FOUNDATIONS The stability, permanence, and maintenance of any pavement depend upon its foundation. If the foundation is weak, the surface will soon settle unequally, forming depressions and ruts. With a good foundation, the condition of the surface will depend upon the material employed for the pavement and upon the manner of laying it.
The essentials necessary to the forming of a good foundation are: (1) The entire removal of all vegetable, perishable, and yielding matter. It is of no use to lay good material on a had substratum.
(2) The drainage of the subsoil wherever necessary. A per manent foundation can be secured only by keeping the subsoil dry; for, where water is allowed to pass into and through it, its weak spots will be quickly discovered and settlement will take place.
(3) The thorough compacting of the natural soil by rolling with a roller of proper weight and shape until it forms a uniform and un yielding surface.
(4) The placing on the natural soil so compacted, a sufficient thickness of an impervious and incompressible material to cut off all communication between the soil and the bottom of the pavement.
The character of the natural soil over which the roadway is to be built has an important bearing upon the kind of foundation and the manner of forming it; each class of soil will require its own special treatment. Whatever its character, it must he brought to a dry and tolerably hard condition by draining and rolling. Sand and gravels which do not hold water, present no difficulty in securing a solid and secure foundation; clays and soils retentive of water are the most difficult. Clay should be excavated to a depth of at least 18 inches below the surface of the finished covering; and the space so excavated should be filled in with sand, furnace slag, ashes, coal dust, oyster shells, broken brick, or other materials which are not excessively absorb ent of water. A clay soil or one retaining water may be cheaply and effectually improved by laying cross-drains with open joints at inter vals of 50 or 100 feet. These drains should be not less than 18 inches below the surface, and the trenches filled with gravel. They should be 4 inches in internal diameter, and should empty into longitudinal drains.
Sand and planks, gravel, and broken stone have been successively used to form' the foundation for pavements; but, although eminently useful materials, their application to this purpose has always been a failure. Being inherently weak and possessing no cohesion, the main reliance for both strength and wear must be placed upon the surface covering. This covering—usually (except in case of sheet asphalt) composed of small units, with joints between them varying from one half an inch to one and a-half inches—possesses no elements of cohe sion; and under the blows and vibrations of traffic the independent units or blocks will settle and be jarred loose. On account of their
porous nature, the subsoil quickly becomes saturated with urine and surface waters, which percolate through the joints; winter frosts up heave them; and the surface of the street becomes blistered and broken up in dozens of places.
Concrete. As a foundation for all classes of pavement (broken stone excepted), hydraulic-cement concrete is superior to any other. When properly constituted and laid, it becomes a solid, coherent mass capable of bearing great weight without crushing. If it fail at all, it must fail altogether. The concrete foundation is the most costly, but this is balanced by its permanence and by the saving in the cost of repairs to the pavement which it supports. It admits of access to subterranean pipes with less injury to the neighboring pavement than any other, for the concrete may be broken through at any point without unsettling the foundation for a considerable distance around it, as is the case with sand or other incoherent material; and when the concrete is replaced and set, the covering may be reset at its proper level, without the un certain allowance for settlement which is necessary in other cases.
Thickness of Concrete. The thickness of the concrete bed must be proportioned by the engineer; it should be sufficient to provide against breaking under transverse strain caused by the settlement of the subsoil. On a well-drained soil, six inches will be found sufficient; but in moist and clayey soils, twelve inches will not be excessive. On such soils a layer of sand or gravel, spread and compacted before pla cing the concrete, will be found very beneficial.
The proportions of the ingredients for concrete used for pavement foundations are usually: 1 part Portland cement 3 parts Sand 7 parts Broken Stone.
Or, 1 part Natural Hydraulic Cement 2 parts Sand 5 parts Broken Stone.
The question is sometimes raised as to whether Natural or Port land cement should be used. Natural cement is more extensively employed on account of its being cheaper in price than Portland. There is no advantage gained in using Portland cement. Concrete should not be laid when the temperature falls below 32' F.
The concrete foundation, after completion, should be- allowed to remain several days before the pavement is placed upon it, in order that the mortar may become entirely set. During setting, the con crete should be protected from the drying action of the sun and wind, and should be kept damp to prevent the formation of drying cracks.