is too high to be carried by top soil layers• When foundation to resist uplift forces

Types of Bridge FoundationShallow Foundation:When, D/B ≤ 1 (Where, D = Depth of foundation & B = Width of Foundation)

Deep Foundation:When, D/B ≥ 4

Types of Bridge FoundationOpen Foundation:

Types of Bridge Foundation

Open Foundation:

Types of Bridge FoundationOpen Foundation – Raft / Mat Type:

Types of Bridge FoundationPile Foundation:

Types of Bridge Foundation

Pile Foundation:

Types of Bridge FoundationPile Foundation:

Types of Bridge FoundationWell Foundation:

Types of Bridge Foundation

Well Foundation:

Selection of Foundation Type

Selection of FoundationSelection Criteria –• Location of Structure – Land (Flyover, ROB etc.), River / Sea, Hills• If on River / Sea – Scour Depth• Geological Conditions• Seismic Zone• Type of structure – Continuous / Simply Supported• If on River / Canal – Possibility of dry river bed / canal closure• Cost of any temporary work (like cofferdam, stream diversion etc.) if required• Available equipment and technology

Most Economic Design, when cost of Superstructure = cost of Substructure and Foundation

Open Foundation

Construction – Open Foundation

L

B

Step – 1: Setting out foundation plan on groundStep – 2: Excavation to required depthStep – 3: Construction of FoundationStep – 4: Back-fill Foundation Trench

Plan

Elevation

Design – Open FoundationStudy of Sub-soil investigation report & calculation of “Safe Bearing Capacity”

Open Foundation Resting on Soil:• Bearing capacity from shear strength criteria• Bearing capacity from maximum allowable settlement criteria

Open Foundation Resting on Rock:• Bearing capacity from crushing strength of rock & fissures / bedding planes• Settlement is generally neglected as no long term settlement

Design – Open FoundationSome important points to remember –

• For foundations resting on Soil – No Tension in Base Pressure is Allowed

• For foundations resting on Rock – Tension is allowed and need to be re-distributed to check the final base pressure (Area out of contact ≤ 20%)

• Allowable base pressure may be Increased in Seismic Cases

• In case of excavation in rock the foundation trenches around the footing shall be filled up with lean concrete.

• The founding level shall be at least 2.0m (for soil) / 1.5m (for soft rock) below the scour level.

• If the proposed foundation is close to any existing bridge / structure, the founding depth should be ≥ the existing foundation depth.

• The theoretical settlement calculation governs over the shear strength criteria to determine allowable bearing pressure.

• The allowable total settlement is 50mm-75mm, but depends on structural requirement also.

• If the foundation is resting on soil, differential settlement of supports need to be considered.

Design – Open FoundationStability Check:

A

P

M

B

F

Overturning Check: SMR / SMO ≥ 2.0

Sliding Check: mR / SF ≥ 1.5

Minimum & Maximum Base Pressure Check –

P/A + ML/ZL + MT/ZT

P/A - ML/ZL - MT/ZT

Design – Open FoundationDesign Base Pressure:

P/A + ML/ZL + MT/ZT

P/A - ML/ZL - MT/ZT

1. Gross Pressure (P1):

2. Pressure due to S/W of Foundation and Soil (P2):

3. Design Base Pressure (P1 – P2):

Pile Foundation

Construction – Pile FoundationPiles can be –• Driven cast-in-situ pile• Bored cast-in-situ pile• Driven precast piles

Minimum Pile Diameter –

Concrete Specification –

Construction – Pile FoundationStep – 1: Construction of piles (upto 1m above cut-off level / upto piling platform)Step – 2: Excavation for pile capStep – 3: Piles to be chipped off upto cut-off levelStep – 4: Placing Reinforcement cage for pile capStep – 5: Construction of pile cap

Design – Pile FoundationStudy of Sub-soil investigation report & calculation of “Allowable Pile Load”

Pile Foundation Resting on Soil:• Ultimate Pile Load = Skin Friction + End Bearing• Uplift Capacity = Skin Friction + Self Weight of Pile• Lateral Capacity = Force required for 5mm deflection at cut-off level / scour level• Factor of Safety – 2.5 for vertical load, 2.0 for uplift and 1.0 for lateral loads

Pile Foundation Resting on Rock:• Ultimate Pile Load = End Bearing + Side Socket Shear• Uplift Capacity = Side Socket Shear + Self Weight of Pile• Factor of Safety – For vertical load 3.0 on End Bearing & 6.0 on Socket Side resistance; For uplift 2.0

Design – Pile FoundationSome important points to remember –• For pile groups, settlement need to be checked and to be restricted to 10-12mm.• In pile group minimum pile spacing = 2.5 x pile dia. (End Bearing Pile)

= 3.0 x pile dia. (Friction Pile)• Negative skin friction shall be taken in account if any compressible layer is present• Theoretical pile capacity need to be verified by Initial (destructive test) load test and Routine (non-destructive test) load test.• Allowable pile load may be increased in seismic cases• Piles need to be designed for a shift of 75mm and tilt of 1 in 150• In case piles resting on rock, differential settlement may be neglected• Minimum thickness of pile cap is greater of 0.6m or 1.5 times diameter of pile• Minimum projection of pile inside pile cap is 50mm

Design – Pile Foundation

Pile and Pile cap forces can be analyzed by mathematical modeling in STAAD / any other recognized software.

• Soil stiffness is represented by spring supports to estimate soil – structure interaction.

• No need to calculate depth of fixity, the forces are directly available from the analysis.

Design – Pile FoundationAnalysis by rivet analogy – considering pile cap as rigid

L

L

T T

Maximum Load on corner pile =

P/n + ML x L / SL2 + MT x T / ST2

Minimum Load on corner pile =

P/n - ML x L / SL2 - MT x T / ST2

Design – Pile FoundationPunching Shear Check:• For Pile• For Pier / Abutment Column

d/2

d/2

Design – Pile FoundationStructural Design by Truss Model:• No need of Shear Reinforcement• 80% of calculated steel to be concentrated in strips linking the pile heads and the remainder uniformly distributed over pile cap.

Design – Pile Foundation

• Piles are designed as compression member along with bending moments.

• Minimum longitudinal steel in pile ≥ 0.4% of gross concrete area

• Transverse steel can be Circular Tie / Spiral Reinforcement

• In high seismic zone Plastic Hinge detailing is required at junction of pile cap and piles

• The test piles need to be designed separately as they do not carry the axial and lateral load together.

Structural Design of Piles:

Detailing – Pile FoundationPile Cap Designed by Bending Theory:

Detailing – Pile FoundationPile Cap Designed by Truss Model:

Detailing – Pile FoundationPile Reinforcement:

Well Foundation / Caissons

Well Foundation – Components

Well CurbBottom Plug

Intermediate Plug

Cutting Edge

Construction – Well FoundationStep – 1: Construction of Cutting Edge & Well Curb on GroundStep – 2: Construction of 1st segment of Steining & excavation inside wellStep – 3: Construction of successive Steining Segments and sinking of well upto founding levelStep – 4: Casting of bottom plugStep – 5: Sand filling and construction of intermediate plugStep – 6: Construction of well cap

Design – Well FoundationStudy of Sub-soil investigation report & calculation of “Safe Bearing Capacity”

Well Foundation Resting on Soil:• Bearing capacity from shear strength criteria• Bearing capacity from maximum allowable settlement criteria

Well Foundation Resting on Rock:• Bearing capacity from crushing strength of rock & fissures / bedding planes• Settlement is generally neglected as no long term settlement

Design – Well FoundationSome important points to remember –• Skin friction is neglected in Safe Bearing Capacity calculation.• In soil no tension in base pressure is allowed. In rocks out of contact area up to 20% of gross area is allowed.• In rocks rock anchor may be provided to overcome tension.• For Well foundation in soil a minimum grip length of 1/3rd the maximum depth of scour below the design scour level is need to be provided.• For Well Foundation in rocks a sump (shear key) of 300mm in hard rock and 600mm in soft rock need to be provided for stability.• Minimum inside diameter of well should not be less than 2.0m for dredging of soil inside.• If the external diameter of a single circular well exceeds 12.0m, then the stresses in steining shall be evaluated by 3D FEM method. • The Steining Section is usually designed as Plain Cement Concrete section, but can be done as RCC also.

Detailing – Well Foundation

Detailing – Well Foundation

Detailing – Well Foundation

Detailing – Well Foundation

Detailing – Well Foundation