46 HIGHWAY CONSTRUCTION 



In excavations of considerable depth, and particularly in soils 

 liable to slips, the slope may be formed in terraces, the horizontal 

 offsets or benches being made a few feet in width with a ditch on 

 the inner side to receive the surface water from the portion of the 

 side slope above them. These benches catch and retain earth 

 that may fall from the slopes above them. The correct forms for the 

 slopes of embankment and excavation are shown in Figs. 21 and 22. 



Covering of Slopes. It is not usual to employ any artificial 

 means to protect the surface of the side slopes from the action of the 

 weather; but it is a precaution which in the end will save much labor 



Fig. 22. Cross-Section for Excavation. 



and expense in keeping the roadways in good order. The simplest 

 means which can be used for this purpose consists in covering the 

 slopes with good sods, or else with a layer of vegetable mould about 

 four inches thick, carefully laid and sown with grass seed. These 

 means are amply sufficient to protect the side slopes from injury 

 when they are not exposed to any other cause of deterioration than 

 the wash of the rain and the action of frost on the ordinary moisture 

 retained by the soil. 



A covering of brushwood or a thatch of straw may also be used 

 with good effect; but from their perishable nature they will require 

 frequent renewal and repairs. 



Where stone is abundant a small wall of stone laid dry may be 

 constructed at the foot of the slopes to prevent any wash from them 

 being carried into the ditches. 



Shrinkage of Earthwork. All materials when excavated 

 increase in bulk, but after being deposited in banks subside or shrink 

 (rock excepted) until they occupy less space than in the pit from 

 which excavated. 



Rock, on the other hand, increases in volume by being broken 

 up, and does not settle again into less than its original bulk. The 

 increase may be taken at 50 per cent. 



