CHOICE OF FOUNDATION.
It is always important that the foundation be suffi cient. The yielding of the base of the pavement means its destruction.
If a firm and durable foundation be employed, the surface may be renewed when necessary or changed from one material to another without disturbing the base, but if the base be weak the surface will be destroyed.
The saving of expense should be at the top rather than at the bottom of a pavement.
The thickness required for the foundation of a pave ment depends upon the nature of the soil upon which it is to rest, and upon the extent and weight of the travel to which it is to be subjected.
When the road-bed is of a retentive material and likely to become wet and soft, the foundation should possess sufficient strength not to be broken through at points where the supporting power of the road-bed may be destroyed by water. It must also be able to resist the action of frost upon the soil below. In such cases 8 or 9 inches of concrete may be necessary. Six inches of good concrete, however, constitute a foundation of considerable strength, and it is only under severe con ditions, poor support and heavy traffic, that a greater depth is necessary.
Under light traffic with good conditions, a less depth may be sometimes used; 4 inches of concrete is fre quently employed to save expense, although 6 is the more common depth. A depth of 4 or 6 inches of well compacted gravel or broken stone is also usually suffi cient where the conditions are such as to admit of the use of a foundation of that character.
It may be here observed that no definite prescrip tion for any pavement, either as to choice of founda tion or as to methods of construction, can fit all cases. What is most successful in one case is quite inappli cable in another. The blind following of particular rules by those not conversant with the principles upon which they are based has been the cause of many fail ures., Judgment must always be used in weighing the focal conditions the problem in hand.