CRACKS IN FLOOR. 167 



or with a very slight batter, inasmuch as the design must be 

 such as to admit of the caisson being floated in or out of its 

 position, the batter on the side walls must therefore be sufficient 

 to enable the caisson to rise to a sufficient height to clear the 

 stems from the stop, without endangering the stability of the 

 caisson. 



For the purpose of keeping the floor of the gate recess and 

 the roller-paths clear of mud, sluiceways are generally arranged 

 in the side walls at the back of the gate recesses in such a 

 manner as to distribute a current of water over the entire area. 

 The scour caused by the adjustment of the lock water level 

 has been found in practice to be sufficient to keep the roller- 

 paths clear of mud. 



Cracks in Floor of Docks and Locks, When the strata is at 

 all of a yielding character, cracks will frequently appear in the 

 floor joints, especially near the centre. These cracks are caused 

 by the settlement of the side walls, due to the greater pressure 

 their weight throws upon the foundations, and inducing a spread- 

 ing or bearing action on the invert of the floor. This tendency 

 to crack can only be met by distributing the weights over as 

 large an area as possible, careful attention to the foundations, 

 and in putting in the backing and the adjustment of the pres- 

 sures due to the weight of the side walls and backing combined, 

 so that the resultant may fall somewhat nearer the front than 

 is usually considered desirable in ordinary retaining- walls. 

 This will induce a slight inward tendency in the side walls, 

 which will, however, be resisted by the invert of the floor. 



Mud Docks, In India, very primitive methods of docking 

 appear to be still in use at some of the minor ports, as the 

 following account 1 of the method pursued at the port of 

 Vizagapatam will show. A large ditch of a little more than 

 the ship's length, and deep enough to contain sufficient water to 

 float the ship, is excavated in the mud at right angles to the 

 river, and into it at high water the vessel is floated. A bund, 

 or dam, consisting of two rows of strong stakes driven into the 

 mud, lined with common palmyra, date, or bamboo mats, and 

 the intermediate space filled with sand, mud, or silt, is then 

 formed across the entrance. The basin being surrounded by a 

 high mud bank formed by the material excavated from the 

 dock, the water level is raised by returning some of the exca- 



1 Abridged from Indian Engineering, vol. Yi. p. 372. 



