272 HARBOUR CONSTRUCTION. 



same way as into sand, but it would yield and continue to sink 

 as more weight was placed upon it. 



When mud is not of great thickness, it is well, as in the case 

 of sand, to remove it, and found upon the more solid ground 

 underneath, unless one or other of the types of mound break- 

 waters be adopted, in which case any settlement which might 

 take place could be easily made up. 



Should a vertical breakwater, however, be a sine qua non, 

 and a sufficiently solid base to found it upon be unobtainable 

 without having recourse to artificial means, the necessary amount 

 of stability may be obtained, if the depth of water be not too 

 great, by driving timber piles say 12 or 13 inches square- 

 about 4 feet apart, centre and centre, over the entire area to be 

 occupied by the breakwater. The heads of these should be cut 

 off perfectly level, and well below the surface of the mud, so as 

 to ensure them against attack by sea-worms. Sills should then 

 be halved into the piles and bolted to them, so as to connect 

 them together and afford support to close 6-inch planking, which 

 should be spiked to the sills so as to form a platform whereon to 

 build the superstructure. Sometimes two layers of planking are 

 laid crossing each other at right angles, and sometimes planking 

 is omitted altogether. In the latter case, the mud is removed 

 for a depth of 4 or 5 feet below the pile-heads, and rich mass 

 concrete is put in. 



The construction of a breakwater upon such a foundation, 

 either by means of blocks or concrete in mass, is then a tolerably 

 straightforward piece of work. 



In a work of this kind, wide and substantial aprons must be 

 provided, to secure it from injury by scour and undermining. 

 Should such take place, or should the worms be allowed access 

 to the timber, the stability of the work would be endangered ; 

 but if the timber- work be kept well down and always buried, no 

 anxiety need be felt in regard to its endurance, as it will be 

 practically imperishable. A row of sheet piling along each side 

 of the foundation will add much to its security. 



There are other methods of dealing with a mud bottom, as, 

 for example, by sinking caissons, as before referred to, or by 

 dredging a trench, either between rows of sheet piling, or of 

 suflicient width to allow for side slopes, until a firm bottom is 

 reached, and then depositing rubble or concrete. 



In some cases the difficulties of a mud bottom may be 



