EMBANKMENTS. 



365 



ing his estimates of the expense. In such cases, 

 it is evident that a knowledge of rocks on the sur- 

 face is not sufficient ; he must know what rocks he 

 at the different depths to which the canal must be 

 cut, and to tliis end he must be acquainted with the 

 order of succession of the different strata. 



Embankments. — In the construction of roads and 

 canals, walls and embankmt^nts often have to be 

 erected ; and from the frequent repairs needed on 

 our canals from the sliding of embankments, we ap- 

 prehend they are often constructed without regard 

 to those rules and principles, on the observance of 

 which durability alone depends. Sir Henry Par- 

 nell remarks, that great care is necessary in ma- 

 king high embankments ; no person should be in- 

 trusted with these works who has not had consid- 

 erable experience as a canal or road maker; for if 

 the base of an embankment is not formed at first to 

 its full breadth, and if the earth is not laid on in 

 regular laye|;s or courses not exceeding four feet 

 in thickness, it is almost certain to slip. 



''There have been but few attempts to make em- 

 bankments by turnpike trustees that do not afford 

 illustrations of this defect, and of a want of knowl- 

 edge of the proper rules by which these works 

 should be managed. No doubt a chief reason for 

 making cuttings and embankments, as is frequently 

 the case, with slopes of one to one, has been to 

 save expense in the purchase of land and moving 

 earth ; but the consequence of making such slopes 

 is, that the earth is constantly slipping, so that, in 

 the end, the expense is always greater in correct- 

 ing the original error than it would have been had 

 proper slopes been made in the first instance. The 

 slopes at which cuttings and embankments can be 

 safely made, depend entirely upon the nature of the 

 soil. In the London and plastic clay formations, it 

 will not be safe to make the slopes of embankments 

 or cuttings that exceed four feet in height, with a 

 H H 2 



