WELL FOUNDATIONS (CAISSONS) Caissons and wells are large diameter foundations adopted in underwater situation such as bridge foundation in river. Three types: open caissons (wells) , box caissons, pneumatic caissons. OPEN CAISSON (WELL) The top and bottom is open during construction. It may of shapes: circular, rectangular, oblong etc. it has a cutting edge which is fabricated at the site and the first segment of shaft is built on it. The soil inside the shaft is dredged by suitable means and another segment is added to it. The process of sinking is continued till it reaches the required depth. Then the bottom is sealed with concrete. The shaft is filled with sand. Advantages: o Can be constructed up to any depth o Low cost of construction Disadvantages: o Progress of construction in boulder deposits is very slow. o The concrete seal placed under water is not effective o The bottom cannot be inspected. BOX CAISSON (FLOATING CAISSON) It is open at the top but closed at the bottom. This type of caisson is cast on land and then towed to the site where it is sunk onto a previously levelled foundation base. Used where loads are not very heavy and a bearing stratum is available at shallow depth. Disadvantages: o foundation bed has to be prepared in advance o bearing capacity of the base has to be properly assessed o protection from scoring action is needed PNEUMATIC CAISSON Has a working chamber at the bottom of the caisson which is kept dry by forcing out water under pressure, thus permitting excavation under dry conditions. Air locks are provided at the top. The caisson gradually sinks as excavation is made. On reaching the final chamber, the working chamber is filled with concrete. Advantages: o Better control in sinking and supervision. o The bootom of the chamber can be sealed off effectively with concrete as it is placed under dry condition. o Obstructions during sinking can be removed easily. Disadvantages: o Costly o Limit on the depth of penetration below water table (about 35 m) COMPONENTS OF A WELL FOUNDATION Well-cap: RCC slab laid at the top of the well staining. It is usually cast monolithically with the staining. It transmits the load of the superstructure to steining.
Steining: main body of the well which transfers the load to subsoil. It also acts as a cofferdam during sinking and provides weight for sinking. Bottom plug: after the well is sunk to the required depth, the base of the well is plugged with concrete. This is called the bottom plug. It transmits load to the subsoil. Curb: the lower wedge-shaped portion of the well steining is called the well curb. The curb facilitates the process of sinking. Dredge hole: the well is sunk by excavating soil from within the well; the hole thus formed is called dredge hole. The dredge hole is later filled with sand which helps in redistributing the load of the superstructure to the bottom plug. Cutting edge: the lowermost portion of the well curb is the cutting edge. It cuts into the soil during sinking. Top plug: a concrete plug covering the sand filling at the top. It provides contact between the well cap and sand filling. FORCES ACTING ON WELL FOUNDATION Well foundation is subjected to dead loads (weight of superstructure and self-weight) and live loads. In addition, it is subjected to the following forces: Wind forces Forces due to water current Forces due to tractive effort of vehicles or by breaking of vehicles Centrifugal forces in case the well is located on a curve Buoyancy Earth pressure Temperature Seismic forces CONSTRUCTION SEQUENCE Open caisson Box caisson
Scanned by CamScanner
Scanned by CamScanner