554 



PRACTICE OF AGRICULTURE. 



Pakt III. 



Sect. IV. Of the Execution of the Works. 



3544. Thejint operation of execution is the setting out of the work bj the resident engi-' 

 neer and surveyor. He will trace and mark the levels accurately of each pound or level 

 reach of the canal, marking them with stakes, and comparing his work with the bench 

 marks, and making two or more of the men who assist him perfectly acquainted with 

 their situations, in case they should be deranged by cattle or otherwise 



3545. The calculations for excavation is the next part of execution. The great desi- 

 deratum in canal-digging is, that the stuff that is dug from one part of the work shall, 

 with the least labor or distance of moving, exactly supply or form the banks that are 

 to be raised in another, so that, on the completion of the work, no spoil banks, or banks 

 of useless soil, shall remain, or any ground be unnecessarily rendered useless by exca- 

 vations or pits. Six different cases will be found frequently to occur in the cutting 

 or forming a canal. In each case the towing-bank (Jig. 449 a.), is wider than the oft"- 

 bank (b) ; and in all, the sides slope one and half feet for one foot in depth, that being 

 found the least slope which can be given. 



3546. Where there is deep cutting on one side (c), or both (d, e), a bench or berm (c, e) 



4^9 



is provided to retain and prevent the loose earth that may moulder down from the upper 

 bank from falling into the canal. The banks are usually made one foot higher than the 

 water is intended to stand in them. 



3547. In level cutting {fig. 449 a, J), the height of the canal should be so contrived, 

 that in any cross section the sum of the areas of the made banks (a, h) should just equal 

 that of tlie area of the section of excavation (i). 



3548. In side-lying ground (fig. 449 c, and fig. 450/), the same object may be attained 

 with a little extra calculation ; and in all other cases {g, h), the engineer will shew the 



perfection of his skill in so conducting the line, that every embankment shall have deep 

 cutting at both, or at least at one of its ends, to furnish the extra stuff with least expense 

 in moving it ; in like manner, every deep cutting (rf, e) should have embankments at 

 one or both of its ends, to receive the extra stuff. 



3549. Before cutting out the lock-spit, or small trench between the several slope holes, 

 as a guide to the men who are to dig, the engineer ought to cause holes to be dug in the 

 line of the canal, near every second or third level peg, or oftener, if the soil be variable, 

 in order to prove the soil to a greater depth, by two or three feet, than the cutting of the 

 canal is to extend ; and each of these the engineer ought carefully to inspect, in order to 

 determine what puddling or lining will be necessary ; and what will be the difficulties of 

 digging, owing to the hardness of the stuff, or to water that must be pumped out, &c. ; 

 all which circumstances, as well as the extra distance that any part of the stuff may re- 

 quire to be moved, must be well considered before the work can be let to the contractors. 



3550. The puddling or lining of a canal, to make it hold water, is a matter of the 

 greatest importance, and we shall consider five cases that are likely to occur or present 

 themselves in the search into the soil that is to be dug, by sinking holes as above- 

 mentioned. The first case we suppose to be that in which the whole is clay, loam, or 

 other water-tight stuff; all soils that will hold water, and not let it soak or percolate 

 freely through them, are called water-tight. Our second case is that in which the 

 whole cutting will be in sand, gravel, loose or open rock, or any other matters that will 

 let water easily through them, and such are called porous soils or stuffs. The third 

 case, we suppose to have a thin stratum of water-tight stuff on the surface, and to have 

 porous stuff for a considerable depth below.. The fourth case may have porous stuff near 

 the surface, and water-tight stuff at the bottom of the canal. The fifth case is that 

 where water-tight stuff appears on the surface ; and below this a stratum of porous stuff, 

 but having again water-tight stuff at no great distance below the intended bottom of the 

 canal. The new raised banks are always to be considered as porous stuff, as, indeed, 

 they will always prove at first, and in a great portion of soils they would ever remain so, 

 unless either puddling or lining was applied ; all ground that has been dug or disturbed, 

 must also be considered as porous. It should also be remarked, that any kind of soil 

 which is perforated much by worms or other insects, should, in canal digging, be consi- 

 dered as porous stuff. 



