OPEN DRAINS, 1 69 



stake can be preserved for reference at any time. The 

 center and side cuts and distance out when recorded in 

 the notes furnish the data required for computing the 

 excavation. If the ground is very irregular, a suffi- 

 cient number of sections must be taken to enable the 

 engineer to compute the contents within reasonable 

 limits of accuracy. 



The slope stakes having been set, the contractor 

 may begin excavation at the limit indicated by them 

 and carry the required slope to the required depth, at 

 which depth the ditch will have the width designated. 



Computing the Cubic Yards of Excavation. — In 

 the usual class of work required for drainage canals it 

 is not necessary to use the lengthy and inconvenient 

 method of computing earthwork by the prismoidal 

 formula. However, in very rough ground it should be 

 used. This may be stated as follows: From the notes 

 compute the end areas of a 100-foot section or station, 

 from these construct the middle area. Add to four 

 times the middle area the area of end sections and 

 take one sixth of the product for the mean area. Frac- 

 tional parts of a station should be treated in the same 

 manner if the field notes have been taken for that pur- 

 pose. The method by end areas is as follows: Add 

 the end areas of any given station and divide by two 

 and the result is the mean area. This in either of the 

 above methods when multiplied by the length of sta- 

 tion and divided by 27 will give the number of cubic 

 yards in the station. The work may be very much 

 shortened by the use of the following table: 



