136 



THE GARDENERS' CHRONICLE OF AMERICA. 



the side ne>t the park the pavement was made 12 feet 

 wide from rail tu curb ; while on the opposite side, where 

 the property was devoted principally to business pur- 

 poses, the width was 18 feet. 



Manifestly the strongest argument for keeping the 

 width of pavements down to a minimum is that of cost, 

 both of construction and maintenance, where the total 

 width between property lines is not cramped. However, 

 for certain reasons mentioned above, mainly esthetic, ap- 

 plying especially tij residence streets, boulevards and 

 highways, the pavement should not occup\- the whole 

 wfdth. 



The boundary lines of roads and streets are often en- 

 croached on bv owners of abutting property and the ac- 

 tual width reduced by projecting steps, entrances, pilas- 

 ters of buildings, etc. The laxity of supervision that per- 

 mits such appropriation of public property to private uses 

 cannot be too severely condemned. An encroachment 

 once permitted is difficult to remove, and as time goes on 

 is more and more objectionable. Encroachment on the 

 public highway should be as vigorously guarded against 

 as on private property. In the design of pavements some- 

 times features of construction are introduced that reduce 

 the effective width, such as badly designed catch-basin 

 inlets, excessive cross slopes, near the gutters, raised 

 crossing and approaches to intersecting sidewalks, etc. 

 There is nothing logical in a design that calls for a catch- 

 basin inlet at the corner of intersecting streets with deep 

 gutters and high curbs, and then, as a means of overcom- 

 ing these unpleasant features, a raised crossing approach 

 that forces traffic out towards the center just where con- 

 gestion is the greatest. 



In the improvement of country and suburban highways 

 more latitude may be taken as to dimensions and charac- 

 ter of pavement than on city streets. Conditions are gen- 

 erally more various. Location, grades, drainage, founda- 

 tion, materials of construction, require more individual 

 study and collection of data. A good road is the most 

 potent factor in the upbuilding of a community, and it 

 should be constructed to carry not only the existing traf- 

 fic, but the increase that will be stimulated by the con- 

 struction of the road itself. 



Correct relation between the service demanded of a 

 road and the elements of design, which include carrying 

 capacity and durability, is essential in the construction of 

 any good road. The proper depth or thickness of pave- 

 ment is a large factor in determining the capacity and 

 durability. Under modern conditions we cannot get away 

 from constant maintenance if we are to have satisfactory 

 roads and streets. Therefore it is impracticable to con- 

 struct so liberally and permanently as to entirely resist 

 wear. The Romans achieved this to a considerable de- 

 gree in their roads, which were several feet thick, built of 

 layers of stone blocks. So their principal roads or some 

 remnants of them exist to-day, but do not meet any of the 

 needs of to-day. They did not know, apparently, much of 

 the art of sub-surface drainage or preparation of the 

 roadbed to receive the stone ; and while it is possible they 

 may have crowned the surface to take away the stirface 

 water, there is little doubt but their roads were rough, 

 dusty, non-resilient and generally unsatisfactory accord- 

 ing to modern standards. Certainly, they would be now, 

 and were then, expensive to construct. At that period 

 there were but few roads, only between important points 

 and principally for military purposes. Now the demand 

 is for roads everywhere built as economically and as ef- 

 ficiently as possible. The thickness, no matter of what 

 material constructed, should be the minimum compatible 

 with intelligent planning. With all the foresight that can 

 be exercised, a road good for this decade will not lie good 



enough for the next decade, all of which is but saying a 

 large part of the road problem is a maintenance problem. 



An efficient thickness of pavement involves all other 

 elements of economical construction — proper underdrain- 

 age, compaction of subgrade, crowning and waterproof- 

 ing of surface, etc. ■ 



With all the kinds of wearing surfaces that have been 

 devised — stone block, brick, creosoted wood, asphalt and 

 tar compositions, oiled and water-bound macadam, and 

 ithe infinite variations thereof — there are only two recog- 

 nized materials for foundations — concrete and macadam. 

 The main difference in the action of the two is that con- 

 crete is practically homogeneous material, the particles 

 positively bonded and knit together so there is a beam or 

 slab action carrying the wheel road over a comparatively 

 broad expanse of base, while macadam is made up of sepa- 

 rate particles which are inter-supporting only and trans- 

 mit the load from one particle to the others beneath. As 

 one particle of stone in the surface may be supported by 

 two, three or four particles immediately below, and each 

 of them on an equal number in the next lower layer, and 

 so on down to the earth beneath, we may reasonably esti- 

 mate the wheel load is spread to the earth base at an angle 

 I if approximately 45 degrees in all directions. Thus, if 

 the contact surface of a wheel is 4 by 6 inches, and the 

 load on each wheel 2,000 pounds, it will be spread over 

 approximately 2 square feet of supporting earth. If the 

 supporting power of the earth is 1,000 pounds per square 

 foot (authorities give from 500 to 2,500 pounds per square 

 foot, according to character of earth, efTectiveness of 

 drainage, etc.). we may conclude our macadam founda- 

 tion is thick enough to carry the load. 



A concrete foundation will obviously spread the load to 

 a greater extent than macadam, on account of its homo- 

 geneity and consequent slab action, and may be made 

 correspondingly thinner. Consideration, however, must 

 be given to the fact that the breaking strength of a 5- or 

 6-inch slab of concrete, made as such concrete usually is, 

 cannot be very high. Cracks occur in concrete founda- 

 tions on account of contraction, settlement, etc., which re- 

 duce the slab action. 



There is another reason why concrete pavement foun- 

 dations can safely be made thinner than macadam, and that 

 is the greater stability of the mass or resistance to lateral 

 displacement. 



If there is a wearing surface of blocks, asphalt compo- 

 sition, or other material, on top of the concrete or mac- 

 adam foundation it will further assist in the spread of the 

 road. The transmission of the load to the foundation 

 through a wearing stn"face of blocks can only be safely 

 calculated in vertical lines with a surface contact the size 

 of the block, on accoimt of the vertical joints between 

 blocks. 



The thickness of the various types of wearing" surface 

 is mainly governed by practical reasons. It is not by any 

 means true that pavement stirfaces generally are to be 

 improved by arbitrarily increasing their thickness. Two 

 inches' is probably the practical and economical thickness 

 for asphalt and asphaltic concrete wearing surfaces, be- 

 cause before the two inches is worn away the inequality of 

 surface becomes a serious objection, and a greater thick- 

 ness would allow it to roll and shift when softened by hot 

 weather. Creosoted wood blocks cannot be made less 

 than Syi or 4 inches thick, because the blocks would split 

 too easily. 



The crown of a pavement is a matter almost entirely 



of surface drainage. Even though a road or street has a 



considerable longitudinal grade, it is necessary to carry 



the surface water to the gutters. The crown probably 



( Continued on pai^e 139.) 



