Geotechnical Engineering

DHARM

808 GEOTECHNICAL ENGINEERING

= (1/2) × 10 × 20(1.9259 – 0.2596) (20 – 18)

= 333 kN/m

Qa = 333 × 7.5 = 2, 497.5 kN

(b) Centre of rotation at the base:

Using Eq. 19.32,

q′max = (1/6)γ′ (Kp′ – Ka)

D

H

3

1

= (1/6) × 10(1.9259 – 0.2596) ×

20

40

3

= 555 kN/m

∴ Qa = 555 × 7.5 = 4162.5 kN

The well is safe against lateral forces since this is greater than 1,800 kN.

Summary of Main Points

1. A Caisson is a type of foundation of the shape of a box, built above ground level and sunk to the
   required depth as a single unit.


2. Caissons are mostly used as foundations for bridge piers and abutments, and water-front struc-
   tures, as also for multi-storey buildings occasionally.


3. Caissons are broadly classified as Open Caissons, Pneumatic Caissons, and Floating or Box
   Caissons.


4. Open Caissons are open both at the top and at the bottom; these are filled with concrete after
   having been sunk to the required level.
   Pneumatic Caissons are those in which water is kept off the working chamber with compressed
   air. Safety precautions and restricted work schedules are essential in this case.
   Floating Caissons are closed at the bottom and open at the top. These are cast on land, launched
   in water, and sunk into position by filling with sand, gravel, or concrete. Stability aspect has to
   be checked for floating caissons.


5. Sand Island Method is popular in the placement of open caissons.


6. Well foundations are similar to open caissons, and are popularly used to support bridge piers
   and abutments in India. They offer certain specific advantages over other types of deep
   foundations.


7. Several plan shapes such as Circular, Rectangular, Hexagonal, Octogonal, Twin-Circular,
   Double-D, Dumb-Well, and Rectangular with multiple dredge holes are commonly used for wells.


8. Besides the steining or body, cutting edge, curb, concrete seal and well cap are the important
   components of a well foundation.


9. The depth of well below the maximum scour level is known as the ‘Grip Length’.


10. Many Hypotheses have been developed for the lateral stability of Well Foundations. However,
    Terzaghi’s Free Rigid Bulkhead Concept is simple, and the IRC Method based on the experimen-
    tal investigations of several research workers is popular.


11. Shifts and tilts during sinking of wells should be remedied to the extent possible and their effect,
    if any, should be considered in the design.