EXPERIMENT STATION RECORD. 



[Vol. 37 



objectionable grasses which grow on the banks tend to sow the entire field. 

 It is best adapted to slopes too steep to permit the use of any form of culti- 

 vated terrace, but it can not be recommended for use on slopes exceeding liO 

 per cent. 



" The narrow-base level-ridge terrace ... to cheap to construct and easy 

 to maintain. However, . . . considerable land is loit to cultivation and the 

 growth of weeds and grasses on the embankments tends to seed the entire 

 field. . . . Outside of these objections, the narrow-base level-ridge terrace, 

 where heavily sodded, renders satisfactory service on pervious soils and slopes 

 not greater than 8 per cent. 



" The broad-ba»e level-rldge terrace . . . has all the advantages of the 

 latter terrace with the added one that no land Is lost to cultivation. ... It 

 is best adapted to use on open, perviou* soils on slopes not exceeding 15 per 

 cent, but under proper conditions of design, construction, and maintenance can 

 be used on any soil and on sloi)es somewhat greater than 15 per cent. . . . 



"The broad-base graded-ridge terrace (the Mangum terrace) . . . not only 

 can be cultlvate<l but it can be crossed at any angle with large farm machinery. 

 Its broad ba.se and flat embankment slopes render it less liable to damage by 

 the flowing water fhgn Ls the c&sv with the narrow-base type. The grade may 

 be either uniform or variable, but iKith practice and theory indicate the vari- 

 able-graded terrace to be superior to the uniform-graded type. The graded 

 teirace is adapted particularly for use on Impervious and worn-out soils, and 

 on shallow open soils with an impervious foundation. . . . 



" By the selection and proper construction of suitable types of terraces 

 erosion can be controlled on bIoir-s up to 20 per cent or even niore. Instances 

 were found where erosion was controlled by the rn^e of terraces on land which 

 had a slope of SO per cent, Ilowever, slopes steeper than 20 per cent usually 

 can be devotwl more profitably to grasses or timber than to cultivali\l crofis. 



" Of all types of terraces, the use of the broad-bjise level-ridge terrace is 

 recommended wherever conditions will permit. This type, supplemented witli 

 efficient tile drains, offers the most ideal method of preventing soil eroelon on 

 any type of soil." 



The causes of cracks in cement-concrete pavements, A. T. Goi-ubeck {West. 

 Enffin., 8 (1917), Nv. 2, pp. S9-6S). — The canse.'^ of craiklug In concrete mads 

 are classlfleil as (1) expansion and a)ntraction due to changes In temperature 

 and moisture content, (2) nonuniform bearing under the slab due to frost 

 action, lack of homogeneity in tlie sub-ba.se and moisture expansion aiid shrink- 

 age of the sub-baae, and (3) bending resulting from heavy loads and impact. 



Tests to determine the amount of friction that can act at the sub-baae of a 

 concrete road when the slab expands and contracts, made by sliding 6 In. by 

 2 ft. by 2 ft. concrete slabs along previously prepared sut>-bases, gave the 

 results shown in the following table : 



Frictiotial resuitance of concrete on variotu sub-bdset. 



Kind of base. 



Co«f- I Mot*. 

 fldent. ; meat. 



Foro«. 



I Coef- 

 i fldent. 



Level clay 



Uneven clay 



Loam 



Level sand 



HDch (,Tavel 

 nch broken stone 

 nch broken stone 



l.SO 

 1.29 

 1.18 

 1.34 

 1.10 

 .92 

 1.78 



Inehe$. 



0.06 



.06 



.05 



.05 

 .05 

 .05 

 .06 



Pound*. 

 1,800 

 1,800 



i,8ai 



1,200 

 1,100 



1,900 



3.07 

 2.07 

 3.07 

 I.3H 

 1.3A 

 1.00 

 2.18 



