If the topography is such that it is evident considerable storm water will flow from the adjacent land to the road ditches, the design must be modified to take this into account. Sometimes such water can be diverted by ditches well back from the road, and thus prevented from flowing into the side ditches along the roadway. It is especially desirable to divert water, which would otherwise flow down the slope of a cut, by means of a ditch on the hill-side above the upper edge of the slope of the cut.
Ditches are not effective unless they afford a free flow throughout their length and have an outlet to a drainage channel of ample capacity. Therefore, ditch grades should be established by survey, especially if the gradient is less than one per cent, and the construction work should be checked to insure that the ditch is actually constructed as planned. A few high places in the ditch will greatly reduce the effectiveness, although these may appear at the time of construction to be slight. Constricted places, such as might be due to a small amount of loose earth left in the ditch, are always to be avoided.
Where the side ditch pa.s.ses from a cut to the berm alongside a fill, the ditch should be excavated throughout in the undisturbed natural soil, five feet or more from the toe of the slope of the fill, and along the filled portion of the road there should be a berm of three or four feet between the toe of the slope of the fill and the near edge of the ditch.
=Underground Water.=--In a preceding paragraph, mention was made of the fact that only a part of the storm water runs off over the surface of the ground, the larger part being absorbed by the soil. The water thus absorbed flows downward through the pores in the soil until it is deflected laterally by some physical characteristic of the soil structure. The movement of underground water is affected by many circ.u.mstances, but only two conditions need be discussed herein.
Underground water, like surface water, tends to attain a level surface, but in so doing it may need to flow long distances through the pores of the soil, and to overcome the resistance incident to so doing some head will be required. That is to say, the water will be higher at some places than at others. If a cut is made in grading the road, the road surface may actually be lower than the ground water level in the land adjoining the road. As a result, the water will seep out of the side slopes in the cut and keep the ditches wet, or even furnish enough water to occasion a flow in the ditch. Similarly, the higher head of the underground water near the top of a hill may result in ground water coming quite close to the surface some distance down the hill. The remedy in both cases is tile underdrains alongside the road to lower the ground water level so that it cannot affect the road surface.
Sometimes the ground water encounters an impervious stratum as it flows downward through the soil, or one that is less pervious than the surface soil. When such is the case, the water will follow along this stratum, and should there be an outcrop of the dense stratum, a spring will be found at that place. This may be on a highway. The impervious stratum may not actually outcrop but may lie only a few feet under the surface of the road, in which case, the road surface will be so water soaked as to be unstable. The so-called "seepy places" so often noted along a road are generally the result of this condition. This condition can be corrected by tile laid so as to intercept the flow at a depth that precludes damage to the road. Commonly, the tile will be laid diagonally across the road some distance above the section where the effect of the water is noted, and will be turned parallel to the road at the ditch line and carried under one of the side ditches to an outlet.
=Tile Drains.=--Where the soil and climatic conditions are such that the roadway at times becomes unstable because of underground water rising to a level not far below the road surface, the ground water level is lowered by means of tile underdrains. The function of the tile drains in such cases is precisely the same as when employed in land drainage; to lower the ground water level.
=Laying Tile.=--The tile lines are usually laid in trenches parallel to the center line of the road near the ditch line and at least 4 feet deep so as to keep the ground water level well down. They must be carefully laid to line and grade. A good outlet must be provided and the last few joints of pipe should be bell-and-spigot sewer pipe with the joints filled with cement mortar. The opening of the tile should be covered with a coa.r.s.e screen to prevent animals from nesting in the tile.
It is frequently necessary to lay a line of tile at the toe of the slope in cuts to intercept water that will percolate under the road from the banks at the sides. In some cases, it is desirable to back-fill the tile trench with gravel or broken stone to insure rapid penetration of surface water to the tile. In other instances, it is advantageous to place catch basins about every three or four hundred feet. These may be of concrete or of tile placed on end or may be blind catch basins formed by filling a section of the trench with broken stone. When a blind catch basin is used, the top should be built up into a mound, and for a tile or concrete catch basin, a grating of the beehive type should be used, so that flow to the tile will not be obstructed by weeds and other trash that is carried to the catch basin.
=Culverts.=--Culverts and bridges are a part of the drainage system and the distinction between the two is merely a matter of size.
Generally, structures of spans less than about eight feet are cla.s.sed as culverts, but the practice is not uniform. In this discussion culverts will be defined as of spans of 8 feet or less.
Numerous culverts are required to afford pa.s.sage for storm water and small streams crosswise of the road, and their aggregate cost is a large item in the cost of road improvement. The size of the waterway of a culvert required in any location will be estimated by an inspection of the stream and existing structure, and by determining the extent and physical characteristics of the drainage area.
Sometimes there is sufficient evidence at the site to indicate quite closely the size required, but this should always be checked by run-off computations. The drainage area contributing water to the stream pa.s.sing through the culvert under consideration is computed from contour maps or from a survey of the ground, and the size of culvert determined by one of the empirical formulas applicable to that purpose. In these formulas, the solution depends upon the proper selection of a factor "C" which varies in accordance with the nature of the drainage area. Two of these that are quite widely used are as follows:
_Myers" Formula: a = CA_
Where _a_ = area of cross section of culvert in square feet. _A_ = area in acres of the drainage area above culvert. _C_ a factor varying from 1 for flat country to 4 for mountainous country or rocky soil, the exact value to be selected after an inspection of the drainage area.
_Talbot"s Formula_: Area of waterway in square feet =
_C_ [Square root of] ((Drainage area in acres)^3)
Transcriber"s Note: The above formula used the mathematical square root symbol in the original. One should read it as "C times the square root of the Drainage area in acres cubed."
_C_ being variable according to circ.u.mstances thus:
"For steep and rocky ground _C_ varies from 2/3 to 1. For rolling agricultural country, subject to floods at times of melting snow, and with length of valley three or four times its width, _C_ is about 1/3, and if stream is longer in proportion to the area, decrease _C_. In districts not affected by acc.u.mulated snow, and where the length of valley is several times its width, 1/5 or 1/6 or even less may be used. _C_ should be increased for steep side slopes, especially if the upper part of the valley has a much greater fall than the channel at the culvert. The value of _C_ to be used in any case is determined after an inspection of the drainage area."
[Ill.u.s.tration: Fig. 2. Design of Pipe Culvert and Bulkhead]
=Length of Culvert.=--The clear length between end walls on a culvert should be at least equal to the width of the roadway between ditches.
This is a minimum of 20 feet for secondary roads and ranges from 24 to 30 feet for main roads. The headwall to the culvert should not be a monument, but should be no higher than needed to prevent vehicles from leaving the roadway at the culvert.
=Farm Entrance Culverts.=--At farm entrances, culverts are required to carry the farm driveway across the side ditch of the road. These culverts are usually about 16 feet along, and should be of a size adequate to take the flow of the side ditch. The farm entrance culvert should be of such design that it can be easily removed to permit cleaning out the ditches with a road grader.
TYPES OF CULVERTS
Culverts constructed of concrete and poured in place are called box culverts because of the rectangular form of the cross section.
Culverts of pre-cast pipe are known as pipe culverts. Several forms of pipe culvert are in general use.
[Ill.u.s.tration: Fig. 3.--Typical Concrete Box Culvert]
=Metal Pipe.=--These may be of cast iron, steel or wrought iron. The cast iron pipe is very durable but expensive and heavy to handle and is not widely used in highway construction. Steel pipe has been employed to a limited extent but its durability is questioned. At least it is known that the pipe made from uncoated, light sheet steel is not very durable. Sheet iron and sheets made from alloy iron coated with spelter have been extensively used and seem to be durable, especially when laid deep enough to eliminate possibility of damage from heavy loads. To insure reasonable resistance to corrosion, the metal sheets should be coated with at least one and one-half ounces of spelter per square foot of sheet and the sheets should not be lighter than 16 gauge for small sizes and should be heavier for the larger sizes.
=Clay and Cement Concrete Pipe.=--The ordinary burned clay bell and spigot pipe that is employed for sewer construction is sometimes used for culverts. It must be very carefully bedded, preferably on a concrete cradle and the joints filled with cement mortar. Culverts of this type have a tendency to break under unusual loads, such as traction engines or trucks. They may be damaged by the pressure from freezing water, particularly when successive freezing and thawing results in the culvert filling with mushy snow, which subsequently freezes.
=Concrete Pipe.=--Reinforced concrete pipe is a satisfactory material for culverts, if the pipe is properly designed. The pipe should be carefully laid on a firm earth bed with earth carefully back-filled and tamped around the pipe. The joints in the pipe should be filled with cement mortar, or should be of a design that will be tight.
=Endwalls for Culverts.=--A substantial retaining wall is placed at each end of the culvert barrel, whatever the type. This is to prevent the end of the culvert from becoming choked with earth and to retain the roadway at the culvert. It also indicates to the drivers the location of the end of the culvert. The endwall extends a foot or more below the floor of the culvert to prevent water from cutting under the barrel. Plain concrete or stone masonry are most commonly used for culvert endwalls.
[Ill.u.s.tration: Fig. 4.--Two Types of Drop Inlet Culvert]
=Reinforced Concrete Box Culverts.=--The pipe culvert is limited in application to the smaller waterways. Reinforced concrete is extensively used for culverts of all sizes, but especially for the larger ones. These are usually constructed with endwalls integral with the barrel of the culvert. Culverts of this type must be designed for the loads antic.i.p.ated to insure suitable strength and stability, and must be constructed of a good quality of concrete. Figs. 2 and 3 show designs for pipe and box culverts.
[Ill.u.s.tration: Fig. 5.--Drop Inlet Culvert]
=Drop Inlet Culverts.=--In some locations erosion has begun in the fields adjacent to a culvert and it will probably continue until the stream above the culvert has eroded to about the level of the floor of the culvert. This is a reason for placing the culvert as high as the roadway will permit, so long as the area above the culvert will be properly drained. Considerable reclamation of land is possible if the culvert is constructed with a box at the inlet and as shown in Fig. 4.
The area up-stream from the culvert will not erode below the level of the top of the box at the inlet end.
Where the stream crossing the road has eroded to considerable depth or has considerable fall, as would sometimes be the case on side hill roads, the culvert barrel would follow the general slope of the ditch but should have a drop inlet. This type of culvert is shown in Fig.
5.
CHAPTER IV
ROAD DESIGN
=Necessity for Planning.=--Sometimes highway improvement is the result of spasmodic and carelessly directed work carried out at odd times on various sections of a road, finally resulting in the worst places being at least temporarily bettered. The grade on the steepest hills is probably reduced somewhat and some of the worst of the low lying sections are filled in and thereby raised. Short sections of surfacing such as gravel or broken stone may be placed here and there. From the standpoint of the responsible official, the road has been "improved,"
but too often such work does not produce an improvement that lasts, and sometimes it is not even of any great immediate benefit to those who use the roads. In nearly every instance such work costs more in money and labor that it is worth.
Lasting improvement of public highways can be brought about only through systematic and correlated construction carried on for a series of years. In other words, there must be a road improvement policy which will be made effective through some agency that is so organized that its policies will be perpetuated and is clothed with enough authority to be capable of enforcing the essential features of good design and of securing the proper construction of improvements.
Details of highway construction and design must vary with many local conditions and types of surface. The limits of grades and the many other details of design may properly be adopted for a specific piece of work only after an adequate investigation of the local requirements and in the light of wide experience in supervising road improvement.
New ideas are constantly being injected into the art of road building, but these are disseminated somewhat slowly, so that valuable devices and improvements in methods remain long unknown except to the comparatively few who have the means for informing themselves of all such developments.
It follows then that the logical system of conducting road improvement is through an agency of continuing personnel which will supervise the preparation of suitable plans and direct the construction in accordance with the most recent experience.
=Road Plans.=--The information shown on the plans prepared for road improvement varies somewhat with the design and with the ideas of the engineer as to what const.i.tutes necessary information, but in general the plans show the existing road and the new construction contemplated in an amount of detail depending princ.i.p.ally upon the character of the construction. Simple plans suffice for grade reduction or reshaping an earth road surface, while for the construction of paved roads, the plans must be worked out in considerable detail. The essential requirement is that there be given on the plans all information necessary to enable the construction to be carried out according to the intentions of the engineer, that all parts of the work fit together, that the culverts are of the proper size and located at the proper places, ditches drain properly, grades are reduced to the predetermined rate, that excavated material is utilized and that an exact record of the work done is retained. Plans are indispensable to economical road construction and the preparation of the plans is the work of the expert in road design, that is, the highway engineer.
=Problem of Design.=--The problem of road design is to prepare plans for a road improvement with the various details so correlated as to insure in the road constructed in accordance therewith the maximum of safety, convenience and economy to the users thereof. The degree to which the design will be effective will depend to a considerable extent upon the financial limitations imposed upon the engineer, but skill and effort on the plans will do a great deal to offset financial handicap and no pains should be spared in the preparation of the plans. Moreover, the plans must afford all of the information needed by the contractor in preparing a bid for the work.
=Preliminary Investigation.=--The first step in road improvement is to secure an adequate idea of the existing conditions on the road or roads involved. The detail to which this information need go will depend entirely upon the purpose of the preliminary investigation, for before a definite plan is prepared, it may be necessary to choose the best from among several available routes. For this purpose, it is not always necessary to make an actual instrument survey of the several routes. A hasty reconnaissance will usually be sufficient. This is made by walking or riding over the road and noting, in a suitable book or upon prepared blanks, the information needed. The items of information recorded will usually be as follows: distances, grades, type of soil on the road and nature of existing surface, character of drainage, location of bridges and culverts and the type of each with notes as to its condition, location of railway crossings and notes as to type, location of intersecting roads, farm entrances, and all similar features that have a bearing on the choice of routes. These data can be obtained in a comparatively short time by a skilled observer who may drive over the road in a motor car. Sometimes it may be desirable to make a more careful study of some certain sections of road and this may be done by waking over the section in question in order to make a more deliberate survey of the features to be considered than is possible when riding in a motor car.
Factors other than relative lengths of routes will obviously determine the cost of improvement and the comparative merits of the improved roads. Some special characteristic of a road, such as bad railroad crossings or a few bad hills, may eliminate a route, or availability of materials along a route may offset disadvantages of alignment or grade.