62 BULLETIN 724, IT. S. DEPARTMENT OF AGRICULTURE. 



under the supervision of the Bureau of Public Roads, showed a total 

 average cost for crushing and quarrying of about $1.25 per cubic 

 yard. The total quantity of crushed stone taken from the nine 

 quarries "was about 55,000 cubic 3^ards. 



V-DRAIN FOUNDATIONS. 



The proper method of constructing V-drain foundations has already 

 been discussed under " Drainage " on page 37. A typical cross sec- 

 tion for such a foundation is shown in figure 13 and the ordinary 

 method of construction is illustrated in Plate V. 



TELFORD FOUNDATIONS. 



The stone used in constructing Telford foundations should be 

 sound and tough, so that when wedged into the foundation indi- 

 vidual pieces of the stone will not crack or be loosened when rolled 

 or when irregularities are broken off with a hammer. Since the 

 relation between the different pa-rticles of stone composing a Tel- 

 .ford foundation necessarily mu^t be almost wholly mechanical and 

 its rigidity, therefore, depend on the interlocking of slight irregu- 

 larities in adjacent stone surfaces, it is evident that if the stone lacks 

 in soundness or toughness, the individual particles might be loosened 

 easily by the shearing off of these irregularities, and whatever load 

 came upon stones so loosened avouIcI be transmitted almost undis- 

 tributed, aiid therefore unreduced, to the subgrade. Such varieties 

 of stone as soft limestone, brittle sandstone, and decayed granite 

 could not, as a rule, be used satisfactorily in a Telford foundation, 

 though, under favorable subgrade conditions, they might be used 

 advantageously in a crushed-stone foundation. 



Satisfactory Telford foundations ma.y be constructed of either 

 quarried or field stone. But, in general, quarried stone is to be pre- 

 ferred, not only because it usually is tougher and more djirable, but 

 because it is easier to obtain a good mechanical bond between fresh 

 stone surfaces than between those which have been exposed to the 

 weather for some time and worn more- or less smooth. It is also 

 easier to secure a firm uniform bearing for pieces of stone having 

 approximately plane surfaces, than where the surfaces are rounded, 

 as is the case with field stone. 



The dimensions individual stones should have need not be fixed 

 very definitely, except that the depth should be equal to the depth 

 of the foundation. In general, the length should not be less than the 

 depth nor greater than about twice the depth. If the length is less 

 than the depth the stone will not be in a stable position, while if the 

 length is greater than about twice the depth it is difficult to insure 

 a uniform bearing. The minimum allowable width depends some- 

 what on the quality of the stone used as well as on the dimen- 

 sions. Where -an exceptionally hard and tough variety of stone is 



