Survey
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Download TRANSPORTATION ENGINEERING II
Transcript
CALIFORNIA BEARING RATIO (CBR) METHOD Typical Testing Machine Introduction • The California Bearing Ratio devised by engineers of the California Division of Highways in nine years period to 1938. • Most universally accepted pavement design methods. • A standard penetration-type loaddeformation test is carried out, and using the values obtained from the test from an empirical design chart, the pavement thickness are calculated. CBR LABORATORY TEST Specimen may be • Prepared • In-situ Mould • • • • 150mm diameter 127.3mm height Separate base plate A collar • The specimen is soaked and the expansion is measured. • Load is applied by the loading frame through a cylinder plunger of 50mm diameter and penetration is measured. • Rate of penetration is maintained at 1.25mm/minute. • Loads are recorded for 2.5 and 5.0mm. • Load is expressed as a percentage of the standard load at the respective deformation level, and is known as the CBR value. CBR Equipment Typical Testing Machine Soaking Samples CBR Equipment CBR Test – Sample Preparation CBR Test – Load-Penetration Measurement California Bearing Ratio (CBR) • Load a piston at a constant rate • See what load it takes to make it penetrate a known amount CBR Equation x CBR 100 y x = material resistance or the unit load on the piston (pressure) for 2.5 mm or 5 mm of penetration y = standard unit load (pressure) for well graded crushed stone. For 2.5 mm of penetration = 1000 psi For 5.0 mm of penetration = 1500 psi Typical CBR Ranges General Soil Type Clean gravels Gravels with fines Clean sands Sands with fines Silts and clays USC Soil Type CBR Range GW 40 – 80 GP 30 – 60 GM 20 – 60 GC 20 – 40 SW 20 – 40 SP 10 – 40 SM 10 – 40 SC 5 – 20 ML 15 or less CL 15 or less OL 5 or less MH 10 or less CH (LL>50%) 15 or less OH 5 or less California Bearing Ratio Equipment California Bearing Ratio Vs Moisture content CBR DESIGN METHOD • In this method, the CBR values are used to determine the total thickness of the flexible pavement and its various layers. • Following are the graphs used in the design of the pavement thickness. Flexible Pavement Design Using CBR (Road Note 31)
