The California Bearing Ratio Method for The Design of Flexible Roads and Runways

Abstract

Introduction One of the most important applications of soil mechanics to road and airfield engineering is to the calculation of the thickness and strength of pavement which has to be placed on a soil formation to provide a stable riding surface. Such calculation is known as pavement design. For the purposes of pavement design, types of construction are divided into two classes: rigid pavements which are partly or entirely composed of concrete, and gexiile pavements which are composed of bituminous materials usually on a base of macadam or pitching. This paper is solely concerned with the design of flexible pavements. In recent years a considerable number of methods of pavement design has been proposed for flexible pavements. Some of these methods are empirical and are based on experience of the performance of different roads or runways. Others are based on theoretical considerations of the stresses or strains involved and the remainder are based on a combination of the theoretical and empirical approaches. All the methods relate the thickness of construction to the strength of the soil on which the pavement is founded (the subgrade) and to the traffic conditions, the severity of which is usually given in terms of the maximum wheel load of the traffic using the pavement. The many methods available can give widely different results. The theoretical aspects involved are not yet fully understood so that it is probably true to say that at present the empirical type of method generally gives more reliable results than the theoretical type. It should be realised that until comparatively recently the design of roads and runways often took little account of the type and condition of the soil foundation and that the use of any rational method of design represents an important advance in road and airfield engineering. It was considered at the Road Research Laboratory that the. first aim should be to produce a reliable empirical method of pavement design for British conditions; the more theoretical approach has not, however, been neglected and a mathematical theory of thestresses in layered elastic systems has been developed by Fox (Ref. 1) and experimental work is in hand to check the values given by this theory. Experience with a number of methods of pavement design has shown that an empirical method, known as the California bearing ratio (C.B.R.) method, was likely to prove the most suitable for use in this country, and this method was therefore selected for study. The present paper describes this method, and the investigations that have been made at the Road Research Laboratory to determine its reliability and the best procedure for its use.