What is California Bearing Ratio (CBR) and its significance?

CBR is a penetration test comparing soil strength to standard crushed stone (CBR = 100%). It measures load required to penetrate a soil sample at specific depth. CBR values indicate subgrade quality: <3% very poor clay, 3-7% poor to fair, 7-20% fair to good, >20% excellent gravelly soil. CBR is used in empirical pavement design methods to determine required thickness - weaker subgrade (lower CBR) needs thicker pavement. Tests performed at field moisture and density conditions.