Well Foundations

Well Foundation for Bridge Structures

• Useful as foundations where uplift loads

are

large

(in

case

of

transmission towers). • It is monolithic and relatively rigid in its structural behaviour,

Types of Wells • Open Caissons (Wells) • Box Caissons and • Pneumatic Caissons

• The top and bottom of the caisson is open at the time of construction, • It can be constructed upto any depth and cost of construction is low, • Progress

of

construction

in

boulder

deposits is very slow, • Concrete seal placed under water is not effective and also the bottom cannot be inspected.

Pneumatic Caissons: • These are of box-shape, closed at the top, with a working chamber at the bottom from which water is kept off with the aid of compressed air. • Thus excavation is facilitated in the dry, and the Caisson sinks as excavation proceeds. • Finally, the working chamber is filled with concrete, upon reaching the final location at the desired depth.

Floating or Box Caissons: • These are also of box-shape, closed at the bottom and open at the top. • This type of Caisson is cast on land, launched in water, towed to the site, and sunk into position by filling it with sand, gravel, concrete, or water.

• Foundation bed has to be prepared in advance. • Bearing capacity of base has to be properly assessed. • Care has to be taken to protect the foundation from scouring action.

Shapes of Wells

1. Steining; It is the body of the well which transfers the loads to the base of the foundation. 2. Curb;

The

lower

portion

of

the

well

steining provided to facilitate sinking. 3. Dredge

hole;

The

well

is

sunk

by

excavating soil from within well. The hole formed due to the excavation of soil is called the dredge hole.

Well Foundation (Also called Caisson, Pier or Monolith)

Bridge Pier River Water River bed level Well cap Well Steining Overburden Soil

( RCC/ Massonary)

Sand Filling

Bottom Plug Rock

Cutting Edge

IS 3955 (1967) requirements; • The dredge hole should be large enough to permit dredging, • The steining thickness should be sufficient to transmit the load and also provide necessary weight for sinking, • It should accommodate the base of structure and not cause undue obstruction to the flow of water, • The overall size should be sufficient to transmit

the

loads to the soil, • It should allow for the permissible tilt and shift of the well

Depth of Well Foundation; Important requirements; • Minimum grip length below the scour depth, • Base pressure to be within permissible limits.

Type of bed soil

Size of particles (mm)

f

Coarse silt Fine Sand

0.04 0.08 0.15

0.35 0.50 0.68

Medium Sand

0.30 0.50

0.96 1.24

Coarse Sand

0.70 1.00 2.00

1.47 1.76 2.49

Gravel

5.00 10.0 20.0

3.89 5.56 7.8

Boulders

50.0 75.0 90.0

12.30 15.20 24.30

• The depth below the scour line in no case be less than 2m for piers and abutments with arches, • 1.2m

for

supporting

piers

and

other

superstructures.

abutments types

of

Method based on elastic method, Assumptions;

• The soil surrounding the well and below the base is perfectly elastic, homogeneous and follows Hooke’s law. • Under design loads, the lateral deflections are so small that the unit soil reaction, p increases linearly with increasing lateral deflection ‘z’ such that p = KH z. • KH increases linearly with depth in cohesionless soil. • The well behaves as a rigid body and is acted upon by a unidirectional horizontal force H and a moment Mo at scour level.