The joints that are necessarily provided in the dam may be classified under two heads.
1. Construction joints, and 2. Contraction joints. 1. Construction Joints.
The concreting of the dam is not done at a stretch but in stages.
Each stage of the concreting is known as lift.
Lift is nothing but thickness of a horizontal layer of concreting laid once.
In concrete dams the thickness of each layer or in other words the lift, is kept about 1.5 m.
The horizontal joint between two successive lifts is known as construction joint. The lift or thickness
of each concreting layer is decided so that cooling of the concrete, while setting, may be effectively accomplished by natural as well as artificial agencies.
Thickness of lower most layer of concreting is kept half i.e. about 0.75 m.
Modern techniques of treatment of surface before laying the new layer of concrete, has almost eliminated the necessities of providing keys or water stops in the construction joints.
2. Contraction Joints.
The main purpose of providing contraction joints is to avoid development of shrinkage cracks in the dam due to changes in the temperature.
Shrinkage cracking of the concrete can be controlled to some extent by properly controlling the temperature and adopting proper methods of curing.
But provision of contraction joints in mass works like dams is invariably essential.
Construction joints may further be classified as transverse contraction joints and longitudinal contraction joints.
(i) Transverse contraction joints.
Direction of the joints is at right angles to the axis of the dam.
The spacing of these joints depends on factors like topographical features, location of the dam, type of cement, climatic conditions, but the general practice is to restrict spacing to about 15 m or the height of the dam whichever is small.
These joints extend throughout the height of the dam.
To insure proper
contact or connection between adjacent parts so that stresses are properly transmitted, the joints are either filled by grouting or by leaving a slot which is filled later when shrinking ceases.
The joints may not be grouted and rendered water tight by introducing water stops or key ways in the joints.
(ii) Longitudinal contraction joint.
These joints are provided along the direction of the axis of the dam.
These joints are considered objectionable from safety point of view as they may coincide very closely with the planes of maximum shear.
Longitudinal joints do not run continuously and they are staggered in plan as shown in Fig. 13.19.
However they are continuous in a vertical direction.
They are laid between two adjacent transverse joints. The spacing of these joints is also limited to about 15 m.
These joints are provided with key ways so as to transmit the principal stresses. S
pacing of key ways vertically is 1.5 m or one key way is to be provided in each lift. Keys.
It is a device, with the help of which shearing stresses from one block are transferred to the adjacent block.
The provision of keys is essential for longitudinal joints but optional for transverse joints.
The adjoining surfaces of each block are given such a shape that they together with transfer of stresses,
cause effective interlocking also. Key ways may be of several shapes such as triangular, trapezoidal or trough shaped.
water stops
The main job of water stops is to prevent leakage from the dam.
They may be made of copper, steel, sheet, lead or of natural rubbers and plastics.
Metal water stops are provided only in the case of non-yielding foundations.
Rubber water stops are used in case the foundation is of yielding type.
Drainage wells filled with asphalt are also sometimes used along with along water bars.
Launch your Graphy