INTRODUCTION
• Deep foundations.
• Provide a solid and massive foundation for heavy loads.
• Useful in situations where the loads have to be transferred to a soil stratum deep below (in case of bridge foundation).
• Can be conveniently installed in a boulder stratum
• Widely used in India as foundations for bridge piers and abutments.
•Also useful as foundations where uplift loads arc large (in case of transmission line towers).
•Being a massive sub-structure, it is monolithic and relatively rigid in its structural behavior.
Types of Wells or Caissons
• Open Caissons (Wells)
• Box Caissons and
• Pneumatic Caissons
Open Caisson or Well
Open Caisson or Well
• Top and bottom of the caisson is open during construction.
• May have any shape in plan: circular, rectangular, oblong etc.
• It has a cutting edge which is fabricated at site and first segment of shall is built on it.
• Soil inside the shaft is dredged by suitable means and another segment is added to it.
• Process of sinking is continued till it reaches the required depth.
• Bottom is sealed with concrete then.
• Shaft is filled with sand.
• It can be constructed up to any depth and cost of construction is relatively low
Disadvantages of Open Caisson or Well
• Progress of construction in boulder deposits is very slow.
• The concrete sealed under water is not effective.
• The bottom can not be inspected
• Progress of construction in boulder deposits is very slow.
• The concrete sealed under water is not effective.
The bottom can not be inspected.
Box Caisson
Box Caisson
• First casted on land and then towed to the site where it is sunk onto a previously levelled foundation base.
• Also called as floating caisson.
• Used where loads are not very heavy and a bearing stratum is available at a shallow depth.
Disadvantages of Box Caisson
• 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
Pneumatic Caisson
• Has a working chamber at the bottom of caisson which is kept dry by forcing out water under pressure, thus permitting excavation under dry conditions.
• Air locks are provided at top.
•Caisson gradually sinks as excavation is made.
•On reaching the final depth, the working chamber is filled with concrete.
• This has an advantage of better control in sinking and supervision.
• The bottom of chamber can be sealed effectively with concrete as it is placed under dry condition.
•Obstructions during sinking, eg., boulders etc. can be removed quite easily
•On reaching the final depth, the working chamber is filled with concrete.
• This has an advantage of better control in sinking and supervision.
• The bottom of chamber can be sealed effectively with concrete as it is placed under dry condition
•Obstructions during sinking, eg., boulders etc., can be removed quite easily
Disadvantages of Pneumatic Caisson
• Cost is very high.
• The limit on the depth of penetration below water level (about 35 m equivalent to a pressure of about 3.5 kg/cm2) is very high.
• These higher pressures are beyond the endurance of human body.
Components of Well Foundation
Well cap
• It is a RCC slab laid on top of the well steining and is usually cast monolithically with the steining
• Transmits the load of superstructure to the steining.
Steining
• It is the main body of well which transfers load to the subsoil,
•Acts as a cofferdam during sinking and provides weight for sinking.
Curb
• The lower wedge - shaped portion of well steining is called the well curb.
• Facilitates the process of sinking
Cutting Edge
• The lowermost portion of the well curb the cutting edge.
• It cuts into the soil during sinking.
Bottom Plug
• After the well is sunk to the required depth, the base of well is plugged with concrete. This is called bottom plug.
• Transmits the load to the subsoil.
Dredge Hole
• The well is sunk by excavating soil from within the well. The hole formed due to the excavation of soil is called the dredge hole.
• It is later filled with sand.
• This sand filling helps in distributing the load of superstructure to the bottom plug.
Top Plug
•It is a concrete plug covering the sand filling usually constructed on top.
• It provides contact between well cap and sand filling.
• Helps in transferring the load through the sand filling.
Shapes of Wells
• The choice of a particular shape of well is dependent mainly on base dimensions of pier or abutment, the ease and cost of construction, tilt and shift during sinking and the magnitude of forces to be resisted.
• Most commonly adopted section of a well is the circular one.
• This has the least perimeter for a given area
of the base and hence is the ideal section in terms of the effort needed during sinking
• Further, as the distance of cutting edge from the dredge hole is equal, sinking is more uniform.
•It is disadvantageous to accommodate a large oblong pier which would require a large diameter well.
• In the case of large oblong piers, two or three independent, circular wells placed very close to each other with a common well cap can be used.
• A combination of independent wells is easy to handle but they have a tendency to tilt towards each other during sinking.
• Tied wells of different shapes are preferred in order to avoid relative tilt between wells.
• Usually the neck portion is kept minimum, keeping in view the dimensions of pier and the dredge holes.
• Double - D and dumb - bell shapes are commonly used shapes of wells.
• In double - D wells, as the curved portion is much smaller as compared to the straight portion, the lateral stability is considerably increased.
• However, the straight portion increases the resistance to sinking and corners pose a problem in dredging
•Double - D shaped wells have advantage in terms of being a monolithic structure.
• Dumb - bell shaped wells have dredge holes as circular.
• Thus, these have the advantage of twin circular wells in terms of ease in dredging.
•Double octagonal wells arc not much in use due to difficulty in construction and the greater resistance offered against sinking on account of increases surface area.
• For piers and abutments of very large sizes, wells with multiple dredge holes are used.
• The code recommends that any shape that fulfils the above requirements be adopted.
• When a group or groups of wells are sunk, the minimum spacing between them depends on the depth of well.
• However, for general guidance in design, a spacing of about 1 m may be used
•IS: 3955 (1967) laid down following requirements for the horizontal cross-section of the well.
• The dredge hole should be large enough to permit drainage.
• The steining thickness should be sufficient to transmit the load and also provide necessary weight for sinking and adequate strength against forces acting on the steining. both during sinking of wells and service.
• It should accommodate the base of structure and not cause undue obstruction to the flow of water.
• The overall size should be suflficient to transmit the loads to the soil.
• It should allow for the permissible tilt and shift of the well.
Depth of a Well Foundation
Two important requirements that influences the depth of a well foundation are:
• Minimum grip length below the scour depth
• Base pressure to be within permissible limits.
The scour depth in a stream should be ascertained either by actual soundings at or near the proposed site during or immediately
alter a Hood before the scour holes have had time to silt up or by theoretical methods, eg., Lacey's formula.
According to Lacey s formula:
For natural streams in alluvial beds, the normal depth of scour d (m) below high flood level (HFL) for regime conditions in a stable channel is given by:
The grip length he taken as one third the maximum scour depth.
• Thus, as per IS: 3955 (1967). the depth of foundation should not be less that 1.33 times the deepest scour below HFL
• This may, however, be reduced if an inerodible stratum such as rock is available at shallow depth.
IRC specifications (1966) recommend the minimum depth of foundation as 1.33 times the anticipated maximum depth of scour below HFL.
• The specifications further state that the depth below scour line should in no case be less than 2 m for piers and abutments with arches.
The depth below scour line should in no case be less than 1.2 m for piers and abutments supporting other types of superstructures.
• The maximum base pressure should be less than the allowable bearing pressure. This is important from the consideration of safety of well.